Changes in prepulse inhibition after local administration of NMDA receptor ligands in the core region of the rat nucleus accumbens

Quinpirole, but not muscimol, infused into the nucleus accumbens disrupts prepulse inhibition of the acoustic startle in rhesus macaques

Sensorimotor gating is the ability to suppress motor responses to irrelevant sensory inputs. This response is disrupted in a range of neuropsychiatric disorders. Prepulse inhibition (PPI) of the acoustic startle response (ASR) is a form of sensorimotor gating in which a low-intensity prepulse immediately precedes a startling stimulus, resulting in an attenuation of the startle response. PPI is conserved across species and the underlying circuitry mediating this effect has been widely studied in rodents. However, recent work from our laboratories has shown an unexpected divergence between the circuitry controlling PPI in rodents as compared to macaques. The nucleus accumbens, a component of the basal ganglia, has been identified as a key modulatory node for PPI in rodents. The role of the nucleus accumbens in modulating PPI in primates has yet to be investigated. We measured whole-body PPI of the ASR in six rhesus macaques following (1) pharmacological inhibition of the nucleus accumbens using the GABAA agonist muscimol, and (2) focal application of the dopamine D2/3 agonist quinpirole (at 3 doses). We found that quinpirole, but not muscimol, infused into the nucleus accumbens disrupts prepulse inhibition in monkeys. These results differ from those observed in rodents, where both muscimol and quinpirole disrupt prepulse inhibition.

Cannabidiol effects on prepulse inhibition in nonhuman primates

Prepulse inhibition (PPI) of acoustic startle reflex is a well-established behavior paradigm to measure sensorimotor gating deficits. PPI is disrupted in several neuropsychiatric disorders, including schizophrenia. PPI tests can be used to screen new drugs for treatment of such disorders. In this review, we discuss how PPI paradigm can help in screening the therapeutic effects of cannabidiol (CBD). We look into recent literature about CBD effects on PPI response in animal models, especially in nonhuman primates. CBD has been shown to modify PPI inN-methyld-aspartate receptor antagonist models for schizophrenia, both in rodents and in nonhuman primates. These results show CBD as a potential drug for the treatment of neurologic disorders that present alterations in sensorimotor system, such as schizophrenia. Moreover, the PPI paradigm seems to be a useful and relative simple paradigm to test the efficacy of CBD as a potential therapeutic drug.

Adolescent isolation rearing produces a prepulse inhibition deficit correlated with expression of the NMDA GluN1 subunit in the nucleus accumbens

Adolescence is a transition period during which social interaction is necessary for normal brain and behavior development. Severely abnormal social interactions during adolescence can increase the incidence of lifelong psychiatric disease. Decreased prepulse inhibition (PPI) is a quantifiable hallmark of some psychiatric illnesses in humans and can be elicited in rodents by isolation rearing throughout the adolescent transition period. PPI is a measure of sensorimotor gating in which the nucleus accumbens (Acb) is crucially involved. The Acb is comprised of core and shell subregions, which receive convergent dopaminergic and glutamatergic inputs. To gain insight into the neurobiological correlates of adolescent adversity, we conducted electron microscopic immunolabeling of dopamine D1 receptors (D1Rs) and the GluN1 subunit of glutamate NMDA receptors in the Acb of isolation-reared (IR) adult male rats. In all animals, GluN1 was primarily located in dendritic profiles, many of which also contained D1Rs. GluN1 was also observed in perisynaptic glia and axon terminals. In IR rats compared with group-reared controls, GluN1 density was selectively decreased in D1R-containing dendrites of the Acb core. Across all animals, dendritic GluN1 density correlated with average percent PPI, implicating endogenous expression of NMDA receptors of the Acb as a possible substrate of the PPI response. These results suggest that adolescent isolation dampens NMDA-mediated excitation in direct (D1R-containing) output neurons of the Acb, and that these changes influence the operational measure of PPI.

Effects of Ketamine on Basal Gamma Band Oscillation and Sensory Gating in Prefrontal Cortex of Awake Rats

Gamma band oscillation (GBO) and sensory gating (SG) are associated with many cognitive functions. Ketamine induces deficits of GBO and SG in the prefrontal cortex (PFC). However, the time-courses of the effects of different doses of ketamine on GBO power and SG are poorly understood. Studies have indicated that GBO power and SG have a common substrate for their generation and abnormalities. In this study, we found that (1) ketamine administration increased GBO power in the PFC in rats differently in the low- and high-dose groups; (2) auditory SG was significantly lower than baseline in the 30 mg/kg and 60 mg/kg groups, but not in the 15 mg/kg and 120 mg/kg groups; and (3) changes in SG and basal GBO power were significantly correlated in awake rats. These results indicate a relationship between mechanisms underlying auditory SG and GBO power.

Syntaxin 1B contributes to regulation of the dopaminergic system through GABA transmission in the CNS

In neuronal plasma membrane, two syntaxin isoforms, HPC-1/syntaxin 1A (STX1A) and syntaxin 1B (STX1B), are predominantly expressed as soluble N-ethylmaleimide-sensitive fusion attachment protein receptors, also known as t-SNAREs. We previously reported that glutamatergic and GABAergic synaptic transmissions are impaired in Stx1b null mutant (Stx1b^-/- ) mice but are almost normal in Stx1a null mutant (Stx1a^-/- ) mice. These observations suggested that STX1A and STX1B have distinct functions in fast synaptic transmission in the central nervous system (CNS). Interestingly, recent studies indicated that Stx1a^-/- or Stx1a^+/- mice exhibit disruption in the monoaminergic system in the CNS, causing unusual behaviour that is similar to neuropsychological alterations observed in psychiatric patients. Here, we studied whether STX1B contributes to the regulation of monoaminergic system and if STX1B is related to neuropsychological properties in human neuropsychological disorders similar to STX1A. We found that monoamine release in vitro was normal in Stx1b^+/- mice unlike Stx1a^-/- or Stx1a^+/- mice, but the basal extracellular dopamine (DA) concentration in the ventral striatum was increased. DA secretion in the ventral striatum is regulated by GABAergic neurons, and Stx1b^+/- mice exhibited reduced GABA release both in vitro and in vivo, disrupting the DAergic system in the CNS of these mice. We also found that Stx1b^+/- mice exhibited reduced pre-pulse inhibition (PPI), which is believed to represent one of the prominent schizotypal behavioural profiles of human psychiatric patients. The reduction in PPI was rescued by DA receptor antagonists. These observations indicated that STX1B contributes to excess activity of the DAergic system through regulation of GABAergic transmission.

