Microinfusion of the non-competitive N-methyl-D-aspartate receptor antagonist MK-801 (dizocilpine) into the dorsal hippocampus of wistar rats does not affect latent inhibition and prepulse inhibition, but increases startle reaction and locomotor activity

Hippocampal Disinhibition Reduces Contextual and Elemental Fear Conditioning While Sparing the Acquisition of Latent Inhibition

Hippocampal neural disinhibition, i.e., reduced GABAergic inhibition, is a key feature of schizophrenia pathophysiology. The hippocampus is an important part of the neural circuitry that controls fear conditioning and can also modulate prefrontal and striatal mechanisms, including dopamine signaling, which play a role in salience modulation. Consequently, hippocampal neural disinhibition may contribute to impairments in fear conditioning and salience modulation reported in schizophrenia. Therefore, we examined the effect of ventral hippocampus (VH) disinhibition in male rats on fear conditioning and salience modulation, as reflected by latent inhibition (LI), in a conditioned emotional response (CER) procedure. A flashing light was used as the conditioned stimulus (CS), and conditioned suppression was used to index conditioned fear. In experiment 1, VH disinhibition via infusion of the GABA-A receptor antagonist picrotoxin before CS pre-exposure and conditioning markedly reduced fear conditioning to both the CS and context; LI was evident in saline-infused controls but could not be detected in picrotoxin-infused rats because of the low level of fear conditioning to the CS. In experiment 2, VH picrotoxin infusions only before CS pre-exposure did not affect the acquisition of fear conditioning or LI. Together, these findings indicate that VH neural disinhibition disrupts contextual and elemental fear conditioning, without affecting the acquisition of LI. The disruption of fear conditioning resembles aversive conditioning deficits reported in schizophrenia and may reflect a disruption of neural processing both within the hippocampus and in projection sites of the hippocampus.

Effects of intra-accumbal administration of dopamine and ionotropic glutamate receptor drugs on delay discounting performance in rats

Nucleus accumbens core (NAcc) has been implicated in impulsive choice, as measured in delay discounting. The role of dopamine (DA) in impulsive choice has received considerable attention, whereas glutamate (Glu) has recently been shown to be an important mediator of discounting. However, research has not examined how DA or Glu receptors in NAcc mediate different aspects of delay discounting performance, that is, (a) sensitivity to reinforcer magnitude and (b) sensitivity to delayed reinforcement. Adult male Sprague-Dawley rats were first trained in a delay discounting task, in which the delay to a large magnitude food reinforcer increased across blocks of trials. Following behavioral training, rats received bilateral implantation of guide cannulas into NAcc. Half of the rats (n = 12) received infusions of the DA-selective ligands SKF 38393 (D1-like agonist: 0.03 or 0.1 μg), SCH 23390 (D1-like antagonist: 0.3 or 1.0 μg), quinpirole (D2-like agonist: 0.3 or 1.0 μg), and eticlopride (D2-like antagonist: 0.3 or 1.0 μg). The other half received infusions of the ionotropic Glu ligands MK-801 (NMDA uncompetitive antagonist: 0.3 or 1.0 μg), AP-5 (NMDA competitive antagonist: 0.3 or 1.0 μg), ifenprodil (noncompetitive antagonist at NR2B-containing NMDA receptors: 0.3 or 1.0 μg), and CNQX (AMPA competitive antagonist: 0.2 or 0.5 μg). Results showed that SCH 23390 (0.3 μg) decreased sensitivity to reinforcer magnitude without altering impulsive choice, whereas ifenprodil (1.0 μg) decreased sensitivity to delayed reinforcement (i.e., impulsive choice). The current results show that DA and NMDA receptors in NAcc mediate distinct aspects of discounting performance. (PsycINFO Database Record

Positive Allosteric Modulation of the Muscarinic M1 Receptor Improves Efficacy of Antipsychotics in Mouse Glutamatergic Deficit Models of Behavior

Current antipsychotics are effective in treating the positive symptoms associated with schizophrenia, but they remain suboptimal in targeting cognitive dysfunction. Recent studies have suggested that positive allosteric modulation of the M₁ muscarinic acetylcholine receptor (mAChR) may provide a novel means of improving cognition. However, very little is known about the potential of combination therapies in extending coverage across schizophrenic symptom domains. This study investigated the effect of the M₁ mAChR positive allosteric modulator BQCA [1-(4-methoxybenzyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid], alone or in combination with haloperidol (a first-generation antipsychotic), clozapine (a second-generation atypical antipsychotic), or aripiprazole (a third-generation atypical antipsychotic), in reversing deficits in sensorimotor gating and spatial memory induced by the N-methyl-d-aspartate receptor antagonist, MK-801 [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine]. Sensorimotor gating and spatial memory induction are two models that represent aspects of schizophrenia modeled in rodents. In prepulse inhibition (an operational measure of sensorimotor gating), BQCA alone had minimal effects but exhibited different levels of efficacy in reversing MK-801-induced prepulse inhibition disruptions when combined with a subeffective dose of each of the three (currently prescribed) antipsychotics. Furthermore, the combined effect of BQCA and clozapine was absent in M₁^-/- mice. Interestingly, although BQCA alone had no effect in reversing MK-801-induced memory impairments in a Y-maze spatial test, we observed a reversal upon the combination of BQCA with atypical antipsychotics, but not with haloperidol. These findings provide proof of concept that a judicious combination of existing antipsychotics with a selective M₁ mAChR positive allosteric modulator can extend antipsychotic efficacy in glutamatergic deficit models of behavior.

