Evaluation of cardiovascular control by neurons in the dorsal medulla of rats

Journal of the Autonomic Nervous System, 29 (1989) 1-12 1 Elsevier

JANS 00983

Evaluation of cardiovascular control by neurons in the dorsal medulla of rats

C h r i s t o p h e r P. Y a r d l e y *, J enn i fe r M. A n d r a d e and L y n n e C. W e a v e r

The John P. Robarts Research Institute and Department of PhysioloKv, UnwersiO" of Western Ontario. London, Ontario, Canada

(Received 10 July 1989) (Revision received 21 September 1989)

(Accepted 22 September 1989)

Key words: Arterial blood pressure; Blood flow; Heart rate; Microinjection; D, L-Homocysteic acid; Glycine; Muscimol

Abstract

The contribution of sympathoexcitatory neurons in the dorsal medulla to the regulation of arterial pressure and the involvement of such neurons in integration of physiological responses or in the genesis of basal vasomotor tone are not well defined. In the present study discharge of neurons in the dorsal medulla of anesthetized rats was increased or decreased by microinjections of amino acids to examine effects on systemic arterial pressure, heart rate and blood flow and conductance of the renal and femoral vascular beds. Microinjections of excitatory D, L-homocysteic acid caused increases in arterial pressure of 18 + 2 mmHg, increases in heart rate ranging from 5-40 beats/rain and renal vasoconstriction; the femoral bed constricted after some injections and dilated in response to others. Injections of the inhibitory amino acid glycine caused no consistent decreases in arterial pressure and heart rate and injections of the y-aminobutyric acid analog, muscimol were ineffective. These data demonstrate that neurons in the dorsal region of the rat medulla can contribute to regulation of arterial pressure and can integrate generalized differential changes in regional vascular resistance, but do not appear to be essential for the genesis of basal vasomotor tone.

Introduction

For more than a hundred years the central nervous system has been thought to set and regulate the level of systemic arterial blood pressure within a narrow range through tonic activity of a medullary vasomotor center [23]. Although this view has remained essentially correct, recently sig-

Correspondence: L.C. Weaver, The John P. Robarts Research Institute, P.O. Box 5015, London, Ontario N6A 5K8, Canada. * Present address: Department of Clinical Trials, Sandoz

(Canada) Ltd., 385 Boulevard Bouchard, Dorval, Quebec H95 1A9, Canada.

nificant advances have been made to identify the precise location of medullary neurons that regulate arterial blood pressure (see ref. 3 for review). Tonic vasomotor tone in the rat appears to be generated within the rostral ventrolateral medulla (RVLM) and the role of other portions of the medulla has been somewhat ignored [3,25,26,29, 30]. Indeed, after an initial period when the dorsal medulla was given equal consideration with the RVLM as the source of vasomotor tone [5,19], this region was perhaps prematurely dismissed as a source of such tone and viewed merely as a region of fibers of passage for the pressor responses originating in the RVLM [9,11]. The dorsal medulla may contribute more than fibers of pas-

0165-1838/89/$03.50 q'? 1989 Elsevier Science Publishers B.V. (Biomedical Division)

sage in view of the work of Gebber and Barman [1,12] suggesting that neurons in the dorsal medulla generate part of the 2 -6 Hz rhythm of sympathetic nerve discharge. However, in comparison to the extensive data presently available on tonically active neurons in the rat RVLM that regulate blood pressure [3], relatively little is known about the role of neurons in the dorsal portion of the rat medulla. In two recent studies of the dorsal and ventrolateral medulla in cats, rabbits and rats, Chai and his coworkers [4,20] have shown that microinjection of excitatory amino acids into the dorsal and dorsomedial medulla can cause pressor responses. Although these studies focussed on medullary organization of the cat, pressor responses were clearly initiated from dorsal medullary neurons in rats. However, the produc- tion of pressor responses by excitation of neurons is not evidence that the neurons fire tonically to support arterial pressure. Furthermore, the involvement of these neurons in regulating arterial pressure during physiological responses also re- mains to be determined; such involvement is likely as the dorsal medullary region appears to be im- portant in one well-known physiological reaction, the defense response [9,16]. The present study was carried out to search for the site of neurons in the dorsal medulla of rats that could be stimulated with excitatory amino acids to elicit increases in arterial blood pressure and heart rate. In another series of experiments more precise information was obtained about the same sites by monitoring not only changes in arterial blood pressure, but also changes in blood flow of different regional vascular beds. Finally, the tonic contribution of these neurons to maintenance of basal levels of arterial blood pressure was tested by inhibiting their discharge with microinjections of the inhibitory amino acid glycine and the "r-aminobutyric acid (GABA) analog muscimol.

