Cholinergic gating of response to auditory stimuli in rat hippocampus

Experimental Medicine Approaches in Early-Phase CNS Drug Development

Traditionally, Phase 1 clinical trials were largely conducted in healthy normal volunteers and focused on collection of safety, tolerability, and pharmacokinetic data. However, in the CNS therapeutic area, with more drugs failing in later phase development, Phase 1 trials have undergone an evolution that includes incorporation of novel approaches involving novel study designs, inclusion of biomarkers, and early inclusion of patients to improve the pharmacologic understanding of novel CNS-active compounds early in clinical development with the hope of improving success in later phase pivotal trials. In this chapter, the authors will discuss the changing landscape of Phase 1 clinical trials in CNS, including novel trial methodology, inclusion of pharmacodynamic biomarkers, and experimental medicine approaches to inform early decision-making in clinical development.

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.

Selective 5HT3 antagonists and sensory processing: a systematic review

Ondansetron is a selective serotonin (5HT3) receptor antagonist that is under evaluation as an adjunctive treatment for schizophrenia, and a novel treatment for hallucinations in Parkinson's disease. Ondansetron reverses sensory gating deficits and improves visuoperceptual processing in animal models of psychosis, but it is unclear to what extent preclinical findings have been replicated in humans. We systematically reviewed human studies that evaluated the effects of ondansetron and other 5HT3 receptor antagonists on sensory gating deficits or sensory processing. Of 11 eligible studies, eight included patients with schizophrenia who were chronically stable on antipsychotic medication; five measured sensory gating using the P50 suppression response to a repeated auditory stimulus; others included tests of visuoperceptual function. Three studies in healthy participants included tests of visuoperceptual and sensorimotor function. A consistent and robust finding (five studies) was that ondansetron and tropisetron (5HT3 antagonist and α7-nicotinic receptor partial agonist) improved sensory gating in patients with schizophrenia. Tropisetron also improved sustained visual attention in non-smoking patients. There was inconsistent evidence of the effects of 5HT3 antagonists on other measures of sensory processing, but interpretation was limited by the small number of studies, methodological heterogeneity and the potential confounding effects of concomitant medication in patients. Despite these limitations, we found strong evidence that selective 5HT3 antagonists (with or without direct α7-nicotinic partial agonist effects) improved sensory gating. Future studies should investigate how this relates to potential improvement in neurocognitive symptoms in antipsychotic naive patients with prodromal or milder symptoms, in order to understand the clinical implications.

Schizophrenia, Bipolar Disorder and Pre-Attentional Inhibitory Deficits

According to recent findings schizophrenia and bipolar disorder as separate disease entities manifest similarities in neuropsychological functioning. Typical disturbances in both disorders are related to sensory gating deficits characterized by decreased inhibitory functions in responses to various insignificant perceptual signals which are experimentally tested by event related potentials (ERP) and measured P50 wave. In this context, recent findings implicate that disrupted binding and disintegration of consciousness in schizophrenia and bipolar disorder that are related to inhibitory deficits reflected in P50 response may explain similarities in psychotic disturbances in both disorders. With this aim, this review summarizes literature about P50 in both schizophrenia and bipolar disorder.

Remodeling of cholinergic input to the hippocampus after noise exposure and tinnitus induction in Guinea pigs

Here, we investigate remodeling of hippocampal cholinergic inputs after noise exposure and determine the relevance of these changes to tinnitus. To assess the effects of noise exposure on the hippocampus, guinea pigs were exposed to unilateral noise for 2 hr and 2 weeks later, immunohistochemistry was performed on hippocampal sections to examine vesicular acetylcholine transporter (VAChT) expression. To evaluate whether the changes in VAChT were relevant to tinnitus, another group of animals was exposed to the same noise band twice to induce tinnitus, which was assessed using gap-prepulse Inhibition of the acoustic startle (GPIAS) 12 weeks after the first noise exposure, followed by immunohistochemistry. Acoustic Brainstem Response (ABR) thresholds were elevated immediately after noise exposure for all experimental animals but returned to baseline levels several days after noise exposure. ABR wave I amplitude-intensity functions did not show any changes after 2 or 12 weeks of recovery compared to baseline levels. In animals assessed 2-weeks following noise-exposure, hippocampal VAChT puncta density decreased on both sides of the brain by 20-60% in exposed animals. By 12 weeks following the initial noise exposure, changes in VAChT puncta density largely recovered to baseline levels in exposed animals that did not develop tinnitus, but remained diminished in animals that developed tinnitus. These tinnitus-specific changes were particularly prominent in hippocampal synapse-rich layers of the dentate gyrus and areas CA3 and CA1, and VAChT density in these regions negatively correlated with tinnitus severity. The robust changes in VAChT labeling in the hippocampus 2 weeks after noise exposure suggest involvement of this circuitry in auditory processing. After chronic tinnitus induction, tinnitus-specific changes occurred in synapse-rich layers of the hippocampus, suggesting that synaptic processing in the hippocampus may play an important role in the pathophysiology of tinnitus.

Gulf War illness associated with abnormal auditory P1 event-related potential: Evidence of impaired cholinergic processing replicated in a national sample

Our team previously reported event-related potential (ERP) and hyperarousal patterns from a study of one construction battalion of the U.S. Naval Reserve who served during the 1991 Persian Gulf War. We sought to replicate these findings in a sample that was more representative of the entire Gulf War-era veteran population, including male and female participants from four branches of the military. We collected ERP data from 40 veterans meeting Haley criteria for Gulf War syndromes 1-3 and from 22 matched Gulf War veteran controls while they performed an auditory oddball task. Reports of hyperarousal from the ill veterans were significantly greater than those from the control veterans, and P1 amplitudes in Syndromes 2 and 3 were significantly higher than P1 amplitudes in Syndrome 1, replicating our previous findings. Many of the contributors to the generation of the P1 potential are also involved in the regulation of arousal and are modulated by cholinergic and dopaminergic systems-two systems whose dysfunction has been implicated in Gulf War illness. These differences among the three syndrome groups where their means were on either side of controls is a replication of our previous ERP study and is consistent with previous imaging studies of this population.

Neural and behavioral mechanisms of proactive and reactive inhibition

Response inhibition is an important component of adaptive behavior. Substantial prior research has focused on reactive inhibition, which refers to the cessation of a motor response that is already in progress. More recently, a growing number of studies have begun to examine mechanisms underlying proactive inhibition, whereby preparatory processes result in a response being withheld before it is initiated. It has become apparent that proactive inhibition is an essential component of the overall ability to regulate behavior and has implications for the success of reactive inhibition. Moreover, successful inhibition relies on learning the meaning of specific environmental cues that signal when a behavioral response should be withheld. Proactive inhibitory control is mediated by stopping goals, which reflect the desired outcome of inhibition and include information about how and when inhibition should be implemented. However, little is known about the circuits and cellular processes that encode and represent features in the environment that indicate the necessity for proactive inhibition or how these representations are implemented in response inhibition. In this article, we will review the brain circuits and systems involved in implementing inhibitory control through both reactive and proactive mechanisms. We also comment on possible cellular mechanisms that may contribute to inhibitory control processes, noting that substantial further research is necessary in this regard. Furthermore, we will outline a number of ways in which the temporal dynamics underlying the generation of the proactive inhibitory signal may be particularly important for parsing out the neurobiological correlates that contribute to the learning processes underlying various aspects of inhibitory control.

