The preclinical profile of asenapine: clinical relevance for the treatment of schizophrenia and bipolar mania

An update on potential pharmacotherapies for cognitive impairment in bipolar disorder

INTRODUCTION

Cognitive impairment is a core feature of bipolar disorder (BD) that impedes recovery by preventing the return to optimal socio-occupational functioning and reducing quality of life. Presently, there are no efficacious treatments for cognitive impairment in BD, but many pharmacological interventions are being considered as they have the potential to target the underlying pathophysiology of the disorder.

AREAS COVERED

This review summarizes the available evidence for pharmacological interventions for cognitive impairment in bipolar disorder. We searched PubMed, MedLine, and PsycInfo from inception to December 1^st, 2022. Traditional treatments, such as lithium, anticonvulsants (lamotrigine), antipsychotics (aripiprazole, asenapine, cariprazine, lurasidone, and olanzapine), antidepressants (vortioxetine, fluoxetine, and tianeptine) and psychostimulants (modafinil), and emerging interventions, such as acetylcholinesterase inhibitors (galantamine and donepezil), dopamine agonists (pramipexole), erythropoietin, glucocorticoid receptor antagonists (mifepristone), immune modulators (infliximab, minocycline and doxycycline), ketamine, metabolic agents (insulin, metformin, and liraglutide), probiotic supplements, and Withania somnifera are discussed.

EXPERT OPINION

The investigation of interventions for cognitive impairment in BD is a relatively under-researched area. In the past, methodological pitfalls in BD cognition trials have also been a critical limiting factor. Expanding on the existing literature and identifying novel pharmacological and non-pharmacological treatments for cognitive impairment in BD should be a priority.

Seventy Years of Antipsychotic Development: A Critical Review

Since the mid-1950s discovery of the first effective antipsychotic medications (APM), we have only been able to improve the tolerability but not the overall efficacy of currently available APMs, as reflected by effectiveness trials in Europe and the United States. This inability to develop more effective APMs is attributable to multiple factors, including failure to create and use assessment tools to assess core symptom domains in schizophrenia, move beyond the dopaminergic hypothesis and to develop "me too" drugs, imposing ill-defined research domain criteria, and lacking federal funding for clinical trials. The classification of APMs is also confusing, including second-generation, partial agonists, and multimodal APMs in the same class of APMs, despite significant differences in their mechanisms of action. Other factors stagnating drug development include inadequate sample sizes to address heterogeneity, lack of statistical measures correlating with clinical significance, using the atheoretical basis of psychiatric diagnoses, failure to control placebo response, and high cost of newer and perhaps more tolerable APMs. Furthermore, there has been a failure to develop early predictors of antipsychotic response and various tools to optimize an APM response. Finally, some mental health providers are also responsible for the suboptimal use of APMs, by using excessive maintenance doses, often with irrational polypharmacy, further compromising effectiveness and medication adherence. However, some bright spots in antipsychotic development include improved tolerability of APMs and long-acting injectables to address the high prevalence of medication nonadherence. This review critically reviews 70 years of antipsychotic development, the reasons behind the failure to develop more effective APMs, and suggestions for future direction.

The Influence of Long-Term Treatment with Asenapine on Liver Cytochrome P450 Expression and Activity in the Rat. The Involvement of Different Mechanisms

