Hormones, genes and the structure of sexual arousal

Behavioural Brain Research 105 (1999) 5–27

Hormones, genes and the structure of sexual arousal

Jonathan Frohlich a,b,*, Sonoko Ogawa a, Maria Morgan a, Leslie Burton b,Donald Pfaff a

a Laboratory of Neurobiology and Beha6ior, The Rockefeller Uni6ersity, 1230 York A6enue, New York NY 10021, USAb Department of Psychology, Fordham Uni6ersity, Bronx, New York, USA

Received 31 March 1999; accepted 28 April 1999

Abstract

Despite the inherent difficulty of connecting individual genes with integrated mammalian behaviors, it has been determined thata series of genes are turned on by estrogenic hormones acting in forebrain. Their products are, in turn, facilitatory for femalereproductive behaviors such as lordosis. The causal routes by which two genes contribute to the control of lordosis behavior, theclassical estrogen receptor gene (ER-a) and a thyroid hormone (TH) receptor gene (TR-b), have been delineated. Beyond themechanisms underlying the expression of concrete, specific natural behaviors, lies the question of sexual motivation. Required asan intervening variable to explain fluctuations in natural behaviors in the face of constant stimuli, motivational states have bothgeneral and specific features. Most theoretical and experimental approaches toward the general aspects of motivation havedepended heavily on concepts of ‘arousal.’ Sexual arousal is likely to depend both on very general, broadly distributed neuronalinfluences and on specific affiliative and sexual tendencies. Is ‘general arousal’ a monolithic, undifferentiated process? In no waycan a review at this time settle such issues, but the reasons behind six new experimental approaches to these questions aredescribed. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Estrogenic hormones; Sexual arousal; Stimuli

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1. Introduction

The difficulties of discerning genetic contributions tothe performance of mammalian social behaviors areclear. The pleiotropy of gene action, redundanciesamong gene product functions, and the lack of under-standing of penetrance, in addition to the plethora ofindirect influences of genes on integrated behaviors allhave been illustrated [166].

Nevertheless, at least six genes expressed in the fore-brain have the following properties: they are turned onby estrogen administration, and their products foster anestrogen-dependent behavior, lordosis [165]. Since es-trogenic facilitation of lordosis behavior requires geneexpression and new protein synthesis, the most parsi-monious summary of these sets of facts would be, thatone way in which estrogen’s facilitate lordosis behavioris to turn on these genes.

In addition to the proof that certain genes are in-volved in controlling the normal performance of lordo-sis behavior, we have evidence as to precisely how theydo it. That is, what are their causal routes? Clearly theclassical estrogen receptor gene (ER-a) and the newlydiscovered estrogen receptor gene (ER-b) act as tran-scription factors in hypothalamic neurons, thus to pro-mote transcription from genes whose products areimportant for lordosis (Fig. 1). The case of the genesfor TH receptors is more complicated. DNA bindingevidence, transcriptional evidence, and data on theproduction of messenger RNAs in brain tissue[51,49,209,210] all indicate that liganded TH receptorscan interfere with estrogen-receptor actions and thusreduce lordosis behavior [50]. Since the TR-b knockoutfemale mouse actually has higher levels of lordosisbehavior and sexual motivation than wild-type con-trols, whereas the opposite is true for the TR-a knock-out mouse, we conclude that the TR-b gene product isthe one responsible for behaviorally important tran-scriptional interference. Thus, a causal route for the

* Corresponding author. Tel.: 1-212-3278666; fax: +212-3278664.E-mail address: [email protected] (J. Frohlich)

0166-4328/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 6 -4328 (99 )00079 -0

J. Frohlich et al. / Beha6ioural Brain Research 105 (1999) 5–276

gene product from the TH receptor-b gene is to inter-fere with ER gene action important for lordosis behav-ior (Fig. 2).

2. Sexual motivation

Experimental strategies for discerning mechanisms ofintegrated behaviors have, very reasonably, focused onthe performance of specific, concrete behaviors. How-ever, to describe reasons for fluctuations in responsefrequency and response strength in the face of a con-stant environment and constant triggering stimuli, mo-tivational concepts are, in a logical fashion, absolutelyrequired. Consider the following analogy with physics:Newton’s Second Law, F=MA, where F is a gravita-tional force. No one has ever ‘seen’ a gravitationalforce. Instead, the gravitational constant is mathemati-cally required in order to describe the observed rela-tionship between mass and acceleration. Likewise,motivation is an intervening variable, an inferred con-cept, logically required to explain changes in responseto a constant stimulus [164]. Demonstration of estro-genic effects on female rat sexual motivation havedepended heavily on the work of Bengt Meyerson andhis colleagues [reviewed in [164]]. Modern work hasused operant techniques to prove the motivation offemales to approach males [141] and to distinguishbetween affiliative and specifically sexual motivationalstates [Matthews, et al., JEAB, 1999 submitted].

What is the structure and what are the subdivisionsof the motivational influences that affect reproductionin the female? Behavioral experimentalists and theoristshave emphasized the general, undifferentiated aspect of

motivation, which Hull called ‘d’ and which Hebbascribed to a general state of arousal. Such a behavioralemphasis followed naturally from early electrophysio-logical studies of the ascending reticular activating sys-tems (RAS), first elucidated by Moruzzi, Magoun andLindsley and their colleagues. In turn, the neurologicalfacts gathered from human patients in coma or invegetative state supported the overall activating influ-ences of ascending reticular influences on behavior (Fig.3).

On the other hand, neurochemists, electrophysiolo-gists, and behaviorists have warned that motivationalconcepts cannot be restricted to a general, undifferenti-ated state of arousal. At least five neurochemical sys-tems are involved: ascending noradrenergic axons, twodopaminergic systems (mesolimbic and nigro-striatal),ascending serotonergic systems, cholinergic influencesfrom the lateral dorsal tegmentum, and histaminergicfibers ascending from the mammillary bodies. Behav-iorally, as well, it has been claimed that different mea-sures of arousal do not always correlate with each otherand that different subsystems for activity, alertness,selective attention, etc. must be operating [169]. Asregards the contributions of ascending arousal systemsto sexual arousal and sexual motivation, therefore,several questions emerge. Under well-controlled condi-tions in female mice, are different measures of arousalhighly correlated with each other? Are they equallysensitive to hormonal and genetic influences? Insofar asdifferent clusters of activational measures are consid-ered, what might they be? Below we have describedapproaches to these measures, divided into sensoryalertness, motoric activity, and emotional reactivity.

Fig. 1. The estrogen receptor (ER) gene products act as transcription factors to mediate the facilitatory effects of estrogens (E) on gene expressionin hypothalamic and forebrain neurons important for reproductive behaviors. Since those gene products are known to facilitate lordosis, thetranscriptional enhancement must be taken as one type of mechanism by which E promotes lordosis. Note that where the hormone enhances geneexpression both for the ligand and for its corresponding receptor, we have the capacity for the two E effects to multiply (× ) each other. By nomeans is this list of hormone-sensitive genes complete. For example, using differential display, Krebs et al. (PNAS, 1999) have identified hsc73as an E-responsive gene in hypothalamic neurons. Therefore, this list is expected to grow.

J. Frohlich et al. / Beha6ioural Brain Research 105 (1999) 5–27 7

Fig. 2. The thyroid hormone receptor b (TR-b) gene product acts as a transcriptional blocker, interfering with effects of ER as a transcriptionfactor (see Fig. 1) in hypothalamic neurons important for behavior. Thus, such transcriptional inhibition could subserve the inhibitory effects ofthyroid hormones on lordosis behavior. Note that current evidence indicates this transcriptional blockage could take place at the level of DNAbinding on estrogen response elements (EREs) or by competition for coactivators. Abbreviations: T4, thyroxine; T3, triodothyronine; E, estrogens;ER, estrogen receptors.

3. Arousal

We can not, in one paper, ‘settle’ the issues surround-ing the physiology of arousal (with us, now, for 50years), nor their application to sexual arousal. How-ever, we can review the scholarly literature supporting areasoned experimental approach to the dimensions un-derlying arousal in female mice.

A fundamental tenet of cognitive psychology is thatbehavior is goal-oriented. While some goals are explicit,such as life preservation, others are implicit, such as selfactualization. Behaviors aimed at achieving these goalsmay be considered motivated behaviors. But what con-struct underlies and provides the impetus for the actual-ization of goal directed, motivated behavior? Accordingto theorists such as Broadbent [24], changes in one’sunderlying state of arousal directly influences perfor-mance on such tasks. Arousal has been defined byHebb [100] as a state which optimizes the processing ofsensory stimuli. Such theorists thus posit arousal to bea unitary construct which exists along a continuumfrom deep coma to extreme excitement [58,67]. At anygiven moment, one’s level of arousal could be situatedat some point along this continuum.

Researchers who approach the construct of arousalfrom a physiological perspective, however, have some-times tended to argue for a modality-specific fractiona-tion of the construct. According to a review by Robbinsand Everett [169], while early physiological researchhad attributed the neurological substrate of Hebb’s[100] unitary arousal construct to the RAS, recentneuroanatomical and neurochemical evidence points toa much greater degree of specificity in the RAS thanoriginally thought. As such, Robbins and Everettprovide evidence for distinct arousal pathways dividedamongst ascending noradrenergic, dopaminergic, sero-

tonergic, and cholinergic systems. Such evidence wastaken to imply the functional fractionation of thearousal construct.

While it may be true that distinct arousal systems canbe identified on physiological and neurochemical levels,when manifest at a behavioral level such fractionationstill may not be relevant. Thus, the assumption byEysenck [67] and Robbins and Everett [169] that physi-ological multiplicity necessarily implies behavioral spe-cificity may be premature and needs to be tested.

