Localizing haloperidol effects on sensorimotor gating in a predictive model of antipsychotic potency

The degree to which a startle response to a loud noise is inhibited by a weak prestimulus is an operational measure of sensorimotor gating. Prepulse inhibition (PPI) can be measured across species and is reduced in schizophrenia patients and dopamine (DA)-activated rats. The ability of DA antagonists to restore PPI in apomorphine (APO)-treated rats correlates highly with their clinical antipsychotic potency. We compared the ability of systemic- vs. intracerebrally (i.c.)-administered haloperidol (HAL) to restore PPI in APO-treated rats. Consistent with previous studies, systemic administration of HAL completely restored PPI in rats treated with APO (0.5 mg/kg s.c.), with an ED50 of approximately 0.02 mg/kg. In an otherwise identical paradigm, HAL failed to fully restore PPI after infusion into either the nucleus accumbens (NACcore or NACshell), NACcore + caudate nucleus (CN), ventral subiculum (VS), medial prefrontal cortex (MPFC), or ventral tegmentum (VTA). A subtotal, but statistically significant restoration of PPI was achieved after HAL infusion into all regions, except the NACshell. Statistically significant effects of i.c. HAL tended to be observed at doses that were only approximately 5-10-fold lower than those at which significant effects were observed after systemic administration. The results suggest that systemically administered HAL may restore PPI in APO-treated rats through its action distributed throughout multiple levels of PPI-regulatory circuitry.

Using an animal model of deficient sensorimotor gating to study the pathophysiology and new treatments of schizophrenia

Certain animal models can greatly enhance our understanding of the neurobiology of schizophrenia and can be used to predict the antipsychotic activity of compounds. Prepulse inhibition (PPI), the reduction in startle produced by a prepulse stimulus, is diminished in schizophrenia patients. Theoretically, deficient PPI in schizophrenia patients is a measure of the loss of sensorimotor gating that may lead to sensory flooding and cognitive fragmentation. In rats, PPI is disrupted by systemic administration of dopamine agonists, serotonin agonists, or glutamate antagonists and by a variety of surgical or pharmacological manipulations of neural circuitry linking the limbic cortex, striatum, pallidum, and pontine reticular formation. This article describes several different ways the loss of PPI in rats can be used as a model for studying the pathophysiology and neurobiology of impaired sensorimotor gating in schizophrenia patients and for predicting antipsychotic activity in novel compounds. First, new experimental strategies may be used to distinguish behavioral profiles of "typical" versus "atypical" antipsychotics. Second, this paradigm can be used to study the effects of early developmental insults--including neonatal lesions and isolated rearing--on the adult emergence of deficient sensorimotor gating. Third, using different animal strains and species, as well as gene "knockout" strategies, greatly increases our ability to understand specific genetic or receptor contributions to the regulation of deficient PPI. Each of these uses of the PPI paradigm is enhanced by studies of the basic brain substrates that regulate PPI in rats and by the increasingly sophisticated assessments of PPI and related measures in schizophrenia spectrum patients.

Reversal of isolation rearing-induced deficits in prepulse inhibition by Seroquel and olanzapine

BACKGROUND

Prepulse inhibition (PPI) of startle provides an operational measure of sensorimotor gating in which a weak stimulus presented prior to a startling stimulus reduces the startle response. PPI deficits observed in schizophrenia patients can be modeled in rats by individual housing from weaning until adulthood. Deficits in PPI produced by isolation rearing can be reversed by antipsychotics.

METHODS

We evaluated the ability of Seroquel and olanzapine to reverse the isolation-induced disruption of PPI. Rats housed for 8 weeks singly or in groups of 3 were tested every 2 weeks after either Seroquel (0, 5.0 mg/kg) or olanzapine (0, 2.5, 5.0 mg/kg). Startle was elicited by 120-dB pulses presented either with or without prepulses (3, 6, or 12 dB above a 65-dB background).

