Differential effects of the D1-DA receptor antagonist SCH39166 on positive and negative symptoms of schizophrenia

Lu AF35700 reverses the phencyclidine-induced disruption of thalamo-cortical activity by blocking dopamine D1 and D2 receptors

Antipsychotic drugs of different chemical/pharmacological families show preferential dopamine (DA) D₂ receptor (D₂-R) vs. D₁ receptor (D₁-R) affinity, with the exception of clozapine, the gold standard of schizophrenia treatment, which shows a comparable affinity for both DA receptors. Here, we examined the ability of Lu AF35700 (preferential D₁-R>D₂-R antagonist), to reverse the alterations in thalamo-cortical activity induced by phencyclidine (PCP), used as a pharmacological model of schizophrenia. Lu AF35700 reversed the PCP-induced alteration of neuronal discharge and low frequency oscillation (LFO, 0.15-4 Hz) in thalamo-cortical networks. Likewise, Lu AF35700 prevented the increased c-fos mRNA expression induced by PCP in thalamo-cortical regions of awake rats. We next examined the contribution of D₁-R and D₂-R to the antipsychotic reversal of PCP effects. The D₂-R antagonist haloperidol reversed PCP effects on thalamic discharge rate and LFO. Remarkably, the combination of sub-effective doses of haloperidol and SCH-23390 (DA D₁-R antagonist) fully reversed the PCP-induced fall in thalamo-cortical LFO. However, unlike with haloperidol, SCH-23390 elicited different degrees of potentiation of the effects of low clozapine and Lu AF35700 doses. Overall, the present data support a synergistic interaction between both DA receptors to reverse the PCP-induced alterations of oscillatory activity in thalamo-cortical networks, possibly due to their simultaneous blockade in direct and indirect pathways of basal ganglia. The mild potentiation induced by SCH-23390 in the case of clozapine and Lu AF35700 suggests that, at effective doses, these agents reverse PCP effects through the simultaneous blockade of both DA receptors.

Efficacy and safety of Lu AF35700 in treatment-resistant schizophrenia: A randomized, active-controlled trial with open-label extension

INTRODUCTION

Treatment resistance constitutes the highest burden of disease within schizophrenia. We hypothesized that the synergistic activity of Lu AF35700 at dopamine D1 and D2 receptors might provide superior antipsychotic effects versus first-line antipsychotic therapy in patients with treatment resistant schizophrenia (TRS), with a benign tolerability profile.

METHODS

This was a randomized, double-blind, active-controlled clinical trial (NCT02717195) followed by a one year open-label safety extension (NCT02892422). Following prospective confirmation of treatment resistance, patients were randomized (1:1:1) to 10 weeks double-blind treatment with Lu AF35700 10 mg or 20 mg, or active comparator (risperidone or olanzapine).

RESULTS

1628 patients were screened for TRS, of which 1092 entered the prospective confirmation period. Of these, 697 were randomized (Lu AF35700 10 mg n = 235, 20 mg n = 232, comparator n = 230) and 395 discontinued before randomization, including 264 (24 %) who responded to treatment. 586 patients completed the double-blind phase, of which 524 entered the open-label extension and 318 completed 1-year of open-label treatment. At the end of the double-blind phase, the mean ± SE change in positive and negative syndrome scale (PANSS) total score was -10.1 ± 0.96 for Lu AF35700 10 mg, -8.22 ± 0.98 for Lu AF35700 20 mg, and - 9.90 ± 0.97 for the comparator group. Treatment differences [95 % CI] versus comparator treatment were non-significant (-0.12 [-2.37; 2.13] and 1.67 [-0.59; 3.94], respectively). The most common adverse events with Lu AF35700 were increased weight and headache. Prolactin values decreased by ≥50 % in both sexes treated with Lu AF35700.

CONCLUSIONS

Despite evidence of antipsychotic efficacy, treatment with Lu AF35700 failed to differentiate from conventional antipsychotic treatment for patients with TRS.

Dopamine D1R Receptor Stimulation as a Mechanistic Pro-cognitive Target for Schizophrenia

Decades of research have highlighted the importance of optimal stimulation of cortical dopaminergic receptors, particularly the D1R receptor (D1R), for prefrontal-mediated cognition. This mechanism is particularly relevant to the cognitive deficits in schizophrenia, given the abnormalities in cortical dopamine (DA) neurotransmission and in the expression of D1R. Despite the critical need for D1R-based therapeutics, many factors have complicated their development and prevented this important therapeutic target from being adequately interrogated. Challenges include determination of the optimal level of D1R stimulation needed to improve cognitive performance, especially when D1R expression levels, affinity states, DA levels, and the resulting D1R occupancy by DA, are not clearly known in schizophrenia, and may display great interindividual and intraindividual variability related to cognitive states and other physiological variables. These directly affect the selection of the level of stimulation necessary to correct the underlying neurobiology. The optimal mechanism for stimulation is also unknown and could include partial or full agonism, biased agonism, or positive allosteric modulation. Furthermore, the development of D1R targeting drugs has been complicated by complexities in extrapolating from in vitro affinity determinations to in vivo use. Prior D1R-targeted drugs have been unsuccessful due to poor bioavailability, pharmacokinetics, and insufficient target engagement at tolerable doses. Newer drugs have recently become available, and these must be tested in the context of carefully designed paradigms that address methodological challenges. In this paper, we discuss how a better understanding of these challenges has shaped our proposed experimental design for testing a new D1R/D5R partial agonist, PF-06412562, renamed CVL-562.

D1-Dopamine Receptor Availability in First-Episode Neuroleptic Naive Psychosis Patients

BACKGROUND

Positron emission tomography studies examining differences in D1-dopamine receptor binding between control subjects and patients with schizophrenia have been inconsistent, reporting higher, lower, and no difference in the frontal cortex. Exposure to antipsychotic medication has been suggested to be a likely source of this heterogeneity, and thus there is a need for studies of patients at early stages of the disorder who have not been exposed to such drugs.

METHODS

Here, we compared 17 healthy control subjects and 18 first-episode neuroleptic naive patients with schizophrenia or schizophreniform psychosis using positron emission tomography and the D1-dopamine receptor radioligand [11C]SCH23390.

RESULTS

We observed a statistically significant difference in the dorsolateral prefrontal cortex. Contrary to our expectations, patients had less D1-dopamine receptor availability with a moderate effect size. In a Bayesian analysis, we show that the data are over 50 times more likely to have occurred under the decrease as opposed to the increase hypothesis. This effect was not global, as our analysis showed that the null hypothesis was preferred over either hypothesis in the striatum.

CONCLUSIONS

This investigation represents the largest single sample of neuroleptic-naive patients examined for D1-dopamine receptor availability using PET and suggests a reduction of prefrontal D1-dopamine receptor density in the pathophysiology of schizophrenia. However, further work will be required to reach a consensus.

Dopamine D1 receptor activation improves adult hippocampal neurogenesis and exerts anxiolytic and antidepressant-like effect via activation of Wnt/β-catenin pathways in rat model of Parkinson's disease

Parkinson's disease (PD) is primarily characterized by midbrain dopamine depletion. Dopamine acts through dopamine receptors (D1 to D5) to regulate locomotion, motivation, pleasure, attention, cognitive functions and formation of newborn neurons, all of which are likely to be impaired in PD. Reduced hippocampal neurogenesis associated with dopamine depletion has been demonstrated in patients with PD. However, the precise mechanism to regulate multiple steps of adult hippocampal neurogenesis by dopamine receptor(s) is still unknown. In this study, we tested whether pharmacological agonism and antagonism of dopamine D1 and D2 receptor regulate nonmotor symptoms, neural stem cell (NSC) proliferation and fate specification and explored the cellular mechanism(s) underlying dopamine receptor (D1 and D2) mediated adult hippocampal neurogenesis in rat model of PD-like phenotypes. We found that single unilateral intra-medial forebrain bundle administration of 6-hydroxydopamine (6-OHDA) reduced D1 receptor level in the hippocampus. Pharmacological agonism of D1 receptor exerts anxiolytic and antidepressant-like effects as well as enhanced NSC proliferation, long-term survival and neuronal differentiation by positively regulating Wnt/β-catenin signaling pathway in hippocampus in PD rats. shRNA lentivirus mediated knockdown of Axin-2, a negative regulator of Wnt/β-catenin signaling potentially attenuated D1 receptor antagonist induced anxiety and depression-like phenotypes and impairment in adult hippocampal neurogenesis in PD rats. Our results suggest that improved nonmotor symptoms and hippocampal neurogenesis in PD rats controlled by D1-like receptors that involve the activation of Wnt/β-catenin signaling.

