Mark A. Geyer

Pharmacological studies of prepulse inhibition models of sensorimotor gating deficits in schizophrenia: a decade in review.

Abstract.Rationale: Patients with schizophrenia exhibit deficits in an operational measure of sensorimotor gating: prepulse inhibition (PPI) of startle. Similar deficits in PPI are produced in rats by pharmacological or developmental manipulations. These experimentally induced PPI deficits in rats are clearly not animal models of schizophrenia per se, but appear to provide models of sensorimotor gating deficits in schizophrenia patients that have face, predictive, and construct validity. In rodents, disruptions in PPI of startle are produced by: stimulation of D2 dopamine (DA) receptors, produced by amphetamine or apomorphine; activation of serotonergic systems, produced by serotonin (5-HT) releasers or direct agonists at multiple serotonin receptors; and blockade of N-methyl-D-aspartate (NMDA) receptors, produced by drugs such as phencyclidine (PCP). Accordingly, dopaminergic, serotonergic, and glutamatergic models of disrupted PPI have evolved and have been applied to the identification of potential antipsychotic treatments. In addition, some developmental manipulations, such as isolation rearing, have provided non-pharmacological animal models of the PPI deficits seen in schizophrenia. Objective: This review summarizes and evaluates studies assessing the effects of systemic drug administrations on PPI in rats. Methods: Studies examining systemic drug effects on PPI in rats prior to January 15, 2001 were compiled and organized into six annotated appendices. Based on this catalog of studies, the specific advantages and disadvantages of each of the four main PPI models used in the study of antipsychotic drugs were critically evaluated. Results: Despite some notable inconsistencies, the literature provides strong support for significant disruptions in PPI in rats produced by DA agonists, 5-HT2 agonists, NMDA antagonists, and isolation rearing. Each of these models exhibits sensitivity to at least some antipsychotic medications. While the PPI model based on the effects of direct DA agonists is the most well-validated for the identification of known antipsychotics, the isolation rearing model also appears to be sensitive to both typical and atypical antipsychotics. The 5-HT PPI model is less generally sensitive to antipsychotic medications, but can provide insight into the contribution of serotonergic systems to the actions of newer antipsychotics that act upon multiple receptors. The deficits in PPI produced by NMDA antagonists appear to be more sensitive to clozapine-like atypical antipsychotics than to typical antipsychotics. Hence, despite some exceptions to this generalization, the NMDA PPI model might aid in the identification of novel or atypical antipsychotic medications. Conclusions: Studies of drug effects on PPI in rats have generated four distinctive models that have utility in the identification of antipsychotic medications. Because each of these models has specific advantages and disadvantages, the choice of model to be used depends upon the question being addressed. This review should help to guide such decisions.

Dopaminergic stimulation disrupts sensorimotor gating in the rat.

Prepulse inhibition is a cross-species phenomenon in which reflex responses to discrete sensory events are modified by weak prestimulation. In experiments designed to investigate the neuropharmacological mechanism of this form of information processing, and its relevance to schizophrenic psychopathology, apomorphine (0.125–4.0 mg/kg) and d-amphetamine (0.5–4.0 mg/kg) were administered to rats in an attempt to modify prepulse inhibition of the acoustic startle response. Rats were presented with 40 ms, 118 dB[A] acoustic pulses which were intermittently preceded by a weak 80 dB[A] acoustic prepulse. Both apomorphine and d-amphetamine induced a significant loss of prepulse inhibition, as reflected by increased pulse-preceded-by-prepulse versus pulse-alone startle magnitudes. Haloperidol (0.1 mg/kg), a specific D2 dopamine receptor antagonist, prevented the effects of 2.0 mg/kg apomorphine on prepulse inhibition, while having little effect by itself. An additional study investigated the effects of chronic intermittent administration of 2.5 mg/kg d-amphetamine. Rats given amphetamine for 8 consecutive days also displayed a loss of prepulse inhibition, with no evidence of tolerance. Finally, prepulse inhibition was examined under high- and low-intensity startle stimulus conditions; apomorphine (1.0 mg/kg) induced a loss of prepulse inhibition under both intensity conditions in approximately equal proportion. The results of these studies suggest a connection between sensorimotor gating, as measured by prepulse inhibition, and dopaminergic overactivity, supporting suggestions that information processing deficits in schizophrenia may be responsible for some psychotic symptoms and their effective treatment by antipsychotic D2 dopamine antagonists.

