Interaction of clozapine with the histamine H3 receptor in rat brain

Canonical and Non-Canonical Antipsychotics' Dopamine-Related Mechanisms of Present and Next Generation Molecules: A Systematic Review on Translational Highlights for Treatment Response and Treatment-Resistant Schizophrenia

Schizophrenia is a severe psychiatric illness affecting almost 25 million people worldwide and is conceptualized as a disorder of synaptic plasticity and brain connectivity. Antipsychotics are the primary pharmacological treatment after more than sixty years after their introduction in therapy. Two findings hold true for all presently available antipsychotics. First, all antipsychotics occupy the dopamine D2 receptor (D2R) as an antagonist or partial agonist, even if with different affinity; second, D2R occupancy is the necessary and probably the sufficient mechanism for antipsychotic effect despite the complexity of antipsychotics' receptor profile. D2R occupancy is followed by coincident or divergent intracellular mechanisms, implying the contribution of cAMP regulation, β-arrestin recruitment, and phospholipase A activation, to quote some of the mechanisms considered canonical. However, in recent years, novel mechanisms related to dopamine function beyond or together with D2R occupancy have emerged. Among these potentially non-canonical mechanisms, the role of Na²⁺ channels at the dopamine at the presynaptic site, dopamine transporter (DAT) involvement as the main regulator of dopamine concentration at synaptic clefts, and the putative role of antipsychotics as chaperones for intracellular D2R sequestration, should be included. These mechanisms expand the fundamental role of dopamine in schizophrenia therapy and may have relevance to considering putatively new strategies for treatment-resistant schizophrenia (TRS), an extremely severe condition epidemiologically relevant and affecting almost 30% of schizophrenia patients. Here, we performed a critical evaluation of the role of antipsychotics in synaptic plasticity, focusing on their canonical and non-canonical mechanisms of action relevant to the treatment of schizophrenia and their subsequent implication for the pathophysiology and potential therapy of TRS.

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

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

Histamine H3 receptor antagonists display antischizophrenic activities in rats treated with MK-801

BACKGROUND

Animal models based on N-methyl-d-aspartate receptor blockade have been extensively used for schizophrenia. Ketamine and MK-801 produce behaviors related to schizophrenia and exacerbated symptoms in patients with schizophrenia, which led to the use of PCP (phencyclidine)- and MK-801 (dizocilpine)-treated animals as models for schizophrenia.

METHODS

The study investigated the effect of subchronic dosing (once daily, 7 days) of histamine H3 receptor (H3R) antagonists, ciproxifan (CPX) (3 mg/kg, i.p.), and clobenpropit (CBP) (15 mg/kg, i.p.) on MK-801 (0.2 mg/kg, i.p.)-induced locomotor activity and also measured dopamine and histamine levels in rat's brain homogenates. The study also included clozapine (CLZ) (3.0 mg/kg, i.p.) and chlorpromazine (CPZ) (3.0 mg/kg, i.p.), the atypical and typical antipsychotic, respectively.

RESULTS

Atypical and typical antipsychotic was used to serve as clinically relevant reference agents to compare the effects of the H3R antagonists. MK-801 significantly increased horizontal locomotor activity, which was reduced with CPX and CBP. MK-801-induced locomotor hyperactivity attenuated by CPX and CBP was comparable to CLZ and CPZ. MK-801 raised striatal dopamine level, which was reduced in rats pretreated with CPX and CBP. CPZ also significantly lowered striatal dopamine levels, although the decrease was less robust compared to CLZ, CPX, and CBP. MK-801 increased histamine content although to a lesser degree. Subchronic treatment with CPX and CBP exhibited further increased histamine levels in the hypothalamus compared to MK-801 treatment alone. Histamine H3 receptor agonist, R-α methylhistamine (10 mg/kg, i.p.), counteracted the effect of CPX and CBP.

CONCLUSIONS

The present study shows the positive effects of CPX and CBP on MK-801-induced schizophrenia-like behaviors in rodents.

