Deficient sensorimotor gating following seizures in amygdala-kindled rats

Epilepsy and its neurobehavioral comorbidities: Insights gained from animal models

It is well established that epilepsy is associated with numerous neurobehavioral comorbidities, with a bidirectional relationship; people with epilepsy have an increased incidence of depression, anxiety, learning and memory difficulties, and numerous other psychosocial challenges, and the occurrence of epilepsy is higher in individuals with those comorbidities. Although the cause-and-effect relationship is uncertain, a fuller understanding of the mechanisms of comorbidities within the epilepsies could lead to improved therapeutics. Here, we review recent data on epilepsy and its neurobehavioral comorbidities, discussing mainly rodent models, which have been studied most extensively, and emphasize that clinically relevant information can be gained from preclinical models. Furthermore, we explore the numerous potential factors that may confound the interpretation of emerging data from animal models, such as the specific seizure induction method (e.g., chemical, electrical, traumatic, genetic), the role of species and strain, environmental factors (e.g., laboratory environment, handling, epigenetics), and the behavioral assays that are chosen to evaluate the various aspects of neural behavior and cognition. Overall, the interplay between epilepsy and its neurobehavioral comorbidities is undoubtedly multifactorial, involving brain structural changes, network-level differences, molecular signaling abnormalities, and other factors. Animal models are well poised to help dissect the shared pathophysiological mechanisms, neurological sequelae, and biomarkers of epilepsy and its comorbidities.

The amygdala modulates prepulse inhibition of the auditory startle reflex through excitatory inputs to the caudal pontine reticular nucleus

Background

Sensorimotor gating is a fundamental pre-attentive process that is defined as the inhibition of a motor response by a sensory event. Sensorimotor gating, commonly measured using the prepulse inhibition (PPI) of the auditory startle reflex task, is impaired in patients suffering from various neurological and psychiatric disorders. PPI deficits are a hallmark of schizophrenia, and they are often associated with attention and other cognitive impairments. Although the reversal of PPI deficits in animal models is widely used in pre-clinical research for antipsychotic drug screening, the neurotransmitter systems and synaptic mechanisms underlying PPI are still not resolved, even under physiological conditions. Recent evidence ruled out the longstanding hypothesis that PPI is mediated by midbrain cholinergic inputs to the caudal pontine reticular nucleus (PnC). Instead, glutamatergic, glycinergic, and GABAergic inhibitory mechanisms are now suggested to be crucial for PPI, at the PnC level. Since amygdalar dysfunctions alter PPI and are common to pathologies displaying sensorimotor gating deficits, the present study was designed to test that direct projections to the PnC originating from the amygdala contribute to PPI.

Results

Using wild type and transgenic mice expressing eGFP under the control of the glycine transporter type 2 promoter (GlyT2-eGFP mice), we first employed tract-tracing, morphological reconstructions, and immunohistochemical analyses to demonstrate that the central nucleus of the amygdala (CeA) sends glutamatergic inputs lateroventrally to PnC neurons, including GlyT2⁺ cells. Then, we showed the contribution of the CeA-PnC excitatory synapses to PPI in vivo by demonstrating that optogenetic inhibition of this connection decreases PPI, and optogenetic activation induces partial PPI. Finally, in GlyT2-Cre mice, whole-cell recordings of GlyT2⁺ PnC neurons in vitro paired with optogenetic stimulation of CeA fibers, as well as photo-inhibition of GlyT2⁺ PnC neurons in vivo, allowed us to implicate GlyT2⁺ neurons in the PPI pathway.

Conclusions

Our results uncover a feedforward inhibitory mechanism within the brainstem startle circuit by which amygdalar glutamatergic inputs and GlyT2⁺ PnC neurons contribute to PPI. We are providing new insights to the clinically relevant theoretical construct of PPI, which is disrupted in various neuropsychiatric and neurological diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01050-z.