RESULTS

Isolation rearing repeatedly disrupted PPI and sometimes increased startle reactivity. Seroquel reversed these deficits without affecting PPI in socially reared controls. Olanzapine (2.5 mg/kg) reversed the isolation rearing-induced PPI deficit and tended to increase basal PPI levels. Both antipsychotics antagonized the isolation rearing-induced increase in startle reactivity.

CONCLUSIONS

Isolation rearing produces deficits in sensorimotor gating in rats that are reversible by atypical antipsychotics, and may therefore aid in identifying new treatments for schizophrenia.

Discrepant findings of clozapine effects on prepulse inhibition of startle: is it the route or the rat?

Studies examined methodological differences that might account for discrepant reports related to the ability of clozapine to restore prepulse inhibition (PPI) of acoustic startle in apomorphine (APO)-treated rats. Changes in PPI after APO and clozapine were compared in Sprague. Dawley (SD) versus Wistar rats. In SD rats, intraperitoneal administration of clozapine (4-12 mg/kg) completely reversed the PPI-disruptive effects of APO (0.5 mg/kg), with significant effects evident at the lowest dose of clozapine. Compared to SD rats, Wistars exhibited a relatively weaker (but statistically significant) disruption of PPI with the same or higher doses of APO and were also less sensitive to the PPI-restorative effects of clozapine. Clozapine administered via subcutaneous route completely restored PPI after APO treatment in SD rats. Discrepant findings with this model can be attributed to differences in rat strain; SD rats exhibit patterns of drug responses in this model that are optimal for examining profiles of putative atypical antipsychotics.

Neuroanatomy of schizophrenia

The articles that appear in this issue offer a framework of insights about the neuroanatomy of schizophrenia from three learned and creative perspectives. All three articles advance our understanding of schizophrenia from a single locus/specific "lesion" model to more advanced perspectives of neural circuit dysfunction models. Goldman-Rakic and Selemon review their own and others' work on structure-activity relationships of the frontal cortex and related working memory dysfunction. This important but sometimes cloudy and complex area is illuminated by their highly specific, informative research. Jones focuses on thalamic abnormalities hypothetically linked to abnormal oscillations in large arrays of cortical and thalamic neurons, a critically important concept in understanding the functional consequences of abnormal (thalamic) brain structure and function. Graybiel describes her interest in abnormal basal ganglia activity-dependent loops that may access the thalamus and set the tone of thalamo-cortical transmission. This view allows for us to understand the "upward" influences on basal ganglia function (and dysfunction) relevant to schizophrenia. These intriguing articles raise a number of issues that await increased data and continued integrating insights. These issues include the need for more information about (1) the developmental timing of lesions or dysfunctions; (2) the extent and regional distribution of abnormalities; (3) the relationship of brain dysfunction to clinical/cognitive abnormalities; and (4) the variable expression of brain abnormalities across the schizophrenia spectrum. These three articles and their authors are at the forefront of our expanding knowledge about the neuroanatomy of schizophrenia and how complex structural and functional deficits are expressed in individuals in the group of schizophrenias.

Reduced prepulse inhibition after electrolytic lesions of nucleus accumbens subregions in the rat

A variety of neurochemical and anatomical studies have shown that the ventral striatal nucleus accumbens (NAcc) can be divided into a predominantly medial 'shell' and a predominantly lateral 'core'. These accumbens subdivisions are innervated by different cortical regions and project to different anatomical targets. Additionally, recent behavioral observations suggest functional differences between the accumbens shell and core. In order to further examine core and shell function, we measured prepulse inhibition (PPI) of the acoustic startle reflex after electrolytic lesions aimed at the central NAcc, the NAcc shell, the NAcc core, and the adjacent anteroventrolateral striatum. Acoustic startle is the contraction of whole-body musculature in response to a sudden, loud auditory stimulus, and PPI is the inhibition of acoustic startle by a weak auditory 'prepulse' administered 30-500 ms prior to the startling stimulus. Previous work has shown that PPI is regulated by the NAcc, and recent observations suggest that PPI is regulated by different neurochemical interactions in the core and shell. PPI was significantly reduced by electrolytic lesions of the central NAcc, as well as by lesions that predominantly damaged either the NAcc shell or the core. Lesions of the anteroventrolateral striatum and lateral edge of the NAcc core did not significantly alter PPI. These findings suggest that, despite neurochemical differences between the two subregions, PPI is regulated by both the NAcc shell and core.

