N-desmethylclozapine, a major metabolite of clozapine, increases cortical acetylcholine and dopamine release in vivo via stimulation of M1 muscarinic receptors

Involvement of decreased muscarinic receptor function in prepulse inhibition deficits in mice reared in social isolation

BACKGROUND AND PURPOSE

We have previously reported that galantamine, a weak acetylcholinesterase inhibitor, improves prepulse inhibition (PPI) deficits in mice reared in social isolation. ACh receptors are involved in the underlying mechanism of PPI, but whether rearing in social isolation causes dysfunction of the cholinergic system is unknown. In this study, we examined the involvement of muscarinic receptors in the improvement of PPI deficits induced by galantamine, and whether the cholinergic system is altered in mice reared in isolation.

EXPERIMENTAL APPROACH

Three-week-old male ddY mice were housed in isolated cages for 6 weeks before the initiation of experiments to create PPI deficits. Cholinergic functions were determined by measuring the behavioural and neurochemical responses to nicotinic and muscarinic receptor agonists.

KEY RESULTS

The improvement by galantamine of social isolation-induced PPI deficits was blocked by scopolamine, a non-selective muscarinic antagonist, and telenzepine, a preferential M₁ receptor antagonist. Activation of M₁ receptors improved social isolation-induced PPI deficits. Social isolation did not affect choline acetyltransferase and acetylcholinesterase activities in the prefrontal cortex and hippocampus, but it reduced the locomotor-suppressive response to muscarinic agonist oxotremorine, but not to nicotine. The isolation also attenuated the M₁ receptor agonist N-desmethylclozapine-induced increase in prefrontal dopamine release.

CONCLUSIONS AND IMPLICATIONS

Galantamine improves PPI deficits of mice reared in social isolation via activation of M₁ receptors. Social isolation reduces the muscarinic, especially M₁, receptor function and this is involved in PPI deficits.

Factors affecting interindividual differences in clozapine response: a review and case report

OBJECTIVE

Clozapine is the most powerful new‐generation antipsychotic. Although this drug leads to great therapeutic benefits, two types of undesirable conditions frequently occur with its use: side effects and resistance to treatment. Therapeutic drug monitoring of clozapine would be very useful to avoid both these situations. The necessity of monitoring the therapy is the result of a wide interindividual variability in the metabolism of clozapine. In this review, we highlight all the conditions underlying this variability, analyzing them one by one.

METHODS

Relevant literature was identified through a search of MEDLINE and PubMed. In addition, the case of a treatment‐resistant patient with accelerated metabolism of clozapine is reported as representative of utility of therapeutic drug monitoring in terms of clozapine dose adjustment.

RESULTS

Genetic polymorphisms of cytochrome P450 enzymes and of neurotransmitter receptors; drug interactions; interactions of clozapine with other substances such as food and drink; smoking; and nonmodifiable variables such as age, ethnicity, and gender have been examined in relation to the existing scientific literature. The laboratory techniques that clinicians could use to identify these variables and adequate therapies are also reviewed.

Clozapine and global cognition in schizophrenia

OBJECTIVE

Clozapine (CLZ) has been shown to have a beneficial effect on cognition in schizophrenia in some studies and a detrimental effect in others. The relative effect and exposure to CLZ and its major metabolite-N-desmethylclozapine (NDMC)-could explain these discrepancies.

METHODS

Using a validated measure of global cognition, we performed 2 binary logistic regression models to assess the relationship among cognition, age, sex, CLZ dose, CLZ and NDMC plasma levels, and their ratio (CLZ/NDMC) in individuals with schizophrenia spectrum disorders. Model 1 included age, sex, CLZ dose, and CLZ and NDMC levels. Model 2 included age, sex, CLZ dose, and CLZ/NDMC.

RESULTS

Among 73 subjects (mean [SD] age, 41.6 [12.0] years), 16 (21.9%) had high cognitive impairment, whereas the rest had low cognitive. In model 1, age and CLZ level were associated with high cognitive impairment (odds ratio [95% confidence interval] for age, 1.079 [1.011-1.152]; CLZ level, 1.010 [1.003-1.017]), whereas NDMC level was associated with its absence (NDMC level, 0.987 [0.977-0.997]). In model 2, age, male sex, and CLZ/NDMC were associated with cognitive impairment (age, 1.083 [1.015-1.154]; sex, 0.178 [0.032-0.994]; CLZ/NDMC, 7.302 [1.823-29.253]). Clozapine dose was not associated with cognition in either model.

CONCLUSIONS

After controlling for age, sex, and dose, CLZ/NDMC was more strongly associated with cognition than CLZ or NDMC levels. N-desmethylclozapine agonist activity versus CLZ antagonist activity at the muscarinic receptors could explain the strength of the association of CLZ/NDMC with cognition.

The antipsychotic potential of muscarinic allosteric modulation

The cholinergic hypothesis of schizophrenia emerged over 50 years ago based on clinical observations with both anticholinergics and pan-muscarinic agonists. Not until the 1990s did the cholinergic hypothesis of schizophrenia receive renewed enthusiasm based on clinical data with xanomeline, a muscarinic acetylcholine receptor M(1)/M(4)-preferring orthosteric agonist. In a clinical trial with Alzheimer's patients, xanomeline not only improved cognitive performance, but also reduced psychotic behaviors. This encouraging data spurred a second clinical trial in schizophrenic patients, wherein xanomeline significantly improved the positive, negative and cognitive symptom clusters. However, the question remained: Was the antipsychotic efficacy due to activation of M(1), M(4) or both M(1)/M(4)? Classical orthosteric ligands lacked the muscarinic receptor subtype selectivity required to address this key question. More recently, functional assays have allowed for the discovery of ligands that bind at allosteric sites, binding sites distinct from the orthosteric (acetylcholine) site, which are structurally less conserved and thereby afford high levels of receptor subtype selectivity. Recently, allosteric ligands, with unprecedented selectivity for either M(1) or M(4), have been discovered and have demonstrated comparable efficacy to xanomeline in preclinical antipsychotic and cognition models. These data suggest that selective allosteric activation of either M(1) or M(4) has antipsychotic potential through distinct, yet complimentary mechanisms.

Regulation of cortical acetylcholine release: insights from in vivo microdialysis studies

Acetylcholine release links the activity of presynaptic neurons with their postsynaptic targets and thus represents the intercellular correlate of cholinergic neurotransmission. Here, we review the regulation and functional significance of acetylcholine release in the mammalian cerebral cortex, with a particular emphasis on information derived from in vivo microdialysis studies over the past three decades. This information is integrated with anatomical and behavioral data to derive conclusions regarding the role of cortical cholinergic transmission in normal behavioral and how its dysregulation may contribute to cognitive correlates of several neuropsychiatric conditions. Some unresolved issues regarding the regulation and significance of cortical acetylcholine release and the promise of new methodology for advancing our knowledge in this area are also briefly discussed.

