ELSEVIER

Prog. Iveuro~Psydlopharmacol &Ed. Psychmt. 2000, Vol. 24, pp. 925-938

Copyright 0 2000 Elsevier Saence Inc.

Pnnted in the USA. All nghts reserved 027%5846/00/%see front matter

PII: SO278-5846(00)00111-1

DIFFERENTIAL EFFECTS OF MK-801 ON CEREBROCORTICAL NEURONAL INJURY IN C57BL/6J, NSA, AND ICR MICE

GAYLE BROSNAN-WATTERS’, TRISHA OGLMI’, DEREK FORD’, LAWRENCE TATEKAWA’. DAVID GILLTAM?, EDWARD J. BILSKY’, and DONALD NASH”

‘Psychology Department, Vanguard University of Southern California, Costa Mesa, California, USA, Department of ‘Psychology and 3Biological Sciences, University of Northern Colorado, Greeley,

Colorado, USA; “Department of Biology, Colorado State University, Fort Collins, Colorado, USA

(Final form, July 2000)

Abstract

Brosnan-Watters, Gayle, Trisha Ogimi, Derek Ford, Lawrence Tatekawa, David Gilliam, Edward J. Bilsky, and Donald Nash: Differential Effects of MK-801 on Cerebrocortical Neuronal Injury in C57BL/6J, NSA, and ICR Mice. Prog. Nemo-Psychopharmacol. & Biol. Psychiatr. 2000, a, pp. 92!?&38. 02000 Elsevier Science Inc.

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Antagonists of the N-methyl-D-aspartate (NMDA) glutamate (Glu) receptor, including [(+)-5- methyl- 10,ll -dihydro-5H-dibenzo[a,d]cyclohepten-5, I O-imine maleate], dizocilpine maleate (MK- 801), injure pyramidal neurons in the posterior cingulateiretrosplenial (PCIRS) cortex when administered systemically to adult rats and mice. These results have, to our knowledge, only been reported previously in Harlan Sprague Dawley albino rats and International Cancer Research (ICR) mice, an outbred albino strain. Male Non-Swiss Albino (NSA) mice, an albino outbred strain, and male C57BL/6J (B6) mice, a pigmented inbred strain, were injected systemically with 1 mgikg of MIS-801 in the first experiment. This dose of MK-801 reliably produces cytoplasmic vacuoles in neurons in layers II1 and IV of the PCRS cortex in 100% of ICR mice treated. There was a significant difference in the number of vacuolated neurons in B6 and NSA mice, as assessed by ANOVA. The NSA were not significantly different than previously examined ICR mice, but the B6 had fewer vacuolated neurons than either of the two outbred strains. In the second experiment, male NSA. [CR. and B6 mice were injected systemically with a high dose, IO mgikg, of MK-80 1. This dose has been demonstrated to result in necrosis in the same population of neurons injured by lower doses of MK-801. An ANOVA indicated that there was a significant difference among the three strains of mice, and a Fisher’s protected t revealed that the B6 mice were significantly different from both the NSA and ICR, but that, with our test, those two strains were indistinguishable. Male ICR, NSA, and B6 mice were tested in the holeboard food search task 5 hours after 1 mg/kg of MK-801. There were significant differences between the strains in performance both pre and posttreatment. The effect of the drug was not statistically significant.

925

926 G. Brosnan-Watters et al.

8. These results suggest that there may be a genetically mediated difference in the reaction to NMDA receptor antagonists, a finding which may be important given the NMDA receptor hypofunction hypothesis for the etiology of schizophrenic symptoms.

Kevwords: cingulate cortex, genetq mice, MK-801, N-methyl-D-aspartate (NMDA), neurotoxicity, schizophrenia

Abbreviations: C57BLi6J (B6), glutamate (Glu), International Cancer Research (ICR), N-methyl-D- aspartate (NMDA), Non-Swiss Albino (NSA), phencyclidine (PCP), posterior cingulate retrosplenial (PCIRS) cortex

Introduction

Antagonists of the N-methyl-D-aspartate (NMDA) glutamate (Glu) receptor, including phencyclidine

(PCP), and analogs ketamine and [(+)-5-methyl-IO,1 I-dihydro-SH-dibenzo[a,d]cyclohepten-5,10-imine

maleate], dizocilpine maleate @K-801), produce a deficit in glutamate function. Systemic administration

of MK-801 and PCP to adult rats and mice results in injury to pyramidal neurons in the posterior

cingulateiretrosplenial (PURS) cortex. This damage is reversible at relatively low doses, (1 mgkg), but

permanent at higher doses (5-10 mg/kg (Wozniak et al., 1996; Brosnan-Watters et al., 1999). Figure 1 is

a photomicrograph which illustrates the neuronal injury.

