Effects of chronic olanzapine and haloperidol differ on the mouse N1 auditory evoked potential

Auditory evoked-potential abnormalities in a mouse model of 22q11.2 Deletion Syndrome and their interactions with hearing impairment

The 22q11.2 deletion is a risk factor for multiple psychiatric disorders including schizophrenia and also increases vulnerability to middle-ear problems that can cause hearing impairment. Up to 60% of deletion carriers experience hearing impairment and ~30% develop schizophrenia in adulthood. It is not known if these risks interact. Here we used the Df1/+ mouse model of the 22q11.2 deletion to investigate how hearing impairment might interact with increased genetic vulnerability to psychiatric disease to affect brain function. We measured brain function using cortical auditory evoked potentials (AEPs), which are commonly measured non-invasively in humans. After identifying one of the simplest and best-validated methods for AEP measurement in mice from the diversity of previous approaches, we measured peripheral hearing sensitivity and cortical AEPs in Df1/+ mice and their WT littermates. We exploited large inter-individual variation in hearing ability among Df1/+ mice to distinguish effects of genetic background from effects of hearing impairment. Central auditory gain and adaptation were quantified by comparing brainstem activity and cortical AEPs and by analyzing the growth of cortical AEPs with increasing sound level or inter-tone interval duration. We found that level-dependent AEP growth was abnormally large in Df1/+ mice regardless of hearing impairment, but other AEP measures of central auditory gain and adaptation depended on both genotype and hearing phenotype. Our results demonstrate the relevance of comorbid hearing loss to auditory brain dysfunction in 22q11.2DS and also identify potential biomarkers for psychiatric disease that are robust to hearing impairment.

Neurophysiological Models in Individuals at Clinical High Risk for Psychosis: Using Translational EEG Paradigms to Forecast Psychosis Risk and Resilience

Over the last several decades, there have been major research efforts to improve the identification of youth and young adults at clinical high-risk for psychosis (CHR-P). Among individuals identified as CHR-P based on clinical criteria, approximately 20% progress to full-blown psychosis over 2-3 years and 30% achieve remission. In more recent years, neurophysiological measures with established sensitivity to schizophrenia have gained traction in the study of CHR-P and its range of clinical outcomes, with the goal of identifying specific biomarkers that precede psychosis onset that 7 chapter, we review studies examining several translational electroencephalography (EEG) and event-related potential (ERP) measures, which have known sensitivity to schizophrenia and reflect abnormal sensory, perceptual, and cognitive processing of task stimuli, as predictors of future clinical outcomes in CHR-P individuals. We discuss the promise of these EEG/ERP biomarkers of psychosis risk, including their potential to provide (a) translational bridges between human studies and animal models focused on drug development for early psychosis, (b) target engagement measures for clinical trials, and (c) prognostic indicators that could enhance personalized treatment planning.

Increased Central Auditory Gain and Decreased Parvalbumin-Positive Cortical Interneuron Density in the Df1/+ Mouse Model of Schizophrenia Correlate With Hearing Impairment

Background

Hearing impairment is a risk factor for schizophrenia. Patients with 22q11.2 deletion syndrome have a 25% to 30% risk of schizophrenia, and up to 60% also have varying degrees of hearing impairment, primarily from middle-ear inflammation. The Df1/+ mouse model of 22q11.2 deletion syndrome recapitulates many features of the human syndrome, including schizophrenia-relevant brain abnormalities and high interindividual variation in hearing ability. However, the relationship between brain abnormalities and hearing impairment in Df1/+ mice has not been examined.

Methods

We measured auditory brainstem responses, cortical auditory evoked potentials, and/or cortical parvalbumin-positive (PV+) interneuron density in over 70 adult mice (32 Df1/+, 39 wild-type). We also performed longitudinal auditory brainstem response measurements in an additional 20 animals (13 Df1/+, 7 wild-type) from 3 weeks of age.

Results

Electrophysiological markers of central auditory excitability were elevated in Df1/+ mice. PV+ interneurons, which are implicated in schizophrenia pathology, were reduced in density in the auditory cortex but not the secondary motor cortex. Both auditory brain abnormalities correlated with hearing impairment, which affected approximately 60% of adult Df1/+ mice and typically emerged before 6 weeks of age.

Conclusions

In the Df1/+ mouse model of 22q11.2 deletion syndrome, abnormalities in central auditory excitability and auditory cortical PV+ immunoreactivity correlate with hearing impairment. This is the first demonstration of cortical PV+ interneuron abnormalities correlating with hearing impairment in a mouse model of either schizophrenia or middle-ear inflammation.

Modelling mouse auditory response dynamics along a continuum of consciousness using a deep recurrent neural network

Objective Understanding neurophysiological changes that accompany transitions between anaesthetized and conscious states is a key objective of anesthesiology and consciousness science. This study aimed to characterize the dynamics of auditory-evoked potential morphology in mice along a continuum of consciousness. Approach Epidural field potentials were recorded from above the primary auditory cortices of two groups of laboratory mice: urethane-anaesthetized (A, n = 14) and conscious (C, n = 17). Both groups received auditory stimulation in the form of a repeated pure-tone stimulus, before and after receiving 10 mg/kg i.p. ketamine (AK and CK). Evoked responses were then ordered by ascending sample entropy into AK, A, CK, and C, considered to reflect physiological correlates of awareness. These data were used to train a recurrent neural network (RNN) with an input parameter encoding state. Model outputs were compared with grand-average event-related potential (ERP) waveforms. Subsequently, the state parameter was varied to simulate changes in the ERP that occur during transitions between states, and relationships with dominant peak amplitudes were quantified. Main results The RNN synthesized output waveforms that were in close agreement with grand-average ERPs for each group (r2 > 0.9, p < 0.0001). Varying the input state parameter generated model outputs reflecting changes in ERP morphology predicted to occur between states. Positive peak amplitudes within 25 to 50 ms, and negative peak amplitudes within 50 to 75 ms post-stimulus-onset, were found to display a sigmoidal characteristic during the transition from anaesthetized to conscious states. In contrast, negative peak amplitudes within 0 to 25 ms displayed greater linearity. Significance This study demonstrates a method for modelling changes in ERP morphology that accompany transitions between states of consciousness using a RNN. In future studies, this approach may be applied to human data to support the clinical use of ERPs to predict transition to consciousness.

Putative TAAR5 agonist alpha-NETA affects event-related potentials in oddball paradigm in awake mice

Trace amines have been reported to be neuromodulators of monoaminergic systems. Trace amines receptor 5 (TAAR5) is expressed in several regions of mice central nervous system, such as amygdala, arcuate nucleus and ventromedial hypothalamus, but very limited information is available on its functional role. The purpose of this study is to examine the effect of TAAR5 agonist alpha-NETA on the generation of mismatch negativity (MMN) analogue in C57BL/6 mice. Event-related potentials have been recorded from awake mice in oddball paradigms before and after the alpha-NETA administration. Alpha-NETA has been found to decrease N40 MMN-like difference, which resulted from the increased response to standard stimuli. An opposite effect has been found for the P80 component: the amplitude increased in response both to standard and deviant stimuli. A significant increase in N40 peak latency after the alpha-NETA administration has been found. This may suggest a reduced speed of information processing similar to the increase in P50 and N100 components latencies in schizophrenia patients. These results provide new evidence for a role of TAAR5 in cognitive processes.