ADIOL protects against 3-NP-induced neurotoxicity in rats: Possible impact of its anti-oxidant, anti-inflammatory and anti-apoptotic actions

Huntington's disease (HD) is a progressive neurodegenerative disorder with a wide spectrum of cognitive, behavioral and motor abnormalities. The mitochondrial toxin 3-nitropropionic acid (3-NP) effectively induces specific behavioral changes and selective striatal lesions similar to that observed in HD. Some neurosteroids, synthesized in neurons and glial cells, previously showed neuroprotective abilities. 5-Androstene-3β-17β-diol (ADIOL) is a major metabolite of dehydroepiandrosterone (DHEA) with previously reported anti-inflammatory, anti-apoptotic and neuroprotective activities. The neuroprotective potential of ADIOL in HD was not previously investigated. Therefore, the present study investigated the neuroprotective effects of ADIOL against 3-NP-induced behavioral changes, oxidative stress, inflammation and apoptosis. Intraperitoneal administration of 3-NP (20mg/kg) for 4 consecutive days in rats caused significant loss in body weight, reduced prepulse inhibition (PPI) of acoustic startle response, locomotor hypoactivity with altered cortical/striatal histological structure, increased cortical/striatal oxidative stress, inflammation and apoptosis. Administration of ADIOL (25mg/kg, s.c.) for two days before 3-NP significantly attenuated the reduction in body weights and PPI, increased locomotor activity and restored cortical/striatal histological structure nearly to normal. Moreover, it displayed anti-oxidant, anti-inflammatory and anti-apoptotic activities as evidenced by the elevation of cortical and striatal reduced glutathione levels, reductions of cortical and striatal malondialdehyde, striatal tumor necrosis factor alpha and interleukin-6 levels. Only a small number of iNOS and caspase-3 positive cells were detected in sections from rats pretreated with ADIOL. This study suggests a potential neuroprotective role of ADIOL against 3-NP-induced Huntington's disease-like manifestations. Such neuroprotection can be attributed to its anti-oxidant, anti-inflammatory and anti-apoptotic activities.

Effect of deep brain stimulation in rats selectively bred for reduced prepulse inhibition

BACKGROUND

Sensorimotor gating, measured as prepulse inhibition (PPI) of the acoustic startle reaction (ASR), is disturbed in certain neuropsychiatric disorders, such as schizophrenia, obsessive compulsive disorder, and Tourette's syndrome (TS). Deep brain stimulation (DBS) of the centromedian-parafascicular complex (CM-Pf), globus pallidus internus (in rats the entopeduncular nucleus - EPN), and the ventral striatum (in rats the nucleus accumbens - NAC) has been used for treatment in TS.

OBJECTIVE

We tested whether DBS of these regions would alleviate breeding-induced low PPI in rats.

METHODS

Rats with breeding-induced low and high PPI were bilaterally implanted with electrodes in the CM-Pf, the EPN, or the NAC. After two weeks, they were stimulated or sham stimulated for epochs of 6 days (in the EPN with a current of 20% below the individual threshold for stimulation-induced side effects, in the NAC or CM-Pf with 100 μA and 150 μA). On the 6th day the rats were tested for PPI of ASR.

RESULTS

Stimulation in the CM-Pf with 150 μA significantly alleviated PPI, while NAC stimulation was less effective. In PPI low rats electrode implantation in the EPN already improved PPI, while subsequent stimulation had no additional effect. Startle reaction of PPI low rats was not affected by stimulation of either region.

CONCLUSION

The CM-Pf and the EPN are important for the modulation of sensorimotor gating in rats with breeding-induced low PPI. These rats may therefore be useful to further investigate the pathophysiological mechanisms of deficient sensorimotor gating and also mechanisms of action of DBS in these circumstances.

Modulation of sensorimotor gating in prepulse inhibition by conditional brain glycine transporter 1 deletion in mice

Inhibition of glycine transporter 1 (GlyT1) augments N-methyl-D-aspartate receptor (NMDAR)-mediated transmission and represents a potential antipsychotic drug target according to the NMDAR hypofunction hypothesis of schizophrenia. Preclinical evaluation of GlyT1 inhibiting drugs using the prepulse inhibition (PPI) test, however, has yielded mixed outcomes. Here, we tested for the first time the impact of two conditional knockouts of GlyT1 on PPI expression. Complete deletion of GlyT1 in the cerebral cortices confers resistance to PPI disruption induced by the NMDAR blocker MK-801 (0.2mg/kg, i.p.) without affecting PPI expression in unchallenged conditions. In contrast, restricting GlyT1 deletion to neurons in forebrain including the striatum significantly attenuated PPI, and the animals remained sensitive to the PPI-disruptive effect of MK-801 at the same dose. These results demonstrate in mice that depending on the regional and/or cell-type specificity, deletion of the GlyT1 gene could yield divergent effects on PPI.

