Carbachol infusion into the dentate gyrus disrupts sensorimotor gating of startle in the rat

Western diet consumption impairs memory function via dysregulated hippocampus acetylcholine signaling

Western diet (WD) consumption during early life developmental periods is associated with impaired memory function, particularly for hippocampus (HPC)-dependent processes. We developed an early life WD rodent model associated with long-lasting HPC dysfunction to investigate the neurobiological mechanisms mediating these effects. Rats received either a cafeteria-style WD (ad libitum access to various high-fat/high-sugar foods; CAF) or standard healthy chow (CTL) during the juvenile and adolescent stages (postnatal days 26-56). Behavioral and metabolic assessments were performed both before and after a healthy diet intervention period beginning at early adulthood. Results revealed HPC-dependent contextual episodic memory impairments in CAF rats that persisted despite the healthy diet intervention. Given that dysregulated HPC acetylcholine (ACh) signaling is associated with memory impairments in humans and animal models, we examined protein markers of ACh tone in the dorsal HPC (HPCd) in CAF and CTL rats. Results revealed significantly lower protein levels of vesicular ACh transporter in the HPCd of CAF vs. CTL rats, indicating chronically reduced ACh tone. Using intensity-based ACh sensing fluorescent reporter (iAChSnFr) in vivo fiber photometry targeting the HPCd, we next revealed that ACh release during object-contextual novelty recognition was highly predictive of memory performance and was disrupted in CAF vs. CTL rats. Neuropharmacological results showed that alpha 7 nicotinic ACh receptor agonist infusion in the HPCd during training rescued memory deficits in CAF rats. Overall, these findings reveal a functional connection linking early life WD intake with long-lasting dysregulation of HPC ACh signaling, thereby identifying an underlying mechanism for WD-associated memory impairments.

Western diet consumption impairs memory function via dysregulated hippocampus acetylcholine signaling

Western diet (WD) consumption during development yields long-lasting memory impairments, yet the underlying neurobiological mechanisms remain elusive. Here we developed an early life WD rodent model to evaluate whether dysregulated hippocampus (HPC) acetylcholine (ACh) signaling, a pathology associated with memory impairment in human dementia, is causally-related to WD-induced cognitive impairment. Rats received a cafeteria-style WD (access to various high-fat/high-sugar foods; CAF) or healthy chow (CTL) during the juvenile and adolescent periods (postnatal days 26-56). Behavioral, metabolic, and microbiome assessments were performed both before and after a 30-day healthy diet intervention beginning at early adulthood. Results revealed CAF-induced HPC-dependent contextual episodic memory impairments that persisted despite healthy diet intervention, whereas CAF was not associated with long-term changes in body weight, body composition, glucose tolerance, anxiety-like behavior, or gut microbiome. HPC immunoblot analyses after the healthy diet intervention identified reduced levels of vesicular ACh transporter in CAF vs. CTL rats, indicative of chronically reduced HPC ACh tone. To determine whether these changes were functionally related to memory impairments, we evaluated temporal HPC ACh binding via ACh-sensing fluorescent reporter in vivo fiber photometry during memory testing, as well as whether the memory impairments could be rescued pharmacologically. Results revealed dynamic HPC ACh binding during object-contextual novelty recognition was highly predictive of memory performance and was disrupted in CAF vs. CTL rats. Further, HPC alpha-7 nicotinic receptor agonist infusion during consolidation rescued memory deficits in CAF rats. Overall, these findings identify dysregulated HPC ACh signaling as a mechanism underlying early life WD-associated memory impairments.

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.

Sensorimotor gating of the startle reflex: what we said 25 years ago, what has happened since then, and what comes next

Our 1992 paper, 'The neural substrates of sensorimotor gating of the startle reflex: a review of recent findings and their implications', reviewed a series of (then) new and preliminary findings from cross-species studies of prepulse inhibition of the startle reflex, and commented on their implications. At the time that the report was composed, PubMed listed about 40 citations for studies using the search term 'prepulse inhibition'. In the ensuing 25 years, the field has added about 2700 such reports, reflecting the substantial growth in interest in prepulse inhibition and its utility across a number of different experimental applications. The 30th anniversary of the Journal of Psychopharmacology provides an opportunity to comment briefly on what was described in that 1992 report, how the field has progressed in the subsequent decades, and the paths forward for studies of prepulse inhibition and its use as an operational measure of sensorimotor gating. Among these future paths, we highlight the use of prepulse inhibition as: an endophenotype for genomic studies, and a biomarker for healthy brain circuitry, which may predict sensitivity to psychotherapeutics. Our 1992 report was highly speculative and based on paper-thin empirical data, yet viewed in a certain light, it appears to have contained a basic roadmap for a journey spanning the next 25 years of prepulse inhibition research… and 'what a long, strange trip it's been'.