Therapy of schizophrenia requires long-term treatment with a relevant antipsychotic drug to achieve a therapeutic effect. The aim of the present study was to investigate the influence of prolonged treatment with the atypical neuroleptic asenapine on the expression and activity of rat cytochrome P450 (CYP) in the liver. The experiment was carried out on male Wistar rats. Asenapine (0.3 mg/kg s.c.) was administered for two weeks. The levels of CYP mRNA protein and activity were determined in the liver and hormone concentrations were measured in the pituitary gland and blood serum. Asenapine significantly decreased the activity of CYP1A (caffeine 8-hydroxylation and 3-N-demethylation), CYP2B, CYP2C11 and CYP3A (testosterone hydroxylation at positions 16β; 2α and 16α; 2β and 6β, respectively). The neuroleptic did not affect the activity of CYP2A (testosterone 7α-hydroxylation), CYP2C6 (warfarin 7-hydroxylation) and CYP2E1 (chlorzoxazone 6-hydroxylation). The mRNA and protein levels of CYP1A2, CYP2B1, CYP2C11 and CYP3A1 were decreased, while those of CYP2B2 and CYP3A2 were not changed. Simultaneously, pituitary level of growth hormone-releasing hormone and serum concentrations of growth hormone and corticosterone were reduced, while that of triiodothyronine was enhanced. In conclusion, chronic treatment with asenapine down-regulates liver cytochrome P450 enzymes, which involves neuroendocrine mechanisms. Thus, chronic asenapine treatment may slow the metabolism of CYP1A, CYP2B, CYP2C11 and CYP3A substrates (steroids and drugs). Since asenapine is metabolized by CYP1A and CYP3A, the neuroleptic may inhibit its own metabolism, therefore, the plasma concentration of asenapine in patients after prolonged treatment may be higher than expected based on a single dose.

Chronic antipsychotic treatment exerts limited effects on the mania-like behavior of dopamine transporter knockdown mice

BACKGROUND

Bipolar disorder is a life-threatening disorder linked to dopamine transporter (DAT) polymorphisms, with reduced DAT levels seen in positron emission tomography and postmortem brains.

AIMS

The purpose of this study was to examine the effects of approved antipsychotics on DAT dysfunction-mediated mania behavior in mice.

METHODS

DAT knockdown mice received either D2-family receptor antagonist risperidone or asenapine and mania-related behaviors were assessed in the clinically-relevant behavioral pattern monitor to assess spontaneous exploration.

RESULTS

Chronic risperidone did not reverse mania-like behavior in DAT knockdown mice. Chronic asenapine reduced mania behavior but this effect was more pronounced in wild-type littermates than in DAT knockdown mice.

CONCLUSION

Taken together, these findings suggest that while acute antipsychotic treatment may be beneficial in management of bipolar mania, more targeted therapeutics may be necessary for long-term treatment. Specific investigation into DAT-targeting drugs could improve future treatment of bipolar mania.

Recent Advances in Marine-Based Nutraceuticals and Their Health Benefits

The oceans have been the Earth's most valuable source of food. They have now also become a valuable and versatile source of bioactive compounds. The significance of marine organisms as a natural source of new substances that may contribute to the food sector and the overall health of humans are expanding. This review is an update on the recent studies of functional seafood compounds (chitin and chitosan, pigments from algae, fish lipids and omega-3 fatty acids, essential amino acids and bioactive proteins/peptides, polysaccharides, phenolic compounds, and minerals) focusing on their potential use as nutraceuticals and health benefits.

Impact of repeated asenapine treatment on FosB/ΔFosB expression in the forebrain structures under normal conditions and mild stress preconditioning in the rat