Preferential relocation of the N-methyl-D-aspartate receptor NR1 subunit in nucleus accumbens neurons that contain dopamine D1 receptors in rats showing an apomorphine-induced sensorimotor gating deficit

Sensorimotor gating as measured by prepulse inhibition (PPI) to startle-evoking auditory stimulation (AS) is disrupted in schizophrenia and in rodents receiving systemic administration of apomorphine, a dopamine D1/D2 receptor agonist, or MK-801, an N-methyl-d-aspartate (NMDA) receptor antagonist. The functional analogies and our prior results showing apomorphine- and AS-induced relocation of the dopamine D1 receptor (D1R) in the nucleus accumbens (Acb) shell suggest that apomorphine and AS may affect the subcellular distribution of the NMDA receptor NR1 subunit, a protein that forms protein-protein interactions with the D1R. We quantitatively compared the electron microscopic immunogold labeling for NR1 in dendritic profiles distinguished with respect to presence of D1R immunoreactivity and location in the Acb shell or core of rats receiving a single s.c. injection of vehicle (VEH) or apomorphine (APO) alone, or combined with AS (VEH+AS, APO+AS). The rats in the APO+AS group were previously shown to have PPI deficits, whereas the rats in the VEH+AS group had normal PPI. A significantly higher percentage of plasmalemmal and a lower percentage of cytoplasmic NR1 immunogold particles were seen in D1R-labeled dendritic spines in the Acb shell of the APO+AS group compared with all other groups. D1R-containing small dendrites in the Acb shell of the APO+AS group also showed a significantly higher density of plasmalemmal and a lower density of cytoplasmic NR1 immunogold particles compared with VEH or APO groups. In the Acb core, the APO+AS group had significantly fewer dendritic spines co-expressing NR1 and D1R compared with VEH or VEH+AS groups. These results, together with our earlier findings, suggest that NMDA receptors are preferentially mobilized in D1R-containing Acb neurons of rats showing apomorphine-induced disruption of PPI in a paradigm using acoustic stimulation.

Strain differences in the gating-disruptive effects of apomorphine: relationship to gene expression in nucleus accumbens signaling pathways

BACKGROUND

Prepulse inhibition (PPI) of startle is a measure of sensorimotor gating that is deficient in certain psychiatric disorders, including schizophrenia. Sprague Dawley (SD) rats are more sensitive to PPI-disruptive effects of apomorphine (APO) at long interstimulus intervals (ISIs) (60-120 msec) and less sensitive to PPI-enhancing effects of APO at short ISIs (10-30 msec) compared with Long Evans (LE) rats.

METHODS

Prepulse inhibition was tested in SD and LE rats after APO (.5 mg/kg) or vehicle in a within- subject design and sacrificed 14 days later. Total RNA was extracted from the nucleus accumbens (NAC). Approximately 700 dopamine-relevant transcripts on the Affymetrix 230 2.0 microarray were analyzed.

RESULTS

As previously reported, SD rats exhibited greater APO-induced PPI deficits at long intervals and less APO-induced PPI enhancement at short intervals compared with LE rats. One hundred four genes exhibited significantly different NAC expression levels in these two strains. Pathway analysis revealed that many of these genes contribute to dopamine receptor signaling, synaptic long-term potentiation, or inositol phosphate metabolism. The expression of some genes significantly correlated with measures of APO-induced PPI sensitivity in either SD or LE rats. The expression of select genes was validated by real-time reverse transcription polymerase chain reaction (RT-PCR).

CONCLUSIONS

Differences in PPI APO sensitivity in SD versus LE rats are robust and reproducible and may be related to strain differences in the expression of genes that regulate signal transduction in the NAC. These genes could facilitate the identification of targets for ameliorating heritable gating deficits in brain disorders such as schizophrenia.

The effects of temporary inactivation of the core and the shell subregions of the nucleus accumbens on prepulse inhibition of the acoustic startle reflex and activity in rats

The nucleus accumbens can be dissociated into at least two subregions: a 'core' and a 'shell'. Using temporary chemical inactivation of these subregions, we investigated whether they are differentially involved in the regulation of prepulse inhibition (PPI) of the acoustic startle reflex and activity. For this purpose, rats were bilaterally implanted with guide cannulae aimed at either the core or the shell and infused with the GABA(A) receptor agonist muscimol (0.5 microg/0.2 microl per side). The control group consisted of vehicle infused and unoperated rats. To ascertain the region selectivity of the infusions, 0.2 microl of [3H]muscimol was infused into either the core or the shell of an additional group of rats. The behavioral results demonstrated that in comparison to the control group, inactivation of the core led to a loss of the prepulse intensity dependency of PPI. Moreover, core inactivation resulted in akinesia directly after infusion, but in hyperactivity 24 and 72 h thereafter in contrast to the control group. In both experiments, inactivation of the shell was ineffective compared to controls. Analysis of the autoradiograms revealed that the spread of drug into the other subregion was minimal, supporting the region selectivity of the inactivation. These results lend further support to the existence of a functional dissociation between the core and the shell, with the former being preferentially involved in PPI and locomotion. The persistent hyperactivity after the muscimol infusion into the core could be explained by compensatory mechanisms taking place in the nucleus accumbens.

Endogenous kynurenic acid disrupts prepulse inhibition

BACKGROUND

Recent studies show that endogenous levels of kynurenic acid (KYNA) are increased in the cerebrospinal fluid of schizophrenic patients. Prepulse inhibition (PPI) of the acoustic startle reflex is an operational measure of sensorimotor gating that is reduced in neuropsychiatric disorders, such as schizophrenia. Previous studies show that administration of N-methyl-D-aspartate (NMDA) receptor antagonists, such as phencyclidine or MK-801, leads to deficits in sensorimotor gating that mimic those observed in schizophrenic patients.

METHODS

The present study examined the effects of the endogenous NMDA receptor antagonist KYNA on startle and PPI in rats. Elevation of endogenous brain levels of KYNA was achieved through intraperitoneal (IP) administration of kynurenine (100 mg/kg), the precursor of KYNA, or by intravenous administration of PNU 156561A (10 mg/kg).

RESULTS

A fourfold increase in brain KYNA levels, as induced by kynurenine or PNU 156561A, significantly reduced PPI. There were no differences in startle magnitudes between control rats and drug-treated rats. The disruption of PPI was restored by administration of the antipsychotic drugs haloperidol (.2 mg/kg, IP) or clozapine (7.5 mg/kg, IP).

CONCLUSIONS

The present results suggest that brain KYNA serves as an endogenous modulator of PPI and are consistent with the hypothesis that KYNA contributes to the pathophysiology of schizophrenia.

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.