Novel Dopamine Therapeutics for Cognitive Deficits in Schizophrenia

Schizophrenia is characterized by profound cognitive deficits that are not alleviated by currently available medications. Many of these cognitive deficits involve dysfunction of the newly evolved, dorsolateral prefrontal cortex (dlPFC). The brains of patients with schizophrenia show evidence of dlPFC pyramidal cell dendritic atrophy, likely reductions in cortical dopamine, and possible changes in dopamine D1 receptors (D1R). It has been appreciated for decades that optimal levels of dopamine are essential for dlPFC working memory function, with many beneficial actions arising from D1R stimulation. D1R are concentrated on dendritic spines in the primate dlPFC, where their stimulation produces an inverted-U dose response on dlPFC neuronal firing and cognitive performance during working memory tasks. Research in both academia and the pharmaceutical industry has led to the development of selective D1 agonists, e.g., the first full D1 agonist, dihydrexidine, which at low doses improved working memory in monkeys. Dihydrexidine has begun to be tested in patients with schizophrenia or schizotypal disorder. Initial results are encouraging, but studies are limited by the pharmacokinetics of the drug. These data, however, have spurred efforts toward the discovery and development of improved or novel new compounds, including D1 agonists with better pharmacokinetics, functionally selective D1 ligands, and D1R positive allosteric modulators. One or several of these approaches should allow optimization of the beneficial effects of D1R stimulation in the dlPFC that can be translated into clinical practice.

A double-blind, placebo-controlled, crossover trial of the selective dopamine D1 receptor antagonist ecopipam in patients with Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is a genetic disorder that has characteristic metabolic, neurologic, and behavioral features. There are multiple behavioral problems including impulsivity, aggressiveness, and severe recurrent self-injurious behavior (SIB). This last behavior varies considerably across subjects and may encompass self-biting, self-hitting, scratching, head banging, and other injurious actions. Current treatments for SIB involve behavioral extinction, sedatives, physical restraints, and removal of teeth. Because these interventions do not reliably control SIB, better treatments are urgently needed. Animal studies have suggested that D1-dopamine receptor antagonists such as ecopipam may suppress SIB. These observations have led to proposals that such drugs might provide effective treatment for in LND. The current study describes the results of a double-blind, three-period, crossover trial of a single dose of ecopipam in subjects with LND. The study was designed for 20 patients, but it was terminated after recruitment of only 10 patients, because interim analysis revealed unanticipated side effects. These side effects were most likely related to starting with a single large dose without any titration phase. Despite the limited data due to early termination, the drug appeared to reduce SIB in most cases. Subjects who completed the trial were eligible to continue the drug in an open-label extension phase lasting a year, and one patient who elected to continue has maintained a striking reduction in SIB for more than a year with no apparent side effects. These results suggest ecopipam could be a useful treatment for SIB in, but further studies are needed to establish an appropriate dosing regimen.

Selective activation of D1 dopamine receptors impairs sensorimotor gating in Long-Evans rats

BACKGROUND AND PURPOSE

Sensorimotor gating is a perceptual process aimed at filtering out irrelevant information. In humans and animal models, this function can be operationally measured through the prepulse inhibition (PPI) of the acoustic startle reflex. Notably, PPI deficits are associated with numerous neuropsychiatric conditions characterized by gating disturbances, including schizophrenia and Tourette syndrome. Ample evidence has shown that dopamine plays a key role in PPI regulation and, in particular, rodent studies indicate that this neurotransmitter modulates PPI through D1 and D2 dopamine receptors. In mice, the relative contributions of these two families of receptors are strain-dependent. Conversely, the role of D1 receptors in the regulation of PPI across different rat strains remains unclear.

EXPERIMENTAL APPROACH

We tested the effects of selective D1 and D2 receptor agonists and antagonists on the startle reflex and PPI of Sprague-Dawley, Wistar and Long-Evans rats.

KEY RESULTS

In contrast with Sprague-Dawley and Wistar rats, the full D1 receptor agonist SKF82958 elicited significant PPI deficits in Long-Evans rats, an effect sensitive to the selective D1 antagonist SCH23390.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that, in Long-Evans rats, D1 receptor activation may be sufficient to significantly impair PPI. These data emphasize the role of D1 receptors in the pathophysiology of neuropsychiatric disorders featuring alterations in sensorimotor gating, and uphold the importance of the genetic background in shaping the role of dopamine receptors in the regulation of this key information-processing function.

LINKED ARTICLES

This article is part of a themed section on Updating Neuropathology and Neuropharmacology of Monoaminergic Systems. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.13/issuetoc.

Functional reversal of (-)-Stepholidine analogues by replacement of benzazepine substructure using the ring-expansion strategy

(-)-Stepholidine is an active ingredient of the Chinese herb Stephania and naturally occurring tetrahydroprotoberberine alkaloid with mixed dopamine receptor D1 agonistic and dopamine receptor D2 antagonistic activities. In this work, a series of novel hexahydrobenzo[4,5]azepino [2,1-a]isoquinolines were designed and synthesized as ring-expanded analogues of (-)-Stepholidine. Initial pharmacological assays demonstrated that a benzazepine replacement was associated with significant increase in selectivity and functional reversal at dopamine receptor D1 . Compound-(-)-15e (Ki  = 5.32 ± 0.01 nm) is more potent than (-)-Stepholidine (Ki  = 13 nm) and was identified as a selective dopamine receptor D1 antagonist (IC50  = 0.14 μm). Moreover, molecular modeling suggested that (-)-15e might exert its dopamine receptor D1 antagonistic activities through interacting with the transmembrane helix 7 of dopamine receptor D1 .

A clinical trial of safety and tolerability for the selective dopamine D1 receptor antagonist ecopipam in patients with Lesch-Nyhan disease

Lesch-Nyhan disease (LND) is an inherited metabolic disorder characterized by the overproduction of uric acid and distinct behavioral, cognitive, and motor abnormalities. The most challenging clinical problem is self-injurious behavior (SIB), which includes self-biting, self-hitting, self-abrasion, and other features. Currently, these behaviors are managed by behavioral extinction, sedatives, physical restraints, and removal of teeth. More effective treatments are needed. Pre-clinical studies have led to the hypothesis that D1-dopamine receptor antagonists may provide useful treatments for SIB in LND. Ecopipam is one such selective D1-dopamine receptor antagonist. This report summarizes results of a dose-escalation study of the safety and tolerability of ecopipam in 5 subjects with LND. The results suggest that ecopipam is well tolerated, with sedation being the most common dose-limiting event. Several exploratory measures also suggest ecopipam might reduce SIB in this population. These results support the hypothesis that D1-dopamine receptor antagonists may be useful for suppressing SIB in LND, and encourage further studies of efficacy.

Phencyclidine-induced disruption of oscillatory activity in prefrontal cortex: Effects of antipsychotic drugs and receptor ligands

The non-competitive NMDA receptor (NMDA-R) antagonist phencyclidine (PCP) markedly disrupts thalamocortical activity, increasing excitatory neuron discharge and reducing low frequency oscillations (LFO, <4Hz) that temporarily group neuronal discharge. These actions are mainly driven by PCP interaction with NMDA-R in GABAergic neurons of the thalamic reticular nucleus and likely underlie PCP psychotomimetic activity. Here we report that classical (haloperidol, chlorpromazine, perphenazine) and atypical (clozapine, olanzapine, quetiapine, risperidone, ziprasidone, aripripazole) antipsychotic drugs--but not the antidepressant citalopram--countered PCP-evoked fall of LFO in the medial prefrontal cortex (mPFC) of anesthetized rats. PCP reduces LFO by breaking the physiological balance between excitatory and inhibitory transmission. Next, we examined the role of different neurotransmitter receptors to reverse PCP actions. D2-R and D1-R blockade may account for classical antipsychotic action since raclopride and SCH-23390 partially reversed PCP effects. Atypical antipsychotic reversal may additionally involve 5-HT1A-R activation (but not 5-HT2A-R blockade) since 8-OH-DPAT and BAYx3702 (but not M100907) fully countered PCP effects. Blockade of histamine H1-R (pyrilamine) and α1-adrenoceptors (prazosin) was without effect. However, the enhancement of GABAA-R-mediated neurotransmission (using muscimol, diazepam or valproate) and the reduction of excitatory neurotransmission (using the mGluR2/3 agonist LY379268 and the preferential kainite/AMPA antagonist CNQX--but not the preferential AMPA/kainate antagonist NBQX) partially or totally countered PCP effects. Overall, these results shed new light on the neurobiological mechanisms used by antipsychotic drugs to reverse NMDA-R antagonist actions and suggest that agents restoring the physiological excitatory/inhibitory balance altered by PCP may be new targets in antipsychotic drug development.