Mitochondrial toxin 3-nitropropionic acid produces startle reflex abnormalities and striatal damage in rats that model some features of Huntington's disease

Systemic administration of the mitochondrial toxin 3-nitropropionic acid (3NP) to rats produces striatal lesions that mimic some aspects of pathology in Huntington's disease (HD). To evaluate whether 3NP-induced lesions cause sensorimotor gating deficits observed in HD, we measured prepulse inhibition (PPI) of the acoustic startle reflex after systemic administration of 3NP (10, 15, or 20 mg/kg) to 5-month-old rats. PPI, the reduction of startle magnitude by a weak auditory prestimulus, is significantly reduced in patients with HD. Two daily injections of 3NP produced gross histologic evidence of striatal lesions in some rats and significantly reduced PPI. Striatal lesions also significantly disrupted amphetamine-induced stereotypy, another index of dorsal striatal function. 3NP thus reproduces a specific objective and quantifiable gating deficit found in patients with HD.

The effects of intra-accumbens neurotensin on sensorimotor gating

Neurotensin is a neuropeptide which coexists with mesolimbic dopamine. Previous studies have shown that centrally administered neurotensin can modulate the activity of mesolimbic dopamine with a profile similar to neuroleptics. For example, infusions of neurotensin into the nucleus accumbens inhibit amphetamine-induced hyperlocomotion. Prepulse inhibition (PPI) occurs when a weak prestimulus ('prepulse') inhibits the amplitude of the startle response to an intense stimulus ('pulse'). PPI is an operational measure of sensorimotor gating which is strongly regulated by mesolimbic dopamine. This study examined the effects of various doses of neurotensin infused into the nucleus accumbens of rats on the prepulse inhibition (PPI) of their acoustic startle reflex. Neurotensin (0.25-5.0 microg) was infused into the nucleus accumbens of rats. Animals then received subcutaneous injections of amphetamine (2 mg/kg) or saline and were placed in startle chambers where measures of startle amplitude and PPI were obtained. Neurotensin increased baseline PPI and blocked amphetamine-induced disruption of PPI in a dose-dependent fashion. The lowest dose of neurotensin tested (0.25 microg) significantly increased baseline PPI and both 0.25 and 1.0 microg neurotensin blocked amphetamine-induced decreases in PPI. The 5.0 microg dose of neurotensin had no significant effect on prepulse inhibition. Neurotensin had no effect on the amplitude of the acoustic startle reflex in amphetamine- or saline-treated rats. The results suggest that intra-accumbens neurotensin has a significant, dose-dependent effect on sensorimotor gating in which lower doses (0.25-1.0 microg) exhibit a neuroleptic-like action.

The modulation of sensorimotor gating deficits by mesolimbic cholecystokinin

The effects of cholecystokinin (CCK) in an animal model of sensorimotor-gating deficits with strong face, construct and predictive validity for schizophrenia were investigated. Prepulse inhibition (PPI) occurs when a weak acoustic lead stimulus inhibits the startle response to a loud startling stimulus. Infusions of sulfated CCK-8 in the posterior nucleus accumbens potentiated apomorphine-induced disruption of PPI but had no effect on baseline PPI or the amplitude of acoustic startle reflex itself. The results provide evidence that mesolimbic CCK may play a role in regulating sensorimotor gating deficits but contradict earlier notions that CCK agonists may have antipsychotic properties and upon which clinical trials of CCK agonists in schizophrenia were based. Rather, these results suggest that antagonists of CCK may display neuroleptic-like actions on deficits in PPI and may hold greater promise as antipsychotics.