Prepulse inhibition deficits of the startle reflex in neonatal ventral hippocampal-lesioned rats: reversal by glycine and a glycine transporter inhibitor

BACKGROUND

Neonatal ventral hippocampal (NVH) lesions in rats induce behavioral abnormalities at adulthood thought to simulate some aspects of the positive, negative, and cognitive deficits classically observed in schizophrenic patients. Such lesions induce a postpubertal emergence of prepulse inhibition (PPI) deficits of the startle reflex reminiscent of the sensorimotor gating deficits observed in a majority of schizophrenic patients. To study the potential involvement of the glycinergic neurotransmission in such deficits, we investigated the capacity of glycine (an obligatory N-methyl-D-aspartate [NMDA] receptor co-agonist) and ORG 24598 (a selective glycine transporter 1 inhibitor) to reverse NVH lesion-induced PPI deficits in rats.

METHODS

Ibotenic acid was injected bilaterally into the ventral hippocampus of 7-day-old pups. Prepulse inhibition of the startle reflex was measured at adulthood.

RESULTS

Glycine (.8 and 1.6 g/kg IP) and ORG 24598 (10 mg/kg IP) fully and partially reversed lesion-induced PPI deficits, respectively.

CONCLUSIONS

These findings confirm that an impaired glutamatergic neurotransmission may be responsible for PPI deficits exhibited by NVH-lesioned rats and support the hypoglutamatergic hypothesis of schizophrenia. They also suggest that drugs acting either directly at the NMDA receptor glycine site or indirectly on the glycine transporter 1 could offer promising targets for the development of novel therapies for schizophrenia.

Schizophrenia: from phenomenology to neurobiology

Schizophrenia is a common and debilitating illness, characterized by chronic psychotic symptoms and psychosocial impairment that exact considerable human and economic costs. The literature in electronic databases as well as citations and major articles are reviewed with respect to the phenomenology, pathology, treatment, genetics and neurobiology of schizophrenia. Although studied extensively from a clinical, psychological, biological and genetic perspective, our expanding knowledge of schizophrenia provides only an incomplete understanding of this complex disorder. Recent advances in neuroscience have allowed the confirmation or refutation of earlier findings in schizophrenia, and permit useful comparisons between the different levels of organization from which the illness has been studied. Schizophrenia is defined as a clinical syndrome that may include a collection of diseases that share a common presentation. Genetic factors are the most important in the etiology of the disease, with unknown environmental factors potentially modulating the expression of symptoms. Schizophrenia is a complex genetic disorder in which many genes may be implicated, with the possibility of gene-gene interactions and a diversity of genetic causes in different families or populations. A neurodevelopmental rather than degenerative process has received more empirical support as a general explanation of the pathophysiology, although simple dichotomies are not particularly helpful in such a complicated disease. Structural brain changes are present in vivo and post-mortem, with both histopathological and imaging studies in overall agreement that the temporal and frontal lobes of the cerebral cortex are the most affected. Functional imaging, neuropsychological testing and clinical observation are also generally consistent in demonstrating deficits in cognitive ability that correlate with abnormalities in the areas of the brain with structural abnormalities. The dopamine and other neurotransmitter systems are certainly involved in the treatment or modulation of psychotic symptoms. These broad findings represent the distillation of a large body of disparate data, but firm and specific findings are sparse, and much about schizophrenia remains unknown.

Differing effects of the cannabinoid agonist, CP 55,940, in an alcohol or Tween 80 solvent, on prepulse inhibition of the acoustic startle reflex in the rat

It has been suggested that cannabinoid agonists increase dopamine (DA) transmission in the mesolimbic dopamine system. However, evidence for such an effect is inconsistent. Prepulse inhibition (PPI) of the acoustic startle reflex is a behavioural paradigm that is modulated by an increase of mesolimbic dopamine. This study sought to ascertain whether or not a cannabinoid agonist, CP 55,940, mimicked the effects of amphetamine (a drug which increases dopamine release) on PPI. The first experiment measured the PPI of 16 male Wistar rats injected (i.p.) with different doses of CP 55,940 in a Latin-square design. A second experiment replicated the effects of the first experiment in a between-subjects design, and also examined the effects of using a 5% alcohol solution as a solvent for cannabinoid agonists, in comparison to the more inert detergent, Tween 80. In both experiments, CP 55,940 in Tween 80 significantly reduced basal activity, increased startle onset latencies and increased PPI, effects opposite to those of amphetamine. These results suggest that the net behavioural effects of cannabinoids are opposite to those of amphetamine. In addition, it was found that 1 ml/kg of a 5% alcohol solution has significant behavioural effects on its own, and reverses the effects of CP 55,940 on PPI.

Tactile prepuff inhibition of startle in children with Tourette's syndrome: in search of an "fMRI-friendly" startle paradigm

BACKGROUND

Functional magnetic resonance imaging (fMRI) studies in neuropsychiatric populations will be enhanced by "on-line" tasks that assess brain activation linked to neurocognitive and psychophysiological functions. In some cases, task modifications may be required for use in an fMRI environment. Prepulse inhibition (PPI) of the startle reflex is an operational measure of sensorimotor gating that is deficient in specific neuropsychiatric disorders, including schizophrenia, Huntington's disease, and Tourette's syndrome (TS). This study examined whether a modified "fMRI-friendly" PPI paradigm is suitable for use in children and adequately sensitive to detect PPI deficits in TS.

METHODS

Bilateral eyeblink PPI was measured in children using chin air puffs to elicit startle and prepuffs to the dorsal hand surface as inhibiting stimuli. This paradigm involved no metallic objects or acoustic stimuli, making it suitable for an fMRI environment that is magnetically sensitive and acoustically complex. Children were also assessed in a "standard" acoustic PPI paradigm.

RESULTS

Robust startle was elicited via either puffs or noise bursts, and these responses were inhibited by prepuffs and prepulses, respectively. Compared to control subjects, children with TS exhibited comparable startle magnitude and habituation but significantly reduced prepuff inhibition and acoustic PPI.

CONCLUSIONS

Sensorimotor gating can be assessed in an "fMRI-friendly" paradigm that detects inhibitory deficits in TS.