Methods

Mapping experiments Mapping of the dorsal region of the medulla

was carried out on 15 male Wistar rats (220-380 g) anesthetized with urethane (1.4 g /kg , i.p.,

Sigma); supplementary doses of 0.15-0.25 g / k g were given intravenously when required. The trachea and femoral artery and vein were cannu- lated. Rats were artificially respired with room air and paralysed during surgery with gallamine tri- ethiodide (8-12 mg/kg , i.v.); muscle relaxation was maintained throughout the experimental period with additional intravenous doses of 2 -4 mg/kg , after assessment of the level of anesthesia. Arterial blood samples were periodically with- drawn to measure pH, pO 2 and pCO 2. Acceptable values were pH, 7.35-7.45; PCO 2, 25-40 mmHg; and P O 2 > 80 mmHg. Acid base disorders were corrected by infusing sodium carbonate, adjusting respiratory volume a n d / o r rate, or respiring animals with O2-enriched room air. An infusion of physiological saline (0.013 m l / m i n ) was started during the surgery and maintained during record- ing periods to compensate for fluid loss. Rectal temperature was kept at approximately 37°C by the use of a heating blanket. Rats were placed in a prone position in a stereotaxic frame (David Kopf Instruments), and the cerebellum was exposed by removing portions of the occipital bone.

Glass micropipettes were prepared from boro- silicate capillary tubing (465/~m i.d., Socorex 851- 5) and the inside diameter of the pipette tips was 20-40 #m. They were filled with either a 0.16-M solution of D, L-homocysteic acid (DLH, Sigma), 1.0 M solution of the inhibitory amino acid glycine (BDH Chemicals), or 0.001 M muscimol (Fluka). D L H and glycine were dissolved in distilled water and muscimol was dissolved in physiological saline. All solutions were corrected to a pH of 7.3-7.5. In some experiments double-barrel glass pipettes were pulled to enable two different drugs to be injected consecutively at the same site. The tips of the glass pipettes were positioned in the medulla according to the rat stereotaxic atlas of Paxinos and Watson [23]. Usually five to eight sites (minimum of 500 /~m between sites) were stimulated in one track. Up to four tracks were completed in each rat. The medulla was explored from its dorsal border to its ventral surface. Mi- croinjections were made from a rostral limit of - 9 . 8 mm (to Bregma) to a caudal limit of - 1 2 . 6 mm (to Bregma) and from 1.0 to 2.5 mm lateral to the midline on both sides of the brain. D L H was

injected in 70-100 nl quantities by pressure injection. Glycine and muscimol were injected in volumes of 100 nl. Injection pressure and pulse duration were controlled by a picospritzer (Gen- eral Valve). The volume of the injection was mea- sured by observing the displacement of the fluid meniscus in the pipette through a custom-built microscope (magnification ×40) fitted with an ocular micrometer. The injection sites were marked by India ink that was added to the DLH, muscimol or glycine. At the end of the experiment the medulla was removed and placed in 10% formol- saline. After fixation, the location of all injection sites was assessed from 50 #m coronal sections of brain cut on a cryostat (Wild Leitz Canada). The sections were stained with neutral red.

Systemic arterial blood pressure was monitored

with the use of a Gould-Statham P23 ID transducer and displayed on a Grass polygraph (Grass Instrument Co.). Heart rate was monitored using a Grass tachograph (7P4H) that was triggered by the arterial pressure pulse.

Regional blood flow experiments In ten rats, in addition to arterial blood pres-

sure and heart rate, changes in regional blood flow in the renal and femoral vascular beds were recorded during microinjections of DLH. These rats were anesthetized with the steroid anesthetic mix- ture, alphaxalone-alphadolone (Saffan; Glaxovet). The dose and method of administration were as previously described [31]. Saffan was used in these studies because it has few effects on the cardiovascular system [6] and because small or selective

Fig. 1. Photomicrograph showing region of three injections 0.5 mm apart from each other. At the most ventral site, 100 nl of DLH evoked an increase in arterial blood pressure and a subsequent injection of 100 nl of glycine evoked a decrease in arterial blood pressure. The India ink stain in this section became pale after counterstaining, but the spread of the injection volume is obvious as

the ink interferred with the uptake of neutral red stain. Bar represents 1.0 ram.

neural influences on vascular resistance have been detected in animals anesthetized with this agent [15,281.