3.1. Theories of arousal

3.1.1. HebbIn discussing his model of the ‘conceptual nervous

system,’ Hebb [99] postulated two frameworks to ac-count for sensory projections to the cortex. In the firstframework, sensory neurons and tracts project quicklyand efficiently through the thalamus to the cortex. Incontrast, the second framework subsumes ‘nonspecific’projections, and represents an arousal system.

In the arousal system, diffuse excitatory impulsesrandomly traverse through an intertwined array ofneurons and synapses, finally terminating in randomlydisparate areas of the cortex. While the communicationof discrete information can obviously not be facilitatedby this system, its purpose is rather to charge the cortexwith background activation and excitation withoutwhich the discrete messages of the first system aremeaningless. While such discrete messages would reachthe sensory cortex, the unexcited remainder of thecortex would not process the information further. Thepairing of appropriate responses to the stimulus wouldnot be retained.

As such, Hebb [99] postulated the operation of twoparallel psychological processes to result from sensory


stimulation. The first of these is a ‘cue function’ whichelicits specific behavioral responses. The second is an‘arousal’ or ‘vigilance function’ which provides theneurological landscape for the activity of the first func-tion. According to Hebb, cue functions are analogousto a steering wheel, while arousal functions areanalogous to a car engine.

3.1.2. DuffyFurthering Hebb’s concept of neurological excitation

in arousal, Elizabeth Duffy [58] conceptualized theliving organism as an ‘energy system.’ This system wasthought to vary along a dimension of activation, whichwas defined as ‘the extent of release of potential energystored in the tissues of the organism, as this is shown inactivity or response.’ Such activity may be covert aswell, such as experienced in anticipatory excitement.Duffy noted various techniques which may be em-ployed to index arousal such as metabolic pattern,skeletal muscle tension, electrical resistance of the skin,electroencephalogram (EEG), blood pressure, andrespiration.

Duffy [58] observed that behavior tended to varyalong dimensions of intensity and duration. With re-spect to direction, behavior was thought to always bedirected either towards or away from some environ-mental situation. Regarding intensity, any given behav-ior could be quantified by the amount of force orenergy expended. Of note, Duffy [58] pointed out thatone of the main objections to the notion of a globalconcept of activation is that a number of researchershave found low intercorrelations between the variousphysiological indices of the construct. In defense, how-ever, Duffy points out that intracorrelations, the degreeof correlation within each individual, are considerablygreater.

3.1.3. Yerkes and DodsonOne of the first theories to account for the relation-

ship between arousal and performance was advancedby Yerkes and Dodson in 1908. Know as the ‘Yerkes–Dodson law,’ this principle maintained that an inverted‘U’ relationship existed between arousal level and per-formance such that when arousal levels are too high ortoo low performance levels would be disrupted. Theprecise slope and apex of the curve was thought to varywith task complexity such that while a moderate levelof arousal is optimal for performance on complextasks, achievement on simple tasks would be enhancedby a somewhat higher level of arousal.

Research on this principle was originally conductedon mice in an animal-discrimination training paradigm.The ability of the mouse to learn to discriminate alonga brightness continuum was the dependent variable.The independent variable manipulated in these studieswas aversive drive, as indexed by the intensity of anelectric shock, and the level of task difficulty. Yerkesand Dodson [207] found that by gradually increasingshock intensity, an inverted ‘U’ function of perfor-mance emerged. Furthermore, maximum performanceresulted from higher shock levels on simple than ondifficult tasks. At the same time, the influence of shockwas more poignant on difficult tasks. More support forthis theory has been obtained when arousal is producedby aversive stimulus rather than by response incentives[67].

3.1.4. EasterbrookA later theory of arousal was developed by Easter-

brook [61]. Easterbrook maintained that an inverserelationship existed between arousal and the range ofenvironmental cues a person could make use of. Hedefined this range as ‘the total number of environmen-

Fig. 3. Proposed logic of motivational influences as they affect reproductive behaviors, including both widely distributed, general systems andspecific affiliative or sexual motivations. Generalized motivational systems are identical with ascending brainstem systems maintaining ‘arousal’.In turn, arousal pathways and their consequent behavioral functions can be subdivided into undifferentiated ‘activating’ influences and morespecific components.


tal cues in any situation that an organism observes,maintains an orientation towards, responds to, or asso-ciates with a response.’ The relative bandwidth of thespectrum of cues attended to, could be increased ordecreased, depending on the complexity of the task athand. Easterbrook felt that an ideal range of cues to beutilized existed for every behavior.

Based on the inverse relationship between arousaland cue utilization, Easterbrook [61] differentiated theconcept of organizing or motivational arousal fromthat of disorganizing or emotional arousal. On taskswhere a narrower range of cues would be beneficial toperformance, arousal would serve a positive, motiva-tional, function. In such instances, arousal could besaid to have a ‘focusing’ or ‘sharpening’ function.When a wider range would be more advantageous,however, increased arousal would have a negative effectand would be disorganizing, or emotional. As such,Easterbrook’s theory highlights the role of attentionalfactors in mediating the relationship between arousaland performance.

Walley and Weiden [200] provided a framework tosupport the relationship between arousal and cue uti-lization as discussed by Easterbrook [61]. In outliningtheir neuropsychological theory of attention, Walleyand Weiden postulated an increase in general arousalduring attention. Central to their theory of attentionwas the concept of encoding which referred to theconversion of sensory input into a form of data thatcould be used by perceptual and cognitive structures. Ahierarchical network of pattern analyzers facilitated aprocess of pattern recognition, of which encoding wasthe highest level. Cognitive masking, the mutual disrup-tion of encoding resulting from the processing of simul-taneous inputs, was thought to have been caused bylateral inhibition of cortical neurons in the highestlevels of the pattern recognition network. Walley andWeiden postulated that increases in arousal lead toincreases in lateral inhibition, and consequently in-creased cognitive masking. As such, heightened arousalwould have a negative effect on multiple and complexcue utilization in attentional processes.

3.1.5. BroadbentAnother major theory was developed by Broadbent

[24]. This theory highlighted the importance of studyingtwo arousing agents occurring concurrently and indi-vidually in order to determine additive effects. Broad-bent postulated the existence of two arousalmechanisms which function in concert with each other.‘Lower mechanisms’ were to be responsible for carryingout the product of carefully planned decisions. Suchmechanisms would be influenced by noise and lack ofsleep. Conversely, ‘upper mechanisms’ monitor and al-ter the parameters of lower mechanisms, enabling themto maintain standard of performance. Dimensions such

as introversion-extroversion and alcohol use impact theupper mechanism. Upper mechanisms can compensatefor problems of lower mechanisms on unpaced taskswhere one can consequently make up for periods ofinefficiency.

In support of this notion, Mirsky and Rosvold [149]found barbiturates (cortical depressants) to have moreof an effect on unpaced than on paced versions of atask. Conversely, tranquilizers (which function to loweractivity in the brain stem) and sleeplessness affect pacedtasks, thereby connoting their identity as of the lowermechanism. One important implication from this di-chotomy is that strictly behavioral indices are problem-atic to determine, because the same level ofperformance can result from either strictly lower mech-anisms, or from the compensatory actions of uppermechanisms.

3.1.6. NaatanenNaatanen [152] argued that the apex and slope of the

inverted ‘U’ function was not a function of arousal perse, but rather was caused by the degree of appropriate-ness of the pattern of activation. Under conditions ofmoderate arousal and optimal performance, the indi-vidual’s behavior would be directed solely towards theexperimental demand. Subsequent increases in arousalwould be detrimental to task performance in that theyprogressively move the direction of behavior fartheraway from the given task. Possible areas of divertedattention include other tasks and other reactions, suchas anxiety reactions, fatigue reactions, and loose associ-ations. It was thought to be this diverted focus whichunderlay the downward slope of the curve during higharousal.

In support of this idea, Naatanen [152] reported anunpublished study where a reaction time task was per-formed while the subject was riding a bicycle. Arousallevel was manipulated by setting the bicycle ergometerto different power levels. At higher levels, reactiontimes increased. Conversely, when the bicycle-ridingtask immediately preceded the reaction-time task, asituation where arousal during the latter task was simi-larly manipulated, no relationship between arousal andreaction time was found. As such. Naatanen concludedthat it was not the absolute level of arousal per sewhich was distracting, but the influence of arousal ondistractability, which was greater in the firstexperiment.

3.1.7. KahnemanKahneman’s theory [115] focused on the interrela-

tionships between motivation, arousal and attention.He emphasized the role of cognitive effort, as well asthe significance of constructs such as task difficulty,incentive, and motivation in the subjective calculationof determining how much effort to employ. Kahneman


noted that as tasks become more complex, the disparitybetween the effort needed to optimally complete the taskand the effort which was available for use, increases.Increased cognitive effort was thought to produce phys-iological arousal. He focused on pupillary dilation as anindex of such arousal. This idea of effort was incorpo-rated within larger constructs, namely allocation policyand evaluation of demands upon capacity. The functionof the latter construct is to supply effort to fulfill taskschosen by the allocation policy. This policy is, in turn,influenced by factors such as involuntary dispositions,transient desires, and attentional narrowing underheightened arousal.

3.1.8. ThayerThayer [189] emphasized the importance of self-re-

ported data in indexing arousal, and devised the activa-tion-deactivation adjective check list (AD-ACL). Thischeck list is comprised of numerous descriptive adjec-tives, from which participants choose in order to reflecttheir feelings of arousal during the test. Factor analysisof this measure revealed that there were four arousalfactors: (a)general activity, (b) high activation, (c) generaldeactivation, and (b) deactivation-sleep. Conceptualizedas ‘a measure of total orgasmic energy release,’ Thayerrelated each of its factors to physiological arousal usingskin conductance, heart rate, muscle action potentials,and finger blood volume as dependent variables, with theindependent variable being sitting quietly or performingmental arithmetic in the presence of noise. Although thephysiological measures correlated well with the AD-ACLfactors, they did not intercorrelate well with each other.Self report measures were found to correlate much better,and were taken to be a superior approach for indexingarousal.