Characteristics of open- and closed-loop smooth pursuit responses among obsessive-compulsive disorder, schizophrenia, and nonpsychiatric individuals

Twenty obsessive-compulsive disorder patients and comparison samples of 20 schizophrenia and 20 nonpsychiatric individuals were presented with (a) a step-ramp task designed to measure smooth pursuit initiation and (b) a regular ramp task designed to measure steady-state tracking performance. Obsessive-compulsive disorder and non-psychiatric individuals had statistically similar pursuit reaction time and average eye accelerations during the open-loop interval. They also had similar closed-loop performance. Schizophrenia patients, however, had delayed pursuit reaction times and reduced eye acceleration during the last 60 ms of the open-loop interval. These findings suggest that brain regions supporting smooth pursuit performance are unimpaired among obsessive-compulsive disorder patients. Furthermore, the deficits found in the schizophrenia patients replicate and extend the results of previous smooth pursuit studies.

Changes in sensorimotor inhibition across the menstrual cycle: implications for neuropsychiatric disorders

The startle reflex is inhibited when the starting noise is preceded 30-500 msec by a weak prepulse. "Prepulse inhibition" (PPI) is reduced in specific neuropsychiatric disorders characterized clinically by impaired inhibition of sensory, motor, or cognitive information. PPI is sexually dimorphic, with men exhibiting significantly more PPI than women. We examined possible neuroendocrine substrates for this sex difference in PPI. The startle reflex, and a measure of visuospatial priming, were measured in 10 men, and in 46 normal women at different points in their menstrual cycle. In women, PPI was significantly reduced in the luteal vs follicular phase of the menstrual cycle. This reduction in PPI was most notable during the period corresponding to midluteal elevations of both estrogen and progesterone. In a task of visuospatial priming, follicular-phase women demonstrated a predominance of inhibition over facilitation, but this pattern reversed across the menstrual cycle.

The basolateral amygdala regulates sensorimotor gating of acoustic startle in the rat

The acoustic startle reflex is a coordinated contraction of the skeletal musculature in response to a sudden, intense sound. One form of startle plasticity, "prepulse inhibition", is the normal suppression of the startle reflex when the intense startling stimulus is immediately preceded by a weak pre-stimulus. Prepulse inhibition is utilized as an operational measure of sensorimotor gating, and is significantly impaired in several neuropsychiatric disorders that are characterized by symptoms associated with central inhibitory deficits. In rats, prepulse inhibition is disrupted by central dopamine activation or by manipulations of limbic cortical structures including the prefrontal cortex and hippocampus. In the present study, we assessed prepulse inhibition in rats after surgical and pharmacologic manipulations of the basolateral amygdala. Quinolinic acid lesions of the basolateral amygdala significantly reduced prepulse inhibition without significantly changing startle amplitude. These lesions also blocked fear-potentiated startle, which is known to be regulated by the basolateral amygdala. The prepulse inhibition-disruptive effects of basolateral amygdala lesions were not reversed by systemic injection of the dopamine antagonist haloperidol at doses that totally restored prepulse inhibition in apomorphine-treated rats. In other studies, intra-amygdala infusion of the competitive N-methyl-D-aspartate antagonist DL-2-amino-5-phosphonovaleric acid (0, 0.15, 1.5, 4.5 microg) dose-dependently reduced prepulse inhibition. These data suggest that the basolateral amygdala regulates sensorimotor gating by mechanisms that are independent of central dopamine hyperactivity.