Prefrontal cortex and reversion of atropine-induced disruption of the degraded contingency effect by antipsychotic agents and N-desmethylclozapine in rats

Interactive context processing is a cognitive ability that is altered in psychotic states, including schizophrenia. This deficit has been linked to prefrontal cortical dysfunction in humans. The degraded contingency effect (DCE) is a simple form of interactive context processing by which contextual information interferes with a target conditioned stimulus for control over conditioned responding. We have previously shown that the DCE was disrupted by the muscarinic receptor antagonist atropine and that this disruption was specifically restored by cholinergic drugs displaying an antipsychotic-like profile, such as physostigmine or xanomeline. The DCE was selectively associated with an increase in Fos immunoreactivity in the medial prefrontal cortex (mPFC), an increase that was not observed in the presence of atropine. Here, we set out to test the actions of typical, atypical and potential antipsychotics on atropine-induced disruption of the DCE and the related mPFC Fos-immunoreactivity profile. Low doses of haloperidol, olanzapine, clozapine and N-desmethylclozapine reversed atropine-induced disruption of the DCE, but with different dose-dependent curves (linear shapes for haloperidol and N-desmethylclozapine, inverted U shapes for olanzapine and clozapine). The level of Fos within the mPFC paralleled the pharmacological profile of the different drugs. Compared to contingent control groups, an increased level of Fos immunoreactivity within the mPFC was observed only with doses that reversed atropine-induced disruption of the DCE. These results suggest that the deficit of interactive context processing, which is a hallmark of psychotic states, might originate from a mere deficit of fundamental associative processes. This deficit might result from a cholinergic blockade of the PFC.

Disruption of mesolimbic regulation of prefrontal cholinergic transmission in an animal model of schizophrenia and normalization by chronic clozapine treatment

Abnormal mesolimbic control of cortical cholinergic activity has been hypothesized to contribute to the cognitive symptoms of schizophrenia. Stimulation of NMDA receptors in nucleus accumbens (NAC) increases acetylcholine (ACh) release in prefrontal cortex (PFC), an activation thought to contribute to attentional processing. Thus, the effects of intra-NAC perfusion of NMDA (250-400 microM) on ACh release in PFC were determined in rats receiving lesions of the ventral hippocampus (VH) as neonates (nVHLX), a neurodevelopmental model of schizophrenia, or as adults (aVHLX). NMDA elevated ACh release (100-150% above baseline) in adults sham-lesioned as neonates or in aVHLX rats. Adult nVHLX were unresponsive to NAC NMDA receptor stimulation. The inability of nVHLX to respond to NMDA emerged over development as a separate experiment demonstrated that evoked ACh release was normal before puberty (100-150% increase) yet, in these same nVHLX animals, absent after puberty. Amphetamine-evoked ACh release was assessed in nVHLX animals to exclude potential limitations in release capacity. Amphetamine produced greater increases in ACh release than in shams, indicating that nVHLX does not impair the capacity of cholinergic neurons to release ACh. Finally, the ability of 13 days of pretreatment with clozapine (1.25 mg/kg/day) to reinstate NMDA-evoked cortical ACh efflux was determined. Clozapine treatment normalized NMDA-evoked ACh release in nVHLX animals. These experiments show that mesolimbic regulation of cortical ACh release is disrupted in postpubertal nVHLX rats and normalized by low-dose treatment of clozapine; supporting the usefulness of nVHLX animals for research on the neuronal mechanisms underlying the cognitive symptoms of schizophrenia.

The effect of clozapine on regional cerebral blood flow and brain metabolite ratios in schizophrenia: relationship with treatment response

The purpose of this study was to investigate the effect of clozapine on regional cerebral blood flow (rCBF) and its relationship with response to treatment. In addition, we aimed to study the influence of clozapine on proton magnetic resonance spectroscopy ((1)H-MRS) findings in the dorsolateral prefrontal cortex (DLPFC) in a subgroup of patients. Psychopathology, neurocognitive functioning, and SPECT imaging of 22 patients were assessed at the baseline and 8 weeks after the initiation of clozapine treatment. In 10 of these patients intermediate-echo (TE: 135 ms) single-voxel (1)H-MRS was also performed at the baseline and after 8 weeks. Clozapine treatment increased the right frontal (superior and medial)/caudate perfusion ratio in the whole group, while it increased bilateral frontal (superior and medial)/caudate perfusion ratios in treatment responders. In addition, percentage changes in left and right frontal (superior and medial)/caudate perfusion ratios compared to the baseline were higher in treatment responders than in non-responders. The improvement in attention was related to the increase in percentage change in the right frontal (superior and medial)/caudate perfusion ratio, while the improvement in verbal fluency was related to the increase in percentage changes in both right and left frontal (superior and medial)/caudate perfusion ratios and to right frontal (superior and medial)/thalamus perfusion. Baseline frontal (superior and medial)/thalamus perfusion could explain 32% of the variability of percentage improvements in psychopathology. (1)H-MRS showed that the baseline PANSS general psychopathology score was inversely correlated with the baseline NAA/Cre ratio. An increased NAA/Cre ratio in DLPFC after 8 weeks of clozapine treatment was also revealed by (1)H-MRS. Our SPECT imaging results suggest the presence of an imbalance in fronto-striato-thalamic circuitry that changes with clozapine, especially in the responders, while (1)H-MRS results indicate a supportive effect of clozapine on neuronal integrity.

Differential effects of M1 and 5-hydroxytryptamine1A receptors on atypical antipsychotic drug-induced dopamine efflux in the medial prefrontal cortex

Systemic administration of the M(1) receptor agonists N-desmethylclozapine (NDMC) and 4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one (AC260584) increase dopamine (DA) efflux in rat medial prefrontal cortex (mPFC). This increase is blocked by systemic administration of both telenzepine, a preferential M(1) receptor antagonist, and N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide (WAY-100635), a 5-hydroxytryptamine(1A) receptor antagonist. The present study sought to determine whether DA efflux in the mPFC induced by the atypical antipsychotic drugs clozapine, risperidone, and olanzapine is also mediated by M(1) receptor stimulation and, specifically, to determine whether these effects are mediated M(1) receptors in the mPFC through use of in vivo microdialysis in awake, freely moving Sprague-Dawley rats. Telenzepine (3 mg/kg) significantly attenuated clozapine- (20 mg/kg), olanzapine- (10 mg/kg), and risperidone- (1.0 mg/kg) induced increases in mPFC DA efflux. Local mPFC perfusion of NDMC, AC260584, clozapine, risperidone, or olanzapine (10-500 microM), significantly increased DA efflux in the mPFC. Local mPFC perfusion of telenzepine (0.1 microM) prevented increases in mPFC DA efflux induced by systemic administration of AC260584 (10 mg/kg), NDMC (20 mg/kg), and clozapine (10 mg/kg), but not by risperidone (1.0 mg/kg) or olanzapine (10 mg/kg). However, local mPFC perfusion of WAY-100635 (0.1 microM) prevented mPFC DA efflux induced by clozapine, risperidone, and olanzapine, but not by AC260584 or NDMC. These results suggest that the AC260584-, NDMC-, and clozapine-induced DA efflux in the mPFC is mediated directly by mPFC M(1) receptors.

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

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

Determination of pharmacokinetic properties of clozapine and norclozapine in Korean schizophrenia patients

There is a wide interethnic variance in the pharmacokinetic profile of clozapine (CLZ), but the accumulated data are limited to some regional populations. In this study, we investigated the pharmacokinetic profile of CLZ in Korean patients and examined the association between serum CLZ parameters and clinical outcome. We assessed 78 Korean patients with schizophrenia who had been taking CLZ medication for more than 6 months. The patients were classified into three groups (good, moderate, and poor responders) according to their Clinical Global Impressions-Improvement scores. The serum concentrations of CLZ and norclozapine were 610.7+/-368.4 and 314.5+/-163.0 ng/ml (mean+/-SD), respectively, showing a large interindividual variation that was affected by dose, age, smoking habits, and sex by variable degrees. The pharmacokinetic profiles of Koreans were similar to those observed in Asians but quite different from those in Caucasians. Investigation on clinical responses revealed that the good or moderate responders clinically improved at a relatively low serum CLZ levels, whereas the poor responders showed less improvement despite the higher doses and serum levels. The metabolic ratio of the good responders was 0.65+/-0.20, higher than the poor responders (P=0.033). In this study, we identified a pharmacokinetic profile of CLZ in Korean schizophrenia patients and found a wide interindividual difference affected by various factors.

