Clozapine modulates midbrain dopamine neuron firing via interaction with the NMDA receptor complex

Schizophrenia Synaptic Pathology and Antipsychotic Treatment in the Framework of Oxidative and Mitochondrial Dysfunction: Translational Highlights for the Clinics and Treatment

Schizophrenia is a worldwide mental illness characterized by alterations at dopaminergic and glutamatergic synapses resulting in global dysconnectivity within and between brain networks. Impairments in inflammatory processes, mitochondrial functions, energy expenditure, and oxidative stress have been extensively associated with schizophrenia pathophysiology. Antipsychotics, the mainstay of schizophrenia pharmacological treatment and all sharing the common feature of dopamine D2 receptor occupancy, may affect antioxidant pathways as well as mitochondrial protein levels and gene expression. Here, we systematically reviewed the available evidence on antioxidants' mechanisms in antipsychotic action and the impact of first- and second-generation compounds on mitochondrial functions and oxidative stress. We further focused on clinical trials addressing the efficacy and tolerability of antioxidants as an augmentation strategy of antipsychotic treatment. EMBASE, Scopus, and Medline/PubMed databases were interrogated. The selection process was conducted in respect of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) criteria. Several mitochondrial proteins involved in cell viability, energy metabolism, and regulation of oxidative systems were reported to be significantly modified by antipsychotic treatment with differences between first- and second-generation drugs. Finally, antioxidants may affect cognitive and psychotic symptoms in patients with schizophrenia, and although the evidence is only preliminary, the results indicate that further studies are warranted.

Activation of ventral tegmental area dopaminergic neurons ameliorates anxiety-like behaviors in single prolonged stress-induced PTSD model rats

Post-traumatic stress disorder (PTSD) is a debilitating psychiatric condition that arises after extremely traumatic events, with clinically significant and lasting impacts on both physical and psychological health. The present study examined the role of ventral tegmental area (VTA) dopaminergic signaling in anxiety-like behaviors and the underlying mechanisms in PTSD model rats. Chemogenetic technology was employed to specifically activate VTA dopamine (DA) neurons in rats subjected to single prolonged stress (SPS), and open field and elevated plus maze tests were applied to evaluate the anxiety-like manifestations. Subsequently, in vivo extracellular electrophysiological analyses were used to examine alterations in the firing characteristics of VTA DA neurons. Chemogenetic activation enhanced the firing and burst rates of VTA DA neurons in SPS-induced PTSD model rats and concomitantly mitigated the anxiety-like behavioral phenotypes. Collectively, these findings reveal a direct association between PTSD-relevant anxiety behaviors and VTA dopaminergic activity, and further suggest that interventions designed to enhance VTA dopaminergic activity may be a potential strategy for PTSD treatment.

Linking Inflammation, Aberrant Glutamate-Dopamine Interaction, and Post-synaptic Changes: Translational Relevance for Schizophrenia and Antipsychotic Treatment: a Systematic Review

Evidence from clinical, preclinical, and post-mortem studies supports the inflammatory/immune hypothesis of schizophrenia pathogenesis. Less evident is the link between the inflammatory background and two well-recognized functional and structural findings of schizophrenia pathophysiology: the dopamine-glutamate aberrant interaction and the alteration of dendritic spines architecture, both believed to be the “quantal” elements of cortical-subcortical dysfunctional network. In this systematic review, we tried to capture the major findings linking inflammation, aberrant glutamate-dopamine interaction, and post-synaptic changes under a direct and inverse translational perspective, a paramount picture that at present is lacking. The inflammatory effects on dopaminergic function appear to be bidirectional: the inflammation influences dopamine release, and dopamine acts as a regulator of discrete inflammatory processes involved in schizophrenia such as dysregulated interleukin and kynurenine pathways. Furthermore, the link between inflammation and glutamate is strongly supported by clinical studies aimed at exploring overactive microglia in schizophrenia patients and maternal immune activation models, indicating impaired glutamate regulation and reduced N-methyl-D-aspartate receptor (NMDAR) function. In addition, an inflammatory/immune-induced alteration of post-synaptic density scaffold proteins, crucial for downstream NMDAR signaling and synaptic efficacy, has been demonstrated. According to these findings, a significant increase in plasma inflammatory markers has been found in schizophrenia patients compared to healthy controls, associated with reduced cortical integrity and functional connectivity, relevant to the cognitive deficit of schizophrenia. Finally, the link between altered inflammatory/immune responses raises relevant questions regarding potential new therapeutic strategies specifically for those forms of schizophrenia that are resistant to canonical antipsychotics or unresponsive to clozapine.

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

Clozapine Increases Nestin Concentration in the Adult Male Rat Hippocampus: A Preliminary Study

Patients with schizophrenia, and rodent models of the disease, both exhibit suppressed neurogenesis, with antipsychotics possibly enhancing neurogenesis in pre-clinical models. Nestin, a cytoskeletal protein, is implicated in neuronal differentiation and adult neurogenesis. We hypothesized that schizophrenia pathogenesis involves nestin downregulation; however, few studies have related nestin to schizophrenia. We assessed nestin protein concentration, prepulse inhibition (PPI), and social interaction in the MK-801 model of schizophrenia, with or without antipsychotic (clozapine) treatment. Adult male Sprague–Dawley rats were intraperitoneally administered saline or MK-801 (0.1 mg/kg) to produce a schizophrenia-like phenotype, with concomitant subcutaneous injections of vehicle or clozapine (5 mg/kg). PPI was assessed on days 1, 8, and 15, and social interaction was assessed on day 4. Hippocampus tissue samples were dissected for Western blotting of nestin concentration. MK-801 alone did not alter nestin concentration, while clozapine alone enhanced hippocampal nestin concentration; this effect was not apparent in animals with MK-801 and clozapine co-administration. MK-801 also produced schizophrenia-like PPI disruptions, some of which were reversed by clozapine. Social interaction deficits were not detected in this model. This is the first report of clozapine-induced enhancements of hippocampal nestin concentration that might be mediated by NMDA receptors. Future studies will explore the impact of neurodevelopmental nestin concentration on symptom onset and antipsychotic treatment.

