Clozapine and some other antipsychotic drugs may preferentially block the same subset of GABA(A) receptors

Tricyclic antipsychotics and antidepressants can inhibit α5-containing GABAA receptors by two distinct mechanisms

Background and Purpose

Many psychotherapeutic drugs, including clozapine, display polypharmacology and act on GABA_A receptors. Patients with schizophrenia show alterations in function, structure and molecular composition of the hippocampus, and a recent study demonstrated aberrant levels of hippocampal α5 subunit‐containing GABA_A receptors. The purpose of this study is to investigate the effects of tricyclic compounds on α5 subunit‐containing receptor subtypes.

Experimental Approach

Functional studies of effects by seven antipsychotic and antidepressant medications were performed in several GABA_A receptor subtypes by two‐electrode voltage‐clamp electrophysiology using Xenopus laevis oocytes. Computational structural analysis was employed to design mutated constructs of the α5 subunit, probing a novel binding site. Radioligand displacement data complemented the functional and mutational findings.

Key Results

The antipsychotic drugs clozapine and chlorpromazine exerted functional inhibition on multiple GABA_A receptor subtypes, including those containing α5‐subunits. Based on a chlorpromazine binding site observed in a GABA‐gated bacterial homologue, we identified a novel site in α5 GABA_A receptor subunits and demonstrate differential usage of this and the orthosteric sites by these ligands.

Conclusion and Implications

Despite high molecular and functional similarities among the tested ligands, they reduce GABA currents by differential usage of allosteric and orthosteric sites. The chlorpromazine site we describe here is a new potential target for optimizing antipsychotic medications with beneficial polypharmacology. Further studies in defined subtypes are needed to substantiate mechanistic links between the therapeutic effects of clozapine and its action on certain GABA_A receptor subtypes.

Antipsychotic drug administration differentially affects [3H]muscimol and [3H]flunitrazepam GABA(A) receptor binding sites

Post-mortem studies of the human brain indicate that certain GABA(A) receptor subtypes may be differentially altered in schizophrenia. Increased binding to the total population of GABA(A) receptors using [3H]muscimol is observed in the post-mortem schizophrenic brain, yet a proportion of these receptors which bind benzodiazepines and are labelled with [3H]flunitrazepam, show decreased or unaltered expression. Data from animal studies suggest that antipsychotic drugs alter GABA(A) receptor expression in a subtype selective manner, but in the opposite direction to that observed in schizophrenia. To broaden our understanding of the effects of antipsychotic drugs on GABA(A) receptors, we examined the saturation binding maximum (B(max)) and binding affinity (K(D)) of [3H]muscimol and [3H]flunitrazepam in the prefrontal cortex (PFC), hippocampus and thalamus of male SD rats that received a sucrose solution containing either haloperidol (1.5 mg/kg), olanzapine (6.5 mg/kg) or no drug daily for up to 28 days using quantitative receptor autoradiography. [3H]Muscimol binding density was increased most prominently in the PFC after 7 days, with larger and more prolonged effects being induced by the atypical antipsychotic drug olanzapine in subcortical regions. While no changes were observed in [3H]muscimol binding in any region after 28 days of drug administration, [3H]flunitrazepam binding density (B(max)) was increased for both antipsychotic treatments in the PFC only. These findings confirm that the subset of GABA(A) receptors sensitive to benzodiazepines are regulated differently from other GABA(A) receptor subtypes following antipsychotic drug administration, in a time- and region-dependent manner.

Neuroleptic drugs in the human brain: clinical impact of persistence and region-specific distribution

