Modulating sensory gating in healthy volunteers: the effects of ketamine and haloperidol

N-methyl-D-aspartate receptor antagonism impairs sensory gating in the auditory cortex in response to speech stimuli

Deficits in early auditory sensory processing in schizophrenia have been linked to N-methyl-D-aspartate receptor (NMDAR) hypofunction, but the role of NMDARs in aberrant auditory sensory gating (SG) in this disorder is unclear. This study, conducted in 22 healthy humans, examined the acute effects of a subanesthetic dose of the NMDAR antagonist ketamine on SG as measured electrophysiologically by suppression of the P50 event-related potential (ERP) to the second (S2) relative to the first (S1) of two closely paired (500 ms) identical speech stimuli. Ketamine induced impairment in SG indices at sensor (scalp)-level and at source-level in the auditory cortex (as assessed with eLORETA). Together with preliminary evidence of modest positive associations between impaired gating and dissociative symptoms elicited by ketamine, tentatively support a model of NMDAR hypofunction underlying disturbances in auditory SG in schizophrenia.

Social preference is maintained in mice with impaired startle reflex and glutamate/D-serine imbalance induced by chronic cerebral toxoplasmosis

Toxoplasma gondii is an opportunistic protozoan pathogen with a wide geographic distribution. The chronic phase of toxoplasmosis is often asymptomatic in humans and is characterized by tissue cysts throughout the central nervous system and muscle cells. T. gondii and other pathogens with tropism for the central nervous system are considered risk factors in the etiology of several neuropsychiatric disorders, such as schizophrenia and bipolar disorder, besides neurological diseases. Currently, it is known that cerebral toxoplasmosis increases dopamine levels in the brain and it is related to behavioral changes in animals and humans. Here we evaluate whether chronic T. gondii infection, using the cystogenic ME-49 strain, could induce behavioral alterations associated with neuropsychiatric disorders and glutamatergic neurotransmission dysfunction. We observed that the startle amplitude is reduced in the infected animals as well as glutamate and D-serine levels in prefrontal cortical and hippocampal tissue homogenates. Moreover, we did not detect alterations in social preference and spontaneous alternation despite severe motor impairment. Thus, we conclude that behavioral and cognitive aspects are maintained even though severe neural damage is observed by chronic infection of C57Bl/6 mice with the ME-49 strain.

Effects of methylphenidate on sensory and sensorimotor gating of initially psychostimulant-naïve adult ADHD patients

Deficient information processing in ADHD theoretically results in sensory overload, which in turn may underlie its symptoms. If this sensory overload is caused by deficient filtering of environmental stimuli, then one would expect finding deficits in P50 gating and prepulse inhibition of the startle reflex (PPI). Previous reports on these measures in ADHD have shown inconsistent findings, which may have been caused by either medication use or comorbidity (e.g. ASD). The primary aim of this study was therefore to explore P50 suppression and PPI in adult, psychostimulant-naïve patients with ADHD without major comorbidity, and to examine the effects of 6 weeks treatment with methylphenidate (MPH) on these measures. A total of 42 initially psychostimulant-naive, adult ADHD patients without major comorbidity and 42 matched healthy controls, were assessed for their P50 gating, PPI, and habituation/sensitization abilities at baseline and after 6 weeks of treatment with methylphenidate. Although six weeks of treatment with MPH significantly reduced symptomatology as well as improved daily life functioning in our patients, it neither significantly affected PPI, P50 suppression nor sensitization, but habituation unexpectedly decreased. The absence of PPI and P50 suppression deficits in our patients in the psychostimulant-naïve state indicates no gating deficits. In turn, this suggests that the difficulties to inhibit distraction of attention by irrelevant stimuli that many patients with (adult) ADHD report, have a different origin than the theoretical causes of sensory overload frequently reported in studies on patients with schizophrenia.

Altered neural oscillations and behavior in a genetic mouse model of NMDA receptor hypofunction

Abnormalities in electroencephalographic (EEG) biomarkers occur in patients with schizophrenia and those clinically at high risk for transition to psychosis and are associated with cognitive impairment. Converging evidence suggests N-methyl-D-aspartate receptor (NMDAR) hypofunction plays a central role in the pathophysiology of schizophrenia and likely contributes to biomarker impairments. Thus, characterizing these biomarkers is of significant interest for early diagnosis of schizophrenia and development of novel treatments. We utilized in vivo EEG recordings and behavioral analyses to perform a battery of electrophysiological biomarkers in an established model of chronic NMDAR hypofunction, serine racemase knockout (SRKO) mice, and their wild-type littermates. SRKO mice displayed impairments in investigation-elicited gamma power that corresponded with reduced short-term social recognition and enhanced background (pre-investigation) gamma activity. Additionally, SRKO mice exhibited sensory gating impairments in both evoked-gamma power and event-related potential amplitude. However, other biomarkers including the auditory steady-state response, sleep spindles, and state-specific power spectral density were generally neurotypical. In conclusion, SRKO mice demonstrate how chronic NMDAR hypofunction contributes to deficits in certain translationally-relevant EEG biomarkers altered in schizophrenia. Importantly, our gamma band findings suggest an aberrant signal-to-noise ratio impairing cognition that occurs with NMDAR hypofunction, potentially tied to impaired task-dependent alteration in functional connectivity.

The Role of N-Methyl-D-Aspartate Receptor Neurotransmission and Precision Medicine in Behavioral and Psychological Symptoms of Dementia

While the world’s population is aging, the prevalence of dementia and the associated behavioral and psychological symptoms of dementia (BPSD) rises rapidly. BPSD are associated with worsening of cognitive function and poorer prognosis. No pharmacological treatment has been approved to be beneficial for BPSD to date. Dysfunction of the N-methyl-D-aspartate receptor (NMDAR)-related neurotransmission leads to cognitive impairment and behavioral changes, both of which are core symptoms of BPSD. Memantine, an NMDAR partial antagonist, is used to treat moderate to severe Alzheimer’s disease (AD). On the other hand, a D-amino acid oxidase inhibitor improved early-phase AD. Whether to enhance or to attenuate the NMDAR may depend on the phases of dementia. It will be valuable to develop biomarkers indicating the activity of NMDAR, particularly in BPSD. In addition, recent reports suggest that gender difference exists in the treatment of dementia. Selecting subpopulations of patients with BPSD who are prone to improvement with treatment would be important. We reviewed literatures regarding the treatment of BPSD, focusing on the NMDAR-related modulation and precision medicine. Future studies examining the NMDAR modulators with the aid of potential biomarkers to tailor the treatment for individualized patients with BPSD are warranted.

Potential Mechanisms for the Ketamine-Induced Reduction of P3b Amplitudes

In specific dosages, the N-methyl-D-aspartate receptor (NMDA) antagonist ketamine can be used to model transient psychotic symptoms in healthy individuals that resemble those of schizophrenia. Ketamine administration also temporarily impairs cognitive functions, which can be studied by event-related potentials (ERPs). ERPs also allow dissecting what stages of information processing are affected by ketamine and what stages remain functional. For tasks requiring the differentiation of targets and non-targets, it has repeatedly been shown that ketamine administration in healthy individuals leads to decreased amplitudes of the ERP component P3b in response to target stimuli. However, it could be argued that this ketamine-induced P3b reduction is the consequence of an increased difficulty to differentiate targets from non-targets, primarily mediated by ketamine's psychotomimetic rather than pharmacological effects. The current review of ERP studies seeks to clarify the issue whether P3b effects of ketamine may indeed be explained as the consequence of an experienced increase in task difficulty or whether alternative mechanisms are perhaps more plausible. The review first summarizes the effects of task difficulty on ERP components related to intentional stimulus categorization (P3b), involuntary attention switches to distractors (P3a), as well as sensory processing (P1, N1). Secondly, the ERP effects of task difficulty are contrasted with those observed in ketamine studies in healthy individuals. Findings show that P3b amplitudes are consistently diminished by an increased task difficulty, as well as after ketamine administration. In contrast and as most important difference, increased task difficulty leads to increased P3a amplitudes to distractors presented in same modality as targets, whereas ketamine leads to reduced P3a amplitudes for such distractors. This dissociation indicates that the decreased P3b amplitudes after ketamine cannot be explained by a drug-induced increase in task difficulty. The conjoint reductions of P3a and P3b amplitudes instead suggest that working memory operations, in particular working memory updating are impaired after ketamine, which is in line with previous behavioral findings.

