A glycinergic intervention potentiates the antiseizure efficacies of MK-801, flurazepam, and carbamazepine

Genetically inbred Balb/c mice differ from outbred Swiss Webster mice on discrete measures of sociability: relevance to a genetic mouse model of autism spectrum disorders

The Balb/c mouse is proposed as a model of human disorders with prominent deficits of sociability, such as autism spectrum disorders (ASDs) that may involve pathophysiological disruption of NMDA receptor-mediated neurotransmission. A standard procedure was used to measure sociability in 8-week-old male genetically inbred Balb/c and outbred Swiss Webster mice. Moreover, because impaired sociability may influence the social behavior of stimulus mice, we also measured the proportion of total episodes of social approach made by the stimulus mouse while test and stimulus mice were allowed to interact freely. Three raters with good inter-rater agreement evaluated operationally defined measures of sociability chosen because of their descriptive similarity to deficits of social behavior reported in persons with ASDs. The data support previous reports that the Balb/c mouse is a genetic mouse model of impaired sociability. The data also show that the behavior of the social stimulus mouse is influenced by the impaired sociability of the Balb/c strain. Interestingly, operationally defined measures of sociability did not necessarily correlate with each other within mouse strain and the profile of correlated measures differed between strains. Finally, "stereotypic" behaviors (i.e. rearing, grooming and wall climbing) recorded during the session of free interaction between the test and social stimulus mice were more intensely displayed by Swiss Webster than Balb/c mice, suggesting that the domains of sociability and "restricted repetitive and stereotyped patterns of behavior" are independent of each other in the Balb/c strain.

D-serine improves dimensions of the sociability deficit of the genetically-inbred Balb/c mouse strain

The Balb/c mouse strain shows quantitative deficits of sociability and is behaviorally-hypersensitive to MK-801 (dizocilpine), a noncompetitive NMDA receptor antagonist. D-Serine (560mg/kg, intraperitoneally), a full agonist for the obligatory glycine co-agonist binding site on the NMDA receptor, increased the amount of time Balb/c mice spend in a compartment containing the enclosed social stimulus mouse and the amount of time Balb/c mice spend exploring (sniffing) an inverted cup containing the enclosed social stimulus mouse in a standard sociability apparatus. These effects of D-serine on the impaired sociability of the Balb/c mouse strain were not due to a "nonspecific" effect on locomotor activity; importantly, the locomotor activity of the Balb/c mouse strain decreases in the presence of an enclosed or freely-moving social stimulus mouse. The data suggest that dimensions of the impaired sociability of the Balb/c mouse strain may be improved by targeted NMDA receptor agonist interventions.

D-Cycloserine improves the impaired sociability of the Balb/c mouse

The genetically inbred Balb/c mouse strain shows evidence of impaired sociability in a standard paradigm. For example, relative to 8-week-old male outbred Swiss-Webster mice, 8 week-old male Balb/c mice spend less time sniffing and in the vicinity of an enclosed 4 week-old male ICR stimulus mouse and, when allowed to interact freely with the stimulus mouse for five minutes, make fewer discrete episodes of social approach and show suppression of locomotor activity. We explored the effect of D-cycloserine (320mg/kg, intraperitoneally), a partial glycine agonist that binds to the obligatory co-agonist glycine binding site on the NMDA receptor, on the sociability of the Balb/c and Swiss-Webster mouse strains in a standard paradigm. The results show that treatment with D-cycloserine increased the locomotor activity of the Balb/c mouse strain in the presence of an enclosed social stimulus mouse and when these mice were allowed to interact freely with each other. Also, D-cycloserine increased the number of discrete episodes of social approach when Balb/c mice were allowed to interact freely with social stimulus mice. However, D-cycloserine had similar effects on measures of sociability in the Swiss-Webster mouse, raising the possibility that the positive effects on the sociability of the Balb/c mouse strain may be mediated by indirect effects on locomotion, arousal, and anxiety.