After discontinuation of neuroleptic agents, their effects are still present for a long time. The exact underlaying mechanisms are still unclear. In two previous studies we measured the concentrations and region-specific distribution of haloperidol (Kornhuber et al. 1999) and levomepromazine (Kornhuber et al. 2006) in postmortem human brain tissues. The aim of the present paper is to compare the results of these two studies. Even after short-term treatment, haloperidol and levomepromazine concentrations reach high levels in human brain tissue. Haloperidol concentrations in brain tissue are 10-30 times higher than the optimum serum concentrations in the treatment of schizophrenia. The brain-to-blood concentration ratio of levomepromazine is about 10. The estimated elimination half-life of these drugs in brain tissue are 6.8 days (haloperidol), 7.9 days (levomepromazine) and 27.8 days for the metabolite desmethyl-levomepromazine, respectively. After two half-lives (about 2 weeks), a considerable amount of drug remains in brain tissue. Haloperidol concentrations appeared to be homogeneously distributed across different brain areas, whereas levomepromazine shows a region-specific distribution, with highest values in the basal ganglia. The persistence of neuroleptic drugs in the human brain might explain their prolonged effects and side effects. The region-specific distribution of levomepromazine may increase our understanding of both the preferential toxicity of neuroleptic drugs against basal ganglia structures and higher basal ganglia volumes in patients treated with neuroleptics.

Effects of subchronic clozapine treatment on long-term potentiation in rat prefrontal cortex

Several studies postulated an interaction of clozapine with N-methyl-D-aspartate (NMDA) receptor-mediated transmission. We previously showed that acute clozapine application on rat prefrontal cortex (PFC) slices increased NMDA receptor-dependent long-term potentiation (LTP) in the prelimbic (PL) area. The present study explores the effects of subchronic clozapine treatment on LTP in the same brain area. After 21 days of treatment (30 mg/kg per day, via drinking water), rats were sacrificed and slices from the PFC were prepared for electrophysiological investigations. To this end, extracellular field potentials in the layer II-V pathway were recorded. In contrast to our previous study with acute application on the slice, subchronic clozapine treatment attenuated LTP as compared to non-treated animals. We interpret these findings to suggest that prolonged treatment with clozapine might result in a compensatory response to the acute facilitating action of clozapine on LTP-mediating processes.

Lack of pro-convulsant activity of the antipsychotic alkaloid alstonine

Psychiatry co-morbidity with epilepsy is common and often requires the combined use of psychotropic and antiepileptic drugs (AEDs). For schizophrenic patients, the occurrence of seizures with atypical agents is highest among antipsychotics, although these agents are more effective in alleviating symptoms (including negative symptoms) and are associated with fewer extrapiramidal effects. The indol alkaloid alstonine is the major component of plants used by traditional Nigerian psychiatrists as anti-dementia drugs. The alkaloid presents an experimental profile very similar to the atypical antipsychotic clozapine. This study aimed to compare the pro-convulsant activity of these two antipsychotic compounds. Through repetitive administration over a 30-day period in a kindling paradigm, it is shown that, unlike clozapine, alstonine does not possess pro-convulsant activity. The data adds to previous suggestions that alstonine deserves to be scrutinized as a model for the development of newer antipsychotics.

Differential effects of long-term treatment with clozapine or haloperidol on GABAA receptor binding and GAD67 expression

One of the most consistent findings in postmortem studies of schizophrenia is increased GABAA receptor binding and reduced glutamic acid decarboxylase (GAD67) expression. Due to long-term antipsychotic treatment before death, these findings may reflect not only the consequences of schizophrenia but also medication effects. To differentiate between these options, we used an animal model and evaluated long-term effects of typical (haloperidol) and atypical (clozapine) antipsychotic drugs on the GABAergic system. A total of 33 adult male rats were treated in three cohorts over a period of 6 months. One cohort of 11 animals received clozapine (45 mg/kg/day), another one received haloperidol (1.5 mg/kg/day) and a third one received pH-adapted minimal concentrations of HCl in the drinking water. Receptor autoradiography of the GABAA receptor ([3H]-muscimol binding) and in situ hybridization in adjacent sections with 35S-labeled cRNA probes of the y-aminobutyric acid (GABA)-producing enzyme, GAD67, was performed. While haloperidol increased GABAA receptor binding in striatum and nucleus accumbens (NA), it suppressed GABAA receptor binding in temporal (TEMPC) and parietal (PARC) cortex. Clozapine induced GABAA receptor binding in infralimbic cortex (ILC) and similar like haloperidol in anterior cingulate cortex (ACC), two regions of the limbic cortex. In addition, either drug increased gene expression of GAD67. It is concluded that antipsychotic drugs differentially alter the GABAergic system, strongly suggesting that drug effects are partially responsible for the up-regulation of GABAA receptor binding in certain brain regions as observed in postmortem brains of schizophrenic patients. However, the reduced GAD67 expression seen in postmortem brains does not appear to reflect drug effects, since our animal model demonstrated increased gene expression.