Psychopharmacology of atypical antipsychotic drugs: From the receptor binding profile to neuroprotection and neurogenesis

The original definition of atypical antipsychotic drugs (APD) was drugs that are effective against positive symptoms in schizophrenia with no or little extrapyramidal symptoms (EPS). However, atypical APD have been reported to be more effective for cognitive dysfunction and negative symptoms in schizophrenia than typical APD, which expands the definition of 'atypicality'. This article provides a critical review of the pharmacology of atypical APD, especially from the viewpoint of receptor binding profiles and neurotransmitter regulations as well as neuroprotection and neurogenesis. A variety of serotonin (5-HT) receptors, such as 5-HT2A / 2C , 5-HT1A , 5-HT6 and 5-HT7 receptors, may contribute to the mechanisms of action of 'atypicality'. The dopaminergic modulations, including a low affinity for dopamine D2 receptors and a partial D2 receptor agonistic action, and glutamatergic regulations may also be involved in the pharmacological backgrounds of 'atypicality'. Atypical APD, but not typical APD, may facilitate cortical neuroprotection and hippocampal neurogenesis, which might be a part of the action mechanisms of atypical APD. The facilitation of cortical neuroprotection and hippocampal neurogenesis induced by atypical APD might be mediated by an increase in the Ser9 phosphorylation of glycogen synthase kinase-3β (GSK-3β). The stimulation of 5-HT1A receptors and/or the blockade of 5-HT2 receptors, which is characteristic of atypical APD, might increase Ser9 phosphorylation of GSK-3β. Moreover, atypical APD increase brain-derived neurotrophic factor (BDNF) levels. BDNF increases Ser9 phosphorylation of GSK-3β and has neuroprotective and neurogenic effects, as in the case of atypical APD. These findings suggest that GSK-3β might play a role in the action mechanisms of atypical APD, in both the 5-HT-dependent and BDNF-dependent mechanisms.

Pharmacology of hallucinations: several mechanisms for one single symptom?

Hallucinations are complex misperceptions, that principally occur in schizophrenia or after intoxication induced by three main classes of drugs: psychostimulants, psychedelics, and dissociative anesthetics. There are at least three different pharmacological ways to induce hallucinations: (1) activation of dopamine D2 receptors (D2Rs) with psychostimulants, (2) activation of serotonin 5HT2A receptors (HT2ARs) with psychedelics, and (3) blockage of glutamate NMDA receptors (NMDARs) with dissociative anesthetics. In schizophrenia, the relative importance of NMDAR and D2R in the occurrence of hallucinations is still debated. Slight clinical differences are observed for each etiology. Thus, we investigated whether the concept of hallucination is homogenous, both clinically and neurobiologically. A narrative review of the literature is proposed to synthesize how the main contributors in the field have approached and tried to solve these outstanding questions. While some authors prefer one explanatory mechanism, others have proposed more integrated theories based on the different pharmacological psychosis models. In this review, such theories are discussed and faced with the clinical data. In addition, the nosological aspects of hallucinations and psychosis are addressed. We suggest that if there may be common neurobiological pathways between the different pharmacological systems that are responsible for the hallucinations, there may also be unique properties of each system, which explains the clinical differences observed.

Schizophrenia: from dopaminergic to glutamatergic interventions

Schizophrenia might be considered a neurodevelopmental disease. However, the fundamental process(es) associated with this disease remain(s) uncertain. Many lines of evidence suggest that schizophrenia is associated with excessive stimulation of dopamine D2 receptors in the associative striatum, with a lack of stimulation of dopamine D1 receptors in prefrontal cortex, and with modifications in prefrontal neuronal connectivity involving glutamate transmission at N-methyl aspartate (NMDA) receptors. This article, whilst briefly discussing the current knowledge of the disease, mainly concentrates on the NMDA hypofunction hypothesis. However, there are also potential consequences for a Dopamine imbalance on NMDA function. Thus, it is proposed that schizophrenia has a complex aetiology associated with strongly interconnected aberrations of dopamine and glutamate transmission.

Pharmacological treatment of schizophrenia: a critical review of the pharmacology and clinical effects of current and future therapeutic agents

Since the introduction of chlorpromazine and throughout the development of the new-generation antipsychotic drugs (APDs) beginning with clozapine, the D(2) receptor has been the target for the development of APDs. Pharmacologic actions to reduce neurotransmission through the D(2) receptor have been the only proven therapeutic mechanism for psychoses. A number of novel non-D(2) mechanisms of action of APDs have been explored over the past 40 years but none has definitively been proven effective. At the same time, the effectiveness of treatments and range of outcomes for patients are far from satisfactory. The relative success of antipsychotics in treating positive symptoms is limited by the fact that a substantial number of patients are refractory to current medications and by their lack of efficacy for negative and cognitive symptoms, which often determine the level of functional impairment. In addition, while the newer antipsychotics produce fewer motor side effects, safety and tolerability concerns about weight gain and endocrinopathies have emerged. Consequently, there is an urgent need for more effective and better-tolerated antipsychotic agents, and to identify new molecular targets and develop mechanistically novel compounds that can address the various symptom dimensions of schizophrenia. In recent years, a variety of new experimental pharmacological approaches have emerged, including compounds acting on targets other than the dopamine D(2) receptor. However, there is still an ongoing debate as to whether drugs selective for singe molecular targets (that is, 'magic bullets') or drugs selectively non-selective for several molecular targets (that is, 'magic shotguns', 'multifunctional drugs' or 'intramolecular polypharmacy') will lead to more effective new medications for schizophrenia. In this context, current and future drug development strategies can be seen to fall into three categories: (1) refinement of precedented mechanisms of action to provide drugs of comparable or superior efficacy and side-effect profiles to existing APDs; (2) development of novel (and presumably non-D(2)) mechanism APDs; (3) development of compounds to be used as adjuncts to APDs to augment efficacy by targeting specific symptom dimensions of schizophrenia and particularly those not responsive to traditional APD treatment. In addition, efforts are being made to determine if the products of susceptibility genes in schizophrenia, identified by genetic linkage and association studies, may be viable targets for drug development. Finally, a focus on early detection and early intervention aimed at halting or reversing progressive pathophysiological processes in schizophrenia has gained great influence. This has encouraged future drug development and therapeutic strategies that are neuroprotective. This article provides an update and critical review of the pharmacology and clinical profiles of current APDs and drugs acting on novel targets with potential to be therapeutic agents in the future.

Dopamine D4 and D5 receptor gene variant effects on clozapine response in schizophrenia: replication and exploration

OBJECTIVES

This study aimed to: 1) replicate previously reported associations between dopamine D4 receptor gene (DRD4) polymorphisms and antipsychotic (AP) response in a clozapine (CLZ) response sample; and 2) explore possible associations of polymorphisms across dopamine D5 receptor gene (DRD5) as well as other DRD4 regions.

METHODS

DRD4 exon III 48-bp, intron I (G)(n), and 120-bp repeat polymorphisms, and three DRD4 single nucleotide polymorphisms (SNPs); and DRD5 (CA/CT/GT)(n) microsatellite and four DRD5 SNPs were assessed using standard genotyping and statistical procedures.

RESULTS

We report evidence, which does not survive correction for multiple testing, supporting previous DRD4 findings. Findings of interest include the 120-bp 1-copy allele, intron I (G)(n) 142-bp/140-bp genotype, and exon III 4R allele with CLZ response. All DRD5 tests were negative.

CONCLUSIONS

Overall, these results suggest a possible minor contribution of DRD4 variants, but not DRD5 variants, towards the AP/CLZ response phenotype.

Clozapine increases reward evaluation but not overall ingestive behaviour in rats licking for sucrose

RATIONALE

Clozapine and the "atypical" antipsychotics are less prone than neuroleptics to induce extrapyramidal motor effects, worsening of the negative symptoms of schizophrenia and dysphoria. This is paralleled by preclinical evidence showing reduced suppression of behaviours aimed at the pursuit of reward, with increased measures of reward efficacy. Serotonin 5-HT2 receptors seem to play a role in determining this profile.

OBJECTIVE

We investigated the effects of clozapine on the microstructure of ingestive behaviour, which might reveal behavioural dimensions, such as reward evaluation and behavioural activation, which might be relevant in explaining its atypical profile. Moreover, we investigated the possibility that coadministration of the typical antipsychotic haloperidol and the 5-HT2A/2C receptor antagonist ritanserin might mimic clozapine effects.

MATERIALS AND METHODS

The effects of clozapine (0.5, 1 and 5 mg/kg) and of the coadministration of haloperidol (0.05 mg/kg) and ritanserin (0.5 and 3 mg/kg) have been examined on the microstructure of licking for a 10% sucrose solution in rats.