Exogenously administered D-serine failed to potentiate the ability of MK-801 to antagonize electrically precipitated seizures in nonhandled control and stressed mice

NMDA receptor hypofunction (NRH) has been implicated in the pathophysiology of schizophrenia because of the ability of phencyclidine (PCP), a noncompetitive NMDA receptor antagonist, to precipitate a schizophreniform psychosis. The possible role that NRH plays in the pathophysiology of schizophrenia stimulated characterization of behaviors elicited by PCP and its analogues. For example, MK-801 (dizocilpine), a noncompetitive NMDA receptor antagonist that binds with higher affinity to the same hydrophobic channel domain as PCP, raises the threshold voltage required for the electrical precipitation of tonic hindlimb extension in mice. This ability of MK-801 is significantly reduced following stress. We showed that an exogenously administered glycine prodrug (i.e., milacemide) was able to potentiate MK-801's antiseizure efficacy in unstressed mice and restore MK-801's antiseizure efficacy in stressed animals. d-Serine may serve as an endogenous agonist for the obligatory glycine co-agonist site on the NMDA receptor complex. Orally administered d-serine has been studied clinically as an adjuvant therapeutic intervention in schizophrenia. Thus, we were surprised at its inability to potentiate MK-801's antiseizure efficacy in either control or stressed animals. These data do not support the development of d-serine as a viable therapeutic intervention for schizophrenia and, possibly, other disorders.

Modulatory effects of d-serine and sarcosine on NMDA receptor-mediated neurotransmission are apparent after stress in the genetically inbred BALB/c mouse strain

Abnormalities of NMDA receptor-mediated neurotransmission are involved in the pathophysiology of schizophrenia, Alzheimer's disease, substance abuse and seizure disorders. The NMDA receptor is implicated in schizophrenia because phencyclidine (PCP), a noncompetitive NMDA receptor antagonist, binds to a hydrophobic domain within the channel, precipitating a schizophreniform psychosis in susceptible persons. Pharmacological, environmental, and genetic variables alter NMDA receptor-mediated neurotransmission. Inbred mouse strains differ in their sensitivity to some of the behavioral effects of MK-801 (dizocilpine), a PCP analogue. The NMDA receptor complex in the BALB/c strain could reflect a unique stoichiometric combination of receptor subunits resulting in a higher proportion of the channels in the open configuration, a higher affinity of MK-801 for its hydrophobic channel domain, and/or a combination of the above. The BALB/c mouse strain, "stressed" mice, and behavioral consequences of MK-801 administration represent models of altered glutamatergic neural transmission. We were interested in examining the effect of stress on the modulatory properties of d-serine and sarcosine. d-Serine is a naturally occurring glycine agonist that modulates the ability of l-glutamate to influence the opening of the NMDA receptor-associated ionophore, and sarcosine is a naturally occurring glycine reuptake inhibitor. The data suggest that 24h after stress, d-serine and sarcosine interact synergistically to reduce MK-801's ability to antagonize electrically precipitated tonic hindlimb extension. Under conditions of stress, modulatory effects of d-serine and sarcosine on the antiseizure effect of MK-801 are observed that are not apparent in the nonstress condition. The results could be relevant to the development of glycinergic interventions for the treatment of neuropsychiatric disorders.

Interaction of stress and strain on glutamatergic neurotransmission: relevance to schizophrenia

Psychosis caused by phencyclidine (PCP) stimulated interest in characterizing rodent behaviors elicited by PCP and its analogues. We have shown that MK-801 antagonizes electrically precipitated seizures (defined as tonic hindlimb extension) and elicits episodes of intense jumping behavior, referred to as "popping," in mice. Moreover, 24 h after stress, MK-801's ability to antagonize electrically precipitated seizures is reduced in outbred NIH Swiss mice. Inbred BALBc mice are more resistant to electrically precipitated seizures than the NIH Swiss strain, and are more sensitive to both MK-801's anticonvulsant effect and ability to elicit popping. In the current experiments, we examined the influence of stress and genetic mouse strain on both MK-801's ability to antagonize electrically precipitated seizures and elicit popping. Stress significantly reduced the threshold voltage for precipitation of seizures in BALBc mice and the anticonvulsant properties of MK-801 in both strains. These data show that factors relevant to schizophrenia and its exacerbation (i.e., acute stress and genetics) influence N-methyl-D-aspartic acid (NMDA) receptor-mediated neurotransmission in intact mice. The BALBc inbred strain of mouse may possess advantages in preclinical screening paradigms designed to assess NMDA receptor agonist interventions for disorders such as schizophrenia. Specifically, stressed BALBc mice showed the greatest behavioral sensitivity to MK-801 with regard to electrically precipitated seizures in the incremental electroconvulsive shock (IECS) paradigm, whereas unstressed BALBc showed the greatest behavioral sensitivity to MK-801 in the "popping" paradigm, relative to BALBc and NIH Swiss mice in the appropriate comparison conditions.