3.2. Beha6ioral paradigms for measuring arousalcomponents

This paper will now review research pertaining tobehavioral paradigms which index arousal components.Paradigms are conceptually distinguished as reflectingeither sensory alertness, motoric activity, or emotionalreactivity. The specific measures selected were chosen fortheir experimental practicality and sensitivity, as well asfor their ability to generate meaningful quantitative data.They are not purported to be exclusive, however. Thatis, we are not claiming that these six experimentalapproaches are the only valid ones – merely that theyare practical, sensitive, and quantitative.

3.2.1. Sensory components and measures

3.2.1.1. Acoustic startle reflexDefinition and measurement of the acoustic startle

reflex. When mice are exposed to an abrupt noise, theytypically demonstrate the acoustic startle reflex. Davis[42] defined this reflex as a brief muscular reaction causedby the sudden onset of an acoustic stimulus. He alsonoted that it is mediated by emotional states such asstress and fear [43].

The acoustic startle response can be measured by astartle reflex system. Such systems have three majorcomponents, a control unit, a startle chamber, and anisolation cabinet. Upon presentation of the stimulus, thestartle response elicits a voltage change on the floor ofthe chamber which is then recorded by the system. Boththe latency and intensity of the response are recorded.

Neurology of the acoustic startle reflex. According toDunn and Berridge [59] intracerebroventricular (ICV)infusion of corticotropin-releasing factor (CRF) elicitschanges on behavioral, physiological, and endocrinolog-ical levels. These changes are similar to those normallyobserved under stressful conditions. In addition, ICVCRF infusion of 1 mg was found by Swerdlow, Geyer,Vale, and Koob [186] to significantly increase the acous-tic startle response in rats. Conversely, this study alsofound pretreatment with the benzodiazepine chlorodi-azepoxide (CDP) to attenuate this effect in a dose-depen-dent fashion. This finding is significant on a clinical levelin that many patients meeting diagnostic criteria forcertain psychiatric disorders such as post-traumatic stressdisorder (PTSD) demonstrate heightened CRF levels intheir cerebro-spinal-fluid [41], as well as more pro-nounced acoustic startle responses relative to normalindividuals [147].

These studies do not demonstrate, however, if or howsuch elevated CSF CRF levels interact with cortical orsubcortical brain regions in order to produce suchsymptoms. In order to answer this question, Lee andDavis [128] investigated the acoustic startle reflex. Previ-ous research using lesion and microinfusion methodol-ogy had revealed ICV infusion of CRF to engender thestartle reflex, with CRF antagonist alpha-helical CRF[185] and electrolytic amygdala lesions [132] to block thisresponse. At the same time, when CRF was infuseddirectly into the amygdala, this reflex was not observed.This indicated that while the amygdala may play anecessary role along the CRF-startle pathway, it was notthe primary target of CRF [132].

Lee and Davis [128] postulated the septum to func-tion as this CRF primary receptor site for three rea-sons. First, it maintains close proximity to the lateralventricle, and both the lateral and medial septum con-tain a significant amount of CRF receptors [31]. Sec-ond, research has indicated a number of rat limbicstructures, including the ventro-lateral septum, to bestimulated by ICV infusion of CRF into the lateralventrical [6]. Third, the septum is thoroughly connected


with cites central to the stress response, such as theparaventricular nucleus of the hypothalamus, the dorsalraphe nucleus, and the locus coeruleus, [184]. Further-more, a study using retrograde or anterograde([3]leucine radioautography) transport methods foundthe horizontal limb of the diagonal band of the septumto send efferents to the amygdala [142].

Some researchers have hypothesized the septum andthe amygdala to function in opposition to each other inthe production of fear and anxiety. Lee, Lin, and Yin[129], for example, found that lesioning the lateralseptum elicited increased tactile startle amplitude,thereby implying an increase in fear. At the same time,Melia, Sananes, and Davis [143] demonstrated thatwhen lesions of the central nucleus of the amygdalaaccompanied such septal lesions, the aforementionedincrease in acoustic startle was attenuated. Therefore,one role of the septum could be to attenuate theamygdala. As such, anxiogenic outcomes of septal le-sions would be caused by the disinhibition of the amyg-dala, and would thus implicate the septum as asignificant locus for CRF binding and for impacting thestartle reflex. This model would be consonant withother research pointing to the inhibition of neuronalactivity following iontophoretic administration of CRFto the lateral septum. Therefore, Lee and Davis [128]hypothesized that ICV infusion of CRF increasedstartle reactions as a function of the disinhibited amyg-dala, which in turn resulted from septal inhibition. Thismodel provided a theoretical framework from which tounderstand the similarity between behaviors normallyseen after the amygdala receives electrical stimulation,and those seen after ICV CRF administration.

The only problem with this hypothesis was that Leeand Davis [128] noted that fiber sparing lesions of themedial septum did not block CRF-enhanced startle.Therefore, fibers that passed through the septum musthave been significant. As a result Lee and Davis fo-cused on the hippocampal projection of the fornixwhich passes through the medial septum. Transectionof these fibers blocked CRF-enhanced startle. Thisimplicated the ventral hippocampus and its efferenttarget areas which communicate via the fornix. In orderto determine which efferent target was significant, Leeand Davis lesioned the bed nucleus of the striata termi-nalus (BNST). This region receives direct projectionsfrom the ventral hippocampus via the fornix. WhileNMDA lesions of this site blocked CRF-startle, chemi-cal lesions of the ventral hippocampus and the amyg-dala did not. This highlighted the significance of theBNST in CRF-enhanced startle.

3.2.1.2. Tactile startle reflexMeasurement of tactile (air-puff ) startle. Tactile

startle can also be measured using a startle reflex sys-tem. The only difference between the two paradigms is

that instead of a sudden sound, an abrupt burst of airis discharged onto the mouse’s neck. The startle re-sponse is recorded as the change in voltage of thestartle chamber floor caused by the abrupt physicalstartle reaction of the animal. As with auditory startle,both the latency and intensity (absolute amount ofvoltage change) of the animal’s response are recorded.

Neurology of tactile (air-puff ) startle. The exposureof rats to threatening circumstances has been shown toelicit both endocrinological and behavioral effects.When placed in such situations rats have been shown torelease adrenocorticotropin (ACTH) and corticosterone(Cort) from the hypothalamo-pituitary-adrenal (HPA)axis into the plasma [40]. In addition, their performanceon tasks thought to index anxiety is altered [131]. It isunclear, however, if these endocrinological and behav-ioral effects are necessarily correlated [71,201].

According to Engelmann et al. [62], airpuff adminis-tration in the startle paradigm represents a moderatestressor which is entirely psychological in nature, as itdoes not involve any physical pain. Furthermore, suchairpuff administration has been demonstrated to galva-nize the HPA axis [191–193]. A study by Engelmann etal. attempted to determine if participation in the air-puff-startle paradigm would therefore, elicit further be-havioral changes as well. To test this, Engelmann et al.correlated airpuff-induced endocrine measures with per-formance on the defensive-withdrawal procedure, aparadigm hypothesized to reflect fear level in the rat[188].

Airpuffs were administered in three blocks, with eachblock separated by 1 min. Within each block, the ratsreceived three airpuffs of 5 sec duration to the side ofthe head. There was a distance of 5 cm between thenozzle of the airpuff device and the head of the animal.Engelmann et al. [62] wore respirators during testing soas not to confound the paradigm by breathing on theanimals. A control group of animals was present in theroom during airpuff administration to equate for con-comitant auditory aspects of the airpuff procedure.

Regarding the behavioral paradigm, the rats werefirst exposed to the defensive-withdrawal paradigm.This procedure requires the rat to leave the tube it isoriginally placed in, and then explore an open field.Degree of exploration is hypothesized to have an in-verse relationship with intrinsic anxiety. The experimen-tal group received airpuff-startle immediately prior toexposure to the behavioral paradigm, while the controlgroup did not.

Results indicated that levels of plasma ACTH in ratssubjected to airpuff increased 10-fold. This reflected asignificantly more pronounced stimulation of the HPAin such rats when compared to controls. Experimentalrats in the defensive-withdrawal paradigm exhibitedsignificantly increased latency to leave the tube, lesstime in the open field, and a general reduction in


activity when compared to control rats. This indicatesthe induction of anxiety by the airpuff-startle. At thesame time, however, this behavioral measure of anxietydid not correlate with endocrinological measures, asboth experimental and control rats demonstrated com-parable elevations in ACTH and CORT. Engelmann etal. [62] report, however, that as such hormones weresampled only at 0 and 12 min, possible differences inrate of rise may have been overlooked. As such, aceiling effect may have obfuscated results.

3.2.2. Motor components and measure

3.2.2.1. Assessment of locomotion beha6iorOpen-field measure. A study by Lee, Tsai, and Chai

[130] characterized the patterns of mice in the open-field chamber. When first placed in the chamber, micetended to ambulate in the margin and avoid the centerregion. Lee, Tsai, and Chai took this to indicate aninitial fear of the center region. In accordance with thishypothesis, the proportion of center to margin time wasincreased as the duration of the experiment continued.This was taken to indicate that as time went on, themice felt progressively more comfortable in theirsurroundings.