Arterial blood pressure was recorded from a carotid artery via a pressure transducer (type 4- 327-L221; Bell and Howell) and heart rate was derived from the pressure pulse by a custom-built instantaneous rate meter [21]. Regional blood flows were recorded from the femoral and renal arteries with cuff-type flow transducers which were con- nected to electromagnetic flow amplifiers (Carolina Medical Electronics Inc.). Zero-flow signals were obtained by occluding the arteries distal to the flow transducers with an occluding snare. Flow transducers were later calibrated in vitro using constant flow perfusion. Regional vascular conductance (blood flow divided by arterial blood

pressure) was computed on-line by custom-built electronic dividers. All cardiovascular variables were registered on pen-recorders (Devices Instru- ment Co.).

Results

Mapping of brainstem with DLH In 15 experiments the dorsal medulla was ex-

plored for sites from which cardiovascular responses were elicited by microinjections of DLH. As the main aim of the mapping was to explore the dorsal portion of the medulla, the majority of injections of DLH were targeted to be at least 2.0 mm dorsal to the most ventral aspect of the medulla. Using this criterion, histological verifica-

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Fig. 3. Cardiovascular responses elicited by microinjection of DLH into the dorsal medulla, Arterial blood pressure responses (left panel) to microinjections of 70 nl of the excitatory amino acid DLH (0.16 M), are shown at each of six points on a diagrammatic coronal section of the rat medulla. The section is 11 mm caudal to bregma. The filled circles represent histologically verified sites of

responses classified as positive and the open circles are sites classified as ineffective. For abbreviations see legend to Fig. 2.

t ion r evea l ed tha t 102 i n j e c t i o n s of D L H w e r e

m a d e in to the d o r s a l ha l f o f the m e d u l l a (Figs . 1

a n d 2). A t 41 of t h e s e s i tes D L H e v o k e d i n c r e a s e s

in a r te r ia l b l o o d p re s su re . T h e i n c r e a s e s in m e a n

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Fig. 2. Diagrammatic coronal sections through the rat medulla illustrating dorsal and ventral sites of injection of DLH (left sides of sections) and glycine or muscimol (right sides of sections). AP coordinates are referenced to bregma. The filled circles on the left side indicate sites at which increases in arterial blood pressure were evoked by DLH. The size of the circle indicates the magnitude of the response; * 5-10 mmHg, • 11-20 mmHg, • > 20 mmHg. Decreases in arterial blood pressure greater than 5 mmHg are shown as filled triangles. In effective stimulation sites are shown as open circles. Injections of glycine are illustrated as filled arrows and injections of muscimol as open arrows. A downward pointing arrow signifies a decrease in arterial pressure greater than 5 mmHg: a horizontal arrow indicates no change in arterial blood pressure. Abbreviations: AC, accessory nucleus; AM, ambiguus nucleus; DMT, dorsomedial tegmental nucleus; FCN, facial nucleus; GI, gigantocellular reticular nucleus; 10, inferior olive; IRT, intermediate reticular nucleus; PG, lateral paragigantocellular nucleus; PCR, parvocellular reticular nucleus; PY, pyramidal tract; RVL, rostroventrolateral reticular nucleus; SOL, solitary tract nucleus; TZ, nucleus trapezoid body; 7N, facial nerve; PDT, posterodorsal

tegmented nucleus; DPG, dorsal paragigantocellular nucleus.

electrode track, small pressure responses were elicited from the dorsal medul la s tar t ing within 0.5 m m of the dorsal surface (Fig. 3); these responses became larger as the electrode was advanced more ventral ly into the vicinity of the parvocellular re- t icular and intermediate reticular nuclei [24]. The larger responses were most consis tent ly evoked from these regions (Fig. 2). The mapping was done with 70-100 nl of D L H because the threshold volume typically needed to cause pressure responses was 50-70 nl. This mapp ing gave good localization of pressor sites with dorso-ventral movements of the electrode as little as 0.5 mm (Fig. 3). Loss of pressor responses by moving the

electrode suggested that excitat ion of neurons by the spread of the D L H following a 100-nl injection was l imited to an area approximate ly 1 m m from the electrode tip. This was supported by the histological evaluat ion as the spread of dye at the inject ion site was also within this range (Fig. 1). In some tracks the dorsal and ventral por t ions of the medul la from which pressor responses could be elicited were clearly dist inguishable. Latency to onset of the increases in arterial blood pressure was in the range of 3 - 1 0 s; responses with laten- cies greater than 15 s were not considered to be within the immedia te vicinity of the inject ion and were not included in the mapping. Changes in