The antiseizure efficacies of MK-801, phencyclidine, ketamine, and memantine are altered selectively by stress

Adaptive changes in the NMDA receptor complex occur in response to exposure to stress. We have previously shown that the ability of MK-801, an uncompetitive NMDA receptor antagonist, to antagonize electrically precipitated tonic hind-limb extension is reduced 24 h after mice are forced to swim for up to 10 min in cold water. The stress-induced reduction of the antiseizure efficacy of MK-801 stimulated the proposal that mice exposed to swim stress may serve as "an intact animal model" of altered or diminished NMDA-mediated neural transmission. In the current investigation, the dose-dependent abilities for the antagonism of electrically precipitated seizures in mice were determined for MK-801, phencyclidine, ketamine, and memantine. Interestingly, a single session of cold water swim stress reduced the antiseizure efficacies of MK-801 and memantine without affecting phencyclidine and ketamine when tested 24 h later. The data do not suggest that stress results in a simple reduction in the number of activated or open channels, but rather alters their size or charge characteristics.

Evaluation of in vivo interactions in mice between flurazepam and two neuroactive steroids

The development of neuroactive steroids as anticonvulsant medications may be useful both as a primary treatment and as an adjuvant to other anticonvulsants. They may be limited, however, by sedative and ataxic side effects. In the current study, 3 alpha-hydroxy-5 beta-pregnan-20-one and alfaxalone, two prototypic neuroactive steroids, were shown to potentiate the ability of flurazepam to antagonize electrically precipitated tonic hindlimb extension in mice at doses that by themselves had little antiseizure efficacy. While alfaxalone alone lacked motor incoordinating effects at a dose (18.0 mg/kg) that potentiated the antiseizure efficacy of flurazepam, the same dose of 3 alpha-hydroxy-5 beta-pregnan-20-one possessed both the ability to potentiate flurazepam's anticonvulsant effect and disrupt mouse rotorod performance. The data suggest that allosteric interactions that have been described in vitro between neuroactive steroids and other modulators of the GABAA receptor complex may have relevance for the intact animal. Finally, the data also suggest that neuroactive steroids could be developed as short-lived adjuvant antiseizure medications in certain critical situations (e.g., medication-refractory status epilepticus). However, the motor incoordinating effects resulting from the combination of neuroactive steroids and flurazepam suggest that their usefulness as adjuvant medications in the chronic therapy of seizure disorders may be limited.

Infusion of NMDA antagonists into the nucleus reticularis pontis oralis inhibits the maximal electroshock seizure response

The nucleus reticularis pontis oralis (RPO) is necessary for the expression of tonic hindlimb extension (THE) in maximal electroshock (MES) seizures of rats. Previous work in this laboratory has demonstrated that both systemic administration and focal RPO microinfusion of D-cycloserine inhibits THE. The purpose of the present study was to characterize specific components of the NMDA receptor/ionophore complex that regulate the anticonvulsant activity mediated by the RPO. Bilateral RPO microinfusion of the competitive NMDA antagonists (-)AP7 and D-CPP as well as the uncompetitive antagonist dizocilpine ((+)MK-801) inhibited THE in a dose-related fashion. Bilateral RPO microinfusion of NMDA did not affect the THE response to MES but did induce convulsions resembling audiogenic seizures in genetically epilepsy prone rats. Bilateral RPO microinfusion of the strychnine-insensitive glycine site partial agonist D-cycloserine and the antagonist 5,7-dichlorokynurenic acid inhibited THE. The strychnine-insensitive glycine partial agonists (+)HA-966 and ACPC, as well as the agonists glycine and D-serine, did not significantly affect the THE response. Strychnine microinfusions in the RPO had no effect on THE. The results support a hypothesis that the RPO is a site of anticonvulsant drug action in MES and indicate that either competitive or uncompetitive NMDA antagonist action regulates the anticonvulsant activity mediated by the RPO. The role of the strychnine-insensitive glycine site in the regulation of the anticonvulsant activity medicated by the RPO is uncertain.