The metabotropic glutamate receptor 5 role on motor behavior involves specific neural substrates

Background

The metabotropic glutamate receptor 5 (mGluR5) is involved in various brain functions, including memory, cognition and motor behavior. Regarding locomotor activity, we and others have demonstrated that pharmacological antagonism of mGluR5 promotes hyperkinesia in mice. Moreover, increased locomotor activity can also be observed in mice following the genetic deletion of mGluR5. However, it is still unclear which specific brain substrates contribute to mGluR5-mediated regulation of motor function.

Results

Thus, to better understand the role of mGluR5 in motor control and to determine which neural substrates are involved in this regulation we performed stereotactic microinfusions of the mGluR5 antagonist, MPEP, into specific brain regions and submitted mice to the open field and rotarod apparatus. Our findings indicate that mGluR5 blockage elicits distinct outcomes in terms of locomotor activity and motor coordination depending on the brain region injected with mGluR5 antagonist. MPEP injection into either the dorsal striatum or dorsal hippocampus resulted in increased locomotor activity, whereas MPEP injection into either the ventral striatum or motor cortex resulted in hypokinesia. Moreover, MPEP injected into the olfactory bulb increased the distance mice traveled in the center of the open field arena. With respect to motor coordination on the rotarod, injection of MPEP into the motor cortex and olfactory bulb elicited decreased latency to fall.

Conclusions

Taken together, our data suggest that not only primarily motor neural substrates, but also limbic and sensory structures are involved in mGluR5-mediated motor behavior.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0113-2) contains supplementary material, which is available to authorized users.

Activation of mGluR2/3 receptors in the ventro-rostral prefrontal cortex reverses sensorimotor gating deficits induced by systemic NMDA receptor antagonists

Prepulse inhibition (PPI) of acoustic startle is an operational measure of sensorimotor gating, which is disrupted in schizophrenia. NMDA receptor (NMDAR) antagonist induced PPI disruption has become an important pharmacological model for schizophrenia; however, knowledge of the underlying mechanism remains incomplete. This study examines the role of NMDAR in the caudal pontine reticular nucleus (PnC) and the medial prefrontal cortex (mPFC) in NMDARs antagonist induced PPI deficits, as well as the NMDA receptor subtypes involved. We administered the NMDA antagonist MK-801 locally into the caudal pontine reticular formation (PnC), where the PPI mediating pathway converges with the primary startle pathway, and into the mPFC prior to behavioural testing. PnC microinjections had no effect on startle and PPI, whereas injections into the ventro-rostral part, but not into the dorso-caudal part of the mPFC, disrupted PPI. These effects could be mimicked by local injection of the NR2B subunit specific antagonist ifenprodil, whereas co-application of MK-801 and the mGluR2/3 agonist LY354740 had no effect on PPI. Moreover, PPI disruptions by systemically administered MK-801 could be reversed by local injections of LY354740 into the ventro-rostral mPFC, but not into the dorso-caudal mPFC. Our results indicate that NR2B subunit containing NMDARs in a specific subregion of the mPFC play a major role in PPI disruptions by systemic NMDAR antagonism. Our results further support the hypothesis that glutamate hyper-function in the mPFC is a main mechanism involved in sensory gating deficits induced by systemic MK-801, supporting the notion that this is an important mechanism in schizophrenia pathology.