Antipsychotic Polypharmacy for the Management of Schizophrenia: Evidence and Recommendations

Schizophrenia is a debilitating illness with a lifetime prevalence estimate of 0.6% and consists of symptoms from the positive, negative, and cognitive domains. Social support, therapy, psychoeducation, and overall case management are very important aspects of the treatment of schizophrenia. However, as abnormalities in neurotransmission are one of the key findings of schizophrenia pathology, pharmacotherapies are cornerstones of the management of schizophrenia. Antipsychotics have been used as the primary pharmacological treatment of schizophrenia. These agents often have a good effect on reducing positive symptoms, but may not markedly improve negative symptoms or cognitive defects. However, at least 20% of individuals with schizophrenia do not experience a substantial response from monotherapy with antipsychotics. Further, despite evolving treatment protocols and advances in early recognition of the disorder, 70% of patients with schizophrenia require long-term, even lifetime, medication to control their symptoms and do not achieve complete recovery. To address these shortcomings, clinicians and research scientists have explored different combinations of treatments, polypharmacy, to improve the treatment of patients. Antipsychotic polypharmacy has been shown to cause more side effects than monotherapy, which is the main reason why most treatment guidelines caution against it. Antipsychotic monotherapy should be strived for and clozapine should be tried at the latest if two monotherapy trials with other antipsychotics have failed and no absolute contraindications exist. If residual symptoms exist despite trials of adequate dose and duration, other reasons that may reduce treatment effect should be ruled out. Long-acting injectables or blood concentration measurements should be considered to affirm compliance and proper serum levels. Antipsychotic polypharmacy should be considered and discussed with patients from whom the aforementioned procedures do not produce a satisfactory treatment result. In some cases, antipsychotic polypharmacy may produce better results than other forms of treatment augmentation, such as benzodiazepines. In particular, combining aripiprazole with clozapine may be effective in reducing treatment side effects or residual symptoms, and this is likely to hold true for combining other partial dopamine D₂ agonists with clozapine as well, although currently scant data exist. More research is needed, both in controlled but also real-world settings, to define optimal antipsychotic polypharmacy and/or other psychotropic treatment augmentation strategies for specific patient groups and situations.

A Role for Xanthurenic Acid in the Control of Brain Dopaminergic Activity

Xanthurenic acid (XA) is a metabolite of the kynurenine pathway (KP) synthetized in the brain from dietary or microbial tryptophan that crosses the blood-brain barrier through carrier-mediated transport. XA and kynurenic acid (KYNA) are two structurally related compounds of KP occurring at micromolar concentrations in the CNS and suspected to modulate some pathophysiological mechanisms of neuropsychiatric and/or neurodegenerative diseases. Particularly, various data including XA cerebral distribution (from 1 µM in olfactory bulbs and cerebellum to 0.1–0.4 µM in A₉ and A₁₀), its release, and interactions with G protein-dependent XA-receptor, glutamate transporter and metabotropic receptors, strongly support a signaling and/or neuromodulatory role for XA. However, while the parent molecule KYNA is considered as potentially involved in neuropsychiatric disorders because of its inhibitory action on dopamine release in the striatum, the effect of XA on brain dopaminergic activity remains unknown. Here, we demonstrate that acute local/microdialysis-infusions of XA dose-dependently stimulate dopamine release in the rat prefrontal cortex (four-fold increase in the presence of 20 µM XA). This stimulatory effect is blocked by XA-receptor antagonist NCS-486. Interestingly, our results show that the peripheral/intraperitoneal administration of XA, which has been proven to enhance intra-cerebral XA concentrations (about 200% increase after 50 mg/kg XA i.p), also induces a dose-dependent increase of dopamine release in the cortex and striatum. Furthermore, our in vivo electrophysiological studies reveal that the repeated/daily administrations of XA reduce by 43% the number of spontaneously firing dopaminergic neurons in the ventral tegmental area. In the substantia nigra, XA treatment does not change the number of firing neurons. Altogether, our results suggest that XA may contribute together with KYNA to generate a KYNA/XA ratio that may crucially determine the brain normal dopaminergic activity. Imbalance of this ratio may result in dopaminergic dysfunctions related to several brain disorders, including psychotic diseases and drug dependence.

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

A Working Hypothesis Regarding Identical Pathomechanisms between Clinical Efficacy and Adverse Reaction of Clozapine via the Activation of Connexin43

Clozapine (CLZ) is an approved antipsychotic agent for the medication of treatment-resistant schizophrenia but is also well known as one of the most toxic antipsychotics. Recently, the World Health Organization’s (WHO) global database (VigiBase) reported the relative lethality of severe adverse reactions of CLZ. Agranulocytosis is the most famous adverse CLZ reaction but is of lesser lethality compared with the other adverse drug reactions of CLZ. Unexpectedly, VigiBase indicated that the prevalence and relative lethality of pneumonia, cardiotoxicity, and seizures associated with CLZ were more serious than that of agranulocytosis. Therefore, haematological monitoring in CLZ patients monitoring system provided success in the prevention of lethal adverse events from CLZ-induced agranulocytosis. Hereafter, psychiatrists must amend the CLZ patients monitoring system to protect patients with treatment-resistant schizophrenia from severe adverse CLZ reactions, such as pneumonia, cardiotoxicity, and seizures, according to the clinical evidence and pathophysiology. In this review, we discuss the mechanisms of clinical efficacy and the adverse reactions of CLZ based on the accumulating pharmacodynamic findings of CLZ, including tripartite synaptic transmission, and we propose suggestions for amending the monitoring and medication of adverse CLZ reactions associated with pneumonia, cardiotoxicity, and seizures.

Glutamate metabolites in treatment resistant schizophrenia: A meta-analysis and systematic review of 1H-MRS studies

Nearly a third to half of schizophrenia patients are non-responsive to first-line antipsychotics and are labelled treatment resistant schizophrenia (TRS). Neurochemical abnormalities in TRS may not be dopaminergic but possibly glutamate (Glu) related. Studies that have examined glutamatergic abnormalities using proton magnetic resonance spectroscopy (¹H-MRS) in TRS, have showed inconsistent results. Hence, we conducted a meta-analysis of ¹H-MRS studies comparing levels of Glu-and its metabolites in the brains of TRS and non-treatment resistant schizophrenia (nTRS) patients. Four eligible studies were included in the analysis. Summary effect size for the group difference between TRS (n = 101, including Ultra-TRS) and nTRS (n = 61) in Glu-levels in the anterior cingulate cortex (ACC) as measured with Hedges's g was 0.21 (95% CI: -0.42 to 0.85; p = 0.5) suggesting absence of significant difference. However, on leave one out analysis, one iteration showed significant difference in Glu-levels between the groups (Hedges's g = 0.46; p = 0.02) with higher Glu-levels in TRS implying significant effect of a single study on the effect size. The higher ACC Glu-in TRS was not associated with symptom severity or antipsychotic administration, indicating a possible trait abnormality. The limited number of datasets comparing Glu-metabolites in other brain regions are narratively described. Our analysis is limited by the significant heterogeneity between studies. Further longitudinal, prospective studies are needed to confirm higher Glu-metabolite levels in ACC in TRS and explore this potential trait abnormality.

Efficacy and cognitive effect of sarcosine (N-methylglycine) in patients with schizophrenia: A systematic review and meta-analysis of double-blind randomised controlled trials

BACKGROUND

Sarcosine (N-methylglycine), a type 1 glycine transporter inhibitor (GlyT1), has shown therapeutic potential for treating schizophrenia; however, studies have reported conflicting results. This meta-analysis aimed to explore the efficacy and cognitive effect of sarcosine for schizophrenia.