Hippocampal and cortical sensory gating in rats: effects of quinpirole microinjections in nucleus accumbens core and shell

Sensory processing disturbances, as measured in the P50/sensory gating paradigm, have been linked to aberrant auditory information processing and sensory overload in schizophrenic patients. In this paradigm, the response to the second of paired-click stimuli is attenuated by an inhibitory effect of the first stimulus. Sensory gating has been observed in most healthy human subjects and normal laboratory rats. Because mesolimbic dopamine has been implicated in other filtering disturbances such as prepulse inhibition of the acoustic startle response and given the fact that amphetamine and apomorphine have been shown to disrupt gating, this study was performed to investigate the role of mesolimbic dopamine in sensory gating. The dopamine D2 receptor agonist quinpirole (10 microg/0.5 microl) was injected bilaterally in nucleus accumbens core and shell and effects on cortical and hippocampal sensory gating were investigated. Also, effects of the dopamine D2 receptor antagonist haloperidol (0.1 mg/kg, subcutaneously) as pretreatment were studied. First, quinpirole significantly reduced both the amplitude to the first click and gating as measured in the cortex and in the hippocampus. There was a tendency for the quinpirole effects on hippocampal gating to be more pronounced in rats injected in the shell. Secondly, haloperidol did not antagonize effects of quinpirole on hippocampal parameters, whereas haloperidol pretreatment fully antagonized quinpirole effects on cortical parameters. In conclusion, gating can be significantly reduced when a dopamine agonist is specifically targeted at mesolimbic dopamine D2 receptors. However, an important consideration is that the dopaminergic effects in the present study on gating are predominantly mediated by the effects on the amplitude to the first click. This has also been suggested for systemic amphetamine injections in rats and schizophrenic patients. This casts doubt on whether dopamine receptor activation affects the putative inhibitory process between the first and the second stimulus.

A neural network approach to the acoustic startle reflex and prepulse inhibition

Prepulse inhibition (PPI) is the normal suppression of the startle reflex when an intense stimulus is preceded by a weak non-startling prestimulus. PPI is widely used as a model for sensorimotor gating processes and has been shown to be impaired in various neuropsychiatric disorders, including schizophrenia. We have reproduced startle-like behavior and basic PPI modifications with a neural network. The network design was constrained by the attempt (1) to use as few connections as possible and (2) to relate neuroanatomical structures to the simulated network. Performance of the network was evaluated by the behavior of the simulated motor neurons in response to prepulse and pulse stimuli presented with various lead intervals and prepulse intensities. A delayed inhibitory pathway via the pedunculopontine nucleus (PPTg) to the caudal pontine reticular nucleus was found to be a necessary but insufficient requirement to reproduce basic PPI output patterns. Additional requirements included (a) a low threshold at or below the caudal pontine reticular formation, (b) signal amplification in the inhibitory pathway and (c) prolongation of activity in the inhibitory pathway. On the grounds of the most appropriate output patterns of the simulations, we propose a mechanism of sustained activation in the PPTg due to recursive connections. Relations between stimuli, behavior (motor output) and the underlying architecture are discussed. Potentially, this modeling technique can be extended to investigate the impact of drugs and higher brain regions on PPI.

Regulation of sensorimotor gating in rats by hippocampal NMDA: anatomical localization

Prepulse inhibition (PPI) of the startle reflex is a measure of sensorimotor gating that is reduced in humans with certain neuropsychiatric disorders, including schizophrenia, and in rats after manipulations of limbic cortico-striato-pallido-pontine circuitry. We have reported that PPI is reduced after specific manipulations of the hippocampal complex (HPC) in rats, but the mechanisms for these effects remain poorly understood. For example, dopaminergic substrates clearly regulate PPI, but the PPI-disruptive effects of intra-HPC carbachol or NMDA are not reversed by D2 receptor antagonists. This study examined the anatomical specificity within the hippocampal complex of the PPI-disruptive effects of NMDA infusion. Startle magnitude and PPI were assessed after acute bilateral infusion of NMDA (0, 0.4 or 0.8 microg) into the dorsal subiculum (DS), region CA1, the ventral subiculum (VS), the rostral entorhinal cortex (ECr) and the caudal entorhinal cortex (ECc). A dorsal-ventral gradient for NMDA effects was observed, with a dose-dependent disruption of PPI after NMDA infusion into the VS or EC, but not the DS, and with intermediate level effects observed after NMDA infusion into CA1. A second set of studies confirmed that the failure of NMDA effects in the DS did not reflect site-related differences in startle magnitude or baseline levels of PPI. These findings demonstrate the importance of the ventral, but not the dorsal HPC, in the glutamatergic regulation of PPI.

Distributed neurodegenerative changes 2-28 days after ventral hippocampal excitotoxic lesions in rats

An enhanced sensitivity to the behavioral effects of dopamine (DA) agonists in adult rats occurs after cytotoxic lesions of the ventral hippocampus (vHPC). While some of these behavioral changes may model specific abnormalities in schizophrenia patients, little is known about the cellular events that underlie vHPC lesion-induced behavioral DA 'supersensitivity'. Neuropathological consequences of excitotoxin lesions of the vHPC were investigated in this study. Adult male rats received vehicle or ibotenic acid infusions into the vHPC, using parameters that produce an enhanced sensitivity to the prepulse inhibition-disruptive effects of the DA agonist apomorphine, 1 month post-lesion. A total of 27 rats were sacrificed, 2, 7, 14, 21 or 28 days post-lesion. Amino-cupric-silver staining demonstrated degenerative changes throughout the hippocampus, and in hippocampal efferent projections to forebrain structures, including the septal nucleus and nucleus accumbens (NAC), and within the olfactory tubercle (OT) and orbital cortex. Silver-impregnated fibers were identified in the substantia nigra reticulata (SNr), NAC, OT, septum and orbital cortex. Some degenerative changes were noted at the earliest time point (2 days post-lesion), while others were delayed in appearance. Adjacent sections stained for tyrosine hydroxylase (TH) immunocytochemistry revealed reduced TH labeling through forebrain DA terminal fields 28 days, but not 14 days after VH lesions. Excitotoxic lesions of the vHPC result in distributed neurotoxic changes in subcortical and cortical brain regions; these changes may contribute to the delayed emergence of DA-mediated behavioral abnormalities in these animals.