Regulation of prepulse inhibition by ventral pallidal projections

The acoustic startle reflex is inhibited by the presentation of a weak auditory prestimulus 30-500 ma prior to the starting stimulus. Previous studies have demonstrated that prepulse inhibition (PPI) of acoustic startle is regulated by GABAergic activity in the ventral pallidum. Ventral pallidal efferents include major projections to the pedunculopontine tegmental nucleus (PPTg), subthalamic nucleus (STN), and mediodorsal thalamus (MD). We used lesion and intracerebral infusion techniques to determine the relevance of these projections to the ventral pallidal regulation of PPI. Consistent with previous results, PPTg lesions significantly reduced PPI in all startle sessions, while MD lesions significantly reduced PPI only under certain experimental conditions. STN lesions failed to alter PPI, but they did significantly disrupt amphetamine-induced locomotion, verifying the behavioral effectiveness of these lesions. Infusion of the GABA-A agonist muscimol into either the PPTg or the MD significantly reduced PPI. Ventral pallidal projections to the PPTg and to the MD thus appear to regulate PPI, possibly via a GABAergic mechanism. Pallidal projections to the STN may regulate other behavioral processes such as locomotor activity, but they do not appear to regulate sensorimotor gating of the acoustic startle reflex.

Seroquel restores sensorimotor gating in phencyclidine-treated rats

Phencylidine (PCP) is a psychotomimetic noncompetitive glutamate antagonist that has been used in studies of the neural substrates of psychosis. Both schizophrenic patients and PCP-treated rats exhibit reduced amounts of prepulse inhibition (PPI) of the startle reflex, which is the normal inhibition of startle that occurs when the starting noise is preceded 30 to 500 msec by a weak prepulse. The present study assessed the effects of seroquel (ICI 204,636), a mixed D2/5-hydroxytryptamine2 antagonist with a preclinical profile suggestive of potential antipsychotic efficacy, on the PCP-induced disruption of PPI. Clozapine, risperidone and haloperidol were also studied as comparison compounds. PCP (1.25 mg/kg) significantly reduced PPI, with prepulses that were 1 to 12 dB above background. Seroquel and clozapine significantly restored PPI in PCP-treated rats, whereas haloperidol and risperidone did not. Similar findings were obtained in studies using separate animals, a slightly lower dose of PCP (1.0 mg/kg) and a high dose of each of these antipsychotics. Separate studies verified that risperidone and haloperidol restored PPI in apomorphine-treated rats. In the present studies, seroquel exhibited a profile consistent with those exhibited by other "atypical" antipsychotics.

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.

Sensorimotor gating in rats is regulated by different dopamine-glutamate interactions in the nucleus accumbens core and shell subregions

The amplitude of the acoustic startle reflex is normally reduced when the startling stimulus is preceded by a weak click or "prepulse'. Prepulse inhibition (PPI) of acoustic startle has been used as an operational measure of sensorimotor gating or inhibition, and is reduced in schizophrenia patients and in rats with central dopamine (DA) activation. The DA agonist-induced disruption of PPI in rats may thus offer a useful animal model to study impaired sensorimotor gating in schizophrenia. We have previously reported that DA-glutamate interactions in the nucleus accumbens (NAC) regulate PPI. The NAC has at least two major subregions-the core and shell-that have distinct anatomical and neurochemical properties. In this study, we compared changes in PPI after manipulations of DA-glutamate activity in these two NAC subregions. Consistent with previous findings, infusion of the non-NMDA agonist AMPA into the NAC core subregion significantly reduced PPI, and this effect was opposed by systemic administration of the D2 antagonist haloperidol. Also consistent with previous reports, infusion of the non-NMDA antagonist CNQX into the NAC core subregion did not alter PPI, but its co-infusion with D-amphetamine (AMPH) attenuated the AMPH-disruption of PPI. In contrast, while PPI was reduced after AMPA infusion into the NAC shell subregion, this effect of AMPA could not be blocked by pretreatment with haloperidol. Infusion of either AMPH or CNQX into the NAC shell subregion reduced PPI independently. The PPI-disruptive effects of intra-shell CNQX infusion were not blocked by haloperidol. The present results suggest striking differences between the NAC core and shell subregions in their neurochemical modulation of sensorimotor gating of acoustic startle in the rat.

