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

Involvement of posterior cingulate cortex in ketamine-induced psychosis relevant behaviors in rats

The involvement of posterior cingulate cortex (PCC) on ketamine-induced psychosis relevant behaviors was investigated in rats. Bilateral infusion of muscimol, a GABA_A receptor agonist, into the PCC significantly antagonized ketamine-induced deficit in prepulse inhibition of a startle reflex (PPI), deficit in gating of hippocampal auditory evoked potentials, and behavioral hyperlocomotion in a dose dependent manner. Local infusion of ketamine directly into the PCC also induced a PPI deficit. Systemic injection of ketamine (3mg/kg,s.c.) induced an increase in power of electrographic activity in the gamma band (30-100Hz) in both the PCC and the hippocampus; peak theta (4-10Hz) power was not significantly altered, but peak theta frequency was increased by ketamine. In order to exclude volume conduction from the hippocampus to PCC, inactivation of the hippocampus was made by local infusion of muscimol into the hippocampus prior to ketamine administration. Muscimol in the hippocampus effectively blocked ketamine-induced increase of gamma power in the hippocampus but not in the PCC, suggesting independent generation of gamma waves in PCC and hippocampus. It is suggested that the PCC is part of the brain network mediating ketamine-induced psychosis related behaviors.

The effects of atomoxetine and methylphenidate on the prepulse inhibition of the acoustic startle response in mice

Atomoxetine (ATM) and methylphenidate (MPD) have been used for the treatment of attention deficit hyperactivity disorder (ADHD). ATM is a selective norepinephrine reuptake inhibitor, whereas MPD is a psychostimulant and acts as a norepinephrine and dopamine reuptake inhibitor. In the present study, we investigated the effects of ATM (1, 3 or 10mg/kg) and MPD (5, 10 or 20mg/kg) on pharmacological mouse models of sensorimotor gating measured by prepulse inhibition (PPI) using the acoustic startle response test. MK-801, a non-competitive N-methyl-d-aspartate receptor antagonist, or apomorphine, a non-competitive dopamine receptor agonist, was used to induce PPI deficits. ATM (3 or 10mg/kg, s.c.) significantly attenuated the MK-801-, but not apomorphine-, induced PPI deficits. In contrast to ATM, MPD did not reverse the PPI deficits induced by either MK-801 or apomorphine. Immunostaining revealed that the number of c-Fos-immunopositive cells was increased in the nucleus accumbens following MK-801 treatment, and this was reversed by the administration of ATM (3mg/kg), but not MPD (10mg/kg). However, neither ATM nor MPD reversed the increased number of c-Fos-immunopositive cells in the nucleus accumbens following apomorphine treatment. These results suggest that the attenuating effect of ATM on the increased c-Fos immunoreactivity in the nucleus accumbens induced by MK-801 may be attributed to the PPI deficit-ameliorating effects of ATM and that ATM would be useful to treat sensorimotor gating-related disorders by improving the patient's attention span or cognitive function.

Corresponding decrease in neuronal markers signals progressive parvalbumin neuron loss in MAM schizophrenia model

Alteration in normal hippocampal (HPC) function attributed to reduced parvalbumin (PV) expression has been consistently reported in schizophrenia patients and in animal models of schizophrenia. However, it is unclear whether there is an overall loss of interneurons as opposed to a reduction in activity-dependent PV content. Co-expression of PV and the constitutively expressed substance P (SP)-receptor protein has been utilized in other models to ascertain the degree of cell survival, as opposed to reduction in activity-dependent PV content, in the HPC. The present study measured the co-expression of PV and SP-receptors in the dentate and dorsal and ventral CA3 subregions of the HPC in the methylazoymethanol acetate (MAM) rat neurodevelopmental model of schizophrenia. In addition, these changes were compared at the post-natal day 27 (PND27) and post-natal day 240 (PND > 240) time points. Brains from PND27 and PND > 240 MAM (n = 8) and saline (SAL, n = 8) treated offspring were immunohistochemically processed for the co-expression of PV and SP-receptors. The dorsal dentate, dorsal CA3 and ventral CA3 subregions of PND27 and PND > 240 MAM rats demonstrated significant reductions in PV but not SP-receptor expression, signifying a loss of PV-content. In contrast, in the ventral dentate the co-expression of PV and SP-receptors was significantly reduced only in PND > 240 MAM animals, suggesting a reduction in cell number. While MAM-induced reduction of PV content occurs in CA3 of dorsal and ventral HPC, the most substantial loss of interneuron number is localized to the ventral dentate of PND > 240 animals. The disparate loss of PV in HPC subregions likely impacts intra-HPC network activity in MAM rats.

