Is clozapine an (partial) agonist at both dopamine D1 and D2 receptors?

Clozapine Withdrawal-Induced Malignant Catatonia or Neuroleptic Malignant Syndrome: A Case Report and a Brief Review of the Literature

ABSTRACT

In our brief literature review, we discuss the changes in the concept of catatonia as well as its various types and symptoms. We also succinctly review the possible symptoms of clozapine withdrawal. In addition, we analyze the main features of the very few published cases of clozapine withdrawal-induced catatonia and the relationship between neuroleptic malignant syndrome and the malignant subtype of catatonia. Furthermore, we present the case of a 29-year-old male patient with schizophrenia in whom a malignant catatonic episode/neuroleptic malignant syndrome (with negativism, stupor, mutism, autonomic signs [eg, fever, hyperhidrosis], and elevated creatine kinase levels) began 5 days after the patient decided arbitrarily to cease his clozapine treatment. His catatonic symptoms quickly (ie, within a few days) resolved after the reinstitution of clozapine. Finally, we attempt to provide a theoretical explanation for the surprising finding in the literature that the withdrawal of clozapine, unlike the withdrawal of any other antipsychotics, may be associated with catatonia (frequently its malignant subtype). The take-home message of our case is that clinicians should bear in mind the risk of catatonia (especially the malignant subtype of it) after the prompt withdrawal of clozapine therapy.

Review of withdrawal catatonia: what does this reveal about clozapine?

Withdrawal symptoms are common upon discontinuation of psychiatric medications. Catatonia, a neuropsychiatric condition proposed to be associated with gamma-aminobutyric acid (GABA) hypoactivity due to its robust response to benzodiazepines, has been described as a withdrawal syndrome in case reports but is not a well-recognized phenomenon. The authors undertook a review of withdrawal catatonia with an aim to understand its presentation as well as the medications and psychoactive substances it is associated with. The review identified 55 cases of withdrawal catatonia, the majority of which occurred upon discontinuation of benzodiazepines (24 cases) and discontinuation of clozapine (20 cases). No other antipsychotic medications were identified as having been associated with the onset of a catatonic episode within 2 weeks following their discontinuation. Increasing GABA activity and resultant GABA receptor adaptations with prolonged use is postulated as a shared pharmacological mechanism between clozapine and benzodiazepines that underlie their association with withdrawal catatonia. The existing evidence for clozapine's activity on the GABA system is reviewed. The clinical presentations of benzodiazepine withdrawal catatonia and clozapine withdrawal catatonia appear to differ and reasons for this are explored. One reason is that benzodiazepines act directly on GABAA receptors as allosteric agonists, while clozapine has more complex and indirect interactions, primarily through effects on receptors located on GABA interneurons. Another possible reason for the difference in clinical presentation is that clozapine withdrawal catatonia may also involve receptor adaptations in non-GABA receptors such as dopamine and acetylcholine. The findings from our review have implications for the treatment of withdrawal catatonia, and treatment recommendations are provided. Further research understanding the uniqueness of clozapine withdrawal catatonia among antipsychotic medication may give some insight as to clozapine's differential mechanism of action.

Dopamine D1 receptor activation improves PCP-induced performance disruption in the 5C-CPT by reducing inappropriate responding