Long-term effect of asenapine (ASE), an atypical antipsychotic drug, on FosB/ΔFosB quantitative variations in the striatum, septum, nucleus accumbens, and prefrontal cortex, was light microscopically evaluated in normal rats and rats preconditioned with chronic unpredictable mild stress (CMS). CMS included restraint, social isolation, crowding, swimming, and cold. The rats were exposed to CMS for 21 days. From the 7th day of CMS, the rats were injected subcutaneously with saline (300μl/rat) or ASE (0.3mg/kg b.w.), twice a day for 14 days. On the 22nd day, i.e. 16-18h after the last treatment, the animals were perfused with fixative and the brains cut into 30μm thick coronal sections. FosB/ΔFosB protein was immunohistochemically visualized by avidin-biotin peroxidase complex (ABC). Four groups of animals were investigated: control+vehicle, control+ASE, CMS+vehicle, and CMS+ASE. Repeated ASE treatment significantly increased the amount of FosB/ΔFosB immunostained cell nuclei in the dorsolateral and dorsomedial striatum and the shell of the nucleus accumbens, followed by strVM and coACC, as assessed by numerical analysis in both total (different size for each structure) and unified (equal size for each structure) brain sectors. The effect of ASE was significantly lowered by CMS preconditioning only in the dorsolateral striatum, dorsomedial striatum, and the shell of the nucleus accumbens, indicated by both total and unified calculations. Although, highest FosB/ΔFosB expression was seen in the prefrontal cortex and lowest in the dorsolateral and ventrolateral septum, no differences between the groups occurred. CMS itself did not affect FosB/ΔFosB expression level. These findings demonstrate for the first time that repeated administration of ASE may result in eliciting of long-lasting FosB/ΔFosB-like transcription factors that could mediate some of the persistent and region-specific changes in brain function, interconnected with chronic drug exposure. However, it cannot be excluded that the impact of repeated ASE exposure might be influenced by an ambient stressogen leverage.

Asenapine: A Review in Schizophrenia

Asenapine (Saphris(®), Sycrest(®)) is an atypical antipsychotic that is administered sublingually twice daily and is approved for schizophrenia in the USA, Japan and other countries, but not in the EU. This article reviews the pharmacology, clinical efficacy and tolerability profile of asenapine in the treatment of adults with schizophrenia. Clinical trials with asenapine have demonstrated efficacy in terms of both positive and negative symptoms of schizophrenia, although findings have not always been consistent. Across three short-term (6-week) studies in acute schizophrenia (including one in Asian patients), asenapine was generally superior to placebo and had broadly similar efficacy to active controls in improving total scores on the Positive and Negative Syndrome Scale. A meta-analysis of four short-term trials with asenapine (that also included a negative study and a failed trial) also showed significant benefit with asenapine over placebo. In longer-term trials and extensions (up to ≈3 years' duration), asenapine was effective relative to placebo in preventing relapse in schizophrenia, but was less effective than olanzapine in patients with schizophrenia or schizoaffective disorder (according to intent-to-treat LOCF analysis). However, in two trials in patients with persistent negative symptoms of schizophrenia, asenapine and olanzapine were similarly effective in reducing negative symptoms at week 26, with asenapine providing better results than olanzapine at week 52 in one of the extensions. The most frequently reported adverse events with asenapine are somnolence, akathisia and oral hypoesthesia. Although potentially associated with more extrapyramidal symptoms, asenapine appears to have less weight gain and metabolic effects than some other antipsychotic agents, such as olanzapine.

Management of depressive symptoms in schizophrenia

BACKGROUND

Although depressive symptoms are a frequently occurring phenomenon in schizophrenia, effective treatments remain an area of clinical need.

OBJECTIVE

To assess the benefit of short-term treatment with the atypical antipsychotic asenapine versus placebo on depressive symptoms in patients with acute schizophrenia in an exacerbated state.

METHODS

Data were pooled from intent-to-treat (ITT) populations of three 6-week, randomized controlled studies with fixed doses of asenapine (ASE; n=427), olanzapine (OLA; n=82), risperidone (RIS; n=54), haloperidol (HAL; n=97), or placebo (PLA; n=254). Change from baseline Calgary Depression Scale for Schizophrenia (CDSS) total score and individual item scores were assessed at Day 21 and Day 42 in the total patient population (n=914), and in patients presenting with a CDSS total score of .6 at baseline (n=248). Mixed model repeated measures (MMRM) analyses were performed on patient data.