The effect of pregabalin on sensorimotor gating in 'low' gating humans and mice

Pregabalin, an anticonvulsant and anxiolytic compound that binds to α2-δ auxiliary subunit Types 1 and 2 of voltage-gated calcium channels, has been shown to reduce excitatory neurotransmission partially through modulation of glutamatergic signaling. Prepulse inhibition (PPI) of startle is an operational measure of sensorimotor gating impacted by disruption of the glutamatergic system and is reduced in schizophrenia patients. Dysregulation of the glutamatergic system has also been implicated in the pathophysiology of schizophrenia. Here we tested the hypothesis that pregabalin may ameliorate PPI in a model of deficient gating in humans and mice. In study 1, 14 healthy human subjects participated in a within subjects, cross-over study with placebo, 50 mg or 200 mg pregabalin treatment prior to undergoing a PPI task. In study 2, 24 C57BL/6 mice underwent a similar procedure with vehicle, 30 and 100 mg/kg dose treatments. In both studies, subjects were assigned to a "Low" or "High" gating group using a median split procedure based on their PPI performance during placebo/vehicle. Drug effects were then examined across these groups. In humans, pregabalin treatment significantly increased PPI performance in the "low gating" group. In mice, pregabalin treatment significantly increased PPI in the low gating group but reduced PPI in the high gating group. Across species, pregabalin treatment improves PPI in subjects with low gating. These data support further exploration of pregabalin as a potential treatment for disorders characterized by sensorimotor gating deficits and glutamatergic hypersignaling, such as schizophrenia.

Projections from ventral hippocampus to medial prefrontal cortex but not nucleus accumbens remain functional after fornix lesions in rats

Sensorimotor gating, as measured by prepulse inhibition (PPI) of startle, is deficient in human beings with schizophrenia and is greatly reduced in rats after bilateral infusion of N-methyl-D-aspartate (NMDA) into the ventral hippocampus (VH). The disruption of PPI by bilateral VH NMDA infusion is blocked by bilateral medial prefrontal cortex (mPFC) lesions, but not by bilateral lesions of the fornix, which is the principal output pathway of the hippocampal formation of the VH. Tract-tracing studies have shown the presence of additional nonfornical pathways by which the VH and neighboring structures of the amygdala may reach forebrain regions that regulate PPI, including the mPFC. To determine whether these nonfornical pathways might mediate forebrain activation after VH NMDA infusion, we examined the effects of bilateral VH NMDA infusion on c-Fos protein expression in the mPFC and nucleus accumbens (NAC) after sham vs. bilateral fornix lesions. Significant increases of c-Fos expression were observed in both the mPFC and NAC after bilateral VH NMDA infusions. Fornix lesions blocked enhanced c-Fos expression in the NAC but not the mPFC after VH NMDA infusion. The results suggest that an intact fornix may be necessary for VH activation of the NAC, but that the VH uses additional nonfornical projections to activate PPI-regulatory circuits within the mPFC.

Pathways from the ventral hippocampus and caudal amygdala to forebrain regions that regulate sensorimotor gating in the rat

The neural substrates regulating sensorimotor gating in rodents are studied in order to understand the basis for gating deficits in clinical disorders such as schizophrenia. N-methyl-D-aspartate (NMDA) infusion into the ventral temporal lobe, including caudal parts of the ventral hippocampal region and amygdala, has been shown to disrupt sensorimotor gating in rats, as measured by prepulse inhibition (PPI) of startle. One working model is that reduced PPI after infusion of NMDA into this region is mediated via its efferents to ventral forebrain structures, i.e. medial prefrontal cortex (mPFC) and nucleus accumbens. Yet, PPI-disruptive effects persist after lesions of the precommissural fornix, the principal output pathway of the hippocampal formation. Here, we aimed to characterize non-fornical forebrain projections from this region that might mediate the PPI-disruptive effects of the ventral temporal lobe. Electrolytic lesions of the precommissural fornix in male Sprague-Dawley rats were followed by infusions of fluorogold into the mPFC or by infusions of biotinylated dextan amine into the ventral temporal lobe. Projections from the ventral subiculum and CA1 regions of the ventral hippocampus to the mPFC and accumbens core and shell were interrupted by fornix lesions. Projections to the mPFC and accumbens from other regions of the ventral temporal lobe, particularly the lateral entorhinal cortex and the embedded olfactory and vomeronasal parts of the caudal amygdala, survived fornix lesions. These additional projections coursed rostrally through the amygdala and emerged via the stria terminalis, interstitial nuclei of the posterior limb of the anterior commissure, and the ventral amygdalofugal pathway. PPI-regulatory portions of the ventral temporal lobe innervate the accumbens and mPFC via multiple routes. It remains to be determined which of these non-fornical projections may be responsible for the persistent regulation of PPI after fornix lesions.