Further characterization of the discriminative stimulus properties of the atypical antipsychotic drug clozapine in C57BL/6 mice: role of 5-HT(2A) serotonergic and alpha (1) adrenergic antagonism

RATIONALE

The discriminative stimulus properties of the atypical antipsychotic drug (APD) clozapine (CLZ) have recently been studied in C57BL/6 mice, a common background strain for genetic alterations. However, further evaluation is needed to fully characterize CLZ's discriminative cue in this strain of mice.

OBJECTIVES

The objectives of the study were to confirm the previous findings using a shorter pretreatment time and to further characterize the receptor mechanisms mediating the discriminative stimulus properties of CLZ by testing APDs, selective ligands, and N-desmethylclozapine (CLZ's major metabolite) in C57BL/6 mice.

MATERIALS AND METHODS

C57BL/6 male mice were trained to discriminate 2.5 mg/kg CLZ (s.c.) from vehicle in a two-lever drug discrimination task.

RESULTS

Generalization testing with CLZ yielded an ED(50) = 1.19 mg/kg. Substitution testing with APDs showed that the atypical APDs quetiapine, sertindole, zotepine, iloperidone, and melperone fully substituted for CLZ (> or =80% CLZ-appropriate responding), but aripiprazole did not. The typical APDs chlorpromazine and thioridazine substituted for CLZ (fluphenazine and perphenazine did not). The serotonin (5-HT) (2A) antagonist M100907 and the alpha(1)-adrenoceptor antagonist prazosin fully substituted for CLZ. The H(1) histaminergic antagonist pyrilamine, dopamine agonist amphetamine, and the selective serotonin reuptake inhibitor fluoxetine did not substitute for CLZ. While N-desmethylclozapine did not substitute for CLZ when tested alone, N-desmethylclozapine plus a low dose of CLZ combined in an additive manner produced full substitution.

CONCLUSIONS

CLZ's discriminative cue in C57BL/6 mice is a "compound" cue mediated in part by antagonism of 5-HT(2A) and alpha(1) receptors.

Discriminative stimulus properties of atypical and typical antipsychotic drugs: a review of preclinical studies

BACKGROUND

Drug discrimination is an increasingly valuable behavioral assay for the preclinical development of antipsychotic drugs. The majority of studies have used the atypical antipsychotic clozapine because it displays robust discriminative stimulus properties and is the "prototypical" or "gold standard" atypical antipsychotic against which other antipsychotics will undoubtedly be compared for many years.

OBJECTIVES

Pharmacological mechanisms mediating the discriminative stimulus properties of antipsychotics used as training drugs and the usefulness of drug discrimination for distinguishing typical and atypical antipsychotics were reviewed.

RESULTS

Clozapine appears to have a compound cue involving antagonism of two or more receptors. While muscarinic receptor antagonism is a prominent factor for mediation of clozapine's cue in rats with a 5.0-mg/kg training dose, there are differences in clozapine's cue with a low training dose and in pigeons and mice. With a low training dose, clozapine has consistently produced full or partial generalization to atypical but not to typical antipsychotics. Although not evaluated as extensively, the atypical antipsychotics quetiapine and ziprasidone also appear to generalize to atypical but not typical antipsychotics. This has not been the case for other antipsychotic drugs (olanzapine, chlorpromazine, haloperidol) used as training drugs.

CONCLUSIONS

There are important differences in discriminative stimulus properties both between and within atypical and typical antipsychotics and across species. While low-dose clozapine discrimination in rats appears to provide a more sensitive behavioral assay for distinguishing atypical from typical antipsychotics, the extent to which clozapine's discriminative stimulus properties are predictive of its antipsychotic effects remains to be determined.

The role of M1 muscarinic cholinergic receptors in the discriminative stimulus properties of N-desmethylclozapine and the atypical antipsychotic drug clozapine in rats

RATIONALE

The discriminative stimulus properties of clozapine (CLZ) have been studied for decades because it remains the prototype for atypical antipsychotic drug effects and yet is unique in many ways, including increased efficacy in treatment-resistant schizophrenia and in reducing suicidality. Recent studies have indicated that the active CLZ metabolite N-desmethylclozapine (NDMC) may play a role in mediating the cognitive efficacy of CLZ and may also have atypical antipsychotic properties.

OBJECTIVES

The present study sought to determine if NDMC has discriminative stimulus properties similar to that of its parent drug CLZ.

MATERIALS AND METHODS

Rats were trained to discriminate 1.25 mg/kg CLZ from vehicle in a two-choice drug discrimination task.

RESULTS

Although NDMC (2.5-20.0 mg/kg) failed to substitute for CLZ, the combination of NDMC (5.0 and 10.0 mg/kg) with a low dose (0.3125 mg/kg) of CLZ produced full substitution (>80% CLZ-appropriate responding) for the 1.25 mg/kg CLZ training dose. Co-administration of the M1-preferring receptor antagonist trihexyphenidyl (6.0 mg/kg) with a 5.0 mg/kg dose of NDMC produced partial substitution (>60% to <80% CLZ-appropriate responding) for CLZ, while administration of trihexyphenidyl alone (0.3-12.0 mg/kg) failed to substitute for CLZ.

CONCLUSIONS

These findings suggest that NDMC produces discriminative stimulus effects that are different from those elicited by its parent drug CLZ. This difference may be due to the agonist properties of NDMC at M(1) muscarinic cholinergic receptors.

Towards an animal model of an antipsychotic drug-resistant cognitive impairment in schizophrenia: scopolamine induces abnormally persistent latent inhibition, which can be reversed by cognitive enhancers but not by antipsychotic drugs

Schizophrenia symptoms segregate into positive, negative and cognitive, which exhibit differential sensitivity to drugs. Recent efforts to identify treatments targeting cognitive impairments in schizophrenia have directed attention to the cholinergic system for its well documented role in cognition. Relatedly, muscarinic antagonists (e.g. scopolamine) produce an 'antimuscarinic syndrome', characterized by psychosis and cognitive impairments. Latent inhibition (LI) is the poorer conditioning to a stimulus resulting from its non-reinforced pre-exposure. LI indexes the ability to ignore irrelevant stimuli and aberrations of this capacity produced by pro-psychotic agents (e.g. amphetamine, MK-801) are used extensively to model attentional impairments in schizophrenia. We recently showed that LI was disrupted by scopolamine at low doses, and this was reversed by typical and atypical antipsychotic drugs (APDs) and the acetylcholinesterase inhibitor physostigmine. Here, at a higher dose (1.5 mg/kg), scopolamine produced an opposite pole of attentional impairment, namely, attentional perseveration, whereby scopolamine-treated rats persisted in expressing LI under strong conditioning that prevented LI expression in controls. Scopolamine-induced persistent LI was reversed by cholinergic and glycinergic cognitive enhancers (physostigmine and glycine) but was resistant to both typical and atypical APDs (haloperidol and clozapine). The latter sets scopolamine-induced persistent LI apart from scopolamine- and amphetamine-induced disrupted LI, which are reversed by both typical and atypical APDs, as well as from other cases of abnormally persistent LI including MK-801-induced persistent LI, which is reversed by atypical APDs. Thus, scopolamine-induced persistent LI may provide a pharmacological LI model for screening cognitive enhancers that are efficient for the treatment of APD-resistant cognitive impairments in schizophrenia.