Fear leads to a deficit of prepulse inhibition of blink reflex in healthy humans

OBJECTIVE

We aimed to analyze whether or not fear conditioning exerts an effect on prepulse inhibition (PPI) of blink reflex (BR). To create fear conditioning, we used fearful faces. Since fearful faces lead to a specific set of fear conditioning, we hypothesized PPI of BR would change under the observation of fearful faces.

METHOD

We included 17 healthy subjects with a mean age of 30.8 ± 6.9 years and seven healthy subjects with a mean age of 57.7 ± 7.3 years between January 2018 and June 2018 and recorded PPI of BR. The recordings were done before observation of any image, during observation of images, and immediately after observation of images. Observation of images included observation of fearful faces for 30 s and a neutral image of a white screen for 30 s (in a randomized order).

RESULTS

There was a R2-PPI deficit during observation of fearful faces in each group whereas R2-PPI fully developed at other time points. R1 amplitude and R2 magnitude were lower during observation of any image compared with baseline and post-observation time points.

CONCLUSION

In conclusion, a deficit of R2-PPI develops during observation of fearful faces in humans which is probably related to activation of the amygdala.

Generalized seizures evoked by nucleus accumbens stimulation induced an opiate-mediated suppression of psychosis relevant behaviors

We have previously demonstrated that kindling of the nucleus accumbens (NAc) induced psychosis relevant behaviors only after one, but not after five, stage-5 seizures, suggesting that five stage-5 NAc-evoked seizures antagonized psychosis relevant behaviors in rats. We hypothesized that brain opioid receptors are responsible for seizure-induced reduction of psychosis relevant behaviors in NAc kindled rats. Rats received NAc kindling until a stage-4 seizure was induced, after which naloxone, a non-specific opioid receptor antagonist, at dose of 1 or 10 mg/kg i.p., or saline (0.3 mL) i.p., was injected 15 min before each kindled seizure. Duration of afterdischarge (AD) was not significantly different among naloxone- and saline-treated groups. However, duration of postictal behavioral depression induced by a stage-5 seizure was significantly shorter in 10 mg/kg naloxone-treated than saline-treated rats, for long (>36 s) AD duration. When tested 3-4 days after five stage-5 seizures, 10 mg/kg naloxone-treated rats, as compared to saline-treated rats, showed a statistically significant loss of gating of hippocampal auditory evoked potentials, and significant reduction of startle response amplitude, but non-significant differences in prepulse inhibition and methamphetamine-induced locomotion. It is inferred that stage-5 seizures, by releasing endogenous opiates, contribute to postictal behavioral depression, and some long-term seizure-induced antipsychotic effects.

Medial septum modulates hippocampal gamma activity and prepulse inhibition in an N-methyl-d-aspartate receptor antagonist model of schizophrenia

We reviewed the participation of the septohippocampal system in an animal model of schizophrenia that was acutely induced by systemic injection of an N-methyl-d-aspartate (NMDA) receptor antagonist such as phencyclidine, MK-801 and ketamine. The NMDA receptor antagonist-induced model of schizophrenia is characterized by behavioral and electrophysiological disruptions, including a decrease in prepulse inhibition of the acoustic startle response (PPI), hyperlocomotion, decrease in gating of hippocampal auditory evoked potentials and robust increase in hippocampal gamma (30-100Hz) oscillations. Similar disruptions were also induced by a single electrographic seizure in the hippocampus. The behavioral and electrophysiological disruptions induced by an NMDA receptor antagonist can be reduced by inactivation or lesion of GABAergic neurons in the medial septum, deep brain stimulation of the medial septum or nucleus accumbens, or positive modulation of GABA_B receptors. Our results suggest a close association between high-amplitude hippocampal gamma oscillations and psychosis-relevant behaviors including PPI loss, behavioral hyperactivity and loss in auditory gating. Abnormal electrophysiology suggests a disruption of somatic and apical dendritic inhibition in the hippocampus, resulting in distorted sensory integration, and impaired cognitive and memory processing. The hippocampus is suggested to be a hub in a brain network that participates in psychosis-relevant behaviors, through its direct projection to the nucleus accumbens, or through indirect connections via the entorhinal, cingulate and prefrontal cortices.