Lee, Tsai, and Chai [130] hypothesized that priorexposure to physiological stress would increase theamount of center activity in mice. This was based onprevious findings by Roth and Katz [173] who demon-strated that while exposure to stressors such as lightand noise decreased initial movement latency, it in-creased center locomotor activity. As such, Lee, Tsai,and Chai exposed mice to footshock or immobilizationstress prior to being placed in the open-field chamber.Results indicated that activity in the total and especiallycenter region was increased regardless of the type ofstressor. Lee, Tsai, and Chai concluded that increasedcenter activity resulted from decreased fearfulness be-cause of the prior stress. As increased stress reflects aheightened level of arousal, we propose that mice whoare more highly aroused will exhibit greater total activ-ity, and possibly increased center activity as well.

Exploratory nosepoke measure. A second motor activ-ity assay is a home cage measure of exploratory nose-poke behavior. In our paradigm, a 1 cm hole has beenpreviously cut out of the far wall of each animal’s homecage. Attached to that hole is a small compartmentwhere an odoriferous substance is placed. As the animalpokes its nose through the hole to smell the substance,an infrared beam is broken and the event is automati-cally recorded on a computer.

Openfield and exploratory nosepoke behavior haveoften been used in conjunction in studies on locomotoractivity. For example, a study by Geyer, Russo, andMasten [86] attempted to both quantify and qualify thebehavioral activity of freely moving rats. Specific be-

haviors investigated were exploratory nosepokes, spa-tial and temporal characteristics of open-field activity,and rearing behavior. Geyer, Russo, and Masten uti-lized the behavioral pattern monitor (BPM) to assessthese characteristics.

Geyer, Russo, and Masten [86] point out that variousfactors must be considered in researching animal activ-ity. One factor they mention is the cause of the behav-ior. Research on exploratory and investigatorybehavior must take into account the fact that activitycould be generated from two sources. It could resultfrom the animal’s intrinsic level of arousal, or from anexternal demand.

A second possible confound is the lack of a cleardefinition as to what the observed behavior indexes.Berlyne [12], for example, argues for the significance ofstudying exploratory behavior, and notes that suchbehavior is correlated positively with the stimulus’ nov-elty and complexity, but negatively with prior exposureto it. The very same instruments used in many studiesto reflect such exploratory behavior, however, havebeen utilized in other studies to connote arousal andemotionality constructs.

As a result, researchers of exploratory behavior havemade use of holeboards. In addition to allowing for theintroduction of complex and novel stimuli, a study byFile and Wardil [70] demonstrated the measure to be avalid index of exploration in mice. Geyer, Russo, andMasten [86] point out that one advantage of addingholepoke measures to locomotor indices is that it allowsthe researcher to dissociate the general arousing orsedating properties of an intervention from its morespecific effects on behavioral responsivity to presentedobjects. For example, while both amphetamine andapomorphine increase locomotor activity, amphetamineincreases and apomorphine decreases the frequency ofholepokes [82,135].

According to Berlyne [12,13], specific explorations ofnearby objects should be qualitatively distinguishedfrom ‘inquisitive’ exploration of remote objects. Thesetwo categories demonstrate distinct rates of habituation[177]. The former category could be reflected in theduration of the various holepokes, while the secondcategory would be quantified in the variety of holesinvestigated within a given time frame. It should benoted, however, that this measure depends on baselineactivity differences, and as such requires concomitantindices of locomotion for comparison. In addition,diverse exploration could be examined by recording theroutes taken by the animal as it explores its environ-ment ([1,125]).

In order to account for the above variables, Geyer,Russo and Masten [86] utilized the BPM to code thebehavior of normal, untreated rats. This device assessesholepokes, rearings, and locomotor behavior. It ac-counts for the rate and duration of responding, as well


as for the pattern of holepokes, rearings, and locomo-tor activity, represented in a Cartesian coordinatesystem.

Geyer, Russo, and Masten [86] report finding a con-sistent mode of behavior in such rats. Locomotiontends to originate in a home area chosen by the subject,such as a corner. Rats reliably choose the same corneras their home base on repeated trials within the sameapparatus, and they maintain consistent explorationpaths. Research pertaining to the effects of stimulantdrugs (amphetamine, scopolamine, apomorphine, caf-feine, and nicotine), point out that some drugs, such asamphetamine (e.g. 5.0 mg/kg) tend to provoke stereo-typical patterns of locomotion [125]. Geyer, Russo, andMasten [86] further point out that rather than annullocomotion patterns previously obtained in normal con-ditions, stimulant drugs replace them with new, evenmore rigidly bound patterns. Within each group, newhome bases and routes are established.

In any case, this study points to the significance ofexamining holepoke behavior as a separate paradigmfrom other types of exploratory behavior. Eachparadigm may reflect a unique and separate aspect oflocomotive behavior. We hypothesize that mice whichare more highly aroused will demonstrate increasedexploratory nosepoke behavior.

Locomotor wheel measure. A third type of motoractivity assay is a measure of running wheel behavior.The proprioceptive and cutaneous cues provided by thewheel make this assay potentially independent of thefirst two.

3.2.2.2. Genetic factors in locomotion in mice. A studyby Crabbe [37] highlighted the significance of geneticfactors in the locomotive behavior of mice in an openfield. In his study, 19 inbred strains of male mice weretested under dull lighting, with the number of light-beam breakage’s as the dependent variable. Mice weretested for 3 min, 30 min after being injected with saline.Analysis of variance indicated significant differences inactivity measures between strains. While the least activestrain, CBA/J, obtained a mean of only 81 crossings,the most active strain, C57BR/cdJ, obtained a mean of211.9. Twenty-four hours later the mice were injectedwith ethanol (2.0 g/kg IP, 20% v/v) and retested after30 min. Obtained mean crossings differences betweendays significantly differed among the strains. Whileactivity decreased for six of the strains, C58/J andBALB/cAnN strains demonstrated increased activity.The remaining strains did not evince significant differ-ences. Further investigation revealed that while DBA/2N mice consistently demonstrated increased behaviorto ethanol irrespective of dosage or delay betweeninjection and assessment, C57BL/6N mice demon-strated transient increases in activity preceding an in-evitable long period of behavioral depression.

This furthers research by Tabakoff and Kiianmaa[187] on open-field activity in ethanol treated mice,where BALB/c and DBA/2 mice were shown to in-crease activity while no effect was found with C57B1/6mice. Furthermore, increased synthesis of striatalDOPA was found in BALB/c mice. This was not foundin the other two strains until a much greater dose ofethanol was administered. DOPA levels were signifi-cantly decreased in all strains, although this was less soin the C57BL/6 mice. These studies, therefore, suggest arelationship between genetics, neurochemistry, and be-havioral stimulation.

An additional method of determining genetic influ-ence is artificial breeding. In a study by DeFries,Wilson, and McClearn [48], mice scoring high or lowon a 2-min open-field test were selectively bred. After30 generations, the average scores of each strain werenearly 40 times apart, with no distribution overlap. Inconjunction with the above studies, this research high-lights the significance of genetic factors in locomotoractivity. As such, behavioral paradigms may interactwith genetic factors in their ability to index arousal.

3.2.2.3. Neurochemistry of locomotor beha6ior. This pa-per will now discuss the neurochemistry of locomotorbehavior. While the neurochemical systems reviewedmay each have other specific effects on arousal, thispaper is only concerned with those effects which can beindexed by purely behavioral techniques. To the extentto which specific behavioral paradigms are significant inreflecting arousal, their underlying neurochemistry issignificant as well.

Dopaminergic systems: the role of dopamine receptors.According to a review by Mink and Thatch [148], bothcortical and subcortical regions are important for theexecution of locomotor activity. Cortical efferent motorneurons are modulated by the basal ganglia. The sub-stantia nigra pars compacta, a subcomponent of thebasal ganglia, is the primary source of ascending do-paminergic input to the dorsal striatum. Dopaminergicneurons of the substantia nigra and the midbrain ven-tral tegmental areas (VTAs) project to a variety oftelencephalic systems such as the neostriatum, cortex,and limbic structures. Various behavioral functions arecontrolled by these DA projections such as spatiallearning and memory, locomotor activity, motivation,reinforcement, and rotational behavior[33,77,145,204,208]. Recent developments in molecularbiology have enhanced our ability to affect the expres-sion of single genes. For example, the utilization ofantisense oligonucleotide vectors allows for the tempo-rary inhibition of gene expression in vivo [38,159].Homologous recombination techniques have enabledthe production of transgenic mice where target genesequences can be destroyed or enhanced during devel-opment [29,122].


Recent advances in molecular biology have lead tothe identification of at least six dopamine receptorisophorms [87,94,181]. Biochemical and pharmacologi-cal studies have indicated that these fall into twoclasses, D1 receptors (D1A, D1B/D5) and D2 receptors(D2L, D2s, D3, and D4). D1 receptors have beenidentified by their affinity for phenyltetrabenzazepine-type drugs (e.g. SCH23390) as well as their coupling toadenylate cyclase, and are found postsynaptically onnon-dopaminergic target cells [5,156]. D2-like receptorsfunction in dopaminergic cells both as autoreceptors,on presynaptic terminals, soma and cell bodies, and aspostsynaptic targets [5,156]. D2 receptors expressaffinity for butyrophenone and benzamide sensitive lig-ands. In addition, their coupling to various secondmessengers allows them to inhibit adenylate cyclase andconductance of potassium channels. Striatal GABAer-gic neurons express dopamine receptors that belong toeither class.

Each category of receptors is further heterogeneousin terms of anatomical localization and expression ofdifferent levels of molecular isoforms [11,124]. Accord-ing to Smith et al. [179], this intimates that the variousreceptor isoforms have distinct roles in dopamine trans-mission. The nature of these distinct roles can be eluci-dated by experimentally manipulating the degree ofgenetic expression of each subtype of dopamine recep-tor, and then observing the functional consequences.