Agonist activity of N-desmethylclozapine at delta-opioid receptors of human frontal cortex

The clozapine metabolite N-desmethylclozapine (NDMC) has been recently shown to act at different neurotransmitter receptors and to display both antagonist and agonist activities. We have previously reported that in cells over-expressing the recombinant delta-opioid receptor NDMC behaved as partial agonist with high intrinsic activity, but its action at the receptors naturally expressed in human brain remained to be investigated. In the present study, we examined whether NDMC was able to bind to and activate delta-opioid receptors in membranes of post-mortem human frontal cortex. In radioligand binding assays, NDMC competition curves displayed high- (K(i)=26 nM) and low-affinity (K(i)=3 microM) components, whose proportion was regulated by guanine nucleotides in an agonist-like fashion. In functional assays, NDMC stimulated [(35)S]GTPgammaS binding (EC(50)=905 nM) and inhibited cyclic AMP formation (EC(50)=590 nM) as effectively as delta-opioid agonists, whereas clozapine was much less potent and efficacious and clozapine N-oxide was completely inactive. The NDMC agonist activity was potently antagonized by the delta-opioid antagonist naltrindole, but not by the micro-opioid receptor antagonist CTAP (D-phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) or the kappa-opioid antagonist nor-binaltorphimine. Moreover, blockade of either acetylcholine muscarinic, dopamine D(2) or serotonin 5HT(1A) receptors failed to affect NDMC agonist activity. These data demonstrate that at clinically relevant concentrations NDMC behaves as an efficacious agonist at delta-opioid receptors of human frontal cortex.

Variability of the in vivo metabolism of clozapine

OBJECTIVES

Clozapine, the gold standard of antipsychotic treatment in treatment-refractory patients with schizophrenia, is metabolized in vivo to clozapine-N-oxide and N-desmethylclozapine (NDMC = norclozapine). N-desmethylclozapine is an active metabolite of clozapine and combines unique pharmacological properties. Because little is known about the rate of metabolic conversion of clozapine in vivo, we assessed the association between clozapine dose and plasma levels for clozapine and NDMC.

METHODS

Plasma levels of clozapine and NDMC were measured in 485 blood samples from 108 patients with schizophrenia treated with clozapine. %NDMC, the ratio of NDMC to total clozapine (NDMC + clozapine), was used as a measure of the in vivo metabolism of clozapine.

RESULTS

Daily clozapine doses correlated significantly with clozapine levels and NDMC levels, whereas %NDMC showed a weaker negative correlation with clozapine dose. The mean %NDMC value was 37.0% +/- 16.8%, with high variability between subjects. Repeated measurements in subjects treated with the same dose of clozapine showed a high within-subject variability of %NDMC.

CONCLUSIONS

Our results suggest a high degree of between-subject and within-subject variability in the metabolism of clozapine in vivo. Direct administration of NDMC may be preferable to reliably achieve sufficient plasma levels of this compound.

Antipsychotic drugs: comparison in animal models of efficacy, neurotransmitter regulation, and neuroprotection

Various lines of evidence indicate the presence of progressive pathophysiological processes occurring within the brains of patients with schizophrenia. By modulating chemical neurotransmission, antipsychotic drugs may influence a variety of functions regulating neuronal resilience and viability and have the potential for neuroprotection. This article reviews the current literature describing preclinical and clinical studies that evaluate the efficacy of antipsychotic drugs, their mechanism of action and the potential of first- and second-generation antipsychotic drugs to exert effects on cellular processes that may be neuroprotective in schizophrenia. The evidence to date suggests that although all antipsychotic drugs have the ability to reduce psychotic symptoms via D(2) receptor antagonism, some antipsychotics may differ in other pharmacological properties and their capacities to mitigate and possibly reverse cellular processes that may underlie the pathophysiology of schizophrenia.

A neurocognitive animal model dissociating between acute illness and remission periods of schizophrenia

RATIONALE

The development and validation of animal models of the cognitive impairments of schizophrenia have remained challenging subjects.

OBJECTIVE

We review evidence from a series of experiments concerning an animal model that dissociates between the disruption of attentional capacities during acute illness periods and the cognitive load-dependent impairments that characterize periods of remission. The model focuses on the long-term attentional consequences of an escalating-dosing pretreatment regimen with amphetamine (AMPH).

RESULTS

Acute illness periods are modeled by the administration of AMPH challenges. Such challenges result in extensive impairments in attentional performance and the "freezing" of performance-associated cortical acetylcholine (ACh) release at pretask levels. During periods of remission (in the absence of AMPH challenges), AMPH-pretreated animals' attentional performance is associated with abnormally high levels of performance-associated cortical ACh release, indicative of the elevated attentional effort required to maintain performance. Furthermore, and corresponding with clinical evidence, attentional performance during remission periods is exquisitely vulnerable to distractors, reflecting impaired top-down control and abnormalities in fronto-mesolimbic-basal forebrain circuitry. Finally, this animal model detects the moderately beneficial cognitive effects of low-dose treatment with haloperidol and clozapine that were observed in clinical studies.

CONCLUSIONS

The usefulness and limitations of this model for research on the neuronal mechanisms underlying the cognitive impairments in schizophrenia and for drug-finding efforts are discussed.

Insulin, IGF-1, and muscarinic agonists modulate schizophrenia-associated genes in human neuroblastoma cells

BACKGROUND

Genes associated with energy metabolism are decreased in schizophrenia brain and human and rodent diabetic skeletal muscle. These and other similarities between diabetes and schizophrenia suggest that an insulin signaling deficit may underlie schizophrenia. We determined with human SH-SY5Y neuroblastoma and astrocyte cell lines whether insulin or other molecules could modulate genes opposite to their change reported in schizophrenia brain.

METHODS

Both cell lines were treated with insulin, insulin-like growth factor (IGF)-1, IGF-2, or brain-derived neurotrophic factor (BDNF). Genes whose expression was found with microarrays to be changed by insulin in a reciprocal manner to their change in schizophrenia were used in a 16-gene miniarray to identify small molecules that might mimic insulin.

RESULTS

Insulin phosphorylated its receptor in the neuroblastoma cells but not in astrocytes and, like IGF-1, increased ERK1/2 and Akt phosphorylation. Insulin and IGF-1 increased the expression of genes decreased in schizophrenia, including those involved in mitochondrial functions, glucose and energy metabolism, hydrogen ion transport, and synaptic function. These gene effects were confirmed and shown to be dose related with the 16-gene miniarrays. Most of 1940 pharmacologically unique compounds failed to alter gene expression, with the exception of muscarinic agonists, which mimicked insulin and IGF-1, and which were blocked by the muscarinic antagonists atropine and telenzepine.

CONCLUSIONS

Stimulation of muscarinic and insulin/IGF-1 receptors alter genes associated with metabolic and synaptic functions in a manner reciprocal to their changes in schizophrenia. Pharmacologic activation of these receptors may normalize genomic alterations in schizophrenia and better address root causes of this disease.