Genetic knockout of the α7 nicotinic acetylcholine receptor gene alters hippocampal long-term potentiation in a background strain-dependent manner

Reduced α7 nicotinic acetylcholine receptor (nAChR) function is linked to impaired hippocampal-dependent sensory processing and learning and memory in schizophrenia. While knockout of the Chrna7 gene encoding the α7nAChR on a C57/Bl6 background results in changes in cognitive measures, prior studies found little impact on hippocampal synaptic plasticity in these mice. However, schizophrenia is a multi-genic disorder where complex interactions between specific genetic mutations and overall genetic background may play a prominent role in determining phenotypic penetrance. Thus, we compared the consequences of knocking out the α7nAChR on synaptic plasticity in C57/Bl6 and C3H mice, which differ in their basal α7nAChR expression levels. Homozygous α7 deletion in C3H mice, which normally express higher α7nAChR levels, resulted in impaired long-term potentiation (LTP) at hippocampal CA1 synapses, while C3H α7 heterozygous mice maintained robust LTP. In contrast, homozygous α7 deletion in C57 mice, which normally express lower α7nAChR levels, did not alter LTP, as had been previously reported for this strain. Thus, the threshold of Chrna7 expression required for LTP may be different in the two strains. Measurements of auditory gating, a hippocampal-dependent behavioral paradigm used to identify schizophrenia-associated sensory processing deficits, was abnormal in C3H α7 knockout mice confirming that auditory gating also requires α7nAChR expression. Our studies highlight the importance of genetic background on the regulation of synaptic plasticity and could be relevant for understanding genetic and cognitive heterogeneity in human studies of α7nAChR dysfunction in mental disorders.

nAChR dysfunction as a common substrate for schizophrenia and comorbid nicotine addiction: Current trends and perspectives

INTRODUCTION

The prevalence of tobacco use in the population with schizophrenia is enormously high. Moreover, nicotine dependence is found to be associated with symptom severity and poor outcome in patients with schizophrenia. The neurobiological mechanisms that explain schizophrenia-nicotine dependence comorbidity are not known. This study systematically reviews the evidence highlighting the contribution of nicotinic acetylcholine receptors (nAChRs) to nicotine abuse in schizophrenia.

METHODS

Electronic data bases (Medline, Google Scholar, and Web of Science) were searched using the selected key words that match the aims set forth for this review. A total of 276 articles were used for the qualitative synthesis of this review.

RESULTS

Substantial evidence from preclinical and clinical studies indicated that dysregulation of α7 and β2-subunit containing nAChRs account for the cognitive and affective symptoms of schizophrenia and nicotine use may represent a strategy to remediate these symptoms. Additionally, recent meta-analyses proposed that early tobacco use may itself increase the risk of developing schizophrenia. Genetic studies demonstrating that nAChR dysfunction that may act as a shared vulnerability factor for comorbid tobacco dependence and schizophrenia were found to support this view. The development of nAChR modulators was considered an effective therapeutic strategy to ameliorate psychiatric symptoms and to promote smoking cessation in schizophrenia patients.

CONCLUSIONS

The relationship between schizophrenia and smoking is complex. While the debate for the self-medication versus addiction vulnerability hypothesis continues, it is widely accepted that a dysfunction in the central nAChRs represent a common substrate for various symptoms of schizophrenia and comorbid nicotine dependence.

Phase 2 Trial of an Alpha-7 Nicotinic Receptor Agonist (TC-5619) in Negative and Cognitive Symptoms of Schizophrenia

OBJECTIVES

This trial was conducted to test the effects of an alpha7 nicotinic receptor full agonist, TC-5619, on negative and cognitive symptoms in subjects with schizophrenia.

METHODS

In 64 sites in the United States, Russia, Ukraine, Hungary, Romania, and Serbia, 477 outpatients (18-65 years; male 62%; 55% tobacco users) with schizophrenia, treated with a new-generation antipsychotic, were randomized to 24 weeks of placebo (n = 235), TC-5619, 5mg (n = 121), or TC-5619, 50 mg (n = 121), administered orally once daily. The primary efficacy measure was the Scale for the Assessment of Negative Symptoms (SANS) composite score. Key secondary measures were the Cogstate Schizophrenia Battery (CSB) composite score and the University of California San Diego Performance-Based Skills Assessment-Brief Version (UPSA-B) total score. Secondary measures included: Positive and Negative Syndrome Scale in Schizophrenia (PANSS) total and subscale scores, SANS domain scores, CSB item scores, Clinical Global Impression-Global Improvement (CGI-I) score, CGI-Severity (CGI-S) score, and Subject Global Impression-Cognition (SGI-Cog) total score.

RESULTS

SANS score showed no statistical benefit for TC-5619 vs placebo at week 24 (5 mg, 2-tailed P = .159; 50 mg, P = .689). Likewise, no scores of CSB, UPSA-B, PANSS, CGI-I, CGI-S, or SGI-Cog favored TC-5619 (P > .05). Sporadic statistical benefit favoring TC-5619 in some of these outcome measures were observed in tobacco users, but these benefits did not show concordance by dose, country, gender, or other relevant measures. TC-5619 was generally well tolerated.

CONCLUSION

These results do not support a benefit of TC-5619 for negative or cognitive symptoms in schizophrenia.

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.

Inhibitory learning is modulated by nicotinic acetylcholine receptors

Prior research has established that stimulating nicotinic acetylcholine receptors can facilitate learning and memory. However, most studies have focused on learning to emit a particular behavior, while little is known about the effects of nicotine on learning to withhold a behavioral response. The present study consisted of a dose response analysis of the effects of nicotine on negative occasion setting, a form of learned inhibition. In this paradigm, rats received one type of training trial in which presentation of a tone by itself was followed immediately by food reward. During the other type of trials, the tone was preceded by presentation of a light and no food was delivered after the tone. Rats gradually learned to approach the cup in anticipation of receiving food reward during presentations of the tone alone, but withheld that behavior when the tone was preceded by the light. Nicotine (0.35 mg/kg) facilitated negative occasion setting by reducing the number of sessions needed to learn the discrimination between trial types and by reducing the rate of responding on non-reinforced trials. Nicotine also increased the orienting response to the light, suggesting that nicotine may have affected the ability to withhold food cup behavior on non-reinforced trials by increasing attention to the light. In contrast to the effects of nicotine, rats treated with mecamylamine (0.125, 0.5, or 2 mg/kg) needed more training sessions to discriminate between reinforced and non-reinforced trials compared to saline-treated rats. The findings indicate that nicotinic acetylcholine receptors may be active during negative occasion setting and that nicotine can potentiate learned inhibition.

The separate and combined effects of monoamine oxidase inhibition and nicotine on P50 sensory gating

INTRODUCTION

The cognitive effects of nicotine in humans remain a topic of great interest, due to the continued prevalence of cigarette smoking in society as well as the hypothesis that cognitively impaired populations such as schizophrenia patients use nicotine as a means of self-medicating against deficits of sensory gating. However, chronic smoking can predispose individuals to robust monoamine oxidase (MAO) inhibition, and thus far, the effect of MAO inhibition on human sensory gating is unknown.

METHODS

In this study, we investigated the effects of both nicotine (6-mg gum) and pharmacologically induced MAO-A inhibition via moclobemide (75 mg) on P50 event-related potential-indexed sensory gating in a sample of 24 healthy non-smoking males.