RESULTS

The observed change from baseline in CDSS total score was significantly larger with ASE.compared to PLA.at both Day 21 (p<0.05) and Day 42 (p<0.01) for the total patient population group, and at Day 21 (p<0.05) in patients with baseline CDSS total score .6. For both populations, there was a significant change from baseline in the CDSS depression item score with ASE.compared to PLA.at Day 21 (p<0.01, all patient population; p<0.05, patients with baseline CDSS .6), and at Day 42 (p<0.01) in the all patient population. Statistically significant changes from baseline, in favor of ASE versus PLA, were also observed in other individual CDSS item scores including hopelessness (p<0.05, Day 21, patients with baseline CDSS .6), self-depreciation (p<0.05, Day 42, all patient population), guilty ideas of reference (p<0.01, Day 42, all patient population), pathological guilt (p<0.01, Day 21, all patient population; p<0.05, Day 21 and Day 42, patients with baseline CDSS score .6), and observed depression (p<0.05, Day 21, all patient population).

CONCLUSIONS

ASE significantly improved a range of depressive symptoms in people with an acute exacerbation of schizophrenia, as measured by the CDSS. ASE may represent a beneficial treatment option for the management of depressive symptoms in patients with schizophrenia.

Effects of repeated asenapine in a battery of tests for anxiety-like behaviours in mice

OBJECTIVE

A number of atypical antipsychotic drugs were demonstrated to have anxiolytic effects in patients and in animal models. These effects were mostly suggested to be the consequence of the drugs' affinity to the serotonin system and its receptors. Asenapine is a relatively new atypical antipsychotic that is prescribed for schizophrenia and for bipolar mania. Asenapine has a broad pharmacological profile with significant effects on serotonergic receptors, hence it is reasonable to expect that asenapine may have some anxiolytic effects. The present study was therefore designed to examine possible effects of asenapine on anxiety-like behaviour of mice.

METHOD

Male ICR mice were repeatedly treated with 0.1 or 0.3 mg/kg injections of asenapine and then tested in a battery of behavioural tests related to anxiety including the open-field test, elevated plus-maze (EPM), defensive marble burying and hyponeophagia tests. In an adjunct experiment, we tested the effects of acute diazepam in the same test battery.

RESULTS

The results show that diazepam reduced anxiety-like behaviour in the EPM, the defensive marble burying test and the hyponeophagia test but not in the open field. Asenapine has anxiolytic-like effects in the EPM and the defensive marble burying tests but had no effects in the open-field or the hyponeophagia tests. Asenapine had no effects on locomotor activity.

CONCLUSION

The results suggest that asenapine may have anxiolytic-like properties and recommends that clinical trials examining such effects should be performed.

Intranasal delivery of asenapine loaded nanostructured lipid carriers: formulation, characterization, pharmacokinetic and behavioural assessment

The aim of the present research work was to develop asenapine (ASM) loaded nanostructured lipid carriers (ANLC) for the delivery of drugs in the brain by an intranasal route to enhance therapeutic efficacy.

Efficacy and safety of asenapine in Asian patients with an acute exacerbation of schizophrenia: a multicentre, randomized, double-blind, 6-week, placebo-controlled study

RATIONALE

Asenapine is a second generation anti-psychotic approved in the USA in 2009 for the treatment of schizophrenia, but its efficacy has not been proven in Asian patients.

OBJECTIVES

The objectives of this study are to evaluate the efficacy and tolerability of asenapine in Asian patients experiencing an acute exacerbation of schizophrenia.

METHODS

In this prospective, double-blind study, patients in Japan, Korea, and Taiwan were randomized (1:1:1) to asenapine 5 mg twice daily (bid), 10 mg bid or placebo for 6 weeks after a 3- to 7-day washout/screening period. The primary endpoint was the mean change in the positive and negative syndrome scale (PANSS) total score from baseline to day 42/treatment end.

RESULTS

Of the 532 participants randomized, 530 received treatment. The primary endpoint was significantly greater with asenapine 5 and 10 mg bid than with placebo (-12.24 and -14.17 vs. -0.95; p < 0.0001). The results of secondary endpoints including PANSS negative subscale scores and PANSS responders at the end of treatment supported the results of the primary endpoint. There were no significant differences in the incidence of treatment-emergent adverse events reported with asenapine 5 and 10 mg bid and placebo (84.6, 80.7, and 81.6 %). There was a mean (± standard deviation) change in weight of -1.76 ± 2.45 kg for placebo, +0.42 ± 2.65 kg for asenapine 5 mg bid, and +0.81 ± 2.89 kg for asenapine 10 mg bid group.