Habituation revisited: an updated and revised description of the behavioral characteristics of habituation.

The most commonly cited descriptions of the behavioral characteristics of habituation come from two papers published almost 40 years ago [Groves, P. M., & Thompson, R. F. (1970). Habituation: A dual-process theory. Psychological Review, 77, 419-450; Thompson, R. F., & Spencer, W. A. (1966). Habituation: A model phenomenon for the study of neuronal substrates of behavior. Psychological Review, 73, 16-43]. In August 2007, the authors of this review, who study habituation in a wide range of species and paradigms, met to discuss their work on habituation and to revisit and refine the characteristics of habituation. This review offers a re-evaluation of the characteristics of habituation in light of these discussions. We made substantial changes to only a few of the characteristics, usually to add new information and expand upon the description rather than to substantially alter the original point. One additional characteristic, relating to long-term habituation, was added. This article thus provides a modern summary of the characteristics defining habituation, and can serve as a convenient primer for those whose research involves stimulus repetition.

Cognitive dysfunction in psychiatric disorders: characteristics, causes and the quest for improved therapy

Studies of psychiatric disorders have traditionally focused on emotional symptoms such as depression, anxiety and hallucinations. However, poorly controlled cognitive deficits are equally prominent and severely compromise quality of life, including social and professional integration. Consequently, intensive efforts are being made to characterize the cellular and cerebral circuits underpinning cognitive function, define the nature and causes of cognitive impairment in psychiatric disorders and identify more effective treatments. Successful development will depend on rigorous validation in animal models as well as in patients, including measures of real-world cognitive functioning. This article critically discusses these issues, highlighting the challenges and opportunities for improving cognition in individuals suffering from psychiatric disorders.

Startle response models of sensorimotor gating and habituation deficits in schizophrenia

Studies of prepulse inhibition and habituation of startle responses elicited by intense stimuli provide some unusual opportunities for cross-species explorations of attentional deficits characteristic of schizophrenic patients. Schizophrenic patients exhibit deficits in both the prepulse inhibition of startle and the habituation of startle. The behavioral plasticity of startle responses and the comparability of the test paradigms used in rats and humans greatly facilitates the development of animal models of specifiable behavioral abnormalities in schizophrenic patients. This review describes two such examples of parallel animal and human models, one involving sensorimotor gating and the other examining behavioral habituation. Evidence is presented supporting the involvement of mesolimbic dopaminergic systems in the modulation of prepulse inhibition or sensorimotor gating and the importance of central serotonergic systems in the habituation of startle.

Isolation rearing of rats produces a deficit in prepulse inhibition of acoustic startle similar to that in schizophrenia

Schizophrenic patients exhibit deficits in the prepulse inhibition of startle, an operational measure of the sensorimotor gating deficits that are theorized to contribute to cognitive disorganization. In rats, the activation of mesolimbic dopamine (DA) disrupts prepulse inhibition, providing a useful model of the similar deficits in sensorimotor gating in schizophrenic patients. Rats reared in isolation exhibit neurochemical and behavioral abnormalities suggestive of hyperactivity in mesolimbic DA systems. In the present studies, rats reared in social groups or in isolation were tested in startle response paradigms using 120 or 105 dB acoustic pulses, some of which were preceded (100 msec) by prepulses that were 2, 4, 8, or 16 dB above the 65 dB background. Isolation-reared animals were hyperreactive only in response to the initial few startle stimuli. The amount of prepulse inhibition was decreased significantly in isolation-reared animals, particularly when midrange 8 dB prepulses were used. A subsequent study replicated the effect of isolation rearing on prepulse inhibition and suggested that the deficit in sensorimotor gating exhibited by isolation-reared animals may be normalized by the administration of the DA antagonist raclopride (0.05 mg/kg). Hence, isolation rearing provides a nonpharmacological way to induce in rats a deficit in sensorimotor gating that is exhibited by schizophrenic patients.