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

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

Differential effects of the antidepressant mirtazapine on amphetamine- and dizocilpine-induced PPI deficits

Prepulse inhibition (PPI) refers to the decrease in motor startle response to salient sensory stimuli (pulses) when they are closely preceded in time by another more modest sensory stimulus (prepulse). PPI deficits can be induced by stimulation of dopamine receptors (e.g., amphetamine or apomorphine) or blockade of NMDA glutamate receptors (e.g., dizocilpine or PCP). Previously we found that antagonists of α(2)-noradrenergic and H(1)-histaminergic receptors significantly attenuate PPI impairments caused by amphetamine or dizocilpine. In the current study we assessed the effects of the antidepressant mirtazapine, which has combined antagonist effects at α(2)-noradrenergic, H(1)-histaminergic and 5-HT serotonergic receptors, on amphetamine- and dizocilpine-induced PPI deficits. In Experiment 1, rats were tested for PPI of the startle response to a tactile air-puff stimulus after auditory prepulses of three different intensities. Drug treatments consisted of combinations of amphetamine (0 and 1mg/kg) and mirtazapine (0, 0.5, 1, 2, and 5mg/kg), with all rats receiving all drug doses and combinations with different counterbalanced orders. In Experiment 2, a different group of rats was tested with drug treatments consisting of combinations of dizocilpine (0 and 0.05 mg/kg) and mirtazapine (0, 0.5, 1, 2, and 5 mg/kg). In Experiment 1 amphetamine (1 mg/kg) significantly reduced PPI whereas mirtazapine caused the opposite effect, with the highest dose of mirtazapine (5 mg/kg) effectively reversing the amphetamine-induced PPI deficit. In Experiment 2 dizocilpine (0.05 mg/kg) significantly reduced PPI, but mirtazapine did not have a significant effect on the inhibition of the startle response. These results indicate that the potential beneficial effects of combined α-adrenergic, 5-HT, and H(1) receptor blockade in counteracting PPI deficits may be associated to cases of sensorimotor gating disorders mediated by dopamine, but not necessarily to NMDA glutamate-induced PPI impairments.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Isolation rearing in rats: effect on expression of synaptic, myelin and GABA-related immunoreactivity and its utility for drug screening via the subchronic parenteral route

Depriving weaned rats of social contact by rearing them in isolation brings about a spectrum of behavioural and neuropathological changes in adulthood which resemble some of the characteristics observed in schizophrenia. Hence, isolation rearing provides a non-pharmacological means to induce in an animal model certain aspects of schizophrenia with a neurodevelopmental origin. We compared the prepulse inhibition and locomotor activity behaviours in group-reared and isolation-reared rats in the context of determining the robustness of any behavioural changes following a subchronic parenteral drug administration protocol. The expression of synaptic, myelin and GABA-related proteins was also assessed in the brains of these rats using semi-quantitative fluorescence immunohistochemistry. Compared to their group-reared counterparts, isolation-reared rats displayed disruption in prepulse inhibition which was lost after repeated testing and subchronic vehicle administration. However, isolation-reared rats showed open-field hyperlocomotion post-subchronic vehicle treatment compared to group-reared rats. Isolation rearing resulted in reduced expression of synaptophysin, synapsin I, myelin basic protein and GABA(B1) receptor proteins, along with an increase in 2',3'-cyclic nucleotide 3'-phosphodiesterase. Of the brain areas examined these observed changes were localised to the hippocampal regions and the substantia nigra. These results suggest an alteration in the synaptic, myelin and GABA-related functions in the brains of isolation-reared rats that displayed behavioural anomalies. Since dysfunction in these systems has also been implicated in schizophrenia, our findings provide additional evidence to support the use of isolation rearing for schizophrenia research; however, its use in the screening of putative antipsychotics following subchronic administration needs to be undertaken warily.