The tyrosine phosphatase STEP: implications in schizophrenia and the molecular mechanism underlying antipsychotic medications

Glutamatergic signaling through N-methyl-D-aspartate receptors (NMDARs) is required for synaptic plasticity. Disruptions in glutamatergic signaling are proposed to contribute to the behavioral and cognitive deficits observed in schizophrenia (SZ). One possible source of compromised glutamatergic function in SZ is decreased surface expression of GluN2B-containing NMDARs. STEP(61) is a brain-enriched protein tyrosine phosphatase that dephosphorylates a regulatory tyrosine on GluN2B, thereby promoting its internalization. Here, we report that STEP(61) levels are significantly higher in the postmortem anterior cingulate cortex and dorsolateral prefrontal cortex of SZ patients, as well as in mice treated with the psychotomimetics MK-801 and phencyclidine (PCP). Accumulation of STEP(61) after MK-801 treatment is due to a disruption in the ubiquitin proteasome system that normally degrades STEP(61). STEP knockout mice are less sensitive to both the locomotor and cognitive effects of acute and chronic administration of PCP, supporting the functional relevance of increased STEP(61) levels in SZ. In addition, chronic treatment of mice with both typical and atypical antipsychotic medications results in a protein kinase A-mediated phosphorylation and inactivation of STEP(61) and, consequently, increased surface expression of GluN1/GluN2B receptors. Taken together, our findings suggest that STEP(61) accumulation may contribute to the pathophysiology of SZ. Moreover, we show a mechanistic link between neuroleptic treatment, STEP(61) inactivation and increased surface expression of NMDARs, consistent with the glutamate hypothesis of SZ.

Long-term behavioral and NMDA receptor effects of young-adult corticosterone treatment in BDNF heterozygous mice

Psychiatric illnesses, such as schizophrenia, are most likely caused by an interaction between genetic predisposition and environmental factors, including stress during development. The neurotrophin, brain-derived neurotrophic factor (BDNF) has been implicated in this illness as BDNF levels are decreased in the brain of patients with schizophrenia. The aim of the present study was to assess the combined effect of reduced BDNF levels and postnatal stress, simulated by chronic young-adult treatment with the stress hormone, corticosterone. From 6 weeks of age, female and male BDNF heterozygous mice and their wild-type controls were chronically treated with corticosterone in their drinking water for 3 weeks. At 11 weeks of age, male, but not female BDNF heterozygous mice treated with corticosterone exhibited a profound memory deficit in the Y-maze. There were no differences between the groups in baseline prepulse inhibition (PPI), a measure of sensorimotor gating, or its disruption by treatment with MK-801. However, an increase in startle caused by MK-801 treatment was absent in male, but not female BDNF heterozygous mice, irrespective of corticosterone treatment. Analysis of protein levels of the NMDA receptor subunits NR1, NR2A, NR2B and NR2C, showed a marked increase of NR2B levels in the dorsal hippocampus of male BDNF heterozygous mice treated with corticosterone. In the ventral hippocampus, significantly reduced levels of NR2A, NR2B and NR2C were observed in male BDNF heterozygous mice. The NMDA receptor effects in hippocampal sub-regions could be related to the spatial memory deficits and the loss of the effect of MK-801 on startle in these mice, respectively. No significant changes in NMDA receptor subunit levels were observed in any of the female groups. Similarly, no significant changes in levels of BDNF or its receptor, TrkB, were found other than the expected reduced levels of BDNF in heterozygous mice. In conclusion, the data show differential interactive effects of reduced levels of BDNF expression and corticosterone treatment on spatial memory and startle in male and female mice, accompanied by significant, but region-specific changes in NMDA receptor subunit levels in the dorsal and ventral hippocampus. These results could be important for our understanding of the interaction of neurodevelopmental stress and BDNF deficiency in cognitive and anxiety-related symptoms of psychiatric illnesses, such as schizophrenia.

Spatial discrimination reversal learning in weanling rats is impaired by striatal administration of an NMDA-receptor antagonist

The striatum plays a major role in both motor control and learning and memory, including executive function and "behavioral flexibility." Lesion, temporary inactivation, and infusion of an N-methyl-d-aspartate (NMDA)-receptor antagonist into the dorsomedial striatum (dmSTR) impair reversal learning in adult rats. Systemic administration of MK-801 disrupts reversal learning in developing rats, as reported in an earlier work by Chadman et al., but it is not known whether NMDA-receptor function within the dmSTR plays a role in this effect. In Experiment 1, reversal learning was dose-dependently impaired following bilateral dmSTR administration of MK-801 (either 2.5 or 5.0 microg) only during the reversal phase relative to saline in postnatal day (P) 26 rats. In Experiment 2, separate groups of P26 rats were trained on the same reversal learning task, but were administered bilateral dmSTR infusions during acquisition only (MK-SAL), reversal only (SAL-MK), both phases (MK-MK), or neither phase (SAL-SAL). The MK-801 effect was specific to the reversal training phase. The drug did not alter acquisition of the initial discrimination. Analysis of the pattern of errors indicates that dmSTR MK-801 treatment increased perseveration of the choice response trained in acquisition. NMDA receptors in the dmSTR play a role in reversal learning in the weanling rat.