A comprehensive analysis of auditory event-related potentials and network oscillations in an NMDA receptor antagonist mouse model using a novel wireless recording technology

There is growing evidence that impaired sensory processing significantly contributes to cognitive deficits found in schizophrenia. Electroencephalography (EEG) has become an important preclinical and clinical technique to investigate the underlying mechanisms of neurophysiological dysfunctions in psychiatric disorders. Patients with schizophrenia show marked deficits in auditory event-related potentials (ERP), the detection of deviant auditory stimuli (mismatch negativity, MMN), the generation and synchronization of 40 Hz gamma oscillations in response to steady-state auditory stimulation (ASSR) and reduced auditory-evoked oscillation in the gamma range. Due to a novel data-logging technology (Neurologger, TSE Systems), it is now possible to record wireless EEG data in awake, free-moving small rodents without any restrictions due to size of the device or attached cables. Recently, a new version of the Neurologger was released with improved performance to record time-locked event-related EEG signals. In this study, we were able to show in mice that pharmacological intervention with the NMDA receptor antagonists Ketamine and MK-801 can impair a comprehensive selection of EEG/ERP readouts (ERP N1 amplitude, 40 Hz ASSR, basal and evoked gamma oscillation, MMN) and therefore mimic the EEG deficits observed in patients with schizophrenia. Our data support the translational value of NMDA receptor antagonists as a model for preclinical evaluation of sensory processing deficits relevant to schizophrenia. Further, the new Neurologger system is a suitable device for wireless recording of clinically relevant EEG biomarkers in freely moving mice and a robust translational tool to investigate novel therapeutic approaches regarding sensory processing deficits related to psychiatric disorders such as schizophrenia.

Effects of Subanesthetic Ketamine Administration on Visual and Auditory Event-Related Potentials (ERP) in Humans: A Systematic Review

Ketamine is a non-competitive N-Methyl-D-Aspartate (NMDA) receptor antagonist whose effect in subanesthetic doses has been studied for chronic pain and mood disorders treatment. It has been proposed that ketamine could change the perception of nociceptive stimuli by modulating the cortical connectivity and altering the top-down mechanisms that control conscious pain perception. As this is a strictly central effect, it would be relevant to provide fresh insight into ketamine's effect on cortical response to external stimuli. Event-related potentials (ERPs) reflect the combined synchronic activity of postsynaptic potentials of many cortical pyramidal neurons similarly oriented, being a well-established technique to study cortical responses to sensory input. Therefore, the aim of this study was to examine the current evidence of subanesthetic ketamine doses on patterns of cortical activity based on ERPs in healthy subjects. To answer the question whether ERPs could be potential markers of the cortical effects of ketamine, we conducted a systematic review of ketamine's effect on ERPs after single and repeated doses. We have searched PubMed, EMBASE and Cochrane Databases and pre-selected 141 articles, 18 of which met the inclusion criteria. Our findings suggest that after ketamine administration some ERP parameters are reduced (reduced N2, P2, and P3 amplitudes, PN and MMN) while others remain stable or are even increased (P50 reduction, PPI, P1, and N1 amplitudes). The current understanding of these effects is that ketamine alters the perceived contrast between distinct visual and auditory stimuli. The analgesic effect of ketamine might also be influenced by a decreased affective discrimination of sensorial information, a finding from studies using ketamine as a model for schizophrenia, but that can give an important hint not only for the treatment of mood disorders, but also to treat pain and ketamine abuse.

Effects of GABA-B receptor positive modulator on ketamine-induced psychosis-relevant behaviors and hippocampal electrical activity in freely moving rats

RATIONALE

Decreased GABA_B receptor function is proposed to mediate some symptoms of schizophrenia.

OBJECTIVES

In this study, we tested the effect of CGP7930, a GABA_B receptor positive allosteric modulator, on ketamine-induced psychosis-relevant behaviors and hippocampal electrical activity in behaving rats.

METHODS

Electrodes were bilaterally implanted into the hippocampus, and cannulae were placed into the lateral ventricles of Long-Evans rats. CGP7930 or vehicle was injected intraperitoneally (i.p.) or intracerebroventricularly (i.c.v.), alone or 15 min prior to ketamine (3 mg/kg, subcutaneous) injection. Paired click auditory evoked potentials in the hippocampus (AEP), prepulse inhibition (PPI), and locomotor activity were recorded before and after drug injection.

RESULTS

CGP7930 at doses of 1 mg/kg (i.p.) prevented ketamine-induced deficit of PPI. CGP7930 (1 mg/kg i.p.) also prevented the decrease in gating of hippocampal AEP and the increase in hippocampal gamma (65-100 Hz) waves induced by ketamine. Unilateral i.c.v. infusion of CGP7930 (0.3 mM/1 μL) also prevented the decrease in gating of hippocampal AEP induced by ketamine. Ketamine-induced behavioral hyperlocomotion was suppressed by 5 mg/kg i.p. CGP7930. CGP7930 alone, without ketamine, did not significantly affect integrated PPI, locomotion, gating of hippocampal AEP, or gamma waves. CGP7930 (1 mg/kg i.p.) increased heterosynaptically mediated paired pulse depression in the hippocampus, a measure of GABA_B receptor function in vivo.

CONCLUSIONS

CGP7930 reduces the behavioral and electrophysiological disruptions induced by ketamine in animals, and the hippocampus may be one of the neural targets where CGP7930 exerts its actions.

Comparing Pharmacological Modulation of Sensory Gating in Healthy Humans and Rats: The Effects of Reboxetine and Haloperidol

Sensory gating is the brain's ability to filter out irrelevant information before it reaches high levels of conscious processing. In the current study we aimed to investigate the involvement of the noradrenergic and dopaminergic neurotransmitter systems in sensory gating. Furthermore, we investigated cross-species reliability by comparing effects in both healthy humans and rats, while keeping all experimental conditions as similar as possible between the species. The design of the human experiment (n=21) was a double-blind, placebo-controlled, cross-over study where sensory gating was assessed following a dose of either reboxetine (8 mg), haloperidol (2 mg), their combination or placebo at four separate visits. Similarly in the animal experiment sensory gating was assessed in rats, (n=22) following a dose of reboxetine (2 mg/kg), haloperidol (0.08 mg/kg), their combination or placebo. The sensory gating paradigms in both experiments were identical. In humans, we found significantly reduced P50 suppression following separate administration of reboxetine or haloperidol, while their combined administration did not reach statistical significance compared with placebo. In the rats, we found a similar significant reduction of sensory gating (N40) following treatment with haloperidol and the combination of haloperidol and reboxetine, but not with separate reboxetine treatment, compared with placebo. Our study indicates that even when experimental conditions are kept as similar as possible, direct human to rat cross-species translation of pharmacological effects on sensory gating is challenging, which calls for more focussed research in this important translational area.