Dual Effects of Limbic Seizures on Psychosis-Relevant Behaviors Shown by Nucleus Accumbens Kindling in Rats

BACKGROUND

A paradox in epilepsy and psychiatry is that temporal lobe epilepsy is often predisposed to schizophrenic-like psychosis, whereas convulsive therapy can relieve schizophrenic symptoms. We have previously demonstrated that the nucleus accumbens is a key structure in mediating postictal psychosis induced by a hippocampal electrographic seizure.

OBJECTIVE/HYPOTHESIS

The purpose of this study is to test a hypothesis that accumbens kindling cumulating in a single (1-time) or repeated (5-times) convulsive seizures have different effects on animal models of psychosis.

METHODS

Electrical stimulation at 60 Hz was applied to nucleus accumbens to evoke afterdischarges until one, or five, convulsive seizures that involved the hind limbs (stage 5 seizures) were attained. Behavioral tests, performed at 3 days after the last seizure, included gating of hippocampal auditory evoked potentials (AEP) and prepulse inhibition to an acoustic startle response (PPI), tested without drug injection or after ketamine (3 mg/kg s.c.) injection, as well as locomotion induced by ketamine or methamphetamine (1 mg/kg i.p.).

RESULTS

Compared to non-kindled control rats, 1-time, but not 5-times, convulsive seizures induced PPI deficit and decreased gating of hippocampal AEP, without drug injection. Compared to non-kindled rats, 5-times, but not 1-time, convulsive seizures antagonized ketamine-induced hyperlocomotion, ketamine-induced PPI deficit and AEP gating decrease. However, both 1- and 5-times convulsive seizures significantly enhanced methamphetamine-induced locomotion as compared to non-kindled rats.

CONCLUSIONS

Accumbens kindling ending with 1 convulsive seizure may induce schizophrenic-like behaviors, while repeated (≥5) convulsive seizures induced by accumbens kindling may have therapeutic effects on dopamine independent psychosis.

Cortical electroconvulsive stimulation alleviates breeding-induced prepulse inhibition deficit in rats

In patients with medical-refractory schizophrenia electroconvulsive therapy (ECT), i.e., the induction of therapeutic seizures via cortical surface electrodes, is effectively used. Electroconvulsive stimulation (ECS) in rodents simulates ECT in humans and is applied to investigate the mechanisms underlying this treatment. Experimentally-induced reduced prepulse inhibition (PPI) of the acoustic startle response (ASR), i.e., the reduction of the startle response to an intense acoustic stimulus when this stimulus is shortly preceded by a weaker not-startling stimulus, serves as an endophenotype for neuropsychiatric disorders that are accompanied by disturbed sensorimotor gating, such as schizophrenia. Here we used rats selectively bred for high and low PPI to evaluate whether bifrontal cortical ECS would affect PPI. For this purpose, cortical screw electrodes were stereotactically implanted above the frontal cortex. After recovery ECS was applied for five consecutive days with stimuli of 1 ms pulse-width, 100 pulses/s, 1 s duration, ranging from 5.5 mA to 10 mA. PPI of ASR was measured one day before ECS, and on days 1, 7, and 14 after the last ECS. In rats with breeding-induced low PPI ECS increased PPI one week after stimulation. In contrast, ECS decreased PPI in rats with high PPI on the first day after stimulation. The reaction to the startle impulse was reduced by ECS without difference between groups. This work provides evidence that rats with breeding-induced high or low PPI could be used to further investigate the underlying mechanisms of ECT in neuropsychiatric disorders with disturbed sensorimotor gating like schizophrenia.