Medial prefrontal administration of MK-801 impairs T-maze discrimination reversal learning in weanling rats

Several executive functions rely on the medial prefrontal cortex (mPFC) in the rat. Aspiration and neurotoxic lesions of the mPFC impair reversal learning in adult rats. Systemic administration of MK-801, an NMDA-receptor antagonist, impairs T-maze reversal learning in weanling rats but the role of mPFC NMDA-receptor antagonism in this effect is not known in either adult or young animals. This set of studies showed that mPFC NMDA receptors are specifically involved in T-maze discrimination reversal in weanling rats. In Experiment 1, 26-day-old rats (P26) demonstrated a dose-dependent impairment following bilateral mPFC administration of either 2.5 or 5.0microg MK-801 or saline (vehicle) during the reversal training phase only. In Experiment 2, P26 rats were trained on the same task, but four groups of rats received bilateral mPFC infusions during acquisition only (MK-SAL), reversal only (SAL-MK), both phases (MK-MK), or neither phase (SAL-SAL). MK-801 impaired performance only when infused during reversal. This suggests that NMDA-receptor antagonism in the mPFC is selectively involved in reversal learning during development and this may account for the previously reported effects of systemic MK-801 on T-maze discrimination reversal in weanling rats.

NMDA receptor involvement in spatial delayed alternation in developing rats

Two experiments examined the effect of the noncompetitive NMDA receptor antagonist, dizocilpine maleate (MK-801), on spatial working memory during development. Rats were trained on spatial delayed alternation (SDA) in a T-maze after ip administration of 0.06 mg/kg MK-801, 0.1 mg/kg MK-801, or saline on postnatal days (P) P23 and P33 (Experiment 1), or following bilateral intrahippocampal administration of 2.5 or 5.0 microg per side MK-801 or saline on P26 (Experiment 2). In Experiment 1, MK-801 dose-dependently impaired SDA learning at both ages. Because the same doses of systemic MK-801 have no effect on T-maze position discrimination learning, impairment of SDA by MK-801 likely reflects disruption of spatial working memory. Both doses of MK-801 abolished acquisition of SDA performance in Experiment 2. Disruption of hippocampal plasticity may account for the effects produced by systemic MK-801 administration. These results confirm and extend earlier lesion studies by implicating plasticity of hippocampal neurons in the ontogeny of spatial delayed alternation.

Acute or subchronic clozapine treatment does not ameliorate prepulse inhibition (PPI) deficits in CPB-K mice with low levels of hippocampal NMDA receptor density

INTRODUCTION

The hypo-glutamatergic hypothesis of schizophrenia is based on clinical similarities between schizophrenia and phencyclidine (PCP)-induced psychosis in mentally healthy humans. Sensorimotor gating, as measured by prepulse inhibition (PPI) of the acoustic startle response (ASR), is impaired in schizophrenic patients. In animals, noncompetitive N-methyl-D: -aspartate (NMDA) antagonists such as PCP disrupt PPI in a way that resembles the defect seen in schizophrenia. In a previous study with inbred mouse strains, low PPI levels have been demonstrated in CPB-K mice possessing low levels of hippocampal NMDA receptor densities. The present study was performed to test whether the low magnitude of PPI in CPB-K mice can be reversed by the atypical antipsychotic drug clozapine (CLZ).

RESULTS

Before any treatment, CPB-K mice displayed a significant (p < 0.001) lower level in PPI and a significant (p < 0.001) higher ASR when compared to BALB/cJ mice known to have high hippocampal NMDA receptor densities. Acute and subchronic effects of a 2-week treatment with CLZ at daily doses of 5 and 10 mg/kg intraperitoneally, respectively, did not reveal any significant alteration of PPI levels in CPB-K mice. Nevertheless, the examination of motor behavior during nonstimulus trials provided a positive control for the drug's effectiveness.

CONCLUSION

In summary, (1) this study confirmed our working hypothesis: Lower levels of hippocampal glutamatergic receptor densities correspond to lower sensorimotor gating in CPB-K mice, and (2) acute or subchronic treatment with CLZ did not elevate low PPI levels in CPB-K mice. Thus, further experiments will concentrate on other antipsychotic drugs to prove the predictive validity of this animal model.

On again, off again effects of gonadectomy on the acoustic startle reflex in adult male rats

Numerous studies have shown sex and/or estrous cycle differences in the acoustic startle reflex (ASR) and its prepulse inhibition (PPI) in humans and animals. However, few have examined the effects of hormone manipulations on these behaviors. This study paired gonadectomy (GDX) in adult male rats with testing for ASR and PPI at 2, 4, 9, 16, 23, 30 and 37 days after surgery. Initial studies of control, GDX and GDX rats given testosterone propionate revealed no group differences in PPI, but did reveal phasic facilitation of the ASR in GDX rats that was greatest on the first and final testing sessions and that was attenuated by testosterone. A second study addressing roles for estrogen and androgen signaling tested new control and GDX rats along with GDX rats given estradiol or the non-aromatizable androgen, 5-alpha-dihydrotestosterone and revealed no group differences in PPI, and increases in ASR in GDX rats that were largest during the first and final testing sessions and that were attenuated by both hormone replacements. However, while responses in GDX rats given testosterone were similar to those of controls, ASR in estradiol- and to a lesser extent in dihydrotestosterone-treated GDX rats were typically lower than in controls. This may suggest that hormone modulation of the ASR requires synergistic estrogen and androgen actions. In the male brain where this can be achieved by local steroid metabolism, the enzymes responsible, e.g., aromatase, could help identify loci in the startle circuitry that may be especially relevant for the hormone modulation observed.