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.

Prepulse inhibition of acoustic startle response in mild cognitive impairment and mild dementia of Alzheimer type

Amnestic mild cognitive impairment (MCI) describes the condition of memory-impaired individuals who otherwise function well and do not meet the clinical criteria for dementia. Such individuals are considered to represent a transitional stage between normal aging and dementia of Alzheimer type (DAT). Neurobiologic changes in amnestic MCI, and their significance for psychophysiologic function, are poorly understood. In this study, the authors compared acoustic prepulse inhibition (PPI) between subjects with amnestic MCI and mild DAT to characterize sensorimotor gating. The acoustic startle reflex, which the authors measured using an accelerometer and electromyogram, involves whole-body movement and eye blink in response to a sudden loud noise (115 dB). PPI is inhibition of this reflex by a softer noise (prepulse; 85 dB) preceding the startle stimulus by 30 ms. PPI was examined in 30 controls, 20 subjects with amnestic MCI, and 20 subjects with mild DAT. Neither amnestic MCI nor mild DAT affected startle movement amplitude. Subjects with amnestic MCI showed significantly enhanced PPI (gating facilitation), while subjects with mild DAT exhibited significantly less PPI than controls (gating deficit). This pattern of PPI changes suggests that neuropathologic changes in the limbic cortex, mainly the entorhinal cortex, at the earliest stage of DAT might be responsible for PPI abnormalities via disturbed regulation of the limbic cortico-striato-pallido-pontine circuitry. Startle PPI changes could be used as a biologic marker for amnestic MCI and mild DAT.

Prefrontal D1 and ventral hippocampal N-methyl-D-aspartate regulation of startle gating in rats

BACKGROUND

Sensorimotor gating, as measured by prepulse inhibition of the startle reflex, is deficient in schizophrenia patients, and in rats after specific manipulations of limbic cortico-striato-pallido-thalamic circuitry. For example, prepulse inhibition in rats is disrupted after D1 blockade in the medial prefrontal cortex, and after N-methyl-D-aspartate infusion into the ventral hippocampus. In the present study, we examined whether these two substrates form part of an integrated circuit regulating sensorimotor gating, which might contribute to the loss of prepulse inhibition in patient populations.

METHODS

Prepulse inhibition was assessed in male Sprague-Dawley rats after systemic or intra-medial prefrontal cortex administration of the D1 antagonist, SCH 23390. Separate rats received intra-medial prefrontal cortex infusion of the retrograde transported label Fluoro-Gold. In rats with sham or electrolytic lesions of the medial prefrontal cortex, prepulse inhibition was tested after infusion of N-methyl-D-aspartate or vehicle into ventral hippocampus regions that were determined to send projections to the medial prefrontal cortex.

RESULTS

Prepulse inhibition was disrupted after systemic SCH 23390 treatment and after infusion of SCH 23390 into medial prefrontal cortex sites within the prelimbic and cingulate cortices. Fluoro-Gold infusion into these medial prefrontal cortex sites labeled cells in the ventral hippocampus complex, including regions CA1 and entorhinal cortex. N-methyl-D-aspartate infusions into these ventral hippocampus regions disrupted prepulse inhibition in rats after sham but not electrolytic lesions of the medial prefrontal cortex.

CONCLUSIONS

Prepulse inhibition appears to be regulated by interacting substrates within the ventral hippocampus and MPFC. Specifically, NMDA activation of the ventral hippocampus appears to disrupt prepulse inhibition in a manner that is dependent on the integrity of infralimbic or cingulate cortical regions that also support a D1-mediated regulation of prepulse inhibition. Conceivably, dysfunction within these hippocampal-frontal circuits may contribute to sensorimotor gating deficits in schizophrenia.

Enhanced locomotor activity in adult rats with neonatal administration ofN-omega-nitro-L-arginine

Nitric oxide (NO) is a neuronal messenger molecule that plays important roles in the development, maintenance, and functional modifications of brain circuits. We investigated whether the NO levels at different postnatal day (P) periods of the brain develop interference with the locomotion in a novel environment during the postpuberal age (P60). First, using the determination of the nitrite accumulation, we evaluated whether treatment with the NO-synthase inhibitor N-nitro-L-arginine (L-NNA) during different neonatal ages (P1 to P3, P4 to P6, and P7 to P9) affected the levels of NO activity in different regions in the neonatal brain of the rat. We then evaluated whether the locomotor activity in the adult rat (P60) is affected by the blocking of the neonatal NO-activity during a specific period of the development of the nervous system. Neonatal rats with L-NNA administration at P4 to P6 and P7 to P9 show a significant decrease in the levels of NO activity in all the brain regions. However, the blocking of NO synthesis during the neonatal period between P4 to P6 produced an increase in the locomotion after puberty. These data suggest that during a specific step in the development of the brain, the NO levels may play a critical role in the structures that control the spontaneous locomotion in a novel environment after puberty.