Glutamatergic dysfunction in schizophrenia: from basic neuroscience to clinical psychopharmacology

The underlying cellular mechanisms leading to frontal cortical hypofunction (i.e., hypofrontality) in schizophrenia remain unclear. Both hypoactive and hyperreactive prefrontal cortical (PFC) states have been reported in schizophrenia patients. Recent proton magnetic resonance spectroscopy studies revealed that antipsychotic-naïve patients with first psychotic episode exhibit a hyperactive PFC. Conversely, PFC activity seems to be diminished in patients chronically exposed to conventional antipsychotic treatments, an effect that could reflect the therapeutic action as well as some of the impairing side effects induced by long-term blockade of dopamine transmission. In this review, we will provide an evolving picture of the pathophysiology of schizophrenia moving from dopamine to a more glutamatergic-centered hypothesis. We will discuss how alternative antipsychotic strategies may emerge by using drugs that reduce excessive glutamatergic response without altering the balance of synaptic and extrasynaptic normal glutamatergic neurotransmission. Preclinical studies indicate that acamprosate, a FDA approved drug for relapse prevention in detoxified alcoholic patients, reduces the glutamatergic hyperactivity triggered by ethanol withdrawal without depressing normal glutamatergic transmission. Whether this effect is mediated by a direct modulation of NMDA receptors or by antagonism of metabotropic glutamate receptor remains to be determined. We hypothesize that drugs with similar pharmacological actions to acamprosate may provide a better and safer approach to reverse psychotic symptoms and cognitive deficits without altering the balance of excitation and inhibition of the corticolimbic dopamine-PFC system. It is predicted that schizophrenia patients treated with acamprosate-like compounds will not exhibit progressive cortical atrophy associated with the anti-dopaminergic effect of classical antipsychotic exposure.

Muscarinic agonists for the treatment of cognition in schizophrenia

It is widely accepted that cholinergic activity at muscarinic receptors is required to maintain cognitive functions, including learning and memory. Memory domains are especially impaired in schizophrenia, which may explain difficulties in psychosocial rehabilitation of individuals with this illness. However, little is known about the mechanism of this impairment. To understand our current knowledge, we reviewed the literature since 1990 via a PubMed search for the terms "muscarinic", "schizophrenia", "cognition", "memory", "learning", and "agonist" in combination. We found 89 basic science/laboratory studies, case reports/series, case-control studies, cross-sectional studies, standardized controlled animal trials, standardized controlled human trials, and reviews. Although further research is required to fully understand the neuropharmacology of the cholinergic system in cognitive function in schizophrenia, we have examined the data currently available. In general, these data suggest that agonist activity at acetylcholine muscarinic type 1 (M1) receptors would enhance memory and learning in schizophrenia. We present an overview of likely side effects of muscarinic agonists. We outline the anticholinergic activity of several available antipsychotics and review the available M1 muscarinic agonists.

Decreased M1 muscarinic receptor density in rat amphetamine model of schizophrenia is normalized by clozapine, but not haloperidol

There is increasing evidence supporting the involvement of the muscarinic-cholinergic system in schizophrenia. We examined the M1 muscarinic receptor density and mRNA expression in brains of a rat amphetamine model of schizophrenia. We also assessed the effect of the model and chronic treatment with haloperidol and clozapine on brain M1 receptor density and gene expression. A significant decrease of about 20% in the density of M1 receptor was detected in the cortex and in the striatum of amphetamine model rats. A significant increase of 33% in the density of the M1 receptor was found in the cortex and striatum of rats treated chronically with clozapine (0.5 mg/kg), but not with haloperidol (25 mg/kg). Chronic clozapine, but not haloperidol, normalized the decrease in M1 receptors observed in amphetamine model rats, in both cortex and striatum. Regulation of the M1 receptor may occur in a post-transcriptional phase. Our findings suggest involvement of both dopaminergic and cholinergic-muscarinic systems in the pathophysiology and pharmacotherapy of schizophrenia.

Better pharmacotherapy for schizophrenia: what does the future hold?

Despite the expansion of available antipsychotic drugs over the past 50 years, functional outcomes for individuals with schizophrenia have not markedly improved. These agents are efficacious for psychosis but do not adequately address other core domains of schizophrenia psychopathology, namely negative symptoms and cognitive impairment, which have a greater impact on functional outcomes, including vocational or academic performance and interpersonal relationships. In addition, treatment-refractory psychosis still precludes functional improvement in many patients. Schizophrenia is a clinical syndrome consisting of these domains, which likely have some disparities in their respective pathophysiologies. This suggests that drug development should look to other molecular targets besides the D2 receptor, which characterizes the mechanism of available medications for schizophrenia. In this report, we review novel pharmacologic approaches that aim to specifically address each individual domain of schizophrenia. The goal of this future pharmacotherapy strategy is to advance outcomes beyond psychosis remission and toward functional recovery.

Repeated low dose of phencyclidine administration impairs spatial learning in mice: blockade by clozapine but not by haloperidol

The effect of phencyclidine (PCP), a non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, was examined in the water maze, a spatial learning and memory task dependent on hippocampal functions. Male adult C57Bl/6J mice received daily (s.c.) injections of either saline or PCP (0.25-4.0 mg/kg) for 12 days. During the last 5 days, the injections were followed by water maze training. Repeated PCP treatments disrupted spatial learning and memory in the 0.5-4.0 mg/kg dose range. Severe sensorimotor disturbances, observed at the 2.0 and 4.0 mg/kg doses of PCP, precluded further swim maze testing. The 0.5 mg/kg but not the 1.0 mg/kg dose of PCP impaired spatial learning and memory without any apparent sensorimotor deficits. PCP, at 1.0 mg/kg, produced impairment in non-spatial learning in the swim maze task and motor disturbances in the rotarod test. Repeated daily treatment with either the "atypical" antipsychotic drug clozapine (0.5 mg/kg i.p.) or the "typical" antipsychotic drug haloperidol (0.05 mg/kg i.p.) failed to influence spatial performances. The spatial impairment caused by the 0.5 mg/kg dose of PCP was blocked by concomitant treatment with clozapine (0.5 mg/kg), but not with haloperidol (0.05 mg/kg). The results suggest that it is possible, at low doses of PCP, to dissociate the spatial learning impairment in the water maze from the adverse behavioral effects of NMDA receptor blockade. This model may provide a basis for the analysis of the mechanisms underlying declarative memory disturbances in schizophrenia and the differences in mechanisms between typical and atypical antipsychotic drugs.

Atypical antipsychotic-induced metabolic side effects: insights from receptor-binding profiles

Atypical antipsychotic drugs offer several notable benefits over typical antipsychotics, including greater improvement in negative symptoms, cognitive function, prevention of deterioration, and quality of life, and fewer extrapyramidal symptoms (EPS). However, concerns about EPS have been replaced by concerns about other side effects, such as weight gain, glucose dysregulation and dyslipidemia. These side effects are associated with potential long-term cardiovascular health risks, decreased medication adherence, and may eventually lead to clinical deterioration. Despite a greater understanding of the biochemical effects of these drugs in recent years, the pharmacological mechanisms underlying their various therapeutic properties and related side effects remain unclear. Besides dopamine D(2) receptor antagonism, a characteristic feature of all atypical antipsychotic drugs, these agents also bind to a range of non-dopaminergic targets, including serotonin, glutamate, histamine, alpha-adrenergic and muscarinic receptors. This review examines the potential contribution of different receptors to metabolic side effects associated with atypical antipsychotic treatment for all seven agents currently marketed in the United States (risperidone, olanzapine, quetiapine, ziprasidone, aripiprazole, paliperidone and clozapine) and another agent (bifeprunox) in clinical development at the time of this publication.