Ventral pallidum mediates amygdala-evoked deficits in prepulse inhibition

Prepulse inhibition (PPI) is an operational measure of sensorimotor gating. It is defined as a reduction in magnitude of a startle response when a startling stimulus is preceded by a weaker "prepulse." PPI has been found to be altered in patients with schizophrenia, autism spectrum disorders, and other neuropsychiatric illnesses. As such, the neural substrates regulating PPI are of particular interest. Previous studies using lesions, selective blockade of N-methyl-d-aspartate (NMDA) receptors, and pharmacological disinhibition have demonstrated that impairment of the function of the basolateral and lateral nuclei of the amygdala (BLA) disrupts PPI. However, transient gamma aminobutyric acid-mediated (GABA-mediated) inactivation of BLA has not been evaluated for effects on PPI. Furthermore, the downstream projection targets that mediate BLA-evoked disruptions of PPI have not been elucidated. Thus, in the present study, we evaluated the effect on PPI of bilateral and unilateral inactivation of BLA, by microinfusion of the GABA-A receptor agonist, muscimol. We found that either bilateral or unilateral inactivation impaired PPI. Because unilateral inactivation was sufficient to impair PPI, we hypothesized that this was due to an indirect activation of a downstream target of BLA, the ventral pallidum (VP). Because VP inhibition normalizes PPI deficits evoked from nucleus accumbens (Kodsi & Swerdlow, 1994), we next tested the degree to which VP inhibition would normalize PPI deficits evoked from BLA. We unilaterally inactivated BLA with concurrent inactivation of VP and found that VP inactivation blocked BLA-evoked deficits in PPI. We suggest that BLA inactivation disrupts PPI through disinhibition of VP.

Kindling of basolateral amygdala but not ventral hippocampus or perirhinal cortex disrupts sensorimotor gating in rats

The neural mechanisms mediating prepulse inhibition (PPI) appear to have relevance to neurological and psychiatric disorders. Patients with temporal lobe epilepsy exhibit psychotic symptoms and disrupted PPI, therefore the present experiments examined the consequences of seizures induced by kindling on PPI. Rats were chronically implanted with an electrode into the basolateral amygdala, perirhinal cortex, or ventral hippocampus and stimulated twice daily until 3 fully generalized, class 5 seizures were elicited. Kindling of basolateral amygdala, but not perirhinal cortex or ventral hippocampus, disrupted PPI when testing began 2min, but not 48h, following the elicitation of the third class 5 seizure. Startle amplitudes were unaffected by kindling. These results suggest that the anatomical origin of seizures is an important factor in determining their potentially disruptive effects on PPI.

Sustained brain-derived neurotrophic factor up-regulation and sensorimotor gating abnormality induced by postnatal exposure to phencyclidine: comparison with adult treatment

Brain-derived neurotrophic factor (BDNF) is involved in synaptic development and plasticity, and alterations in BDNF expression or signaling are implicated in drug addiction and psychiatric diseases, such as depression and schizophrenia. In this study, we administered phencyclidine to postnatal and adult rats with different time schedules, and determined the correlations between BDNF expression and the behavioral effects. Both single and repeated phencyclidine injections into adult rats induced BDNF up-regulation in the corticolimbic system and a decrease in prepulse inhibition, both of which were transient. In contrast, subchronic postnatal administration increased BDNF protein and mRNA levels in the hippocampus and entorhinal cortex, which were sustained until 8 weeks of age. In parallel, the postnatal rats treated with phencyclidine developed a persistent decrease in prepulse inhibition at the adult stage. The chronic BDNF increase appeared to contribute to the prepulse inhibition abnormality, as subchronic BDNF infusion into the hippocampus of normal rats mimicked the prepulse inhibition deficits. This study suggests that phencyclidine exposure during brain development induces sustained BDNF up-regulation in the limbic system with a biological link to sensorimotor gating deficits.