RESULTS

Clozapine failed to affect whole ingestion as revealed by the lack of effect on lick number. However, it increased reward evaluation at the dose of 1 mg/kg, as revealed by increased mean bout size. Haloperidol resulted in a decreased bout size. Ritanserin failed to exert any effects either alone or when coadministered with haloperidol.

CONCLUSION

The ability of clozapine to increase reward evaluation might contribute to explain its atypical profile both in the clinical setting and in preclinical studies. These results suggest that 5-HT2A/2C receptors are not involved in the observed effect.

Mechanisms of action of antipsychotic drugs of different classes, refractoriness to therapeutic effects of classical neuroleptics, and individual variation in sensitivity to their actions: Part I

Many issues remain unresolved about antipsychotic drugs. Their therapeutic potency scales with affinity for dopamine D2 receptors, but there are indications that they act indirectly, with dopamine D1 receptors (and others) as possible ultimate targets. Classical neuroleptic drugs disinhibit striatal cholinergic interneurones and increase acetyl choline release. Their effects may then depend on stimulation of muscarinic receptors on principle striatal neurones (M4 receptors, with reduction of cAMP formation, for therapeutic effects; M1 receptors for motor side effects). Many psychotic patients do not benefit from neuroleptic drugs, or develop resistance to them during prolonged treatment, but respond well to clozapine. For patients who do respond, there is a wide (>ten-fold) range in optimal doses. Refractoriness or low sensitivity to antipsychotic effects (and other pathologies) could then arise from low density of cholinergic interneurones. Clozapine probably owes its special actions to direct stimulation of M4 receptors, a mechanism available when indirect action is lost.

In vitro findings of alterations in intracellular calcium homeostasis in schizophrenia

The pathogenesis of schizophrenia involves several complex cellular mechanisms and is not well understood. Recent research has demonstrated an association between primary disturbances characteristic of the disease, including altered dopaminergic and glutamatergic neurotransmission, and impairments in neuronal calcium (Ca(2+)) homeostasis and signaling. Emerging Ca(2+) hypothesis links and unifies various cellular processes involved in the pathogenesis of schizophrenia and suggests a central role of dysregulation of Ca(2+) homeostasis in the etiology of the disease. This review explores the in vitro data on Ca(2+) homeostasis and signaling in schizophrenia. Major limitation in this research is the lack of schizophrenia markers and validated disease models. As indicated in this review, one way to overcome these limitations may be analyses of Ca(2+) signalosomes in peripheral cells from schizophrenia patients. Validation of animal models of schizophrenia may permit the application of advanced Ca(2+) imaging techniques in living animals.

Dopamine on D2-like receptors "reboosts" dopamine D1-like receptor-mediated behavioural activation in rats licking for sucrose

BACKGROUND

The analysis of licking microstructure provides measures, such as duration and number of licking bouts, which might reveal the former an evaluation process and the latter an approach response. Dopamine D2-like receptor antagonists reduce the duration of licking bouts and mimic the effect of reducing sucrose concentration, while conflicting results are reported on the effects of dopamine D1-like receptor antagonists. The aim of this study is to examine the roles of dopamine D1-like and D2-like receptors in the activation of reward-associated responses and in reward evaluation, through the study of licking microstructure.

METHODS

The effects of the dopamine D2-like receptor antagonists raclopride (0.025-0.25 mg/kg), the D1-like antagonist SCH 23390 (0.01-0.04 mg/kg) and the antipsychotic drug haloperidol (0.02-0.05 mg/kg), have been examined on the microstructure of licking for a 10% sucrose solution in rats.

RESULTS

The results confirm that dopamine D2-like receptor antagonists reduce the duration of licking bouts and reveal that while SCH 23390 reduced licking exclusively by reducing bout number, raclopride produced on this measure an extinction mimicry effect similar to that observed in instrumental responding for different rewards.

DISCUSSION

These results are consistent with the hypothesis that the level of activation of the responses to the reward-associated cues depends on dopamine D1-like receptor stimulation, and is updated, or "reboosted", on the basis of a dopamine D2-like receptor-mediated evaluation process occurring during the consummatory transaction with the reward.

Enhanced latent inhibition in dopamine receptor-deficient mice is sex-specific for the D1 but not D2 receptor subtype: implications for antipsychotic drug action

Latent inhibition (LI) is reduced learning to a stimulus that has previously been experienced without consequence. It is an important model of abnormal allocation of salience to irrelevant information in patients with schizophrenia. In rodents LI is abolished by psychotomimetic drugs and in experimental conditions where LI is low in controls, its expression is enhanced by antipsychotic drugs with activity at dopamine (DA) receptors. It is however unclear what the independent contributions of DA receptor subtypes are to these effects. This study therefore examined LI in congenic DA D1 and D2 receptor knockout (D1 KO and D2 KO) mice. Conditioned suppression of drinking was used as the measure of learning in the LI procedure. Both male and female DA D2 KO mice showed clear enhancement of LI reproducing antipsychotic drug effects in the model. Unexpectedly, enhancement was also seen in D1 KO female mice but not in D1 KO male mice. This sex-specific pattern was not replicated in locomotor or motor coordination tasks nor in the effect of DA KOs on baseline learning in control groups indicating some specificity of the effect to LI. These data suggest that the dopaminergic mechanism underlying LI potentiation and possibly antipsychotic action may differ between the sexes, being mediated by D2 receptors in males but by both D1 and D2 receptors in females. These data suggest that the DA D1 receptor may prove an important target for understanding sex differences in the mechanisms of action of antipsychotic drugs and in the aetiology of aberrant salience allocation in schizophrenia.

Dopamine type-1 receptor binding in major depressive disorder assessed using positron emission tomography and [11C]NNC-112

The dopamine type-1 receptor has been implicated in major depressive disorder (MDD) by clinical and preclinical evidence from neuroimaging, post mortem, and behavioral studies. To date, however, selective in vivo assessment of D(1) receptors has been limited to the striatum in MDD samples manifesting anger attacks. We employed the PET radioligand, [(11)C]NNC-112, to selectively assess D(1) receptor binding in extrastriatal and striatal regions in a more generalized sample of MDD subjects. The [(11)C]NNC-112 nondisplaceable binding potential (BP(ND)) was assessed using PET in 18 unmedicated, currently depressed subjects with MDD and 19 healthy controls, and compared between groups using MRI-based region-of-interest analysis. The mean D(1) receptor BP(ND) was reduced (14%) in the left middle caudate of the MDD group relative to control group (p<0.05). Among the MDD subjects D(1) receptor BP(ND) in this region correlated negatively with illness duration (r=-0.53; p=0.02), and the left-to-right BP(ND) ratio correlated inversely with anhedonia ratings (r=-0.65, p=0.0040). The D(1) receptor BP(ND) was strongly lateralized in striatal regions (p<0.002 for main effects of hemisphere in accumbens area, putamen, and caudate). In post hoc analyses, a group-by-hemisphere-by-gender interaction was detected in the dorsal putamen, which was accounted for by a loss of the normal asymmetry in depressed women (F=7.33, p=0.01). These data extended a previous finding of decreased striatal D(1) receptor binding in an MDD sample manifesting anger attacks to a sample selected more generally according to MDD criteria. Our data also more specifically localized this abnormality in MDD to the left middle caudate, which is the target of afferent neural projections from the orbitofrontal and anterior cingulate cortices where neuropathological changes have been reported in MDD. Finally, D(1) receptor binding was asymmetrical across hemispheres in healthy humans, compatible with evidence that dopaminergic function in the striatum is lateralized during reward processing, voluntary movement, and self-stimulation behavior.

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.

The pipeline and future of drug development in schizophrenia

While the current antipsychotic medications have profoundly impacted the treatment of schizophrenia over the past 50 years, the newer atypical antipsychotics have not fulfilled initial expectations, and enormous challenges remain in long-term treatment of this debilitating disease. In particular, improved treatment of the negative symptoms and cognitive dysfunction in schizophrenia which greatly impact overall morbidity is needed. In this review we will briefly discuss the current pipeline of drugs for schizophrenia, outlining many of the strategies and targets currently under investigation for the development of new schizophrenia drugs. Many of these compounds have great potential as augmenting agents in the treatment of negative symptoms and cognition. In addition, we will highlight the importance of developing new paradigms for drug discovery in schizophrenia and call for an increased role of academic scientists in discovering and validating novel drug targets. Indeed, recent breakthroughs in genetic studies of schizophrenia are allowing for the development of hypothesis-driven approaches for discovering possible disease-modifying drugs for schizophrenia. Thus, this is an exciting and pivotal time for the development of truly novel approaches to drug development and treatment of complex disorders like schizophrenia.