Hippocampal lesions enhance startle gating-disruptive effects of apomorphine in rats: a parametric assessment

Prepulse inhibition of the startle reflex is an operational measure of sensorimotor gating that is impaired in schizophrenia patients and dopamine agonist-treated rats. Previous reports demonstrated an enhanced sensitivity to the prepulse inhibition-disruptive effects of the D(1)/D(2) agonist apomorphine in adult rats four weeks after cytotoxic lesions of the hippocampus, but left unanswered several important questions regarding the nature of this apparent lesion-induced dopamine supersensitivity. Because of the potential importance of this model to current theories of the pathophysiology of schizophrenia, studies now assessed specific features of this effect of hippocampus lesions on prepulse inhibition in rats. The enhanced prepulse inhibition-disruptive effects of apomorphine in ventral hippocampus-lesioned rats were unaffected by startle pulse intensity, suggesting an independence of this lesion effect from potential ceiling effects of elevated startle magnitude. These lesion effects were observed four weeks post-lesion, but not two weeks post-lesion, suggesting a delayed development of this phenomenon. No enhancement of apomorphine sensitivity was observed in rats four weeks after lesions restricted to the dorsal hippocampus; in contrast, these lesions significantly increased "no-drug" levels of prepulse inhibition. Ventral hippocampus-lesioned rats exhibited a significant reduction in prepulse inhibition after subthreshold doses of either the selective D(2)-family agonist quinpirole or the partial D(1) agonist SKF 38393, suggesting that activation of either receptor family is adequate for the expression of this effect of ventral hippocampus lesions. This may be an important paradigm for understanding the contribution of ventral hippocampus dysfunction to the neurobiology of impaired sensorimotor gating in neuropsychiatric populations.

The neurobiology of startle

Startle is a fast response to sudden, intense stimuli and probably protects the organism from injury by a predator or by a blow. The acoustic startle response (ASR) of mammals is mediated by a relatively simple neuronal circuit located in the lower brainstem. Neurons of the caudal pontine reticular nucleus (PnC) are key elements of this primary ASR pathway. The ASR in humans and animals has a non-zero baseline, that is, the response magnitude can be increased or decreased by a variety of pathological conditions and experimental manipulations. Therefore, the ASR has been used as a behavioral tool to assess the neuronal basis of behavioral plasticity and to model neuropathological dysfunctions of sensorimotor information processing. Cross-species examples for the increase of the ASR magnitude are sensitization, fear-potentiation and drug-induced enhancement. Examples for the reduction of the ASR magnitude are habituation, prepulse inhibition, drug-induced inhibition and the attenuation by positive affect. This review describes the neuronal basis underlying the mediation of the ASR, as well as the neuronal and neurochemical substrates of different phenomena of enhancement and attenuation of the ASR. It also attempts to elucidate the biological background of these forms of behavioral plasticity. Special emphasis is put on the potential relevance of ASR modulations for the understanding of human psychiatric and neurological diseases.

Disruption of prepulse inhibition following N-methyl-D-aspartate infusion into the ventral hippocampus is antagonized by clozapine but not by haloperidol: a possible model for the screening of atypical antipsychotics

The present study tested the effects of the typical neuroleptic haloperidol and an atypical neuroleptic clozapine on ventral hippocampus stimulation-induced disruption of prepulse inhibition (PPI). Bilateral infusions of 0.7 microg NMDA into the ventral hippocampus disrupted PPI. The impairment of PPI following the infusion was completely normalized 24 h after the infusion. This disruption of PPI was antagonized by clozapine (5.0 mg/kg), but not by haloperidol (0.2 mg/kg). Since disruption of PPI is considered to constitute an animal model of schizophrenia that is related to the deficit of sensorimotor gating observed in schizophrenic patients, these results suggest that PPI disruption induced by intra-ventral hippocampal infusions of NMDA may serve as an animal model for the selective detection of atypical antipsychotics.

Cross-species studies of sensorimotor gating of the startle reflex

Sensorimotor gating of the startle reflex can be assessed across species, using similar stimuli to elicit comparable response characteristics. As measured by prepulse inhibition (PPI), gating is reduced in patients with some neuropsychiatric disorders, and in rats after manipulations of limbic cortex, striatum, pallidum, or pontine tegmentum. This limbic "CSPP" circuitry can be studied in rats to reveal the neurochemical and neuroanatomical substrates regulating PPI at a high level of resolution. This detailed circuit information is used as a "blueprint" to identify substrates that may lead to PPI deficits in psychiatric-disordered humans. Some human disorders with identifiable, localized lesions in CSPP circuitry, for example, Huntington's disease, provide direct validation for this cross-species model. Studies have begun to assess the pharmacological homology of PPI across species, as an initial step towards translating detailed neural circuit information from rats to humans. These initial studies suggest the possibility that the effects of dopaminergic (DAergic) drugs on PPI (reducing PPI) may be homologous across species; nicotinic drugs may also produce similar effects on PPI across species (increasing PPI). By contrast, the effects of glutamatergic and serotonergic drugs may exhibit disparate effects on PPI across species. The use of DAergic agonists in human studies is complicated by their significant side effects, but new studies demonstrate that several "human friendly" direct DA agonists disrupt PPI in rats and are thus good candidates for further studies of the cross-species homology of the DAergic regulation of PPI. In this manner, PPI can be used to probe the sensitivity of DAergic systems, and perhaps other CSPP elements, across normal and neuropsychiatric-disordered populations.

Ligands of the NMDA receptor-associated glycine recognition site and motor behavior

Motor behavior critically depends on glutamatergic functions in the basal ganglia (BG). The dorsal and ventral striatum--the main input structures of the BG--are involved in modulation of stereotyped sniffing behavior, locomotion, catalepsy and prepulse inhibition. The effects of the NMDA receptor have been well characterized in respect to motor behavior in the past. The function of the allosteric glycine site was however disregarded until now, because brain penetrating ligands were missing. The present study summarized the motor behavioral profile of several glycine site ligands (7-chlorokynurenate, ACEA 1021, MRZ-2/576, (+) HA-966, D-cycloserine and felbamate). It is shown that through blockade of the glycine site of the NMDA receptor a distinct behavioral profile can be obtained.