The Role of Nicotine in Schizophrenia

Schizophrenia is associated with by severe disruptions in thought, cognition, emotion, and behavior. Patients show a marked increase in rates of smoking and nicotine dependence relative to nonaffected individuals, a finding commonly ascribed to the potential ameliorative effects of nicotine on symptom severity and cognitive impairment. Indeed, many studies have demonstrated improvement in patients following the administration of nicotine. Such findings have led to an increased emphasis on the development of therapeutic agents to target the nicotinic system as well as increasing the impetus to understand the genetic basis for nicotinic dysfunction in schizophrenia. The goal of this review article is to provide a critical summary of evidence for the role of the nicotinic system in schizophrenia. The first part will review the role of nicotine in normalization of primary dysfunctions and endophenotypical changes found in schizophrenia. The second part will provide a summary of genetic evidence linking polymorphisms in nicotinic receptor genes to smoking and schizophrenia. The third part will summarize attempts to treat schizophrenia using agents specifically targeting nicotinic and nicotinic receptor subtypes. Although currently available antipsychotic treatments are generally able to manage some aspects of schizophrenia (e.g., positive symptoms) they fail to address several other critically effected aspects of the disease. As such, the search for novel mechanisms to treat this disease is necessary.

Optimizing the glutamatergic challenge model for psychosis, using S+ -ketamine to induce psychomimetic symptoms in healthy volunteers

The psychomimetic effects that occur after acute administration of ketamine can constitute a model of psychosis and antipsychotic drug action. However, the optimal dose/concentration has not been established and there is a large variety in outcome measures. In this study, 36 healthy volunteers (21 males and 15 females) received infusions of S(+)-ketamine or placebo to achieve pseudo-steady state concentrations of 180 and 360 ng/mL during two hours. The target of 360 ng/mL induced increasingly more intensive effects than expected, and the targets were subsequently reduced to 120 and 240 ng/mL, which were considered tolerable. There was a clear, concentration-dependent psychomimetic effect as shown on all subscales of the positive and negative syndrome scale (e.g. positive subscale +43.7%, 95%CI 34.4-53.7%, p < 0.0001 for 120 ng/mL and +70.5%, 95%CI 59.0-82.8%, p < 0.0001 for 240 ng/mL) and different visual analogue scales. The startle reflex was inhibited (prepulse inhibition) by both main target concentrations to a similar extent, suggesting a maximum effect. Ketamine was found to constitute a robust model for induction of psychomimetic symptoms and the optimal concentration range for a drug interaction study would be between 100 and 200 ng/mL.

Reverse translation of clinical electrophysiological biomarkers in behaving rodents under acute and chronic NMDA receptor antagonism

Electroencephalogram (EEG) stands out as a highly translational tool for psychiatric research, yet rodent and human EEG are not typically obtained in the same way. In this study we developed a tool to record skull EEG in awake-behaving rats in a similar manner to how human EEG are obtained and then used this technique to test whether acute NMDA receptor antagonism alters rodent EEG signals in a similar manner as in humans. Acute MK-801 treatment elevated gamma power and reduced beta band power, which closely mirrored EEG data from healthy volunteers receiving acute ketamine. To explore the mechanisms behind these oscillatory changes, we examined the effects of GABA-A receptor blockade, finding that picrotoxin (PTX) recapitulated the decrease in sound-evoked beta oscillations observed with acute MK-801, but did not produce changes in gamma band power. Chronic treatment with either PTX or MK-801 did not affect frequency-specific oscillatory activity when tested 24 h after the last drug injection, but decreased total broadband oscillatory power. Overall, this study validated a novel platform for recording rodent EEG and demonstrated similar oscillatory changes after acute NMDA receptor antagonism in both humans and rodents, suggesting that skull EEG may be a powerful tool for further translational studies.

Sleep deprivation disrupts prepulse inhibition and induces psychosis-like symptoms in healthy humans

Translational biomarkers, such as prepulse inhibition (PPI) of the acoustic startle response, are playing an increasingly important role in the development of antipsychotic drugs for schizophrenia and related conditions. However, attempts to reliably induce a PPI deficit by psychotomimetic drugs have not been successful, leaving an unmet need for a cross-species psychosis model sensitive to this widely studied surrogate treatment target. Sleep deprivation (SD) might be such a model as it has previously been shown to induce PPI deficits in rats, which could be selectively prevented with antipsychotic but not anxiolytic or antidepressant compounds. Here, in a first proof-of-concept study we tested whether SD induces a deficit in PPI and an increase in psychosis-like symptoms in healthy humans. In two counterbalanced sessions, acoustic PPI and self-reported psychosis-like symptoms (Psychotomimetic States Inventory) were measured in 24 healthy human volunteers after a normal night's sleep and after a night of total SD. SD decreased PPI (p = 0.001) without affecting the magnitude or habituation of the startle response (all p > 0.13). SD also induced perceptual distortions, cognitive disorganization, and anhedonia (all p < 0.02). Thus, extending previous rodent work, we conclude that SD, in combination with the PPI biomarker, might be a promising translational surrogate model for psychosis as this method represents a possibility to partially and reversibly mimic the pathogenesis of psychotic states.

D-serine prevents cognitive deficits induced by acute stress

Increasing evidence indicates that acute stress disrupts cognitive functions mediated by glutamate-NMDA receptors, although the mechanisms are not fully understood. Here we investigated whether d-serine and glycine, the endogenous co-agonists of the NMDA receptor, are regulated by acute stress. We studied the biochemical and behavioral effects of acute restraint stress in C57BL/6 mice. Acute restraint stress decreased d-serine levels in the prefrontal cortex and glycine levels in the hippocampus. Behaviorally, acute stress impaired memory consolidation in the object recognition task and prepulse inhibition of the startle response. Importantly, d-serine administration (1 g/kg, i.p.) prevented both stress-induced impairments. Taken together, our results show for the first time an interplay between stress and d-serine and warrant further research on the role of d-serine in stress-related disorders.

Electrophysiological and behavioral responses to ketamine in mice with reduced Akt1 expression

RATIONALE

A number of studies have associated reduced Akt1 expression with vulnerability for schizophrenia. Although mice with deletion of a single copy of the Akt1 gene (Akt1(+/-)) show reduced Akt1 expression relative to wild-type (WT) animals, the extent to which these mice show schizophrenia-like phenotypic changes and/or increased susceptibility to epigenetic or non-genetic factors related to schizophrenia is unknown.

OBJECTIVES

Mutant mice were assessed on electroencephalographic/event-related potential (EEG/ERP) and behavioral (acoustic startle and pre-pulse inhibition) measures relevant to schizophrenia. Mice were also assessed following exposure to the NMDA receptor antagonist ketamine, a potent psychotomimetic drug, in order to assess the role of reduced Akt1 expression as a vulnerability factor for schizophrenia. Methods Akt1(+/-), Akt1(-/-), and WT mice received a series of paired-click, white noise stimuli, following ketamine (50 mg/kg) and saline injections. EEG was analyzed for ERPs and event-related power. Akt1(+/-) and WT mice were also assessed on PPI following ketamine (50 mg/kg) or saline injection.

RESULTS

Akt1(+/-) and Akt1(-/-) mice displayed reduced amplitude of the P20 component of the ERP to the first click of a paired-click stimulus, as well as reduced S1-S2 difference for P20 and N40 components, following ketamine. Mutant mice also showed increased reduction in gamma synchrony and theta suppression following ketamine. Akt1(+/-) mice displayed reduced pre-pulse inhibition.

CONCLUSIONS

Reduced genetic expression of Akt1 facilitated ketamine-induced changes of EEG and behavior in mice, suggesting that reduced Akt1 expression can serve as a vulnerability factor for schizophrenia.

P50 suppression and its neural generators in antipsychotic-naive first-episode schizophrenia before and after 6 months of quetiapine treatment

BACKGROUND

Numerous studies have demonstrated sensory gating deficits in schizophrenia. However, only a few longitudinal studies report on the effects of antipsychotic treatment on sensory gating deficits and their results are inconsistent. In the present study, P50 suppression and its neural generators were investigated in antipsychotic-naïve first-episode patients with schizophrenia before and after 6 months of treatment with quetiapine.