The Effect of Subchronic Dosing of Ciproxifan and Clobenpropit on Dopamine and Histamine Levels in Rats

The present study was designed to investigate the effect of once daily for 7-day (subchronic treatment) dosing of histamine H₃ receptor antagonists, ciproxifan (CPX) (3 mg/kg, i.p.), and clobenpropit (CBP) (15 mg/kg, i.p), including clozapine (CLZ) (3.0 mg/kg, i.p.) and chlorpromazine (CPZ) (3.0 mg/kg, i.p.), the atypical and typical antipsychotic, respectively, on MK-801(0.2 mg/kg, i.p.)-induced locomotor activity, and dopamine and histamine levels in rats. Dopamine and histamine levels were measured in striatum and hypothalamus, respectively, of rat brain. Atypical and typical antipsychotics were used to serve as clinically relevant reference agents to compare the effects of the H3 receptor antagonists. MK-801-induced increase of horizontal activity was reduced with CPX and CBP. The attenuation of MK-801-induced locomotor hyperactivity produced by CPX and CBP was comparable to CLZ and CPZ. MK-801 raised dopamine levels in the striatum, which was reduced in rats pretreated with CPX and CBP. CPZ also lowered striatal dopamine levels, though the decrease was less robust compared to CLZ, CPX and CBP. MK-801 increased histamine content although to a lesser degree. Subchronic treatment with CPX and CBP exhibited further increase in histamine levels in the hypothalamus compared to the MK-801 treatment alone. Histamine H₃ receptor agonist, R-α methylhistamine (10 mg/kg, i.p.) counteracted the effects of CPX and CBP. In conclusion, the subchronic dosing of CPX/CBP suggests some antipsychotic-like activities as CPX/CBP counteracts the modulatory effects of MK-801 on dopamine and histamine levels and prevents MK-801-induced hyperlocomotor behaviors.

Effects of NMDA receptor antagonists and antipsychotics on high frequency oscillations recorded in the nucleus accumbens of freely moving mice

RATIONALE

Abnormal oscillatory activity associated with N-methyl-D-aspartate (NMDA) receptor hypofunction is widely considered to contribute to the symptoms of schizophrenia.

OBJECTIVE

This study aims to characterise the changes produced by NMDA receptor antagonists and antipsychotics on accumbal high-frequency oscillations (HFO; 130-180 Hz) in mice.

METHODS

Local field potentials were recorded from the nucleus accumbens of freely moving mice.

RESULTS

Systemic injection of ketamine and MK801 both dose-dependently increased the power of HFO and produced small increases in HFO frequency. The atypical antipsychotic drug, clozapine, produced a robust dose-dependent reduction in the frequency of MK801-enhanced HFO, whilst haloperidol, a typical antipsychotic drug, had little effect. Stimulation of NMDA receptors (directly or through the glycine site) as well as activation of 5-HT1A receptors, reduced the frequency of MK801-enhanced HFO, but other receptors known to be targets for clozapine, namely 5-HT2A, 5-HT7 and histamine H3 receptors had no effect.

CONCLUSIONS

NMDA receptor antagonists and antipsychotics produce broadly similar fundamental effects on HFO, as reported previously for rats, but we did observe several notable differences. In mice, HFO at baseline were weak or not detectable unlike rats. Post-injection of NMDA receptor antagonists HFO was also weaker but significantly faster. Additionally, we found that atypical antipsychotic drugs may reduce the frequency of HFO by interacting with NMDA and/or 5-HT1A receptors.

A pharmacogenetic study of risperidone on histamine H3 receptor gene (HRH3) in Chinese Han schizophrenia patients

Evidence suggests that the human histamine H3 receptor (HRH3) may be involved in the pharmacodynamics of risperidone and influence clinical efficacy. More information on the pharmacogenetics of this receptor may therefore be useful in developing individualized therapy. However, to our knowledge, no study has been reported in this area. The aim of this investigation was to clarify whether H3 receptor polymorphism could affect risperidone efficacy. We genotyped tag single nucleotide polymorphisms (SNPs) of the HRH3 gene (rs3787429 and rs3787430) and analyzed their association with the reduction of Brief Psychiatric Rating Scale (BPRS) score in Chinese Han schizophrenia patients (N = 129), following an eight-week period of risperidone monotherapy. The confounding effects of non-genetic factors were estimated, and then the significant one was included as the covariate for adjustment in statistical analysis. Baseline symptom score was the only significant confounding effect and thus the covariate. After adjustment, significant association of HRH3 with antipsychotic efficacy was detected (for rs3787429, p = 0.013, 0.087 after 4 weeks and 8 weeks of treatment, respectively; for rs3787430, p = 0.024, 0.010 after 4 weeks and 8 weeks of treatment, respectively) and stood up to conservative Bonferroni correction. Our results demonstrate that polymorphism of the HRH3 gene may be a potential genetic marker for predicting the therapeutic effect of risperidone, and suggest novel pharmacological links between HRH3 and risperidone. Further studies with larger samples and different ethnic populations are warranted to confirm our results.