Multiple limbic regions mediate the disruption of prepulse inhibition produced in rats by the noncompetitive NMDA antagonist dizocilpine

Prepulse inhibition (PPI), a phenomenon in which a weak prestimulus decreases the startle response to an intense stimulus, provides an operational measure of sensorimotor gating (a process by which an organism filters sensory information) and is diminished in schizophrenia and schizotypal patients. The psychotomimetic phencyclidine and its potent congener dizocilpine are noncompetitive antagonists of the NMDA receptor complex, and they disrupt PPI in rodents, mimicking the clinically observed PPI deficit. The neuroanatomical substrates mediating the PPI-disruptive effects of noncompetitive NMDA antagonists are unknown. The present study sought to identify brain regions subserving the disruption of PPI produced by noncompetitive NMDA antagonists in rats. PPI was measured in startle chambers immediately after bilateral infusion of dizocilpine (0, 0.25, 1.25, and 6.25 microgram/0.5 microliter/side) into one of six brain regions: amygdala, dorsal hippocampus, medial prefrontal cortex, nucleus accumbens, ventral hippocampus, and dorsomedial thalamus. Dizocilpine significantly decreased PPI after infusion into the amygdala or dorsal hippocampus. A trend toward PPI disruption was observed with administration into medial prefrontal cortex. In contrast, no change in PPI was produced by dizocilpine infusion into nucleus accumbens, ventral hippocampus, or dorsomedial thalamus. Startle reactivity was increased by dizocilpine infusion into amygdala, dorsal hippocampus, nucleus accumbens, and dorsomedial thalamus, but not medial prefrontal cortex. These findings indicate that multiple limbic forebrain regions mediate the ability of noncompetitive NMDA antagonists to disrupt PPI and that the PPI-disruptive and the startle-increasing effects of dizocilpine are mediated by different central sites.

Deficient sensorimotor gating following seizures in amygdala-kindled rats

BACKGROUND

Human patients with limbic epilepsy may develop a psychosis. We combined animal models for epileptogenesis and schizophrenia to investigate possible mechanisms underlying the occurrence of psychoses in epileptics. Since the dysfunction of sensorimotor gating is the basis of some psychotic symptoms, we tested if epileptogenesis or acute seizures influence sensorimotor gating in rats, measured as prepulse inhibition (PPI) of the acoustic startle response (ASR). PPI is the reduction of the ASR that is observed when a startling pulse is preceded by a nonstartling prepulse. Reduced PPI was found in schizophrenics and in rats under certain conditions.

METHODS

We investigated the effects on PPI of different models of limbic epileptogenesis (repeated stimulation of the basolateral amygdala, treatment with pentylenetetrazole, injection of kainate).

RESULTS

PPI was normal in chronic epileptic rats 1 week after the last generalized seizure. Impaired PPI was found in amygdala-kindled rats 10 min after seizures. The ASR amplitude in the absence of prepulses was increased in kainate-treated rats, but not in the other groups.

CONCLUSIONS

Chemical epileptogenesis or repeated stimulation of the amygdala per se did not disrupt sensorimotor gating, but the recent occurrence of seizures in amygdala-kindled rats compromised sensorimotor gating in a way compatible with psychotic states in humans.

The ventral pallidum mediates disruption of prepulse inhibition of the acoustic startle response induced by dopamine agonists, but not by NMDA antagonists

Prepulse inhibition (PPI) of the acoustic startle response is observed when the startling noise pulse is preceded by a weak, non-startling stimulus. PPI has been considered as a measure for sensorimotor gating mechanisms. Disruption of PPI can be found in schizophrenic patients as well as after blockade of NMDA receptors or stimulation of dopamine receptors in rats. The neuronal circuitry which regulates PPI consists of cortico-limbic brain structures where the nucleus accumbens (NAC) plays a key role. The NAC exerts its modulating effects on PPI by way of a projection from the ventral pallidum (VP) to the pedunculopontine tegmental nucleus (PPTg). We recently postulated that the reduction of PPI by intra-NAC infusion of glycine-site NMDA antagonists is not mediated by the VP. We tested here this hypothesis in rats with excitotoxic lesions of the VP which were systemically treated with apomorphine or MK-801 or received intraNAC infusions of dopamine or the glycine-site NMDA antagonist 7-chlorokynurenic acid. Lesioned rats showed a marked deficit in PPI after MK-801 and 7-chlorokynurenate treatment but not after apomorphine or dopamine injection, in contrast to sham-lesioned controls showing deficits in PPI under all conditions. These data provide behavioral evidence for the existence of a pathway which does not include the VP for the mediation of sensorimotor gating deficits. We propose that a direct connection between the NAC and PPTg may be responsible for the effects of NMDA/glycine receptor blockade, whereas the VP is an indispensable relay for the disruptive effects on PPI exerted by the NAC dopamine system.

Induction of Fos-protein in the forebrain and disruption of sensorimotor gating following N-methyl-D-aspartate infusion into the ventral hippocampus of the rat

Several neuropsychiatric disorders, including schizophrenia, are characterized by sensorimotor gating deficits. Prepulse inhibition of the acoustic startle response is an operational measure assessing sensorimotor gating and has been found to be reduced in schizophrenic patients. Much attention has therefore been paid to the neuronal mechanisms underlying the disruption of prepulse inhibition. The activity of limbic forebrain structures such as the septohippocampal system, the prefrontal cortex, and the nucleus accumbens has been the main focus of recent research into the regulation of prepulse inhibition in rats. We here provide a functional anatomical picture of forebrain structures probably involved in the regulation of prepulse inhibition. Stimulation of the ventral hippocampus with a subconvulsive dose of N-methyl-D-aspartate caused a significant and long-lasting disruption of prepulse inhibition. Immunostaining of the c-Fos protein revealed a characteristic pattern of neuronal activity in various forebrain areas, including the nucleus accumbens and different frontal cortical areas after hippocampal stimulation. Based on the present findings, we conclude that the overactivity within a network of cortico-limbic forebrain structures compromises the normal processing of sensory stimuli by disrupting a neuronal filter mechanism. Interestingly, there is a considerable overlap between the pattern of neuronal activity observed in our study and the brain pathology in schizophrenics reported in the literature.