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Fig. 4. Cardiovascular responses evoked from the same site in the dorsal medulla by consecutive microinjections of DLH and glycine. The upper portion is a diagrammatic coronal section of the rat medulla showing the histologically verified injection site. The lower panels show arterial blood pressure (BP) and heart rate responses to a 70-nl injection of DLH (panel A) and subsequent injection of 100 nl of glycine 35 min later (panel B). Recovery from the effect of glycine 15 min after injection is shown in panel C. The injection

time is shown by the marker under the panels. Abbreviations as in Fig. 2.

heart rate often occurred with the pressure responses elicited from the dorsal medulla. Of 41 pressor responses, 25 were accompanied by increases in heart rate in the range of 5-40 beats / ra in (mean: 12_+ 3 beats /min) , two were accompanied by minor decreases in heart rate (mean: 5 +_ 10 bea t s /min) and in the remaining 14 trials no changes in heart rate occurred with the pressor responses. Intact baroreceptor reflexes in these rats may have offset potential increases in heart rate.

Stimulation in 16 sites caused decreases in arterial pressure of 13 +_ 1 mmHg. These depressor sites were often in, or in the immediate vicinity of, the nucleus tractus solitarius, although occasion- ally at other more ventral sites in the medulla. Decreases in heart rate of 11 + 2 bea ts / ra in accompanied ten depressor responses and no change in heart rate occurred with the remaining six. Vagolytic actions of gallamine may have obscured some decreases in heart rate.

Responses to glycine and muscimol At 29 sites in the medulla, either glycine or

muscimol was injected (100 nl) unilaterally to inhibit any ongoing neural firing. Histological verification revealed that 21 of these sites were 2 mm or more dorsal from the most ventral surface of the medulla (Figs. 1, 2 and 4). Of the eight glycine injections in the dorsal medulla, only three caused decreases in blood pressure. These changes ranged from - 5 to - 2 8 m m H g (mean: - 1 5 + 7 mmHg). The largest of these responses and the site of injection is shown in Fig. 4. The heart rate also decreased by 5 and 10 bea t s /min following two of these effective glycine injections, but remained unchanged during the third (Fig. 4). Of 13 muscimol injections into the dorsal medulla, only one produced a 10-mmHg decrease in mean arterial blood pressure.

Regional hemodynamic responses to stimulation of the dorsal medulla

In ten rates, microelectrodes filled with D L H were targeted at regions in the dorsal medulla that had been shown in the mapping experiments to contain neurons that could be stimulated to elicit increases in mean arterial blood pressure. At 12

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Fig. 5. Diagrammatic coronal sections through the rat medulla showing DLH injection sites that elicited increases in arterial blood pressure and changes in renal and femoral vascular conductance. Fully filled squares show injection sites at which arterial blood pressure and heart rate increased and renal vasoconstriction (decrease in conductance) and femoral vasodilatation (increase in conductance) occurred. Half-filled squares show sites at which there was an increase in arterial blood pressure and heart rate with generalized vasoconstriction in the renal and femoral vascular beds (decrease in conduc-

tance), Abbreviations as in Fig. 2.

histologically verified sites (Fig. 5), injection of D L H evoked an increase in arterial blood pressure (39 + 6 mmHg) and heart rate (38 + 8 bea ts /min) . These increases in blood pressure and heart rate were greater than those in the first group of rats because animals anesthetized with Saffan have brisk cardiovascular responses (see Methods). The increases in arterial blood pressure and heart rate were accompanied by two distinctive patterns of change in regional blood flow and conductance. At seven of these sites generalized vasoconstruc- tion occurred, which was characterized by a decrease in both the renal (mean: - 3 7 + 7%) and femoral (mean: - 5 0 + 1 1 % ) vascular conductance. At the other five sites, femoral conductance increased (mean: +115 _+ 22%) while renal con-

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~W[ (rain) Fig. 6. Pressor response, tachycardia and differential changes in vascular flow and conductance elicited by DLH injection (100 nl) into the dorsal medulla. Records from above down: blood flow (FLOW) and vascular conductance (CON) in femoral (FEM) and renal (REN) beds, heart rate (HR) and arterial blood pressure (BP). Time and injection of DLH are

indicated under the traces.

duc tance decreased (mean: - 3 6 _ + 4%) (Fig. 6). The d i f ferent ia l pa t t e rn of regional h e m o d y n a m i c response in these five rats was a c c o m p a n i e d by pup i l l a ry d i l a t a t ion and exop tha lmus and, in this respect , the whole pa t t e rn of response was ident i- cal to the defense reac t ion that has been observed in the rat [16,17,31,32].