Altered histamine H3 receptor radioligand binding in post-mortem brain samples from subjects with psychiatric diseases

BACKGROUND AND PURPOSE

Histamine is a modulatory neurotransmitter in the brain. Auto- and hetero-histamine H3 receptors are present in human brain and are potential targets of antipsychotics. These receptors may also display disease-related abnormalities in psychiatric disorders. Here we have assessed how histamine H3 receptors in human brain may be affected in schizophrenia, bipolar disorder, major depression.

EXPERIMENTAL APPROACH

Histamine H3 receptor radioligand binding assays were applied to frozen post-mortem prefrontal and temporal cortical sections and anterior hippocampal sections from subjects with schizophrenia, bipolar disorder, major depression and matched controls.

KEY RESULTS

Compared with the controls, increased H3 receptor radioligand binding was found in dorsolateral prefrontal cortex of schizophrenic subjects (especially the ones who were treated with atypical antipsychotics), and bipolar subjects with psychotic symptoms. No differences in H3 receptor radioligand binding were found in the temporal cortex. In hippocampal formation of control subjects, H3 receptor radioligand binding was prominent in dentate gyrus, subiculum, entorhinal cortex and parasubiculum. Decreased H3 binding was found in the CA4 area of bipolar subjects. Decreased H3 binding in CA2 and presubiculum of medication-free bipolar subjects was also seen.

CONCLUSIONS AND IMPLICATIONS

The results suggest that histamine H3 receptors in the prefrontal cortex take part in the modulation of cognition, which is impaired in schizophrenic subjects and bipolar subjects with psychotic symptoms. Histamine H3 receptors probably regulate connections between hippocampus and various cortical and subcortical regions and could also be involved in the neuropathology of schizophrenia and bipolar disorder.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Antipsychotic-like profile of thioperamide, a selective H3-receptor antagonist in mice

Experimental and clinical evidence points to a role of central histaminergic system in the pathogenesis of schizophrenia. The present study was designed to study the effect of histamine H(3)-receptor ligands on neuroleptic-induced catalepsy, apomorphine-induced climbing behavior and amphetamine-induced locomotor activities in mice. Catalepsy was induced by haloperidol (2 mg/kg p.o.), while apomorphine (1.5 mg/kg s.c.) and amphetamine (2 mg/kg s.c.) were used for studying climbing behavior and locomotor activities, respectively. (R)-alpha-methylhistamine (RAMH) (5 microg i.c.v.) and thioperamide (THP) (15 mg/kg i.p.), per se did not cause catalepsy. Administration of THP (3.75, 7.5 and 15 mg/kg i.p.) 1 h prior to haloperidol resulted in a dose-dependent increase in the catalepsy times (P < 0.05). However, pretreatment with RAMH significantly reversed such an effect of THP (15 mg/kg i.p.). RAMH per se showed significant reduction in locomotor time, distance traveled and average speed but THP (15 mg/kg i.p.) per se had no effect on these parameters. On amphetamine-induced hyperactivity, THP (3.75 and 7.5 mg/kg i.p.) reduced locomotor time, distance traveled and average speed (P < 0.05). Pretreatment with RAMH (5 microg i.c.v.) could partially reverse such effects of THP (3.75 mg/kg i.p.). Climbing behavior induced by apomorphine was reduced in animals treated with THP. Such an effect was, however, reversed in presence of RAMH. THP exhibited an antipsychotic-like profile by potentiating haloperidol-induced catalepsy, reducing amphetamine-induced hyperactivity and reducing apomorphine-induced climbing in mice. Such effects of THP were reversed by RAMH indicating the involvement of histamine H(3)-receptors. Findings suggest a potential for H(3)-receptor antagonists in improving the refractory cases of schizophrenia.