N-desmethylclozapine an M1 receptor agonist enhances nitric oxide's cardiac vagal facilitation in the isolated innervated rat right atrium

We have previously determined that neuronal nitric oxide (NO) may partly mediate its established cholinergic effect via activation of muscarinic type 1 (M1) receptors located at the preganglionic/postganglionic synapse. In this series of experiments we set out to confirm this finding using an M1 agonist. Experiments were carried out on the isolated vagally innervated right atrium in the presence of atenolol (4 microM). The right vagus was stimulated at 4, 8, 16, 32 Hz; pulse duration 1 ms at 20 V for 20 s and the effect on cardiac interval (ms) assessed. N-desmethylclozapine (100 nM), a potent M1 agonist, enhanced the vagally induced increase in cardiac interval, a lower concentration of 50 nM had no significant effect on cardiac interval. This effect was prevented by pre-treatment of the atria with the neuronal NO synthase inhibitor 1 (2-trifluoromethylphenyl)imidazole (TRIM) at 0.14 mM. The vagal stimulation protocol was repeated in order to rule out a reduction in vagal effectiveness which may have been due to the experimental stimulation protocol used in this study. TRIM (0.14 mM) alone causes a small but significant attenuation of the vagally induced increase in cardiac interval. These results show that agonism of M1 receptors on cardiac vagal preganglionic fibres enhances vagal cardiac effects which can be prevented by a neuronal NO inhibitor.

Molecular targets for treating cognitive dysfunction in schizophrenia

Cognitive impairment is a core feature of schizophrenia as deficits are present in the majority of patients, frequently precede the onset of other positive symptoms, persist even with successful treatment of positive symptoms, and account for a significant portion of functional impairment in schizophrenia. While the atypical antipsychotics have produced incremental improvements in the cognitive function of patients with schizophrenia, overall treatment remains inadequate. In recent years, there has been an increased interest in developing novel strategies for treating the cognitive deficits in schizophrenia, focusing on ameliorating impairments in working memory, attention, and social cognition. Here we review various molecular targets that are actively being explored for potential drug discovery efforts in schizophrenia and cognition. These molecular targets include dopamine receptors in the prefrontal cortex, nicotinic and muscarinic acetylcholine receptors, the glutamatergic excitatory synapse, various serotonin receptors, and the gamma-aminobutyric acid (GABA) system.

Partial agonist actions of aripiprazole and the candidate antipsychotics S33592, bifeprunox, N-desmethylclozapine and preclamol at dopamine D2Lreceptors are modified by co-transfection of D3receptors: potential role of heterodimer formation

Aripiprazole and the candidate antipsychotics, S33592, bifeprunox, N-desmethylclozapine (NDMC) and preclamol, are partial agonists at D(2) receptors. Herein, we examined their actions at D(2L) and D(3) receptors expressed separately or together in COS-7 cells. In D(2L) receptor-expressing cells co-transfected with (D(3) receptor-insensitive) chimeric adenylate cyclase-V/VI, drugs reduced forskolin-stimulated cAMP production by approximately 20% versus quinpirole (48%). Further, quinpirole-induced inhibition was blunted by aripiprazole and S33592, confirming partial agonist properties. In cells co-transfected with equal amounts of D(2L)and D(3) receptors (1 : 1), efficacies of aripiprazole and S33592 were attenuated. Further, in cells co-transfected with D(2L) and an excess of D(3) receptors (1 : 3), aripiprazole and S33592 were completely inactive, and they abolished the actions of quinpirole. Likewise, bifeprunox, NDMC and preclamol lost agonist properties in cells co-transfected with D(2L)and D(3) receptors. Accordingly, at split D(2trunk)/D(3tail) and D(3trunk)/D(2tail) chimeras, agonist actions of quinpirole were blocked by aripiprazole and S33592 that, like bifeprunox, NDMC and preclamol, were inactive alone. Conversely, when a 12 amino acid sequence in the third intracellular loop of D(3) receptors was replaced by the homologous sequence of D(2L) receptors, aripiprazole, S33592, bifeprunox, NDMC and preclamol inhibited cAMP formation by approximately 20% versus quinpirole (42%). Moreover, at D(2L) receptor-expressing cells co-transfected with modified D(3i3(D2)) receptors, drugs behaved as partial agonists. To summarize, low efficacy agonist actions of aripiprazole, S33592, bifeprunox, NDMC and preclamol at D(2L) receptors are abrogated upon co-expression of D(3) receptors, probably due to physical association and weakened coupling efficacy. These findings have implications for the functional profiles of antipsychotics.

Lurasidone (SM-13496), a novel atypical antipsychotic drug, reverses MK-801-induced impairment of learning and memory in the rat passive-avoidance test

Lurasidone (SM-13496) is a novel atypical antipsychotic with high affinities to dopamine D2, serotonin 5-HT7, 5-HT2A, 5-HT1A receptors and alpha2C adrenoceptor. In this study, the effects of lurasidone on the rat passive-avoidance response and its impairment by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine) were evaluated and compared with those of other antipsychotics. The passive-avoidance response was examined by measuring the step-through latency, 1 day after the animals received foot-shock training. When given before the training session, lurasidone did not affect the passive-avoidance response at any dose tested (1-30 mg/kg, p.o.). All the other atypical antipsychotics examined (i.e., risperidone, olanzapine, quetiapine, clozapine and aripiprazole), however, significantly reduced the step-through latency at relatively high doses. A pre-training administration of lurasidone significantly and dose-dependently reversed the MK-801-induced impairment of the passive-avoidance response. At doses lower than those that affected the passive-avoidance response, risperidone, quetiapine, and clozapine partially reduced the MK-801-induced impairment, whereas haloperidol, olanzapine, and aripiprazole were inactive. In addition, the post-training administration of lurasidone was as effective in countering the MK-801 effect as the pre-training administration, suggesting that lurasidone worked, at least in part, by restoring the memory consolidation process disrupted by MK-801. These results suggest that lurasidone is superior to other antipsychotics in improving the MK-801-induced memory impairment and may be clinically useful for treating cognitive impairments in schizophrenia.

Evaluation of N-desmethylclozapine as a potential antipsychotic--preclinical studies

There is growing interest in N-desmethylclozapine (NDMC), the major metabolite of clozapine, as a unique antipsychotic because it acts in vitro as a 5-HT(2) antagonist and as a partial agonist to dopamine D(2) and muscarinic receptors. To explore this, we compared NDMC to a typical (haloperidol), atypical (clozapine), and partial-agonist atypical (aripiprazole) antipsychotic in preclinical models. The comparison was carried out using: brain D(2) and 5-HT(2) receptor occupancy; animal models predictive of antipsychotic efficacy (amphetamine-induced hyperlocomotion (AIL) and conditioned avoidance response (CAR) models); measures predictive of side effects (catalepsy and prolactin elevation); and molecular markers predictive of antipsychotic action (striatal Fos induction). NDMC (10-60 mg/kg/s.c.) showed high 5-HT(2) (64-79%), but minimal D(2) occupancy (<15% at 60 mg/kg) 1 h after administration. In contrast to other antipsychotics, NDMC was not very effective in reducing AIL or CAR and showed minimal induction of Fos in the nucleus accumbens. However, like atypical antipsychotics, it showed no catalepsy, prolactin elevation, and minimal Fos in the dorsolateral striatum. It seems unlikely that NDMC would show efficacy as a stand-alone antipsychotic, however, its freedom from catalepsy and prolactin elevation, and its unique pharmacological profile (muscarinic agonism) may make it feasible to use this drug as an adjunctive treatment to existing antipsychotic regimens.