Hippocampal dopamine receptors modulate cFos expression in the rat nucleus accumbens evoked by chemical stimulation of the ventral hippocampus

Recently, we have shown that D1 and D2 receptors in the ventral hippocampus (VH) modulate both the locomotor activation and the increase in dopamine (DA) levels in the rat nucleus accumbens (NAc) induced by NMDA stimulation of the VH. In the present study we analyze the possible role of VH D1 and D2 receptors in the modulation of the cFos expression in NAc (core and shell subregions) and in dorsal striatum. This was assessed by immunohistochemical analysis of cFos expression in the rat brains after retro-dialysis application of NMDA (50mM, 10 min) into VH, in absence and in presence of either the D1/D5 receptor antagonist SCH 23390 (100 and 250 microM, 60 min) or the D2 receptor antagonist raclopride (100 and 250 microM, 60 min). NMDA induced a robust increase in the cFos expression in the NAc shell, both in the ipsilateral and contralateral side. No statistically significant increases were observed in the NAc core and in the dorsal striatum. Simultaneous application of SCH 23390 and NMDA into the VH attenuated the NMDA-evoked cFos expression in NAc shell. In contrast, raclopride had no significant effect. Our present results show that the NMDA receptor mediated effects in the VH require D1 receptors and suggest that DA in VH strongly modulates the excitatory outputs from this brain area.

Alterations in behavioral responses to a cholinergic agonist in post-pubertal rats with neonatal ventral hippocampal lesions: relationship to changes in muscarinic receptor levels

Excitotoxic neonatal ventral hippocampal (NVH) lesion in rats is considered as a putative animal model of schizophrenia as lesioned animals show characteristic post-pubertal emergence of neurochemical and behavioral abnormalities analogous to some of those seen in this disease. Converging evidence points to the involvement of central cholinergic system in this neuropsychiatric disorder, and our previous studies have suggested that cholinergic neurotransmission may be altered in post-pubertal NVH lesioned rats. We investigated here muscarinic receptor reactivity in NVH lesioned animals by measuring the effects of the muscarinic receptor agonist oxotremorine on physiological responses known to be modulated by these receptors such as body temperature, salivation, tremor, pain, and prepulse inhibition of the acoustic startle (PPI). Quantitative receptor autoradiography revealed that post-pubertal NVH lesioned animals display increased levels of [3H]pirenzepine/M1-like and [3H]AFDX-384/M2-like receptor binding sites in the striatum, nucleus accumbens, and in subareas of the dorsal hippocampus. Moreover, in response to the systemic administration of oxotremorine (0.25 mg/kg), post-pubertal NVH lesioned rats exhibited increases in salivation and tremor, and a greater reduction in body temperature compared to sham control animals. Increases in the hot-plate latency were also observed suggesting enhanced antinociceptive effects of oxotremorine in post-pubertal NVH lesioned animals. Finally, oxotremorine (0.1 and 0.25 mg/kg) disrupted PPI in post-pubertal sham control rats while the muscarinic receptor antagonist biperiden (0.5 and 1.0 mg/kg) normalized this behavior in NVH lesioned rats. Taken together, these findings reveal that post-pubertal NVH lesioned rats display enhanced muscarinic receptor responsiveness, which may relate to some behavioral abnormalities reported in this animal model.

Modulating sensorimotor gating in healthy volunteers: the effects of desipramine and haloperidol