RESULTS

Ratio score (rP50) measured gating revealed significant improvement in auditory stimulus suppression after combined nicotine and MAO-A inhibition compared to placebo and to the nicotine-alone condition. This nicotine + MAO-A inhibition-induced efficient gating was consistent regardless of participants' baseline (placebo) gating efficiency, despite the observation that nicotine in the absence of MAO-A inhibition exhibited a detrimental effect on gating in participants with high baseline suppression ratios.

CONCLUSION

Nicotine and monoamine oxidase-inhibiting agents in tobacco smoke appear to exert a synergistic effect on sensory gating, which may contribute to the elevated dependence rates seen in populations with cognitive deficits such as schizophrenia.

CDP-choline: effects of the procholine supplement on sensory gating and executive function in healthy volunteers stratified for low, medium and high P50 suppression

Diminished auditory sensory gating and associated neurocognitive deficits in schizophrenia have been linked to altered expression and function of the alpha-7 nicotinic acetycholinergic receptor (α7 nAChR), the targeting of which may have treatment potential. Choline is a selective α7 nAChR agonist and the aim of this study was to determine whether cytidine 5'-diphosphocholine (CDP-choline), or citicoline, a dietary source of choline, increases sensory gating and cognition in healthy volunteers stratified for gating level. In a randomized, placebo-controlled, double-blind design involving acute administration of low, moderate doses (500 mg, 1000 mg) of CDP-choline, 24 healthy volunteers were assessed for auditory gating as indexed by suppression of the P50 event-related potential (ERP) in a paired-stimulus (S1, S2) paradigm, and for executive function as measured by the Groton Maze Learning Task (GMLT) of the CogState Schizophrenia Battery. CDP-choline improved gating (1000 mg) and suppression of the S2 P50 response (500 mg, 1000 mg), with the effects being selective for individuals with low gating (suppression) levels. Tentative support was also shown for increased GMLT performance (500 mg) in low suppressors. These preliminary findings with CDP-choline in a healthy, schizophrenia-like surrogate sample are consistent with a α7 nAChR mechanism and support further trials with choline as a pro-cognitive strategy.

Histological correlates of N40 auditory evoked potentials in adult rats after neonatal ventral hippocampal lesion: animal model of schizophrenia

The neonatal ventral hippocampal lesion (NVHL) is an established neurodevelopmental rat model of schizophrenia. Rats with NVHL exhibit several behavioral, molecular and physiological abnormalities that are similar to those found in schizophrenics. Schizophrenia is a severe psychiatric illness characterized by profound disturbances of mental functions including neurophysiological deficits in brain information processing. These deficits can be assessed by auditory evoked potentials (AEPs), where schizophrenics exhibit abnormalities in amplitude, duration and latency of such AEPs. The aim of the present study was to compare the density of cells in the temporal cerebral cortex and the N40-AEP of adult NVHL rats versus adult sham rats. We found that rats with NVHL exhibit significant lower amplitude of the N40-AEP and a significant lower number of cells in bilateral regions of the temporal cerebral cortex compared to sham rats. Because the AEP recordings were obtained from anesthetized rats, we suggest that NVHL leads to inappropriate innervation in thalamic-cortical pathways in the adult rat, leading to altered function of cortical networks involved in processing of primary auditory information.

Normalizing effects of EVP-6124, an α-7 nicotinic partial agonist, on event-related potentials and cognition: a proof of concept, randomized trial in patients with schizophrenia

Cognitive impairment is a cause of significant disability in patients with schizophrenia. To date, no drug has been approved for this indication by the U.S. Food and Drug Administration. This article presents findings suggesting that a medication targeting the alpha-7 nicotinic acetylcholine receptor (α7 nAChR) might meet this need. This single-center, randomized, parallel-group, double-blind,placebo-controlled study examined 21 medically stable patients with schizophrenia or schizoaffective disorder treated with second generation antipsychotics. Patients were treated with a daily dose of either 0.3 mg (n=8) or 1.0 mg (n=9) of EVP-6124, an α7 nAChR partial agonist, or placebo (n=4). Treatment continued for 21 days while patients continued their usual antipsychotic medication: aripiprazole (10-30 mg/day), olanzapine (10-20 mg/day), paliperidone (3-12 mg/day), or risperidone (2-16 mg/day). Cognitive test performance, eventrelated electroencephalographic (EEG) potentials, clinical symptoms, laboratory values, and clinical side effects were measured. EVP-6124 was well tolerated and showed positive, and in some cases, dose-dependent effects on several EEG responses, especially the Mismatch Negativity and P300 potentials. Positive effects were also found in performance on cognitive tests that measured non-verbal learning, memory, and executive function. This study is an example of the type of early proof of concept trial that is done to enable drug developers to evaluate whether to continue research on an agent. Based on the promising findings in this study, larger phase II studies were initiated to further test the pro-cognitive effects of EVP-6124 in patients with chronic schizophrenia.

CLINICAL TRIALS REGISTRATION

Safety, Tolerability, and Pharmacokinetic Study of EVP-6124 in Patients with Schizophrenia, NCT01556763 http://clinicaltrials.gov/ct2/show/NCT01556763?term=NCT01556763&rank=1.

Sensory disturbances, inhibitory deficits, and the P50 wave in schizophrenia

Sensory gating disturbances in schizophrenia are often described as an inability to filter redundant sensory stimuli that typically manifest as inability to gate neuronal responses related to the P50 wave, characterizing a decreased ability of the brain to inhibit various responses to insignificant stimuli. It implicates various deficits of perceptual and attentional functions, and this inability to inhibit, or "gate", irrelevant sensory inputs leads to sensory and information overload that also may result in neuronal hyperexcitability related to disturbances of habituation mechanisms. These findings seem to be particularly important in the context of modern electrophysiological and neuroimaging data suggesting that the filtering deficits in schizophrenia are likely related to deficits in the integrity of connections between various brain areas. As a consequence, this brain disintegration produces disconnection of information, disrupted binding, and disintegration of consciousness that in terms of modern neuroscience could connect original Bleuler's concept of "split mind" with research of neural information integration.

Association study of polymorphisms in the alpha 7 nicotinic acetylcholine receptor subunit and catechol-o-methyl transferase genes with sensory gating in first-episode schizophrenia

The purpose of the current study was to explore the association of auditory P50 sensory gating (P50) and prepulse inhibition (PPI) of schizophrenia with polymorphisms in the CHRNA7 and COMT genes. One hundred and fourty patients with schizophrenia participated in this study. They were administered the tests P50 and PPI. Moreover, three single nucleotide polymorphisms (SNPs) (rs2337980, rs1909884 and rs883473) in CHRNA7 and three SNPs (rs4680, rs737865 and rs165599) in COMT were selected to be genotyped by polyacrylamide gel microarray techniques. P50 index showed significant reduction in S2 amplitude between wild-type and mutation groups in the COMT rs4680. S1 amplitude of mutation group in the COMT rs737865 was also lower compared to wild-type group. PPI index revealed a shorter pulse latency of mutation group in the rs4680. The suppression ratio of mutation group was lower in COMT rs165599. Negative findings were shown between comparisons in all the CHRNA7 SNPs. We find that P50 and PPI may be influenced by COMT rs4680 polymorphisms in schizophrenia; more excitingly, we find that P50 might be influenced by COMT rs737865 polymorphisms and PPI may be influenced by COMT rs165599 polymorphisms in schizophrenia, and their mutations are associated with the reduction of the risk of P50 or PPI defects in schizophrenia. Futher studies with a larger number of subjects are needed to verify the present findings.