To our knowledge, this effect has only been demonstrated in Harlan Sprague Dawley albino rats and

ICR (International Cancer Research) mice (an albino outbred strain from Harlan Sprague Dawley, similar

genetically to CD1 mice) (Olney et al., 1991; Wozniak et al., 1996).

It has been hypothesized that a deficit in glutamate function, specifically hypofimction of the NMDA

receptor, may be a mediating factor in the pathology of schizophrenia (Olney and Farber, 1995; Coyle,

1996). Evidence for this hypothesis includes the fact that NMDA receptor antagonists such as PCP and

analogs such as ketamine produce a psychosis which resembles schizophrenia in producing both negative

and positive symptoms (Olney and Farber, 1995; Coyle, 1996; Farber et al., 1996; Ishimaru and Toru,

1997; Heresco-Levy and Javitt, 1998). Interestingly, in mice, some behaviors produced by the NMDA

antagonist MK-801 might be construed as resembling some form of psychosis. Included is an explosive

popping behavior seen in many mice after MK-801 administration, and the fact that after some doses of

M-801, mice will walk off the edge of an elevated flat surface, seeming to be unaware that they are

doing so (Deutsch and Hitri, 1993; Deutsch et al., 1993). Other evidence for this hypothesis includes the

brain injury which results from the administration of m-801 and other NMDA receptor antagonists

(Wozniak et al., 1996; Brosnan-Walters et al., 1999), which seems to follow a developmental sequelae

similar to schizophrenia. Similar to schizophrenia, it is not manifest until early adulthood (Olney and

Differential effects of MK-80 1 in mice 927

Farber, 1995). The brain injury is only fully expressed when the animal is adult, and does not seem to

occur at all in very young animals (Farber et al., 1995).

Fig 1. This is a photomicrograph (100X magnification) of a representative section of mouse cortex depicting pyramidal neurons in layer III of the PC/M cortex of a mouse injected with 1 m&g MK-801 5 hr earlier. Conspicuous vacuoles (note arrows) can be seen in the cytoplasm of these pyramidal neurons. At 5 hr posttreatment, the appearance of the injured neurons, at the light microscope level, is the same at either the 1 or 10 mgkg dose.

928 G. Brosnan-Watters et al.

It is generally accepted that there is a strong heritability component in schizophrenia (Tsuang et al.,

1991) (Asherson et al., 1995). To our knowledge, however, there has previously been no demonstrated

link between the genetic component in schizophrenia and the glutamate hypotheses. In these studies, the

authors compared the reaction of three strains of mice to MK-801 at two different doses. The finding of a

presumably genetically mediated difference in brain injury in reaction to this drug may suggest new ways

of investigating the pathogenesis of schizophrenia.

Methods

Animals

Exneriment 1: Four to five month old male NSA mice (n=8 treated, n=2 controls) and B6 mice (n=8

treated, n=2 controls) (from colonies at the University of Northern Colorado) that were maintained on

continuous access to food and water in a light (12 hr on/off) and temperature controlled environment were

used in this study.

Experiment 2: Four to five month old male NSA (n=5), B6 (n=5), (same source) and ICR (n=3) mice

(from Harlan Sprague Dawley) maintained under similar conditions, were used in the second experiment,

and two mice from each strain served as controls.

Experiment 3: Male B6 (n=7 treated, 9 control), NSA (n=6 treated, 7 control), and ICR (n= 6 treated

and 6 control) mice which were approximately three months old at the beginning of habituation

(described below) from the same sources as experiment 2 served as subjects for behavioral testing. They

were maintained in the same vivarium, under similar conditions as the mice used for Experiments 1 and 2

until the start of behavioral testing, at which point they were food restricted as described below.

Drugs

MK-801 (purchased from Research Biochemicals International, One Strathmore Road, Natick, MA,

USA) was dissolved in distilled water, in a concentration of 0.1 or 1.0 mgiml, and injected

in a volume of 10 ml/kg of body weight.