In schizophrenia both an involvement of a reduced prefrontal dopaminergic activity and an enhanced noradrenergic activity have been suggested. In addition, patients suffering from schizophrenia show reduced sensorimotor gating and reduced habituation. If there is a causality between these neurotransmitters and these processes, then either a reduction in dopaminergic activity or an enhanced noradrenergic activity in healthy volunteers would result in reduced sensorimotor gating and reduced habituation. In the present study, a group of 12 healthy male volunteers was tested four times in a prepulse inhibition (PPI) paradigm 2.5 h following administration of placebo/placebo, placebo/desipramine (50 mg), placebo/haloperidol (2 mg) and desipramine (50 mg)/haloperidol (2 mg). A significant reduction of percentage PPI was found in all active treatments compared with placebo/placebo, while no treatment effects on habituation were found. Furthermore, a significant increase in heart rate was found in both desipramine treatments, from 120 min following oral intake onwards. Both desipramine and haloperidol reduced PPI, which suggests that an enhanced noradrenergic activity and a reduced dopaminergic activity lead to a reduction in sensorimotor gating. Since reduced sensorimotor gating is found in schizophrenia, these results supply further evidence for a reduced prefrontal dopaminergic activity and an enhanced noradrenergic activity in schizophrenia. Furthermore, the combination of haloperidol and desipramine did not have a synergistic effect on PPI, which indicates an interaction between the compounds. The site for this interaction is most likely located in the prefrontal cortex, since evidence is accumulating that extracellular dopamine concentration is regulated by noradrenergic terminals, particularly in the frontal areas of the brain. Since no effects on habituation were found, this suggests that neither enhanced noradrenergic nor decreased dopaminergic activity is involved in this process.

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.

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.

Effects of an adenosine A2A receptor blockade in the nucleus accumbens on locomotion, feeding, and prepulse inhibition in rats

The nucleus accumbens (NAc) subserves behaviors governed by natural rewards, i.e., feeding or exploration, and has been implicated in control of prepulse inhibition (PPI), a measure of sensorimotor gating. The present study sought to determine whether a tonic stimulation of adenosine A(2A) receptors in the rat NAc is involved in control of spontaneous locomotor activity, feeding behavior, and PPI. To this end, bilateral microinfusions of a prodrug (MSX-3) (3 microg and 5 microg in 1 microl per side) of the selective A(2A) receptor antagonist MSX-2 or vehicle (1 microl per side) were administered into the NAc. Results show that blockade of intra-NAc adenosine A(2A) receptors by a high (5 microg), but not by a low (3 microg), dose of MSX-3 increased locomotor activity in an open field, reduced food intake, and delayed intake onset in food-deprived rats examined in a test cage with standard laboratory chow. Furthermore, PPI was significantly disrupted after intra-NAc infusion of 5 microg, but not 3 microg, MSX-3. These findings suggest that locomotor activity as well as intact PPI and feeding behavior rely on tonic activation of intra-NAc A(2A) receptors. The data add further support to the view that adenosine is a tonically active modulator of striatal function through actions on A(2A) receptors.

Hippocampal modulation of sensorimotor processes

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

Aberrant responses to acoustic stimuli in mice deficient for neural recognition molecule NB-2

NB-2, a member of the contactin subgroup in the immunoglobulin superfamily, is expressed specifically in the postnatal nervous system, reaching a maximum level at 3 weeks postnatal. NB-2 displays neurite outgrowth-promoting activity in vitro. To assess its function in the nervous system, we generated mutant mice in which a part of the NB-2 gene was ablated and replaced with the tau-LacZ gene. The general appearance of NB-2-deficient mice and their gross anatomical features were normal. The LacZ expression patterns in heterozygous mice revealed that NB-2 is preferentially expressed in the central auditory pathways. In the audiogenic seizure test, NB-2-deficient mice exhibited a lower incidence of wild running, but a higher mortality rate than the wild-type littermates. c-Fos immunohistochemistry demonstrated that neural excitability induced by the audiogenic seizure test in the NB-2-deficient mice was prominently attenuated in both the dorsal and external cortices of the inferior colliculus, where enhanced neural excitability was observed in the wild-type mice. In response to pure-tone stimulation after priming, NB-2-deficient mice exhibited a diffuse and low level of c-Fos expression in the central nucleus of the inferior colliculus, which was distinctly different from the band-like c-Fos expression corresponding to the tonotopic map in the wild-type littermates. Taken together, these results suggest that a lack of NB-2 causes impairment of the neuronal activity in the auditory system.

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

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

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

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

Hyperactivity and disruption of prepulse inhibition induced by N-methyl-D-aspartate stimulation of the ventral hippocampus and the effects of pretreatment with haloperidol and clozapine