‘Despite differences in signal transduction mecha-nisms, the contribution of D1- and D2-like receptors tolocomotion through the direct and indirect striatopalli-dal projections is generally considered synergistic’ [118].Both D1 and D2 receptor antagonists can induce hy-pokinesia or akinesia [25,76,174]. Kelly et al. [118]pointed out, however, that current pharmacologicalagents do not demonstrate precise specificity for anydopamine receptor subtypes. As such, while infusion ofD2 antisense oligodeoxynucleotides in mice [211] resultsin an incomplete attenuation of striatal D2 receptors,spontaneous locomotor activity was nonetheless re-duced. Both categories of receptors must be stimulatedin order to counterindicate such akinesia [174,182].

Conversely, either an increase in basal activity [206]or no change in locomotion [56] has been found inanimals having undergone selective nullification of D1receptors via genetic targeting. Very little research hasbeen carried out on the relationship between the otherdopamine receptor subtypes and locomotion.

In order to further investigate the role of D2 recep-tors in locomotion, Kelly et al. [118] utilized mice whichas a result of gene targeting had been born withoutfunctional D2 receptors. Results indicated that onopen-field measures, horizontal movement of ho-mozygous D2 deficient mice was half the amount foundfor vehicle controls. At the same time, such activity wassignificantly more than what was found for haloperidol-

treated control mice, which demonstrated clear locomo-tor decrements. It was noteworthy that the reductionsin locomotion caused by D2 receptor depletion wassignificantly less than that resulting from the neuro-chemical blocking of D2-like receptors by haloperidolin wild-type mice. In order to explain this puzzlingfinding, Kelly et al. [118] postulated that the mutantmice may have developed certain CNS adaptations, andthereby been able to behaviorally compensate.

Smith et al. [179] note that many other studies havealso found that neurochemically altering the expressionof various dopamine receptor or transporter proteins inmice leads to clear alterations in overall levels of motoractivity [205,206]. These findings intimate that suchgenetic alteration leads to disruptions of dopaminergictransmission in the nucleus accumbens/ventral striatum.At the same time, however, broader aspects of behaviorhave been largely ignored in these studies. In light ofthis, Smith et al. [179] investigated the effect of D1Areceptor-deficiency on more widespread behavioral in-dices, in order to allow for the expression of the func-tions of the mesotelecephalic DA projections. As such,they employed open-field measures of exploration andactivity, a measure of sensorimotor orienting, Morriswater maze place and cue learning, and an olfactorydiscrimination test of associative learning.

Results indicated that homozygous mice were im-paired in the ability to initiate spontaneous activity,and demonstrated significant decrements in the abilityto respond to environmental stimuli. On open-fieldmeasures, a trend for decreased overall locomotion wasfound. Furthermore, heterozygous mice demonstrated asimilar pattern of results. In some instances, these micedemonstrated more pronounced impairments than thehomozygous ones. These findings extend the scope ofthose of earlier studies indicating the significance ofdopamine receptors in locomotor activity.

The role of dopamine neurons. A study by Horvitz,Stewart, and Jacobs [104] attempted to elucidate therole of ventral tegmental dopamine neurons in theresponse of cats to visual and auditory stimuli. TheVTA is the source of mesolimbocortical dopamine neu-rons. While previous research had demonstrated therole of the VTA in the neural circuitry of reward,Horvitz, Stewart, and Jacobs were interested in deter-mining the non-reward utility of such neurons. In theirstudy, VTA dopamine neurons demonstrated increasedexcitatory responding following visual and auditorystimulation.

These findings were taken to further the conclusionsof Freeman and Bunney [75] who demonstrated in-creased burst firing of VTA dopamine neurons subse-quent to manual stimulation of the rat vibrissae.Freeman and Bunney also found VTA burst activityduring periods where the rat seemed to demonstrateattention, and subsequent to presentation of whistling,


clapping and hissing sounds. Horvitz, Stewart, andJacobs [104] similarly observed increased dopamineburst firing when the cat seemed to be attending toenvironmental stimuli.

The significance of dopamine in attentional systems,however, further extends to the realm of behavior.Hooks and Kalivas [102], for example, found thatmicroinjection of dopamine into the ventral pallidumand nucleus accumbens engendered concomitant in-creases in locomotion and exploration in novel situa-tions. Similarly, Fink, and Smith [72] demonstrated thatdenervation of mesolimbocortical dopamine neurons bybilaterally microinjecting 6-hydroxydopamine into theanterolateral hypothalamus followingdesmethylimpramine pretreatment served to curtail in-vestigatory activity. Furthermore, Clarke, Jakubovic,and Fibiger [35] demonstrated that such lesions coun-terindicate increases in locomotion normally producedby dopamine agonists. In conjunction with the findingsof Horvitz, Stewart, and Jacobs [104] reviewed above, itwould appear that sensory stimuli activate mesolimbicdopamine neurons and their output at target areasinduces locomotor exploratory activity [104].

Dopamine and the nucleus accumbens. Dopaminergicpathways have been shown to originate in the midbrainand project to areas of the forebrain involved in theinitiation of movement as well as other facets of behav-ioral arousal and reinforcement [7,9,170]. The nucleusaccumbens septi has been reported to play a pivotalrole in dopaminergic forebrain function [134]. Thisstructure shares many aspects with its dorsal neighborthe caudate-putamen complex, otherwise known as theneostriatum [154]. The anterior cerebral cortex projectsafferents to the striatum, which in turn projects to thethalamus and then back to cortex. Prefrontal cortexneurons terminate in the ventral striatal-accumbensarea [4].

The nucleus accumbens is anatomically multifaceted,with a core and a shell region. These distinctions aremore clear in mid-accumbens [101,144]. Deutsch andCameron [52] reported that while the shell has higherconcentrations of dopamine and serotonin, the coredemonstrates greater dopamine and serotonin utiliza-tion. Furthermore, whereas haloperidol treatment facil-itated dopamine utilization more significantly in thecore, the shell showed increased utilization in responseto short duration immobilization stress.

A study by Campbell, Villavicencio, Yeghiayan, Ba-likian, and Baldessarini [27] sought to clarify the role ofregional variations in locomotion by injection of a setamount of dopamine within each anatomically definedregion. They found locomotor hyperactivity resultingfrom dopamine injection to be strongly related to thedorsomedial area. This was determined by calculatingthe degree of responding as well as by noting thatresponses to dopamine from adjacent and ventromedialsites were identical to control injections.

Admitting that discrete pharmacological manipula-tion of accumben subregions is difficult, Campbell,Villavicencio, Yeghiayan, Balikian, and Baldessarini[27] nevertheless offered hypotheses to explain theirresults. They postulated that significant factors included‘variations in the distribution of cell types affected byDA, local density and functional efficacy of specific DAreceptors, local intercellular connectivity, or efferent orafferent connections with more distant sites.’ Further-more, investigation of the role of D1 and D2-typereceptors in mediating locomotor activity and behav-ioral reinforcement elicited from rat accumbens sug-gested that both receptors needed to be activated foroptimal response.

Serotonergic systems: the role of serotonin neurons.According to a review by Soubrie [180], researchershave traditionally conceptualized serotonergic path-ways as inhibiting-motor systems. In one such study,Thiebot, Hamon, and Soubrie [190] investigated therole of serotonin pathways in behavior that had becomeinhibited as a result of punishment. In their study,when rats would press a lever to receive food they wereadministered an electric shock, which was then pairedwith the turning off of the room lights. Subsequently,the rats demonstrated a decrease in lever pressing whenthe lights were turned off. Such decreased behavior wasattenuated, however, when a benzodiazepine (CDP)was injected into the dorsal raphe, a locus of ascendingserotonergic pathways. This was taken to indicate thatserotonergic neurons had been responsible for the ini-tial behavioral inhibition. Further evidence for this ideawas obtained by the additional finding that 5,7-DHTlesioning prior to training rendered the benzodiazepinetreatment powerless.

Concurring evidence for the inhibitory role of sero-tonin could be drawn from a study by Clarke and File[34]. When rats were injected with 5,7-dihydrox-ytryptamine, a serotonin depletor, in the lateral septum,an increase in social-interaction, and consequently loco-motor behavior, was observed. This pattern was muchlike that seen after benzodiazepine administration. Con-versely, when NA and DA were depleted by 6-hydroxy-dopamine, social-interaction behavior did not increase.This highlights the role of serotonin in inhibiting boththe motivation for social interaction and the productionof locomotor behavior.

While these arguments may seem compelling, Geyer[79] noted that five lines of evidence point to thecontrary. First, research has demonstrated that thevarious serotonergic pathways in the brain may eachhave different functions [78,85,111,136]. Second, thepostulation of the inhibitory effect of serotonin isrooted in research pointing to hyperactivity in animalsfollowing the lesioning of the median raphe nucleus[78,111,136]. However, further research has demon-strated that neurotoxic lesions which engender a similar


degree of forebrain serotonin depletion as in the formerstudies do not result in hyperactivity as indexed byinvestigatory behavior on a holeboard [84]. In addition,serotonin depletion via para-chlorophenylalanine(PCPA) has been shown to either increase [69] ordecrease [26] locomotor activity depending on otherenvironmental and behavioral factors. Third, raphefiring rates and arousal, as manipulated by chemicallyinducing various degrees of muscle atonia, have beenshown to be positively correlated [183,195]. Fourth, theperceptual stimulation induced by hallucinogens havebeen shown to result from the activity of serotoninagonists, rather than overall serotonergic inhibition[44,108]. This implies an increase in the arousal ofsensory and cognitive systems by the activation ofserotonin pathways. Fifth, Geyer [79] asserts that do-pamine and serotonin receptor agonists seem to havesimilar effects in the treatment of schizophrenicpatients.