Effects of clozapine, haloperidol and iloperidone on neurotransmission and synaptic plasticity in prefrontal cortex and their accumulation in brain tissue: an in vitro study

The mode of action of the antipsychotic drugs clozapine, haloperidol and iloperidone was investigated in layer V of prefrontal cortex slices using extracellular field potential, intracellular sharp-electrode as well as whole-cell voltage clamp recording techniques. Intracellular investigations on a broad range of concentrations revealed that the typical neuroleptic haloperidol at higher concentrations significantly depressed the excitatory postsynaptic component induced by electrical stimulation of layer II. This was not seen with the atypical neuroleptics clozapine and iloperidone. None of the three compounds had any effect on the resting membrane potential, spike amplitude or input resistance at relevant concentrations. Synaptic plasticity was assessed by means of extracellular field potential recordings. Clozapine significantly facilitated the potentiation of synaptic transmission, whereas haloperidol and iloperidone showed no effects. In line with its facilitating effect on synaptic plasticity, it could be demonstrated by whole-cell voltage clamp recordings that clozapine increased N-methyl-D-aspartic acid receptor-mediated excitatory postsynaptic currents in the majority of prefrontal cortical neurones. These investigations were made with neuroleptic drugs applied to the bath in the micromolar concentration range in order to approach clinical brain concentrations that are reached after administration of therapeutic doses. The drug concentrations reached in the slices after the experiments were assessed by means of high-pressure liquid chromatography coupled with mass-spectrometric detection. Surprisingly, drug accumulation in the in vitro preparation was of similar degree as reported in vivo. In conclusion, the typical neuroleptic haloperidol significantly depressed excitatory synaptic transmission in layer V neurones of the prefrontal cortex. In contrast, the two atypical neuroleptics iloperidone and clozapine revealed no depressing effects. This feature of the atypical neuroleptics might be beneficial since a hypofunctionality of this brain area is thought to be linked with the pathophysiology of schizophrenia. Additionally, clozapine facilitated long-term potentiation, which might be linked with the clinically observed beneficial effects on certain cognitive parameters. The clozapine-induced increase of N-methyl-D-aspartic acid receptor-mediated currents suggests that clozapine facilitates the induction of long-term potentiation. Furthermore, the present study points to the importance of considering the significant accumulation of neuroleptic drugs in in vitro studies.

Inhibition of skeletal muscle nicotinic receptors by the atypical antipsychotic clozapine

We have previously observed that certain atypical antipsychotic drugs reduce the amplitude and duration of miniature end-plate currents (EPCs) at the frog neuromuscular junction (Effects of atypical antipsychotics on vertebrate neuromuscular transmission, Nguyen, Q.-T., Yang, J., Miledi, R. Neuropharmacology 42, 2002, 670-676), therefore suggesting that these drugs act on nicotinic acetylcholine receptors. In this study we examined the effects of the atypical antipsychotic clozapine on nicotinic receptors of frog neuromuscular end-plates or in Xenopus oocytes expressing the alpha(1)beta(1)gamma delta mouse skeletal muscle nicotinic receptor. At neuromuscular junctions, postsynaptic currents were reduced by micromolar concentrations of clozapine. This compound also acted presynaptically by increasing the quantal content of EPCs of muscles without noticeably affecting paired-pulse facilitation. In oocytes, clozapine inhibited alpha(1)beta(1)gamma delta receptors with an IC(50) of 10 microM and a Hill coefficient of 1. Blockage of alpha(1)beta(1)gamma delta receptors by clozapine bears several hallmarks of open-channel blockers, including faster response decays, strong voltage dependence of the block, large rebound currents upon wash, and reduction of peak responses even at saturating concentrations of acetylcholine. However, clozapine increased the EC(50) for acetylcholine and its blocking effect was enhanced by preincubation. These results suggest that clozapine antagonizes muscle nicotinic receptors by blocking open channels, and possibly also by another mechanism which still remains to be investigated.