CONCLUSIONS

Asenapine was effective and generally well tolerated when used for the treatment of acute exacerbations of schizophrenia in Asian patients.

The Role of Dopamine and Its Dysfunction as a Consequence of Oxidative Stress

Dopamine is a neurotransmitter that is produced in the substantia nigra, ventral tegmental area, and hypothalamus of the brain. Dysfunction of the dopamine system has been implicated in different nervous system diseases. The level of dopamine transmission increases in response to any type of reward and by a large number of strongly additive drugs. The role of dopamine dysfunction as a consequence of oxidative stress is involved in health and disease. Introduce new potential targets for the development of therapeutic interventions based on antioxidant compounds. The present review focuses on the therapeutic potential of antioxidant compounds as a coadjuvant treatment to conventional neurological disorders is discussed.

Asenapine for the Acute Treatment of Pediatric Manic or Mixed Episode of Bipolar I Disorder

OBJECTIVE

To evaluate asenapine versus placebo in 403 patients aged 10 to 17 years with bipolar I disorder currently in manic or mixed episodes.

METHOD

In this double-blind, placebo-controlled, international trial, patients were randomized 1:1:1:1 to placebo, asenapine 2.5, 5, or 10 mg b.i.d. (twice daily). Primary efficacy measure was change from baseline in Young-Mania Rating Scale (YMRS) total score at day 21. Analyses of patients with/without attention-deficit/hyperactivity disorder (ADHD) and with/without stimulant use were performed.

RESULTS

The mean difference in asenapine versus placebo in YMRS was -3.2 (p = .0008), -5.3 (p < .001), and -6.2 (p < .001) for asenapine 2.5, 5, and 10 mg b.i.d., respectively. Treatment-emergent adverse events with an incidence ≥5% and at least twice placebo were somnolence, sedation, hypoesthesia oral, paresthesia oral, and increased appetite. The asenapine groups had a higher incidence of ≥7% weight gain (range, 8.0%-12.0%) versus placebo (1.1%; p < .05). The mean change from baseline in fasting insulin was larger for patients treated with asenapine than those with placebo (asenapine 2.5 mg b.i.d.: 73.375 pmol/L; asenapine 5 mg b.i.d.: 114.042 pmol/L; asenapine 10 mg b.i.d.: 59.846 pmol/L; placebo: 3.690 pmol/L). The mean changes from baseline for lipid parameters and glucose were also larger in asenapine groups than in the placebo group. No safety differences were observed with respect to ADHD and stimulant use.

CONCLUSION

All asenapine doses versus placebo were superior based on change in YMRS at day 21. Asenapine was generally well tolerated in patients aged 10 to 17 years with bipolar I disorder in manic or mixed states. Increases in weight and fasting insulin were associated with asenapine. Clinical trial registration information-Efficacy and Safety of Asenapine Treatment for Pediatric Bipolar Disorder; http://clinicaltrials.gov; NCT01244815.

Pharmacological approaches to manage persistent symptoms of major depressive disorder: rationale and therapeutic strategies

Major depressive disorder (MDD) is a highly prevalent chronic psychiatric illness associated with significant morbidity, mortality, loss of productivity, and diminished quality of life. Typically, only a minority of patients responds to treatment and meet criteria for remission as residual symptoms may persist, the result of an inadequate course of treatment and/or the presence of persistent side effects. The foremost goal of treatment should be to restore patients to full functioning and eliminate or relieve all MDD symptoms, while being virtually free of troublesome side effects. The current available pharmacological options to manage persistent depressive symptoms include augmentation or adjunctive combination strategies, both of which target selected psychobiological systems and specific mood and somatic symptoms experienced by the patient. As well, non-pharmacological interventions including psychotherapies may be used in either first-line or adjunctive approaches. However, the evidence to date with respect to available adjunct therapies is limited by few studies and those published have utilized only a small number of subjects and lack enough data to allow for a consensus of expert opinion. This underlines the need for further longer term, large population-based studies and those that include comorbid populations, all of which are seen in real world community psychiatry.