The laminar histamine receptor system in human prefrontal cortex suggests multiple levels of histaminergic regulation

Human prefrontal cortex is essential for high brain functions and its activity is modulated by multiple neurotransmitters, including histamine. However, the histamine receptors in this brain area have not been systematically studied so far. In situ hybridization and receptor binding autoradiography were employed to map and quantify the mRNA expression and receptor binding of three of the four histamine receptors (H(1), H(2), H(3)). mRNA expression and receptor binding of these three histamine receptors displayed characteristic laminar distribution patterns. Both H(1) and H(3) receptor mRNAs were mainly expressed in the deeper layers (H(1) in laminae V and VI; H(3) in lamina V), where most of the corticothalamic projections originate, whereas H(2) receptor mRNA was primarily expressed in the superficial layer II. Receptor ligand binding of these three histamine receptors displayed relatively even distribution patterns throughout the gray matter. However, higher densities of H(1) and H(3) receptor radioligand binding sites were seen in the middle layers III and IV that receive abundant thalamic inputs and where some of the apical dendrites of the deep-layer pyramidal neurons terminate, whereas higher density of H(2) receptor radioligand binding sites was seen in the superficial layers I-III. The results, together with data on histaminergic regulation of thalamic oscillations suggest that histamine regulates both cortico-cortical and thalamo-cortical circuits. As histamine receptors are also abundant in thalamus, histamine may be involved also in human diseases of the thalamocortical system.

Molecular cloning and characterization of a novel type of histamine receptor preferentially expressed in leukocytes

Recently cDNA encoding the histamine H3 receptor was isolated after 15 years of considerable research. However, several studies have proposed heterogeneity of the H3 receptor. We report here the molecular cloning and characterization of a novel type of histamine receptor. A novel orphan G-protein-coupled receptor named GPRv53 was obtained through a search of the human genomic DNA data base and analyzed by rapid amplification of cDNA ends (RACE). GPRv53 possessed the features of biologic amine receptors and had the highest homology with H3 receptor among known G-protein-coupled receptors. Mammalian cells expressing GPRv53 were demonstrated to bind and respond to histamine in a concentration-dependent manner. In functional assays, not only an H3 receptor agonist, R-(alpha)-methylhistamine, but also a H3 receptor antagonist, clobenpropit, and a neuroleptic, clozapine, activated GPRv53-expressing cells. Tissue distribution analysis revealed that expression of GPRv53 is localized in the peripheral blood leukocytes, spleen, thymus, and colon, which was totally different from the H3 receptor, whose expression was restricted to the brain. The discovery of the GPRv53 receptor will open a new phase of research on the physiological role of histamine.

Non-imidazole histamine H3 ligands. Part I. Synthesis of 2-(1-piperazinyl)- and 2-(hexahydro-1H-1,4-diazepin-1-yl)benzothiazole derivatives as H3-antagonists with H1 blocking activities

New 2-(1-Piperazinyl)- and 2-(hexahydro-1H-1,4-diazepin-1-yl)benzothiazoles were prepared and tested as H1- and H3-receptor antagonists. A number of compounds showed weak H1-antagonistic activity, with pA2 values ranging from 5.5 to 6.1. The simple alkyl substituted, 2-[1-(4-methyl and 4-ethyl)piperazinyl] analogues show increasing, moderate H3-antagonistic activity (pA2 = 6.0, and pA2 = 7.0). The compounds with 4-phenylalkyl substitution, for both the piperazinyl and the hexahydro-1H-1,4-diazepin-1-yl homologues series, regardless of the different physicochemical properties of the para substituents at the phenyl ring, showed weak H3-antagonistic activity with pA2 values ranging from 4.4 to 5.6.

In vivo modulation of rat hypothalamic histamine release by the histamine H3 receptor ligands, immepip and clobenpropit. Effects of intrahypothalamic and peripheral application

We investigated the effect of the new potent and selective histamine H3 receptor agonist, immepip, and the histamine H3 receptor antagonist, clobenpropit, on in vivo neuronal histamine release from the anterior hypothalamic area of urethane-anesthetized rats, using microdialysis. Intrahypothalamic perfusion with immepip at concentrations of 1 and 10 nM reduced histamine release to 75% and 35% of its basal level, respectively. Peripheral injection of immepip (5 mg/kg) caused a sustained decrease in histamine release of 50%. Clobenpropit potently increased histamine release after intrahypothalamic perfusion. The maximal increase in histamine release was 2-fold, observed at a concentration of 10 nM clobenpropit. Peripheral injection of clobenpropit (5-15 mg/kg) increased histamine release to about 150% of the basal value. A more marked increase in histamine release was found after injection of the histamine H3 receptor antagonist, thioperamide (5 mg/kg). In conclusion, intrahypothalamic perfusion of the histamine H3 receptor agonist, immepip and the histamine H3 receptor antagonist, clobenpropit, potently and oppositely modulated in vivo histamine release from the anterior hypothalamic area. The decreased histamine release after peripheral injection of immepip indicates that this novel agonist readily crosses the blood-brain barrier, making it a potential candidate for in vivo histamine H3 receptor studies. The differential increase in histamine release after peripheral injection of clobenpropit and thioperamide is discussed.