Trace Amine-Associated Receptor 1 Agonist Modulates Mismatch Negativity-Like Responses in Mice

The trace amine-associated receptor 1 (TAAR1) is a G protein-coupled receptor widely expressed in the mammalian brain, particularly in limbic system and monoaminergic areas. It has proven to be an important modulator of dopaminergic, serotoninergic, and glutamatergic neurotransmission and is considered to be a potential useful target for the pharmacotherapy of neuropsychiatric disorders, including schizophrenia. One of the promising schizophrenia endophenotypes is a deficit in neurocognitive abilities manifested as mismatch negativity (MMN) deficit. This study examines the effect of TAAR1 partial agonist RO5263397 on the MMN-like response in freely moving C57BL/6 mice. Event-related potentials (ERPs) were recorded from awake mice in the oddball paradigm before and after RO5263397 administration. The RO5263397 (but not saline) administration increased the N40 amplitude in response to deviant stimuli. That provided the MMN-like difference at the 36-44 ms interval after the injection. The pitch deviance-elicited changes before the injection and in the control paradigm were established for the P68 component. After TAAR1 agonist administration the P68 amplitude in response both to standard and deviant stimuli was increased. These results suggest that the MMN-like response in mice may be modulated through TAAR1-dependent processes (possibly acting through the direct or indirect glutamate NMDA receptor modulation), indicating the TAAR1 agonists potential antipsychotic and pro-cognitive activity.

Chronic paroxetine treatment prevents disruption of methamphetamine-sensitive circadian oscillator in a transgenic mouse model of Huntington's disease

Circadian abnormalities seen in Huntington's disease (HD) patients are recapitulated in several HD transgenic mouse models. In mice, alongside the master clock located in the suprachiasmatic nucleus (SCN), two other oscillators may influence circadian behaviour. These are the food-entrainable oscillator (FEO) and the methamphetamine-sensitive circadian oscillator (MASCO). SCN- and MASCO- (but not FEO-) driven rhythms are progressively disrupted in the R6/2 mouse model of HD. MASCO-driven rhythms are induced by chronic treatment with low dose of methamphetamine and characterised by an increase in period length to greater than 24 h. Interestingly, the rhythms mediated by MASCO deteriorate earlier than those mediated by the SCN in R6/2 mice. Here, we used a pharmacological strategy to investigate the mechanisms underlying MASCO-driven rhythms in WT mice. In contrast to methamphetamine, chronic cocaine was ineffective in generating a MASCO-like component of activity although it markedly increased locomotion. Furthermore, neither blocking dopamine (DA) receptors (with the DA antagonist haloperidol) nor blocking neurotransmission by inhibiting the activity of vesicular monoamine transporter (with reserpine) prevented the expression of the MASCO-driven rhythms, although both treatments downregulated locomotor activity. Interestingly, chronic treatment with paroxetine, a serotonin-specific reuptake inhibitor commonly used as antidepressant in HD, was able to restore the expression of MASCO-driven rhythms in R6/2 mice. Thus, MASCO-driven rhythms appear to be mediated by both serotoninergic and dopaminergic systems. This supports the idea that abnormalities in MASCO output may contribute to both the HD circadian and psychiatric phenotype.

Sensory processing in autism spectrum disorders and Fragile X syndrome-From the clinic to animal models

Brains are constantly flooded with sensory information that needs to be filtered at the pre-attentional level and integrated into endogenous activity in order to allow for detection of salient information and an appropriate behavioral response. People with Autism Spectrum Disorder (ASD) or Fragile X Syndrome (FXS) are often over- or under-reactive to stimulation, leading to a wide range of behavioral symptoms. This altered sensitivity may be caused by disrupted sensory processing, signal integration and/or gating, and is often being neglected. Here, we review translational experimental approaches that are used to investigate sensory processing in humans with ASD and FXS, and in relevant rodent models. This includes electroencephalographic measurement of event related potentials, neural oscillations and mismatch negativity, as well as habituation and pre-pulse inhibition of startle. We outline robust evidence of disrupted sensory processing in individuals with ASD and FXS, and in respective animal models, focusing on the auditory sensory domain. Animal models provide an excellent opportunity to examine common mechanisms of sensory pathophysiology in order to develop therapeutics.

Modulation of mGlu2 Receptors, but Not PDE10A Inhibition Normalizes Pharmacologically-Induced Deviance in Auditory Evoked Potentials and Oscillations in Conscious Rats

Improvement of cognitive impairments represents a high medical need in the development of new antipsychotics. Aberrant EEG gamma oscillations and reductions in the P1/N1 complex peak amplitude of the auditory evoked potential (AEP) are neurophysiological biomarkers for schizophrenia that indicate disruption in sensory information processing. Inhibition of phosphodiesterase (i.e. PDE10A) and activation of metabotropic glutamate receptor (mGluR2) signaling are believed to provide antipsychotic efficacy in schizophrenia, but it is unclear whether this occurs with cognition-enhancing potential. The present study used the auditory paired click paradigm in passive awake Sprague Dawley rats to 1) model disruption of AEP waveforms and oscillations as observed in schizophrenia by peripheral administration of amphetamine and the N-methyl-D-aspartate (NMDA) antagonist phencyclidine (PCP); 2) confirm the potential of the antipsychotics risperidone and olanzapine to attenuate these disruptions; 3) evaluate the potential of mGluR2 agonist LY404039 and PDE10 inhibitor PQ-10 to improve AEP deficits in both the amphetamine and PCP models. PCP and amphetamine disrupted auditory information processing to the first click, associated with suppression of the P1/N1 complex peak amplitude, and increased cortical gamma oscillations. Risperidone and olanzapine normalized PCP and amphetamine-induced abnormalities in AEP waveforms and aberrant gamma/alpha oscillations, respectively. LY404039 increased P1/N1 complex peak amplitudes and potently attenuated the disruptive effects of both PCP and amphetamine on AEPs amplitudes and oscillations. However, PQ-10 failed to show such effect in either models. These outcomes indicate that modulation of the mGluR2 results in effective restoration of abnormalities in AEP components in two widely used animal models of psychosis, whereas PDE10A inhibition does not.

Brown Norway rats, a putative schizophrenia model, show increased electroencephalographic activity at rest and decreased event-related potential amplitude, power, and coherence in the auditory sensory gating paradigm

In recent schizophrenia clinical research, electroencephalographic (EEG) oscillatory activities induced by a sensory stimulus or behavioral tasks have gained considerable interest as functional and pathophysiological biomarkers. The Brown Norway (BN) rat is a putative schizophrenia model that shows naturally low sensorimotor gating and deficits in cognitive performance, although other phenotypes have not been studied. The present study aimed to investigate the neurophysiological features of BN rats, particularly EEG/event-related potential (ERP). EEG activity was recorded at rest and during the auditory sensory gating paradigm under an awake, freely moving condition. Frequency and ERP analysis were performed along with time-frequency analysis of evoked power and intertrial coherence. Compared with Wistar-Kyoto rats, a well-documented control line, BN rats showed increased EEG power at rest, particularly in the theta and gamma ranges. In ERP analysis, BN rats showed reduced N40-P20 amplitude but normal sensory gating. The rats also showed reduced evoked power and intertrial coherence against auditory stimuli. These results suggest that BN rats show features of EEG/ERP measures clinically relevant to schizophrenia and may provide additional opportunities for translational research.