METHODS

Thirty-four antipsychotic-naïve first-episode schizophrenia patients and age and gender matched healthy controls were tested in an auditory sensory gating paradigm at baseline and after 6 months. During this period, the patients were treated with quetiapine, while controls received no treatment. Sixteen patients completed the study.

RESULTS

Patients showed significant reduced P50 suppression compared with controls at baseline but not at follow-up. Furthermore, a significant positive correlation between baseline P50 suppression and dose of quetiapine at follow-up was found. P50 suppression in patients receiving above median dosages of quetiapine increased significantly from baseline to follow-up. At baseline, a frontocentral source was significantly more active in patients than in controls at the time of the testing stimulus.

CONCLUSIONS

The present findings suggest that P50 suppression deficits are already present at an early stage of schizophrenia. Furthermore, particularly those patients with more severe gating deficits appeared to need higher dosages of quetiapine, although their clinical symptoms did not seem to indicate this. Quetiapine treatment significantly improved these gating deficits. Furthermore, a frontocentral source in the brain appeared to be involved in the deficient P50 gating of the patients.

The computational anatomy of psychosis

This paper considers psychotic symptoms in terms of false inferences or beliefs. It is based on the notion that the brain is an inference machine that actively constructs hypotheses to explain or predict its sensations. This perspective provides a normative (Bayes-optimal) account of action and perception that emphasizes probabilistic representations; in particular, the confidence or precision of beliefs about the world. We will consider hallucinosis, abnormal eye movements, sensory attenuation deficits, catatonia, and delusions as various expressions of the same core pathology: namely, an aberrant encoding of precision. From a cognitive perspective, this represents a pernicious failure of metacognition (beliefs about beliefs) that can confound perceptual inference. In the embodied setting of active (Bayesian) inference, it can lead to behaviors that are paradoxically more accurate than Bayes-optimal behavior. Crucially, this normative account is accompanied by a neuronally plausible process theory based upon hierarchical predictive coding. In predictive coding, precision is thought to be encoded by the post-synaptic gain of neurons reporting prediction error. This suggests that both pervasive trait abnormalities and florid failures of inference in the psychotic state can be linked to factors controlling post-synaptic gain - such as NMDA receptor function and (dopaminergic) neuromodulation. We illustrate these points using biologically plausible simulations of perceptual synthesis, smooth pursuit eye movements and attribution of agency - that all use the same predictive coding scheme and pathology: namely, a reduction in the precision of prior beliefs, relative to sensory evidence.

Genetic models of sensorimotor gating: relevance to neuropsychiatric disorders

Sensorimotor gating, or the ability of a sensory event to suppress a motor response, can be measured operationally via prepulse inhibition (PPI) of the startle response. PPI is deficient in schizophrenia patients as well as other neuropsychiatric disorders, can be measured across species, and has been used widely as a translational tool in preclinical neuropharmacological and genetic research. First developed to assess drug effects in pharmacological and developmental models, PPI has become one of the standard behavioral measures in genetic models of schizophrenia and other neuropsychiatric disorders that exhibit PPI deficits. In this chapter we review the literature on genetic models of sensorimotor gating and discuss the utility of PPI as a tool in phenotyping mutant mouse models. We highlight the approaches to genetic mouse models of neuropsychiatric disease, discuss some of the important caveats to these approaches, and provide a comprehensive table covering the more recent genetic models that have evaluated PPI.

The effects of sertindole on sensory gating, sensorimotor gating, and cognition in healthy volunteers

Sensory gating, indexed by P50 suppression, and sensorimotor gating, indexed by prepulse inhibition (PPI), are impaired in schizophrenia spectrum disorders. There is considerable evidence that schizophrenia patients treated with atypical antipsychotics exhibit relatively less gating deficits than do other patients with schizophrenia. Some recent studies have investigated the effects of antipsychotic medications on gating in healthy volunteers exhibiting low levels of gating, rather than in patients. Therefore, the current study investigated the influence of sertindole versus placebo in two separate experimental sessions, on PPI, P50 suppression, and cognition in 30 male volunteers stratified for low and high baseline gating levels. Sertindole increased PPI and P50 suppression in healthy subjects exhibiting low baseline PPI and low baseline P50 suppression, respectively, while sertindole attenuated gating in subjects exhibiting high baseline gating. Furthermore, subjects exhibiting low PPI chose worse strategies in a spatial working memory task. These findings suggest that mixed D(2)/5-HT(2) receptor antagonists enhance both PPI and P50 suppression in a way that enhances it in healthy subjects exhibiting low baseline gating. Furthermore, the results militate in favor of the concomitant assessment of PPI, P50 suppression and cognitive measures while investigating the effect of antipsychotic medication in healthy subjects.

Stability of prepulse inhibition and habituation of the startle reflex in schizophrenia: a 6-year follow-up study of initially antipsychotic-naive, first-episode schizophrenia patients

Deficits in information processing appear to be core features in the pathogenesis of schizophrenia. Prepulse inhibition (PPI) and habituation of the startle reflex are operational measures of early information processing. Impaired PPI in schizophrenia has been replicated in many studies and is regarded as an endophenotype for schizophrenia. However, reports on the stability of PPI over a longer period of time are lacking, both for patients with schizophrenia and for healthy subjects. The current study examined 25 initially drug-naive, first-episode schizophrenia patients and 23 healthy matched controls. Three PPI measures [stimulus onset asynchrony (SOA) 30, 60, 120 ms] and habituation were assessed at baseline, and again after 6 yr. Sixteen patients and 17 healthy controls completed the study, and 13 patients and 17 healthy controls were included in the final analysis. The schizophrenia patients had PPI deficits compared to controls at baseline. After 6 yr, no significant group differences were found. PPI had increased significantly in the patients and had decreased significantly in controls. In addition, patients showed significantly less habituation than controls while habituation did not change in patients or controls. The present results show that PPI in drug-naive, first-episode schizophrenia patients can improve significantly over time. As PPI increased in patients over the same period that it decreased in controls, it is likely that the increase was caused by disease-related factors such as disease process, clinical state, or medication.

Mouse pharmacological models of cognitive disruption relevant to schizophrenia

Schizophrenia is a debilitating cognitive disorder. The link between cognitive debilitation and functional outcome in patients with schizophrenia has prompted research to develop procognitive therapies. It is hoped that by improving cognition in these patients, their functional outcome will also improve. Although no established treatments exist as yet, progress has been made toward understanding how to evaluate putative compounds in the clinic. Genetic mouse models and pharmacological rat models of cognitive disruption are being developed that may help to evaluate these putative compounds preclinically. Considering the increased number of genetic mouse models relevant to schizophrenia, there is a need to evaluate pharmacological manipulations on cognition in mice. Here we review the current literature on mouse pharmacological models relevant to schizophrenia. In this review, we discuss where different pharmacological effects between rats and mice on cognitive tasks are observed and assess the validity offered by these models. We conclude that the predictive validity of these models is currently difficult to assess and that much more needs to be done to develop useful mouse pharmacological models of cognitive disruption in schizophrenia.

How human electrophysiology informs psychopharmacology: from bottom-up driven processing to top-down control

This review surveys human event-related brain potential (ERP) and event-related magnetic field (ERF) approaches to psychopharmacology and psychopathology, and the way in which they complement behavioral studies and other neuroimaging modalities. The major paradigms involving ERP/ERF are P50 suppression, loudness-dependent auditory evoked potential (LDAEP), mismatch negativity (MMN), P300, mental chronometry, inhibitory control, and conflict processing (eg, error-related negativity (ERN)). Together these paradigms cover a range of more bottom-up driven to more top-down controlled processes. A number of relationships between the major neurotransmitter systems and electrocortical mechanisms are highlighted. These include the role of dopamine in conflict processing, and perceptual processing vs motor preparation; the role of serotonin in P50 suppression, LDAEP, and MMN; glutamate/NMDA and MMN; and the role of acetylcholine in P300 generation and memory-related processes. A preliminary taxonomy for these relationships is provided, which should be helpful in attuning possible new treatments or new applications of existing treatments to various disorders.