METHODS

In this study, PubMed, Cochrane Systematic Reviews, and Cochrane Collaboration Central Register of Controlled Clinical Trials were searched electronically for double-blinded randomised controlled trials that used sarcosine for treating schizophrenia. We used the published trials up to November 2019 to investigate the efficacy of sarcosine in schizophrenia. We pooled studies by using a random-effect model for comparing sarcosine treatment effects. Patients who were diagnosed with schizophrenia according to the criteria of the Diagnostic and Statistical Manual of Mental Disorders-Fourth Edition were recruited. Clinical improvement and cognitive function scores between baseline and after sarcosine use were compared using the standardised mean difference (SMD) with 95% confidence intervals (CIs). The heterogeneity of the included trials was evaluated through visual inspection of funnel plots and through the I² statistic.

RESULTS

We identified seven trials with 326 participants with schizophrenia meeting the inclusion criteria. All these studies evaluated the overall clinical symptoms, and four of them evaluated overall cognitive functions. Sarcosine use achieved more significant effects than the use of its comparators in relieving overall clinical symptoms (SMD = 0.51, CI = 0.26-0.76, p < 0.01). Moreover, studies with the low Positive and Negative Syndrome Scale range of 70-79 showed significant effect size (ES)s of 0.67 (95% CI: 0.03-1.31, p = 0.04). In addition, trials enrolling patients with stable clinical symptoms had significant ESs: 0.53 (95% CI: 0.21-0.85, p < 0.01). Add-on sarcosine combined with first- and second-generation antipsychotics, except clozapine, had a positive effect. For overall cognitive functions, sarcosine showed a positive but insignificant effect compared with its comparators (SMD = 0.27, CI = -0.06 to 0.60, p = 0.10). The effects were correlated with increased female proportions and decreased illness duration, albeit nonsignificantly.

CONCLUSIONS

The meta-analysis suggests that sarcosine may be associated with treatment effect on overall clinical symptoms in patients with schizophrenia but not cognitive functions.

Focus on Clozapine Withdrawal- and Misuse-Related Cases as Reported to the European Medicines Agency (EMA) Pharmacovigilance Database

BACKGROUND

Clozapine is of high clinical relevance for the management of both treatment-resistant schizophrenia and psychotic disturbances with concurrent drug misuse. Although the molecule presents with a range of well-known side-effects, its discontinuation/withdrawal syndrome has been only anecdotally described.

AIMS

the 2005-2018 European Medicines Agency (EMA) dataset of Adverse Drug Reactions (ADRs) was analyzed to identify and describe possible clozapine withdrawal- and misuse-/abuse-/dependence-related issues.

METHOD

A descriptive analysis of clozapine-related ADRs was performed when available, data on ADRs' outcome, dosage, and possible concomitant drug(s) were considered.

RESULTS

Out of 11,847 clozapine-related ADRs, some 599 (5.05%) were related to misuse/abuse/dependence/withdrawal issues, including 258 withdrawal-related (43.1%); 241 abuse-related (40.2%); and 80 intentional product misuse-related (13.3%) ADRs. A small number of overdose- and suicide-related ADRs were reported as well. Clozapine was typically (69.2%) identified alone, and most (84.7%) fatalities/high-dosage intake instances were reported in association with a history of substance abuse.

CONCLUSIONS

Previous suggestions about the possibility of a clozapine discontinuation/withdrawal occurrence are here supported, but further studies are needed. However, the misuse/abuse cases here identified might be difficult to interpret, given the lack of studies highlighting the possible recreational use of clozapine. The high-dosage intake, fatal outcomes and clozapine/polydrug abuse issues reported here may, however, be a reason for concern.

Molecular targets of atypical antipsychotics: From mechanism of action to clinical differences

The introduction of atypical antipsychotics (AAPs) since the discovery of its prototypical drug clozapine has been a revolutionary pharmacological step for treating psychotic patients as these allow a significant recovery not only in terms of hospitalization and reduction in symptoms severity, but also in terms of safety, socialization and better rehabilitation in the society. Regarding the mechanism of action, AAPs are weak D₂ receptor antagonists and they act beyond D₂ antagonism, involving other receptor targets which regulate dopamine and other neurotransmitters. Consequently, AAPs present a significant reduction of deleterious side effects like parkinsonism, hyperprolactinemia, apathy and anhedonia, which are all linked to the strong blockade of D₂ receptors. This review revisits previous and current findings within the class of AAPs and highlights the differences in terms of receptor properties and clinical activities among them. Furthermore, we propose a continuum spectrum of "atypia" that begins with risperidone (the least atypical) to clozapine (the most atypical), while all the other AAPs fall within the extremes of this spectrum. Clozapine is still considered the gold standard in refractory schizophrenia and in psychoses present in Parkinson's disease, though it has been associated with adverse effects like agranulocytosis (0.7%) and weight gain, pushing the scientific community to find new drugs as effective as clozapine, but devoid of its side effects. To achieve this, it is therefore imperative to characterize and compare in depth the very complex molecular profile of AAPs. We also introduce relatively new concepts like biased agonism, receptor dimerization and neurogenesis to identify better the old and new hallmarks of "atypia". Finally, a detailed confrontation of clinical differences among the AAPs is presented, especially in relation to their molecular targets, and new means like therapeutic drug monitoring are also proposed to improve the effectiveness of AAPs in clinical practice.

Treatment-Resistant Schizophrenia Patients Show Elevated Anterior Cingulate Cortex Glutamate Compared to Treatment-Responsive

INTRODUCTION

Resistance to antipsychotic treatment is a significant clinical problem in patients with schizophrenia with approximately 1 in 3 showing limited or no response to repeated treatments with antipsychotic medication. The neurobiological basis for treatment resistance is unknown but recent evidence implicates glutamatergic function in the anterior cingulate cortex. We examined glutamate levels of chronically ill treatment-resistant patients directly compared to treatment-responsive patients.

METHODS

We acquired proton magnetic resonance spectroscopy (1H-MRS) at 3 Tesla from 21 treatment-resistant and 20 treatment-responsive patients. All participants had a DSM-IV diagnosis of schizophrenia. Treatment-resistant patients were classified using the modified Kane criteria. The groups were matched for age, sex, smoking status, and illness duration.

RESULTS

Glutamate to creatine ratio levels were higher in treatment-resistant patients (Mean [SD] = 1.57 [0.24]) than in treatment-responsive patients (Mean[SD] = 1.38 [0.23]), (T[35] = 2.34, P = .025, 2-tailed), with a large effect size of d = 0.76. A model assuming 2 populations showed a 25% improvement in the fit of the Akaike weights (0.55) over a model assuming 1 population (0.44), producing group values almost identical to actual group means.

DISCUSSION

Increased anterior cingulate glutamate level is associated with treatment-resistant schizophrenia. This appears to be a stable neurobiological trait of treatment-resistant patients. We discuss possible explanations for glutamatergic dysfunction playing a significant role in resistance to conventional antipsychotic treatments, which are all dopamine-2 receptor blockers. Our findings suggest that glutamatergic treatments may be particularly effective in resistant patients and that 1H-MRS glutamate indices can potentially have clinical use.