Attentional deficits contribute significantly to the functional disability of schizophrenia patients. The 5-choice continuous performance test (5C-CPT) measures attention in mice, rats, and humans, requiring the discrimination of trial types that either require a response or the inhibition of a response. The 5C-CPT, one version of human continuous performance tests (CPT), enables attentional testing in rodents in a manner consistent with humans. Augmenting the prefrontal cortical dopaminergic system has been proposed as a therapeutic target to attenuate the cognitive disturbances associated with schizophrenia. Using translational behavioural tasks in conjunction with inducing conditions relevant to schizophrenia pathophysiology enable the assessment of pro-attentive properties of compounds that augment dopaminergic activity. Here, using a repeated phencyclidine (PCP) treatment regimen and the 5C-CPT paradigm, we assess the pro-attentive properties of SKF 38393, a dopamine D1 receptor agonist, in rats. We show that repeated PCP treatment induces robust deficits in 5C-CPT performance indicative of impaired attention. Pre-treatment with SKF 38393 partially attenuates the PCP-induced deficits in 5C-CPT performance by reducing false alarm responding and increasing response accuracy. Impaired target detection was still evident in SKF 38393-treated rats however. Thus, augmentation of the dopamine D1 system improves PCP-induces deficits in 5C-CPT performance by selectively reducing aspects of inappropriate responding. These findings provide evidence to support the hypothesis that novel therapies targeting the dopamine D1 receptor system could improve aspects of attentional deficits in schizophrenia patients.

Dysconnection in schizophrenia: from abnormal synaptic plasticity to failures of self-monitoring

Over the last 2 decades, a large number of neurophysiological and neuroimaging studies of patients with schizophrenia have furnished in vivo evidence for dysconnectivity, ie, abnormal functional integration of brain processes. While the evidence for dysconnectivity in schizophrenia is strong, its etiology, pathophysiological mechanisms, and significance for clinical symptoms are unclear. First, dysconnectivity could result from aberrant wiring of connections during development, from aberrant synaptic plasticity, or from both. Second, it is not clear how schizophrenic symptoms can be understood mechanistically as a consequence of dysconnectivity. Third, if dysconnectivity is the primary pathophysiology, and not just an epiphenomenon, then it should provide a mechanistic explanation for known empirical facts about schizophrenia. This article addresses these 3 issues in the framework of the dysconnection hypothesis. This theory postulates that the core pathology in schizophrenia resides in aberrant N-methyl-D-aspartate receptor (NMDAR)-mediated synaptic plasticity due to abnormal regulation of NMDARs by neuromodulatory transmitters like dopamine, serotonin, or acetylcholine. We argue that this neurobiological mechanism can explain failures of self-monitoring, leading to a mechanistic explanation for first-rank symptoms as pathognomonic features of schizophrenia, and may provide a basis for future diagnostic classifications with physiologically defined patient subgroups. Finally, we test the explanatory power of our theory against a list of empirical facts about schizophrenia.

Activation of dopamine D2 receptors in the CNS inhibits sympathetic cutaneous vasomotor alerting responses (SCVARs), contributing to clozapine's SCVAR-inhibiting action

Sympathetic neural outflow to thermoregulatory cutaneous vascular beds is selectively activated when the individual is aroused, so that cutaneous blood flow is characterized by sudden alerting-related falls to near zero levels ("SCVARs", sympathetic cutaneous vasomotor alerting responses). Our previous work shows that clozapine, an atypical antipsychotic drug used in schizophrenia, profoundly inhibits SCVARs. Clozapine, conventionally assumed to have a dopamine D(2) receptor antagonist action, also increases baseline cutaneous blood flow and lowers body temperature. However dopamine D(2) receptor agonists lower temperature, suggesting that a dopamine D(2)agonist action might also reduce SCVARs. The present study determined whether a dopamine D(2)agonist action contributes to clozapine's SCVAR-inhibiting effect. SCVARs were measured in conscious rats with a Doppler ultrasonic flow probe chronically implanted around the base of the artery, with probe wires passing subcutaneously to a headpiece. Doppler signals were monitored via a flexible connection between the headpiece and a swivel device in the roof of the cage. Apomorphine (0.1-0.5 mg/kg), quinpirole (0.05-0.25 mg/kg) and 7-OH-DPAT (0.02-0.5 mg/kg) dose-dependently reduced SCVARs. Pre-treatment with the dopamine receptor antagonist spiperone (20 microg/kg) but not the D(1) antagonist SCH-23390 or the peripheral dopamine D(2) antagonist domperidone, abolished this effect. Spiperone pre-treatment reduced the SCVAR-inhibiting action of clozapine (0.06-1.0 mg/kg). Chlorpromazine (0.1-10 mg/kg) also dose-dependently inhibited SCVARs, but this effect was not reduced by pre-treatment with spiperone. Mechanisms underlying clozapine's SCVAR-inhibiting effect include dopamine D(2) receptor agonism, not dopamine D(2) receptor antagonism, calling into question the mechanism of the drug's therapeutic action in schizophrenia.