The medial septum mediates impairment of prepulse inhibition of acoustic startle induced by a hippocampal seizure or phencyclidine

The involvement of the septohippocampal system on the impaired sensorimotor gating induced by phencyclidine (PCP) or by an electrically induced hippocampal seizure was examined in behaving rats. An impaired sensorimotor gating, measured by prepulse inhibition (PPI) of the acoustic startle response, was observed following a hippocampal afterdischarge (AD) or systemic injection of PCP and was accompanied with an increase in hippocampal gamma waves (30-70 Hz). The medial septum infusion with muscimol (0.25 microg), a GABA(A) receptor agonist, 15 min prior to PCP or a hippocampal AD, prevented the impairment of sensorimotor gating and the increase in gamma waves. By itself, muscimol (0.25 microg) injection into the medial septum did not affect PPI, although it significantly suppressed spontaneous gamma waves. In order to identify subpopulations of neurons mediating the sensorimotor gating deficit and the hippocampal gamma wave increase, 0.14-0.21 microg of p75 antibody conjugated to saporin (192 IgG-saporin) was injected into the medial septum to selectively lesion the septohippocampal cholinergic neurons. Neither the PPI deficit nor the gamma wave increase induced by PCP or a hippocampal AD was affected by 192 IgG-saporin lesion of the medial septum. It is concluded that increase in neural activity in the medial septum participates in the impairment of sensorimotor gating and the increase in hippocampal gamma waves induced by PCP or a hippocampal AD. It is suggested that the GABAergic but not the cholinergic septohippocampal neurons mediate the sensorimotor gating deficit.

Prepulse inhibition deficits in GAD65 knockout mice and the effect of antipsychotic treatment

Recent postmortem studies in humans suggest that defects in GABAergic neurotransmission might contribute to the neuropathology associated with schizophrenia. Disturbances in GABAergic systems may also contribute to the sensorimotor gating deficits classically observed in schizophrenic patients, including deficits in prepulse inhibition (PPI). To explore the relationship, the current study examined the integrity of PPI and startle habituation in knockout (KO) mice that lack the GABA synthesizing enzyme glutamic acid decarboxylase 65 (GAD 65). GAD65 KO mice displayed normal baseline and habituated startle responses, which did not differ from GAD65 wild-type (WT) or heterozygous (HET) mice. However, GAD65 KO mice showed robust deficits in PPI which were reversed by the atypical antipsychotic agent clozapine. These results lend support to the view that abnormalities in GABAergic systems might contribute to the basic pathophysiological mechanisms in schizophrenia.

The ventral hippocampal regulation of prepulse inhibition and its disruption by apomorphine in rats are not mediated via the fornix

Prepulse inhibition (PPI) of startle is a measure of sensorimotor gating that is impaired in schizophrenia. We have reported that PPI is regulated by the ventral hippocampus (VH) and that the PPI disruptive effects of the dopamine agonist apomorphine are enhanced 4 weeks after excitotoxic lesions of the VH. The mechanisms responsible for the VH influence on PPI are not understood, but have been ascribed to interactions between the VH and nucleus accumbens. In the present study, we examined whether the VH influence on PPI and its dopaminergic regulation is dependent on the integrity of the VH-accumbens projection via the fornix. First, the PPI-disruptive effects of intra-VH NMDA infusion were assessed after sham or electrolytic transection of the fornix. Second, the PPI-disruptive effects of apomorphine were assessed 1 month after excitotoxic or electrolytic lesions of the VH, or after fornix transection. Intra-VH N-methyl-D-aspartate infusion significantly disrupted PPI; this effect was unaffected by fornix lesions. The PPI-disruptive effects of apomorphine were significantly enhanced by excitotoxic or electrolytic lesions of the VH, but not by fornix transection. The influence of the VH on PPI and its dopaminergic regulation does not appear to be mediated via the fornix. The enhanced sensitivity to the PPI-disruptive effects of apomorphine after VH lesions is not dependent on excitotoxin-induced changes in the VH or its downstream projections.