Chronic Antipsychotic Treatment in the Rat - Effects on Brain Interleukin-8 and Kynurenic Acid

Schizophrenia is associated with activation of the brain immune system as reflected by increased brain levels of kynurenic acid (KYNA) and proinflammatory cytokines. Although antipsychotic drugs have been used for decades in the treatment of the disease, potential effects of these drugs on brain immune signaling are not fully known. The aim of the present study is to investigate the effects of chronic treatment with antipsychotic drugs on brain levels of cytokines and KYNA. Rats were treated daily by intraperitoneally administered haloperidol (1.5 mg/kg, n = 6), olanzapine (2 mg/kg, n = 6), and clozapine (20 mg/kg, n = 6) or saline (n = 6) for 30 days. Clozapine, but not haloperidol or olanzapine-treated rats displayed significantly lower cerebrospinal fluid (CSF) levels of interleukin-8 compared to controls. Whole brain levels of KYNA were not changed in any group. Our data suggest that the superior therapeutic effect of clozapine may be a result of its presently shown immunosuppressive action. Further, our data do not support the possibility that elevated brain KYNA found in patients with schizophrenia is a result of antipsychotic treatment.

Clozapine and glycinamide prevent MK-801-induced deficits in the novel object recognition (NOR) test in the domestic rabbit (Oryctolagus cuniculus)

Studies in humans indicate that acute administration of sub-anesthetic doses of ketamine, an NMDA receptor antagonist, provokes schizophrenic-like symptoms in healthy volunteers, and exacerbates existing symptoms in individuals with schizophrenia. These and other findings suggest that NMDA receptor hypofunction might participate in the pathophysiology of schizophrenia, and have prompted the development of rodent pharmacological models for this disorder based on acute or subchronic treatment with NMDA receptor antagonists, as well as the development of novel pharmacotherapies based on increasing extrasynaptic glycine concentrations. In the present study, we tested whether acute hyperlocomotory behavior and/or deficits in the novel object recognition (NOR) task, induced in male rabbits by the acute subcutaneous (s.c.) administration of MK-801 (0.025 and 0.037 mg/kg s.c., respectively), were prevented by prior administration of the atypcial antipsychotic, clozapine (0.2mg/kg, s.c.), or the glycine pro-drug glycinamide (56 mg/kg, s.c.). We found that clozapine fully prevented the MK-801-induced hyperlocomotion, and both clozapine and glycinamide prevented MK-801-induced deficits in the NOR task. The present results show that MK-801-induced hyperlocomotion and deficits in the NOR task in the domestic rabbit demonstrate predictive validity as an alternative animal model for symptoms of schizophrenia. Moreover, these results indicate that glycinamide should be investigated in pre-clinical models of neuropsychiatric disorders such as schizophrenia, obsessive compulsive disorder and anxiety disorders, where augmentation of extrasynaptic glycine concentrations may have therapeutic utility.

Neuroprogression in schizophrenia: Pathways underpinning clinical staging and therapeutic corollaries

OBJECTIVE

Whilst dopaminergic dysfunction remains a necessary component involved in the pathogenesis of schizophrenia, our current pharmacological armoury of dopamine antagonists does little to control the negative symptoms of schizophrenia. This suggests other pathological processes must be implicated. This paper aims to elaborate on such theories.

METHODS

Data for this review were sourced from the electronic database PUBMED, and was not limited by language or date of publication.

RESULTS

It has been suggested that multiple 'hits' may be required to unveil the clinical syndrome in susceptible individuals. Such hits potentially first occur in utero, leading to neuronal disruption, epigenetic changes and the establishment of an abnormal inflammatory response. The development of schizophrenia may therefore potentially be viewed as a neuroprogressive response to these early stressors, driven on by changes in tryptophan catabolite (TRYCAT) metabolism, reactive oxygen species handling and N-methyl d-aspartate (NMDA) circuitry. Given the potential for such progression over time, it is prudent to explore the new treatment strategies which may be implemented before such changes become established.

CONCLUSIONS

Outside of the dopaminergic model, the potential pathogenesis of schizophrenia has yet to be fully elucidated, but common themes include neuropil shrinkage, the development of abnormal neuronal circuitry, and a chronic inflammatory state which further disrupts neuronal function. Whilst some early non-dopaminergic treatments show promise, none have yet to be fully studied in appropriately structured randomized controlled trials and they remain little more than potential attractive targets.

Electrochemical studies of ascorbic acid, dopamine, and uric acid at a dl-norvaline-deposited glassy carbon electrode

The electrochemical behavior of ascorbic acid (AA), dopamine (DA), and uric acid (UA) on a dl-norvaline-modified glassy carbon electrode (GCE) was studied by cyclic voltammetry. The bare GCE failed to distinguish the oxidation peaks of AA, DA, and UA in phosphate-buffered solution (pH 5.0), while the dl-norvaline-modified GCE could separate them efficiently. In differential pulse voltammetric (DPV) measurements, the modified electrode resolved the overlapped voltammetric responses of AA, DA, and UA into three well-defined voltammetric peaks. Under optimum conditions, the anodic peak currents of DPV for AA, DA, and UA were proportional to the concentration in the range of 20–400, 1–40, and 15–180 μmol/L, respectively, with a correlation coefficient (r) of around 0.998. The detection limits were 5, 0.3, and 10 μmol/L (S/N = 3) for AA, DA, and UA, respectively. Satisfactory results were achieved for the determination of AA in vitamin C tablets, DA in a dopamine ampoule sample, and UA in human blood serum samples.

In situ detection of dopamine using nitrogen incorporated diamond nanowire electrode

Significant difference was observed for the simultaneous detection of dopamine (DA), ascorbic acid (AA), and uric acid (UA) mixture using nitrogen incorporated diamond nanowire (DNW) film electrodes grown by microwave plasma enhanced chemical vapor deposition. For the simultaneous sensing of ternary mixtures of DA, AA, and UA, well-separated voltammetric peaks are obtained using DNW film electrodes in differential pulse voltammetry (DPV) measurements. Remarkable signals in cyclic voltammetry responses to DA, AA and UA (three well defined voltammetric peaks at potentials around 235, 30, 367 mV for DA, AA and UA respectively) and prominent enhancement of the voltammetric sensitivity are observed at the DNW electrodes. In comparison to the DPV results of graphite, glassy carbon and boron doped diamond electrodes, the high electrochemical potential difference is achieved via the use of the DNW film electrodes which is essential for distinguishing the aforementioned analytes. The enhancement in EC properties is accounted for by increase in sp(2) content, new C-N bonds at the diamond grains, and increase in the electrical conductivity at the grain boundary, as revealed by X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure measurements. Consequently, the DNW film electrodes provide a clear and efficient way for the selective detection of DA in the presence of AA and UA.