In vivo electrophysiological investigations into the role of histamine in the dentate gyrus of the rat

Drugs acting at the three known classes of histamine receptors were injected intracerebroventricularly into the rat. The effects of these drugs upon synaptic potentials recorded from the dentate gyrus of the freely-moving rat were determined. Population spikes and field excitatory postsynaptic potentials were recorded from the granule cell layer of the dentate gyrus following stimulation of the perforant path. Drugs, dissolved in 0.9% NaCl were applied into the lateral cerebral ventricle in a volume of 5 microl over a period of 6 min. The histamine H1 receptor antagonist mepyramine (0.4 or 0.8 microg) had no significant effect on population spikes or field excitatory postsynaptic potentials. In contrast the H2 receptor antagonist cimetidine (3.25, 6.5 or 13 microg) showed a biphasic effect. At the lower doses (3.25 or 6.5 microg) a small (15%) depression of the field excitatory postsynaptic potentials and population spikes was observed beginning about 1 h following the infusion. At the highest dose tested (13 microg) a marked increase of the population spike was observed beginning immediately following the infusion and lasting for 90 min. Application of the H3 receptor agonist R-alpha-methylhistamine (0.2 microg) depressed the field excitatory postsynaptic potentials (15% at 4 h post-injection) and even more strongly the population spike (50%). Surprisingly, at higher doses (0.4 and 0.8 microg) no effect was seen. The H3 receptor antagonist thioperamide (0.41 and 0.82 microg) did not cause an increase in synaptic potentials but rather at the highest dose a small depression occurred at later time points (2-4 h following the infusion). At the lower dose (0.41 microg) thioperamide blocked the effect of R-alpha-methylhistamine (0.2 microg). These results show that the histaminergic system modulates information flow through the dentate gyrus in a complex manner involving both histamine H2 and H1 receptors.

Pharmacological characterisation of the histamine H3 receptor in the rat hippocampus

The purpose of this report was to pharmacologically characterise the histamine H3 in the rat hippocampus using radioligand binding studies with the H3 receptor antagonist [125I]iodophenpropit and the H3 receptor mediated inhibition of [3H]noradrenaline release. A dissociation constant of 0.33 nM and a maximal number of binding sites of 125 fmol/mg protein were found for [125I]iodophenpropit. Competition studies showed stereoselectivity for the (R) and (S) enantiomers of alpha-methylhistamine and 10 microM of GTPgammaS shifted the curve of (R)-alpha-methylhistamine rightwards. Up to 1 microM, (R)-alpha-methylhistamine displaced only 30% whereas the tested H3-antagonists displaced 50-60% of the total [125I]iodophenpropit bound. This indicates the presence of an additional non-H3 receptor binding site(s) for [125I]iodophenpropit in the rat hippocampus. This secondary site shows low affinity for H3 agonists, but high affinity for the tested H3 antagonists. Electrically evoked [3H]acetylcholine release was shown in slices of rat hippocampus. No H3 receptor modulation of [3H]acetylcholine release from hippocampal slices was detectable. However, H3 receptor activation inhibited 42% of the electrically-evoked [3H]noradrenaline release in rat hippocampal slices. The inhibition of [3H]noradrenaline release was effectively antagonized by the H3 antagonists thioperamide and burimamide. We describe the pharmacological identification of the histamine H3 receptor in the rat hippocampus and its similarities and differences from the cortical H3 receptor. These studies enable us to investigate changes in density and functionality of the hippocampal H3 receptor under (patho)physiological conditions.