Sensorimotor gating and habituation in antipsychotic-naive, first-episode schizophrenia patients before and after 6 months' treatment with quetiapine

Impaired prepulse inhibition of the startle reflex (PPI) in schizophrenia has been replicated in many studies. However, previous results may have been influenced by course of illness, and antipsychotic medication. Studies on antipsychotic-naive, first-episode schizophrenia patients are lacking, since these patients are so difficult to recruit. Furthermore, longitudinal studies are few, and their results are inconsistent: some results indicating a reduction of PPI deficits by treatment with atypical antipsychotics, while others do not. This study reports on PPI, habituation and sensitization of the human startle reflex in a large group of antipsychotic-naive, first-episode schizophrenia patients, and the effect of subsequent treatment with quetiapine. Thirty-four antipsychotic-naive, first-episode schizophrenia patients (24 males, 10 females), and age- and gender-matched healthy controls were tested in a psychophysiological test battery at baseline and again after 6 months. During this period, the patients were treated with quetiapine, while the controls received no treatment. Sixteen patients completed the study. At baseline, male patients showed significantly lower PPI than controls. Treatment with quetiapine for 6 months increased male PPI to a level where it was no longer statistically different from the controls. The much smaller group of females did not show PPI deficits at baseline. In addition, compared to controls, patients appeared highly aroused and showed a strong yet non-significant trend for reduced sensitization at baseline, but not at follow-up. Patients and controls showed similar levels of habituation, both at baseline, and at follow-up. These findings indicate that PPI deficits are already present from the earliest stage of clinical onset of schizophrenia, before the patients have received any antipsychotic treatment. In addition, following 6 months' treatment with quetiapine these PPI deficits were normalized. Furthermore, the results suggest that schizophrenia patients in the antipsychotic-naive state show reduced levels of sensitization, yet normal levels of habituation.

Substance use in adolescence and psychosis: clarifying the relationship

Adolescence is a time of exploration of the self, and this exploration may involve the use of alcohol and drugs. Sadly, for some, adolescence also marks the first signs of a psychosis. The temporal proximity between the onset of substance use and of psychosis has been the cause of much debate. Here we review the association of alcohol, cannabis, stimulants, and other drugs with psychosis, and we conclude that the use of cannabis and the amphetamines significantly contributes to the risk of psychosis.

Sensory gating and sensorimotor gating in medication-free obsessive-compulsive disorder patients

Obsessive-compulsive disorder (OCD) is associated with deficits in inhibition mechanisms. This is reflected in reports showing impaired sensorimotor and sensory gating in OCD patients, as measured with prepulse inhibition (PPI) of the startle reflex and P50 suppression paradigms. However, most of the patients in these studies used medication and the results were not controlled for menstrual cycle phase in women. In this study PPI and P50 suppression were tested in 25 medication-free OCD patients and 25 healthy controls, using auditory stimuli and controlling for menstrual cycle effects. Subgroups were established, based on clinical variables (e.g. 'washers' and 'checkers'). No impairments in PPI or P50 suppression were found in the OCD group when compared with healthy controls. However, a subgroup of OCD patients ('checkers', n=12) showed increased P50 suppression. It was concluded that sensorimotor and sensory gating is not impaired in drug-free OCD patients, taking into account the menstrual cycle effects in women. These results do not support hypotheses linking deficits in these inhibition paradigms and the pathogenesis of OCD. The finding of an increased P50 suppression in the subgroup of 'checkers' deserves further investigation and underlines the value of studying subgroups of OCD.

Assessment of auditory sensory processing in a neurodevelopmental animal model of schizophrenia--gating of auditory-evoked potentials and prepulse inhibition

The use of translational approaches to validate animal models is needed for the development of treatments that can effectively alleviate cognitive impairments associated with schizophrenia, which are unsuccessfully treated by the current available therapies. Deficits in pre-attentive stages of sensory information processing seen in schizophrenia patients, can be assessed by highly homologues methods in both humans and rodents, evident by the prepulse inhibition (PPI) of the auditory startle response and the P50 (termed P1 here) suppression paradigms. Treatment with the NMDA receptor antagonist PCP on postnatal days 7, 9, and 11 reliably induce cognitive impairments resembling those presented by schizophrenia patients. Here we evaluate the potential of early postnatal PCP (20mg/kg) treatment in Lister Hooded rats to induce post-pubertal deficits in PPI and changes, such as reduced gating, in the P1 suppression paradigm in the EEG. The results indicate that early postnatal PCP treatment to rats leads to a reduction in PPI of the acoustic startle response. Furthermore, treated animals were assessed in the P1 suppression paradigm and produced significant changes in auditory-evoked potentials (AEP), specifically by an increased P1 amplitude and reduced P2 (P200 in humans) gating. However, the treatment neither disrupted normal P1 gating nor reduced N1 (N100 in humans) amplitude, representing two phenomena that are usually found to be disturbed in schizophrenia. In conclusion, the current findings confirm measures of early information processing to show high resemblance between rodents and humans, and indicate that early postnatal PCP-treated rats show deficits in pre-attentional processing, which are distinct from those observed in schizophrenia patients.

Gamma and delta neural oscillations and association with clinical symptoms under subanesthetic ketamine

Several electrical neural oscillatory abnormalities have been associated with schizophrenia, although the underlying mechanisms of these oscillatory problems are unclear. Animal studies suggest that one of the key mechanisms of neural oscillations is through glutamatergic regulation; therefore, neural oscillations may provide a valuable animal-clinical interface on studying glutamatergic dysfunction in schizophrenia. To identify glutamatergic control of neural oscillation relevant to human subjects, we studied the effects of ketamine, an N-methyl-D-aspartate antagonist that can mimic some clinical aspects of schizophrenia, on auditory-evoked neural oscillations using a paired-click paradigm. This was a double-blind, placebo-controlled, crossover study of ketamine vs saline infusion on 10 healthy subjects. Clinically, infusion of ketamine in subanesthetic dose significantly increased thought disorder, withdrawal-retardation, and dissociative symptoms. Ketamine significantly augmented high-frequency oscillations (gamma band at 40-85 Hz, p=0.006) and reduced low-frequency oscillations (delta band at 1-5 Hz, p<0.001) compared with placebo. Importantly, the combined effect of increased gamma and reduced delta frequency oscillations was significantly associated with more withdrawal-retardation symptoms experienced during ketamine administration (p=0.02). Ketamine also reduced gating of the theta-alpha (5-12 Hz) range oscillation, an effect that mimics previously described deficits in schizophrenia patients and their first-degree relatives. In conclusion, acute ketamine appeared to mimic some aspects of neural oscillatory deficits in schizophrenia, and showed an opposite effect on scalp-recorded gamma vs low-frequency oscillations. These electrical oscillatory indexes of subanesthetic ketamine can be potentially used to cross-examine glutamatergic pharmacological effects in translational animal and human studies.