Association study of four dopamine D1 receptor gene polymorphisms and clozapine treatment response

Dopamine D1 receptors (D1) in the prefrontal cortex have been implicated in the modulation of cognitive processes as well as both positive and negative symptoms of schizophrenia. Therefore pharmacologic agents with potent D1 effects such as clozapine may influence the symptoms of schizophrenia (SCZ). Genetic variation in the D1 receptor gene (DRD1) may help to explain some of the variability in patient response to antipsychotics (APs). This study investigates the effect of four single nucleotide polymorphisms (SNPs) in DRD1 on clozapine response in two distinct SCZ populations (Caucasian and African American) refractory or intolerant to conventional APs. This study included 183 Caucasian and 49 African American schizophrenics diagnosed using the Diagnostic and Statistical Manual of Mental Disorders (revised third or fourth edition). Genotyping was determined by 5'-exonuclease fluorescence assays. Within each population genotype, allele, allele +/- and haplotype frequencies were compared against dichotomous and quantitative measures of treatment response. Linkage disequilibrium analysis was also performed. In the Caucasian sample, no associations were observed for individual SNP tests. However, a rare three-marker haplotype predicted poor response. In the African American sample, the rs265976 variant and another three-marker haplotype were associated with cLozapine response. Although we did not find an association between the rs4532 SNP (-48 A/G, recognized by a DdeI restriction cut site) and cLozapine response as reported by Potkin et al. (2003), a trend in the same direction was observed as well. Our findings suggest that the rs4532 SNP may have a small effect if any. Further studies in larger, independent samples are required to validate these findings.

Neuroreceptor imaging studies in schizophrenia

The ability of SPECT and PET to image specific biomolecules in the living brain provides a unique tool for clinical researchers. It is therefore not surprising that the use of neuroreceptor-imaging techniques has become more widespread over the past decade. This article reviews the application of these techniques to the study of schizophrenia. The design of neuroreceptor-imaging studies performed in the field of schizophrenia research can be broadly divided into two categories: (1) studies of pathophysiology and (2) studies of pharmacology. The former examines neuroreceptor and neurotransmitter parameters in individuals with schizophrenia compared to control subjects in order to provide a better understanding of the disease process. Studies of pharmacology seek to elucidate the mechanism of action for the treatments utilized in schizophrenia. This review will consider both studies of pathophysiology and pharmacology, with a discussion of the application of these techniques to drug development.

A single 20 mg dose of dihydrexidine (DAR-0100), a full dopamine D1 agonist, is safe and tolerated in patients with schizophrenia

The potential of dopamine D(1) receptor agonists to have beneficial effects on cognitive function has been suggested by a body of preclinical evidence. We now report the use of dihydrexidine (DAR-0100), the first full D(1) agonist, in a pilot study assessing single low dose safety and tolerability in patients with schizophrenia. A within-subject cross-over design was used in 20 adults (18-65 years) with SCID-IV diagnosed schizophrenia. Subjects were outpatients with a moderate level of residual negative symptoms, and were on stable dosing of non-D(1)-blocking antipsychotic drugs. Following screening, subjects were hospitalized for 48 h, and at 0800 h each morning scanned on a 3 T MRI scanner for resting brain perfusion, followed by a Blood Oxygen Level Dependent (BOLD) fMRI scan during an N-Back working memory task. They then received 20 mg subcutaneously (SC) of dihydrexidine or placebo over 15 min, followed by 45 min of intermittent MRI scans of perfusion and BOLD activity during the working memory task. Blood was drawn for serum drug levels and subjects were evaluated for clinical and cognitive changes. The procedure was repeated using the opposite challenge 2 days later. Dihydrexidine was well tolerated with no serious adverse events although three subjects had mild dizziness and five subjects experienced nausea. There was no significant effect of drug on clinical interview ratings or delayed (afternoon) neuropsychological performance. No medication interactions were seen. Thus, a single subcutaneous dose of dihydrexidine is tolerated and safe in patients with schizophrenia and does not produce delayed clinical or neuropsychological improvements.

Mechanism of action of atypical antipsychotic drugs and the neurobiology of schizophrenia

Atypical antipsychotics have greatly enhanced the treatment of schizophrenia. The mechanisms underlying the effectiveness and adverse effects of these drugs are, to date, not sufficiently explained. This article summarises the hypothetical mechanisms of action of atypical antipsychotics with respect to the neurobiology of schizophrenia.When considering treatment models for schizophrenia, the role of dopamine receptor blockade and modulation remains dominant. The optimal occupancy of dopamine D(2) receptors seems to be crucial to balancing efficacy and adverse effects - transient D(2) receptor antagonism (such as that attained with, for example, quetiapine and clozapine) is sufficient to obtain an antipsychotic effect, while permanent D(2) receptor antagonism (as is caused by conventional antipsychotics) increases the risk of adverse effects such as extrapyramidal symptoms. Partial D(2) receptor agonism (induced by aripiprazole) offers the possibility of maintaining optimal blockade and function of D(2) receptors. Balancing presynaptic and postsynaptic D(2) receptor antagonism (e.g. induced by amisulpride) is another mechanism that can, through increased release of endogenous dopamine in the striatum, protect against excessive blockade of D(2) receptors. Serotonergic modulation is associated with a beneficial increase in striatal dopamine release. Effects on the negative and cognitive symptoms of schizophrenia relate to dopamine release in the prefrontal cortex; this can be modulated by combined D(2) and serotonin 5-HT(2A) receptor antagonism (e.g. by olanzapine and risperidone), partial D(2) receptor antagonism or the preferential blockade of inhibitory dopamine autoreceptors. In the context of the neurodevelopmental disconnection hypothesis of schizophrenia, atypical antipsychotics (in contrast to conventional antipsychotics) induce neuronal plasticity and synaptic remodelling, not only in the striatum but also in other brain areas such as the prefrontal cortex and hippocampus. This mechanism may normalise glutamatergic dysfunction and structural abnormalities and affect the core pathophysiological substrates for schizophrenia.

Under the curve: Critical issues for elucidating D1 receptor function in working memory

It has been postulated that spatial working memory operates optimally within a limited range of dopamine transmission and D1 dopamine receptor signaling in prefrontal cortex. Insufficiency in prefrontal dopamine, as in aging, and excessive transmission, as in acute stress, lead to impairments in working memory that can be ameliorated by D1 receptor agonist and antagonist treatment, respectively. Iontophoretic investigations of dopamine's influence on the cellular mechanisms of working memory have revealed that moderate D1 blockade can enhance memory fields in primate prefrontal pyramidal neurons while strong blockade abolishes them. The combined behavioral and physiological evidence indicates that there is a normal range of dopamine function in prefrontal cortex that can be described as an "inverted-U" relationship between dopamine transmission and the integrity of working memory. Both in vivo and in vitro studies have demonstrated a role for dopamine in promoting the excitability of prefrontal pyramidal cells and facilitating their N-methyl-d-aspartate inputs, while simultaneously restraining recurrent excitation and facilitating feedforward inhibition. This evidence indicates that there is a fine balance between the synergistic mechanisms of D1 modulation in working memory. Given the critical role of prefrontal function for cognition, it is not surprising that this balancing act is perturbed by both subtle genetic influences and environmental events. Further, there is evidence for an imbalance in these dopaminergic mechanisms in multiple neuropsychiatric disorders, particularly schizophrenia, and in related nonhuman primate models. Elucidating the orchestration of dopamine signaling in key nodes within prefrontal microcircuitry is therefore pivotal for understanding the influence of dopamine transmission on the dynamics of working memory. Here, we explore the hypothesis that the window of optimal dopamine signaling changes on a behavioral time-scale, dependent upon current cognitive demands and local neuronal activity as well as long-term alterations in signaling pathways and gene expression. If we look under the bell-shaped curve of prefrontal dopamine function, it is the relationship between neuromodulation and cognitive function that promises to bridge our knowledge between molecule and mind.