Different serine and glycine metabolism in patients with schizophrenia receiving clozapine

Dysfunction of the N-methyl-d-aspartate receptor, which is modulated by excitatory amino acids (EAA), is involved in the pathophysiology of schizophrenia. The effects of antipsychotics on EAA metabolism are uncertain. Positive clinical effects of treatment with antipsychotics were not always associated with changes in EAA serum levels in patients with schizophrenia in clinical trials. To examine EAA serum levels in relation to the intensity of psychotic symptoms and the type of medication received we compared these variables among patients with schizophrenia (n = 49) treated with first (FGA) or second (SGA) generation antipsychotics or clozapine. Glutamate, aspartate, glycine, total serine and d-serine serum levels were measured by High Performance Liquid Chromatography. The Positive and Negative Syndrome Scale (PANSS) and the Scale for the Assessment of Negative Symptoms (SANS) were used to assess symptoms of schizophrenia. Lower average levels of glycine and total serine were found in the serum of patients receiving clozapine when compared to the groups of patients treated with FGA or SGA. There were no differences in serum glutamate, aspartate or d-serine levels or in the intensity of schizophrenic symptoms assessed by PANSS or SANS among the groups of patients treated with FGA or SGA or clozapine. Lower glycine and total serine serum levels could be caused by the particular characteristics of the population of patients receiving clozapine rather than as an effect of the clozapine. The results suggest selective deficiency of l-serine synthesis in the patients with resistance to non-clozapine treatment. It might be an unique biochemical and pathophysiological characteristic of the treatment-resistance in schizophrenia.

Clozapine functions through the prefrontal cortex serotonin 1A receptor to heighten neuronal activity via calmodulin kinase II-NMDA receptor interactions

Aberrant dopamine release in the prefrontal cortex (PFC) is believed to underlie schizophrenia, but the mechanistic pathway through which a widely used antipsychotic, clozapine (Clz), evokes neurotransmitter-releasing electrical stimulation is unclear. We analyzed Clz-evoked regulation of neuronal activity in the PFC by stimulating axons in layers IV and V and recording the electrical effect in the post-synaptic pyramidal cells of layers II and III. We observed a Clz-evoked increase in population spike (PS), which was mediated by serotonin 1A receptor (5-HT(1A)-R), phospholipase Cβ, and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Immunoblotting demonstrated that the Clz-activation of CaMKII was 5-HT(1A)-R-mediated. Intriguingly, the NMDA receptor (NMDA-R) antagonist (±)2-amino-5-phosphonovaleric acid (APV) eliminated the Clz-mediated increase in PS, suggesting that the 5-HT(1A)-R, NMDA-R and CaMKII form a synergistic triad, which boosts excitatory post-synaptic potential (EPSP), thereby enhancing PS. In corroboration, Clz as well as NMDA augmented field EPSP (fEPSP), and WAY100635 (a 5-HT(1A)-R antagonist), APV, and a CaMKII inhibitor eliminated this increase. As previously shown, CaMKII binds to the NMDA-R 2B (NR2B) subunit to become constitutively active, thereby inducing α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor recruitment to the post-synaptic membrane and an increase in fEPSP. Co-immunoprecipitation demonstrated that Clz potentiates interactions among CaMKII, NR2B, and 5-HT(1A)-R, possibly in the membrane rafts of the post-synaptic density (PSD), because pretreatment with methyl-β-cyclodextrin (MCD), an agent that disrupts rafts, inhibited both co-immunoprecipitation as well as fEPSP. In summary, Clz functions in the PFC by orchestrating a synergism among 5-HT(1A)-R, CaMKII, and NMDA-R, which augments excitability in the PFC neurons of layers II/III.

Clozapine and N-methyl-D-aspartate have positive modulatory actions on their respective discriminative stimulus properties in C57BL/6 mice

The impairment of N-Methyl-D-Aspartate receptors is thought to contribute to negative symptoms and cognitive deficits. In vitro studies suggest that atypical antipsychotic drugs like clozapine may help to alleviate these deficits by enhancing glutamatergic function. The present study examined the in vivo interaction of clozapine with N-Methyl D-aspartate by training one group of C57BL/6 mice to discrimination 2.5 mg/kg clozapine from vehicle and another group to discriminate 30 mg/kg N-Methyl D-aspartate from vehicle in a two-lever drug discrimination task. Cross-generalization testing revealed that N-Methyl D-aspartate (3-56 mg/kg) failed to substitute for clozapine in the clozapine-trained mice, while clozapine (0.625 mg/kg) produced partial substitution in the N-Methyl D-aspartate-trained mice. Interestingly, administration of a low, non-generalizing dose of each training drug in combination with the full range of doses of the alternate training drug produced full and dose-dependent substitution in both clozapine- and N-Methyl D-aspartate-trained mice. The α(1) antagonist prazosin fully and dose-dependently substituted for both clozapine and N-Methyl D-aspartate. These results suggest that the shared discriminative stimulus properties between clozapine and N-Methyl D-aspartate may be mediated through indirect mechanisms, possibly in part through α(1) adrenergic antagonism.

Pharmacological manipulation of kynurenic acid: potential in the treatment of psychiatric disorders

The kynurenine pathway constitutes the main route of tryptophan degradation and generates the production of several neuroactive compounds; quinolinic acid is an excitotoxic NMDA receptor agonist, 3-hydroxykynurenine is a free-radical generator and kynurenic acid (KYNA) is an antagonist at glutamate and nicotinic receptors. In low micromolar concentrations, KYNA blocks the glycine site of the NMDA receptor and the nicotinic alpha(7) acetylcholine receptor. Knowledge regarding kynurenine metabolites and their involvement in neurophysiological processes has increased dramatically in recent years. In particular, endogenous KYNA appears to tightly control firing of midbrain dopamine neurons and to be involved in cognitive functions. Thus, decreased endogenous levels of rat brain KYNA have been found to reduce firing of these neurons, and mice with a targeted deletion of kynurenine aminotransferase II display low endogenous brain KYNA levels concomitant with an increased performance in cognitive tests. It is also suggested that kynurenines participate in the pathophysiology of psychiatric disorders. Thus, elevated levels of KYNA have been found in the CSF as well as in the post-mortem brain of patients with schizophrenia. Advantages in understanding how kynurenines can be pharmacologically manipulated may provide new possibilities in the treatment of psychiatric disorders, such as schizophrenia.

The immunological basis of glutamatergic disturbance in schizophrenia: towards an integrated view

This overview presents a hypothesis to bridge the gap between psychoneuroimmunological findings and recent results from pharmacological, neurochemical and genetic studies in schizophrenia. In schizophrenia, a glutamatergic hypofunction is discussed to be crucially involved in dopaminergic dysfunction. This view is supported by findings of the neuregulin- and dysbindin genes, which have functional impact on the glutamatergic system. Glutamatergic hypofunction is mediated by NMDA (N-methyl-D-aspartate) receptor antagonism. The only endogenous NMDA receptor antagonist identified up to now is kynurenic acid (KYN-A). KYN-A also blocks the nicotinergic acetycholine receptor, i.e. increased KYN-A levels can explain psychotic symptoms and cognitive deterioration. KYN-A levels are described to be higher in the CSF and in critical CNS regions of schizophrenics. Another line of evidence suggests that of the immune system in schizophrenic patients is characterized by an imbalance between the type-1 and the type-2 immune responses with a partial inhibition of the type-1 response, while the type-2 response is relatively over-activated. This immune constellation is associated with the inhibition of the enzyme indoleamine 2,3-dioxygenase (IDO), because type-2 cytokines are potent inhibitors of IDO. Due to the inhibition of IDO, tryptophan is predominantly metabolized by tryptophan 2,3-dioxygenase (TDO), which is located in astrocytes, but not in microglia cells. As indicated by increased levels of S100B, astrocytes are activated in schizophrenia. On the other hand, the kynurenine metabolism in astrocytes is restricted to the dead-end arm of KYN-A production. Accordingly, an increased TDO activity and an accumulation of KYN-A in the CNS of schizophrenics have been described. Thus, the immune-mediated glutamatergic-dopaminergic dysregulation may lead to the clinical symptoms of schizophrenia. Therapeutic consequences, e.g. the use of antiinflammatory cyclooxygenase-2 inhibitors, which also are able to directly decrease KYN-A, are discussed.