Effects of neuronal nicotinic acetylcholine receptor allosteric modulators in animal behavior studies

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation-conducting transmembrane channels from the cys-loop receptor superfamily. The neuronal subtypes of these receptors (e.g. the α7 and α4β2 subtypes) are involved in neurobehavioral processes such as anxiety, the central processing of pain, food intake, nicotine seeking behavior, and a number of cognitive functions like learning and memory. Neuronal nAChR dysfunction is involved in the pathophysiology of many neurological disorders, and behavioral studies in animals are useful models to assess the effects of compounds that act on these receptors. Allosteric modulators are ligands that bind to the receptors at sites other than the orthosteric site where acetylcholine, the endogenous agonist for the nAChRs, binds. While conventional ligands for the neuronal nAChRs have been studied for their behavioral effects in animals, allosteric modulators for these receptors have only recently gained attention, and research on their behavioral effects is growing rapidly. Here we will discuss the behavioral effects of allosteric modulators of the neuronal nAChRs.

Cholinergic gating of hippocampal auditory evoked potentials in freely moving rats

As perturbations in auditory filtering appear to be a candidate trait marker of schizophrenia, there has been considerable interest in the development of translational rat models to elucidate the underlying neural and neurochemical mechanisms involved in sensory gating. This is the first study to investigate the effects of the non-selective muscarinic antagonist scopolamine, the muscarinic M1 antagonist biperiden and the cholinesterase inhibitor donepezil (also in combination with scopolamine and biperiden) on auditory evoked potentials (AEPs) and sensory gating. In the saline condition, only the N50 peak displayed sensory gating. Scopolamine and biperiden both disrupted sensory gating by increasing N50 amplitude for the S2 click. Donepezil was able to fully reverse the effects of biperiden on N50 sensory gating, but had residual effects when combined with scopolamine; i.e., it enhanced sensory gating by increasing N50 amplitude of the S1 stimulus. Donepezil by itself improved sensory gating by enhancing N50 amplitude of S1, and reducing N50 amplitude of the S2 click. In conclusion, due to its relatively more selective effects biperiden is to be preferred over scopolamine as a means for pharmacologically inducing cholinergic impairments in auditory processing in healthy rats. Changes in auditory processing and sensory gating induced by cholinergic drugs may serve as a translational model for aging instead of schizophrenia.

Genetic matters: thirty years of progress using mouse models in nicotinic research

This research update summarizes thirty years of studies on genetic influences on responses to the acute or chronic administration of nicotine. Early studies established that various inbred mice are differentially sensitive to the effects of the drug. Classical genetic analyses confirmed that nicotine effects on locomotion, body temperature and seizures are heritable. A significant inverse correlation between the locomotor and hypothermic effects and the density of nicotine binding sites suggested that differential expression α4β2-neuronal nicotinic acetylcholine receptor (nAChR) mediated some of this genetic variability. Subsequent studies with α4 and β2 nAChR null (decreased sensitivity) and gain of function mutants (increased sensitivity) supports the role of the α4β2*nAChR subtype. However, null mutant mice still respond to nicotine, indicating that other nAChR subtypes also mediate these responses. Mice differing in initial sensitivity to nicotine also differ in tolerance development following chronic treatment: those mice that are initially more sensitive to nicotine develop tolerance at lower treatment doses than less sensitive mice, indicating that tolerance is an adaptive response to the effects of nicotine. In contrast, the sensitivity of mice to pre-pulse inhibition of acoustic startle response is correlated with the expression of α7-nAChR. While genetic variability in nAChR expression and function is an important factor contributing to differences in response to nicotine, the observations that altered activity of opioid, glutamate, and cannabinoid receptors among others also change nicotine sensitivity reinforces the proposal that the genetics of nicotine response is more complex than differences in nAChRs.

A randomized exploratory trial of an α-7 nicotinic receptor agonist (TC-5619) for cognitive enhancement in schizophrenia

This exploratory trial was conducted to test the effects of an alpha7 nicotinic receptor partial agonist, TC-5619, on cognitive dysfunction and negative symptoms in subjects with schizophrenia. In the United States and India, 185 outpatients (18-60 years; male 69%; 46% tobacco users) with schizophrenia treated with quetiapine or risperidone monotherapy were randomized to 12 weeks of placebo (n=91) or TC-5619 (n=94; orally once daily 1 mg day 1 to week 4, 5 mg week 4 to 8, and 25 mg week 8 to 12). The primary efficacy outcome measure was the Groton Maze Learning Task (GMLT; executive function) of the CogState Schizophrenia Battery (CSB). Secondary outcome measures included: CSB composite score; Scale for Assessment of Negative Symptoms (SANS); Clinical Global Impression-Global Improvement (CGI-I); CGI-severity (CGI-S); and Subject Global Impression-Cognition. GMLT statistically favored TC-5619 (P=0.036) in this exploratory trial. SANS also statistically favored TC-5619 (P=0.030). No other secondary outcome measure demonstrated a drug effect in the total population; there was a statistically significant drug effect on working memory in tobacco users. The results were typically stronger in favor of TC-5619 in tobacco users and occasionally better in the United States than in India. TC-5619 was generally well tolerated with no clinically noteworthy safety findings. These results support the potential benefits of TC-5619 and alpha7 nicotinic receptor partial agonists for cognitive dysfunction and negative symptoms in schizophrenia.

Effects of the novel α7 nicotinic acetylcholine receptor agonist ABT-107 on sensory gating in DBA/2 mice: pharmacodynamic characterization

Nicotinic acetylcholine receptor (nAChR) agonists improve sensory gating deficits in animal models and schizophrenic patients. The aim of this study was to determine whether the novel and selective α7 nAChR full agonist 5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy]pyridazin-3-yl)-1H-indole (ABT-107) improves sensory gating deficits in DBA/2 mice. Sensory gating was measured by recording hippocampal-evoked potential P20-N40 waves and determining gating test/conditioning (T/C) ratios in a paired auditory stimulus paradigm. ABT-107 at 0.1 μmol/kg (average plasma concentration of 1.1 ng/ml) significantly improved sensory gating by lowering T/C ratios during a 30-min period after administration in unanesthetized DBA/2 mice. ABT-107 at 1.0 μmol/kg was ineffective at 30 min after administration when average plasma levels were 13.5 ng/ml. However, the 1.0 μmol/kg dose was effective 180 min after administration when plasma concentration had fallen to 1.9 ng/ml. ABT-107 (0.1 μmol/kg) also improved sensory gating in anesthetized DBA/2 mice pretreated with α7 nAChR-desensitizing doses of nicotine (6.2 μmol/kg) or ABT-107 (0.1 μmol/kg) itself. Moreover, repeated b.i.d. dosing of ABT-107 (0.1 μmol/kg) was as efficacious as a single dose. The acute efficacy of ABT-107 (0.1 μmol/kg) was blocked by the nAChR antagonist methyllycaconitine, but not by the α4β2 nAChR antagonist dihydro-β-erythroidine. These studies demonstrate that ABT-107 improves sensory gating through the activation of nAChRs, and efficacy is sustained under conditions of repeated dosing or with prior nAChR activation with nicotine.