Localized injections of midazolam into the amygdala and hippocampus induce differential changes in anxiolytic-like motor activity in mice

Various strains of mice display a reliable increase in motor activity in response to benzodiazepines given at low to moderate doses. This hyperactivity has been described as both an anxiolytic-associated increase in exploratory activity and a nonspecific stimulant effect controlled by central neural mechanisms separate from those involved in the anxiolytic-like effects. The purpose of the current study was to investigate the neural circuitry underlying the hyperactivity effects of benzodiazepines in mice. Specifically, we examined the relationship between anxiety and motor activity after bilateral intra-amygdala or intra-hippocampal microinjections of the nonselective full benzodiazepine receptor agonist midazolam in C57BL/6 mice. Behavioral measures of anxiety and motor activity in open field were examined in mice given localized injections of 0, 2, 8 or 32 nmol of midazolam directed into the amygdala or hippocampus. Midazolam injected into the amygdala at the low dose produced an anxiolytic-like effect, as reflected by an increase in central open field activity. Higher doses injected into the amygdala produced a motor-depressant action, indicative of a drug-induced sedative effect. Infusions into the hippocampus produced a biphasic effect on motor activity with the two lower doses of midazolam producing a motor-stimulant action and the high dose producing a motor-depressant effect. Hippocampus injections produced no anxiolytic-like effects. The current findings demonstrate that injections of midazolam produced a regional dissociation of the anxiety-related and motor-related parameters and provide evidence that the stimulant and anxiolytic effects of benzodiazepines are independent phenomena regulated by different central mechanisms.

Effects of fimbria-fornix, hippocampus, and amygdala lesions on discrimination between proximal locations

The conditioned cue preference (CCP) task was used to study the information required to discriminate between spatial locations defined by adjacent arms of an 8-arm radial maze. Normal rats learned the discrimination after 3 unreinforced preexposure (PE) sessions and 4 food paired-unpaired training trials. Fimbria-fornix lesions made before, but not after, PE, and hippocampus lesions made at either time, blocked the discrimination, suggesting that the 2 structures processed different information. Lateral amygdala lesions made before PE facilitated the discrimination. This amygdala-mediated interference with the discrimination was the result of a conditioned approach response that did not discriminate between the 2 arm locations. A hippocampus/fimbria-fornix system and an amygdala system process different information about the same learning situation simultaneously and in parallel.

Regional dissociations within the hippocampus--memory and anxiety

The amnestic effects of hippocampal lesions are well documented, leading to numerous memory-based theories of hippocampal function. It is debatable, however, whether any one of these theories can satisfactorily account for all the consequences of hippocampal damage: Hippocampal lesions also result in behavioural disinhibition and reduced anxiety. A growing number of studies now suggest that these diverse behavioural effects may be associated with different hippocampal subregions. There is evidence for at least two distinct functional domains, although recent neuroanatomical studies suggest this may be an underestimate. Selective lesion studies show that the hippocampus is functionally subdivided along the septotemporal axis into dorsal and ventral regions, each associated with a distinct set of behaviours. Dorsal hippocampus has a preferential role in certain forms of learning and memory, notably spatial learning, but ventral hippocampus may have a preferential role in brain processes associated with anxiety-related behaviours. The latter's role in emotional processing is also distinct from that of the amygdala, which is associated specifically with fear. Gray and McNaughton's theory can in principle incorporate these apparently distinct hippocampal functions, and provides a plausible unitary account for the multiple facets of hippocampal function.