Animal Models of Psychosis: Current State and Future Directions

Psychosis is an abnormal mental state characterized by disorganization, delusions and hallucinations. Animal models have become an increasingly important research tool in the effort to understand both the underlying pathophysiology and treatment of psychosis. There are multiple animal models for psychosis, with each formed by the coupling of a manipulation and a measurement. In this manuscript we do not address the diseases of which psychosis is a prominent comorbidity. Instead, we summarize the current state of affairs and future directions for animal models of psychosis. To accomplish this, our manuscript will first discuss relevant behavioral and electrophysiological measurements. We then provide an overview of the different manipulations that are combined with these measurements to produce animal models. The strengths and limitations of each model will be addressed in order to evaluate its cross-species comparability.

Auditory event-related signals in mouse ERG recordings

PURPOSE

In murine disease models, particularly in cases when retinal electrical activity is reduced, an event-related component becomes apparent that does not change with the stimulus intensity in electroretinogram (ERG) recordings. In this work, we show that this electric component is evoked by the sound of the flash discharge rather than the light flash itself.

METHODS

Wild-type mice (C57BL/6), mice with rod function only (Cnga3 (-/-)), mice lacking any photoreceptor function (Cnga3 (-/-) rho (-/-)), and mice with no auditory function (Cdh23 (vAlb/vAlb) ) were examined with Xenon flash ERG systems. An acoustic noise generator was used to mask discharge sounds.

RESULTS

ERG recording modalities were identified where usually no discernible response can be elicited. These include photopic conditions in Cnga3 (-/-) mice, photopic conditions together with very low stimulus intensities in C57BL/6 mice, and both scotopic and photopic conditions in Cnga3 (-/-) rho (-/-) mice. However, in all of these cases, small signals, featuring an initial a-wave like deflection at about 20 ms and a subsequent b-wave like deflection peaking at about 40 ms after the flash, were detected. In contrast, such signals could not be detected in deaf Cdh23 (vAlb/vAlb) mice. Furthermore, masking the Xenon discharge sound by continuous acoustic noise led to a loss of the event-related signals in a reversible manner.

CONCLUSIONS

We could identify an auditory event-related component, presumably resembling auditory evoked potentials, as a major source of ERG signals of non-visual origin in mice. This finding may be of particular importance for the analysis and interpretation of ERG data in mice with reduced visual responses.

Animal models and measures of perceptual processing in schizophrenia

This paper summarizes the discussions regarding animal paradigms for assessing perception at the seventh meeting of the Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia (CNTRICS). A breakout group at the meeting addressed candidate tests in animals that might best parallel the human paradigms selected previously in the CNTRICS program to assess two constructs in the domain of perception: gain control and visual integration. The perception breakout group evaluated the degree to which each of the nominated tasks met pre-specified criteria: comparability of tasks across multiple species; construct validity; neuroanatomical homology between species; and dynamic range across parametric variation.

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.

The PDE5 inhibitor vardenafil does not affect auditory sensory gating in rats and humans

RATIONALE

Sensory gating is an adaptive mechanism of the brain to prevent overstimulation. Patients suffering from clinical disorders such as Alzheimer's disease or schizophrenia exhibit a deficit in gating, which indicates not only an impairment in basic information processing that might contribute to the cognitive problems seen in these patients. Phosphodiesterase type 5 inhibitors (PDE5-Is) have been shown to improve cognition in rodents in various behavioural tasks and might consequently be an interesting target for cognition enhancement. However, the effects of PDE5-Is on sensory gating are not known yet.

OBJECTIVES

This work aims to study the effects of PDE5 inhibition on auditory sensory gating in rats and humans.

METHODS

In the rat study, vehicle or 0.3-3 mg/kg of the PDE5-I vardenafil was given orally 30 min before testing and electrode locations were the vertex, hippocampus and the striatum. The human subjects received placebo, 10-20 mg vardenafil 85 min before testing and sensory gating was measured at the cortex (Fz, Fcz and Cz) electrodes.

RESULTS

Significant gating was only found for the N1 component in rats while all three peaks P1, N1 and P2 showed gating in humans, i.e. the response to the second sound click was decreased as compared with the first for these deflections. Administration of vardenafil did neither have an effect on sensory gating in rats nor in humans.

CONCLUSIONS

These findings imply that positive effects of PDE5 inhibition on cognition are not mediated by more early phases of information processing.

Nicotine receptor subtype-specific effects on auditory evoked oscillations and potentials

Background

Individuals with schizophrenia show increased smoking rates which may be due to a beneficial effect of nicotine on cognition and information processing. Decreased amplitude of the P50 and N100 auditory event-related potentials (ERPs) is observed in patients. Both measures show normalization following administration of nicotine. Recent studies identified an association between deficits in auditory evoked gamma oscillations and impaired information processing in schizophrenia, and there is evidence that nicotine normalizes gamma oscillations. Although the role of nicotine receptor subtypes in augmentation of ERPs has received some attention, less is known about how these receptor subtypes regulate the effect of nicotine on evoked gamma activity.

Methodology/Principal Findings

We examined the effects of nicotine, the α7 nicotine receptor antagonist methyllycaconitine (MLA) the α4β4/α4β2 nicotine receptor antagonist dihydro-beta-erythroidine (DHβE), and the α4β2 agonist AZD3480 on P20 and N40 amplitude as well as baseline and event-related gamma oscillations in mice, using electrodes in hippocampal CA3. Nicotine increased P20 amplitude, while DHβE blocked nicotine-induced enhancements in P20 amplitude. Conversely, MLA did not alter P20 amplitude either when presented alone or with nicotine. Administration of the α4β2 specific agonist AZD3480 did not alter any aspect of P20 response, suggesting that DHβE blocks the effects of nicotine through a non-α4β2 receptor specific mechanism. Nicotine and AZD3480 reduced N40 amplitude, which was blocked by both DHβE and MLA. Finally, nicotine significantly increased event-related gamma, as did AZD3480, while DHβE but not MLA blocked the effect of nicotine on event-related gamma.

Conclusions/Significance

These results support findings showing that nicotine-induced augmentation of P20 amplitude occurs via a DHβE sensitive mechanism, but suggests that this does not occur through activation of α4β2 receptors. Event-related gamma is strongly influenced by activation of α4β2, but not α7, receptor subtypes, while disruption of N40 amplitude requires the activation of multiple receptor subtypes.

NMDA antagonists recreate signal-to-noise ratio and timing perturbations present in schizophrenia

RATIONALE

There is increasing evidence that functional deficits in schizophrenia may be driven by a reduction in the signal-to-noise ratio (SNR) and consistent timing of neural signals. This study examined the extent to which exposure to the NMDA receptor antagonists ketamine and MK801, frequently used pharmacological models of schizophrenia, recreate deficits in electrophysiological markers of disturbed brain circuits that are thought to underlie the illness. Furthermore, this study characterizes the specificity of these differences across the frequency spectrum so as to help identify the nature of selective circuit abnormalities that mediate each oscillatory response as relevant to schizophrenia.

DESIGN

Mouse EEG was recorded during exposure to repeated auditory stimuli after injection of either vehicle or drug. The dose-response relationship for each electrophysiological measure was determined for ketamine and MK-801. Time-frequency analyses were performed to assess baseline, total, and evoked power and intertrial coherence (ITC) at low (5-10 Hz) and high (35-80 Hz)-frequencies.

RESULTS

High frequency evoked and total power was decreased by MK-801 and ketamine in a dose-dependent fashion. High frequency baseline power was increased by MK-801 and ketamine in a dose-dependent fashion. Similar to evoked power, high frequency inter-trial coherence was dose-dependently decreased by both drugs. Low frequency ITC was only decreased by ketamine.