Nicotinic interactions with antipsychotic drugs, models of schizophrenia and impacts on cognitive function

People with schizophrenia often have substantial cognitive impairments, which may be related to nicotinic receptor deficits, (alpha7 and alpha4beta2), documented in the brains of people with schizophrenia. The large majority of people with schizophrenia smoke cigarettes. Thus, nicotinic interactions with antipsychotic drugs are widespread. Complementary co-therapies of novel nicotinic ligands are being developed to add to antipsychotic therapy to treat the cognitive impairment of schizophrenia. Thus, it is critical to understand the interaction between nicotinic treatments and antipsychotic drugs. Nicotinic interactions with antipsychotic drugs, are complex since both nicotine and antipsychotics have complex actions. Nicotine stimulates and desensitizes nicotinic receptors of various subtypes and potentiates the release of different neurotransmitters. Antipsychotics also act on a verity of receptor systems. For example, clozapine acts as an antagonist at a variety of neurotransmitter receptors such as those for dopamine, serotonin, norepinepherine and histamine. In a series of studies, we have found that in normally functioning rats, moderate doses of clozapine impair working memory and that clozapine blocks nicotine-induced memory and attentional improvement. Clozapine and nicotine can attenuate each other's beneficial effects in reversing the memory impairment caused by the psychototmimetic drug dizocilpine. A key to the clozapine-induced attenuation of nicotine-induced cognitive improvement appears to be its 5HT(2) antagonist properties. The selective 5HT(2) antagonist ketanserin has a similar action of blocking nicotine-induced memory and attentional improvements. It is important to consider the interactions between nicotinic and antipsychotic drugs to develop the most efficacious treatment for cognitive improvement in people with schizophrenia.

Galantamine enhances dopaminergic neurotransmission in vivo via allosteric potentiation of nicotinic acetylcholine receptors

Clinical studies suggest that adjunct galantamine may improve negative and cognitive symptoms in schizophrenia. These symptoms may be related to impaired dopaminergic function in the prefrontal cortex. Indeed, galantamine has been shown to increase dopamine release in vitro. Galantamine is an allosteric modulator of nicotinic acetylcholine receptors (nAChRs) and, at higher doses, an acetylcholine esterase (AChE) inhibitor. We have previously shown that nicotine, through stimulation of nAChRs in the ventral tegmental area (VTA), activates midbrain dopamine neurons and, hence, potentiation of these receptors could be an additional mechanism by which galantamine can activate dopaminergic pathways. Therefore, the effects of galantamine (0.01-1.0 mg/kg s.c.) on dopamine cell firing were tested in anaesthetized rats. Already at a low dose, unlikely to result in significant AchE inhibition, galantamine increased firing activity of dopaminergic cells in the VTA. The effect of galantamine was prevented by the nAChR antagonist mecamylamine (1.0 mg/kg s.c.), but not the muscarinic receptor antagonist scopolamine (0.1 mg/kg s.c.), and it was not mimicked by the selective AChE inhibitor donepezil (1.0 mg/kg s.c.). Our data thus indicate that galantamine increases dopaminergic activity through allosteric potentiation of nAChRs. Galantamine's effect was also prevented by the alpha7 nAChR antagonist methyllycaconitine (6.0 mg/kg i.p.) as well as the N-methyl-D-aspartate antagonist CGP39551 (2.5 mg/kg s.c.), indicating a mechanism involving presynaptic facilitation of glutamate release. In parallel microdialysis experiments, galantamine was found to increase extracellular levels of dopamine in the medial prefrontal cortex. These results may have bearing on the enhancement of negative and cognitive symptoms in schizophrenia.

Nicotinic-antipsychotic drug interactions and cognitive function

In summary, neuronal nicotinic systems are important for a variety of aspects of cognitive function impacted by antipsychotic drugs. It has been demonstrated that antipsychotic drugs have memory and attentional impairing effects when given to unimpaired subjects. Nicotine can reduce some of these impairments, but antipsychotic drug administration can also attenuate nicotine effects. We have found that nicotinic agonists selective for alpha7 and alpha4beta2 receptor subtypes significantly improve learning and memory. Serotonergic actions of antipsychotic drugs may decrease efficacy of nicotinic co-treatments. When the antipsychotic drug clozapine and nicotine are administered to subjects with cognitive impairments caused by NMDA glutamate receptor blockade or hippocampal dysfunction they can significantly attenuate the attentional and memory impairments. Nicotine has been shown in our studies to reverse the memory impairment caused by acute clozapine-induced memory improvement. Acute risperidone and haloperidol has been shown to attenuate nicotine-induced memory improvement. We have determined the role of hippocampal alpha7 and alpha4beta2 nicotinic receptors in the neural basis of nicotinic antipsychotic interactions. Local acute and chronic hippocampal infusion of either nicotinic alpha7 or alpha4beta2 antagonists cause significant spatial working memory impairment. Chronic hippocampal nicotinic antagonist infusions have served as a model of persistent decreases in nicotinic receptor level seen in schizophrenia and Alzheimer's disease. Clozapine attenuated the memory deficit caused by chronic suppression of hippocampal alpha4beta2 receptors while the amnestic effects of clozapine were potentiated by chronic suppression of hippocampal alpha7 receptors. Nicotinic co-treatment may be a useful adjunct in the treatment of schizophrenia, to attenuate cognitive impairment of schizophrenia. Nicotine as well as selective nicotinic alpha7 and alpha4beta2 receptor agonists significantly improve working memory and attentional function. Nicotine treatment was found to be effective in attenuating the attentional and memory impairments caused by the psychototmimetic NMDA antagonist dizocilpine (MK-801), a model of the cognitive impairment of schizophrenia. Studies of the interactions of antipsychotic drugs with nicotinic agents provided quite useful information concerning possible co-treatment of people with schizophrenia with nicotinic therapy. Nicotine was found to significantly attenuate the memory impairments caused by the antipsychotic drugs clozapine and olanzapine. Interestingly, nicotine-induced cognitive improvement was significantly attenuated by the antipsychotic drug clozapine. One of the principal effects of clozapine is to block 5HT2 receptors. Ketanserin a 5HT2 antagonist significantly attenuated nicotine-induced improvements in attention and memory. Thus it appears that antipsychotic drugs with actions blocking 5HT2 receptors may limit the efficacy of nicotinic co-treatments for cognitive enhancement.