One method employed to determine the relationshipbetween serotonergic systems and locomotor arousalhas been to specifically apply appropriate agonists toserotonin receptors. Different receptor subtypes areassociated with different results. For example, when5-HT1A agonists are administered at low doses, theytend to decrease motor activity. At high doses, how-ever, they engender the opposite effect. On the otherhand, 5-HT-1B agonists result in reliable increases inlocomotor activity in a dose-dependent fashion[157,167]. Oberlander, Blaquiere, and Pujol [157] postu-lated that such activation must result from post-synap-tic serotonin receptors which had become supersensitiveas a result of deafferation following 5,7-DHT lesioning.5-HT2 receptor agonists do not affect motor activity,but increase the animals sensitivity to threatening com-ponents of the testing situation [81]. Consequently,investigatory activity in novel, but not familiar, envi-ronments is curtailed by hallucinogens [2].

In contrast to the focus on post-synaptic receptors inthe previous studies, Geyer [79] highlighted the signifi-cance of the discharge of presynaptic serotonin causedby indirect serotonin agonists. Such drugs includeMDMA (Ecstacy) and MDE (Eve). According toGeyer [79], the release of presynaptic serotonin causedby these drugs leads to a significant locomotor activa-tion in rats that appears to result from released sero-tonin acting on 5-HT1B receptors.

Studies using the BPM in rats indicate that MDMA-like drugs produce significant increases in locomotion[80,88]. MDMA induced hyperactivity is accompaniedby a significant reduction of investigation responses.Rearing and investigatory holepoke behavior are sig-nificantly attenuated by MDMA and MDE [88]. Thislatter finding is significant in that it allows us to distin-guish the cause of the former finding from that of apossible concomitant dopamine release, as dopamine-

releasing agents tend to increase locomotor and hole-poke activity [86]. In addition, dose-dependentincrements in locomotion produced by MDMA orMDE are further characterized by significantly alteredspatial patterning of locomotion; animals reliably movearound the perimeter of the chamber in unusuallystraight paths [88], unlike patterns seen after am-phetamine administration [83].

A study by Dringenberg, Hargreaves, Baker, Cooley,and Vanderwolf [57] further investigated the role ofserotonin in locomotor activity. Various researchershave utilized PCPA to investigate this issue. PCPAfunctions to prevent the biosynthesis of tryptophanhydroxylase, the enzyme responsible for serotonin syn-thesis, and hence leads to depletion of serotonin levels[119]. Despite a large number of such studies, however,contradictory results have been obtained. VariousPCPA doses and procedures used to assess locomotoractivity have been shown to both decrease and increaselocomotor activity. When rats are assessed in familiarenvironments, doses of 150–400 mg/kg PCPA areshown to increase activity as indexed by a runningwheel [69,110,140]. In unfamiliar environments, noveltyresponses such as grooming, rearing, and exploratorybehavior are often exhibited in rats [19,36]. In thissituation, increases [69], decreases [140] and no change[120] in such behavior have been reported followingPCPA administration.

Pharmacological investigations have implicated as-cending serotonergic pathways in motor functions.Vanderwolf [197] demonstrated an activating effect ofsuch neurons on the neocortex and the hippocampus.This effect was shown to be closely related to specificmotor performances.

In the Dringenberg et al. [57] study, PCPA-treatedrats were assessed in open-field exploratory behavior,running-wheel activity, and sensorimotor tests such asreactivity to stimuli applied directly to the animals feet.Results indicated that doses between 150–1000 mg/kgall reduced spontaneous exploratory and running wheelbehavior. Dringenberg et al. noted that for some behav-iors, doses of 150 or 500 mg/kg were not sufficient toproduce this effect, although it could be reliably pro-duced at higher doses. Furthermore, 76% of corticalserotonin could be depleted without any significanteffect on a number of complex behaviors in the rat.Therefore, inadequate serotonin depletion as a result ofinsufficient PCPA may explain the lack of decreases inbehavior in some previous studies. In addition, Drin-genberg et al. assert that pre- and post-synaptic com-pensatory mechanisms may obfuscate results.

Although they did not find increased running wheelactivity in this study, Dringenberg et al. [57] did findPCPA-treated rats to exhibit hyperreactivity to cuta-neous stimuli (i.e. Q-tip touching feet). They used thislatter finding to explain previous research indicating


PCPA treatment to increase running-wheel behavior[69] by the idea that the running wheel generally pro-vides a constant source of proprioceptive and cuta-neous stimulation, much like the stimulation applied bythe Q-tip. As such, PCPA may curtail only spontaneousmotor activity, such as open-field exploratory activity.

This idea receives support from neuroanatomicalstudies as well. Research has indicated locomotor wheeland open-field exploratory activity to function throughdifferent neuroanatomical pathways. Lesioning tech-niques have been used to demonstrate the dissociationbetween these systems. Dirlam [55], for example, foundseptal and tegmental lesions to especially decrease run-ning wheel behavior, while sparing cage and mazelocomotor activity. Lynch [138] isolated a pathwayspanning from the ventral frontal cortex through themedial forebrain bundle, whose destruction inhibitedrunning-wheel behavior but not stabilimeter cage activ-ity. Furthermore, Capobianco and Hamilton [30] foundlesioning of the fornix, medial forebrain bundle, ordiagonal band, to all increase running-wheel behavior,while only the former increased stabilimeter activity.

Rosencrans [172] further highlighted the relationshipbetween serotonin levels and locomotor activity in hisfinding that forebrain serotonin levels are higher inmore active rats. Similarly, Vanderwolf [197] noted thatelectrical stimulation of 5HT containing cells, pharma-cological elevation of central 5HT levels, and agonistadministration, resulted in the activation of the electro-corticogram in conjunction with movement but notimmobility.

Various studies investigating the effects of pharmaco-logically increasing CNS serotonin levels have found a‘serotonin behavior syndrome’ [107,109]. This syn-drome is characterized by ‘splaying of the hind limbs,alternating treading movements of the forelimbs, side toside movements of the head, elevation of the tail,tremors, generalized muscle spasms, hyperplaxia, ex-ophthalmos, salivation and ejaculation’ [198]. Jacobsand Klemfuss [109] found this constellation of behav-iors to remain intact even following decerebration. Fur-thermore, Deakin and Green [47] were able to elicit thissyndrome by administering a serotonin receptor ago-nist, even after the ablation of descending spinal sero-tonergic projections by 5,7-dihydroxytryptamine. Assuch, Jacobs and Klemfuss concluded that almost theentire syndrome is mediated by neuronal pathways inthe pons, medulla, and spinal cord.

Deakin and Green [47] also found that serotoninreceptor antagonists methysergide and methergoline ob-fuscated many of the behavioral aspects of this syn-drome. At the same time, however, the treated ratsbecame hyperactive and demonstrated greater locomo-tor activity. Of note, such heightened locomotion ischaracteristically present at the outset of the serotoninsyndrome (before such antagonists are introduced), but

soon dissipates in favor of the above symptoms[93,168]. Vanderwolf, McLauchlin, Dringenberg, andBaker [198] argue that this finding implies that sero-tonin-induced increases in locomotion would normallyoccur if not for their being obviated by the serotoninbehavioral syndrome. Thus, increased brain serotoninengenders hyperlocomotion related to cerebral corticalactivation, but its activation is prevented by simulta-neous secretion of spinal serotonin.

The roles of serotonin receptors. According to a re-view by Soubrie [180], serotonergic systems have beencommonly understood to inhibit motor activity. Rem-pel, Callaway, and Geyer [167] point out, however, thatrecent studies have demonstrated the opposite effect,and that observed 3,4-methylenedioxymethylam-phetamine (MDNA)-induced hyperactivity may not re-sult from increased dopamine release, but rather fromthat of presynaptic 5-HT. Rempel, Callaway, andGeyer offer evidence in support of this hypothesis.First, research characterizing animal behavior using theBPM indicates clear distinctions between the patternsof behavior observed after the administration of am-phetamine-like substances which increase dopamine re-lease [88] from hyperactivity caused by indirect 5-HTagonists. Second, MDMA-induced hyperactivity, whichnormally results from MDMA treatment does not oc-cur if the animal has been treated with SSRI’s such asfluoxitine, or with 5-HT synthesis inhibitor-parachlorophenylalanine [26]. At the same time, thesepretreatments either potentiate or have no effect onhyperactivity induced by dopamine.

These studies indicate that the increased locomotionobserved after administration of methylenedioxy-substi-tuted phenylalkylamines is a result of their function asindirect serotonin agonists [167]. But which types ofserotonin receptors are involved in this process? As aresult of differential binding to [3H]5-HT and[3H]spiroperidol ligands, Peroutka and Snyder [163]demonstrated that two major categories of serotoninreceptors are extant, 5-HT1, and 5HT-2. Further radi-oligand research demonstrated that three subtypes of5-HT1 receptors, 5-HT1A,B,C, exist in rats [162]. Incircumstances when MDMA increases locomotion,both 5-HT1A and 5-HT1C/2 receptor agonists reliablylead to a reduction in locomotion in the BPM [137,151].Conversely, systemic administration of the 5-HT1B ag-onist RU 24969 has been demonstrated to engenderdose-dependent increases in locomotor levels in miceand rats [95] ostensibly via postsynaptic 5-HT-1B recep-tors [158,194].