Electrical stimulation of the hippocampus disrupts prepulse inhibition in rats: frequency- and site-dependent effects

Prepulse inhibition (PPI) is a normal reduction in the startle response produced when a brief, low intensity stimulus is presented prior to a startle-evoking stimulus. PPI is often disrupted in humans diagnosed with schizophrenia. As similar stimuli elicit PPI in rodents and humans, interventions in rodents that disrupt PPI may reveal aspects of neuronal dysfunction relevant to schizophrenia. Stimulation of the ventral hippocampus (vHip) with NMDA significantly increases dopamine (DA) efflux in the nucleus accumbens (NAc) and disrupts PPI, whereas NMDA infusion into the dorsal hippocampus (dHip) fails to alter PPI. Our previous research shows that brief periods of 20 Hz electrical vHip stimulation also significantly increase NAc DA efflux. The present experiments assessed the effects of stimulating the vHip or dHip on PPI and NAc DA efflux. As predicted, 20 Hz stimulation (10 s, 300 microA) of the vHip, but not the dHip, reversibly disrupted PPI. In contrast, 2 Hz stimulation (100 s, 300 microA) of the vHip failed to affect PPI. Microdialysis experiments revealed that 20 Hz stimulation of the vHip increased NAc DA efflux only in the hemisphere ipsilateral to the stimulating electrode, whereas 20 Hz stimulation of the dHip failed to affect NAc DA efflux. These data demonstrate the regional specificity and frequency-dependent effects of hippocampal activity on PPI. Additionally, it is intriguing that both chemical and electrical stimulation of the vHip disrupt PPI and increase NAc DA efflux, however, the relevance of these changes in NAc DA efflux to the disruption of PPI remains to be determined.

Baclofen reverses the reduction in prepulse inhibition of the acoustic startle response induced by dizocilpine, but not by apomorphine

RATIONALE

Since baclofen, the prototypical GABA(B) receptor agonist, is known to reduce the activity of dopaminergic mesolimbic neurons, a putative antipsychotic property of this compound has been suggested, but the evidence for this is still controversial.

OBJECTIVES

The aim of the present study was to elucidate the effects of baclofen on the prepulse inhibition (PPI) of the acoustic startle response (ASR), a behavioral paradigm considered to be one of the most powerful tools for the evaluation of sensorimotor gating and for the screening of antipsychotics.

METHODS

We tested the effects of baclofen (1.25, 2.5, 5 and 10 mg/kg IP) in rats, per se and in co-treatment with some of the substances known to induce a robust reduction of PPI, such as apomorphine (0.25 mg/kg SC) and dizocilpine (0.1 mg/kg SC). Finally, in order to ascertain whether the effects of baclofen could be ascribed to its activity on GABA(B) receptors, we analyzed whether its action could be prevented by pretreatment with SCH 50911, a selective GABA(B) receptor antagonist (20 mg/kg IP). All the experiments were carried out using standard procedures for the assessment of PPI of the ASR.

RESULTS

Baclofen per se produced no significant change in PPI parameters. Moreover, while no effect on apomorphine-mediated alterations in PPI parameters was observed, baclofen proved able to reverse dizocilpine-induced PPI disruption, and this effect was significantly prevented by SCH 50911. On the other hand, this last compound exhibited no effects per se at the same dose.

CONCLUSIONS

These results indicate that GABA(B) receptors are implicated in the neurobiological circuitry accounting for glutamatergic action in sensorimotor gating, and therefore can be proposed as putative new targets in the pharmacological therapy of psychotic disorders. Further studies should be addressed to evaluate more closely the clinical efficacy of baclofen in this respect.

Dorsal hippocampus and classical fear conditioning to tone and context in rats: effects of local NMDA-receptor blockade and stimulation

Consistent with the importance of the hippocampus in learning more complex stimulus relations, but not in simple associative learning, the dorsal hippocampus has commonly been implicated in classical fear conditioning to context, but not to discrete stimuli, such as a tone. In particular, a specific and central role in contextual fear conditioning has been attributed to mechanisms mediated by dorsal hippocampal N-methyl-D-aspartate (NMDA)-type glutamate receptors. The present study characterized the effects of blockade or tonic stimulation of dorsal hippocampal NMDA receptors by bilateral local infusion of the noncompetitive NMDA receptor antagonist MK-801 (dizocilpine maleate; 6.25 microg/side) or of NMDA (0.7 microg/side), respectively, on classical fear conditioning to tone and context in Wistar rats. Freezing was used to measure conditioned fear. Regardless of whether conditioning was conducted with tone-shock pairings or unsignaled footshocks (background or foreground contextual conditioning), both NMDA and MK-801 infusion before conditioning resulted in reduced freezing during subsequent exposure to the conditioning context. Freezing during subsequent tone presentation in a new context, normally resulting from conditioning with tone-shock pairings, was not impaired by MK-801 but was strongly reduced by NMDA infusion before conditioning; this freezing was also reduced by NMDA infusion before tone presentation (in an experiment involving NMDA infusions before conditioning and subsequent tone presentation to assess the role of state-dependent learning). It was assessed whether unspecific infusion effects (altered sensorimotor functions, state dependency) or infusion-induced dorsal hippocampal damage contributed to the observed reductions in conditioned freezing. Our data suggest that formation of fear conditioning to context, but not tone, requires NMDA receptor-mediated mechanisms in the dorsal hippocampus. As indicated by the effects of NMDA, some dorsal hippocampal processes may also contribute to fear conditioning to tone. The role of the dorsal hippocampus and local NMDA receptor-mediated processes in fear conditioning to tone and context is discussed in comparison with ventral hippocampal processes.