AC260584 (4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one), a selective muscarinic M1 receptor agonist, increases acetylcholine and dopamine release in rat medial prefrontal cortex and hippocampus

Both muscarinic and nicotinic receptors are implicated in cognition. We have previously suggested that stimulation of the muscarinic M1 receptor has a beneficial effect on cognition, based upon evidence that the muscarinic M1 receptor agonist of N-desmethylclozapine, the major metabolite of clozapine, may contribute to the ability of clozapine to improve some domains of cognition in schizophrenia. Present study examined the effectiveness of a new muscarinic M1 receptor agonist, 4-[3-(4-butylpiperidin-1-yl)-propyl]-7-fluoro-4H-benzo[1,4]oxazin-3-one (AC260584), to increase the release of acetylcholine and dopamine in the rat medial prefrontal cortex and hippocampus. Using microdialysis in awake, freely moving rats, AC260584, 3 and 10, but not 1 mg/kg (s.c.), significantly increased dopamine release in the medial prefrontal cortex and hippocampus. However, only the high dose of AC260584, 10 mg/kg (s.c.), significantly increased acetylcholine release in these regions. Moreover, the increases in acetylcholine release produced by AC260584, 10 mg/kg, were attenuated by the muscarinic M1 receptor antagonist telenzepine (3 mg/kg, s.c.) but not by the 5-HT1A receptor antagonist N-[2-(4-2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridyl) cyclohexanecarboxamide (WAY100635, 0.2 mg/kg, s.c.). However, the increase in dopamine release produced by 10 mg/kg AC260584 was blocked by both telenzepine and WAY100635. In addition, pretreatment with the atypical antipsychotic drug risperidone (0.1 mg/kg, s.c.) potentiated AC260584 (1.0 mg/kg, s.c.)-induced acetylcholine and dopamine release in the medial prefrontal cortex. These findings suggest that the muscarinic M1 receptor agonist property of AC260584 contributes to its enhancement of cortical acetylcholine and dopamine efflux. Therefore, AC260584, as well as other muscarinic M1 receptor agonists, may be a valuable target for the development of drugs which can improve the cognitive deficits in schizophrenia and perhaps other neuropsychiatric disorders, as well.

Glutamate and dopamine dysregulation in schizophrenia--a synthesis and selective review

The dopamine hypothesis of schizophrenia is the principal explanatory model of antipsychotic drug action. Recent discoveries extend our understanding of the neurochemistry of schizophrenia, with increasing evidence of dysfunction in glutamate and GABA as well as dopamine systems. In this review, we study the evidence for dopaminergic dysfunction in schizophrenia, drawing data from neurochemical imaging studies. We also review the NMDA receptor hypofunction hypothesis of schizophrenia as a supplementary explanatory model for the illness. We examine predictions made by the NMDA receptor hypofunction hypothesis and consider how they fit with current neurochemical findings in patients and animal models. We consider the case for glutamatergic excitotoxicity as a key process in the development and progression of schizophrenia, and suggest ways in which glutamate and dopamine dysregulation may interact in the condition.

N-Desmethylclozapine, a major clozapine metabolite, acts as a selective and efficacious delta-opioid agonist at recombinant and native receptors

The present study examined the effects of N-desmethylclozapine (NDMC), a biologically active metabolite of the atypical antipsychotic clozapine, at cloned human opioid receptors stably expressed in Chinese hamster ovary (CHO) cells and at native opioid receptors present in NG108-15 cells and rat brain. In CHO cells expressing the delta-opioid receptor (CHO/DOR), NDMC behaved as a full agonist both in stimulating [(35)S]GTPgammaS binding (pEC(50)=7.24) and in inhibiting cyclic AMP formation (pEC(50)=6.40). NDMC inhibited [(3)H]naltrindole binding to CHO/DOR membranes with competition curves that were modulated by guanine nucleotides in an agonist-like manner. Determination of intrinsic efficacies by taking into consideration both the maximal [(35)S]GTPgammaS binding stimulation and the extent of receptor occupancy at which half-maximal effect occurred indicated that NDMC had an efficacy value equal to 82% of that of the full delta-opioid receptor agonist DPDPE, whereas clozapine and the other clozapine metabolite clozapine N-oxide displayed much lower levels of agonist efficacy. NDMC exhibited poor agonist activity and lower affinity at the kappa-opioid receptor and was inactive at mu-opioid and NOP receptors. In NG108-15 cells, NDMC inhibited cyclic AMP formation and stimulated the phosphorylation of extracellular signal-regulated kinase 1/2 by activating the endogenously expressed delta-opioid receptor. Moreover, in membranes of different brain regions, NDMC stimulated [(35)S]GTPgammaS binding and regulated adenylyl cyclase activity and the effects were potently antagonized by naltrindole. These data demonstrate for the first time that NDMC acts as a selective and efficacious delta-opioid receptor agonist and suggest that this unique property may contribute, at least in part, to the clinical actions of the atypical antipsychotic clozapine.

Discriminative stimulus properties of S32504, a novel D3/D2 receptor agonist and antiparkinson agent, in rats: attenuation by the antipsychotics, aripiprazole, bifeprunox, N-desmethylclozapine, and by selective antagonists at dopamine D2 but not D3 receptors

RATIONALE

Drug-discrimination studies have proven instructive in the characterization of psychotropic agents, a procedure applied herein to the novel antiparkinson agent, S32504. This highly selective agonist at dopamine D(3) and (less potently) D(2) receptors displays potent antiparkinson, neuroprotective and antidepressant properties (Millan et al., J Pharmacol Exp Ther 309:936-950, 2004a; Millan et al., J Pharmacol Exp Ther 309:903-920, 2004b; Millan et al., J Pharmacol Exp Ther 309:921-935, 2004c).

OBJECTIVES

To generate a discriminative stimulus (DS) with S32504 and undertake substitution/antagonism studies with diverse antiparkinson and antipsychotic agents.

MATERIALS AND METHODS

Using a two-lever, fixed-ratio 10 schedule, rats were trained to recognize S32504 (0.04 mg/kg, s.c.) from saline.

RESULTS

S32504 displayed dose-dependent and stereospecific substitution in comparison to its less active racemic form, (+/-) S31411, and to its inactive (-) distomer, S32601. Apomorphine, and the selective D(3)/D(2) receptor agonists, ropinirole, PD128,907, 7-OH-DPAT and CGS15855A, fully (=80%) substituted for S32504, whereas D(4) and D(1)/D(5) receptor agonists were ineffective. The selective D(3) vs D(2) receptor partial agonist, BP897, did not substitute for S32504 and the selective D(3) receptor antagonists, S33084, SB277,011, GR218,231, PNU99194A and S14297, did not block its DS properties. By contrast, S32504 lever selection was blocked by the preferential D(2) vs D(3) receptor antagonists, L741,626 and S23199, and by the D(2)/D(3) antagonists, raclopride and haloperidol. The D(2)/D(3) receptor partial agonists and antipsychotics, aripiprazole, bifeprunox, N-desmethylclozapine and preclamol did not substitute for S32504: indeed, they dose-dependently attenuated its DS properties.

CONCLUSION

The antiparkinson agent, S32504, displays DS properties principally mediated by high-efficacy activation of D(2) receptors Antipsychotics known to act as partial agonists at D(2)/D(3) receptors attenuate DS properties of S32504, actions reflecting their low efficacy at these sites.