Event-related potential patterns associated with hyperarousal in Gulf War illness syndrome groups

An exaggerated response to emotional stimuli is one of the several symptoms widely reported by veterans of the 1991 Persian Gulf War. Many have attributed these symptoms to post-war stress; others have attributed the symptoms to deployment-related exposures and associated damage to cholinergic, dopaminergic, and white matter systems. We collected event-related potential (ERP) data from 20 veterans meeting Haley criteria for Gulf War Syndromes 1-3 and from 8 matched Gulf War veteran controls, who were deployed but not symptomatic, while they performed an auditory three-condition oddball task with gunshot and lion roar sounds as the distractor stimuli. Reports of hyperarousal from the ill veterans were significantly greater than those from the control veterans; different ERP profiles emerged to account for their hyperarousability. Syndromes 2 and 3, who have previously shown brainstem abnormalities, show significantly stronger auditory P1 amplitudes, purported to indicate compromised cholinergic inhibitory gating in the reticular activating system. Syndromes 1 and 2, who have previously shown basal ganglia dysfunction, show significantly weaker P3a response to distractor stimuli, purported to indicate dysfunction of the dopaminergic contribution to their ability to inhibit distraction by irrelevant stimuli. All three syndrome groups showed an attenuated P3b to target stimuli, which could be secondary to both cholinergic and dopaminergic contributions or disruption of white matter integrity.

Alpha7 nicotinic acetylcholine receptors modulate motivation to self-administer nicotine: implications for smoking and schizophrenia

Individuals diagnosed with schizophrenia have an exceptionally high risk for tobacco dependence. Postmortem studies show that these individuals have significant reductions in α7 nicotinic acetylcholine receptors (nAChRs) in several brain areas. Decreased α7-mediated function might not only be linked to schizophrenia but also to increased tobacco consumption. The purpose of this study was to determine whether pharmacological blockade of α7 nAChRs would increase motivation of rats to intravenously self-administer nicotine (NIC) during a progressive ratio schedule of reinforcement (PR). Before PR, rats received local infusions of 0, 10, or 20 pmol of a selective α7 nAChR antagonist, α-conotoxin ArIB [V11L,V16D] (ArIB) into the nucleus accumbens (NAc) shell or the anterior cingulate cortex, brain areas that contribute to motivation for drug reward. We additionally sought to determine whether local infusion of 0, 10, or 40 nmol of a selective α7 nAChR agonist, PNU 282987, into these brain areas would decrease motivation for NIC use. Infusion of ArIB into the NAc shell and anterior cingulate cortex resulted in a significant increase in active lever pressing, breakpoints, and NIC intake, suggesting that a decrease in α7 nAChR function increases motivation to work for NIC. In contrast, PNU 282987 infusion resulted in reductions in these measures when administered into the NAc shell, but had no effect after administration into the anterior cingulate cortex. These data identify reduction of α7 nAChR function as a potential mechanism for elevated tobacco use in schizophrenia and also identify activation of α7 nAChRs as a potential strategy for tobacco cessation therapy.

Regulation of GABAergic inputs to CA1 pyramidal neurons by nicotinic receptors and kynurenic acid

Impaired α7 nicotinic acetylcholine receptor (nAChR) function and GABAergic transmission in the hippocampus and elevated brain levels of kynurenic acid (KYNA), an astrocyte-derived metabolite of the kynurenine pathway, are key features of schizophrenia. KYNA acts as a noncompetitive antagonist with respect to agonists at both α7 nAChRs and N-methyl-D-aspartate receptors. Here, we tested the hypothesis that in hippocampal slices tonically active α7 nAChRs control GABAergic transmission to CA1 pyramidal neurons and are sensitive to inhibition by rising levels of KYNA. The α7 nAChR-selective antagonist α-bungarotoxin (α-BGT; 100 nM) and methyllycaconitine (MLA; 10 nM), an antagonist at α7 and other nAChRs, reduced by 51.3 ± 1.3 and 65.2 ± 1.5%, respectively, the frequency of GABAergic postsynaptic currents (PSCs) recorded from CA1 pyramidal neurons. MLA had no effect on miniature GABAergic PSCs. Thus, GABAergic synaptic activity in CA1 pyramidal neurons is maintained, in part, by tonically active α7 nAChRs located on the preterminal region of axons and/or the somatodendritic region of interneurons that synapse onto the neurons under study. L-Kynurenine (20 or 200 μM) or KYNA (20-200 μM) suppressed concentration-dependently the frequency of GABAergic PSCs; the inhibitory effect of 20 μM L-kynurenine had an onset time of approximately 35 min and could not be detected in the presence of 100 nM α-BGT. These results suggest that KYNA levels generated from 20 μM kynurenine inhibit tonically active α7 nAChR-dependent GABAergic transmission to the pyramidal neurons. Disruption of nAChR-dependent GABAergic transmission by mildly elevated levels of KYNA can be an important determinant of the cognitive deficits presented by patients with schizophrenia.

Multiple genes in the 15q13-q14 chromosomal region are associated with schizophrenia

OBJECTIVE

The chromosomal region, 15q13-q14, including the α7 nicotinic acetylcholine receptor gene, CHRNA7, is a replicated region for schizophrenia. This study fine-mapped genes at 15q13-q14 to determine whether the association is unique to CHRNA7.

METHODS

Family-based and case-control association studies were performed on Caucasian-non-Hispanic and African-American individuals from 120 families as well as 468 individual patients with schizophrenia and 144 well-characterized controls. Single-nucleotide polymorphism (SNP) markers were genotyped, and association analyses carried out for the outcomes of schizophrenia, smoking, and smoking in schizophrenia.

RESULTS

Three genes were associated with schizophrenia in both ethnic populations: TRPM1, KLF13, and RYR3. Two SNPs in CHRNA7 were associated with schizophrenia in African-Americans, and a second SNP in CHRNA7 was significant for an association with smoking and smoking in schizophrenia in Caucasians.

CONCLUSION

Results of these studies support association of the 15q13-q14 region with schizophrenia. The broad positive association suggests that more than one 15q gene may be contributing to the disorder, either in combination or through a regulatory mechanism.

Nicotinic mechanisms in the treatment of psychotic disorders: a focus on the α7 nicotinic receptor

Nicotine is heavily abused by persons with schizophrenia. Nicotine better enables people with schizophrenia to filter out extraneous auditory stimuli. Nicotine also improves prepulse inhibition when compared to placebo. Nicotine similarly increases the amplitude of patients' duration mismatch negativity. The 15q13-14 region of the genome coding for the α7 nicotinic receptor is linked to schizophrenia. Multiple single nucleotide polymorphisms have been identified in this 15q13-14 gene promoter region that are more frequently present in people with schizophrenia than in normal controls. Abnormalities in expression and regulation of central nicotinic cholinoceptors with decreased α7 binding in multiple brain regions are also present. Nicotine enhances cognition in schizophrenia. Alternative agents that activate the nicotinic receptor have been tested including 3-[2,4-dimethoxybenzylidene]anabaseine (DMXB-A). This compound improved attention, working memory, and negative symptoms in an add-on study in nonsmoking patients with schizophrenia. There are multiple other nicotinic agents, including positive allosteric modulators, in the preclinical stages of development. Finally, the effects of varenicline and clozapine and their relation to smoking cessation are discussed.