CONCLUSIONS

Both ketamine and MK-801 cause alterations in high-frequency baseline (noise), total (signal), and evoked (signal) power resulting in a loss of high frequency SNR that is thought to primarily reflect local circuit activity. These changes indicate an inappropriate increase in baseline activity, which can also be interpreted as non-task related activity. Ketamine induced a loss of intertrial coherence at low frequencies, indicating a loss of consistency in low-frequency circuit mechanisms. As a proportion of baseline power, both drugs had a relative shift from low to high frequencies, reflecting a change in the balance of brain activity from coordination of global regions to a pattern of discoordinated, autonomous local activity. These changes are consistent with a pattern of fragmented regional brain activity seen in schizophrenia.

Ketamine Modulates Theta and Gamma Oscillations

Ketamine, an N-methyl-d-aspartate (NMDA) receptor glutamatergic antagonist, has been studied as a model of schizophrenia when applied in subanesthetic doses. In EEG studies, ketamine affects sensory gating and alters the oscillatory characteristics of neuronal signals in a complex manner. We investigated the effects of ketamine on in vivo recordings from the CA3 region of mouse hippocampus referenced to the ipsilateral frontal sinus using a paired-click auditory gating paradigm. One issue of particular interest was elucidating the effect of ketamine on background network activity, poststimulus evoked and induced activity. We find that ketamine attenuates the theta frequency band in both background activity and in poststimulus evoked activity. Ketamine also disrupts a late, poststimulus theta power reduction seen in control recordings. In the gamma frequency range, ketamine enhances both background and evoked power, but decreases relative induced power. These findings support a role for NMDA receptors in mediating the balance between theta and gamma responses to sensory stimuli, with possible implications for dysfunction in schizophrenia.

Mouse behavioral endophenotypes for schizophrenia

An endophenotype is a heritable trait that is generally considered to be more highly, associated with a gene-based neurological deficit than a disease phenotype itself. Such, endophenotypic deficits may therefore be observed in the non-affected relatives of disease patients. Once endophenotypes have been established for a given illness, such as schizophrenia, mechanisms of, action may then be established and treatment options developed in order to target such measures. The, current paper describes and assesses the merits and limitations of utilizing behavioral and, electrophysiological endophenotypes of schizophrenia in mice. Such endophenotypic deficits include: decreased auditory event related potential (ERP) amplitude and gating (specifically, that of the P20, N40, P80 and P120); impaired mismatch negativity (MMN); changes in theta and gamma frequency, analyses; decreased pre-pulse inhibition (PPI); impaired working and episodic memories (for instance, novel object recognition [NOR], contextual and cued fear conditioning, latent inhibition, Morris and, radial arm maze identification and nose poke); sociability; and locomotor activity. A variety of, pharmacological treatments, including ketamine, MK-801 and phencyclidine (PCP) can be used to, induce some of the deficits described above, and numerous transgenic mouse strains have been, developed to address the mechanisms responsible for such endophenotypic differences. We also, address the viability and validity of using such measures regarding their potential clinical implications, and suggest several practices that could increase the translatability of preclinical data.

Mouse model predicts effects of smoking and varenicline on event-related potentials in humans

BACKGROUND

Nicotine alters auditory event-related potentials (ERPs) in rodents and humans and is an effective treatment for smoking cessation. Less is known about the effects of the partial nicotine agonist varenicline on ERPs.

METHODS

We measured the effects of varenicline and nicotine on the mouse P20 and varenicline and smoking on the human P50 in a paired-click task. Eighteen mice were tested following nicotine, varenicline, and their combination. One hundred and fourteen current smokers enrolled in a placebo-controlled within-subject crossover study to test the effects of varenicline during smoking and abstinence. Thirty-two subjects participated in the ERP study, with half receiving placebo first and half varenicline first (VP).

RESULTS

Nicotine and varenicline enhanced mouse P20 amplitude, while nicotine improved P20 habituation by selectively increasing the first-click response. Similar to mice, abstinence reduced P50 habituation relative to smoking by reducing the first-click response. There was no effect of varenicline on P50 amplitude during abstinence across subjects. However, there was a significant effect of medication order on P50 amplitude during abstinence. Subjects in the PV group displayed reduced P50 during abstinence, which was blocked by varenicline. However, subjects in the VP group did not display abstinence-induced P50 reduction.

CONCLUSIONS

Data suggest that smoking improves sensory processing. Varenicline mimics amplitude changes associated with nicotine and smoking but fails to alter habituation. The effect of medication order suggests a possible carryover effect from the previous arm. This study supports the predictive validity of ERPs in mice as a marker of drug effects in human studies.

Neurophysiological measures of sensory registration, stimulus discrimination, and selection in schizophrenia patients

Cortical Neurophysiological event related potentials (ERPs) are multidimensional measures of information processing that are well suited to efficiently parse automatic and controlled components of cognition that span the range of deficits exhibited in schizophrenia patients. Components following a stimulus reflect the sequence of neural processes triggered by the stimulus, beginning with early automatic sensory processes and proceeding through controlled decision and response related processes. Previous studies employing ERP paradigms have reported deficits of information processing in schizophrenia across automatic through attention dependent processes including sensory registration (N1), automatic change detection (MMN), the orienting or covert shift of attention towards novel or infrequent stimuli (P3a), and attentional allocation following successful target detection processes (P3b). These automatic and attention dependent information components are beginning to be recognized as valid targets for intervention in the context of novel treatment development for schizophrenia and related neuropsychiatric disorders. In this review, we describe three extensively studied ERP components (N1, mismatch negativity, P300) that are consistently deficient in schizophrenia patients and may serve as genetic endophenotypes and as quantitative biological markers of response outcome.

Predator odor modulates auditory event-related potentials in mice

Animals process information from different sensory modalities, requiring integration of signals and assignment of significance. People with schizophrenia perceive sensory information without external stimuli (hallucinations) and attribute meaning to coincidental events (referential delusions), suggesting deficits in sensory integration. We investigate sensory integration deficits by measuring the impact of olfactory cues on auditory processing in a mouse model of schizophrenia. N-methyl-D-aspartate-NR1 knockdown and wild-type mice were exposed to predator odor during auditory event-related potentials. Both groups reduced N1 event-related potential amplitude in the presence of predator odor, indicating that mice appropriately integrate olfactory and auditory stimuli. NR1 knockdown mice do not have deficits in this task, suggesting that sensory integration may rely on non-N-methyl-D-aspartate receptor mediated circuits.

Ondansetron results in improved auditory gating in DBA/2 mice through a cholinergic mechanism

The 5-HT(3) receptor antagonist, ondansetron, has been shown to correct the auditory gating deficit in medicated schizophrenia patients. Inhibition of 5-HT(3) receptors releases acetylcholine, the endogenous ligand for nicotinic acetylcholine receptors. The schizophrenia-related auditory gating deficit is modulated, in part, by nicotinic acetylcholine receptors, as is the mouse (DBA/2) model of the deficit. The present study assessed the effects of both acute and chronically administered ondansetron on auditory gating in DBA/2 mice. Auditory gating is defined as a decrease in amplitude of response to the second of a paired identical auditory stimulus presented 0.5 s following an initial auditory stimulus. Acute ondansetron administration at the lowest dose (0.1 mg/kg, IP) tested had no effect, while other doses (0.33 and 1 mg/kg, IP) produced improvements in auditory gating. The improvements were produced through both an increase in response to the first auditory stimulus and a decrease in the response to the second auditory stimulus. Co-administration of an alpha7 nicotinic acetylcholine receptor antagonist, alpha-bungarotoxin, or the alpha4beta2 nicotinic acetylcholine receptor antagonist dihydro-beta-erythroidine, with the 0.33 mg/kg dose of ondansetron blocked the improvement in auditory gating produced by ondansetron alone. There was no difference in response between the chronically injected mice and naive mice. Both showed improved auditory gating, thus, demonstrating no "carry over" effect of daily injections. These data demonstrate that indirect stimulation of nicotinic acetylcholine receptors by ondansetron can improve auditory gating parameters in DBA/2 mice.