As the 5-HT-1B subtype is the only one associatedwith increased locomotion, Rempel, Callaway, andGeyer [167] hypothesized it to be the site of action ofendogenous serotonin released by indirect 5-HT ago-nists such as MDMA. As such, they attempted todetermine if the previously observed BPM behavioral


profile of 5-HT agonists would be mimicked by treat-ment with RU24969. This profile consists of increasedlocomotion characterized by bidirectionally traversingstraight paths, decreased exploratory holepokes, anddecreased rearing activity, and is distinct from patternstypically produced by scopolamine, caffeine, and director indirect dopamine agonists [86]. In accordance withtheir hypothesis, results indicated treatment with 5HT-1B agonist RU24969 to engender a pattern of behaviorsimilar to that typical of 5HT agonists.

3.2.3. Affecti6e components: fear conditioning

3.2.3.1. Definition and procedure of fear conditioning.Fear conditioning, a form of aversive classical condi-tioning, is often employed by researchers investigatingthe neuroanatomy and neurochemistry of fear in ani-mals. In this procedure, a noxious unconditioned stimu-lus (US), such as an electric shock, which elicits fearwhen presented alone, is paired with a previously neu-tral stimulus such as a tone, the conditioned stimulus(CS). After as few as a single pairing the fear responsecomes to be elicited by the CS alone. Rats and micetend to demonstrate such fear by a number of behav-iors such as changes in heart rate and blood pressure,and freezing [126].

LeDoux [126] points out that it is not the conditionedresponse (CR) itself that is learned in this procedure.Rather, it is a hard-wired evolutionarily adaptive re-sponse. Conditioning merely enlarges the number ofstimuli sufficient to elicit such a response.

3.2.3.2. Neural Circuitry of Fear Conditioning.. Accord-ing to LeDoux, Cicchetti, Xagorarias, and Romanaski[127], the amygdala is the neuroanatomical locus wheresensory and autonomic fear input is translated into abehavioral output. Research by Iwata, Chida, andLeDoux [106] has highlighted the role of the centralnucleus of the amygdala in the pathway for such behav-ioral response. Campeau and Davis [28] have demon-strated that lesions of this region interfere with freezingand other fear related responses, while Iwata, Chida,and LeDoux have demonstrated stimulation of thisarea to engender the opposite effect. Campeau andDavis further demonstrated that the basolateral com-plex of the amygdala serves a critical function in thetransmission of sensory information from subcorticaland cortical regions to the central nucleus of theamygdala.

Further research has indicated that the associationsformed between neutral and noxious stimuli during fearconditioning are forged in the amygdala [45,127]. Thisidea was based on research indicating that polymodalassociation areas send advanced sensory information tothe lateral and basal nuclei of the amygdala [196].Lateral, basolateral, and central nuclei receive input

directly from distinct subcortical sensory areas([3,14,127]). Brain-stem nuclei that regulate heart rate,blood pressure, and breathing (dorsal motor nucleus ofthe vagus, lateral hypothalamus, parabrachial nucleus)also project to the central nucleus. Furthermore, whenN-methyl-D-asperate antagonists, which normally blocklong-term potentiation and hence learning, are injectedinto the amygdala during conditioning trials, fear con-ditioning does not occur [68,150]. Finally, innate fear isalso dependent on the amygdala, as research has shownthat rats with amygdala lesions do not demonstratenormal innate fear of cats [21].

According to Rosen and Schulkin [171], when brainareas involved in attention are activated, responses tosensory stimulation are increased. They attribute this toa concomitant decreasing of the threshold, and increas-ing of the receptive fields of neurons in such areas. Assuch, research has demonstrated that certain sensorycortical neurons which normally would not fire for agiven sensory stimulus, do fire for such stimuli afterfear conditioning has been undergone [203].

Rosen and Schulkin [171] point out that amygdalastimulation can increase arousal and attention in manyanimals [73,178]. Furthermore, Kapp, Whalen, Supple,and Pascoe, [117] have reported that the central nucleusis involved in the attainment of a greater level ofnonspecific arousal and attention during fear. This wasdemonstrated by conditioned emotional responses, e.g.increased levels of freezing following stimulation. Assuch, it is the hypothesis of this study that higher levelsof arousal in mice will be associated with increased fearconditioning as reflected by behavioral indices of fear,such as an increase in the amount of freezing.

3.2.3.3. Neurochemistry of Fear Conditioning.. To theextent to which differential degrees of fear acquisitionreflect an underlying difference in arousal, the elucida-tion of the neurochemistry of fear conditioning isimportant.

The role of dopamine receptors. While various studieshave demonstrated that neuroleptic drugs can interferewith conditioning [64,74,133], the mechanism of disrup-tion is unclear. Some researchers maintain that thesedrugs disrupt the incentive motivational properties ofpotentially rewarding stimuli [17,18], while others pos-tulate the impairment to be in the ability to initiate aresponse [20].

In evidence of the latter position, Blackburn, Phillips,and Fibiger [20] demonstrated that administration torats of pimozide, a dopamine receptor antagonist, de-creased the conditioned preparatory response of enter-ing a food compartment. Both the latency and the totalnumber of entries into the compartment after presenta-tion of the CS was decreased. Blackburn, Phillips, andFibiger maintained that because unconditioned feedingbehavior remained unchanged following an identical


dose of pimozide, the obtained attenuation of condi-tioning could not have resulted from a difference in therats overall level of arousal. Horvitz and Ettenberg[103] point out, however, that as the effect of the drugon behavior was not assessed prior to the onset of theCS, specific motor or sensorimotoric abilities may havebeen concomitantly impaired, thus leading to the ob-tained results.

In order to further investigate this issue, Horvitz andEttenberg [103] presented hungry rats with a CS (alter-ation of room illumination) prior to feeding. Trainingpersisted until the rats demonstrated reliable increasesin locomotor activity subsequent to CS presentation.During the experimental procedure, open-field locomo-tor activity was assessed prior to and following CSpresentation in pimozide and vehicle animals. Hungryanimals significantly increased locomotor activity fol-lowing the CS. Horvitz and Ettenberg inferred that thischange was a result of motivational properties of theCS, as opposed to differences in the stimulus conditionsthemselves, because untrained rats did not demonstratesimilar locomotor increase.

Administration of pimozide did not elicit group dif-ferences (trained vs untrained rats) in the proportion ofincreased behavior following CS presentation, althoughthe overall level of activity was lower in the pimozidegroups. This implies that the incentive motivationalaspects of conditioning was not disrupted by pimozide.Horvitz and Ettenberg [103] further point out that theeffects of the drug could not be explained by an upperlimit having been placed on the animal’s potentialbehavioral capacity. The reason for this is that whiledrugged animals demonstrated decreased prestimuluslocomotor activity, their activity levels were able tosignificantly increase upon exposure to the CS. Thus,the prestimulus attenuation must have resulted from adecrease in the motivational properties of normal be-havioral output elicited by the environment.

These results do not imply that previously foundrelationships between dopamine and reinforcement (e.g.[10,46,65]) are spurious. A distinction between theHorvitz and Ettenberg [103] study and such previousstudies could be that in previous studies dopamine wasshown to play a role in the acquisition of conditionedproperties by stimuli. The Horvitz and Ettenberg studyimplies, however, that after the CS has already ob-tained motivational aspects, it can tap into this pairingthrough non-DA pathways.

The role of glutamate le6els. As summarized in Kan-del, Schwartz, and Jessell [116], glutamate is a majorexcitatory transmitter in the brain. Four types of gluta-mate receptors are extant. The major excitatory actionof glutamate, however, occurs on the kainate andquisqualate A receptors. Glutamate also binds to theNDMA receptor.

Saul’skaya and Marsden [175] reported a study wherea conditioning paradigm was carried out in rats suchthat contextual stimuli were paired with an electricshock, and thus came to elicit the conditioned emo-tional response on their own. In vivo intracerebraldialysis indicated increased dopamine release in themedial region of the nucleus accumbens. This releasereached its apex at the conclusion of behavioral testing,and lasted for 80 min. In addition, the study found thatwhen MK-801, a glutamate receptor inhibitor, was usedto inhibit N-methyl-D-asperate receptors in the nucleusaccumbens, dopamine release was curtailed after theinitial 20 min.

In accordance with this finding, Saul’skaya andMarsden [175] hypothesized that this effect of MK-801could be explained by a delayed reaction of nucleusaccumbens glutamatergic afferents to the CR, which inturn would be responsible for the neurochemical re-sponse after 20 min. In fact, previous research hadindicated an interplay between dopaminergic and gluta-matergic afferents in the nucleus accumbens [22]. Assuch, Saul’skaya and Marsden utilized a conditionedfear paradigm, with electric shock being paired with atone. Results indicated that the training phase wascharacterized by a decrease in extracellular glutamateconcentration in the nucleus accumbens. Twelve min-utes after being returned to their home cages, however,the rats evinced a 5-min increase in extracellular gluta-mate in this area. Soon thereafter, glutamate levelsreturned to baseline. These findings were taken to sup-port their hypothesis.

Saul’skaya and Marsden [175] further noted thatduring the period of increased glutamate levels, behav-ioral activity, such as grooming, rearlings, and search-ing for food, increased. At the same time, electric shockalso induced hyperactivity, even though nucleus accum-bens glutamate levels were decreased at this time.Saul’skaya and Marsden noted that this indicates thatthe relationship between glutamate levels and activitymay be moderated by the motivation behind thebehavior.

The role of genetics. In a study by Paylor, Tracy,Wehner, and Rudy [160], DBA/2 and C57L/6 micewere compared on two paradigms of fear conditioning.These two paradigms were contextual fear and auditorycue conditioning. The development of contextual fearwas assumed to involve the hippocampus, as Wehner,Sleight, and Upchurch [202] demonstrated thehippocampus to be important in spatial learning andmemory. While both strains demonstrated equivalentconditioning in the latter paradigm, DBA/2 miceevinced less conditioning to contextual fear. This high-lighted the significance of genetic factors in fear condi-tioning paradigms.