Procedures

Experiment 1: MK-801 (1 mgikg) was administered systemically via subcutaneous injection, control

mace were injected with vehicle, and 5 hours posttreatment all mice were deeply anesthetized with 14%

Differential effects of MK-801 in mice 929

chloral hydrate. Intracardiac perfusion was performed using a solution of 2.5% paraformaldehyde and

1 5% glutaraldehyde in 0.1 M sodium phosphate buffer Brams were cut on a slicing device (Rodent

Brain Matrix for mouse 69022, Electron Microscopy Sciences. Fort Washington, PA, USA) and post-

fixed in osmium tetroxide, dehydrated in alcohol, cleared in toluene, and embedded in araldlte. Brams

were sectioned at 1 pm on an ultramicrotome and stained with methyleneiblue azure II. Coronal sections

were taken from the PCiRS cortex at rostrocaudal level 2.5 according to the atlas of Slotnick and Leonard

(1975) just posterior to where the corpus collosum ceases to decussate. Using light microscopy, the

number of vacuolated neurons was counted in one section from each mouse by an observer who did not

know the treatment condltlon of the mouse. This area of the PC’RS cortex has been chosen because it

seems to be particularly sensltlve to the effects of NMDA-antagonist induced intracytoplasmic

vacuolization, and because such distinctive landmarks help to insure that the sections evaluated are level-

matched (from the same area of each brain)

Experiment 2: The procedure was the same, except that the dose of h4K-801 was IO mgikg

Experiment 3: Mice were maintained on continuous access to food and water until the beginning of

experiment, The rotating holeboard food search task has been described previously (Brosnan-Watters et

al., 1996, Wozniak et al., 1996; Brosnan-Watters and Wozmak, 1997, Brosnan-Watters et al., 1999).

Briefly, the apparatus consists of a square floor that contains 16 holes arranged in a 4 x 4 matrix enclosed

by P/~XI&IS sides In the 4-comer configuration, used in the present experiment, an opaque l’l~x~glus

Insert is placed on the floor of the apparatus covering all but the four comer holes. There is a Froot I,oop

(Kellogg’s) in every exposed hole which is made inaccessible by being placed under a screen at the

bottom of the hole. The screen allows the odor of the food to emanate from the hole, but does not allow

access to it. When an individual hole 1s baited, a piece of I;roo/ Loop is placed on top of the screen,

making the food accessible. The entire apparatus rests on a turntable so that it can be rotated easily. The

start tube is placed m the center of the apparatus.

Mice were weighed during handling and habituation to establish baseline body weights before bemg

exposed to food restnction, and were weighed daily during the experimental procedure. Food restrlction

was accomplished by depriving mice overnight the first day of food restriction, then feeding an amount of

food dally which would lower their body weights to approximately 85 to 90% of free feeding weight, and

then would maintain them at that weight for the duration of the experiment.

Mice were handled and allowed to explore the hole board apparatus for three min per day for one week

During this time, all the holes were covered. During the next phase, mice were food restricted as

previously described, and for 10 days were placed in the apparatus with all four holes open, and with a

930 G. Brosnan-Watters et al.

piece of Froot Loop in every hole. They were allowed three min to find and eat the reinforcement from

the four holes.

Afler handling and habituation, the mice were tramed on the 4-comer version of the hole board task

where each mouse was required to learn the spatial location of the one hole out of four that was balted

(contained a Froot Loop piece) on every trial. A trial consisted of releasing a mouse from the start tube

and allowing it to poke its head into the holes until it retrieved the reinforcer or until 3 min elapsed. If a

mouse poked its head into the baited hole and retrieved the reinforcer on its first poke for a given trial, a

correct trial was recorded. If a mouse poked its head up to eye Level (operational definition of a hate

poke) into a non-baited hole on its first hole poke, this was an incorrect trial, but the mouse was allowed

to continue to poke until it retrieved the reward. In order to prevent mice from using odor or other

proximal cues to locate the correct hole, the apparatus was washed with a scented detergent and rotated 90

degrees (on a random basis) between each trial. Thus, although the actual balted hole was different from

the one used on the immediately preceding trial, it was always located m the same position relative to

distal cues in the room. The total number of pokes it took the mouse to retrieve the reward was recorded.

A massed trials protocol was used where training continued for up to 1.5 hr or until the mouse reached a

criterion of 8 correct trials out of 9 consecutive trials. Only mice that were able to reach criterion within

the 1.5 hr time limit were included in the study. Twenty four hr after acquisition training, the mice were

given a retention test using the same criterion to determine ifthey would perform as if they remembered

the spatial location of the baited hole learned on the previous day.