The effect of haloperidol on the histaminergic neuron system in the rat brain

In this study, the effect of haloperidol on histamine (HA) levels, histidine decarboxylase (HDC) activities and the bindings of [3H]-(R)-alpha-methylhistamine ([3H]-(R)-alpha-MeHA) to histamine H3 receptors were investigated in the rat brain. Administration of 10 mg/kg of haloperidol decreased HA levels in the rat striatum and diencephalon, but increased HDC activities in rat striatum and diencephalon, although that of 5 mg/kg did not change them. Meanwhile, haloperidol inhibited the bindings of [3H]-(R)-alpha-MeHA to H3 receptor sites in the rat striatal membrane with a Ki value of 10.5 +/- 0.45 microM. These findings suggest that only a high dose of haloperidol increases HA synthesis and release as a histamine H3 receptor antagonist in the rat brain.

Brain penetration of the histamine H3 receptor antagonists thioperamide and clobenpropit in rat and mouse, determined with ex vivo [125I]iodophenpropit binding

We investigated the brain penetration of the histamine H3 receptor antagonists thioperamide and clobenpropit using ex vivo [125I]iodophenpropit binding. Homogenates of the rat cortex, striatum and mouse whole brain were prepared 1 h after subcutaneous injection of the H3 antagonists and incubated with [125I]iodophenpropit, a radiolabeled H3 receptor antagonist, to determine the H3 receptor occupancy. Specific [125I]iodophenpropit binding to the rat cortex and striatum was inhibited by thioperamide with IC30 values of 1.0 and 1.5 mg/kg, respectively. Clobenpropit also inhibited [125I]iodophenpropit binding, but was less potent (IC30: 18 and 19 mg/kg in the rat cortex and striatum, respectively) than thioperamide. Similar results were obtained in experiments with mouse whole brain (3.5 and 13 mg/kg for thioperamide and clobenpropit), indicating that there is no important species differences in the brain penetration of these drugs between rats and mice. These findings suggest that after peripheral injection both in rat and mouse thioperamide penetrates the blood-brain barrier more efficiently compared to clobenpropit.

Kindling with clozapine: behavioral and molecular consequences

Clozapine is an 'atypical' neuroleptic that improves symptoms of many patients with schizophrenia whose illness is resistant to treatment with other neuroleptics. Unlike the 'typical neuroleptics (chlorpromazine, haloperidol), clozapine does not induce extrapyramidal symptoms such as Parkinsonism and tardive dyskinesia in humans or catalepsy in the rat. However, clozapine frequently causes epileptiform EEG changes and causes seizures in 3-5% of patients treated with this drug in therapeutic doses. Clozapine also induces dose dependent myoclonus in the partially restrained rat. In the experiments reported here, partially restrained rats were administered repeated alternate day or weekly low, fixed doses of clozapine (1 mg/kg). This dose initially caused no behavioral change. Following the third and subsequent administrations, the same dose elicited an increasing number of myoclonic seizure-like jerks reaching 140/h following the 15th injection in rats receiving the same low dose of clozapine on alternate days and 160/h following the 9th injection in animals that received the same dose once weekly. These effects are consistent with kindling, i.e. a progressive increase of brain excitability following repeated administration of a fixed subconvulsive dose of an excitatory agent. Clozapine kindled animals exhibited a significantly different pattern of early gene expression in ventral tegmental area, origin of the mesolimbic-mesocortical dopamine system and in the anterior thalamic nuclei, compared with saline treated controls subjected to exactly the same recording conditions. The evidence of central nervous system excitation with clozapine may be important to the unique therapeutic effect of this atypical antipsychotic in the treatment of symptoms, especially the deficit symptoms, of schizophrenia.

[3H]-thioperamide as a radioligand for the histamine H3 receptor in rat cerebral cortex