The effects of neuroleptics on the GABA-induced Cl- current in rat dorsal root ganglion neurons: differences between some neuroleptics

1. Several neuroleptics inhibited the 3 microM gamma-aminobutyric acid induced-chloride current (GABA-current) on dissociated rat dorsal root ganglion neurons in whole-cell patch-clamp investigations. 2. The IC(50) for clozapine, zotepine, olanzapine, risperidone and chlorpromazine were 6.95, 18.26, 20.30, 106.01 and 114.56 microM, respectively. The values for the inhibitory effects of neuroleptics on the GABA (3 microM)-current, which were calculated by the fitting Hill's equations where the concentrations represent the mean therapeutic blood concentrations, were ranked clozapine>zotepine>chlorpromazine>olanzapine>risperidone. These inhibitory effects, weighted with the therapeutic concentrations of neuroleptics, were correlated with the clinical incidences of seizure during treatment with neuroleptics. 3. Clozapine reduced the picrotoxin-inhibiton, and may compete with a ligand of the t-butylbicyclophosphorothionate (TBPS) binding site. 4. Haloperidol and quetiapine did not affect the peak amplitude of the GABA (3 microM)-current. However, haloperidol reduced the clozapine-inhibition, and may antagonize ligand binding to TBPS binding site. 5. Neuroleptics including haloperidol and quetiapine enhanced the desensitization of the GABA (3 microM)-current. However, haloperidol and quetiapine at 100 microM inhibited the desensitization at the beginning of application. 6. Blonanserin (AD-5423) at 30 and 50 microM potentiated the GABA (3 microM)-current to 170.1+/-6.9 and 192.0+/-10.6% of the control current, respectively. Blonanserin shifted GABA concentration-response curve leftward. Blonanserin only partly negatively interacted with diazepam. The blonanserin-potentiation was not reversed by flumazenil. Blonanserin is not a benzodiazepine receptor agonist. 7. The various effects of neuroleptics on the GABA-current may be related to the clinical effects including modifying the seizure threshold.

Clozapine inhibits synaptic transmission at GABAergic synapses established by ventral tegmental area neurones in culture

Elucidation of the mechanism of action of the atypical antipsychotic clozapine is complicated by the finding that this molecule interacts with multiple targets including dopaminergic and serotonergic receptors. Binding studies have suggested that clozapine also antagonises GABA(A) receptors, but physiological evidence for such a block at functional synapses is lacking. In this study, we explored this antagonism by using electrophysiological techniques on GABAergic neurones of the ventral tegmental area in culture. Inhibitory post-synaptic currents (IPSCs) evoked in isolated GABAergic neurones were found to be dose-dependently inhibited by clozapine. Compatible with a post-synaptic mechanism, we found that membrane currents evoked by exogenous applications of GABA were similarly dose-dependently inhibited by clozapine. An analysis of miniature inhibitory post-synaptic currents (mIPSCs) showed that clozapine reduced the amplitude of quantal events in a way similar to SR-95531, a specific GABA(A) receptor antagonist. Both drugs caused a similar leftward shift of the cumulative probability distribution of mIPSC amplitudes. This suggests that clozapine acts on both synaptic and extrasynaptic GABA(A) receptors. In conclusion, our work demonstrates that clozapine produces a functional antagonism of GABA(A) receptors at synapses. Because this effect occurs at concentrations that could be found in the brain of patients treated with clozapine, a reduction in GABAergic synaptic transmission could be implicated in the therapeutic actions and/or side-effects of clozapine.