A chronic iron-deficient/high-manganese diet in rodents results in increased brain oxidative stress and behavioral deficits in the morris water maze

Iron deficiency (ID) is especially common in pregnant women and may even persist following childbirth. This is of concern in light of reports demonstrating that ID may be sufficient to produce homeostatic dysregulation of other metals, including manganese (Mn). These results are particularly important considering the potential introduction of the Mn-containing gas additive, methyl cyclopentadienyl manganese tricarbonyl (MMT), in various countries around the world. In order to model this potentially vulnerable population, we fed female rats fed either control (35 mg Fe/kg chow; 10 mg Mn/kg chow) or low iron/high-manganese (IDMn; 3.5 mg Fe/kg chow; 100 mg Mn/kg chow) diet, and examined whether these changes had any long-term behavioral effects on the animals' spatial abilities, as tested by the Morris water maze (MWM). We also analyzed behavioral performance on auditory sensorimotor gating utilizing prepulse inhibition (PPI), which may be related to overall cognitive performance. Furthermore, brain and blood metal levels were assessed, as well as regional brain isoprostane production. We found that treated animals were slightly ID, with statistically significant increases in both iron (Fe) and Mn in the hippocampus, but statistically significantly less Fe in the cerebellum. Additionally, isoprostane levels, markers of oxidative stress, were increased in the brain stem of IDMn animals. Although treated animals were indistinguishable from controls in the PPI experiments, they performed less well than controls in the MWM. Taken together, our data suggest that vulnerable ID populations exposed to high levels of Mn may indeed be at risk of potentially dangerous alterations in brain metal levels which could also lead to behavioral deficits.

Gap Detection Methods for Assessing Salicylate-Induced Tinnitus and Hyperacusis in Rats

Purpose

A variety of options for behavioral assessment of tinnitus in laboratory animals are available to researchers today. These options are briefly reviewed, followed by data suggesting that gap detection procedures might be used to efficiently measure acute, salicylate-induced tinnitus and possibly hyperacusis in rats.

Method

Fischer Brown Norway rats ( n = 10) were given intraperitoneal injections of 350 mg/kg sodium salicylate on 2 consecutive days, and the effects on gap detection were observed across 9 different frequency bands. Pretest, posttest, and washout data were collected. An additional 4 rats were each given 4 different doses of sodium salicylate (0, 150, 250, and 300 mg/kg), and gap detection and prepulse inhibition were measured.

Results

Significant gap detection deficits were observed from pre- to posttest that were consistent with tinnitus. Consistent gap detection deficits were found using broadband noise backgrounds, while significant improvements in responding to frequency-specific test bands were found. Similar effects were repeated in the dose response portion of the study.

Conclusions

Gap detection procedures efficiently measured salicylate-induced changes in behavior that were consistent with the presence of tinnitus. In addition, the reliable, stronger responses at many frequencies after salicylate injections suggest the possibility of measuring a hyperacusis-like phenomenon using these methods.

Deficits in parvalbumin and calbindin immunoreactive cells in the hippocampus of isolation reared rats

Post-mortem studies have provided evidence for abnormalities of the gamma-aminobutyric acid (GABA)-ergic system in schizophrenia. The calcium-binding proteins (CBPs), parvalbumin (PV), calbindin (CB) and calretinin (CR) can be used as markers for specific subpopulations of GABAergic neurons in the brain. Isolation rearing of rats is a non-pharmacological, non-lesion manipulation that leads to deficits in prepulse inhibition of the startle reflex (PPI) and other behavioural and neurochemical alterations reminiscent of schizophrenia. Female rats were reared in social housing (groups of three) or singly for 11 weeks post weaning and PPI was measured. Brains were removed and hippocampal CBP- containing neurons determined following immunocytochemical staining. Compared to socially housed rats, isolated rats exhibited PPI deficits and reductions in PV and CB-immunoreactive cells in the hippocampus, with no significant change in CR. These findings demonstrate selective abnormalities of sub-populations of GABAergic interneurons in the hippocampus of isolation reared rats, which resemble the neuronal deficits seen in this region in schizophrenia.

Intrahippocampal injections of Tat: Effects on prepulse inhibition of the auditory startle response in adult male rats

The presence of human immunodeficiency virus (HIV-1) in the brain mediates the pathogenesis of HIV-associated dementia complex (HAD), partially through the viral toxins gp120 and Tat. This study characterized potential deficits in sensorimotor gating, as measured by prepulse inhibition (PPI), following hippocampal administration of Tat. Adult, male Sprague-Dawley rats were bilaterally injected with 50 microg Tat or saline (1 microl volume), into the hippocampus. Following 7 weeks of recovery, all animals were tested using the auditory startle response (ASR) with habituation, control, and PPI trials. Assessment of ASR habituation [100dB(A) white noise stimulus, 70dB(A) background, 5-min acclimation period, 36 habituation trials with fixed interstimulus interval (ISI) of 10 s] demonstrated a significant approximately 50% reduction in the overall peak ASR amplitude, but no change in peak ASR latency, nor an effect on the rate of habituation. PPI measures demonstrated robust alterations in sensorimotor gating. The PPI test (ISI of 0, 8, 40, 80, 120, or 4000 ms, 6-trial blocks, Latin-square) showed an attenuated response on peak ASR amplitude during the control trials (0 and 4000 ms ISI), but not on the PPI trials (8-120 ms ISI). Most striking was the rightward shift in ISI for maximal inhibition of the response (chi2(1)=4.7, p<or=0.03). There was no significant peak ASR latency effect during the control trials (<1 ms) of the PPI test, although there was the suggestion of a slowing of the response (4 ms, approximately 15%) across PPI trials. Collectively, the present data suggest that intrahippocampal injections of Tat have adverse effects on cognitive processing, as indexed by sensorimotor gating.