Chronic ketamine impairs fear conditioning and produces long-lasting reductions in auditory evoked potentials

Ketamine is an NMDA receptor antagonist with a variety of uses, ranging from recreational drug to pediatric anesthetic and chronic pain reliever. Despite its value in the clinical setting, little is known about the immediate and long-lasting effects of repeated ketamine treatment. We assessed the effects of chronic administration of a subanesthetic dose of ketamine on contextual fear conditioning, detection of pitch deviants and auditory gating. After four, but not two, weeks of daily ketamine injections, mice exhibited decreased freezing in the fear conditioning paradigm. Gating of the P80 component of auditory evoked potentials was also significantly altered by treatment condition, as ketamine caused a significant decrease in S1 amplitude. Additionally, P20 latency was significantly increased as a result of ketamine treatment. Though no interactions were found involving test week, stimulus and treatment condition, these results suggest that repeated ketamine administration impairs fear memory and has lasting effects on encoding of sensory stimuli.

Neuregulin 1 transgenic mice display reduced mismatch negativity, contextual fear conditioning and social interactions

INTRODUCTION

Neuregulin-1 (NRG1) is one of susceptibility genes for schizophrenia and plays critical roles in glutamatergic, dopaminergic and GABAergic signaling. Using mutant mice heterozygous for Nrg1 (Nrg1(+/-)) we studied the effects of Nrg1 signaling on behavioral and electrophysiological measures relevant to schizophrenia.

EXPERIMENTAL PROCEDURE

Behavior of Nrg1(+/-) mice and their wild type littermates was evaluated using pre-pulse inhibition, contextual fear conditioning, novel object recognition, locomotor, and social choice paradigms. Event-related potentials (ERPs) were recorded to assess auditory gating and novel stimulus detection.

RESULTS

Gating of ERPs was unaffected in Nrg1(+/-) mice, but mismatch negativity in response to novel stimuli was attenuated. The Nrg1(+/-) mice exhibited behavioral deficits in contextual fear conditioning and social interactions, while locomotor activity, pre-pulse inhibition and novel object recognition were not impaired.

SUMMARY

Nrg1(+/-) mice had impairments in a subset of behavioral and electrophysiological tasks relevant to the negative/cognitive symptom domains of schizophrenia that are thought to be influenced by glutamatergic and dopaminergic neurotransmission. These mice are a valuable tool for studying endophenotypes of schizophrenia, but highlight that single genes cannot account for the complex pathophysiology of the disorder.

Event-related oscillations as risk markers in genetic mouse models of high alcohol preference

Mouse models have been developed to simulate several relevant human traits associated with alcohol use and dependence. However, the neurophysiological substrates regulating these traits remain to be completely elucidated. We have previously demonstrated that differences in the event-related potential (ERP) responses can be found that distinguish high-alcohol preferring from low alcohol preferring mice that resemble differences seen in human studies of individuals with high and low risk for alcohol dependence. Recently, evidence of genes that affect event-related oscillations (EROs) and the risk for alcohol dependence has emerged, however, to date EROs have not been evaluated in genetic mouse models of high and low alcohol preference. Therefore, the objective of the present study was to characterize EROs in mouse models of high (C57BL/6 [B6] and high alcohol preference 1 [HAP-1] mice) and low (DBA/2J [D2] and low alcohol preference-1 [LAP-1] mice) alcohol preference. A time-frequency representation method was used to determine delta, theta and alpha/beta ERO energy and the degree of phase variation in these mouse models. The present results suggest that the decrease in P3 amplitudes previously shown in B6 mice, compared to D2 mice, is related to reductions in evoked delta ERO energy and delta and theta phase locking. In contrast, the increase in P1 amplitudes reported in HAP-1 mice, compared to LAP-1 mice, is associated with increases in evoked theta ERO energy. These studies suggest that differences in delta and theta ERO measures in mice mirror changes observed between groups at high- and low-risk for alcoholism where changes in EROs were found to be more significant than group differences in P3 amplitudes, further suggesting that ERO measures are more stable endophenotypes in the study of alcohol dependence. Further studies are needed to determine the relationship between expression of these neurophysiological endophenotypes and the genetic profile of these mouse models.

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.

Event-related oscillations in mice: effects of stimulus characteristics

Event-related oscillations (EROs) are rhythmic changes that are evoked by sensory and/or cognitive processes that influence the dynamics of the EEG. EROs are estimated by a decomposition of the EEG signal into phase and magnitude information for a range of frequencies and then changes in those frequencies are characterized over a millisecond time scale with respect to task events. EROs have been demonstrated to be sensitive measures of both normal and abnormal cognitive functioning in humans but have not been fully described in mice. The results of these studies demonstrate that EROs can be generated in cortical sites in mice in the delta, theta, alpha/beta frequency ranges in response to auditory stimuli. Oscillations in the 7.5-40 Hz frequencies were significantly affected in the 0-50 ms time range in response to differences in tone frequency. Whereas, changes in tone loudness produced changes in oscillations in the 7.5-40 Hz frequencies in the 350-800 ms range. No significant changes in EROs were found to differences in tone probability. These studies suggest that EROs are an electrophysiological assay sensitive to tone characteristics and as such may be suitable for the exploration of the effects of genetic or neuropharmacological manipulations on neurosensory processing in mice.

Role of beta2-containing nicotinic acetylcholine receptors in auditory event-related potentials

RATIONALE

Nicotine improves sensory processing in schizophrenic individuals, as measured by changes in auditory event-related potentials (ERPs). Nicotine administration also alters ERPs in mice by increasing the amplitude and gating of the P20 ERP component while decreasing the amplitude of the N40 ERP component. Less is known about the role of specific nicotinic acetylcholine receptor (nAChR) subtypes.

OBJECTIVES

In this study, we examined whether nAChRs containing the beta2 subunit contribute to nicotine's effects on auditory ERPs.

MATERIALS AND METHODS

We tested the effect of nicotine in wild-type mice and mice lacking the beta2 nAChR subunit. Mice underwent stereotaxic implantation of stainless steel electrodes located in the CA3 region of the hippocampus, and 50 paired click stimuli were delivered during each drug condition.

RESULTS

There was no significant difference in P20 or N40 amplitude or gating between genotypes during the control condition, suggesting that beta2-containing receptors are not essential for the baseline auditory ERP response. Nicotine increased P20 amplitude and enhanced gating in wild-type and beta2 knockout mice, but only decreased N40 amplitude in wild-type mice. There was no effect of nicotine on N40 gating in either genotype.

CONCLUSIONS

beta2-containing receptors are necessary for nicotine's effects on the N40 component of the mouse auditory ERP. These results suggest that beta2-containing nAChRs modulate sensory processing and may serve as a therapeutic target in schizophrenic individuals.