Schizophrenia-like behavioral changes after partial hippocampal kindling

The effect of hippocampal kindling on behavioral changes following 10 and 21 hippocampal afterdischarges (ADs) or electrographic seizures was examined in behaving rats. As compared to control, non-stimulated rats, 21 but not 10 hippocampal ADs resulted in a decrease in social contact, an increase in social isolation, and an increase in climbing and chasing behavior tested in an open field 3 days after cessation of kindling. Porsolt forced swimming test was not different among the control, 10- or 21-AD groups of rats. A deficit in sensorimotor gating, measured by prepulse inhibition of an acoustic startle, was observed in kindled as compared to control rats at 2 weeks after 21 ADs, but not after 10 ADs. Similarly, methamphetamine (1 mg/kg i.p.) induced higher locomotor activity in kindled rats, as compared to controls, after 21 ADs but not after 10 ADs. Spontaneous locomotor activity in a novel cage, without drug administration, was not different between kindled and control rats. These findings suggest that behavioral alterations after repeated hippocampal electrographic seizures may be mediated by increased dopaminergic functions, which may also mediate the psychiatric symptoms in human epileptic patients.

Quetiapine produces a prolonged reversal of the sensorimotor gating-disruptive effects of basolateral amygdala lesions in rats

Prepulse inhibition (PPI) of startle is impaired in schizophrenia and in rats after manipulations of limbic cortical and subcortical regions. The atypical antipsychotic quetiapine was used to reverse PPI deficits after basolateral amygdala (BLA) lesions in rats. BLA quinolinic acid lesions significantly disrupted PPI 1 week postsurgery. Tests with quetiapine (0 vs. 7.5 mg/kg) in a within-subject design 2-3 weeks postsurgery revealed a normalization of PPI. Carry-over effects lasted up to 3 weeks, with a return of lesion-induced deficits by Week 5 postsurgery. This dose of quetiapine also blocked the PPI-disruptive effects of phencyclidine. PPI deficits after BLA lesions are reversed by quetiapine, in a manner that is sustained beyond its acute pharmacological effects and which may be mediated downstream from the BLA.

Prepulse inhibition deficits in patients with panic disorder

Prepulse inhibition (PPI) is an operational measure of sensorimotor gating that is reduced in several neuropsychiatric disorders that are characterized by deficits in inhibition or gating of intrusive or irrelevant stimuli. Clinically, panic disorder (PD) patients have been described as having difficulties in inhibition of responding to sensory and cognitive events. Because such difficulties may be due to failures in early stages of information processing, we examined PPI in patients with PD. Acoustic startle reactivity, habituation, and PPI (30-, 60-, 120-, 240-, and 2,000-ms interstimulus intervals) were assessed in patients with panic disorder (m/f = 10, 10) and age- and gender-matched healthy controls (m/f = 11, 10). PD patients were assessed with structured clinical interview for DSM-IV criteria with benzodiazepine treatment as an exclusion criterion. Panic disorder patients exhibited normal startle reactivity, reduced habituation, and significantly reduced PPI in the 30-, 60-, and 240-ms prepulse conditions. Within the PD group, the patients with high trait and state anxiety exhibited less PPI than patients with low trait and state anxiety. Furthermore, in PD patients, decreased PPI correlated significantly with high trait but not state anxiety. These data indicate that early stages of sensory information processing are abnormal in patients with PD. These observed deficits in PPI could reflect a more generalized difficulty in suppressing or gating information in panic disorder. The correlation between high trait anxiety and deficient PPI supports the hypothesis that sensorimotor gating abnormalities are an enduring feature of subjects with PD.