Effect of acute asenapine treatment on Fos expression in the forebrain structures under normal conditions and mild stress preconditioning in the rat

Asenapine (ASE) is a novel atypical antipsychotic drug approved for the treatment of schizophrenia and bipolar disorder. Stress is an inseparable part of the human life, which may interfere with the therapeutic effect of different drugs. The aim of the present study was: (1) to delineate the quantitative and qualitative profiles of the ASE effect on Fos expression in the striatum, septum, nucleus accumbens, and the prefrontal cortex and (2) to find out whether a chronic unpredictable variable mild stress (CMS) preconditioning may modify the effect of acute ASE treatment. Stress paradigms included restrain, social isolation, crowding, swimming, and cold. The animals were exposed to CMS for 21 days and on the 22nd day received an injection of vehicle (saline 300 μl/rat s.c.) or ASE (0.3mg/kg s.c.). They were sacrificed 90 min after the treatments. Fos protein was visualized by avidin biotin peroxidase (ABC). Four groups of animals were investigated: controls+vehicle, controls+ASE, CMS+vehicle, and CMS+ASE. The number of Fos labeled neurons was calculated per total investigated area, which was selective for each structure, and also recalculated per unified sector. ASE treatment induced significant and very similar increase of the Fos expression in both ASE control and ASE CMS animals in comparison with saline control and CMS ones. Moreover, ASE induced regional differences in the number of Fos-positive neurons. In both ASE groups most pronounced response in the number of Fos profiles occurred in the dorsolateral striatum, ventrolateral septum, shell of the nucleus accumbens, and the medial prefrontal cortex. Mild Fos response was seen in the dorsomedial and ventromedial striatum and core of the nucleus accumbens. No response was seen in the dorsolateral septum. The present paper demonstrates for the first time the character of the Fos distribution in the forebrain structures induced by acute ASE treatment as well as ASE response to 21 days CMS preconditioning. The study provides an important comparative background that may help in the further understanding of the effect of ASE on the brain activation as well as its responsiveness to CMS challenges.

Asenapine sensitization from adolescence to adulthood and its potential molecular basis

Asenapine is a new antipsychotic drug that induces a long-lasting behavioral sensitization in adult rats. The present study investigated the developmental impacts of adolescent asenapine treatment on drug sensitivity and on 3 proteins implicated in the action of antipsychotic drugs (i.e. brain-derived neurotrophic factor (BDNF), dopamine D2 receptor, and ΔFosB) in adulthood. Male adolescent Sprague-Dawley rats (postnatal days, P 43-48) were first treated with asenapine (0.05, 0.10 or 0.20mg/kg, sc) and tested in the conditioned avoidance or PCP (2.0mg/kg, sc)-induced hyperlocomotion tasks for 5 days. After they became adults (∼P 76), asenapine sensitization was assessed in a single avoidance or PCP-induced hyperlocomotion challenge test with all rats being injected with asenapine (0.10mg/kg, sc). Rats were then sacrificed 1 day later and BDNF, D2 and ΔFosB in the prefrontal cortex, striatum and hippocampus were examined using Western blotting. In adolescence, repeated asenapine treatment produced a persistent and dose-dependent inhibition of avoidance response, spontaneous motor activity and PCP-induced hyperlocomotion. In the asenapine challenge test, adult rats treated with asenapine (0.10 and 0.20mg/kg) in adolescence made significantly fewer avoidance responses and showed a stronger inhibition of spontaneous motor activity than those previously treated with saline. However, no group difference in the levels of BDNF, D2 and ΔFosB expression was found. These findings suggest that although adolescent asenapine treatment for a short period of time induces a robust behavioral sensitization that persists into adulthood, such a long-term effect is not likely to be mediated by BDNF, D2 and ΔFosB.