Glycine transporter I inhibitor, N-methylglycine (sarcosine), added to clozapine for the treatment of schizophrenia

BACKGROUND

Agonists at the N-methyl-D-aspartate (NMDA)-glycine site (D-serine, glycine, D-alanine and D-cycloserine) and glycine transporter-1 (GlyT-1) inhibitor (N-methylglycine, or called sarcosine) both improve the symptoms of stable chronic schizophrenia patients receiving concurrent antipsychotics. Previous studies, however, found no advantage of D-serine, glycine, or D-cycloserine added to clozapine. The present study aims to determine the effects of sarcosine adjuvant therapy for schizophrenic patients receiving clozapine treatment.

METHODS

Twenty schizophrenic inpatients enrolled in a 6-week double-blind, placebo-controlled trial of sarcosine (2 g/day) which was added to their stable doses of clozapine. Measures of clinical efficacy and side-effects were determined every other week.

RESULTS

Sarcosine produced no greater improvement when co-administered with clozapine than placebo plus clozapine at weeks 2, 4, and 6. Sarcosine was well tolerated and no significant side-effect was noted.

CONCLUSIONS

Unlike patients treated with other antipsychotics, patients who received clozapine treatment exhibit no improvement by adding sarcosine or agonists at the NMDA-glycine site. Clozapine possesses particular efficacy, possibly related to potentiation of NMDA-mediated neurotransmission. This may contribute to the clozapine's unique clinical efficacy and refractoriness to the addition of NMDA-enhancing agents.

Schizophrenia as an inflammation-mediated dysbalance of glutamatergic neurotransmission

This overview tries to bridge the gap between psychoneuroimmunological findings and recent results from pharmacological, neurochemical and genetic studies in schizophrenia. Schizophrenia is a disorder of dopaminergic neurotransmission, but modulation of the dopaminergic system by glutamatergic neurotransmission seems to play a key role. This view is supported by genetic findings of the neuregulin- and dysbindin genes, which have functional impact on the glutamatergic system. Glutamatergic hypofunction, however, is mediated by the N-methyl-D-aspartate (NMDA)-receptor antagonism. The only endogenous NMDA receptor antagonist identified up to now is kynurenic acid (KYNA). Despite the NMDA receptor antagonism, KYNA also blocks, in lower doses, the nicotinergic acetycholine receptor, i.e., increased KYNA levels can explain psychotic symptoms and cognitive deterioration. KYNA levels are described to be higher in the cerebrospinal fluid (CSF) and in critical central nervous system (CNS) regions of schizophrenics as compared to controls. Another line of evidence suggests that a (prenatal) infection is involved in the pathogenesis of schizophrenia. Due to an early sensitization process of the immune system or to a (chronic) infection, which is not cleared through the immune response, an immune imbalance between the type-1 and the type-2 immune responses takes place in schizophrenia. The type-1 response is partially inhibited, while the type-2 response is over-activated. This immune constellation is associated with inhibition of the enzyme indoleamine dioxygenase (IDO), because IDO - located in astrocytes and microglial cells - is inhibited by type-2 cytokines. IDO catalyzes the first step in tryptophan metabolism, the degradation from tryptophan to kynurenine, as does tryptophan 2,3-dioxygenase (TDO). Due to the inhibition of IDO, tryptophan-kynurenine is predominantly metabolized by TDO, which is located in astrocytes, not in microglial or other CNS cells. In schizophrenia, astrocytes in particular are activated, as increased levels of S100B appear. Additionally, they do not have the enzymatic equipment for the normal metabolism-route of tryptophan. Due to the lack of kynurenine hydroxylase (KYN-OHase) in astrocytes, KYNA accumulates in the CNS, while the metabolic pathway in microglial cells is blocked. Accordingly, an increase of TDO activity has been observed in critical CNS regions of schizophrenics. These mechanisms result in an accumulation of KYNA in critical CNS regions. Thus, the immune-mediated glutamatergic-dopaminergic dysregulation may lead to the clinical symptoms of schizophrenia. Therapeutic consequences, e.g., the use of anti-inflammatory cyclo-oxygenase-2 inhibitors, which can also decrease KYNA directly, are discussed.

Effects of COX-1 and COX-2 inhibitors on the firing of rat midbrain dopaminergic neurons--possible involvement of endogenous kynurenic acid

Kynurenic acid (KYNA) is an endogenous glutamate-receptor antagonist with a preferential action at the glycine-site of the NMDA-receptor. In the present in vivo study, the importance of brain KYNA to modulate the activity of dopamine (DA) neurons in the ventral tegmental area (VTA) was analyzed by utilizing the decrease in brain KYNA formation induced by the cyclooxygenase (COX)-2 inhibitor parecoxib. A reduction in brain KYNA concentration (39-44%) by parecoxib (25 mg/kg, i.v., 1 h or, i.p., 3.5 h) was associated with a decreased firing rate and burst firing activity. In concordance, an increase in brain KYNA concentration (150-300%), induced by the COX-1 inhibitor indomethacin (50 mg/kg, i.v., 1 h or, i.p., 3.5 h), produced opposite effects, that is, increased firing rate and burst firing activity. The decrease and increase in neuronal firing of VTA DA neurons by the COX-inhibitors was reversed by L-701,324 (antagonist at the NMDA-glycine site; 0.06-2 mg/kg, i.v.) and by D-cycloserine (partial agonist at the NMDA-glycine site; 2-32 mg/kg, i.v.), respectively. In addition, the parecoxib-induced decrease in firing rate and burst firing activity was effectively blocked by pretreatment with kynurenine (5 mg/kg, i.p., 30 min), the immediate precursor of KYNA. Present results suggest that the action of COX-inhibitors on the firing of VTA DA neurons are linked to their effects on KYNA formation and that endogenous KYNA is tonically modulating the neuronal activity of VTA DA neurons. Such a modulatory action of KYNA should be of importance for the functioning of mesocorticolimbic DA pathway.

Somatic augmentation strategies in clozapine resistance--what facts?

BACKGROUND

Polypharmacy without evidence-based support is sometimes needed for patients treated with 40% to 70% clozapine who are clozapine nonresponders. Several somatic augmentation strategies are proposed in the scientific literature, with different levels of evidence for safety and efficacy.

OBJECTIVES

The purpose of the present study is to review the available literature on the efficacy and safety of clozapine augmentation with somatic agents other than antipsychotics. The following classes of agents are considered: (1) mood stabilizers, (2) antidepressants, (3) electroconvulsive therapy and repetitive transcranial magnetic stimulation, (4) glutamatergic agents, (5)fatty acids supplements, and (6) benzodiazepines.