Neonatal hippocampal Tat injections: developmental effects on prepulse inhibition (PPI) of the auditory startle response

The current estimate of children (<15 years) living with HIV and AIDS is 2.2 million. The major source of infection occurs through vertical transmission of the virus from mother to child during delivery [UNAIDS/WHO, 2005. AIDS Epidemic Update. UNAIDS, Geneva]. Recent studies have shown that timing of HIV-1 infection might be related to the onset and rate of progression of CNS disease. The effects of HIV on the brain are thought to be mediated indirectly through the viral toxins Tat and gp120. This study characterized developmental effects on PPI following intrahippocampal administration of Tat. On postnatal day (P)1, one male and one female pup from each of eight Sprague-Dawley litters were bilaterally injected with 50 microg Tat or saline (1 microl volume). Animals were tested for PPI of the auditory startle response (ASR) (ISIs of 0, 8, 40, 80, 120, and 4000 ms, six trial blocks, Latin-square design) on days 30, 60 and 90. Tat altered PPI and the pattern of alterations was different for males and females. For males, a leftward shift was evident in the ISI for maximal inhibition of the response on day 30 and on day 60 (chi(2)(1)=4.7, p< or =.03, and chi(2)(1)=5.3, p< or =.02, respectively), but not on day 90. For females, Tat altered peak ASR latency across PPI trials (8-120 ms) at all days of testing (30, 60, and 90 days of age), as indexed by orthogonal component analyses, indicating less modulation of PPI by ISI. Data collected from a second group that were tested only once at 90 days of age, suggested that the observed adverse Tat effects for males and females early in development were maintained with age. Thus, the diminishing TAT effect on PPI at day 90 in a longitudinal study design was attributed to repeated testing, rather than 'recovery of function'. Collectively, the data suggested that hippocampal Tat injections in neonatal rats produced alterations in the pre-attentive process of sensorimotor gating, as indexed by PPI.

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.

Gap detection deficits in rats with tinnitus: A potential novel screening tool

The study describes a novel method for tinnitus screening in rats by use of gap detection reflex procedures. The authors hypothesized that if a background acoustic signal was qualitatively similar to the rat's tinnitus, poorer detection of a silent gap in the background would be expected. Rats with prior evidence of tinnitus at 10 kHz (n = 14) exhibited significantly worse gap detection than controls (n = 13) when the gap was embedded in a background similar to their tinnitus. No differences between tinnitus and control rats were found with 16 kHz or broadband noise backgrounds, which helped to rule out explanations related to hearing loss or general performance deficits. The results suggest that gap detection reflex procedures might be effective for rapid tinnitus screening in rats.

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.

Delayed onset of prepulse inhibition deficits following kainic acid treatment on postnatal day 7 in rats

Abnormal activity in corticolimbic circuits during development may be a predisposing factor for schizophrenia. Permanent or temporary lesions of limbic structures such as the ventral hippocampus and basolateral amygdala in rats on postnatal day (PND) 7 result in functional changes similar to some behavioural and cognitive signs of schizophrenia. The present experiments tested whether transient increases in the neural activity of corticolimbic circuits on PND 7 would result in similar behavioural changes. Long-Evans rats were treated with either kainic acid (KA, 1.5 mg/kg, i.p.) or saline on PND 7 and tested for prepulse inhibition (PPI) of the acoustic startle response and spontaneous locomotor activity both in a novel environment and following amphetamine treatment before puberty (PND 35) and in early adulthood (PND 56). In subgroups of animals PPI was also measured following apomorphine administration (0.2 mg/kg) and spatial learning and memory were tested in the water maze. Rats treated with KA were indistinguishable from saline-treated animals on PND 35. However, on PND 56, KA-treated animals showed a subtle consistent decrease in PPI relative to control animals, but did not show increased sensitivity to the disruptive effects of a low dose of apomorphine on PPI. Locomotor responses to novelty or amphetamine were not reliably altered in the KA-treated animals. KA- and saline-treated animals performed similarly in the water maze. These results support the hypothesis that neural hyperactivity on PND 7 in rats causes behavioural changes in early adulthood that resemble some symptoms of schizophrenia. These pharmacological data suggest that the changes are not mediated by postsynaptic alterations in mesolimbic dopamine transmission.