Diminished cerebral inhibition in neonates associated with risk factors for schizophrenia: parental psychosis, maternal depression, and nicotine use

Diminished inhibitory gating of cerebral auditory evoked responses is transmitted in families with psychoses as an endophenotype related to the genetic risk for these illnesses. To assess whether the endophenotype is already expressed in infants of parents with psychotic illness and to assess effects of other known risk factors for schizophrenia, ie, maternal cigarette smoking and depression, inhibitory gating of cerebral auditory evoked responses was evaluated by comparing the P1 evoked responses to the first and second of paired auditory stimuli. Cerebral evoked responses were recorded during active sleep from 22 infants with a parent diagnosed with a psychotic illness and 129 infants with parents with no such history. Of these infants, 25 were prenatally exposed to nicotine (16 from the comparison group and 9 from the group with parental psychosis). Mothers of 35 infants had diagnoses of major depressive disorder. Parental psychosis (P = .032) and exposure to maternal smoking (P = .012) both resulted in diminished inhibitory gating in infant offspring. Compared to infants of mothers who did not smoke and who had neither parental psychosis nor maternal depression, diminished inhibitory gating was observed in infants with parental psychosis (P = .027) and in infants with maternal depression (P = .049). Diminished inhibitory gating of auditory evoked response in infants who have risk factors for schizophrenia mirrors reports of its familial transmission in adults. The results further indicate that the phenotypic expression of familial genetic and environmental risks for psychosis is already manifest very early in development.

Differential sensory gating functions between smokers and non-smokers among drug-naive first episode schizophrenic patients

Although an acute effect of cigarette smoking and nicotine on sensory gating of schizophrenias has been investigated in published papers, the chronic effect of cigarette smoking on this phenomenon has not yet been reported. We report the effects of chronic cigarette smoking, without new acute exposure before testing, on sensory gating using the P50 auditory evoked potential in a group of drug-naive first episode schizophrenic smokers and healthy smokers. Sensory gating was evaluated using auditory P50 suppression elicited using the conditioning (S1)-testing (S2) paradigm. Fifty six male drug-naive first episode schizophrenic patients were compared to 41 healthy male controls. Patients were classified into subgroups of current smokers (n=18) and non-smokers (n=38) to explore the effects of smoking on sensory gating. All subjects did not smoke a cigarette for at least 1h prior to testing. Schizophrenic patients showed an increased S2 amplitude and a poorer sensory gating as measured by both S2/S1 ratio and S1-S2 difference of P50 amplitude, as compared to healthy controls. However, smokers showed an increased S1 amplitude and better sensory gating than did non-smokers both in schizophrenia patients and healthy controls. Our findings support a sensory gating deficit among first episode schizophrenic patients. However, it was less pronounced among schizophrenic patients who were current cigarette smokers, suggesting a positive effect of chronic cigarette smoking on ameliorating this sensory gating deficit in schizophrenia. Our findings of the present study present new evidence supporting the self-medication hypothesis of self-medication by cigarette smoking in schizophrenia to possibly ameliorate pre-existing functional deficits.

Targeting the nicotinic alpha7 acetylcholine receptor to enhance cognition in disease

A promising drug target currently under investigation to improve cognitive deficits in neuropsychiatric and neurological disorders is the neuronal nicotinic alpha7 acetylcholine receptor (α7nAChR). Improving cognitive impairments in diseases such as Alzheimer's (AD) and schizophrenia remains a large unmet medical need, and the α7nAChR has many properties that make it an attractive therapeutic target. The α7nAChR is a ligand gated ion channel that has particularly high permeability to Ca(2+) and is expressed in key brain regions involved in cognitive processes (e.g., hippocampus). The α7nAChRs are localized both pre-synaptically, where they can regulate neurotransmitter release, and post-synaptically where they can activate intracellular signaling cascades and influence downstream processes involved in learning and memory. In particular, activation of the α7nAChR with small molecule agonists enhances long-term potentiation, an in vitro model of synaptic plasticity, and improves performance across multiple cognitive domains in rodents, monkeys, and humans. Positive allosteric modulation of the α7nAChR offers an alternate approach to direct agonism that could prove to be particularly beneficial in certain disease populations where smoking nicotine is prevalent (e.g., schizophrenia) and could interfere with an orthosteric agonist approach. The current review focuses on the neurobiology of the α7nAChR, its role in cognition and the development status of some of the most promising molecules advancing for the treatment of cognitive dysfunction in disease.

Neural systems governed by nicotinic acetylcholine receptors: emerging hypotheses

Cholinergic neurons and nicotinic acetylcholine receptors (nAChRs) in the brain participate in diverse functions: reward, learning and memory, mood, sensory processing, pain, and neuroprotection. Nicotinic systems also have well-known roles in drug abuse. Here, we review recent insights into nicotinic function, linking exogenous and endogenous manipulations of nAChRs to alterations in synapses, circuits, and behavior. We also discuss how these contemporary advances can motivate attempts to exploit nicotinic systems therapeutically in Parkinson's disease, cognitive decline, epilepsy, and schizophrenia.

Involvement of the cholinergic system in conditioning and perceptual memory

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

Continuous administration of a selective alpha7 nicotinic partial agonist, DMXBA, improves sensory inhibition without causing tachyphylaxis or receptor upregulation in DBA/2 mice

Stimulation of nicotinic receptors, specifically the alpha7 subtype, improves sensory inhibition and cognitive function in receptor deficient humans and rodents. However, stimulation with a full agonist, such as nicotine, produces rapid tachyphylaxis of the P20N40-measured sensory inhibition process. 3-(2,4-dimethoxybenzylidine) anabaseine (DMXBA, also GTS-21) selectively activates the alpha7 nicotinic receptor, and in acute administration studies, has been shown to improve deficient sensory inhibition in both humans and rodents with repeated dosing. Unlike nicotine, this partial agonist acted without inducing tachyphylaxis. Here, we assessed the ability of DMXBA to improve sensory inhibition in DBA/2 mice after 7 days of continuous administration via a subcutaneously implanted osmotic minipump. When assessed on day 8, mice receiving saline showed the characteristic deficient sensory inhibition seen with untreated DBA/2 mice. The 25- and 50-mg/ml infusion concentrations of DMXBA, but not the 100-mg/ml, produced significantly improved sensory inhibition in the mice, exclusively through a decrease in test amplitude. No concentration significantly upregulated hippocampal alpha7 receptor levels. DMXBA levels in the brain were higher than plasma at 2 of the 3 concentrations infused. These data suggest that continuous exposure to DMXBA does not significantly affect the underlying responsiveness of the sensory inhibition pathway to this partial agonist, nor cause receptor upregulation, at these relatively low brain concentrations. The ability of DMXBA to maintain its effectiveness during constant administration conditions may be due to an ability to activate alpha7 receptors at low concentrations, and consequently low fractional occupancy of the five possible binding sites on this homomeric receptor.

Genetic Association Study of the Alpha 7 Nicotinic Receptor (CHRNA7) with the Development of Schizophrenia and Bipolar Disorder in Korean Population

OBJECTIVE

CHRNA7 has been shown to be a strong candidate gene for schizophrenia and bipolar disorder. It is located on chromosome 15q13-q14, which is one of the replicated linkage spots for schizophrenia and bipolar disorder.

METHODS

We conducted an association study to determine whether previous positive association is replicable in the Korean population. We included 254 patients with schizophrenia, 193 patients with bipolar disorder type I, 38 patients with bipolar disorder type II, 64 schizoaffective disorder patients, and 349 controls. All subjects were ethnically Korean. A total of 898 subjects were included, and genotyping was done for three single nucleotide polymorphisms (SNPs) of CHRNA7. These three intronic SNPs were rs2337506 (A/G), rs6494223 (C/T), and rs12916879 (A/G).