1. The purpose of the present study was to characterize the binding of the histamine H3 receptor antagonist, [3H]-thioperamide, to rat cerebral cortical membranes. 2. The binding of [3H]-thioperamide to rat cerebral cortical membranes reached equilibrium after incubation with [3H]-thioperamide after 8-10 h at 4 degrees C. Equilibrium was maintained for up to 18 h of incubation. Addition of 1 microM (R)-alpha-methylhistamine rapidly dissociated [3H]-thioperamide from its binding sites. From these kinetic experiments a dissociation constant of 0.3 nM was obtained for [3H]-thioperamide. 3. Saturation experiments with [3H]-thioperamide using 1 microM (R)-alpha-methylhistamine to define nonspecific binding were best analysed according to a single site model. A dissociation constant (KD) of 0.80 +/- 0.06 nM (n = 3) and a maximal number of binding sites (Bmax) of 73 +/- 20 fmol mg-1 protein (n = 3) were obtained for the binding of [3H]-thioperamide to rat cerebral cortical membranes. 4. Saturation experiments with [3H]-thioperamide using 0.3 microM iodophenpropit to define nonspecific binding were best analysed according to a two site model. For the high affinity [3H]-thioperamide site a KD value of 1.1 +/- 0.3 nM (n = 3) and Bmax value of 162 +/- 108 fmol mg-1 protein (n = 3) were obtained whereas KD and Bmax values for the low affinity site were 96 +/- 19 nM and 4346 +/- 3092 fmol mg-1 protein (n = 3), respectively. 5. Using 5 nM [3H]-thioperamide, the binding was hardly displaced by H3 agonists within concentration-ranges expected to bind to the histamine H3 receptor. Under these conditions, [3H]-thioperamide binding was fully displaced by various H3-antagonists, yet most H3 antagonists showed Ki values different from those expected for the histamine H3 receptor. 6. Using 0.3 nM [3H]-thioperamide, 50-60% of the total binding was potently displaced by the H3 agonists histamine, (R)-alpha-methylhistamine, (S)-alpha-methylhistamine, imetit and immepip. Displacement of the binding of 0.3 nM [3H]-thioperamide binding exhibited clear stereoselectivity for the R and S isomers of alpha-methylhistamine. 7. Binding of 0.3 nM [3H]-thioperamide was completely displaced by several H3 antagonists (thioperamide, iodophenpropit, iodoproxyfan, and burimamide) and biphasic displacement curves were obtained; the Ki values for the high affinity site corresponded well with the expected values for the H3 receptor. Antagonists fully displaced the binding of 5 nM [3H]-thioperamide with affinities comparable to the low affinity site found with 0.3 nM [3H]-thioperamide. 8. Ondansetron and haloperidol did not displace binding of 5 nM [3H]-thioperamide at concentrations at which the former are known to bind to 5-HT3 or sigma receptors, respectively. On the other hand, nonselective cytochrome P450 inhibitors displaced the binding of 5 nM [3H]-thioperamide from both rat cerebral cortical membranes and rat liver microsomes. 9. It is concluded that the histamine H3 antagonist, [3H]-thioperamide, can be used as a radioligand to study the histamine H3 receptor in rat brain, provided that subnanomolar concentrations are used in displacement studies. Moreover, the specific binding should be defined with an H3 agonist, since most H3 antagonists share with [3H]-thioperamide a low affinity, high density, non-H3 receptor binding site(s) in rat brain. The latter is probably due to binding to cytochrome P450 isoenzymes.

Binding of clozapine metabolites and analogues to the histamine H3 receptor in rat brain cortex

Following up the finding that the non-imidazole drug clozapine shows a considerable histamine H3 receptor antagonistic activity, a series of analogues and metabolites (clozapine-N-oxide, and N-desmethylclozapine) were tested for their affinity towards the H3 receptor using the radiolabelled H3 antagonist [125I]-iodophenpropit. Qualitative structure affinity relationships are derived for the tested compounds. In the clozapine molecule four structurally different moieties may be considered. In comparison with the affinity for the H3 receptor shown by clozapine, the following main conclusions can be drawn: The 4-piperazinyl region does not allow substituents longer than a CH3 or electronegative atoms such as an O (as in clozapine-N-oxide); the lack of the CH3 group (as in N-desmethylclozapine) also reduces the affinity for H3 receptors. Substitutions at the 5-diazepine position do not drastically alter the affinity for the H3 receptor, although a basic nitrogen is favoured over CH2, O, or S. The 8 position in ring I is an important modulatory site for H3 affinity; electronegative substituents such as chloro and fluoro in this aromatic ring increase the affinity. When these substituents are, however, present at position X2 in the ring, they disable binding to the H3 receptor. The two major clozapine metabolites (clozapine-N-oxide, and N-desmethylclozapine) will not be responsible for a possible contribution of the H3 receptor antagonism to the clinical profile of clozapine.