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.

Effects of acute procyclidine administration on prepulse inhibition of the startle response in schizophrenia: a double-blind, placebo-controlled study

Prepulse inhibition (PPI) of the startle response refers to a reduction in response to a strong stimulus (pulse) if this is preceded shortly by a weak non-startling stimulus (prepulse). Consistent with theories of deficiencies in early stages of information processing, PPI is found to be reduced in patients with schizophrenia. Atypical antipsychotics are found to be more effective than typical antipsychotics in improving PPI in this population. Anticholinergic drugs are often used to control extrapyramidal symptoms induced by antipsychotic medication, especially by typical antipsychotics, in schizophrenic patients and are known to disrupt cognitive functions in both normal and schizophrenic populations. The effect of anticholinergics on PPI in schizophrenia has not yet been examined. This study determined the effects of procyclidine, an anticholinergic drug, on PPI in patients with schizophrenia given risperidone or quetiapine and not on any anticholinergic drugs, employing a placebo-controlled, cross-over design. Under double-blind conditions, subjects were administered oral 15 mg procyclidine and placebo on separate occasions, 2 weeks apart, and tested for acoustic PPI (prepulse 8 dB and 15 dB above the background and delivered with 30-ms, 60-ms and 120-ms prepulse-to-pulse intervals). Procyclidine significantly impaired PPI compared to placebo (assessed as percentage reduction) with 60-ms prepulse-to-pulse trials and increased the latencies to response peak across all trials. The use of anticholinergics needs to be carefully controlled/examined in investigations of information processing deficits using a PPI model and reduced to the minimum level in clinical care of schizophrenia.

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.

Reduced prepulse inhibition in rats with entorhinal cortex lesions

The relationship between the entorhinal cortex and prepulse inhibition (PPI) as well as dopaminergic participation in this relationship were examined. PPI is an operational measure of sensorimotor gating in which a robust response to a startling auditory pulse stimulus is inhibited when the stimulus is preceded by a weak prepulse. PPI can be measured in various species and is reduced in several neuropsychiatric disorders and in dopamine-activated rats. The entorhinal cortex was damaged bilaterally using ibotenic acid, and acoustic startle experiments were performed during treatment with haloperidol or saline on day 21 after the ibotenic acid injection. Neither this injection nor haloperidol affected the amplitude of the startle movement. Bilateral entorhinal cortex lesions reduced PPI, while haloperidol partially restored it. The entorhinal cortex and the sensorimotor gating system therefore may be related via dopaminergic circuits, possibly including the nucleus accumbens. Further, as the entorhinal cortex provides the major extrinsic synaptic input to the rat hippocampus, disease involvement of this region may severely affect cognition in various disorders including schizophrenia.

Early maternal deprivation and prepulse inhibition: the role of the postdeprivation environment

Early postnatal maternal deprivation leads to a variety of biochemical and behavioural alterations in the offspring, some of which do not develop until adulthood, like deficits in prepulse inhibition. Since a number of these deficits are similar to abnormalities observed in schizophrenic patients, maternal deprivation has been proposed as an interesting model for schizophrenia. However, little is still known about the processes that determine these long-term consequences. Previous experiments showed that the strain of rats and the deprivation procedure are important factors. In the present set of experiments, we focussed on the postdeprivation period. We showed that rearing normal Wistar rats in social isolation from weaning disrupts prepulse inhibition. However, if maternally deprived Wistar rats were reared in social isolation, the prepulse inhibition was normal. We further showed that if only half of the litters were maternally deprived at postnatal day 9, the animals had only a small disruption in prepulse inhibition compared to animals that came from litters where all the animals were deprived. In a final experiment, we crossfostered maternally deprived mothers to nondeprived pups and vice versa. This experiment showed that both the nondeprived pups raised by a deprived mother and the deprived pups raised by a nondeprived mother had small deficits in prepulse inhibition. Taken together, these data clearly show that the postdeprivation period is of crucial importance for the development of prepulse inhibition deficits in maternally deprived rats. We present a working model in order to explain the long-term behavioural consequences of maternal deprivation.