RESULTS

Case controls and small-size clinical trials largely dominate the literature, limiting the power to draw conclusions concerning safety issues and the meaning of negative studies. Moreover, variable definitions of clozapine resistance, heterogeneous outcome measures, and short duration of treatment trials are additional limitations.

CONCLUSION

Generally, adjunctive strategies for clozapine-resistant patients remain based on scarce evidence of efficacy and significant safety concerns. Low-frequency repetitive transcranial magnetic stimulation, fatty acids supplements, and mirtazapine showed good tolerability and some efficacy, but the results need replication.

Inhibition of the glycine transporter GlyT-1 potentiates the effect of risperidone, but not clozapine, on glutamatergic transmission in the rat medial prefrontal cortex

Clinical studies suggest that the efficacy of the atypical antipsychotic drug (APD) risperidone (but not clozapine) can be augmented by adjunctive treatment with agonists at the glycine site of the N-methyl-D-aspartate (NMDA) receptor. By using intracellular recording, we have investigated the effect of the glycine transporter-1 (GlyT-1) inhibitor N [3-(4'-fluorophenyl)-3-(4'phenylphenylphenoxy) propyl] sarcosine (NFPS) on NMDA-induced currents in pyramidal cells of the medial prefrontal cortex (mPFC), both when given alone and in combination with either risperidone or clozapine. Both risperidone and clozapine enhanced the NMDA-induced currents. The concentration-response curves were biphasic, and the maximal effect of clozapine on the NMDA-induced currents was significantly larger than the maximal effect of risperidone. NFPS also significantly potentiated the NMDA-induced currents, when given alone. Moreover, NFPS (1 microM) augmented the effect of both the maximal (20 nM), and a submaximal (10 nM), concentration of risperidone. In contrast, NFPS did not potentiate either the effect of the maximal (100 nM) or a submaximal (80 nM) concentration of clozapine on the NMDA-induced currents. These data may explain the beneficial clinical results of using glycine reuptake antagonists as adjuvant treatment to risperidone. Our findings also suggest that risperidone and clozapine may affect NMDA receptor-mediated neurotransmission differently in the mPFC.

Subchronic treatment with kynurenine and probenecid: effects on prepulse inhibition and firing of midbrain dopamine neurons

Acute elevation of the endogenous NMDA-receptor antagonist kynurenic acid (KYNA) is associated with an increased neuronal activity of rat ventral tegmental area (VTA) dopamine (DA) neurons and disruption in prepulse inhibition (PPI). In the present study, the effects of subchronic exposure to kynurenine and probenecid (20 mg/kg/day and 10 mg/kg/day, respectively for 14 days), aiming at increasing brain KYNA turnover, on rat VTA dopaminergic firing and on PPI were investigated. This treatment increased neuronal firing of VTA DA neurons, changed the response of these neurons to systemically administered nicotine (3-400 microg/kg, i.v.) and tended to disrupt PPI. Present results show that the effect on firing of VTA DA neurons by acutely elevated levels of brain KYNA also persists following subchronic exposure. In addition, no adaptive changes seem to occur with regard to the electrophysiological effects of KYNA on VTA DA neurons following subchronic treatment with kynurenine and probenecid.

Modulators of the glycine site on NMDA receptors, D-serine and ALX 5407, display similar beneficial effects to clozapine in mouse models of schizophrenia

RATIONALE

Schizophrenia is characterized by disturbances in sensorimotor gating and attentional processes, which can be measured by prepulse inhibition (PPI) and latent inhibition (LI), respectively. Research has implicated dysfunction of neurotransmission at the NMDA-type glutamate receptor in this disorder.

OBJECTIVES

This study was conducted to examine whether compounds that enhance NMDA receptor (NMDAR) activity via glycine B site, D-serine and ALX 5407 (glycine transporter type 1 inhibitor), alter PPI and LI in the presence or absence of an NMDAR antagonist, MK-801.

METHODS

C57BL/6J mice were tested in a standard PPI paradigm with three prepulse intensities. LI was measured in a conditioned emotional response procedure by comparing suppression of drinking in response to a noise in mice that previously received 0 (non-preexposed) or 40 noise exposures (preexposed) followed by two or four noise-foot shock pairings.

RESULTS

Clozapine (3 mg/kg) and D-serine (600 mg/kg), but not ALX 5407, facilitated PPI. MK-801 dose dependently reduced PPI. The PPI disruptive effect of MK-801 (1 mg/kg) could be reversed by clozapine and ALX 5407, but not by D-serine. All the compounds were able to potentiate LI under conditions that disrupted LI in controls. MK-801 induced abnormal persistence of LI at a dose of 0.15 mg/kg. Clozapine, D-serine, and ALX 5407 were equally able to reverse persistent LI induced by MK-801.

CONCLUSIONS

D-Serine and ALX 5407 display similar effects to clozapine in PPI and LI mouse models, suggesting potential neuroleptic action. Moreover, the finding that agonists of NMDARs and clozapine can restore disrupted LI and disrupt persistent LI may point to a unique ability of the NMDA system to regulate negative and positive symptoms of schizophrenia.

N-Methyl-D-aspartate receptors as a target for improved antipsychotic agents: novel insights and clinical perspectives

RATIONALE

Activation of "co-agonist" N-methyl-D-aspartate (NMDA) and Glycine(B) sites is mandatory for the operation of NMDA receptors, which play an important role in the control of mood, cognition and motor function.

OBJECTIVES

This article outlines the complex regulation of activity at Glycine(B)/NMDA receptors by multiple classes of endogenous ligand. It also summarizes the evidence that a hypoactivity of Glycine(B)/NMDA receptors contributes to the pathogenesis of psychotic states, and that drugs which enhance activity at these sites may possess antipsychotic properties.

RESULTS

Polymorphisms in several genes known to interact with NMDA receptors are related to an altered risk for schizophrenia, and psychotic patients display changes in levels of mRNA encoding NMDA receptors, including the NR1 subunit on which Glycine(B) sites are located. Schizophrenia is also associated with an overall decrease in activity of endogenous agonists at Glycine(B)/NMDA sites, whereas levels of endogenous antagonists are elevated. NMDA receptor "open channel blockers," such as phencyclidine, are psychotomimetic in man and in rodents, and antipsychotic agents attenuate certain of their effects. Moreover, mice with genetically invalidated Glycine(B)/NMDA receptors reveal similar changes in behaviour. Finally, in initial clinical studies, Glycine(B) agonists and inhibitors of glycine reuptake have been found to potentiate the ability of "conventional" antipsychotics to improve negative and, albeit modestly, cognitive and positive symptoms. In contrast, therapeutic effects of clozapine are not reinforced, likely since clozapine itself enhances activity at NMDA receptors.

CONCLUSIONS

Reduced activity at NMDA receptors is implicated in the aetiology of schizophrenia. Correspondingly, drugs that (directly or indirectly) increase activity at Glycine(B) sites may be of use as adjuncts to other classes of antipsychotic agent. However, there is an urgent need for broader clinical evaluation of this possibility, and, to date, there is no evidence that stimulation of Glycine(B) sites alone improves psychotic states.