Latent inhibition in schizophrenia

Latent inhibition (LI) refers to the retarded acquisition of a conditioned response that occurs if the subject being tested is first preexposed to the to-be-conditioned stimulus (CS) without the paired unconditioned stimulus (UCS). Because the 'irrelevance' of the to-be-conditioned stimulus is established during non-contingent preexposure, the slowed acquisition of the CS-UCS association is thought to reflect the process of overcoming this learned irrelevance. Latent inhibition has been reported to be diminished in acutely hospitalized schizophrenia patients. If acutely hospitalized schizophrenia patients are preexposed to the CS, they learn the association as fast as, and perhaps faster than, patients who are not preexposed to the CS. This finding has been interpreted as reflecting the inability of acute schizophrenia patients to ignore irrelevant stimuli. In this study, the LI paradigm was identical to the one used in previous reports of LI deficits in schizophrenia patients (Baruch et al., 1988). Latent inhibition was observed in normal control subjects (n = 73), including individuals identified as 'psychosis-prone' based on established screening criteria, and in anxiety (n = 19) and mood disorder (n = 13) patients. Learning scores (trials to criterion) in "acutely' hospitalized as well as "chronic' hospitalized schizophrenia patients (n = 45) were significantly elevated in both preexposed and non-preexposed subjects, compared to controls. Acute schizophrenia patients exhibited intact LI. Separate cohorts of acute and chronic schizophrenia patients (n = 23) and normal controls (n = 34) exhibited intact LI when tested in a new, easier-to-acquire computerized LI paradigm. These results fail to identify specific LI deficits in schizophrenia patients, and raise the possibility that previously observed LI deficits in schizophrenia patients may reflect, at least in part, performance deficits related to learning acquisition.

Do D1/D2 interactions regulate prepulse inhibition in rats?

Prepulse inhibition (PPI) of the startle reflex is an operational measure of sensorimotor gating that is reduced in schizophrenia patients and in dopamine (DA)-activated rats. We previously found that PPI is disrupted by systemic administration of the D2 agonist quinpirole, but not by the D1 agonist SKF 38393. In this report we further characterize the D1 and D2 substrates and their potential interactions in the regulation of PPI in rats. PPI is reduced by concomitant administration of the D1 agonist SKF 38393 (5 mg/kg; relative affinity D1:D2 = 50:1) and by a subthreshold dose (0.1 mg/kg) of the D2 agonist quinpirole, but not by either drug given alone at these doses. Pretreatment with the D2 antagonist raclopride (0.05 mg/kg), but not the D1 antagonist SCH 23390 (0.05 mg/kg), blocks the SKF 38393/quinpirole synergistic reduction of PPI. The relative D1 agonist SKF 82958 (5 mg/kg; relative affinity D1:D2 = 10:1) disrupts PPI, and this effect of SKF 82958 is reversed by the D2 antagonist raclopride but not by the D1 antagonist SCH 23390. Consistent with a recent report (Hoffman and Donovan 1994), the PPI-disruptive effects of the D1/D2 agonist apomorphine (0.5 mg/kg) could be blocked by pretreatment with the D1 antagonist SCH 23390. Surprisingly the PPI-disruptive effects of quinpirole are also opposed by pretreatment with SCH 23390. Our present findings confirm that D2 receptors are important for the regulation of PPI in rats, but they also suggest that there exists a synergistic interaction between D1 and D2 substrates in the regulation of PPI. D1 receptors might modulate PPI in a "rate-dependent" manner in which tonic D1 activity is essential for the full manifestation of the D2-mediated modulation of PPI. However, D1 receptors do not appear to participate in the modulatory mechanisms of sensorimotor gating as an independent substrate.