Clozapine's antipsychotic effects do not depend on blockade of 5-HT3 receptors

Sixteen known 5-HT3 receptor blockers, including clozapine, fully or partially reverse the inhibitory effect of 1 microM GABA on [35S]TBPS binding, indicating that they are also GABA(A) antagonists, some of them selective for subsets of GABA(A) receptors. The 5-HT3 receptor blocker, ondansetron, has been reported to produce some antipsychotic and anxiolytic effects. However, no antipsychotic effects have been reported for a large number of highly potent 5-HT3 receptor blockers. Like clozapine, ondansetron partially reverses the inhibitory effect of GABA on [35S]TBPS binding. Additivity experiments suggest that ten 5-HT3 receptor blockers tested at low concentrations preferentially block subtypes of GABA(A) receptors that are among those blocked by clozapine. Wiley and Porter (29) reported that MDL-72222, the most potent GABA(A) antagonist described here, partially generalizes (71%) with clozapine in rats trained to discriminate an interoceptive clozapine stimulus, but only at a dose that severely decreases responding. Tropisetron (ICS-205,930) exhibits both GABA-positive and GABA-negative effects. R-(+)-zacopride is 6-fold more potent than S-(-)-zacopride as a GABA(A) antagonist. We conclude that the observed antipsychotic and, possibly, anxiolytic effects of some 5-HT3 receptor blockers are due to selective antagonism of certain GABA(A) receptors, and not to blockade of 5-HT3 receptors. We speculate that the anxiolytic and sedative effects of clozapine and several other antipsychotic drugs may be due to selective blockade of alpha1beta2gamma2 GABA(A) receptors which are preferentially located on certain types of GABAergic interneurons (probably parvalbumin positive). Blockade of these receptors will increase the inhibitory output of these interneurons. So far, no highly potent GABA(A) antagonists with clozapine-like selectivity have been identified. Such compounds may exhibit improved clozapine-like antipsychotic activity.

Clozapine and several other antipsychotic/antidepressant drugs preferentially block the same 'core' fraction of GABA(A) receptors

Clozapine and several other antipsychotic/antidepressant drugs that fully or partially block GABA(A) receptors were tested at concentrations that reversed the inhibitory effect of 1 microM GABA on 35S-t-butylbicyclophosphorothionate ([35S]TBPS) binding to rat forebrain membranes only about 20-30%, here designated "core" fractions. Clozapine at 10 microM reverses 1 microM GABA 25+/-4.0% (n = 23) (its "core" fraction). Fourty three compounds were tested alone, and pairwise together with 10 microM Clozapine. The "core" fractions of some of the compounds yielded significant additive reversals together with 10 microM Clozapine, while others did not. A group of 14 compounds of which 7 are clinically effective antipsychotic drugs, including Chlorprothixene, Clomacran, Clopipazan, Fluotracen, Sulforidazine, Thioproperazine, and cis-Thiothixene, were statistically non-additive with 10 microM Clozapine, suggesting that all of these drugs selectively block the same core population of GABA(A) receptors as Clozapine. These non-additivities also suggest that Clozapine at 10 microM fully saturates a subset of GABA(A) receptors blocked by 1 microM GABA. Therefore, Clozapine probably blocks 2 or more types of GABA(A)receptors, but only half of the receptors that are sensitive to 1 microM GABA. A second group of 12 compounds of which 6 are clinically active antidepressant/antipsychotic drugs including Amoxapine, Clothiapine, Dibenzepine, Inkasan (Metralindole), Metiapine and Zimelidine were slightly, but significantly, additive with Clozapine suggesting that these compounds block most of Clozapine's core fraction, plus a small additional fraction. A third group consisted of ten compounds that yielded larger (R > 80) and statistically highly significant additivities with Clozapine. Complete additivity was obtained with Bathophenanthroline disulfonate, and Isocarboxazid, suggesting that they block GABA(A) receptors other than those blocked by 10 microM Clozapine. Seven "classical" GABA(A) receptor blockers, also tested at concentrations yielding 21 to 33% reversal alone, were all significantly additive with 10 microM Clozapine, but in no case was the additivity complete. The largest additivity was obtained with Pitrazepine (21%) and the smallest with Tubocurarine (9%). These results provide further support for the notion that selective blockade of the same subset of GABA(A) receptors may contribute to the clinical antipsychotic/antidepressant effects of Clozapine. The deltaB(opt) values for Clozapine are 50+/-1.7% and 26+/-2.6% (n = 3) in whole rat forebrain and cerebellum, respectively, confirming that clozapine-sensitive GABA(A) receptors are unevenly distributed in the brain. The sedative and anxiolytic properties of Clozapine and other antipsychotic drugs may be due to selective blockade of GABergic disinhibition at certain interneurons.