N-methyl-d-aspartic acid receptor antagonist-induced frequency oscillations in mice recreate pattern of electrophysiological deficits in schizophrenia

INTRODUCTION

Electrophysiological responses to auditory stimuli have provided a useful means of elucidating mechanisms and evaluating treatments in psychiatric disorders. Deficits in gating during paired-click tasks and lack of mismatch negativity following deviant stimuli have been well characterized in patients with schizophrenia. Recently, analyses of basal, induced, and evoked frequency oscillations have gained support as additional measures of cognitive processing in patients and animal models. The purpose of this study is to examine frequency oscillations in mice across the theta (4-7.5 Hz) and gamma (31-61 Hz) bands in the context of N-methyl-d-aspartic acid receptor (NMDAR) hypofunction and dopaminergic hyperactivity, both of which are thought to serve as pharmacological models of schizophrenia.

EXPERIMENTAL PROCEDURES

Electroencephalograms (EEG) were recorded from mice in five treatment groups that consisted of haloperidol, risperidone, amphetamine, ketamine, or ketamine plus haloperidol during an auditory task. Basal, induced and evoked powers in both frequencies were calculated.

RESULTS

Ketamine increased basal power in the gamma band and decreased the evoked power in the theta band. The increase in basal gamma was not blocked by treatment with a conventional antipsychotic. No other treatment group was able to fully reproduce this pattern in the mice.

CONCLUSIONS

Ketamine-induced alterations in EEG power spectra are consistent with abnormalities in the theta and gamma frequency ranges reported in patients with schizophrenia. Our findings support the hypothesis that NMDAR hypofunction contributes to the deficits in schizophrenia and that the dopaminergic pathways alone may not account for these changes.

A novel electrophysiological model of chemotherapy-induced cognitive impairments in mice

PURPOSE

Chemotherapeutic agents are known to produce persistent cognitive deficits in cancer patients. However, little progress has been made in developing animal models to explore underlying mechanisms and potential therapeutic interventions. We report an electrophysiological model of chemotherapy-induced cognitive deficits using a sensory gating paradigm, to correspond with performance in two behavioral tasks.

EXPERIMENTAL DESIGN

Mice received four weekly injections of methotrexate and 5-fluorouracil. Whole-brain event-related potentials (ERPs) were recorded throughout using a paired-click paradigm. Mice underwent contextual fear conditioning (CFC) and novel-object recognition testing (NOR).

RESULTS

Chemotherapy-treated animals showed significantly impaired gating 5 weeks after drug treatments began, as measured by the ratio of the first positive peak in the ERP (P1) minus the first negative peak (N1) between first and second auditory stimuli. There was no effect of drug on the amplitude of P1-N1 or latency of P1. The drug-treated animals also showed significantly increased freezing during fear conditioning and increased exploration without memory impairment during novel object recognition.

CONCLUSIONS

Chemotherapy causes decreased ability to gate incoming auditory stimuli, which may underlie associated cognitive impairments. These gating deficits were associated with a hyperactive response to fear conditioning and reduced adaptation to novel objects, suggesting an additional component of emotional dysregulation. However, amplitudes and latencies of ERP components were unaffected, as was NOR performance, highlighting the subtle nature of these deficits.

Deviance-elicited Changes in Event-related Potentials are Attenuated by Ketamine in Mice

Background: People with schizophrenia exhibit reduced ability to detect change in the auditory environment, which has been linked to abnormalities in N-methyl-D-aspartate (NMDA) receptor-mediated glutamate neurotransmission. This ability to detect changes in stimulus qualities can be measured with electroencephalography using auditory event-related potentials (ERPs). For example, reductions in the N100 and mismatch negativity (MMN), in response to pitch deviance, have been proposed as endophenotypes of schizophrenia. This study examines a novel rodent model of impaired pitch deviance detection in mice using the NMDA receptor antagonist ketamine. Methods: ERPs were recorded from unanesthetized mice during a pitch deviance paradigm prior to and following ketamine administration. First, N40 amplitude was evaluated using stimuli between 4 and 10 kHz to assess the amplitude of responses across the frequency range used. The amplitude and latency of the N40 were analyzed following standard (7 kHz) and deviant (5–9 kHz) stimuli. Additionally, we examined which portions of the ERP are selectively altered by pitch deviance to define possible regions for the mouse MMN. Results: Mice displayed increased N40 amplitude that was followed by a later negative component between 50 and 75 msec in response to deviant stimuli. Both the increased N40 and the late N40 negativity were attenuated by ketamine. Ketamine increased N40 latency for both standard and deviant stimuli alike. Conclusions: The mouse N40 and a subsequent temporal region have deviance response properties similar to the human N100 and, possibly, MMN. Deviance responses were abolished by ketamine, suggesting that ketamine-induced changes in mice mimic deviance detection deficits in schizophrenia.

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.

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.

In vitro and in vivo demonstration of risperidone implants in mice

BACKGROUND

Non-adherence with medication is a critical limitation in current long-term treatment of schizophrenia and a primary factor in poor quality-of-life outcomes. However, few treatments have addressed this shortcoming using an implantable drug delivery approach. The goal of this study was to provide in vitro and in vivo proof of concept for a long-term implantable risperidone delivery system in mice.

METHODS

Implantable formulations of risperidone were created using the biodegradable polymer Poly Lactic co Glycolic Acid (PLGA) combined with various drug loads. Implant bioactivity was tested using in vitro release and stability studies, as well as in vivo pharmacokinetic and behavioral studies in mice.

RESULTS

The pattern of risperidone release is influenced by various parameters, including polymer composition and drug load. In vitro measures demonstrate that risperidone is stable in implants under physiological conditions. Behavioral measures demonstrate the bioactivity of risperidone implants delivering 3 mg/kg/day in mice, while pharmacokinetic analyses indicate that reversibility is maintained throughout the delivery interval.

CONCLUSIONS

The current report suggests that implantable formulations are a viable approach to providing long-term delivery of antipsychotic medications based on in vivo animal studies and pharmacokinetics. Implantable medications demonstrated here can last two months or longer while maintaining coherence and removability past full release, suggesting a potential paradigm shift in the long-term treatment of 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.

Effects of nicotine vary across two auditory evoked potentials in the mouse

BACKGROUND

Schizophrenia patients display sensory processing deficits, reduced alpha7-nicotine receptor expression, and increased incidence of smoking, prompting investigation of nicotine receptor agonists as possible treatments. We evaluated the effects of acute and chronic nicotine, using an animal model that incorporates genetic variation for sensory processing and nicotine sensitivity.

METHODS

C57BL/6J and DBA/2Hsd mice received 2 weeks of 4.2 mg/kg chronic nicotine or saline. Auditory evoked potentials were recorded before and after acute nicotine injection of 1.05 mg/kg on day 14, with a paired-click paradigm (S1/S2). Amplitude and gating of the P20 and N40 were compared between conditions.

RESULTS

Acute nicotine increased the amplitude and gating of the P20 and decreased the amplitude and gating of the N40 across all groups, primarily by acting on S1. Chronic nicotine attenuated the effects of acute nicotine on the N40.

CONCLUSIONS

Our data support the notion that the mouse P20 shares pharmacological response properties with the human P50. In addition, findings suggest that nicotine might increase the initial sensory response (S1), with a resulting improvement in gating of some components.