This study re-examined the hyperactivity and disruption of prepulse inhibition induced by N-methyl-D-aspartate stimulation of the rat ventral hippocampus and compared how both effects were affected by pretreatment with either haloperidol or clozapine. While the hyperactivity is thought to depend on dopamine receptor activation in the nucleus accumbens, the dopamine D2-class receptor blocker haloperidol failed to antagonize the disruption of prepulse inhibition in previous studies. However, an ameliorative effect of the atypical neuroleptic clozapine on disruption of prepulse inhibition was suggested by our previous experiments [Zhang et al. (1999) NeuroReport 10, 1-6]. In the present study, bilateral infusion of N-methyl-D-aspartate (0.5microg/side) into the ventral hippocampus of Wistar rats increased open field locomotor activity and disrupted prepulse inhibition. Both effects were observed immediately after infusion but disappeared 24h later. Injection of haloperidol (0.2mg/kg) or clozapine (5mg/kg), 45min prior to N-methyl-D-aspartate infusion, totally antagonized the hyperactivity but did not affect the disruption of prepulse inhibition. We conclude that dopaminergic mechanisms are differentially involved in the hyperactivity and disruption of prepulse inhibition induced by N-methyl-D-aspartate stimulation of the ventral hippocampus. Activation of accumbal dopamine receptors, which is blocked by clozapine and haloperidol to a comparable extent, seems to be crucial for the hyperactivity but not the disruption of prepulse inhibition. The present finding that both clozapine and haloperidol failed to antagonize the disruption of prepulse inhibition induced by N-methyl-D-aspartate stimulation of the ventral hippocampus is discussed with respect to our previous contrary finding concerning the ameliorative effect of clozapine and with respect to the disruption of prepulse inhibition in rats being considered as a model of sensorimotor gating deficits in schizophrenia.

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.

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

Latent inhibition (the retarded conditioning to a stimulus following its repeated non-reinforced pre-exposure) and prepulse inhibition (the reduction in the startle response to an intense acoustic stimulus when this stimulus is immediately preceded by a prepulse) reflect cognitive and sensorimotor gating processes, respectively, and are deficient in schizophrenic patients. The disruption of latent inhibition and prepulse inhibition in the rat is used as an animal model for the attentional deficits associated with schizophrenia. The present study tested the extent to which latent inhibition and prepulse inhibition, startle reaction and locomotor activity in the open field were affected by infusing the non-competitive N-methyl-D-aspartate receptor antagonist MK-801 (dizocilpine) into the dorsal hippocampus of Wistar rats. We used the same dose of MK-801 (6.25microg/0.5microl per side) previously found to be effective in the disruption of prepulse inhibition when infused into the dorsal hippocampus of Sprague-Dawley rats [Bakshi V. P. and Geyer M. A. (1998) J. Neurosci. 18, 8394-8401; Bakshi V. P. and Geyer M. A. (1999) Neuroscience 92, 113-121]. Bilateral infusion of MK-801 into the dorsal hippocampus did not disrupt latent inhibition. Furthermore, in contrast to previous studies, we failed to find a significant disruption of prepulse inhibition after MK-801 infusion into the dorsal hippocampus, although MK-801 infusion was effective in increasing the startle amplitude as well as locomotor activity in an open field. From our results, we suggest that N-methyl-D-aspartate receptor-mediated processes within the dorsal hippocampus are not necessary for the normal maintenance of the attentional processes reflected by latent inhibition and prepulse inhibition.

Role of the substantia nigra pars reticulata in sensorimotor gating, measured by prepulse inhibition of startle in rats

The substantia nigra pars reticulata (SNR) is one of the major output nuclei of the basal ganglia. It connects the dorsal and ventral striatum with the thalamus, superior colliculus and pontomedullary brainstem. The SNR is therefore in a strategic position to regulate sensorimotor behavior. We here assessed the effects of SNR lesions on prepulse inhibition (PPI) of the acoustic startle response (ASR), stereotypy and locomotion in drug-free rats, as well as after systemic administration of the dopamine agonist DL-amphetamine (2 mg/kg), and the NMDA receptor antagonists dizocilpine (0.16 mg/kg) and CGP 40116 (2 mg/kg). SNR lesions reduced PPI, enhanced spontaneous sniffing and potentiated the locomotor stimulation by dizocilpine and CGP 40116. PPI was impaired by dizocilpine and CGP 40116 in controls. The ASR was enhanced in controls by dizocilpine and amphetamine. SNR lesions prevented the enhancement of the ASR by amphetamine. A second experiment tested the hypothesis that the SNR mediates PPI via a GABAergic inhibition of the startle pathway. Infusion of the GABA(B) antagonist phaclofen but not the GABA(A) antagonist picrotoxin into the caudal pontine reticular nucleus reduced PPI. Hence, lesion of the SNR reduces sensorimotor gating possibly by elimination of a nigroreticular GABAergic projection interacting with GABA(B) receptors. Moreover, destruction of the SNR enhances the motor stimulatory effects of amphetamine and of the NMDA antagonists dizocilpine and CGP 40116. We conclude that the SNR exerts a tonic GABAergic inhibition on sensorimotor behavior that is regulated by the dorsal and the ventral striatum.