RESULTS

There was only one marginally significant association; this association was between rs12916879 and bipolar disorder type I in the male subgroup. In both the allele and genotype distributions, we found a weak signal (Chi-squared=3.57, df=1, p=0.06 for allele, Chi-squared=7.50, df=2, p=0.02 for genotype) only. Unphased haplotype analysis could not provide additional support for this finding. No SNP was associated with schizophrenia or any other affected groups in this Korean sample. The associative finding is marginal and inconclusive.

CONCLUSION

We could not replicate positive association in other ethnic groups previously studied. This suggests possible heterogeneity in the genes associated with schizophrenia and bipolar disorders. Because of structural complexity of the CHRNA7 gene and the limited statistical power of this study, further genetic studies with more SNPs and larger samples covering various populations, along with more fine molecular exploration of the CHRNA7 gene structure, are required.

Distinct neural generators of sensory gating in schizophrenia

Although malfunctioning of inhibitory processes is proposed as a pathophysiological mechanism in schizophrenia and has been studied extensively with the P50 gating paradigm, the brain regions involved in generating and suppressing the P50 remain unclear. The current investigation used EEG source analysis and the standard S1-S2 paradigm to clarify the neural structures associated with P50 gating in 16 schizophrenia patients and 14 healthy subjects. Based on prior research, the superior temporal gyrus, hippocampus, dorsolateral prefrontal cortex, thalamus, and their dipole moments were evaluated. In modeling the P50, a neural network involving all four brain regions provided the best goodness-of-fit across both groups. In healthy subjects, the P50 ratio score correlated positively with the hippocampal dipole moment ratio, whereas a significant association with the DLPFC dipole moment ratio was observed in schizophrenia patients. In each instance, the neural structure was found to account for unique variance in explaining the P50 ratio, along with some suggestion of DLPFC involvement in healthy subjects.

Efeitos comportamentais, cognitivos e psicofisiológicos dos canabinoides: relevância para a psicose e a esquizofrenia

Avanços recentes no conhecimento sobre a função do receptor de canabinoide renovaram o interesse na associação entre cannabis e psicose. Linhas convergentes de evidências sugerem que os canabinoides podem produzir uma ampla gama de sintomas transitórios positivos, negativos e cognitivos assemelhados aos de esquizofrenia. Os canabinoides também produzem alguns déficits psicofisiológicos sabidamente presentes na esquizofrenia. É igualmente claro que em indivíduos com um transtorno psicótico estabelecido, os canabinoides podem exacerbar sintomas, desencadear recaídas e ter consequências negativas no curso da doença. Evidências crescentes sugerem que a exposição precoce e pesada à cannabis pode aumentar o risco de se desenvolver um transtorno psicótico como a esquizofrenia. A relação entre exposição à cannabis e esquizofrenia preenche alguns, mas não todos os critérios usuais de causalidade. Porém, a maioria das pessoas que utilizam cannabis não desenvolve esquizofrenia e muitas pessoas diagnosticadas com esquizofrenia nunca utilizaram cannabis. Portanto, é provável que a exposição à cannabis seja uma "causa componente" que interage com outros fatores para "causar" esquizofrenia ou outro transtorno psicótico, mas não é nem necessária nem suficiente para fazê-lo sozinha. No entanto, na ausência de causas conhecidas da esquizofrenia e com as implicações de políticas de saúde pública, se tal vínculo for estabelecido, as causas componentes, tais como a exposição a canabinoide, devem continuar sendo um foco de estudos futuros. Finalmente, são necessárias mais pesquisas para identificar os fatores subjacentes à vulnerabilidade à psicose relacionada a canabinoide e para elucidar os mecanismos biológicos subjacentes a esse risco.

Role of the NMDA-receptor in Prepulse Inhibition in the Rat

Kynurenic acid (KYNA) is an endogenous metabolite of tryptophan. Studies have revealed increased brain KYNA levels in patients with schizophrenia. Prepulse inhibition (PPI) is a behavioral model for sensorimotor gating and found to be reduced in schizophrenia. Previous studies have shown that pharmacologically elevated brain KYNA levels disrupt PPI in the rat. The aim of the present study was to investigate the receptor(s) involved in this effect. Rats were treated with different drugs selectively blocking each of the sites that KYNA antagonizes, namely the glutamate recognition site of the N-methyl-D-aspartate receptor (NMDAR), the α7* nicotinic acetylcholine receptor (α7nAChR) and the glycine site of the NMDAR. Kynurenine (200 mg/kg) was given to replicate the effects of increased levels of KYNA on PPI. In order to block the glutamate recognition site of the NMDAR, CGS 19755 (10 mg/kg) or SDZ 220–581 (2.5 mg/kg) were administered and to antagonize the α7nAChR methyllycaconitine (MLA; 6 mg/kg) was given. L-701,324 (1 and 4 mg/kg) or 4-Chloro-kynurenine (4-Cl-KYN; 25, 50 and 100 mg/kg), a drug in situ converted to 7-Chloro-kynurenic acid, were used to block the glycine-site of the NMDAR. Administration of SDZ 220-581 or CGS 19755 was associated with a robust reduction in PPI, whereas L-701,324, 4-Cl-KYN or MLA failed to alter PPI. Kynurenine increased brain KYNA levels 5-fold and tended to decrease PPI. The present study suggests that neither antagonism of the glycine-site of the NMDA receptor nor antagonism of the α7nAChR disrupts PPI, rather with regard to the effects of KYNA, blockade of the glutamate recognition-site is necessary to reduce PPI.

Varenicline and P50 auditory gating in medicated schizophrenic patients: a pilot study

Most schizophrenic patients have a deficit in auditory sensory gating that appears to be mediated by the alpha-7 nicotinic receptor. This pilot study examines the effects of varenicline, an alpha-7 agonist, on the P50 auditory evoked potential in six schizophrenic patients. The study was canceled because of concerning side effects consistent with those reported by the FDA. However, in this small group of subjects, varenicline did not consistently enhance P50 auditory gating.

Pharmacological strategies for enhancing cognition in schizophrenia

Researchers have long recognized that individuals with schizophrenia experience challenges in a wide range of cognitive domains, and research on cognitive impairment in schizophrenia is not a recent phenomena. However, the past 10-20 years have seen an increasing recognition of the central importance of cognition to understanding function and outcome in this illness (Green et al. in Schizophr Bull 26:119-136, 2000), an awareness that has shifted the emphasis of at least some work on schizophrenia. More specifically, there has been a rapidly growing body of work on methods of enhancing cognition in schizophrenia, as a means to potentially facilitate improved outcome and quality of life for individuals with this debilitating illness. The current chapter reviews the results of a range of studies examining adjunctive pharmacological treatments to enhance cognition in schizophrenia using a range of designs, including single-dose studies, open-label repeated dosing studies, and double-blind parallel group and crossover designs with repeated dosing. Although many of the single-dose and open-label studies have suggested positive cognitive effects from a range of agents, few of the larger-scale double-blind studies have generated positive results. The current state of results may reflect the need to identify alternative molecular mechanisms for enhancing cognition in schizophrenia or the need to reconceptualize the ways in which pharmacological agents may improve cognition in this illness, with a concomitant change in the traditional clinical trial study design used in prior studies of cognitive enhancement in schizophrenia.