Latent inhibition disruption by MK-801 in a conditioned taste-aversion paradigm

N-Methyl-D-aspartate (NMDA) receptors appear to be involved in CS processing and memory consolidation. The present paper analyzed the effect of the non-competitive NMDA receptor antagonist Dizocilpine maleate (MK-801) on Latent Inhibition (LI)-retarded learning of a CS-US association after to-be-CS preexposures at time of testing, using Wistar rats as experimental subjects. If NMDA receptors are involved in CS processing, MK-801 administration should affect LI. In fact, previous experiments revealed that a 2.0mg/kg MK-801 dose, administered 20 h before preexposure and conditioning, abolished LI in a conditioned taste-aversion paradigm. In the present paper, MK-801 (0.2 mg/kg) was either injected after preexposure, after conditioning, or after both preexposure and conditioning stages. LI was abolished when MK-801 was injected after preexposure, but not when it was injected after conditioning. These results support the role of NMDA receptors in CS processing and memory consolidation.

The "two-headed" latent inhibition model of schizophrenia: modeling positive and negative symptoms and their treatment

RATIONALE

Latent inhibition (LI), namely, poorer performance on a learning task involving a previously pre-exposed non-reinforced stimulus, is disrupted in the rat by the dopamine (DA) releaser amphetamine which produces and exacerbates psychotic (positive) symptoms, and this is reversed by treatment with typical and atypical antipsychotic drugs (APDs) which on their own potentiate LI. These phenomena are paralleled by disrupted LI in normal amphetamine-treated humans, in high schizotypal humans, and in schizophrenia patients in the acute stages of the disorder, as well as by potentiated LI in normal humans treated with APDs. Consequently, disrupted LI is considered to provide an animal model of positive symptoms of schizophrenia with face, construct and predictive validity.

OBJECTIVES

To review most of the rodent data on the neural substrates of LI as well as on the effects of APDs on this phenomenon with an attempt to interpret and integrate these data within the framework of the switching model of LI; to show that there are two distinct LI models, disrupted and abnormally persistent LI; to relate these findings to the clinical condition.

RESULTS

The nucleus accumbens (NAC) and its DA innervation form a crucial component of the neural circuitry of LI, and are involved at the conditioning stage. There is a clear functional differentiation between the NAC shell and core subregions whereby damage to the shell disrupts LI and damage to the core renders LI abnormally persistent under conditions that disrupt LI in normal rats. The effects of shell and core lesions parallel those produced by lesions to the major sources of input to the NAC: entorhinal cortex lesion, like shell lesion, disrupts LI, whereas hippocampal lesion, like core lesion, produces persistent LI with changes in context, and basolateral amygdala (BLA) lesion, like core lesion, produces persistent LI with extended conditioning. Systemically induced blockade of glutamatergic as well as DA transmission produce persistent LI via effects exerted at the conditioning stage, whereas enhancement of DA transmission disrupts LI via effects at the conditioning stage. Serotonergic manipulations can disrupt or potentiate LI via effects at the pre-exposure stage. Both typical and atypical APDs potentiate LI via effects at conditioning whereas atypical APDs in addition disrupt LI via effects at pre-exposure. Schizophrenia patients can exhibit disrupted or normal LI as a function of the state of the disorder (acute versus chronic), as well as persistent LI.

CONCLUSIONS

Different drug and lesion manipulations produce two poles of abnormality in LI, namely, disrupted LI under conditions which lead to LI in normal rats, and abnormally persistent LI under conditions which disrupt it in normal rats. Disrupted and persistent LI are differentially responsive to APDs, with the former reversed by both typical and atypical APDs and the latter selectively reversed by atypical APDs. It is suggested that this "two-headed LI model" mimics two extremes of deficient cognitive switching seen in schizophrenia, excessive and retarded switching between associations, mediated by dysfunction of different brain circuitries, and can serve to model positive symptoms of schizophrenia and typical antipsychotic action, as well as negative symptoms of schizophrenia and atypical antipsychotic action.