Activation of the retrohippocampal region in the rat causes dopamine release in the nucleus accumbens: disruption by fornix section

There is a well-described projection from the retrohippocampus (subiculum and entorhinal cortex) to the nucleus accumbens that is involved in the control of psychomotor behaviour, and is implicated in the aetiology of schizophrenia. Cortical abnormalities are widely reported in the brains of schizophrenic patients, but it is unclear whether they are the cause or consequence of those changes in subcortical systems that are treated with neuroleptic drugs. We have, therefore, conducted a series of microdialysis experiments in anaesthetized rats to determine whether infusion of the excitotoxin, N-methyl-D-aspartate, into the retrohippocampus increases mesolimbic dopamine release. We found a clear and reproducible increase in extracellular dopamine in the nucleus accumbens following N-methyl-D-aspartate (2.5 microg), that was abolished when we sectioned the fimbria-fornix. Furthermore, inhibition of gamma-aminobutyric acid receptors following retrohippocampus administration of bicuculline (4 microg), also increased dopamine in the nucleus accumbens. The dopamine response to bicuculline was accompanied by an increase in dopamine metabolism, was long lasting, and also reduced by fornix section.The response to both N-methyl-D-aspartate and bicuculline depends on the integrity of the projection from the retrohippocampus to the nucleus accumbens. The results provide an underlying mechanism whereby a primary insult in the temporal cortex, caused by excessive N-methyl-D-aspartate receptor stimulation, can produce a hyperdopaminergic state. In addition, an increase in subiculo-accumbens activity evoked by bicuculline may also explain why patients with limbic epilepsy can develop a psychosis.

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.

Lesions of the rat piriform cortex prevent long-lasting sensorimotor gating deficits induced by stimulation of the ventral hippocampus

Prepulse inhibition (PPI) is a cross-species operational measure of sensorimotor gating. Reduced PPI is found in schizophrenics and can be induced experimentally in rats. Stimulation of the rat ventral hippocampus (VH) with N-methyl-D-aspartate (NMDA) results in long-lasting PPI deficits (carry-over effect). Since we have previously shown that this carry-over effect was accompanied by increased expression of c-Fos in the piriform cortex (PIR), we here tested the effects of lesions of the PIR on PPI after stimulation of the VH. PIR lesioned rats still showed disruption of PPI after injection of NMDA into the VH. However, the carry-over effect observed in controls was prevented by PIR lesions. These data suggest that the PIR is important for long-lasting alterations in brain functioning leading to impaired sensorimotor gating.

Characterization of phenytoin-resistant kindled rats, a new model of drug-resistant partial epilepsy: influence of genetic factors

It has been recently shown that a subpopulation of amygdala-kindled Wistar rats can be selected which do not respond to phenytoin with an increase in afterdischarge threshold (ADT). Such non-responders could be a perfect model for studying the mechanisms of pharmacoresistance of complex partial seizures. Further studies on these rats suggested that the lack of anticonvulsant response was not due to the influence of experimental factors, but is an inherent property of each rat. In this study the influence of genetic factors on the pharmacoresistance to phenytoin by breeding Wistar rats which have been selected for their ability to consistently respond or not respond to phenytoin is examined. Male and female Wistar rats were implanted with bipolar electrodes in the basolateral amygdala and kindled. The fully kindled rats were repeatedly tested for their ability to respond to phenytoin (75 mg/kg i.p.) with an ADT increase. Responders and non-responders were mated and the offspring underwent the same kindling and repeated phenytoin testing procedure. Altogether, four generations of kindled rats were studied. The incidence of responses to phenytoin, i.e. ADT increased by more than 20% of control, was significantly higher in the F2 generation of the responder line compared to the non-responder line, but the number of responders and non-responders in the offspring generations F1-F3 did not significantly increase. The data suggest that the ability to respond or not to respond to phenytoin is genetically determined, although it does not follow a simple scheme of inheritance. The low reproductive success of the kindled and phenytoin-treated rats made it impossible to achieve a strain of phenytoin non-responders.

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.