Haloperidol counteracts the ketamine-induced disruption of processing negativity, but not that of the P300 amplitude

Antagonists of the N-methyl-D-aspartate (NMDA) receptors such as ketamine, induce abnormalities in healthy subjects similar to those found in schizophrenia. However, recent evidence, suggests that most of the currently known NMDA antagonists have a broader receptor profile than originally thought. Besides exerting an antagonistic effect on NMDA receptors, they have agonistic effects on dopamine D2 receptors. Can haloperidol (D2 antagonist) counteract the disruptive effects of ketamine on psychophysiological parameters of human attention? In a randomized, double-blind, placebo-controlled experiment 18 healthy male volunteers received placebo/placebo, placebo/ketamine (0.3 mg/kg i.v.) and haloperidol (2 mg)/ketamine (0.3 mg/kg i.v.) on three separate test days, after which they were tested in an auditory selective-attention paradigm. Haloperidol/ketamine reduced task performance compared to placebo/placebo, while the task performance in these two treatments did not differ from placebo/ketamine. Furthermore, placebo/ketamine reduced processing negativity compared to both placebo/placebo and haloperidol/ketamine, while processing negativity did not differ between placebo/placebo and haloperidol/ketamine treatments. However, both placebo/ketamine and haloperidol/ketamine reduced P300 amplitude compared to placebo/placebo, while P300 amplitude did not differ between placebo/ketamine and haloperidol/ketamine treatments. The combined effects of haloperidol and ketamine reduced task performance, suggesting that this is dependent on dopaminergic D2 activity, probably in the prefrontal cortex. In addition, ketamine reduced both P300 amplitude and processing negativity. In contrast to the P300 amplitude, the disruptive effects of ketamine on processing negativity could be prevented by pretreatment with haloperidol. The current results suggest that ketamine reduced P300 amplitude by its antagonistic effect on glutamatergic activity, while it reduced processing negativity by its agonistic effect on dopaminergic D2 activity.

Neurophysiological and neurochemical animal models of schizophrenia: focus on glutamate

Deficits in N-methyl-d-aspartate receptor (NMDAR) function play a critical role in the pathophysiology of schizophrenia. Animal models are needed to investigate possible mechanisms underlying NMDA dysfunction in schizophrenia as well as development of new therapeutic approaches. A major difficulty in developing animal models for schizophrenia is the identification of quantifiable measures that can be tested in a similar fashion in both humans and animals. The majority of animal models utilize analogous measures, wherein species-specific behaviors are used as presumed parallel manifestations of a common underlying construct. In vivo microdialysis and electrophysiology represent two methodologies in which homologous measures can instead be obtained in both animals and humans. In both techniques, well-validated, NMDA-sensitive measures are analyzed in rodents using probes implanted directly into cortex or subcortical structures. We discuss the currently available data from studies that used these methods in non-human primate and rodent glutamate models. In addition, we emphasize the possible relevance of the amphetamine-challenge studies to positive symptoms and of EEG measures to cognitive deficits in schizophrenia.

Assessment of NMDA receptor NR1 subunit hypofunction in mice as a model for schizophrenia

N-methyl-D-aspartate receptors (NMDARs) play a pivotal role in excitatory neurotransmission, synaptic plasticity and brain development. Clinical and experimental evidence suggests a dysregulation of NMDAR function and glutamatergic pathways in the pathophysiology of schizophrenia. We evaluated electrophysiological and behavioral properties of NMDAR deficiency utilizing mice that express only 5-10% of the normal level of NMDAR NR1 subunit. Auditory and visual event related potentials yielded significantly increased amplitudes for the P20 and N40 components in NMDAR deficient (NR1(neo)-/-) mice suggesting decreased inhibitory tone. Compared to wild types, NR1(neo)-/- mice spent less time in social interactions and showed reduced nest building. NR1(neo)-/- mice displayed a preference for open arms of a zero maze and central zone of an open field, possibly reflecting decreased anxiety-related behavioral inhibition. However, locomotor activity did not differ between groups in either home cage environment or during behavioral testing. NR1(neo)-/- mice displayed hyperactivity only when placed in a large unfamiliar environment, suggesting that neither increased anxiety nor non-specific motor activation accounts for differential behavioral patterns. Data suggest that NMDAR NR1 deficiency causes disinhibition in sensory processing as well as reduced behavioral inhibition and impaired social interactions. The behavioral signature in NR1(neo)-/- mice supports the impact of impaired NMDAR function in a mouse model with possible relevance to negative symptoms in schizophrenia.

Prepulse inhibition and genetic mouse models of schizophrenia

Mutant mouse models related to schizophrenia have been based primarily on the pathophysiology of schizophrenia, the known effects of antipsychotic drugs, and candidate genes for schizophrenia. Sensorimotor gating deficits in schizophrenia patients, as indexed by measures of prepulse inhibition of startle (PPI), have been well characterized and suggested to meet the criteria as a useful endophenotype in human genetic studies. PPI refers to the ability of a non-startling "prepulse" to inhibit responding to the subsequent startling stimulus or "pulse." Because of the cross-species nature of PPI, it has been used primarily in pharmacological animal models to screen putative antipsychotic medications. As techniques in molecular genetics have progressed over the past 15 years, PPI has emerged as a phenotype used in assessing genetic mouse models of relevance to schizophrenia. In this review, we provide a selected overview of the use of PPI in mouse models of schizophrenia and discuss the contribution and usefulness of PPI as a phenotype in the context of genetic mouse models. To that end, we discuss mutant mice generated to address hypotheses regarding the pathophysiology of schizophrenia and candidate genes (i.e., hypothesis driven). We also briefly discuss the usefulness of PPI in phenotype-driven approaches in which a PPI phenotype could lead to "bottom up" approaches of identifying novel genes of relevance to PPI (i.e., hypothesis generating).

Using the MATRICS to guide development of a preclinical cognitive test battery for research in schizophrenia

Cognitive deficits in schizophrenia are among the core symptoms of the disease, correlate with functional outcome, and are not well treated with current antipsychotic therapies. In order to bring together academic, industrial, and governmental bodies to address this great 'unmet therapeutic need', the NIMH sponsored the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative. Through careful factor analysis and consensus of expert opinion, MATRICS identified seven domains of cognition that are deficient in schizophrenia (attention/vigilance, working memory, reasoning and problem solving, processing speed, visual learning and memory, verbal learning and memory, and social cognition) and recommended a specific neuropsychological test battery to probe these domains. In order to move the field forward and outline an approach for translational research, there is a need for a "preclinical MATRICS" to develop a rodent test battery that is appropriate for drug development. In this review, we outline such an approach and review current rodent tasks that target these seven domains of cognition. The rodent tasks are discussed in terms of their validity for probing each cognitive domain as well as a brief overview of the pharmacology and manipulations relevant to schizophrenia for each task.

Oxytocin as a natural antipsychotic: a study using oxytocin knockout mice

It has been previously suggested that oxytocin (Oxt) may act as a natural antipsychotic. To test this hypothesis, we investigated whether disruption of the oxytocin gene (Oxt-/-) made mice more susceptible to the psychosis-related effects of amphetamine (Amp), apomorphine (Apo) and phencyclidine (PCP). We examined drug-induced changes in the prepulse inhibition (PPI) of the startle reflex, a measure of sensorimotor gating deficits characteristic of several psychiatric and neurological disorders, including schizophrenia. We found that treatment with Amp, Apo and PCP all had effects on PPI. However, in Oxt-/- mice, but not Oxt+/+ mice, PCP treatment resulted in large PPI deficits. As PCP is a noncompetitive N-methyl-D-aspartic acid receptor antagonist, these findings suggest that the absence of Oxt alters the glutamatergic component of the PPI.