Effects of MK801 and neuroleptics on prepulse inhibition: re-examination in two strains of rats

Disruption of prepulse inhibition (PPI) induced by NMDA receptor antagonists, such as MK801, has been used as an animal model of positive and negative symptoms of schizophrenia. Previous studies suggested that atypical, but not typical, neuroleptics can selectively restore MK801-induced PPI disruption and that such selectivity may depend on strain differences. The present study re-examined PPI disruption by systemic MK801 in Wistar (WS) and Sprague-Dawley (SD) strains, and addressed the issue whether clozapine (atypical), compared to haloperidol (typical), effectively antagonizes MK801-induced PPI disruption. In addition, we tested the effects of bilateral microinfusion of MK801 into the ventral hippocampus in WS. Systemic MK801 disrupted PPI in both strains. Neither clozapine nor haloperidol antagonized MK801-induced PPI in either strain. Our clozapine data do not agree with previous reports of clozapine's ability to antagonize MK801-induced PPI disruption. Similar to previous results with SD, MK801 infusion into the ventral hippocampus failed to affect PPI in WS. In our view, the selective ability of atypical neuroleptics to restore PPI disruption by NMDA antagonists, and to serve as a tool for identifying possible atypical neuroleptics, requires further examination. PPI disruption with systemic MK801 may be due to the blockade of NMDA receptors in multiple brain sites.

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.

CROC-4: a novel brain specific transcriptional activator of c-fos expressed from proliferation through to maturation of multiple neuronal cell types

A novel, brain-specific cDNA, denoted CROC-4, was cloned from human brain by a contingent replication of cDNA procedure capable of detecting transcriptional activators of the human c-fos proto-oncogene promoter. CROC-4 encoded an 18-kDa serine/threonine-rich polypeptide containing a P-loop motif and an SH3-binding region with phosphorylation sites for a variety of protein kinases (cdc2, CDK2, MAPK, CDK5, protein kinase C, Ca(2+)/calmodulin protein kinase 2, casein kinase 2) involved in cell proliferation and differentiation. Immunohistochemistry revealed that during early development, expression was associated with proliferating and migrating cells throughout the rodent brain, initially appearing in the proliferative ventricular zones. During late development and in adult human brain, CROC-4 was expressed in diverse brain regions including the thalamus, subthalamic nucleus, corpus callosum, substantia nigra, caudate nucleus, amygdala, and hippocampus. The association of CROC-4 expression with proliferating regions of developing brain and retention in regions of the adult brain, as well as the punctate nuclear location, suggest that CROC-4 participates in brain-specific c-fos signaling pathways involved in cellular remodeling of brain architecture.

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.

Ventral subiculum administration of the somatostatin receptor agonist MK-678 increases dopamine levels in the nucleus accumbens

Somatostatin (or somatotropin-release inhibitory factor, SRIF) binding and in situ hybridisation studies have indicated a high expression of receptor subtypes throughout the rat brain and, in particular, in subregions of the hippocampus and subiculum. In vitro, somatostatin and related peptides, including seglitide (MK-678), hyperpolarize subicular neurones of the burst firing type-a response, which may have functional consequences for their output. One major projection from the subiculum is to the nucleus accumbens. The functional consequence of somatostatin receptor stimulation in the ventral subiculum has been assessed by measuring extracellular levels of dopamine in the ipsilateral nucleus accumbens. In anaesthetised rats, administration of seglitide (MK-678), a somatostatin analogue with selectivity for the SRIF-1 receptor (comprising somatostatin sst2, sst3 and sst5 subtypes) significantly increased extracellular levels of dopamine in the ipsilateral nucleus accumbens shell. The result suggests that hyperpolarization of subicular neurones by MK-678 may lead to activation of the subiculo-accumbens projection system, and an associated increase in dopaminergic function.

Chemical stimulation of the ventral hippocampus elevates nucleus accumbens dopamine by activating dopaminergic neurons of the ventral tegmental area