One week after pretreatment retention the mice were divided into groups matched on the basis of trials

to criterion. Half were injected (SC) with 1 mg/kg M-801, and the other group was injected with vehicle

Five hr after each injection, the mice were run individually, but in order to control for time of day, they

were run in pairs where the order of testing (drugged/nondrugged) was randomly determined. The

protocol for the posttreatment acquisition and retention was identical to the pretreatment acquisition and

retention, except that mice were trained to a new hole. The mice were tested by an experimenter who did

not know the treatment status of the ammals. The final number of animals in each group was in some

cases affected by non-performing mice (operationally defined as an animal which did not poke in any hole

for 3 consecutive trials).

Statistical Analvses

In Experiments 1 and 2, counts of injured neurons in the PC/RS cortex of the mice were subjected to an

analysis of variance with one between subjects variable, strain, with Fisher’s protected t used post hoc to

determine the direction of the results. In Experiment 3, trials to criterion were analyzed using a repeated

measures ANOVA with two between subjects variables, strain and treatment, and one within subjects

variable, pre and posttreatment.

Differential effects of MK-801 in mice 931

Results

Experiment I

A total of 16 mice, 8 each of NSA and B6, were analyzed in each drugged group, and 2 mice of each

strain served as controls. There were no vacuolated neurons in any of the control brains of either strain.

The number of vacuolated neurons was determined by counting the neurons containing vacuoles in a level

matched representative full coronal section from the brain of each mouse, using light microscopy. There

was a significant difference between the B6 and NSA mice in the number of vacuolated neurons

evidenced, with the B6 having significantly fewer vacuolated neurons than the NSA mice (mean B6=7,

mean NSA = 41.5, F(1,14) =29.47, p = .OOOl). Because we had previously examined ICR mice (n=6)

using the same dose of drug at the same time point, another ANOVA was done adding these data

(Brosnan-Watters et al., 1999). There was a significant difference between the groups, F(2,19) = 14.07, p

= .0002. Fisher’s protected t test revealed that the B6 had significantly fewer injured neurons than the

NSA and ICR, but the ICR and NSA were not significantly different from each other. Details may be

seen in Fig 2.

86 NSA ICR

B6 N=8 NSA N=8 ICR N=6

Fig 2. Mean number of vacuolated neurons in representative sections of PCYRS cortex of B6 and NSA mice 5 hours posttreatment with 1 mg/kg MK-801. Also included in the graph is data from the previous experiment wherein the same dose of M-801 was administered to ICR mice, and the same procedures were carried out. As can be seen, the NSA and ICR mice were similar in their reaction to MK-801, and the B6 were significantly different from NSA and ICR mice (*=significant at c.01).

932 G. Brosnan-Watters et al.

Experiment 2

A total of 19 mice, 5 NSA, 5 B6, and 3 ICR were analyzed in each group, with 2 control mice in each

strain. No vacuolated neurons were seen in control mice. The number of vacuolated neurons was

determined as above. An ANOVA indicated that there was a significant difference among the three strains

of mice F(2,12) = 14.13, p = .0012, and a Fisher’s protected t revealed that the B6 mice were significantly

different from both the NSA and ICR, p = c.01, but that those two strains were indistinguishable. Details

can be seen in Fig 3.

0 1

B6 N=5 NSA N=5 ICR N=3

NiA IdR

Fig 3 Mean number of vacuolated neurons in representative sections of PC&KS cortex of B6, NSA, and ICR mice 5 hours posttreatment with 10 mg/kg MK-801. As can be seen, the B6 mice had significantly fewer injured neurons than either the NSA or ICR mice (*=significant at <.O 1).

Experiment 3

An overall ANOVA, comparing the three strains of mice, treated vs non-treated, on their pre and post

treatment scores revealed a significant difference between the strains of mice in trials to criterion,

F(2,3)=3.7, p= ,039, but there was no effect of treatment, F(1,2)=2.53, Pz.121. Post hoc test (Tukey’s

HSD) revealed that the performance of the ICR mice was significantly different than that of the B6 and

ICR. Individual analyses of variance on each strain’s performance revealed no effect of the drug

treatment. The performance of B6 and NSA mice was somewhat poorer posttreatment, but not

Differential effects of MK-80 1 in mice 933

significantly so, and ICR mice, both treated and control, performed significantly better posttreatment,

F(1)=9.513, p=.O12 These results can be seen in Fig 4.