Studies on the hippocampal formation: From basic development to clinical applications: Studies on schizophrenia

The hippocampal formation plays a critical role in cognitive function. The developmental events that shape the hippocampal formation are continuing to be elucidated and their implications for brain function are emerging as well as applying those advances to interventions that have important possibilities for the treatment of brain dysfunction. The story told in this chapter is about the use of the in oculo transplant method to illuminate intrinsic and extrinsic features that underlie the development of the dentate gyrus and adjacent hippocampus and the role of one molecule in the hippocampus and schizophrenia. Schizophrenia, originally conceptualized as a dysfunction in dopaminergic neurotransmission, is now known to involve multiple neuronal systems. Dysfunction of hippocampal neurons is emerging as one of its signature pathological features. Basic insights into the development and function of hippocampal interneurons form the basis of a new treatment initiative for this illness. Evidence for the role of the alpha 7-nicotinic acetylcholine receptor in the development and function of these neurons in rodents has led to human trials of nicotinic agonists for cognitive dysfunction in schizophrenia and the possibility of improving hippocampal development in children at risk for schizophrenia by perinatal supplementation with choline, which can act as an alpha 7-nicotinic acetylcholine receptor agonist.

Ketamine-induced deficit of auditory gating in the hippocampus of rats is alleviated by medial septal inactivation and antipsychotic drugs

RATIONALE

Gating of sensory responses is impaired in schizophrenic patients and animal models of schizophrenia. Ketamine, an N-methyl-D-aspartate receptor antagonist, is known to induce schizophrenic-like symptoms including sensory gating deficits in humans.

OBJECTIVE

This study aims to investigate the mechanisms underlying ketamine's effect on gating of auditory evoked potentials in the hippocampus of freely moving rats.

METHODS

Gating was measured by the ratio of the test-click response (T) to the conditioning-click response (C), or T/C, with T and C measured as peak amplitudes.

RESULTS

Ketamine (1, 3, or 6 mg/kg s.c.) injection dose-dependently increased T/C ratio as compared to saline injection (s.c.). T/C ratio was 0.48 +/- 0.05 after saline injection and 0.73 +/- 0.17 after ketamine (3 mg/kg s.c.) injection. The increase in T/C ratio after ketamine was blocked by prior inactivation of the medial septum with GABA(A) receptor agonist muscimol or by systemic administration of antipsychotic drugs, including chlorpromazine (5 mg/kg i.p.), haloperidol (1 mg/kg i.p.), or clozapine (7.5 mg/kg i.p.). Infusion of muscimol into the medial septum or injection of an antipsychotic drug alone did not affect the T/C ratio. However, rats with selective lesion of the septohippocampal cholinergic neurons by 192 IgG-saporin showed a significantly higher T/C ratio (0.86 +/- 0.10) than sham lesion rats (0.26 +/- 0.07), and ketamine did not further increase T/C ratio in rats with septohippocampal cholinergic neuron lesion.

CONCLUSIONS

Ketamine's disruption of hippocampal auditory gating was normalized by inactivation of the medial septum; in addition, septal cholinergic neurons participate in normal auditory gating.

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.

THE EFFECT OF IMMOBILIZATION STRESS ON SENSORY GATING IN MICE

Central sensory filtering processes can be demonstrated using a paired stimulus paradigm. Normal humans show a diminished, vertex-recorded mid-latency (50 ms) of auditory evoked potential to the second of paired clicks (0.5 s apart), a phenomenon termed as auditory gating. A loss of 50 ms in auditory gating is strongly related to psychosis. The N40 auditory evoked potential (EP) in rats has been used to develop an animal model for the study of sensory gating mechanisms. Previous animal studies of auditory gating have used psychotomimetic drug administration to induce sensory gating. However, a nonpharmacologic model of deficient gating would be advantageous. In the present study we investigated the effect of immobilization stress on sensory gating in twelve adult male mice. Evoked responses to the paired auditory click stimuli from vertex location of scalp were recorded using a silver needle electrode, a bioelectric amplifier, and an analog-digital converter. The mice were exposed to immobilization stress (IS) for 3 h. Data showed that the N40 potential was depressed in response to the second of the paired stimuli before application immobilization stress. At the end of the 3-h immobilization, the depression of the second N40 response was not observed. It was concluded that sensory gating is present in the mice and acutely disrupted by stressful stimuli, as shown in human subjects and rats.

Atypical antipsychotics as noncompetitive inhibitors of alpha4beta2 and alpha7 neuronal nicotinic receptors

It has been suggested that the interaction of antipsychotic medications with neuronal nicotinic receptors may increase the cognitive dysfunction associated with schizophrenia and may explain why current therapies only partially address this core feature of the illness. In the present studies we compared the effects of the atypical antipsychotics quetiapine, clozapine and N-desmethylclozapine to those of the typical antipsychotics haloperidol and chlorpromazine on the alpha4beta2 and alpha7 nicotinic receptor subtypes. The binding of [(3)H]-nicotine to rat cortical alpha4beta2 receptors and [(3)H]-methyllycaconitine to rat hippocampal alpha7 receptors was not affected by any of the compounds tested. However, Rb(+) efflux evoked either by nicotine or the selective alpha4beta2 agonist TC-1827 from alpha4beta2 receptors expressed in SH-EP1 cells and nicotine-evoked [(3)H]-dopamine release from rat striatal synaptosomes were non-competitively inhibited by all of the antipsychotics. Similarly, alpha-bungarotoxin-sensitive epibatidine-evoked [(3)H]-norepinephrine release from rat hippocampal slices and acetylcholine-activated currents of alpha7 nicotinic receptors expressed in oocytes were inhibited by haloperidol, chlorpromazine, clozapine and N-desmethylclozapine. The inhibitory effects on nicotinic receptor function produced by the antipsychotics tested occurred at concentrations similar to plasma levels achieved in schizophrenia patients, suggesting that they may lead to clinically relevant effects on cognition.

Nicotine and attention: event-related potential investigations in nonsmokers

Research into the effects of nicotine and smoking on cognition has largely confirmed the subjective reports of smoking in smokers on mental functions, showing smoking abstinence to disrupt and smoking/nicotine to restore cognitive functioning. Evidence of performance improvements in nonsmokers has provided partial support for the absolute effects of nicotine on cognitive processes, which are independent of withdrawal relief, but the mechanisms underlying its pro-cognitive properties still remain elusive. The attentional facilitation frequently reported with smoking/nicotine may be indirectly related to its diffuse arousal-enhancing actions, as evidenced by electroencephalographic (EEG) fast frequency power increments, or it may reflect nicotine's direct modulating effects on specific neural processes governing stimulus encoding, selection and rejection. Event-related potential (ERP) components extracted during the performance of cognitive tasks have proven to be sensitive to early pre-attentive and later attention-dependent processes that are not otherwise reflected in behavioral probes. To date, the majority of ERP studies have been conducted with smokers using passive non-task paradigms or relatively non-demanding "oddball" tasks. This paper will emphasize our recent ERP investigations with acute nicotine polacrilex (6 mg) administered to nonsmokers, and with a battery of ERP and behavioral performance paradigms focusing on intra- and inter-modal selective attention and distraction processes. These ERP findings of nicotine-augmented early attentional processing add support to the contention that nicotine may be be used by smokers as a "pharmacological tool" for tuning cognitive functions relating to the automatic and controlled aspects of sensory input detection and selection.