3.3. Hormones and arousal

3.3.1. Estrogen and serotonergic systemsMuch of the research reported above has demon-

strated the significance of serotonin and serotonin re-ceptors in arousal. As such, hormones that influenceserotonin levels may in turn influence arousal levels.Recent research has pointed to a significant interplaybetween estrogen and serotonin levels and receptors.

A study by Biegon, Reches, Snyder, and McEwen[16] investigated the effects of estrogen and proges-terone on serotonergic and noradrenergic receptor den-sity. Female ovariectomized mice were treated for 2weeks with estrogen, progesterone, or both. Analysis ofreceptor density revealed the estrogen condition to en-gender increased 5HT2 receptor and decreased 5HT1receptor concentration. Progesterone blocked the in-crease of 5HT2 receptors, but did not forestall 5HT1decrease. Similarly, when frontal cortex serotonin con-centrations were assessed in these rats, a significantdecrease in serotonin levels was found in the estrogenand combined condition. This observation is consonantwith findings by Kueng, Wirz-Justice, Menze, and Cha-puis-Arndt [123] that serotonin levels decreased duringproestrus. Biegon et al. [16] asserted, however, thatsuch presynaptic changes would probably not be solelyresponsible for the observed postsynaptic change inreceptor density.

Various other studies have corroborated the relation-ship between estrogen and serotonin levels. For exam-ple, a study by Johnson and Crowey [112] investigatedthe relationship between estradiol and serotoninturnover in individual hypothalamic and forebrain nu-clei. The impetus for this investigation was to clarify apossible mediating effect of serotonin on the relation-ship between estradiol and circulating levels of luteiniz-ing hormone (LH) and prolactin concentrations (PRL).As such, Johnson and Crowley depleted serotonin levelsin the afformentioned brain areas and sought to deter-mine if the previously demonstrated relationship be-tween estradiol and LH and PRL would remain.

Results indicated that estrogen treatment increasedserotonin turnover in the medial preoptic, ventrome-dial, and cortical amygdaloid nuclei. This was parallelto an increase in PRL and a decrease in LH secretion.Conversely, when serotonin levels were depleted in themedial preoptic and cortical amygdaloid regions, PRLlevels did not increase as much as normal upon estro-gen treatment, while LH was unaffected. Of interest,however, is the functional relationship found betweenestrogen and serotonin.

The fundamental relationship between estrogen andserotonin was further expounded in a study by Bethea[15] on ovariectomized female macaques treated withestrogen and progesterone. Bethea found estrogen-in-ducible progestin receptors on most of the serotonin

neurons in dorsal and ventral regions of the raphenuclei. Additionally demonstrated was that the expres-sion in this region of tryptophan hydroxylase, an en-zyme essential to serotonin synthesis, was regulated byovarian hormones.

3.3.2. Estrogen and dopaminergic systemsIn addition to its effect on the serotonergic system,

estrogen also strongly influences the dopaminergic sys-tem. As one of the important functions of dopamine isto regulate the basal ganglia, which in turn regulatessequential movement, estrogen plays an important rolein such ability.

The relation between estrogen and motor perfor-mance in humans has been investigated in numerousstudies testing women on motor tasks at various phasesof the menstrual cycle. Studies investigating sensorimo-tor speed by using simple reaction-time paradigms havegenerally not found a significant difference across thecycle [105,121]. In studies investigating more complexmotor tasks, however, a significant effect has beenfound. During the late-follicular phase, a period charac-terized by increased estrogen and decreased proges-terone levels, women were shown to demonstrateimproved articulatory and fine motor skills [96,97].When tested during the midluteal phase, a time whenboth estrogen and progesterone are high, Hampson andKimura [98] found enhanced achievement on measuresof speeded motor coordination, coupled with decre-ments on perceptual-spatial tasks.

According to a review by Van Hartesveldt and Joyce[199] estrogen is an important regulating factor in twodopamine systems in the brain. The first of these is thetubero-infundibular dopamine (TIDA) system. Do-pamine released by the TIDA system is transportedthrough the hypophysial portal system to the anteriorpituitary gland where it decreases prolactin release[23,139,155]. TIDA is inhibited by estrogen administra-tion, which therefore, engenders increased release ofprolactin [60].

The second DA system modulated by estrogen is themesostriatal dopamine system. Evidence for this wasfirst gleaned from studies suggesting gender differencesin striatum dopamine levels in both mice and rats[39,92]. A study by Jori, Colturani, Dolfini, andRutczynski [113] found dopamine concentrations to begreater during diestrous than during proestrus and es-trus, the concentration of homovanillic acid (HVA), themetabolite of dopamine, to be highest during estrus,and dopamine turnover to be most rapid during estrus.According to a review by Van Hartsveldt and Joyce[199], during proestrus, when plasma estrogen is high,concentrations of striatal dopamine and dopamineturnover are at their lowest levels. Approximately 12–24 h after the proestrus increase in estrogen, dopamineturnover significantly increases.


Research has attempted to partial out the influenceof estrogen from that of other hormones which fluctu-ate during the estrous cycle (e.g. progesterone, PRL).These studies have injected estrogen into rodents andmeasured presynaptic striatal dopamine levels. Resultshave indicated the influence of estrogen on mesostriataldopamine activity to be time dependent. For example,Van Hartsveldt and Joyce [199] noted that if tested6–18 h after acute or chronic estrogen treatment, adecrease in striatal TH activity is found in conjunctionwith increased striatal dopamine turnover. Shorter orlonger latencies did not result in such effects because offeedback mechanisms [146] or the influence of estrogenon other hormones such as prolactin [32,161].

In addition to such presynaptic effects of estrogen onmesostriatal dopamine release, postsynaptic effects havealso been observed. Specifically, Van Hartsveldt andJoyce [199] pointed out that estrogen administrationattenuated the normal effect of dopamine agonists onstriatal neurons. In addition, research has demonstratedthat the influence of dopamine agonists and antagonistson striatal acetylcholine (ACh) levels can be regulatedby estrogen [66]. According to Van Hartesveldt andJoyce, estrogen does not affect the synthesis of ACh,thus implying that such observed effects must resultfrom inhibition of dopamine agonists.

Aside from its effects on dopamine levels and func-tion, estrogen also influences the density of striataldopamine receptors. DiPaolo, Carmichael, Labrie, andRaynaud [53] found the density of striatal dopaminereceptors to increase during periods of estrogen treat-ment. Gordon and Perry [91] found a further increase48 h or more after such treatment had ended.

The effects of estrogen on dopaminergic systems canbe observed on a behavioral level as well. Peripheralinjection of DA agonists such as apomorphine or dex-amphetamine into rats have tended to yield stereotypedbehaviors such as compulsive gnawing behavior [63], aswell as locomotor behavior [176]. Furthermore, Gordon[89] found that while estradiol administered in smalldoses decreased the intensity and duration of stereo-typed behaviors 24 h later, large doses increased suchbehavior if tested 48 h or later.

The notion of estrogen attenuating the excitatoryeffect of DA on stereotyped behaviors has receivedfurther evidence from studies of female rat ovariec-tomy. Ovariectomized female rats, who are thus lackingcirculating estrogen, have been reported [89,90] to showmarked increases of such behavior. This increase be-comes even greater over time [90]. Naik, Kelkar, andSheth [153] demonstrated ovariectomy in mice to in-crease stereotyped behaviors facilitated by dopamineagonists, an effect which is attenuated by estradioladministration.

Conversely, evidence has also been obtained indicat-ing that estrogen increases DA-mediated activity.DiPaolo, Rouillard, and Bedard [54] found that oneinjection of 17b estradiol (30 ng/0.2 ml, s.c.) lead to arapid elevation of dopamine turnover in the striatumand nucleus accumbens of rats. Becker et al. [8] foundthat when estrogen was injected directly into the stria-tum of ovariectomized rats, sensorimotor coordinationincreased. Such apparent contradictions were explainedby Van Hartesveldt and Joyce [199] by the finding thatthe relationship between dopamine-dependent behaviorand estrogen treatment is dependent on estrogen dosageand the time delay between estrogen injection andbehavioral observations [89].

3.3.3. Thyroid hormone and arousalThe significance of TH in arousal can be demon-

strated by investigating the behavioral correlates ofhyper and hypothyroidism, as well as by studying theeffects of TRH administration on behavior.

A study by Khan et al. [114] investigated the effect ofTRH administration on arousal. In this study, rats weredivided into four conditions. In the first condition theywere injected with pentobarbital (IP PB) alone. In thesecond condition they received IP BP as well as TRH.The next two conditions were identical except that theanimals received electroconvulsive shock (ECS) prior toinjection. After ECS, rats typically lose their rightingreflex for a short while. This is the reflex to stand upafter being placed in a supine position. Arousal wasindexed by the amount of time after ECS until therighting reflex returned, as well as the latency of the ratto begin exploring the T-maze it was placed in. De-creased latency for both behaviors was taken to indi-cate increased arousal. Results indicated thatirrespective of ECS, rats given TRH had decreasedlatency for return of the righting reflex. In addition, theECS+TRH group demonstrated significantly de-creased latency to explore the maze when compared tothe control group. These results were taken to indicatethe significance of TRH on arousal.

4. Outlook

Adequate documentation of various potential com-ponents of arousal, sensory, motor, and affective, canbe found to justify future investigations of hormone-sensitive genes as they might influence arousal. Further,the question of whether sensory, motor, and affectivefunctions in arousal are equally sensitive to hormonaland genetic manipulations remains open for futureinvestigation. Once dimensions of arousal in femalemice are thoroughly documented, potential hormoneeffects can be better understood.


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