I36 Control

n=9

B6 Treated

n=7

NSA

Control n=7

NSA Treated

n=6

ICR Control

n=6

ICR Treated

n=6

Fig 4 Mean number of trials it took each strain of mouse to reach criterion of 8 of 9 consecutive correct trials in the rotating holeboard food search task There was a significant difference between the three strains of mice in their trials to criterion overall, but there was no effect of the drug treatment.

Discussion

In these studies, the authors have compared the effects of two doses of MK-801 on three strains of mice.

Previously, the effects of I mg/kg and 10 mg/kg of MK-801, both behaviorally and histologically, had

been demonstrated in male ICR mice (Wozniak et al., 1996; Brosnan-Watters et al., 1999). In the present

experiments, we compared the histological effect of MK-801 in male B6 and NSA mice at 5 hr

posttreatment with I mgikg of MK-801, as had been done previously with male ICR mice. We found a

significant difference between the B6 and NSA, with B6 mice demonstrating much less susceptibility to

the brain injury as determined by the number of vacuolated neurons in a representative level-matched

section from each mouse brain. The NSA were similar to the previously examined ICR, and the B6 were

significantly different from both NSA and ICR. We also compared the effects of 10 mg/kg of MK-801 in

B6, NSA, and ICK mice at 5 hr posttreatment. The same difference was found.

934 G. Brosnan-Watters et al.

Previous Experiments

The higher dose of MK-801 has been demonstrated, in male ICR mice, to produce necrosis in neurons in

the PCiRS cortex and to chronically impair learning (Wozniak et al., 1996). In that study, necrotic

neurons were identified using hematoxylin and eosin (H & E) staining 4 days posttreatment. In the

present experiment, the brains were examined just 5 hr posttreatment. Thus, we can not be sure that the

injured neurons were permanently damaged. It would be reasonable to assume that this would be the case

because the appearance of the injured neurons in the B6 and NSA mice was similar to that of the ICR

mice, where necrosis had been demonstrated after the 10 mgikg dose. However, further experiments will

be necessary to confirm that conclusion. Further, it has been demonstrated that the higher dose of MK-801

produces disseminated corticolimbic neurodegeneration in adult Sprague Dawley rats (Wozniak et al.,

1998). In this study, brains were not examined for damage in other corticolimbic areas, and therefore, it

was not determined if the strain difference in reaction to MK-801 would also be seen in other areas. In

rats, there is also a difference between the sexes in reaction to the drug, and it would be interesting to see

if that difference occurs in mice, and if there are strain differences in

that area, also.

Factors Which May Contribute to Differences in Sensitivitv to MK-80 1

The fact that the B6 mice do not seem as sensitive to the injury produced by MK-801 could be due to a

number of factors. The B6 may metabolize MK-801 faster or the pharmacokinetics of the B6 may be

different than that of the other strains, producing decreased CNS bioavailability of the drug. However,

others have demonstrated similar behavioral effects of MK-801 in B6 mice and DBA/2J mice (Shen and

Phillips, 1998) and have reported that MK-801 does have other specific effects in 86 mice (Szabo et al.,

1994) These reports suggest that B6 are affected behaviorally similar to other mice. Anecdotally, it was

observed during the present experiments that all strains appeared to have the same behavioral sequelae

after treatment with MK-801. These behaviors were not quantified, but consisted of stereotypical

grooming within minutes after injection with the drug, followed in some cases by “popping behavior”

described by others (Deutsch and Hitri, 1993) in some mice, and then by a posture which seems common

to all mice after administration of 1 mg/kg and higher doses. This posture includes the mouse leaning its

head up against a comer of the plastic cage, and rhythmically moving its front paws in repetitive,

alternating pawing motions.

Behavioral Testing

The results of the behavioral testing are difficult to interpret due to the small number of mice in each

treated and nontreated group. It has been previously demonstrated that 1 mg/kg of MK-801 results in

impaired learning in ICR mice when measured by trials to criterion in the holeboard food search task 5 hr

Differential effects of MK-801 in mice 935

posttreatment (Brosnan-Watters et al., 1999). The results of the current experiment did not replicate those

findings. However, the effects of MIS-801 are extremely variable, often resulting in outlyers in either

direction, thus a larger number of animals may be necessary to have enough power to detect effects in

either direction. The fact that the total number of animals in each strain revealed a significant effect of

strain may offer support for this possibility Differences between the strains was not unexpected, because

B6 mice have been reported by others to be particularly good at spatial learning tasks (Ammassari-Teule

et al., 1993). However, although visual inspection of the data might suggest that this was true, the only

statistically significant difference was between the ICR and the other two strains. Further experiments

with larger numbers of animals in each condition in each strain will be necessary.