Ocular motor responses to unpredictable and predictable smooth pursuit stimuli among patients with obsessive-compulsive disorder

This study was conducted to evaluate the smooth pursuit system functioning of patients with obsessive-compulsive disorder (OCD). For Study 1, 12 subjects with OCD and 12 nonpsychiatric subjects were administered 9-deg-per-sec ramp stimuli to elicit smooth pursuit eye movements. Consistent with a previous report, patients with OCD did not significantly differ from nonpsychiatric subjects on pursuit gain, or frequency of corrective and intrusive saccades. Patients with OCD, however, had smaller catch-up saccades during smooth pursuit than nonpsychiatric subjects. For Study 2, 12 subjects with OCD and 12 nonpsychiatric subjects were administered 2 different triangle wave stimuli with target velocities of 12 (0.2 Hz) deg per sec and 24 (0.4 Hz) deg per sec. Subjects with OCD and nonpsychiatric subjects did not significantly differ on any variable in the slow target velocity condition. When following 24-deg-per-sec targets, however, patients with OCD had significantly lower pursuit gain than the nonpsychiatric subjects. Results from Study 1 and 2 are consistent with the hypothesis that patients with OCD have a modest smooth pursuit deficit that is elicited only while following faster velocity targets.

Increased sensitivity to the sensorimotor gating-disruptive effects of apomorphine after lesions of medial prefrontal cortex or ventral hippocampus in adult rats

Sensorimotor gating of the startle reflex is impaired in humans with schizophrenia and in rats after mesolimbic D2 dopamine receptor activation. The loss of startle gating after D2 activation in rats has been used as an animal model of impaired sensorimotor gating in schizophrenia, because the ability of antipsychotics to restore startle gating in D2-activated rats correlates significantly with antipsychotic clinical potency. Substantial evidence indicates that the pathophysiology of schizophrenia includes structural and functional deficits in prefrontal and temporal regions, particularly the dorsolateral prefrontal cortex and the hippocampus and parahippocampal gyrus. The present study assessed startle gating in adult rats after ibotenic acid lesions of the medial prefrontal cortex or ventral hippocampus. Medial prefrontal cortex lesioned rats exhibited normal startle amplitude and normal sensorimotor gating, as reflected by prepulse inhibition (PPI) of the startle reflex. Hippocampus lesioned rats exhibited elevated startle amplitude, and similar to rats with medial prefrontal cortex lesions, did not show significant changes in basal PPI. Low doses of the mixed dopamine agonist apomorphine did not significantly reduce PPI in sham lesioned rats, but significantly disrupted PPI in both medial prefrontal cortex- and ventral hippo-campus lesioned rats. These data are consistent with the hypothesis that cell damage in frontal and temporal cortex increases the sensitivity to the sensorimotor gating-disruptive effects of dopamine receptor activation.

Neonatal excitotoxic hippocampal damage in rats causes post-pubertal changes in prepulse inhibition of startle and its disruption by apomorphine

Neonatal excitotoxic hippocampal damage in the rat results in postpubertal onset of a variety of abnormal behaviors related to excessive dopaminergic transmission in the mesolimbic/nigrostriatal system, and thus may be considered an animal model of some aspects of schizophrenia. Because sensorimotor gating is impaired in adult patients with schizophrenia and in rats with experimentally induced mesolimbic dopamine hyperactivity, the present experiments investigated the effects of neonatal (postnatal day 7, PD7) ibotenic acid (3 micrograms) lesions of the ventral hippocampus (VH) on the amplitude and prepulse inhibition (PPI) of acoustic startle in prepubertal (PD35) and postpubertal (PD56) rats. Startle was elicited using 105 and 118-dB pulses alone or preceded by 4, 8, or 16 dB above-background prepulses in rats treated with vehicle or apomorphine (APO; 0.025 or 0.1 mg/kg SC). At PD35, PPI in VH-lesioned rats did not differ significantly from these measures in sham operated rats. Apomorphine significantly increased startle amplitude and reduced PPI in both sham operated and VH-lesioned rats at PD35. At PD56, startle amplitude in VH-lesioned rats was not significantly different from controls, but PPI was reduced significantly compared to controls. Ventral hippocampus lesioned rats also exhibited an exaggerated reduction in PPI after treatment with APO. These findings provide further evidence of postpubertal impairments that may be related to increased mesolimbic dopamine transmission and receptor sensitivity in rats with neonatal hippocampal damage, and provide further support for the fidelity of this animal model of schizophrenia.