Metabotropic glutamate receptors in the hippocampus and nucleus accumbens are involved in generating seizure-induced hippocampal gamma waves and behavioral hyperactivity

The involvement of metabotropic glutamate receptor (mGluR) subtypes in the generation of hippocampal EEG (30-100 Hz) and behaviors induced by a hippocampal afterdischarge (AD) was examined in freely behaving rats. A hippocampal AD induced an increase in gamma waves (30-100 Hz) for 20 min, accompanied by behavioral hyperactivity. Bilateral intracerebroventricular (i.c.v.) infusion of (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG), a group I and II mGluR antagonist, 30 min before a hippocampal AD, significantly suppressed both the increase in gamma waves and the behavioral hyperactivity. The hippocampal theta rhythm, the spontaneous hippocampal gamma waves, and evoked field potential oscillations of approximately 40 Hz were not affected by MCPG. Pre-infusion (i.c.v.) of (2S)-alpha-ethylglutamic acid (EGLU; a group II mGluR antagonist), but not (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; a group I mGluR antagonist), suppressed the postictal increase of both hippocampal gamma waves and behaviors. MCPG was infused locally into different brain structures in order to specify its target sites. Intra-hippocampal infusion of MCPG, or EGLU, blocked the increase in both gamma waves and behaviors. Infusion of MCPG into the nucleus accumbens suppressed the postictal behavioral hyperactivity without affecting the increase in hippocampal gamma waves. MCPG injected into the medial septum blocked neither postictal gamma activity nor behavioral hyperactivity. It is suggested that the group II mGluRs in the hippocampus are involved in generation of the postictal hippocampal gamma waves, while behavioral hyperactivity is partly mediated by mGluRs in the nucleus accumbens. However, spontaneous gamma and theta waves in the normal hippocampus are not mediated by mGluRs.

The role of hippocampal dopamine receptors in prepulse inhibition

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

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

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

Behavioral effects of psychomotor stimulants in rats with dorsal or ventral subiculum lesions: locomotion, cocaine self-administration, and prepulse inhibition of startle

Compelling evidence suggests a primary role for the mesoaccumbens dopaminergic pathway in the behavioral effects of amphetamine and cocaine, but the roles of other projections to the accumbens, including those arising in the hippocampal formation, are less clear. The authors evaluated the effects of discrete excitotoxic lesions of either the dorsal or ventral subiculum on the locomotor activating, reinforcing, and sensorimotor gating-disruptive effects of psychomotor stimulant drugs. Whereas dorsal subiculum-lesioned rats were hyperactive in tests of exploratory locomotion and startle reactivity, ventral subiculum-lesioned rats exhibited an attenuated locomotor response to amphetamine, moderately impaired acquisition of cocaine self-administration, and reduced levels of prepulse inhibition of startle. These 2 behavioral profiles overlap considerably with those previously observed in rats with lesions of the rostrodorsal and caudomedial accumbens, respectively, and suggest that projections from dorsal subiculum to accumbens core and ventral subiculum to accumbens shell exert distinct influences on behavioral responses that are amplified by psychomotor stimulant drugs.

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.

Structural and functional abnormalities of the hippocampal formation in rats with environmentally induced reductions in prepulse inhibition of acoustic startle

The effects of social isolation on prepulse inhibition of acoustic startle (PPI), electrophysiology and morphology of subicular pyramidal neurons and the densities of interneuronal sub-types in the hippocampal formation were examined. Wistar rats (male weanlings) were housed socially (socials, n=8) or individually (isolates, n=7). When tested eight weeks later, PPI was lower in isolates. Rats then received terminal anaesthesia before slices of hippocampal formation were made in which the electrophysiological properties of a total of 108 subicular neurons were characterized. There were no differences in neuronal sub-types recorded in socials compared with isolates. Intrinsically burst-firing and regular spiking pyramidal neurons were examined in detail. There were no differences in resting membrane potential or input resistance in isolates compared with socials but action potential height was reduced and action potential threshold raised in isolates. A limited morphological examination of Neurobiotin-filled intrinsically burst-firing neurons did not reveal differences in cell-body area or in number of primary dendrites. Sections from the contralateral hemispheres of the same rats were stained with antibodies to calretinin, parvalbumin and the neuronal isoform of nitric oxide synthase (nNOS). In isolates, the density of calretinin positive neurons was increased in the dentate gyrus but unchanged in areas CA3, CA1 and subiculum. Parvalbumin and nNOS positive neuronal densities were unchanged. Hence in rats with environmentally induced reductions in PPI there are structural and functional abnormalities in the hippocampal formation. If the reduction in PPI stems from these abnormalities, and reduced PPI in rats is relevant to schizophrenia, then drugs that correct the reported electrophysiological changes might have antipsychotic effects.

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.