The neural networks underlying auditory sensory gating

One of the most consistent electrophysiological deficits reported in the schizophrenia literature is the failure to inhibit, or properly gate, the neuronal response to the second stimulus of an identical pair (i.e., sensory gating). Although animal and invasive human studies have consistently implicated the auditory cortex, prefrontal cortex and hippocampus in mediating the sensory gating response, localized activation in these structures has not always been reported during non-invasive imaging modalities. In the current experiment, event-related FMRI and a variant of the traditional gating paradigm were utilized to examine how the gating network differentially responded to the processing of pairs of identical and non-identical tones. Two single-tone conditions were also presented so that they could be used to estimate the HRF for paired stimuli, reconstructed based on actual hemodynamic responses, to serve as a control non-gating condition. Results supported an emerging theory that the gating response for both paired-tone conditions was primarily mediated by auditory and prefrontal cortex, with potential contributions from the thalamus. Results also indicated that the left auditory cortex may play a preferential role in determining the stimuli that should be inhibited (gated) or receive further processing due to novelty of information. In contrast, there was no evidence of hippocampal involvement, suggesting that future work is needed to determine what role it may play in the gating response.

Neurophysiological endophenotypes across bipolar and schizophrenia psychosis

The search for liability genes of the world's 2 major psychotic disorders, schizophrenia and bipolar disorder I (BP-I), has been extremely difficult even though evidence suggests that both are highly heritable. This difficulty is due to the complex and multifactorial nature of these disorders. They encompass several intermediate phenotypes, some overlapping across the 2 psychotic disorders that jointly and/or interactively produce the clinical manifestations. Research of the past few decades has identified several neurophysiological deficits in schizophrenia that frequently occur before the onset of psychosis. These include abnormalities in smooth pursuit eye movements, P50 sensory gating, prepulse inhibition, P300, mismatch negativity, and neural synchrony. Evidence suggests that many of these physiological deficits are distinct from each other. They are stable, mostly independent of symptom state and medications (with some exceptions) and are also observed in non-ill relatives. This suggests a familial and perhaps genetic nature. Some deficits are also observed in the BP-I probands and to a lesser extent their relatives. These deficits in physiological measures may represent the intermediate phenotypes that index small effects of genes (and/or environmental factors). The use of these measures in genetic studies may help the hunt for psychosis liability genes and clarify the extent to which the 2 major psychotic disorders share etio-pathophysiology. In spite of the rich body of work describing these neurophysiological measures in psychotic disorders, challenges remain: Many of the neurophysiological phenotypes are still relatively complex and are associated with low heritability estimates. Further refinement of these physiological phenotypes is needed that could identify specific underlying physiological deficits and thereby improve their heritability estimates. The extent to which these neurophysiological deficits are unique or overlap across BP-I and schizophrenia is unclear. And finally, the clinical and functional consequences of the neurophysiological deficits both in the probands and their relatives are not well described.

Early auditory sensory processing deficits in mouse mutants with reduced NMDA receptor function

Cognitive deficits in schizophrenia include impairments at automatic, preattentive stages of sensory information processing. These deficits are evident in the prepulse inhibition- (PPI) and habituation of the auditory startle response paradigm, the paired tone paradigm in the EEG, and the peak recovery function of auditory evoked potentials (AEP). Administration of NMDA receptor antagonists reliably disrupts PPI and habituation of the startle, but not gating of AEPs in rodents. In the peak recovery paradigm, patients with schizophrenia and primates treated with NMDA receptor antagonists show reduced maximal response at long interstimulus intervals (ISI), but normal responses at short ISIs. Thus reduced NMDA receptor signalling may underlie alterations in these paradigms observed in schizophrenia. We tested the paradigms mentioned in mouse mutants with reduced expression of the NR1 subunit of the NMDA receptor (N = 15) and their wild-type littermates (N = 16). The NR1 mutant mice showed impaired habituation and PPI of the auditory startle response, as well as impaired gating in the paired tone paradigm. Deficits between the two gating measures did not correlate, corroborating previous evidence that these paradigms measure distinct processes. In the peak recovery paradigm, the NR1 mutants showed increased responses of the AEPs P1 and N1 at short ISIs but no difference between groups were observed at long ISIs. In conclusion, the NR1 hypomorphic mice modelled sensory and sensorimotor gating and startle habituation deficits observed in schizophrenia, but failed to model alterations in the peak recovery function.

Models of schizophrenia in humans and animals based on inhibition of NMDA receptors

The research of the glutamatergic system in schizophrenia has advanced with the use of non-competitive antagonists of glutamate NMDA receptors (phencyclidine, ketamine, and dizocilpine), which change both human and animal behaviour and induce schizophrenia-like manifestations. Models based on both acute and chronic administration of these substances in humans and rats show phenomenological validity and are suitable for searching for new substances with antipsychotic effects. Nevertheless, pathophysiology of schizophrenia remains unexplained. In the light of the neurodevelopmental model of schizophrenia based on early administration of NMDA receptor antagonists it seems that increased cellular destruction by apoptosis or changes in function of glutamatergic NMDA receptors in the early development of central nervous system are decisive for subsequent development of psychosis, which often does not manifest itself until adulthood. Chronic administration of antagonists initializes a number of adaptation mechanisms, which correlate with findings obtained in patients with schizophrenia; therefore, this model is also suitable for research into pathophysiology of this disease.

Haloperidol differentially modulates prepulse inhibition and p50 suppression in healthy humans stratified for low and high gating levels

Schizophrenia patients exhibit deficits in sensory gating as indexed by reduced prepulse inhibition (PPI) and P50 suppression, which have been linked to psychotic symptom formation and cognitive deficits. Although recent evidence suggests that atypical antipsychotics might be superior over typical antipsychotics in reversing PPI and P50 suppression deficits not only in schizophrenia patients, but also in healthy volunteers exhibiting low levels of PPI, the impact of typical antipsychotics on these gating measures is less clear. To explore the impact of the dopamine D2-like receptor system on gating and cognition, the acute effects of haloperidol on PPI, P50 suppression, and cognition were assessed in 26 healthy male volunteers split into subgroups having low vs high PPI or P50 suppression levels using a placebo-controlled within-subject design. Haloperidol failed to increase PPI in subjects exhibiting low levels of PPI, but attenuated PPI in those subjects with high sensorimotor gating levels. Furthermore, haloperidol increased P50 suppression in subjects exhibiting low P50 gating and disrupted P50 suppression in individuals expressing high P50 gating levels. Independently of drug condition, high PPI levels were associated with superior strategy formation and execution times in a subset of cognitive tests. Moreover, haloperidol impaired spatial working memory performance and planning ability. These findings suggest that dopamine D2-like receptors are critically involved in the modulation of P50 suppression in healthy volunteers, and to a lesser extent also in PPI among subjects expressing high sensorimotor gating levels. Furthermore, the results suggest a relation between sensorimotor gating and working memory performance.

Neurophysiological biomarkers for drug development in schizophrenia

Schizophrenia represents a pervasive deficit in brain function, leading to hallucinations and delusions, social withdrawal and a decline in cognitive performance. As the underlying genetic and neuronal abnormalities in schizophrenia are largely unknown, it is challenging to measure the severity of its symptoms objectively, or to design and evaluate psychotherapeutic interventions. Recent advances in neurophysiological techniques provide new opportunities to measure abnormal brain functions in patients with schizophrenia and to compare these with drug-induced alterations. Moreover, many of these neurophysiological processes are phylogenetically conserved and can be modelled in preclinical studies, offering unique opportunities for use as translational biomarkers in schizophrenia drug discovery.

The effects of increased central serotonergic activity on prepulse inhibition and habituation of the human startle response

Sensorimotor gating is critical to normal brain functioning, and disruptions are associated with certain mental illnesses, such as schizophrenia. Prepulse inhibition of the acoustic startle reflex (ASR) (PPI) is an operational measure of sensorimotor gating, of which evidence for a serotonergic modulation is currently inconsistent. In a double-blind placebo-controlled crossover design, 18 healthy male volunteers received either placebo or a dose of 10 mg of escitalopram (SSRI), after which they were tested in both PPI and habituation of the startle reflex paradigms. No significant differences between the two treatments were observed on PPI, although escitalopram was found to significantly delay habituation of the ASR. In the current study, escitalopram was found to delay habituation, but it did not affect PPI in healthy male volunteers. As escitalopram is a highly specific SSRI, the results suggest that an increased serotonergic activity disrupts habituation, but not PPI in healthy volunteers.