Prepulse inhibition in the rat is regulated by ventral and caudodorsal striato-pallidal circuitry

It has been shown that the acoustic startle reflex is inhibited by weak auditory prepulses presented 30-500 ms prior to the startling stimulus and that this prepulse inhibition (PPI) is modulated by ventral striato-pallidal circuitry. However, dorsal striatal modulation of PPI has not been examined. Cell-specific lesions and intracerebral drug infusions were used to elucidate striatal modulation of PPI. Quinolinic acid lesions of the ventral and caudodorsal striatum significantly decreased PPI, whereas lesions of the rostrodorsal and middorsal striatum did not significantly alter PPI. Infusion of the GABA-A antagonist picrotoxin into the ventral and caudal dorsal pallidum also significantly reduced PPI, whereas rostral pallidal picrotoxin infusion had no significant effect. Thus, PPI in the rat seems to be modulated by both ventral and caudodorsal striato-pallidal circuitry, but not by rostrodorsal or middorsal striato-pallidal projections.

Presynaptic dopamine-glutamate interactions in the nucleus accumbens regulate sensorimotor gating

Prepulse inhibition (PPI) is the normal reduction in startle reflex that occurs when a startling stimulus is preceded by a weak prepulse. PPI is reduced in patients with schizophrenia and in rats after central dopamine (DA) activation. The DA agonist-induced disruption of PPI in rats may thus model some features of impaired sensorimotor gating in schizophrenia. Ascending DAergic and descending glutamatergic fibers converge within the nucleus accumbens (NAC), and interactions at this DA-glutamate interface have been implicated in the pathophysiology of schizophrenia. In this study, we examined the role of NAC DA-glutamate interactions in the regulation of PPI in rats. Intra-NAC infusion of the non-NMDA antagonist, CNQX, attenuated the PPI-disruptive effects of d-amphetamine (AMPH), but CNQX did not affect PPI when injected alone, nor did it reverse the PPI-disruptive effects of the direct D2/D3 agonist quinpirole. Intra-NAC infusion of the non-NMDA agonist AMPA significantly reduced PPI. The PPI-disruptive effects of AMPA were blocked by haloperidol and by 6-hydroxydopamine (6OHDA) lesions of the NAC. These data suggest that the PPI-disruptive effects of AMPH are dependent on tonic non-NMDA receptor activation in the NAC, and that non-NMDA receptor activation in the NAC results in a DA-dependent reduction in PPI. The parsimonious interpretation of these data is that non-NMDA glutamate receptors in the NAC facilitate presynaptic DA function, and that this DA-glutamate interaction is a critical regulatory substrate of sensorimotor gating.

Ventral pallidal GABA-A receptors regulate prepulse inhibition of acoustic startle

Prepulse inhibition (PPI) of the acoustic startle reflex occurs when a weak auditory stimulus is presented 30-500 ms before the startling stimulus. Previous studies have shown that PPI is modulated by GABAergic projections from the ventral striatum to the ventral pallidum (VP). To evaluate the anatomical and pharmacological substrates of pallidal modulation of PPI, we measured PPI after intrapallidal infusion of GABA-B and GABA-A antagonists. Intrapallidal infusion of the GABA-B antagonist, 2-OH-saclofen (0.025-0.10 microgram), did not significantly alter PPI, startle amplitude or peak startle latency. Infusion of the GABA-A antagonist, picrotoxin (0.02-0.08 microgram), into the medial or central VP significantly reduced PPI; this effect appeared somewhat weaker after picrotoxin infusion into the lateral VP and was absent after infusion into the adjacent fundus striatum (FS). There was no significant effect of picrotoxin infusion into any of the VP sites or FS on startle amplitude or peak startle latency. Thus, ventral striato-pallidal GABAergic modulation of PPI appears to be mediated solely by GABA-A receptors and this modulatory substrate is predominantly distributed across the medial and central portions of the VP.