How antipsychotics work-from receptors to reality

How does a small molecule blocking a few receptors change a patients' passionately held paranoid belief that the FBI is out to get him? To address this central puzzle of antipsychotic action, we review a framework linking dopamine neurochemistry to psychosis, and then link this framework to the mechanism of action of antipsychotics. Normal dopamine transmission has a role in predicting novel rewards and in marking and responding to motivationally salient stimuli. Abnormal dopamine transmission alters these processes and results in an aberrant sense of novelty and inappropriate assignment of salience leading to the experience of psychosis. Antipsychotics improve psychosis by diminishing this abnormal transmission by blocking the dopamine D2/3 receptor (not D1 or D4), and although several brain regions may be involved, it is suggested that the ventral striatal regions (analog of the nucleus accumbens in animals) may have a particularly critical role. Contrary to popular belief, the antipsychotic effect is not delayed in its onset, but starts within the first few days. There is more improvement in the first 2 weeks, than in any subsequent 2-week period thereafter. However, a simple organic molecule cannot target the complex phenomenology of the individual psychotic experience. Antipsychotics diminish dopamine transmission and thereby dampen the salience of the pre-occupying symptoms. Therefore, in the initial stage of an antipsychotic response, the patients experience a detachment from symptoms, a relegation of the delusions and hallucinations to the back of their minds, rather than a complete erasure of the symptoms. Only with time, and only in some, via the mediation of new learning and plasticity, is there a complete resolution of symptoms. The implications of these findings for clinical care, animal models, future target discovery and drug development are discussed.

Targeting prefrontal cortical dopamine D1 and N-methyl-D-aspartate receptor interactions in schizophrenia treatment

The prefrontal cortex plays a principal role in higher cognition and particularly in the fast online manipulation of appropriate information to guide forthcoming behavior. Dysfunction of this process represents a main feature in the pathophysiology of schizophrenia. Both dopamine D1 and N-methyl-D-aspartate (NMDA) receptors in the prefrontal cortex play a critical role in synaptic plasticity, memory mechanisms, and cognition. Recent data have shown that D1 and NMDA receptors interact bidirectionally and may greatly influence the output of the prefrontal cortex. Hypofunction of these receptor systems in the prefrontal cortex is found in schizophrenia. This review attempts to summarize some of the latest findings on the cellular mechanisms that underlie D1-NMDA receptor interactions. These findings have provided potential therapeutic strategies that aim to functionally up-regulate D1 and/or NMDA receptor safely via selective activation of D1 receptors or coagonist activation of NMDA receptors through blockade of the glycine transporter-1.

Mechanism of action of antipsychotic drugs: from dopamine D(2) receptor antagonism to glutamate NMDA facilitation

BACKGROUND

The fundamental pathologic processes associated with schizophrenia remain uncertain.

OBJECTIVE

The goal of this article was to review imaging evidence suggesting that schizophrenia is associated with excessive stimulation of D(2) receptors, as well as imaging experiments supporting the hypothesis that this dysregulation might be secondary to N- methyl-d-aspartate (NMDA) dysfunction.

CONCLUSIONS

Recent imaging data support the association of schizophrenia with a dopamine endophenotype involving excessive subcortical dopamine function. Animal and imaging data are consistent with the idea that this abnormality might be secondary to a synaptic disconnectivity involving the prefrontal cortex, which is well modeled by NMDA antagonist administration. In turn, this dopamine dysregulation might worsen synaptic connectivity and NMDA function. Thus, both glutamate/dopamine and dopamine/glutamate interactions may be relevant to schizophrenia pathophysiology and treatment. A deficit in glutamate transmission may lead to the dopamine endophenotype associated with this illness, and dopamine alterations in turn might exacerbate glutamate transmission deficits. The view that NMDA alterations are primary and dopamine alterations are secondary is probably oversimplistic, as both sets of abnormalities reinforce each other. A consequence of this general model is that direct intervention to support NMDA function might be beneficial as an augmentation strategy for the treatment of schizophrenia. Thus, it is proposed that schizophrenia is associated with strongly interconnected abnormalities of glutamate and dopamine transmission: NMDA hypofunction in the prefrontal cortex and its connections might generate a pattern of dysregulation of dopamine systems that, in turn, further weakens NMDA-mediated connectivity and plasticity.

Mechanisms of action of second generation antipsychotic drugs in schizophrenia: insights from brain imaging studies

Multiple lines of evidence including recent imaging studies suggest that schizophrenia is associated with an imbalance of the dopaminergic system, entailing hyperstimulation of striatal dopamine (DA) D2 receptors and understimulation of cortical DA D1 receptors. This DA endophenotype presumably emerges from the background of a more general synaptic dysconnectivity, involving alterations in N-methyl-d-aspartate (NMDA) and glutamatergic (GLU) functions. Equally important is the fact that this DA dysregulation might further impair NMDA transmission. The first generation antipsychotic (FGA) drugs are characterized by high affinity to and generally high occupancy of D2 receptors. The efficacy of FGAs is limited by a high incidence of extrapyramidal side-effects (EPS). Second generation antipsychotic (SGA) drugs display reduced EPS liability and modest but clinically significant enhanced therapeutic efficacy. Compared to FGAs, the improved therapeutic action of SGAs probably derives from a more moderate D2 receptor blockade. We will review the effects of SGAs on other neurotransmitter systems and conclude by highlighting the importance of therapeutic strategies aimed at directly increasing prefrontal DA, D1 receptor transmission or NMDA transmission to enhance the therapeutic effect of moderate D2 receptor antagonism.

Treatments for schizophrenia: a critical review of pharmacology and mechanisms of action of antipsychotic drugs

The treatment of schizophrenia has evolved over the past half century primarily in the context of antipsychotic drug development. Although there has been significant progress resulting in the availability and use of numerous medications, these reflect three basic classes of medications (conventional (typical), atypical and dopamine partial agonist antipsychotics) all of which, despite working by varying mechanisms of actions, act principally on dopamine systems. Many of the second-generation (atypical and dopamine partial agonist) antipsychotics are believed to offer advantages over first-generation agents in the treatment for schizophrenia. However, the pharmacological properties that confer the different therapeutic effects of the new generation of antipsychotic drugs have remained elusive, and certain side effects can still impact patient health and quality of life. Moreover, the efficacy of antipsychotic drugs is limited prompting the clinical use of adjunctive pharmacy to augment the effects of treatment. In addition, the search for novel and nondopaminergic antipsychotic drugs has not been successful to date, though numerous development strategies continue to be pursued, guided by various pathophysiologic hypotheses. This article provides a brief review and critique of the current therapeutic armamentarium for treating schizophrenia and drug development strategies and theories of mechanisms of action of antipsychotics, and focuses on novel targets for therapeutic agents for future drug development.

Glutamate, dopamine, and schizophrenia: from pathophysiology to treatment

The fundamental pathological process(es) associated with schizophrenia remain(s) uncertain, but multiple lines of evidence suggest that this condition is associated with (1) excessive stimulation of striatal dopamine (DA) D2 receptors, (2) deficient stimulation of prefrontal DA D1 receptors and, (3) alterations in prefrontal connectivity involving glutamate (GLU) transmission at N-methyl-d-aspartate (NMDA) receptors. This chapter first briefly discusses the current knowledge status for these abnormalities, with emphasis on results derived from clinical molecular imaging studies. The evidence for hyperstimulation of striatal D2 receptors rests on strong pharmacological evidence and has recently received support from brain imaging studies. The hypothesis of deficient prefrontal cortex (PFC) D1 receptor stimulation is almost entirely derived from preclinical studies. Preliminary imaging data compatible with this hypothesis have recently emerged. The NMDA hypofunction hypothesis originates mainly from indirect pharmacological data. The interactions between DA and GLU systems relevant to schizophrenia are then reviewed. Animal and imaging data supporting the general model that the putative DA imbalance in schizophrenia (striatal excess and cortical deficiency) might be secondary to NMDA hypofunction in the PFC and its connections are presented. Equally important are the potential consequences of this DA imbalance for NMDA function in the striatum and the cortex, which are subsequently discussed. In conclusion, it is proposed that schizophrenia is associated with strongly interconnected abnormalities of GLU and DA transmission: NMDA hypofunction in the PFC and its connections might generate a pattern of dysregulation of DA systems that, in turn, further weakens NMDA-mediated connectivity and plasticity.

Prefrontal DA transmission at D1 receptors and the pathology of schizophrenia

The current view on the dopamine (DA) hypothesis of schizophrenia postulates a cortical/subcortical imbalance: subcortical mesolimbic DA projections might be hyperactive, resulting in hyperstimulation of D2 receptors and positive symptoms, whereas mesocortical DA projections to the prefrontal cortex might be hypoactive, resulting in hypostimulation of D1 receptors, negative symptoms, and cognitive impairment. Although the subcortical abnormalities are relatively well established now, the evidence for cortical hypodopaminergia is just emerging. This article will review current evidence for prefrontal hypodopaminergia in schizophrenia, with special emphasis on positron emission tomography (PET) studies measuring cortical D1 receptors in schizophrenia. The presentation of the clinical data will be introduced by a brief overview of the function of prefrontal DA systems, both at the cellular and cognitive level. The impact of antipsychotic drugs on prefrontal DA function will also be reviewed. We will conclude with the formulation of several models of altered prefrontal DA transmission at D1 receptors in schizophrenia.