Discussion

The present s tudy has shown that microinjec- t ion of the exc i ta to ry amino acid D L H into the dorsa l region of the medu l l a can cause increases in ar ter ia l b lood pressure which are usual ly accom- pan ied by increases in hear t rate. The use of

micro in jec t ions of exc i ta tory aminoac ids has, over the last decade, rep laced electr ical s t imula t ion as a tool for m a p p i n g regions of the b ra in involved in ca rd iovascu la r regulat ion. Both g lu t ama te and D L H , which s t imula te cell bodies ra ther than fibers of passage, have been used successful ly to ident i fy sites in the rat bra in con ta in ing neurona l e lements regula t ing ar ter ia l b lood pressure [13,17,26,29,30]. In par t icular , D L H has been shown to elicit a subs tant ia l increase in the f ir ing ra te of neurons with very l i t t le a da p t a t i on [10]. There fore the responses evoked by D L H in the present experiments are cons idered to be due to exci ta t ion of neuronal groups wi thin the vic ini ty of the micro- p ipe t t e tip. The loss of p ressor responses with movements of the e lec t rode as l i t t le as 0.5 ram, his tological examina t ion of e lec t rode t racks and dye marks of inject ion sites, p lus theoret ica l esti- mates of the spread at inject ion sites of 100 nl of fluid [22], suggest that our in ject ions of 70 -100 nl of D L H were ac t iva t ing neurons up to 1 m m from the p ipe t te tip. Therefore, p ressor responses produced by dorsa l medu l l a ry inject ions in this s tudy must have been caused by ac t ions on neurons in the dorsa l medul la . Increases in ar ter ia l b lood pressure and hear t rate were cons is ten t ly el ici ted f rom sites within 1 m m of the most dorsa l aspect of the medul la and react ive sites also occur red more ventral ly, toward the well d o c u m e n t e d pres- sot region of the rost ra l vent ro la te ra l medu l l a [3,25,26,29,30]. However , in a n u m b e r of t racks the dorsa l and ventral pressor regions were clear ly dis t inct (Fig. 3). The most cons is ten t and largest increases in ar ter ial b lood pressure were el ici ted in the vicini ty of the parvoce l lu la r re t icular nucleus. Because inject ion volumes of 70 -100 nl of D L H were needed to cause responses , the neurons caus- ing the responses p r o b a b l y are not loca ted in compac t groups and may have been few in number. Fur the rmore , the magn i tude of response which could be el ici ted from the dorsa l neurons was not as great as can be el ici ted f rom the R V L M . There- fore, the con t r ibu t ion of these dorsa l neurons to ca rd iovascu la r cont ro l appea r s to be less than that of neurons in the R V L M .

Our ident i f ica t ion of a dorsa l region in the rat medul la con ta in ing neurons that could be exci ted to elicit pressor responses conf i rms and extends

the work of Chai and his coworkers [4,20], but differs from previous studies by Ross et al. in the rat [25,26]. These authors did not inject excitatory amino acids into the dorsal region. Instead, increases in arterial blood pressure were elicited by electrical stimulation at sites in a dorsal region of the medulla and the authors considered this area to be only a region containing fibers relaying the pressor responses from the ventral medulla. This conclusion was based on the finding that adrenaline synthesizing neurons were found only in the ventral medulla and that bundles of phenylethanolamine-N-methyltransferase-containing axons were found projecting from this region through the dorsal medulla. These adrenaline-containing neurons and axons were assumed to mediate increases in arterial blood pressure initiated from the RVLM. This interpretation may be partially correct, but the findings do not preclude the ex- istence of other vasopressor neurons in the dorsal medulla. Because the region of the dorsal medulla from which we evoked pressor response does not appear to contain catecholaminergic cells [18], the dorsal medullary neurons mediating pressor responses which we observed must contain other neurotransmitters.