Evidence for the involvement of the hippocampus in the pathophysiology of schizophrenia

The hippocampus, a medial temporal lobe structure, is often considered to play an important role in the pathophysiology of schizophrenia. Recent developments of neuroimaging and molecular postmortem techniques have significantly increased our ability to study the role of discrete brain regions in the pathophysiology of schizophrenia. This article describes animal models, structural, histological, molecular biology, and neuropsychological evidence for the involvement of the hippocampus in the pathophysiology of schizophrenia. The major findings in schizophrenic patients are decreased volumes, hypometabolism, and cytoarchitectural abnormalities which are more robust on the left hippocampus, as well as verbal memory impairment. It is yet to be determined whether these changes are neurodevelopmental or neurodegenerative in nature. Overall, these findings indicate that there are subtle changes in the hippocampus of schizophrenic patients. More comprehensive and focused hippocampal research in schizophrenia is required to elucidate the contribution of this intriguing brain structure to the pathophysiology of schizophrenia.

Toward understanding the biology of a complex phenotype: rat strain and substrain differences in the sensorimotor gating-disruptive effects of dopamine agonists

Sensorimotor gating, measured by prepulse inhibition (PPI) of the startle reflex, is reduced in schizophrenia patients and in rats treated with dopamine agonists. Strain differences in the sensitivity to the PPI-disruptive effects of dopamine agonists may provide insight into the genetic basis for human population differences in sensorimotor gating. We reported strain differences in the sensitivity to the PPI-disruptive effects of the D1/D2 agonist apomorphine in adult rats, with greater sensitivity in Harlan Sprague Dawley (SDH) versus Wistar (WH) rats. However, Kinney et al. (1999) recently reported opposite findings, using Bantin-Kingman Sprague Dawley (SDBK) and Wistar (WBK) rats; in fact, SDBK rats did not exhibit clear apomorphine-induced reductions in sensorimotor gating. These new findings of Kinney et al. (1999) directly conflict with over 15 years of results from our laboratories and challenge interpretations from a large body of literature. The present studies carefully assessed drug effects on sensorimotor gating in SD versus W strains, across rat suppliers (H vs BK). Significantly greater SDH than WH apomorphine sensitivity in PPI measures was observed in both adult and 18 d pups, confirming that these strain differences are both robust and innate. These strain differences in apomorphine sensitivity were not found in adult BK rats. Supplier differences in sensitivity (SDH > SDBK) were also evident in the PPI-disruptive effects of D1 but not D2-family agonists; PPI was clearly disrupted by quinpirole in both SDH and SDBK rats. These findings demonstrate robust, innate, neurochemically specific, and apparently heritable phenotypic differences in an animal model of sensorimotor gating deficits in human neuropsychiatric disorders.

Animal models of deficient sensorimotor gating: what we know, what we think we know, and what we hope to know soon

Sensorimotor gating of the startle reflex can be studied in humans and laboratory animals using measures of prepulse inhibition (PPI) of the startle reflex. PPI is reduced in patients with specific neuropsychiatric disorders and in rats after manipulation of the limbic cortex, striatum, pallidum or pontine tegmentum. Studies are rapidly identifying the neurochemical and neuroanatomical substrates regulating PPI in laboratory animals; this detailed circuit information has been used as a 'blueprint' to identify possible candidate substrates responsible for PPI deficits in psychiatrically disordered humans. In parallel, studies have also begun to assess the homology of pharmacological effects on PPI across species, as an initial step towards translating detailed neural circuit information from rats to humans. Despite this rapid progress, there is an increasing danger of overlooking important methodological and interpretative issues that could impact either positively or negatively on the ultimate utility of models based on measures of PPI. Some of these issues--ranging from the cross-species methods for quantifying specific variables to the relevance of genetic drift to animal and human studies of PPI--and their implications for future studies are the focus of this review.

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.

Cytotoxic lesion of the medial prefrontal cortex abolishes the partial reinforcement extinction effect, attenuates prepulse inhibition of the acoustic startle reflex and induces transient hyperlocomotion, while sparing spontaneous object recognition memory in the rat

The partial reinforcement extinction effect refers to the increase in resistance to extinction of an operant response acquired under partial reinforcement relative to that acquired under continuous reinforcement. Prepulse inhibition of the acoustic startle response refers to the reduction in startle reactivity towards an intense acoustic pulse stimulus when it is shortly preceded by a weak prepulse stimulus. These two behavioural phenomena appear to be related to different forms of attentional processes. While the prepulse inhibition effect reflects an inherent early attentional gating mechanism, the partial reinforcement extinction effect is believed to involve the development of acquired inattention, i.e. the latter requires the animals to learn about what to and what not to attend. Impairments in prepulse inhibition and the partial reinforcement extinction effect have been independently linked to the neuropsychology of attentional dysfunctions seen in schizophrenia. The proposed neural substrates underlying these behaviourial phenomena also appear to overlap considerably: both focus on the nucleus accumbens and emphasize the functional importance of its limbic afferents, including that originating from the medial prefrontal cortex, on accumbal output/activity. The present study demonstrated that cytotoxic medial prefrontal cortex lesions which typically damaged the prelimbic, the infralimbic and the dorsal anterior cingulate areas could lead to the abolition of the partial reinforcement extinction effect and the attenuation of prepulse inhibition. The lesions also resulted in a transient elevation of spontaneous locomotor activity. In contrast, the same lesions spared performance in a spontaneous object recognition memory test, in which the lesioned animals displayed normal preference for a novel object when the novel object was presented in conjunction with a familiar object seen 10 min earlier within an open field arena. The present results lend support to the hypothesis that medial prefrontal cortex dysfunction might be related to some forms of attentional abnormality central to the symptomatology of schizophrenia. Relevance of the present findings in relation to the neural substrates underlying the partial reinforcement extinction effect and prepulse inhibition is further discussed.