Towards a muscarinic hypothesis of schizophrenia

Although the neurotransmitter dopamine plays a prominent role in the pathogenesis and treatment of schizophrenia, the dopamine hypothesis of schizophrenia fails to explain all aspects of this disorder. It is increasingly evident that the pathology of schizophrenia also involves other neurotransmitter systems. Data from many streams of research including pre-clinical and clinical pharmacology, treatment studies, post-mortem studies and neuroimaging suggest an important role for the muscarinic cholinergic system in the pathophysiology of schizophrenia. This review will focus on evidence that supports the hypothesis that the muscarinic system is involved in the pathogenesis of schizophrenia and that muscarinic receptors may represent promising novel targets for the treatment of this disorder.

Role of postsynaptic serotonin1A receptors in risperidone-induced increase in acetylcholine release in rat prefrontal cortex

Most atypical antipsychotic drugs increase acetylcholine release in the prefrontal cortex, but the detailed mechanism is still unknown. The present study examined the role of serotonin (5-HT)1A receptors in risperidone-induced increases in acetylcholine release in rat prefrontal cortex. Systemic administration of risperidone at doses of 1 and 2 mg/kg increased acetylcholine release in the prefrontal cortex in a dose-dependent manner. This increase was antagonized by systemic administration of high doses (1 and 3 mg/kg) of N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl)cyclohexanecarboxamide (WAY100635), a 5-HT1A receptor antagonist/dopamine D4 receptor agonist, but not by a low dose (0.1 mg/kg) of the antagonist which antagonizes preferentially presynaptic 5-HT1A autoreceptors. Furthermore, local application of WAY100635 into the prefrontal cortex also attenuated risperidone-induced increases in acetylcholine release. WAY100635 alone did not affect acetylcholine release in the prefrontal cortex. On the other hand, local application of risperidone (3 and 10 microM), the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (1 and 10 microM), and the dopamine D4 receptor antagonist 3-(4-(4-iodophenyl)piperazine-1-yl)methyl-1H-pyrrolo[2,3-b]pyridine (1 and 10 microM) into the cortex did not affect acetylcholine release in the prefrontal cortex. These results suggest that risperidone increases acetylcholine release in the prefrontal cortex through a complex mechanism which is enhanced by prefrontal 5-HT1A receptor activation.

5-HT6 receptor antagonist SB-399885 potentiates haloperidol and risperidone-induced dopamine efflux in the medial prefrontal cortex or hippocampus

Many studies suggest that the 5-HT6 receptors are involved, along with other 5-HT receptors, in the pathophysiology and pharmacotherapy of schizophrenia. It is a putative therapeutic target of atypical antipsychotic drugs, notably clozapine, as well as some other psychotropic agents. Preferential potentiation of dopamine (DA) efflux in the medial prefrontal cortex (mPFC) and hippocampus (HIP) has been suggested to contribute to the ability of atypical antipsychotic drugs (APDs), e.g. clozapine, risperidone, olanzapine and ziprasidone, to improve cognitive function in schizophrenia. The present study demonstrated that SB-399885, a selective 5-HT6 receptor antagonist, at doses of 3 and 10 mg/kg, had no effect on cortical DA release in freely moving rats. However, both doses of SB-399885 slightly but significantly increased DA release in the HIP. Of particular interest, SB-399885, 3 mg/kg, significantly potentiated the ability of a typical antipsychotic drug haloperidol, a D2 receptor antagonist, at a dose of 0.1 mg/kg, to increase DA release in the HIP but not the mPFC. The atypical antipsychotic drug risperidone, a multireceptor antagonist, which lacks 5-HT6 receptor antagonist properties, at doses of 0.1, 0.3 and 1.0 mg/kg, produced a bell-shaped dose response effect on DA efflux in the mPFC and HIP. SB-399885 potentiated risperidone (1.0 mg/kg)-induced DA efflux in both regions. The increase in the HIP, but not the mPFC, DA efflux by 0.3 mg/kg risperidone was also potentiated by SB-399885, 3 mg/kg. These results suggest that the combined blockade of 5-HT6 and D2 receptors may contribute to the potentiation of haloperidol- and risperidone-induced DA efflux in the mPFC or HIP. The present data provides additional evidence in support of a possible therapeutic role for 5-HT6 receptor antagonism, as an addition on therapy, to enhance cognitive function in schizophrenia.

Cholinergic circuits and signaling in the pathophysiology of schizophrenia

Central cholinergic signaling has long been associated with aspects of memory, motivation, and mood, each affected functions in neuropsychiatric disorders such as schizophrenia. In this chapter, we review evidence related to the core hypothesis that dysregulation of central cholinergic signaling contributes to the pathophysiology of schizophrenia. Although central cholinergic circuits are resistant to simplification-particularly when one tries to parse the contributions of various classes of cholinergic receptors to disease related phenomena--the potential role of ACh signaling in Schizophrenia pathophysiology deserves careful consideration for prospective therapeutics. The established role of cholinergic circuits in attentional tuning is considered along with recent work on how the patterning of cholinergic activity may modulate corticostriatal circuits affected in schizophrenia.

Discriminative stimulus properties of the atypical antipsychotic drug clozapine in rats trained to discriminate 1.25 mg/kg clozapine vs. 5.0 mg/kg clozapine vs. vehicle

Clozapine, the prototype for atypical antipsychotic drugs, is used in the drug discrimination paradigm as a model for screening atypical from typical antipsychotic drugs. Previous drug discrimination studies in rats have shown that a 1.25 mg/kg clozapine training dose provides full stimulus generalization (i.e.) >or=80% condition-appropriate responding) to most atypical antipsychotic drugs, although a 5.0 mg/kg clozapine training dose appears necessary to provide stimulus generalization to other atypical antipsychotic drugs. The present study sought to characterize the pharmacological mechanisms that mediate these clozapine training doses. In rats trained to discriminate 1.25 vs. 5.0 mg/kg clozapine vs. vehicle in a three-choice drug discrimination task, various receptor-selective compounds were tested for stimulus generalization. The antidepressant mianserin was also tested. Full stimulus generalization from the 1.25 mg/kg clozapine training dose occurred only to mianserin (98.8%). Partial substitution (i.e. >or=60% and <80% condition-appropriate responding) to the 5.0 mg/kg clozapine training dose occurred for the muscarinic receptor antagonist scopolamine. The combined total percentage of responding on the 1.25 and 5.0 mg/kg clozapine levers, however, was well above the full substitution criteria at the 0.25, 0.5, and 1.0 mg/kg scopolamine doses. The M1 agonist N-desmethylclozapine, the nicotinic antagonist mecamylamine, the D1 antagonist SCH 23390, the D4 antagonist LU 38-012, the 5-HT1A agonist (+)-8-OH-DPAT, the 5-HT1A antagonist WAY 100 635, the 5-HT2A/2B/2C antagonist ritanserin, the 5-HT6 antagonist RO4368554, the alpha1 antagonist prazosin, the alpha2 antagonist yohimbine, and the histamine H1 antagonist pyrilamine all failed to substitute for either the 1.25 or the 5.0 mg/kg clozapine training doses. These results are consistent with previous evidence that antidepressant drugs have a tendency to substitute for clozapine and that muscarinic receptor antagonism may mediate the discriminative stimulus properties of 5.0 mg/kg clozapine. The lack of stimulus generalization from either clozapine training dose to other receptor-selective compounds, however, fails to explain how this model screens atypical from typical antipsychotic drugs and suggests that the discriminative stimulus properties of clozapine consist of a compound cue.