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.

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.

Characterisation of Gs activation by dopamine D1 receptors using an antibody capture assay: antagonist properties of clozapine

Herein, we examined the direct coupling of human dopamine D1 receptors to G(s) proteins using an antibody capture assay together with a detection technique employing scintillation proximity assay beads. Using a specific antibody, dopamine (DA) and the selective dopamine D1 receptor agonists, 6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SKF81297) and 3-allyl-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SKF82958), behaved as high-efficacy agonists ( approximately 100%) in stimulating guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]GTP gamma S) binding to G(s) in L-cells, whereas 2,3,4,5,-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine (SKF38393) displayed partial agonist properties (70%). The action of dopamine was specifically mediated by human dopamine D1 receptors inasmuch as the selective human dopamine D1 receptor antagonist, (R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-benzazepine-7-ol (SCH23390), blocked dopamine-induced [35S]GTP gamma S binding to G(s) with a pK(B) (9.29) close to its pK(i) (9.33). The antipsychotic agents, clozapine and haloperidol, displayed no intrinsic activity when tested alone and inhibited dopamine-stimulated G(s) activation with pK(B)'s of 6.7 and 7.3, respectively, values close to their pK(i) values at these sites. In conclusion, the use of an anti-G(s) protein immunoprecipitation assay coupled to scintillation proximity assays allows direct evaluation of the functional activity of dopamine D1 receptors ligands at the G protein level. Employing this novel technique, the typical and atypical antipsychotics, clozapine and haloperidol, respectively, both exhibited antagonist properties at dopamine D1 receptors.

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.

Affinity of cyamemazine, an anxiolytic antipsychotic drug, for human recombinant dopamine vs. serotonin receptor subtypes

Animal studies indicate that the anxiolytic properties of the antipsychotic agent cyamemazine may result from blockade of serotonin 5-HT(2C) receptors and to a lesser extent from blockade of serotonin 5-HT(3) receptors. Here, we used human recombinant receptors to determine the relative affinity of cyamemazine for serotonin and dopamine receptor subtypes. In addition, cyamemazine was tested in other brain receptor types and subtypes which are considered to mediate central nervous systems effects of drugs. Hence, cyamemazine affinity was determined in human recombinant receptors expressed in CHO cells (hD(2), hD(3), and hD(4.4) receptors, h5-HT(1A), h5-HT(2A), h5-HT(2C), and h5-HT(7), and hM(1), hM(2), hM(3), hM(4), and hM(5) receptors), L-cells (hD(1) receptor), and HEK-293 cells (h5-HT(3) receptors) or natively present in N1E-115 cells (5-HT(3) receptors) or in rat cerebral cortex (non-specific alpha(1)- and alpha(2)-adrenoceptors, GABA(A) and GABA(B) receptors, H(3) histamine receptors), and guinea-pig cerebellum (H(1) central and H(2) histamine receptors) membranes. Similarly to atypical antipsychotics, cyamemazine exhibited high affinity for: (i) h5-HT(2A) receptors (K(i)=1.5+/-0.7 nM, mean+/-SEM, N=3) and this was four times higher than for hD(2) receptors (K(i)=5.8+/-0.8 nM), (ii) h5-HT(2C) receptors (K(i)=11.8+/-2.2nM), and (iii) 5-HT(7) receptors (K(i)=22 nM). Conversely, cyamemazine exhibited very low affinity for h5-HT(3) receptors (K(i)=2.9+/-0.4 microM). In conclusion, similarly to atypical antipsychotic agents, cyamemazine, possesses high affinity for h5-HT(2A), h5-HT(2C), and h5-HT(7) receptors, a feature which can explain its low propensity to cause extrapyramidal adverse reactions in clinical practice. The high affinity for h5-HT(2C) receptors, but not for h5-HT(3) receptors, can account for the anxiolytic activity of cyamemazine in human subjects.