Asenapine review, part I: chemistry, receptor affinity profile, pharmacokinetics and metabolism

INTRODUCTION

Asenapine is a second-generation (atypical) antipsychotic currently marketed for the treatment of schizophrenia and bipolar mania/mixed episodes.

AREAS COVERED

The purpose of this review is to describe the chemistry, pharmacodynamics, and pharmacokinetics of asenapine.

EXPERT OPINION

Asenapine has a complex pharmacodynamic profile with affinities at multiple dopamine, serotonin, histamine, and α-adrenergic receptors, all at which asenapine functions as an antagonist. Sublingual asenapine tablets are absorbed in the oral mucosa, with a Tmax occurring between 30 and 90 min. Terminal half-life is approximately 24 h. Asenapine has multiple inactive metabolites, produced via direct glucuronidation (primarily via UGT1A4), demethylation, and oxidative metabolism (primarily via CYP1A2). Hepatic and renal routes contribute approximately equally to the elimination of asenapine and its metabolites. Two notable drug-drug interactions are evident: asenapine (an inhibitor of CYP2D6) can increase plasma levels of paroxetine, and fluvoxamine (a CYP1A2 inhibitor) can increase plasma levels of asenapine. Caution is required when coadministering asenapine with drugs that are both substrates and inhibitors of CYP2D6.

Acute and chronic effects of NMDA receptor antagonists in rodents, relevance to negative symptoms of schizophrenia: a translational link to humans

Negative symptoms of schizophrenia remain an unmet clinical need as they are common, persistent, respond poorly to existing treatments and lead to disability. Blunted affect, alogia, asociality, anhedonia and avolition are regarded as key negative symptoms despite DSM-IV-TR specifying a more limited range. The key to development of improved therapies is improved animal models that mimic the human condition in terms of behaviour and pathology and that predict efficacy of novel treatments in patients. Accumulating evidence shows that NMDA receptor (NMDAR) antagonists mimic cognitive deficits of relevance to schizophrenia in animals, along with associated pathological changes. This review examines evidence for the ability of NMDAR antagonists to mimic anhedonia and asociality, two negative symptoms of schizophrenia, in animals. The use of various species, paradigms and treatment regimens are reviewed. We conclude that sub-chronic treatment with NMDAR antagonists, typically PCP, induces social withdrawal in animals but not anhedonia. NMDAR antagonists have further effects in paradigms such as motivational salience that may be useful for mimicking other aspects of negative symptoms but these require further development. Sub-chronic treatment regimens of NMDAR antagonists also have some neurobiological effects of relevance to negative symptoms. It is our view that a sub-chronic treatment regime with NMDAR antagonists, particularly PCP, with animals tested following a wash-out period and in a battery of tests to assess certain behaviours of relevance to negative symptoms and social withdrawal (the animal equivalent of asociality) is valuable. This will enhance our understanding of the psycho and neuropathology of specific negative symptom domains and allow early detection of novel pharmacological targets.

The role of dopamine in schizophrenia from a neurobiological and evolutionary perspective: old fashioned, but still in vogue

Dopamine is an inhibitory neurotransmitter involved in the pathology of schizophrenia. The revised dopamine hypothesis states that dopamine abnormalities in the mesolimbic and prefrontal brain regions exist in schizophrenia. However, recent research has indicated that glutamate, GABA, acetylcholine, and serotonin alterations are also involved in the pathology of schizophrenia. This review provides an in-depth analysis of dopamine in animal models of schizophrenia and also focuses on dopamine and cognition. Furthermore, this review provides not only an overview of dopamine receptors and the antipsychotic effects of treatments targeting them but also an outline of dopamine and its interaction with other neurochemical models of schizophrenia. The roles of dopamine in the evolution of the human brain and human mental abilities, which are affected in schizophrenia patients, are also discussed.