New developments in the pharmacotherapy of schizophrenia

This review summarizes current key research strategies and the most prominently pursued new potential treatments for schizophrenia. First, new routes of administration for second generation antipsychotics are presented. These include rapidly dissolving tablets, drops and sirups as well as new intramuscular formulations. Newly available short acting and long acting (depot) antipsychotics complement oral antipsychotics so that the full spectrum of routes of administration is now available for second generation antipsychotics. Next to antipsychotic polypharmacy, in which two or more antipsychotics are combined, pharmacological add-on treatments, mainly with benzodiazepines, antidepressants and mood stabilizers enjoy increasing popularity. Most of this practice is driven by personal preferences, clinical experience and marketing rather than evidence based medicine. New pharmacological mechanisms currently utilized in advanced states of development include partial dopamine D2-receptor agonism, supplementation with glutamatergic agents, estrogen and omega-3-fatty acids. While the concept of partial D1-agonism has already led to the successful launch of a new antipsychotic, aripiprazole, the other attempts to improve therapeutic response in schizophrenia patients have so far provided equivocal results. It is argued that they may be helpful for certain subgroups or specific symptoms of schizophrenia patients. In conclusion, many exciting new pharmacological leads are currently pursued and this will very likely augment the options for treating patients with schizophrenia.

Effects of the selective dopamine D(1) antagonists NNC 01-0112 and SCH 39166 on latent inhibition in the rat

Dopamine D(1) receptor blockade does not appear to be a prerequisite for antipsychotic activity since many clinically effective antipsychotics have little or no affinity for this receptor subtype. Clozapine, however, which has minimal liability for extrapyramidal symptoms, possesses affinities of similar order for D(1) and D(2) receptors. In earlier animal models used to predict antipsychotic effect, selective D(1) antagonists have shown effects similar to standard antipsychotics with preferential D(2) or mixed D(1)/D(2) antagonism. We investigated the effects of haloperidol (0.1 mg/kg) and two selective D(1) antagonists, NNC 01-0112 (0.05, 0.1 and 0.2 mg/kg) and SCH 39166 (0.02, 0.2 and 2.0 mg/kg), on latent inhibition (LI) in rats. LI is a behavioural paradigm in which repeated nonreinforced preexposure to a stimulus retards subsequent associations to that stimulus. Disrupted LI has been suggested as a model for the attentional deficits in schizophrenia. Using preexposure to a flashing light stimulus, which subsequently served as a conditioned stimulus for suppression of water licking, we demonstrated a clear LI effect with haloperidol but with neither of the two D(1) antagonists. Since selective D(1) antagonists are not clinically effective, these results add further credibility for the relevance of LI as an animal model of psychosis.

Prefrontal dopamine D1 receptors and working memory in schizophrenia

Studies in nonhuman primates documented that appropriate stimulation of dopamine (DA) D1 receptors in the dorsolateral prefrontal cortex (DLPFC) is critical for working memory processing. The defective ability of patients with schizophrenia at working memory tasks is a core feature of this illness. It has been postulated that this impairment relates to a deficiency in mesocortical DA function. In this study, D1 receptor availability was measured with positron emission tomography and the selective D1 receptor antagonist [11C]NNC 112 in 16 patients with schizophrenia (seven drug-naive and nine drug-free patients) and 16 matched healthy controls. [11C]NNC 112 binding potential (BP) was significantly elevated in the DLPFC of patients with schizophrenia (1.63 +/- 0.39 ml/gm) compared with control subjects (1.27 +/- 0.44 ml/gm; p = 0.02). In patients with schizophrenia, increased DLPFC [11C]NNC 112 BP was a strong predictor of poor performance at the n-back task, a test of working memory. These findings confirm that alteration of DLPFC D1 receptor transmission is involved in working memory deficits presented by patients with schizophrenia. Increased D1 receptor availability observed in patients with schizophrenia might represent a compensatory (but ineffective) upregulation secondary to sustained deficiency in mesocortical DA function.

Selective up-regulation of dopamine D1 receptors in dendritic spines by NMDA receptor activation

Glutamate, by activating N-methyl-d-aspartate (NMDA) receptors, alters the balance between dopamine D1 and D2 receptor signaling, but the mechanism responsible for this effect has not been known. We report here, using immunocytochemistry of primary cultures of rat neostriatal neurons, that activation of NMDA receptors recruits D1 receptors from the interior of the cell to the plasma membrane while having no effect on the distribution of D2 receptors. The D1 receptors were concentrated in spines as shown by colocalization with phalloidin-labeled actin filaments. The effect of NMDA on D1 receptors was abolished by incubation of cells in calcium-free medium and was mimicked by the calcium ionophore ionomycin. Recruitment of D1 receptors from the interior of the cell to the membrane was confirmed by subcellular fractionation. The recruited D1 receptors were functional as demonstrated by an increase in dopamine-sensitive adenylyl cyclase activity in membranes derived from cells that had been pretreated with NMDA. These results provide evidence for regulated recruitment of a G protein-coupled receptor in neurons, provide a cell biological basis for the effect of NMDA on dopamine signaling, and reconcile the conflicting hyperdopaminergic and hypoglutamatergic hypotheses of schizophrenia.

Atypical Antipsychotics: Mechanism of Action

Background:

Al though the prin ci pal brain tar get that all an tipsy chotic drugs at tach to is the do pamine D2 re cep tor, tra di tional or typi cal an tipsy chot ics, by at tach ing to it, in duce ex tra py r a mi dal signs and symp toms (EPS). They also, by bind ing to the D2 re cep tor, ele vate se rum pro lac tin. Atypi cal a ntipsy chot ics given in dos ages within the clini cally ef fec tive range do not bring about these adverse clini cal ef fects. To un der stand how these drugs work, it is im por tant to ex am ine the atypical an tipsychot ics' mecha nism of ac tion and how it dif fers from that of the more typi cal drugs.

Method:

This re view analy zes the af fini ties, the oc cu pan cies, and the dis so cia tion time- course of vari ous an tipsy chot ics at do pa mine D2 re cep tors and at se ro tonin (5-HT) re cep tors, both in the test tube and in live pa tients.

Results:

Of the 31 an tipsy chot ics ex am ined, the older tra di tional an tipsy chot ics such as tri fluperazine, pi mozide, chlor pro maz ine, fluphe nazine, ha loperi dol, and flupen thixol bind more tightly than dopamine it self to the do pa mine D2 re cep tor, with dis so cia tion con stants that are lower than that for do pa mine. The newer, atypi cal an tipsy chot ics such as queti apine, re moxi pride, clo zapine, ol a nzap ine, sert in dole, zi pra si done, and amisul pride all bind more loosely than do pa mine to the do pamine D 2 re cep tor and have dis so cia tion con stants higher than that for do pa mine. These tight and loose bind ing data agree with the rates of an tipsy chotic dis so cia tion from the human- cloned D2 recep tor. For in stance, ra dio ac tive ha loperi dol, chlor pro maz ine, and ra clo pride all dis s o ci ate very slowly over a 30- minute time span, while ra dio ac tive queti apine, clo zap ine, re moxi pride, and amisul pride dis so ci ate rap idly, in less than 60 sec onds. These data also match clini cal brain-imaging find ings that show ha loperi dol re main ing con stantly bound to D2 in hu mans un der go ing 2 posi tron emis sion to mo gra phy (PET) scans 24 hours apart. Con versely, the oc cu pa tion of D2 by clo zap ine or queti apine has mostly dis ap peared af ter 24 hours.

Conclusion:

Atypi cals clini cally help pa tients by tran siently oc cu py ing D2 re cep tors and then rap idly dis so ci at ing to al low nor mal do pa mine neu ro trans mis sion. This keeps pro lac tin lev els nor mal, spares cog ni tion, and ob vi ates EPS. One the ory of atypi cal ity is that the newer drugs block 5-HT2A re ceptors at the same time as they block do pa mine re cep tors and that, some how, this serotonin- dopam i n e bal ance con fers atypi cal ity. This, how ever, is not borne out by the re sults. While 5-HT2A re cep tors are read ily blocked at low dos ages of most atypi cal an tipsy chotic drugs (with the im por tant ex ceptions of re moxi pride and amisul pride, nei ther of which is avail able for use in Can ada) the do s ages at which this hap pens are be low those needed to al le vi ate psy cho sis. In fact, the an tipsy chotic thresh old oc cu pancy of D 2 for an tipsy chotic ac tion re mains at about 65% for both typi cal and atypi cal an tipsychotic drugs, re gard less of whether 5-HT2A re cep tors are blocked or not. At the same time, the a ntipsy chotic thresh old oc cu pancy of D2 for elic it ing EPS re mains at about 80% for both typi cal and atypi cal an tipsy chot ics, re gard less of the oc cu pancy of 5-HT2A re cep tors.