The relationship between urine excretion and biogenic amines and their metabolites in cerebrospinal fluid of schizophrenic patients

Concentrations of norepinephrine and metabolites of biogenic amines were measured in lumbar cerebrospinal fluid of 30 patients with chronic schizophrenia, nine of whom were polyuric. The mean level of norepinephrine was two-fold higher (p < or = 0.025) in polyuric patients than in patients whose excretion of urine was within the normal range. CSF levels of histamine's primary metabolite, tele-methylhistamine, an index of brain histaminergic activity, were positively correlated (p < 0.005) with daily urine volume. These results are consistent with the known influence of norepinephrine and histamine on fluid regulation and suggest that norepinephrine and histamine may be involved in psychogenic polydipsia-polyuria in schizophrenic patients.

Mechanisms of action of atypical antipsychotic drugs: a critical analysis

Various criteria used to define atypical antipsychotic drugs include: 1) decrease, or absence, of the capacity to cause acute extrapyramidal motor side effects (acute EPSE) and tardive dyskinesia (TD); 2) increased therapeutic efficacy reflected by improvement in positive, negative, or cognitive symptoms; 3) and a decrease, or absence, of the capacity to increase prolactin levels. The pharmacologic basis of atypical antipsychotic drug activity has been the target of intensive study since the significance of clozapine was first appreciated. Three notions have been utilized conceptually to explain the distinction between atypical versus typical antipsychotic drugs: 1) dose-response separation between particular pharmacologic functions; 2) anatomic specificity of particular pharmacologic activities; 3) neurotransmitter receptor interactions and pharmacodynamics. These conceptual bases are not mutually exclusive, and the demonstration of limbic versus extrapyramidal motor functional selectivity is apparent within each arbitrary theoretical base. This review discusses salient distinctions predominantly between prototypic atypical and typical antipsychotic drugs such as clozapine and haloperidol, respectively. In addition, areas of common function between atypical and typical antipsychotic drug action may also be crucial to our identification of pathophysiological foci of the different dimensions of schizophrenia, including positive symptoms, negative symptoms, and neurocognitive deficits.

The moderate affinity of clozapine at H3 receptors is not shared by its two major metabolites and by structurally related and unrelated atypical neuroleptics

We determined the affinity and/or potency of two metabolites of clozapine (clozapine-N-oxide and N-desmethylclozapine) and of five atypical neuroleptics, chemically related (olanzapine) or unrelated to clozapine (remoxipride, risperidone, thioridazine, zotepine), at H3 receptors. The specific binding of 3H-N alpha-methylhistamine to rat brain cortex homogenates was inhibited by the seven compounds; the pKi values were: N-desmethylclozapine (5.33); clozapine-N-oxide (4.18); olanzapine (5.45); thioridazine (4.91); zotepine (4.75); remoxipride (4.51) and risperidone (4.43). Three compounds were examined in a functional H3 receptor model as well. The electrically evoked tritium overflow from superfused mouse brain cortex slices, which represents quasi-physiological noradrenaline release, was not affected by N-desmethylclozapine (3.2 and 10 microM), clozapine-N-oxide (3.2-100 microM) and olanzapine (3.2-32 microM). On the other hand, the three compounds shifted to the right the concentration-response curve of histamine for its inhibitory effect on the evoked overflow; the apparent pA2 values were 5.84, 4.21 and 5.80, respectively. The present study shows that five atypical neuroleptics of different chemical classes and the two major metabolites of clozapine possess a lower affinity and/or antagonistic potency at H3 receptors than clozapine itself (pKi 6.15, pA2 6.33; Kathmann M, Schlicker E, Göthert M (1994). Psychopharmacology 116: 464-468).

5-HT1A receptors are not involved in clozapine's lack of cataleptogenic potential

Clozapine and 8-OH-DPAT antagonized haloperidol-induced catalepsy in rats (ED50 10 and 0.1 mg/kg s.c., respectively). Whereas the selective 5-HT1A receptor antagonist WAY 100635 (0.1 mg/kg s.c.) completely antagonized the inhibitory effect of 8-OH-DPAT, clozapine's effect was not affected. On the other hand, clozapine and 8-OH-DPAT inhibited ultrasonic vocalization in rats (ED50 0.7 and 0.03 mg/kg s.c., respectively), which effects were antagonized by WAY 100635. The lack of catalepsy of clozapine, therefore, cannot be addressed primarily to clozapine's agonistic activity at 5-HT1A receptors.