Relationship between P50 suppression and the cortical silent period

Deficient inhibitory neurotransmission has been demonstrated in schizophrenia through electroencephalography (e.g. P50 suppression) and transcranial magnetic stimulation (e.g. short-interval cortical inhibition and the cortical silent period). It is not known whether these inhibitory paradigms are related despite evidence suggesting that both are coordinated through gamma-aminobutyric acid inhibitory neurotransmission. We explored the relationship between P50 suppression, short-interval cortical inhibition and the cortical silent period in 21 healthy participants using previously published methods. P50 suppression was significantly correlated with cortical silent period (r=-0.49, P=0.02) but not with short-interval cortical inhibition. As both P50 suppression and the cortical silent period have been linked to gamma-aminobutyric acidB receptor-mediated inhibitory neurotransmission, these data highlight the importance of this receptor subtype in the pathophysiology of schizophrenia.

(+/-) Ketamine-induced prepulse inhibition deficits of an acoustic startle response in rats are not reversed by antipsychotics

Prepulse inhibition (PPI) is the reduction in the startle response caused by a low intensity non-startling stimulus (the prepulse) which is presented shortly before the startle stimulus and is an operational measure of sensorimotor gating. PPI is impaired in psychiatric disorders such as schizophrenia. Ketamine, a non-competitive N-methyl-D-aspartate antagonist has been shown to induce schizophrenia-like behavioural changes in humans and PPI deficits in rats, which can be reversed by antipsychotics. Thus, ketamine-induced PPI deficits in rats may provide a translational model of schizophrenia. The aim of this study was to investigate the effects of antipsychotic drugs and drugs known to alter the glutamate system upon ketamine-induced PPI deficits in rats. Rats were habituated to the PPI procedure [randomized trials of either pulse alone (110 dB/50 ms) or prepulse + pulse (80 dB/10 ms)]. Animals were assigned to pre-treatments based on the level of PPI on the last habituation test and balanced across startle chambers. Ketamine (1-10 mg/kg s.c; 15 min ptt) increased startle amplitude and induced PPI deficits at 6 and 10 mg/kg. PPI deficits induced by ketamine at 6 mg/kg were not attenuated by clozapine (2.5-10 mg/kg s.c.; 60 min ptt), risperidone (0.1-1 mg/kg i.p.; 60 min ptt), haloperidol (0.1-1 mg/kg i.p.; 60 min ptt), lamotrigine (3-30 mg/kg p.o.; 60 min ptt), or SB-271046-A (5-20 mg/kg p.o.; 2 hour ptt) nor potentiated by 2-methyl-6-(phenylethynyl)-pyridine (3-10 mg/kg i.p.; 30 min ptt). These results suggest that under these test conditions ketamine-induced PPI deficits in rats is relatively insensitive and does not represent a translational model for drug discovery in schizophrenia.

Prepulse inhibition of the startle reflex and its attentional modulation in the human S-ketamine and N,N-dimethyltryptamine (DMT) models of psychosis

Patients with schizophrenia exhibit diminished prepulse inhibition (PPI) of the acoustic startle reflex and deficits in the attentional modulation of PPI. Pharmacological challenges with hallucinogens are used as models for psychosis in both humans and animals. Remarkably, in contrast to the findings in schizophrenic patients and in animal hallucinogen models of psychosis, previous studies with healthy volunteers demonstrated increased levels of PPI after administration of low to moderate doses of either the antiglutamatergic hallucinogen ketamine or the serotonergic hallucinogen psilocybin. The aim of the present study was to investigate the influence of moderate and high doses of the serotonergic hallucinogen N,N-dimethyltryptamine (DMT) and the N-methyl-D-aspartate antagonist S-ketamine on PPI and its attentional modulation in humans. Fifteen healthy volunteers were included in a double-blind cross-over study with two doses of DMT and S-ketamine. Effects on PPI and its attentional modulation were investigated. Nine subjects completed both experimental days with the two doses of both drugs. S-ketamine increased PPI in both dosages, whereas DMT had no significant effects on PPI. S-ketamine decreased and DMT tended to decrease startle magnitude. There were no significant effects of either drug on the attentional modulation of PPI. In human experimental hallucinogen psychoses, and even with high, clearly psychotogenic doses of DMT or S-ketamine, healthy subjects failed to exhibit the predicted attenuation of PPI. In contrast, PPI was augmented and the startle magnitude was decreased after S-ketamine. These data point to important differences between human hallucinogen models and both animal hallucinogen models of psychosis and naturally occurring schizophrenia.

Inhibitory gating of single unit activity in amygdala: effects of ketamine, haloperidol, or nicotine

BACKGROUND

Inhibitory gating is thought to be a basic process for filtering incoming stimuli to the brain. Little information is currently available concerning local neural networks of inhibitory gating or the intrinsic neurochemical substrates involved in the process.

METHODS

The goal of the present study was to examine the pharmacological aspects of inhibitory gating from single units in the amygdala. We tested the effects of ketamine (80 mg/kg) and haloperidol (1 mg/kg) on inhibitory gating. Additionally, we examined the effect of nicotine (1.2 mg/kg) on single unit gating in this same brain structure.

RESULTS

We found that in one subset of neurons, ketamine administration significantly reduced tone responsiveness with a subsequent loss of inhibitory gating, whereas the other subset persisted in both auditory responding and gating albeit at a weaker level. Haloperidol and nicotine had very similar effects, exemplified by a dramatic increase in the response to the initial "conditioning" tone with a subsequent improvement in inhibitory gating.

CONCLUSIONS

Tone responsiveness and inhibitory gating persists in a subset of neurons after glutamate N-methyl-D-aspartate receptor blockade. Dopamine and nicotine modulate gating in these normal animals and have similar effects of enhancing responsiveness to auditory stimulation at the single unit and evoked potential level.

Region-specific reduction in entorhinal gamma oscillations and parvalbumin-immunoreactive neurons in animal models of psychiatric illness

Psychiatric illnesses, particularly schizophrenia, are associated with disrupted markers for interneuronal function and interneuron-mediated brain rhythms such as gamma frequency oscillations. Here we investigate a possible link between these two observations in the entorhinal cortex and hippocampus by using a genetic and an acute model of psychiatric illness. Lysophosphatidic acid 1 receptor-deficient (LPA1-deficient) mice show psychomotor-gating deficits and neurochemical changes resembling those seen in postmortem schizophrenia studies. Similar deficits are seen acutely with antagonism of the NMDA subtype of glutamate receptor. Neither model induced any change in power or frequency of gamma rhythms generated by kainate in hippocampal slices. In contrast, a dramatic decrease in the power of gamma oscillations was seen in superficial, but not deep, medial entorhinal cortex layers in both models. Immunolabeling for GABA, parvalbumin, and calretinin in medial entorhinal cortex from LPA1-deficient mice showed an approximately 40% reduction in total GABA- and parvalbumin-containing neurons, but no change in the number of calretinin-positive neurons. This deficit was specific for layer II (LII). No change in the number of neurons positive for these markers was seen in the hippocampus. Acute NMDA receptor blockade, which selectively reduces synaptic drive to LII entorhinal interneurons, also disrupted gamma rhythms in a similar manner in superficial entorhinal cortex, but not in hippocampus. These data demonstrate an area-specific deficit in gamma rhythmogenesis in animal models of psychiatric illness and suggest that loss, or reduction in function, of interneurons having a large NMDA receptor expression may underlie the network dysfunction that is seen.