Mecamylamine blocks nicotine-induced enhancement of the P20 auditory event-related potential and evoked gamma

Cigarette smoking is significantly more prevalent in individuals with schizophrenia than in non-affected populations. Certain neurocognitive deficits and disruptions common in schizophrenia may be altered by smoking, leading to the hypothesis that schizophrenics engage in smoking behavior to alleviate specific neurocognitive symptoms of the disorder. Additionally, research suggests that individuals with schizophrenia have altered auditory event-related potentials (ERPs) and abnormalities in evoked gamma oscillations which are both indices of sensory information processing. This study was conducted to examine the effect of acute administration of nicotine and the non-specific nicotinic antagonist mecamylamine on the P20 and N40 components of the ERP and evoked gamma oscillations in mice. Acute nicotine (1 mg/kg) significantly increased P20 amplitude, an effect that was blocked by pretreatment with mecamylamine (2 mg/kg). Additionally, acute nicotine increased the normal burst of evoked gamma following an auditory stimulus. The increase in evoked gamma was also blocked by mecamylamine pretreatment. Although acute nicotine decreased amplitude of the N40 component, this decrease was not attenuated by mecamylamine. These results replicate findings that nicotine may enhance early sensory information processing through the nicotinic acetylcholinergic receptor system in an established model (ERPs) and extend these findings in an emerging, novel model (evoked gamma oscillations) of sensory information processing. The results also support the hypothesis that nicotine may be beneficial to individuals with deficits in neurocognitive functions, such as those suffering from schizophrenia.

Impaired attentional modulation of auditory evoked potentials in N-methyl-D-aspartate NR1 hypomorphic mice

In human neurophysiology, auditory event-related potentials (AEPs) are used to investigate cognitive processes such as selective attention. Selective attention to specific tones causes a negative enhancement of AEPs known as processing negativity (PN), which is reduced in patients with schizophrenia. The evidence suggests that impaired selective attention in these patients may partially depend on deficient N-methyl-D-aspartate receptor (NMDAR)-mediated signaling. The goal of this study was to corroborate the involvement of the NMDAR in selective attention using a mouse model. To this end, we first investigated the presence of PN-like activity in C57BL/6J mice by recording AEPs during a fear-conditioning paradigm. Two alternating trains of tones, differing in stimulus duration, were presented on 7 subsequent days. One group received a mild foot shock delivered within the presentation of one train (conditioning train) on days 3-5 (conditioning days), while controls were never shocked. The fear-conditioned group (n= 9) indeed showed a PN-like activity during conditioning days manifested as a significant positive enhancement in the AEPs to the stimuli in the conditioning train that was not observed in the controls. The same paradigm was then applied to mice with reduced expression of the NMDAR1 (NR1) subunit and to a wild-type control group (each group n= 6). The NR1 mutants showed an associative AEP enhancement, but its magnitude was significantly reduced as compared with the magnitude in wild-type mice. We conclude that electrophysiological manifestations of selective attention are observable yet of different polarity in mice and that they require intact NMDAR-mediated signaling. Thus, deficient NMDAR functioning may contribute to abnormal selective attention in schizophrenia.

Mice expressing constitutively active Gsalpha exhibit stimulus encoding deficits similar to those observed in schizophrenia patients

People with schizophrenia display sensory encoding deficits across a broad range of electrophysiological and behavioral measures, suggesting fundamental impairments in the ability to transduce the external environment into coherent neural representations. This inability to create basic components of complex stimuli interferes with a high fidelity representation of the world and likely contributes to cognitive deficits. The current study evaluates the effects of constitutive forebrain activation of the G(s)alpha G-protein subunit on auditory threshold and gain using acoustic brainstem responses and cortically generated N40 event-related potentials to assess the role of cyclic AMP signaling in sensory encoding. Additionally, we examine the ability of pharmacological treatments that mimic (amphetamine) or ameliorate (haloperidol) positive symptoms of schizophrenia to test the hypothesis that the encoding deficits observed in G(s)alpha transgenic mice can be normalized with treatment. We find that G(s)alpha transgenic mice have decreased amplitude of cortically generated N40 but normal acoustic brainstem response amplitude, consistent with forebrain transgene expression and a schizophrenia endophenotype. Transgenic mice also display decreased stimulus intensity response (gain) in both acoustic brainstem response and N40, indicating corticofugal influence on regions that lack transgene expression. N40 deficits in transgenic animals were ameliorated with low dose haloperidol and reversed with higher dose, suggesting dopamine D2 receptor-linked Gi activity contributes to the impairment. Consistent with this hypothesis, we recreated the G(s)alpha transgenic deficit in wild type animals using the indirect dopamine agonist amphetamine. This transgenic model of sensory encoding deficits provides a foundation for identifying biochemical contributions to sensory processing impairments associated with schizophrenia.

Corticosterone modulates auditory gating in mouse

Previous studies suggest that circulating glucocorticoids may influence the encoding and processing of sensory stimuli. The current study investigated this hypothesis by measuring the generation (amplitude), gating (recovery cycle), and sensitivity (intensity function) of auditory evoked responses in C57BL/6 mice treated with chronic corticosterone (0, 1, 5, 15, or 30 mg/kg/day for 14 days). We found that low-dose corticosterone (5 but not 1 mg/kg/day) enhanced the amplitude and improved gating of evoked potentials without affecting the intensity function. In comparison, higher doses (15 and 30 mg/kg/day) decreased the amplitude and impaired gating of evoked potentials, also without altering the stimulus intensity function. At all doses, lower amplitudes of evoked potentials were significantly correlated with higher circulating corticosterone levels. These data highlight the need to consider serum glucocorticoid levels when assessing human disease states associated with aberrations of information processing such as schizophrenia and depression.

Convergence and divergence in the neurochemical regulation of prepulse inhibition of startle and N40 suppression in rats

Prepulse inhibition of startle ('PPI'), a cross-species measure of sensorimotor gating, is impaired in schizophrenia patients. Suppression of P50 event-related potentials (ERPs) in response to the second of two clicks ('P50 gating') is also impaired in schizophrenia. Suppression of N40 ERPs to the second of two clicks ('N40 gating') is thought by some to be a rat homolog of human P50 gating. Emerging evidence suggests differences in the neurobiology of deficits detected by PPI vs P50 (or N40) gating. We recorded PPI and N40 gating contemporaneously in rats, to assess convergence and divergence in the neurochemical regulation of these measures. Dose-response studies examined the effects of apomorphine (APO), phencyclidine (PCP) or the 5HT2A agonist DOI on PPI, and on motor responses to stimuli (S1 and S2) that elicit N40 gating. Effects of optimal drug doses on PPI and N40 gating were then assessed in other rats with implanted cortical surface electrodes. APO, PCP and DOI caused dose-dependent disruptions of both PPI and gating of motor responses to N40 stimuli. Reduced PPI reflected diminished prepulse effectiveness, demonstrated by increased startle levels on prepulse+pulse trials. In contrast, reduced gating of motor responses to N40 stimuli reflected a reduced motor response to S1. In separate rats, robust PPI, N40 potentials and N40 gating could be detected within one test. PPI and N40 gating were disrupted by APO, PCP, and DOI. Again, drug effects on PPI reflected increased startle on prepulse+pulse trials, while those on N40 gating reflected reduced ERP responses to S1. In conclusion, when PPI and N40 gating were studied concurrently in rats, drug effects on PPI reflected reduced inhibition of startle by the prepulse, while diminished N40 gating reflected S1 response suppression. Despite similarities in drug sensitivity, these results suggest that distinct neurobiological mechanisms underlie drug-induced deficits in PPI and N40 gating.