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

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

Behavioral studies following lesions of the mesolimbic and mesostriatal serotonergic pathways

The behavior of rats with selective lesions of either the dorsal (B7), median (B8), or lateral (B9) raphe nuclei was compared to that of sham-lesioned controls in a variety of experimental situations. As described previously, the extent of damage to the midbrain raphe nuclei was determined by fluorescence histochemistry, and the tryptophan hydroxylase and tyrosine hydroxylase activities of 6 forebrain regions were measured for each rat. None of the lesions affected tyrosine hydroxylase activity. Lesions of B7, which reduced tryptophan hydroxylase in the striatum, thalamus, cortex, and hypothalamus, had no significant effect on any of the behavioral measures. Lesions of B9, although twice as large, neither reduced forebrain tryptophan hydroxylase significantly nor affected any of the behavioral variables. However, B8 lesions, which reduced hippocampal, septal, cortical, and hypothalamic tryptophan hydroxylase, had behavioral effects similar to those reported after combined raphe lesions parachlorophenylalanine. Median raphe-lesioned rats were hyperactive when placed in a novel environment and throughout the dark phase of the light/dark cycle. With respect to locomotor activity, B8-lesioned rats were also hyper-responsive to amphetamine. When placed in a stabilimeter and subjected to repeated air puff stimuli, rats with B8 lesions exhibited larger startle responses. Furthermore, only B8-lesioned animals perseverated when given two unreinforced trials in a Y-maze. All these histologic, biochemical, and behavioral variables were assessed individually for all 39 animals, and a multivariate correlational analysis incorporating the data of this and the preceding paper is presented here. These experiments suggest that the mesolimbic serotonergic pathway originating in B8 subserves some of the inhibition necessary to dampen behavioral responsivity.

Corticotropin-releasing factor potentiates acoustic startle in rats: Blockade by chlordiazepoxide

A series of experiments was performed to investigate the effects of corticotropin-releasing factor (CRF) on the amplitude of the acoustic startle response (ASR) in rats. Intracerebroventricular (ICV) administration of 1 μg rat CRF significantly potentiated acoustic startle amplitude; these effects were reversed in a dose-dependent manner by pretreatment with the benzodiazepine chlordiazepoxide (CDP). Doses of CDP that anatgonized CRF-potentiated ASR did not lower startle baseline or antagonize amphetamine-or strychnine-potentiated ASR. These results suggest that CRF has “anxiogenic” properties and may serve as a neuroendocrine modulator of stress-enhanced behaviors.

Convergent functional genomics of schizophrenia: from comprehensive understanding to genetic risk prediction

We have used a translational convergent functional genomics (CFG) approach to identify and prioritize genes involved in schizophrenia, by gene-level integration of genome-wide association study data with other genetic and gene expression studies in humans and animal models. Using this polyevidence scoring and pathway analyses, we identify top genes (DISC1, TCF4, MBP, MOBP, NCAM1, NRCAM, NDUFV2, RAB18, as well as ADCYAP1, BDNF, CNR1, COMT, DRD2, DTNBP1, GAD1, GRIA1, GRIN2B, HTR2A, NRG1, RELN, SNAP-25, TNIK), brain development, myelination, cell adhesion, glutamate receptor signaling, G-protein–coupled receptor signaling and cAMP-mediated signaling as key to pathophysiology and as targets for therapeutic intervention. Overall, the data are consistent with a model of disrupted connectivity in schizophrenia, resulting from the effects of neurodevelopmental environmental stress on a background of genetic vulnerability. In addition, we show how the top candidate genes identified by CFG can be used to generate a genetic risk prediction score (GRPS) to aid schizophrenia diagnostics, with predictive ability in independent cohorts. The GRPS also differentiates classic age of onset schizophrenia from early onset and late-onset disease. We also show, in three independent cohorts, two European American and one African American, increasing overlap, reproducibility and consistency of findings from single-nucleotide polymorphisms to genes, then genes prioritized by CFG, and ultimately at the level of biological pathways and mechanisms. Finally, we compared our top candidate genes for schizophrenia from this analysis with top candidate genes for bipolar disorder and anxiety disorders from previous CFG analyses conducted by us, as well as findings from the fields of autism and Alzheimer. Overall, our work maps the genomic and biological landscape for schizophrenia, providing leads towards a better understanding of illness, diagnostics and therapeutics. It also reveals the significant genetic overlap with other major psychiatric disorder domains, suggesting the need for improved nosology.