Glutamate Hypothesis

The results of the histologic examination are particularly intriguing, given recent developments which

support a glutamate hypothesis of schizophrenia (Javitt and Zukin, 1990; Olney and Farber, 1995; Coyle,

1996; Hirsch et al., 1997). For instance, ketamine is a dissociative anesthetic which has been used for

children which acts by antagonizing the NMDA receptor. When ketamine is administered to adults, it

results in an “emergence reaction” which resembles psychosis. This does not happen with prepubertal

children (Olney and Farber, 1995). Thus, the effects of NMDA receptor antagonists follow a

developmental sequelae similar to that of schizophrenia, which usually does not appear until the

individual is post-puberty. Further, phencyclidine (PCP) and MK-80 1, which also act as NMDA receptor

antagonists, produce a psychosis, usually temporary, in adult humans, and result in exacerbations of

psychosis in non-symptomatic schizophrenics (Olney and Farber, 1995; Coyle, 1996; Ishimaru and Toru,

1997; Heresco-Levy and Javitt, 1998).

The finding that there is a difference in the amount of brain injury resulting from both a relatively low

and a very high dose of MK-801 depending on the strain of mouse examined raises interesting

possibilities for further research. Clearly, the different strains of mice are genetically different, and thus

the difference in the effect of the drug would seem to be mediated by genetic differences. These findings

open the possibility for the finding of a link between the glutamate hypothesis for schizophrenia and the

well-accepted genetic component of schizophrenia

(Asherson et al., 1995).

NMDA Recentor Hvnofunction and Behavior

There have been tantalizing glimpses into behavior which might suggest a genetic component to the

NMDA receptor hypofunction hypothesis, although they have not, to our knowledge, been well-explored.

For instance, a difference in prepulse inhibition among several strains of mice was demonstrated by

936 G. Brosnan-Watters et al

Paylor and Crawley (1997) and others have demonstrated that MK-801 disrupts prepulse inhibition in

mice (Geyer et al., 1990). Prepulse inhibition normally occurs when a weak stimulus occurring before a

normally startling stimulus inhibits the response to the starthng stimulus, and is similar in both humans

and rodents. This normal response IS drsrupted in certain neuropsychratric populations, including those

with schizophrema (Geyer et al., 1990). The fact that MK-801 can produce rmpairments of prepulse

inhrbrtron suggests the involvement of the glutamatergic system in this behavior. Further, others have

reported that dopamine-glutamate interactions play a role in prepulse inhibitron (Wan et al., 1995) It

would be Interesting to see if inbred strains which differ in the amount of prepulse

Inhibition normally shown would vary in their reaction to MK-801 when tested in this paradrgm.

It is possible that using inbred strains of mice to investigate the involvement of the NMDA receptor’s

role in the pathogenesis of schizophrenia from a genetic standpoint may offer opportunities to ask new

questions using this model. Future experiments investigating differences in neuropathological reaction to

MK-801 should look at other strains of mice, preferably inbred strains which are relatively well-

characterized genetically. Quantitative trait loci (QTL) analysis might be used to determine if some

particular gene could be mediating the different effects of MK-801 on neuropathology in different strains.

Conclusions

We conclude that the fact that there is a difference between NSA, ICR, and B6 mice in the number of

injured neurons in the PCYRS cortex as a result of administration of MK-801 suggests the possibility that

there may be a genetic component to the susceptibility of the different strains to the NMDA receptor

antagonist. There is, however, the possibility that some other, unsuspected, systematic variable is at

work. Thus it is important to complete more experiments using these and other strains of mice to

investigate these differences, and also to investigate the possibility of other chemically mediated

differences in brains in reaction to NMDA receptor antagonists.

Acknowledpemeots

Support for thus research was provided in part by grants from the Research Corporation of the

University of Northern Colorado (GBW) and the Nash Foundation (GBW). Support for the writing of

this article was provided by a grant from the AAUW Educational Foundation (GBW). In addition, we

wish to thank John Olney and David Wozniak for the use of their laboratories and for their support during

the initial stages of this work.

Differential effects of MK-80 1 in mice 937

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Inquiries and Reprints Requests should be addressed to:

Gayle Brosnan-Watters, Ph.D. Department of Psychology Vanguard University of Southern California Costa Mesa, CA 92626 Tel 714-556-3610, Ext 454 Fax 714-966-6316 E-mail gbrosnanwatters@vanguard.edu