Hypericum extract reverses S-ketamine-induced changes in auditory evoked potentials in humans - possible implications for the treatment of schizophrenia

BACKGROUND

Auditory evoked potentials (AEP) provide a correlate of cognitive dysfunction in schizophrenia. Both cognitive dysfunction and AEP-characteristics might be related to reduced glutamatergic neurotransmission as induced by glutamate-antagonist like ketamine. Hypericum extract LI160 has demonstrated a ketamine-antagonising effect. We examined whether LI160 reverses changes of a low dose ketamine on AEP in healthy subjects.

METHODS

We performed a double-blind randomized treatment with either 2 x 750 mg LI 160 or placebo given one week, using a crossover design, in 16 health subjects. A test-battery including AEPs, the oculodynamic test (ODT) and a cognitive test were performed before and after an infusion with 4 mg of S-ketamine over a period of 1 hour.

RESULTS

S-ketamine lead to a significant decrease in the N100-P200 peak to peak (ptp) amplitude after the placebo treatment, whereas ptp was significantly increased by S-ketamine infusion in the LI160 treated subjects. The ODT and the cognitive testing revealed no significant effect of ketamine-infusion and therefore no interaction between treatment groups.

CONCLUSIONS

AEP measures are sensitive means to assess the effect of low dose ketamine. Provided that ketamine mimics cognitive deficits in schizophrenia, LI160 might be effective to treat these symptoms.

Ketamine produces lasting disruptions in encoding of sensory stimuli

The current study analyzed the acute, chronic, and lasting effects of ketamine administration in four inbred mouse strains (C3H/HeHsd, C57BL/6Hsd, FVB/Hsd, and DBA/2Hsd) to evaluate vulnerability to ketamine as a drug of abuse and as a model of schizophrenia. Serum half-life of ketamine was similar between all strains (approximately 13 min). Also, the ratio of brain-to-serum ketamine levels was 3:1. Examination of multiple phases of auditory processing using auditory-evoked potentials (AEPs) following acute ketamine (0, 5, and 20 mg/kg) treatment revealed C3H/HeHsd mice to be most vulnerable to ketamine-induced alterations in AEPs, whereas FVB/Hsd mice exhibited the least electrophysiological sensitivity to ketamine. Overall, the precortical P1-evoked potential component increased in amplitude and latency, whereas the cortically generated N1 and P2 components decreased in amplitude and latency following acute ketamine across all strains. Brain catecholamine analyses indicated that ketamine decreased hippocampus epinephrine levels in C3H/HeHsd but elevated hippocampus epinephrine levels in FVB/Hsd, suggesting one potential mechanism for AEP vulnerability to ketamine. Based on results of the acute study, the immediate and lasting effects of chronic low-dose ketamine on AEPs were examined among C3H/HeHsd (sensitive) and FVB/Hsd (insensitive) mice. We observed a decrement of the N1 amplitude that persisted at least 1 week after the last exposure to ketamine across both strains. This lasting deficit in information processing occurred in the absence of acute changes among the FVB/Hsd mice. Implications for both ketamine abuse and N-methyl-D-aspartate hypofunction models of schizophrenia are discussed.

Tropisetron improves deficient inhibitory auditory processing in DBA/2 mice: role of alpha 7 nicotinic acetylcholine receptors

RATIONALE

Deficient inhibitory processing of the P50 auditory evoked potential is a pathophysiological feature of schizophrenia. Several lines of evidence suggest that alpha 7 nicotinic receptors play a critical role in this phenomenon. Similar to schizophrenic patients, DBA/2 mice spontaneously exhibit a deficit in inhibitory processing of the P20-N40 auditory evoked potential, which is thought to be a rodent analog of the human P50 auditory evoked potential.

OBJECTIVE

The present study was undertaken to examine whether tropisetron, a partial agonist at alpha 7 nicotinic receptors and an antagonist at 5-hydroxytryptamine-3 receptors, improves this deficit in DBA/2 mice.

RESULTS

Administration of tropisetron (1 mg/kg i.p.) significantly improved the deficient inhibitory processing of the P20-N40 auditory evoked potential in DBA/2 mice. Coadministration of methyllycaconitine (MLA; 3 mg/kg i.p.), a partially selective antagonist at alpha 7 nicotinic receptors, significantly blocked the normalizing effect of tropisetron. Furthermore, MLA alone did not alter the deficient inhibitory processing of the P20-N40 auditory evoked potential in DBA/2 mice.

CONCLUSIONS

The data suggest that tropisetron improves the deficient inhibitory processing of the P20-N40 auditory evoked potential in DBA/2 mice by effects on alpha 7 and perhaps alpha 4 beta 2 nicotinic receptors. Tropisetron may be useful for the treatment of deficient inhibitory processing in schizophrenia.

Phosphodiesterase inhibitors: a novel mechanism for receptor-independent antipsychotic medications

OVERVIEW

All current antipsychotic medications work by binding to Gi-coupled dopamine (DA) D2 receptors. Such medications are thought to affect cellular function primarily by decreasing DA-mediated regulation of intracellular cyclic adenosine monophosphate (cAMP).However, several studies indicate that cAMP signal transduction abnormalities in schizophrenia may not be limited to D2-containing cells. The current study examines the potential of using non-receptor-based agents that modify intracellular signal transduction as potential antipsychotic medications.

METHODS

The indirect DA agonist amphetamine has been used to model the auditory sensory processing deficits in schizophrenia. Such pharmacologically induced abnormalities are reversed by current antipsychotic treatments. This study examines the ability of the phosphodiesterase-4 inhibitor, rolipram, to reverse amphetamine-induced abnormalities in auditory-evoked potentials that are characteristic of schizophrenia.

RESULTS

Rolipram reverses amphetamine-induced reductions in auditory-evoked potentials.

CONCLUSION

This finding could lead to novel approaches to receptor-independent treatments for schizophrenia.

Monoamine reuptake inhibition and nicotine receptor antagonism reduce amplitude and gating of auditory evoked potentials

BACKGROUND

Sensory encoding deficits have been extensively studied as endophenotypic markers of schizophrenia using auditory evoked potentials. In order to increase understanding of the neurochemical basis of such deficits, we utilized an animal model to test whether monoamine reuptake inhibition and nicotine receptor antagonism reduce the amplitude and gating of the P20 and N40 auditory components.

METHODS

C57BL/6J mice received 12 days of chronic vehicle, bupropion, haloperidol or bupropion plus haloperidol. Auditory evoked potentials were then recorded in alert mice to measure the amplitude and gating of evoked components during a paired click paradigm similar to tasks used to measure the P50 and N100 auditory potentials in schizophrenia. Evoked potentials were recorded prior to and following acute nicotine.

RESULTS

Bupropion reduced the amplitude and gating of the N40 evoked potential in mice, similar to the P50 and N100 endophenotypes associated with sensory encoding deficits in schizophrenia. This deficit was fully reversed only by the combination of haloperidol and nicotine, suggesting that dopamine reuptake inhibition and nicotine antagonism both contribute to the observed phenotype. Furthermore, nicotine increased P20 amplitude across all groups supporting a role for nicotine agonists in pre-attentive sensory encoding deficits.

CONCLUSIONS

We propose that the combination of monoamine inhibition and nicotine receptor antagonism may serve as a useful model for preclinical screening of pharmaceutical compounds aimed at treating sensory encoding deficits in schizophrenia.