Effects of phencyclidine (PCP) and (+)MK-801 on sensorimotor gating in CD-1 mice

1. Male CD-1 mice were tested for prepulse inhibition (PPI) following administration of PCP and the PCP site ligand, (+)MK-801, as well as the dopamine (DA) agonist (-)-apomorphine and DA releaser d-amphetamine. Similar to reports in rats, PCP (0.36-36.0 mumol/kg), (+)MK-801 (0.03-3.0 mumol/kg), (-)-apomorphine (3.3 and 10.0 mumol/kg) and d-amphetamine (3.0 and 8.0 mumol/kg) significantly reduced PPI when administered prior to testing. 2. Because PCP also binds to sigma receptors, the authors tested the sigma ligand (+)-3-PPP at (118 mumol/kg) which marginally increased the PPI. 3. Haloperidol (1.1 mumol/kg) pretreatment attenuated the reduction in PPI following (-)-apomorphine (10.0 mumol/kg), however no effects of haloperidol or clozapine pretreatment on (+)MK-801 disruption of PPI were observed. 4. Because of the pharmacological similarities between mouse data and previously published rat data, it is concluded that the mouse is a viable alternative to the rat for testing PPI.

The acoustic startle response in rats--circuits mediating evocation, inhibition and potentiation

This review describes the neuronal mechanisms underlying the mediation and modulation of the acoustic startle response (ASR) in rats. The combination of anatomical, physiological and behavioral methods has identified pathways which mediate and modulate the ASR. The ASR is mediated by a relatively simple, oligosynaptic pathway located in the lower brainstem which activates spinal and cranial motor neurons. An important element of the pathway which mediates the ASR is the caudal nucleus of the pontine reticular formation (PnC). Interestingly, this nucleus is also the target of input from various brain nuclei which are involved in the modulation (e.g. fear-potentiation, sensitization, habituation, prepulse inhibition and pleasure-attenuation) of the ASR. Hence, the PnC can be described as a sensorimotor interface, where the transition of sensory input into the motor output can be directly influenced by excitatory or inhibitory afferents. On the basis of these facts we conclude that the ASR may be a valuable model for the study of general principles of sensorimotor-motivational information processing at the behavioral and neurophysiological level in mammals.

The ventral subiculum modulation of prepulse inhibition is not mediated via dopamine D2 or nucleus accumbens non-NMDA glutamate receptor activity

Prepulse inhibition of the acoustic startle reflex is an operational measure of sensorimotor gating. The neural substrates of prepulse inhibition may be relevant to the pathophysiology of neuropsychiatric disorders that are characterized by sensorimotor gating deficits, including schizophrenia. Studies have demonstrated abnormalities within the hippocampal formation of schizophrenia patients, and animal studies have revealed that the hippocampus, and specifically the ventral subiculum, regulates prepulse inhibition. The ventral subiculum sends a dense glutamatergic projection to the nucleus accumbens, and the nucleus accumbens is known to potently regulate prepulse inhibition via dopaminergic and non-N-methyl-D-aspartate (non-NMDA) glutamatergic mechanisms. In the present study, we examined whether the hippocampal regulation of prepulse inhibition is mediated through subiculo-accumbens glutamatergic efferents. Intra-ventral subiculum infusion of NMDA dose dependently reduced prepulse inhibition, and this effect of NMDA was reversed by co-infusion of the NMDA receptor antagonist D,L-amino-5-phosphonovaleric acid (AP5). The prepulse inhibition-disruptive effect of intra-ventral subiculum NMDA infusion was not prevented by infusion of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) into the nucleus accumbens core or shell subregions. Pretreatment with the D2 receptor antagonist haloperidol also failed to block the prepulse inhibition-disruptive effects of intra-ventral subiculum NMDA infusion. Thus, the present findings suggest that while prepulse inhibition is regulated by NMDA activity in the ventral subiculum, this effect does not appear to be mediated via nucleus accumbens dopamine D2 receptors or via nucleus accumbens non-NMDA glutamatergic substrates.

Examination of drug-induced and isolation-induced disruptions of prepulse inhibition as models to screen antipsychotic drugs

Prepulse inhibition (PPI) of an acoustic startle response is impaired in schizophrenics. PPI can also be studied in the rat, and is impaired by dopamine (DA) D2/3 receptor agonists such as apomorphine. This disruption is reversed by DA antagonists, leading to proposals that this approach may be a useful means to identify novel antipsychotics. There is also evidence to suggest a role of serotonergic (5-HT) and glutamatergic systems in schizophrenia, and accordingly PPI can be disrupted by the 5-HT2 agonist DOI, and the non-competitive NMDA antagonist, dizocilpine. In the present study we have examined the effect of four antipsychotic drugs, haloperidol (0.1-0.3 mg/kg), raclopride (0.03-0.3 mg/kg), risperidone (0.3-3 mg/kg) and clozapine (0.0001-10 mg/kg), against the PPI disruptions induced by apomorphine (0.5 mg/kg), DOI (3 mg/kg) and dizocilpine (0.15 mg/kg). Furthermore, these drugs have been examined for their ability to restore a PPI deficit produced by housing rats under conditions of social isolation. All drugs except clozapine reversed an apomorphine-induced disruption. However, clozapine and risperidone, but not raclopride and haloperidol, reversed a DOI-induced disruption. Only risperidone was effective in restoring a PPI deficit produced by dizocilpine. In contrast to the drug-induced disruptions which were differentially sensitive to the various neuroleptics, isolation-induced disruptions were restored by each drug. These results support the idea that non-drug induced disruptions of PPI, such as social isolation, may be a more viable approach to identify novel antipsychotics.