Time-dependence of risperidone and asenapine sensitization and associated D2 receptor mechanism

When an antipsychotic drug is given repeatedly and intermittently, there is often a long-term increase in its behavioral efficacy, termed antipsychotic sensitization. With the passage of time, the magnitude of antipsychotic sensitization may increase or decrease based on the principle of time-dependent sensitization (TDS) or memory decay, respectively. In the present study, we examined the time-dependent feature and possible dopamine D2 receptor mechanism of sensitization induced by risperidone and asenapine in the conditioned avoidance response test. Well-trained male adult Sprague-Dawley rats were first repeatedly treated with risperidone (1.0mg/kg) or asenapine (0.2mg/kg) and tested for avoidance response daily for 5 consecutive days. Eight, 18 or 38 days after the 5th drug treatment, all rats were retested drug-free to assess the long-term impact of prior risperidone or asenapine treatment. Drug-pretreated rats had significantly lower avoidance than vehicle-pretreated ones on this test, and the group differences increased with the passage of time. In the subsequent drug challenge test at 10, 20 or 40 days after the 5th drug treatment, all rats were injected with a low dose of risperidone (0.3mg/kg) or asenapine (0.1mg/kg). Drug-pretreated rats again made significantly fewer avoidances than controls, confirming the antipsychotic sensitization effect. Finally, in the quinpirole (a D2/3 receptor agonist, 1.0mg/kg, sc)-induced hyperlocomotion test, risperidone-pretreated rats exhibited a significantly higher level of motor activity than the vehicle-pretreated ones. These findings suggest that risperidone and asenapine sensitization is long-lasting, follows the TDS principle, and is likely mediated by D2 receptor supersensitivity.

Repeated asenapine treatment produces a sensitization effect in two preclinical tests of antipsychotic activity

Among several commonly used atypical antipsychotic drugs, olanzapine and risperidone cause a sensitization effect in the conditioned avoidance response (CAR) and phencyclidine (PCP)-induced hyperlocomotion paradigms--two well established animal tests of antipsychotic drugs, whereas clozapine causes a tolerance effect. Asenapine is a novel antipsychotic drug recently approved for the treatment of schizophrenia and manic disorders. It shares several receptor binding sites and behavioral features with other atypical antipsychotic drugs. However, it is not clear what type of repeated effect (sensitization or tolerance) asenapine would induce, and whether such an effect is transferrable to other atypicals. In this study, male adult Sprague-Dawley rats were first repeatedly tested with asenapine (0.05, 0.10 or 0.20 mg/kg, sc) for avoidance response or PCP (3.20 mg/kg, sc)-induced hyperlocomotion daily for 5 consecutive days. After 2-3 days of retraining/drug-free recovery, they were then challenged with asenapine (0.10 mg/kg, sc), followed by olanzapine (0.50 mg/kg, sc) and clozapine (2.50 mg/kg, sc). During the 5-day drug test period (the induction phase), repeated asenapine treatment progressively increased its inhibition of avoidance response and PCP-induced hyperlocomotion in a dose-dependent fashion. On the asenapine and olanzapine challenge tests (the expression phase), rats previously treated with asenapine still showed significantly lower avoidance response and lower PCP-induced hyperlocomotion than those previously treated with vehicle. An increased reactivity to clozapine challenge in prior asenapine-treated rats was also found in the PCP-induced hyperlocomotion test. These findings suggest that asenapine is capable of inducing a sensitization effect and a cross-sensitization to olanzapine and clozapine (to a lesser extent). Because the behavioral profile of asenapine in both tests is similar to that of olanzapine, but different from that of clozapine, we suggest that asenapine resembles olanzapine to a greater extent than clozapine in its therapeutic and side effect profiles.