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.

Ontogeny of isolation rearing-induced deficits in sensorimotor gating in rats

Presentation of a weak stimulus immediately before a startling stimulus decreases the magnitude of the resultant startle response. This phenomenon, termed prepulse inhibition (PPI), provides an operational measure of sensorimotor gating, and is deficient in schizophrenia patients. Clinically observed PPI deficits can be modeled in rodents by housing rats individually from weaning until adulthood. The developmental time course of isolation rearing-induced PPI deficits, however, is unknown. The present studies characterized the ontogeny of isolation-induced PPI deficits and hyperactivity. Separate groups of Sprague-Dawley and Lister hooded rats were either singly housed (ISO) or socially housed (SOC, groups of two to three per cage) upon weaning and then maintained in these housing conditions for different periods of time until assessment of PPI and locomotor activity; animals were tested at time points that roughly corresponded to before puberty (2 weeks postweaning), during puberty (4 weeks postweaning), or after puberty (6-7 weeks post weaning). PPI deficits were seen in Sprague-Dawley ISO rats at either the 4- or 6-, but not the 2-week time points. In contrast, hyperactivity was noted in these animals starting at the 2-week time point. Lister rats showed the same general pattern of ISO-induced effects, with ISO-induced hyperactivity (observed 4 weeks postweaning) preceding ISO-induced PPI deficits (observed 7 weeks postweaning). Therefore, ISO produces dissociable effects on PPI and locomotor activity, with PPI deficits emerging only during or after puberty. ISO might thus provide a useful noninvasive tool with which to study the neural substrates of delayed-onset sensorimotor gating abnormalities.

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.

A hyperglycinergic rat model for the pathogenesis of schizophrenia: preliminary findings

There is evidence of high glycine concentrations in the brains and periphery of schizophrenics. In the forebrain, glycine plays a major role as a co-agonist with glutamate at the excitatory N-methyl-D-aspartate (NMDA) receptors. This activity of glycine is involved in the normal functioning of the brain in adulthood and during neurodevelopment, and it may also cause neurotoxicity and brain abnormalities when its concentrations are high. To test the hypothesis that the high glycine concentrations observed in schizophrenics play an etiologic role in schizophrenia, an animal model was tested where rats were made hyperglycinic from life in utero to adulthood. The hyperglycinic rats showed abnormalities in sensory gating mechanisms, enlarged cerebral ventricles and diminished hippocampal dimensions. All of these abnormalities closely parallel observations reported in patients with schizophrenic psychoses. These results from a rat model suggest an etiologic role for high glycine concentration in the behavior and brain abnormalities of schizophrenic patients.

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

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

Deficient sensorimotor gating following seizures in amygdala-kindled rats

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Impaired sensorimotor gating in patients with non-epileptic seizures

The aim of the study was to identify possible disturbances of sensorimotor gating and habituation of the eye blink startle response, in patients with non-epileptic seizures (NES). Prepulse inhibition (PPI) of the startle reflex, as an operational measure of sensorimotor gating and habituation was studied in 21 patients with NES and in 22 healthy control subjects. Six NES patients were taking antiepileptic drugs at the time of testing. PPI was significantly impaired in the NES group compared to the control group, with deficits being greater in unmedicated patients. There was a trend for medicated NES patients to show higher PPI than unmedicated patients, but this was not significant. Habituation was intact in both medicated and unmedicated NES patients. It is proposed that deficits of information processing related to sensorimotor gating in patients with NES may be associated with abnormalities within the limbic system-basal ganglia circuitry which has been shown to be the substrate of 'gating' assessed by PPI. It was also found that NES patients had greater psychopathology than the control group when rated by the Traumatic Symptom Checklist (TSC-40). Overall, the anxiety subscale was the only element that was negatively correlated with PPI. It is suggested that anxiety may contribute to impairment of PPI in patients with NES.

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.