The role of the dorsal medulla in cardiovascular control also has been studied in rabbits. Kumada et al. [18] lesioned the dorsal medulla, particularly in the vicinity of the nucleus reti- cularis parvocellularis and observed profound hypotension and abolition of the vasomotor component of the cerebral ischemic response. They con- cluded that the integrity of the dorsal medulla was essential for the maintenance of resting arterial blood pressure. This hypothesis was quickly re- jected following additional studies in the rabbit [9] demonstrating that lesions of the dorsal medulla also attenuated pressor responses elicited from the RVLM. Further neuroanatomical studies [11] led to the conclusion that the dorsal medulla of the rabbit, like the rat, was a region containing a high density of vasomotor axons rather than a true integrative region containing neurons. However, our results in the rat and other more recent studies in rabbits, cats and rats [4,14,20] clearly demonstrate that neurons in the dorsal medulla can integrate cardiovascular responses. The location of

pressor neurons in the dorsal medulla of the rabbit and cat is similar to the region in the rat.

Because a change in arterial blood pressure can result from different combinations of changes in regional vascular resistance, we monitored regional blood flow and conductance in the renal and femoral beds as well as systemic arterial pressure in a series of experiments. Indeed, we found that the increase in arterial blood pressure and heart rate elicited by stimulation of the dorsal medulla could be associated with two different cardiovascular response patterns. At the majority of effective stimulation sites, generalized vasoconstriction with decreases in both renal and femoral conductance occurred. At the other effective sites however, although the renal vascular bed constricted, the hind-limb vascular bed vasodi- lated. This differential pattern of response is iden- tical to the 'visceral alerting response' associated with the defense reaction [16,17,31,32]. Further- more, the pupillary dilatation and exopthalmus that were observed to occur during the response are also associated with the full defense reaction [17,31]. A previous study in the cat showed that regions in the dorsal medulla can be electrically stimulated to elicit defense reactions [8], and our results also show that portions of the rat dorsal medulla can integrate differential cardiovascular response patterns, possibly associated with this reaction. Because the locations of sites producing generalized vasoconstriction overlapped those producing differential responses, and because the number of stimulation sites was relatively small, functionally distinct regions of the dorsal medulla could not be separated anatomically in our study.

The pathways from neurons in the dorsal medulla mediating the cardiovascular responses are not certain. Anatomical and electrophysiologi- cal studies provide no evidence for a major direct projection from this dorsal medullary region to the spinal preganglionic neurons [1,12,14]. This apparent lack of direct projection to the spinal cord implies that efferent projections mediating the cardiovascular responses must relay at another site. Barman and Gebber have identified neurons with a firing pattern that correlated with sympathetic nerve activity in the dorsal regions of the medulla [1,12]. Moreover, the same authors have

10

identified neurons in the same dorsal region that appear to activate neurons in the RVLM and contribute to the 2-6 Hz rhythm that is character- istic of sympathetic nerve discharge. Therefore, one possible relay from the dorsal pressor region is the RVLM.

The importance of 'pressor' regions of the dorsal medulla in maintenance of basal vasomotor tone was investigated in the present study and no major role was demonstrated. When the inhibitory amino acid glycine and the GABA agonist muscimol were microinjected into 'pressor' regions of the dorsal medulla, glycine produced decreases in blood pressure in only three of eight trials and muscimol was essentially ineffective. These results provide little evidence that tonic activity of neurons in the dorsal medulla is essential for maintaining basal vasomotor tone. More- over, Talman and Robertson [27] recently re- ported that microinjections of glycine into the NTS of the rat can cause decreases in arterial blood pressure. This finding prompts the sugges- tions that two of the three depressor responses to glycine injections in our experiments may have been effects on NTS neurons, as the effective injection sites were close to this nucleus. An alter- native explanation for our results may be that urethane-anesthetized rats are particularly depen- dent upon the RVLM for vasomotor tone and are less responsive to control from other CNS regions as suggested recently by Cochrane et al. [7].

In summary, we conclude that neurons in the dorsal medulla of the rat, particularly in the vicinity of the parvocellular reticular nucleus, contribute to integration of neural influences on the cardiovascular system. Output from these neurons can result in generalized or differential control of regional vascular resistance. However, the neurons in this region of the dorsal medulla do not appear to be essential for the genesis of basal vasomotor tone.

Acknowledgements

The authors thank Thomas Norry for his assis- tance with preparation of the figures and Lisa Kleinknecht for typing the manuscript. This re-

search was supported by grants from the Heart and Stroke Foundation of Ontario and the Medi- cal Research Council of Canada. L.C.W. is a Career Investigator of the Heart and Stroke Foundation of Ontario and J.M.A. held a John D. Schultz Science Student Scholarship from the Heart and Stroke Foundation of Ontario. The study of regional hemodynamic responses was done in the laboratory of Professor S.M. Hilton at the University of Birmingham, U.K.

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Evaluation of cardiovascular control by neurons in the dorsal medulla of rats