Dual-probe microdialysis (with HPLC and electrochemical detection) in freely moving rats and single-unit recording in anesthetized rats were used to study the extent to which impulse flow through the ventral tegmental area (VTA) contributes to elevations in nucleus accumbens (NAS) dopamine (DA) evoked by stimulation of the ventral subiculum (VS). During perfusion of artificial extracellular fluid into the VTA, injections of 0.74 microgram of the excitatory amino acid NMDA into the VS elevated accumbens DA to >150% of basal values. During intra-VTA perfusion of either 1 microM tetrodotoxin (which blocks impulse flow) or 1 mM kynurenic acid (which blocks excitatory glutamate receptors), injections of NMDA into the VS failed to elevate accumbens DA. Thus, increased impulse flow through VTA DA neurons, mediated by excitatory glutamate inputs to this region, appears critical for VS stimulation to elevate NAS DA. Increased impulse flow through VTA DA neurons was confirmed using single-unit recording in anesthetized rats. Intra-VS NMDA injections increased the firing rates of 45% (14 of 31), decreased the firing rates of 13% (4 of 31), and had no effect on 42% (13 of 31) of VTA DA neurons. Increases in firing rates were evident within 15 min of NMDA injections, a time at which VS NMDA injections elevate accumbens DA in awake animals. The results of the present experiments identify the VTA as a critical site through which outputs from the VS modulate NAS dopaminergic neurotransmission.

Locomotor activity and accumbens Fos expression driven by ventral hippocampal stimulation require D1 and D2 receptors

Numerous studies have suggested that excitatory projections from the ventral hippocampus to the nucleus accumbens modulate locomotor activity in rats. Furthermore, the ability of ventral hippocampal neurons to alter locomotor activity may involve the dense dopaminergic innervation found in the nucleus accumbens. The purpose of this study was to: (i) more fully characterize the locomotor effects of acute alterations in ventral hippocampal activity; (ii) ascertain the influence of dopamine agonists and antagonists on locomotor changes produced by altered ventral hippocampal activity; and (iii) use immediate early gene induction to determine whether dopamine antagonists alter the response of nucleus accumbens neurons to ventral hippocampal stimulation. By comparing a variety of excitatory amino acid agonists, it was found that ventral hippocampal infusion of N-methyl-D-aspartate elevated locomotor activity in a subconvulsive manner, while other excitatory amino acid receptor agonists did not. Inactivation of the ventral hippocampus achieved by lidocaine infusion did not suppress ongoing locomotor activity, nor did it affect amphetamine-induced increases in locomotor activity. Increases in locomotor activity induced by ventral hippocampal N-methyl-D-aspartate infusion were blocked by systemic administration of haloperidol (a D2 receptor antagonist), SCH-23390 (a D1 receptor antagonist) or reserpine. Cellular expression of the protein product of the immediate early gene, c-fos, was dramatically increased in the nucleus accumbens shell after ventral hippocampal N-methyl-D-aspartate infusion, and haloperidol, SCH-23390 and reserpine attenuated this effect. These results suggest that the increases, but not decreases, in ventral hippocampal activity have a measurable effect on ongoing rates of locomotion, and that this effect requires both D1 and D2 receptors. Moreover, the studies of Fos expression suggest that dopamine receptor antagonists attenuate neuronal responses to ventral hippocampal stimulation within the nucleus accumbens, a brain region important in the generation and maintenance of locomotor activity.

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.

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.

Impaired learning and memory and altered hippocampal neurodevelopment resulting from interleukin-2 gene deletion

Interleukin-2 (IL-2), the protypical T cell growth factor and immunoregulatory cytokine produced by lymphocytes, has been implicated as a brain neurotrophic factor and neuromodulator. The consequences of the absence of endogenous IL-2 on brain development and function were unknown. Brain IL-2 receptors are enriched in the hippocampal formation, an area critical for the acquisition and consolidation of spatial learning and memory. Thus, we tested the hypothesis that mice lacking IL-2 would exhibit alterations in hippocampal-dependent learning and neurodevelopment. Compared with C57BL/6-IL-2+/+ wild-type mice, we observed that C57BL/6-IL-2-/- gene knockout mice had markedly impaired spatial learning and memory in the Morris water maze. No significant deficits in parameters of learning and memory performance were found in severe combined immunodeficient (SCID) mice (C57BL/6scid), however, suggesting that the impaired spatial learning and memory exhibited by IL-2 knockout mice is not attributable to generalized immunodeficiency resulting from the absence of endogenous IL-2. Examination of other domains of behavioral performance showed that the IL-2 knockout and wildtype mice did not differ in measures of fearfulness or locomotor activity in an elevated plus maze, or in reflexive startle responses to auditory stimuli--although prepulse inhibition of acoustic startle (PPI) was increased significantly in IL-2 knockout mice. The spatial learning and memory impairment in IL-2 knockout mice was accompanied by reductions in hippocampal infrapyramidal mossy neuronal fiber length, a factor shown previously to correlate positively with spatial learning ability. These findings indicate that, in addition to being a pivotal cytokine in immune regulation, IL-2 may play a role in the development and regulation of brain neurons involved in spatial learning and memory.