Hippocampal modulation of sensorimotor processes

While the hippocampus makes unique contributions to memory, it has also long been associated with sensorimotor processes, i.e. innate processes involving control of motor responses to sensory stimuli. Moreover, hippocampal dysfunction has been implicated in neuropsychiatric diseases, such as schizophrenia and anxiety disorders, primarily characterized by non-mnemonic deficits in the processing of and responding to sensory information. This review is concerned with the hippocampal modulation of three sensorimotor processes in rats-locomotor activity, prepulse inhibition (PPI) of the startle reflex, and the startle reflex itself-whose alterations are related to human psychosis or anxiety disorders. Its main purpose is to present and discuss the picture emerging from studies examining the effects of pharmacological manipulations of the dorsal and ventral hippocampus by local drug microinfusions. While a role of the hippocampus in regulating locomotor activity, PPI, and startle reactivity has also been suggested based on the effects of hippocampal lesions, the microinfusion studies have revealed additional important details of this role and suggest modifications of notions based on lesion studies. In summary, the microinfusion studies corroborate that hippocampal mechanisms can directly influence locomotor activity, PPI, and startle reactivity, and that aberrant hippocampal function may contribute to neuropsychiatric diseases, in particular psychosis. The relation between different sensorimotor processes and hippocampal neurotransmission, the role of ventral and dorsal hippocampus, and the extrahippocampal mechanisms mediating the hippocampal modulation of different sensorimotor processes can partly be dissociated. Thus, the hippocampal modulation of these sensorimotor processes appears to reflect multiple operations, rather than one unitary operation.

Prepulse inhibition in rats with temporary inhibition/inactivation of ventral or dorsal hippocampus

Prepulse inhibition (PPI) of the acoustic startle response is a measure of sensorimotor gating and is decreased in neuropsychiatric diseases, including schizophrenia. Hippocampal involvement in PPI has been the subject of several studies, in particular, as aberrant hippocampal activity has been associated with schizophrenia. In rats, chemical stimulation of the ventral hippocampus reduced PPI, while normal PPI was found following hippocampal lesions, suggesting that ventral hippocampal overactivity is detrimental for PPI, but that normal hippocampal activity does not contribute substantially to PPI. In the present study, we investigated the importance of hippocampal activity for PPI by examining PPI in Wistar rats with temporarily decreased hippocampal activity, aiming to avoid compensatory processes that may occur with permanent lesions. Bilateral ventral or dorsal hippocampal infusions of the gamma-aminobutyric acid A (GABA(A)) receptor agonist muscimol (1 microg/side) or the sodium-channel blocker tetrodotoxin (TTX, 10 ng/side) reduced PPI. This reduction is probably neuroleptic-resistant since haloperidol and clozapine did not antagonize the muscimol-induced decreases in PPI. PPI reduction by muscimol inhibition or TTX inactivation of the dorsal or ventral hippocampus indicates that hippocampal activity contributes to sensorimotor gating, suggesting intact PPI after permanent hippocampal lesions to reflect compensatory processes. The data are discussed with respect to hippocampal dysfunction in schizophrenia.

The role of hippocampal dopamine receptors in prepulse inhibition

Although it has long been realized that the hippocampal formation receives a projection from the midbrain dopaminergic cell groups and contains mRNA for all five dopamine receptors, the functional role of this dopaminergic projection has not been studied so far. The present study aimed to investigate the role of dopamine receptors in the dorsal CA1 area of the hippocampus in prepulse inhibition. The results show that local application of amphetamine reduced prepulse inhibition without affecting the baseline startle amplitude. This effect of amphetamine could be reversed by coadministration of the D1 antagonist SCH23390. Moreover, local application of the D1 agonist SKF81297 also disrupted prepulse inhibition without altering basal startle amplitude. These data clearly suggest that the hippocampal D1 receptor plays an important role in prepulse inhibition. The effects of amphetamine could not be reversed by coadministration of the D2 antagonist sulpiride. Interestingly, the D2/3 agonist quinpirole did reduce prepulse inhibition, again without affecting basal startle amplitude. Because quinpirole has a much higher affinity for the D3 receptor than does sulpiride, it is suggested that the D3 receptor might be involved in this effect.

Effects of hippocampal N-methyl-D-aspartate infusion on locomotor activity and prepulse inhibition: differences between the dorsal and ventral hippocampus

Prepulse inhibition (PPI) of the acoustic startle response and open-field locomotor activity were measured after bilateral infusion of N-methyl-D-aspartate into the ventral (0.10, 0.25, 0.50 microg/side) and dorsal (0.10, 0.25, 0.50, 0.70 microg/side) hippocampus of Wistar rats. Dose-dependent hyperactivity and disruption of PPI--behavioral effects related to psychotic symptoms--were observed after ventral infusions but were virtually absent after dorsal infusions. This functional dorsal-ventral difference might be related to the different connections of the dorsal and ventral hippocampus with the amygdala, nucleus accumbens, and prefrontal cortex, which have been implicated in the regulation of locomotor activity and PPI. Hippocampal overactivity has been associated with schizophrenia. The findings suggest that overstimulation of the ventral hippocampal projections may contribute to behavioral outcomes related to psychotic symptoms.