Relevance:

The “fast- off-D 2” the ory, on the other hand, pre dicts which an tipsy chotic com pounds will or will not pro duce EPS and hy per pro lac ti ne mia and which com pounds pres ent a rela tiv ely low risk for tar dive dyski ne sia. This the ory also ex plains why L- dopa psy cho sis re sponds t o low atypi cal an tipsy chotic dos ages, and it sug gests vari ous in di vidu al ized treat ment strategies.

Aberrant behavioral effects of a dopamine D1 receptor antagonist and agonist in monkeys: evidence of uncharted dopamine D1 receptor actions

BACKGROUND

Basic research indicates a role for dopamine (DA) D1 antagonism in the treatment of schizophrenia. Clinical trials have not confirmed any role. Besides the defining second messenger (adenylyl cyclase [AC]), DA D1 receptors are linked to other effectors (e.g., phospholipase C [PLC]). Differing actions of DA D1 antagonists upon differing effectors could explain conflicting results between the lab/clinic.

METHODS

In a monkey model in which behavioral effects of DA D1 antagonists/agonists have been well characterized we examined: 1) SKF 83959, biochemically, a DA D1 antagonist, behaviorally a DA D1 agonist, and 2) SKF 83822, biochemically, a DA D1 agonist, which, unlike all previously tested DA D1 agonists, does not also stimulate PLC. SKF 83959 and SKF 83822 were given alone and combined with DA D1 and D2 agonists, antagonists, and dextroamphetamine (AMP).

RESULTS

SKF 83959 acted as a DA D1 agonist (induced oral dyskinesia given alone, counteracted DA D1 antagonist [NNC 756], induced dystonia, and did not inhibit AMP induced behaviors). SKF 83822, unlike previously studied DA D1 agonists, did not induce dyskinesia, but resulted in a state of extreme arousal and locomotor activation without stereotypy, effectively counteracted by NNC 756, but not by SKF 83959 nor raclopride (DA D2 antagonist).

CONCLUSIONS

It is hypothesized that: 1) dyskinesia is linked to PLC stimulation; 2) DA D1 agonism can play a role in the induction of psychosis, via a mechanism linked neither to AC nor PLC, and 3) DA D1 antagonists differ in antipsychotic potential, possibly via this unidentified mechanism.

The psychopharmacology-molecular biology interface: exploring the behavioural roles of dopamine receptor subtypes using targeted gene deletion ('knockout')

In the absence of selective agonists and antagonists able to discriminate between individual members of the D1-like and D2-like families of dopamine receptor subtypes, functional parcellation has remained problematic. 'Knockout' of these subtypes by targeted gene deletion offers a new approach to evaluating their roles in the regulation of behaviour. Like any new technique, 'knockout' has associated with it a number of methodological limitations that are now being addressed in a systematic manner. Studies on the phenotype of D1(A/1), D(1B/5), D2, D3 and D4 'knockouts' at the level of spontaneous and agonist/antagonist-induced behaviour are reviewed, in terms of methodological issues, neuronal implications and potential clinical relevance. Dopamine receptor subtype 'knockout' is a nascent technology that is now beginning to fulfil its potential. It is being complemented by more systematic phenotypic characterisation at the level of behaviour and additional, molecular biologically-based approaches.

Blunted brain metabolic response to ketamine in mice lacking D(1A) dopamine receptors

The interaction of glutamatergic and dopamine neurotransmission is thought to have relevance to both the pathophysiology and pharmacotherapy of schizophrenia. For example, subanesthetic doses of the N-methyl-D-aspartate receptor (NMDA-R) antagonist ketamine induce schizophrenia-like behavioral effects in humans and both behavioral and brain metabolic activation in rodents. Blockade of NMDA-R results in dopamine release, and antipsychotic drugs that block dopamine neurotransmission decrease NMDA-R antagonist-induced behavioral activation. The involvement of dopamine receptors in brain metabolic activation induced by ketamine is, however, unknown. The present study used D(1A) knockout mice to determine the role of dopamine D(1A) receptors in the effects of subanesthetic doses of ketamine on both behavioral responses and on alterations in regional [14C]2-deoxyglucose (2-DG) uptake. There was less ketamine-induced behavioral activation in D(1A) knockout mice than in wild-type mice. In wild-type mice, ketamine (30 mg/kg) induced dramatic increases in 2-DG uptake in limbic cortical regions, hippocampal formation, nucleus accumbens, basolateral amygdala, and caudal parts of the substantia nigra pars reticulata. D(1A) knockout mice exhibited blunted metabolic activation in response to ketamine in a neuroanatomically specific manner. The selective D(1) antagonist, SCH23390 (0.3 mg/kg), inhibited both ketamine-induced brain metabolic activation and behavioral responses in the wild-type mice, with a similar neuroanatomical specificity observed in the D(1A) knockout mice. Thus, the neuroanatomically selective role that D(1A) receptors play in ketamine-induced behavior and regional brain metabolic activation in mice provides a useful model for further studies of how the D(1A) receptor function may be altered in schizophrenia.

Change of dopamine receptor mRNA expression in lymphocyte of schizophrenic patients

BACKGROUND

Though the dysfunction of central dopaminergic system has been proposed, the etiology or pathogenesis of schizophrenia is still uncertain partly due to limited accessibility to dopamine receptor. The purpose of this study was to define whether or not the easily accessible dopamine receptors of peripheral lymphocytes can be the peripheral markers of schizophrenia.

RESULTS

44 drug-medicated schizophrenics for more than 3 years, 28 drug-free schizophrenics for more than 3 months, 15 drug-naïve schizophrenic patients, and 31 healthy persons were enrolled. Sequential reverse transcription and quantitative polymerase chain reaction of the mRNA were used to investigate the expression of D3 and D5 dopamine receptors in peripheral lymphocytes. The gene expression of dopamine receptors was compared in each group. After taking antipsychotics in drug-free and drug-naïve patients, the dopamine receptors of peripheral lymphocytes were sequentially studied 2nd week and 8th week after medication. In drug-free schizophrenics, D3 dopamine receptor mRNA expression of peripheral lymphocytes significantly increased compared to that of controls and drug-medicated schizophrenics, and D5 dopamine receptor mRNA expression increased compared to that of drug-medicated schizophrenics. After taking antipsychotics, mRNA of dopamine receptors peaked at 2nd week, after which it decreases but the level was above baseline one at 8th week. Drug-free and drug-naïve patients were divided into two groups according to dopamine receptor expression before medications, and the group of patients with increased dopamine receptor expression had more severe psychiatric symptoms.

CONCLUSIONS

These results reveal that the molecular biologically-determined dopamine receptors of peripheral lymphocytes are reactive, and that increased expression of dopamine receptor in peripheral lymphocyte has possible clinical significance for subgrouping of schizophrenis.

Acute administration of SCH23390 increases D(1) receptors on nonpyramidal neurons in rat mPFC

Atypical antipsychotic drugs (APDs) such as clozapine and olanzapine antagonize both D(1) and D(2) receptors; however, little is known regarding their pharmacologic effect on specific neuronal elements within the local circuitry of corticolimbic regions, such as medial prefrontal cortex (mPFC). To characterize the effect of short-term antagonism of the D(1) receptor a high-resolution autoradiographic technique was used to assess the density (B(max)) and affinity (K(d)) of this receptor on pyramidal cells (i.e., large neurons (LNs, >/=100 microm(2))), nonpyramidal cells (i.e., small neurons (SNs, <100 microm(2))) and in the surrounding neuropil (NPL) of layer VI in rat mPFC. Either normal saline or the selective D(1) antagonist SCH23390 (1.0 mg/kg/day) were administered for 48 h via Alzet osmotic pumps. Frozen sections were incubated in [(3)H]SCH23390 (1-8 nM) in the presence or absence of the competitive inhibitor SKF38393 (10 microM). A microscopic adaptation to Scatchard analysis revealed a significant increase (82%) in B(max) for neuronal cell bodies (P < 0.05), but not for neuropil of drug-treated animals. Further analysis indicated that the increase in B(max) was present on SNs (94%, P < 0.05), but not LNs in SCH23390-treated rats. In contrast, K(d) values for LNs, SNs, and NPL were not significantly altered by drug treatment. Since the vast majority of SNs are nonpyramidal in nature, short-term administration of a selective D(1) antagonist seems to be associated with a preferential upregulation of this receptor on interneurons. Overall, these results are consistent with the hypothesis that the mechanism of action of atypical antipsychotic medications involves changes in D(1) receptor activity associated with local circuit neurons in rat mPFC.