Antipsychotics and single-cell activity in the rat superior colliculus

Schizophrenic patients have problems with saccadic eye movements that can be characterized as a loss of control over the saccadic system. Preliminary clinical results suggest that antipsychotics can either disrupt or improve saccadic performance. The brain mechanism through which antipsychotics might affect saccades is the subject of study. The superior colliculus (SC) is crucially involved in the generation of saccades. Previous experimental studies showed that the substantia nigra reticulata (SNr), a structure with profound inhibitory influence on the SC, is differentially affected by classical and atypical antipsychotics. In this study, we investigated the potential effects of atypical antipsychotics (clozapine, olanzapine, and risperidone) and a classical antipsychotic (haloperidol) on the firing rate of SC cells in the rat. In anesthetized rats, we performed extracellular recordings on spontaneous active neurons in the intermediate and deep layers of the SC. After subcutaneous injection of an antipsychotic drug, changes in firing rate were compared with responses upon saline injection. Olanzapine (1.0 mg/kg), risperidone (0.3 and 1.0 mg/kg), and haloperidol (0.5 mg/kg) did not significantly alter cell activity, but clozapine (10.0 mg/kg) induced a short-lasting but significant decrease. Except for clozapine, the effects of antipsychotics on the SC were nonsignificant and therefore independent of the effects in the SNr. Our results support the notion that clozapine is different from the other atypical antipsychotics.

New (sulfonyloxy)piperazinyldibenzazepines as potential atypical antipsychotics: chemistry and pharmacological evaluation

A series of 2- or 8-trifluoromethylsulfonyloxy (TfO) and 2- or 8-methylsulfonyloxy (MsO) 11-piperazinyldibenzodiazepines, -oxazepines, and -thiazepines were synthesized and evaluated in pharmacological models for their potential clozapine-like properties. In receptor binding assays, the 2-TfO analogues (18a, GMC2-83; 24, GMC3-06; and previously reported GMC1-169, 9a) of the dibenzazepines have profiles comparable to that of clozapine, acting on a variety of CNS receptors except they lack M1 receptor affinity. Introduction of 2-TfO to clozapine leads to compound 9e (GMC61-39) which has a similar binding profile as that of clozapine including having M1 receptor affinity. Interestingly, the MsO analogues, as well as the 8-TfO analogues, have no or weak dopaminergic and serotonergic affinities, but all 8-sulfonyloxy analogues do have M1 affinities. In behavioral studies performed to indicate the potential antipsychotic efficacy and the propensity to induce EPS, 2-TfO analogues blocked effectively the apomorphine-induced climbing in mice in a dose-dependent manner with ED50 values (mg/kg) of 2.1 sc for 9a, 1.3 po for 18a, 2.6 sc for 24, and 8.2 sc for 9e. On the other hand, they showed a clear dose separation with regard to their ED50 values (mg/kg) for indicating catalepsy in rats (>44 sc for 9a, 28 po for 18a, 30 sc for 24, and >50 sc for 9e, respectively), thus implicating a more favorable therapeutic ratio (K/A, ED50 climbing/ED50 catalepsy) in comparison with typical neuroleptics such as haloperidol and isoclozapine. Furthermore, compound 18a was also demonstrated to be an orally potent DA antagonist with an ED50 value of 0.7 mg/kg po in the ex vivo L-DOPA accumulation model. The present study contributes to the SAR of 11-piperazinyldibenzazepines, and the 2-TfO analogues of 11-piperazinyldibenzazepines are promising candidates as clozapine-like atypical antipsychotics with low propensity to induce EPS.