Effect of typical and atypical antipsychotic drugs on 5-HT2 receptor density in rat cerebral cortex

Presynaptic 5-HT2A-mGlu2/3 Receptor–Receptor Crosstalk in the Prefrontal Cortex: Metamodulation of Glutamate Exocytosis

The glutamatergic nerve endings of a rat prefrontal cortex (PFc) possess presynaptic 5-HT_2A heteroreceptors and mGlu2/3 autoreceptors, whose activation inhibits glutamate exocytosis, and is measured as 15 mM KCl-evoked [³H]D-aspartate ([³H]D-asp) release (which mimics glutamate exocytosis). The concomitant activation of the two receptors nulls their inhibitory activities, whereas blockade of the 5-HT_2A heteroreceptors with MDL11,939 (1 μM) strengthens the inhibitory effect elicited by the mGlu2/3 receptor agonist LY329268 (1 μM). 5-HT_2A receptor antagonists (MDL11,939; ketanserin; trazodone) amplify the impact of low (3 nM) LY379268. Clozapine (0.1–10 μM) mimics the 5-HT_2A agonist (±) DOI and inhibits the KCl-evoked [³H]D-asp overflow in a MDL11,939-dependent fashion, but does not modify the (±) DOI-induced effect. mGlu2 and 5-HT_2A proteins do not co-immunoprecipitate from synaptosomal lysates, nor does the incubation of PFc synaptosomes with MDL11,939 (1 μM) or clozapine (10 µM) modify the insertion of mGlu2 subunits in synaptosomal plasma membranes. In conclusion, 5-HT_2A and mGlu2/3 receptors colocalize, but do not physically associate, in PFc glutamatergic terminals, where they functionally interact in an antagonist-like fashion to control glutamate exocytosis. The mGlu2/3-5-HT_2A metamodulation could be relevant to therapy for central neuropsychiatric disorders, including schizophrenia, but also unveil cellular events accounting for their development, which also influence the responsiveness to drugs regimens.

Clozapine, nimodipine and endosulfan differentially suppress behavioral defects caused by gain-of-function mutations in a two-pore domain K+ channel (UNC-58)

Two-pore domain K⁺ channels (K2Ps) regulate the resting membrane potential in excitable cells and determine ease of depolarization. Gain-of-function (gf) mutations in one of these channels (unc-58) in C. elegans switch it to a Na⁺ conductance channel and cause tremors, paralysis and other defects. We hypothesized that it should be possible to identify drugs that corrected these defects in unc-58(gf) mutant animals by blocking or modulating the over-active channels. We examined dispersal of animals on food because the absence of effective forward locomotion is the most obvious defect. In addition, we quantified egg release over 24 h. Starting with a known inhibitor of mammalian K2Ps and directed structure-based screening, we evaluated numerous drugs in these assays. Loratadine, which inhibits human KCNK18, significantly improved movement as did methiothepin. We confirmed that endosulfan, a GABA-A receptor antagonist, corrected locomotion in the unc-58(gf) strains. Based on structural similarities to other hits, we found that clozapine, loxapine and amoxapine potently suppressed abnormal phenotypes. Curiously, nimodipine, a Ca⁺⁺-channel blocker, dramatically improved movement and egg laying in unc-58(e665), but not unc-58(n495) animals. Molecular modeling provided initial insights into a possible basis for this difference based on the location of the e665 and n495 mutations. This research may lead to identification of novel K2P modulators and potential leads for drug discovery.

Development and Validation of a GC-MS Method for the Detection and Quantification of Clotiapine in Blood and Urine Specimens and Application to a Postmortem Case

Introduction. Clotiapine is an atypical antipsychotic of the dibenzothiazepine class introduced in a few European countries since 1970, efficient in treatment-resistant schizophrenic patients. There is little published data on the therapeutic and toxic concentrations of this drug. Aims. The aim of the present study is the development and validation of a method that allows the detection and quantification of clotiapine in blood and urine specimens by gas chromatography-mass spectrometry (GC-MS). Methods. Validation was performed working on spiked postmortem blood and urine samples. Samples were extracted with liquid-liquid extraction (LLE) technique at pH 8.5 with n-hexane/dichloromethane (85/15 v/v) and analysis was followed by GC-MS. Methadone-d9 was used as internal standard. Results. The limit of detection (LOD) was 1.2 and 1.3 ng/mL for urine and blood, respectively, while the lower limit of quantification (LLOQ) was 3.9 and 4.3 ng/mL, respectively. Linearity, precision, selectivity, accuracy, and recovery were also determined. The method was applied to a postmortem case. The blood and urine clotiapine concentrations were 1.32 and 0.49 μg/mL, respectively. Conclusions. A reliable GC-MS method for the detection and quantification of clotiapine in blood and urine samples has been developed and fully validated and then applied to a postmortem case.

Clozapine acts as an agonist at serotonin 2A receptors to counter MK-801-induced behaviors through a βarrestin2-independent activation of Akt

The G protein-coupled serotonin 2A receptor (5-HT2AR) is a prominent target for atypical antipsychotic drugs, such as clozapine. Although clozapine is known to inhibit 5-HT2AR signaling through G protein-dependent mechanisms, it differs from classic GPCR antagonists, in that it also induces 5-HT2AR internalization and activates Akt signaling via a 5-HT2AR-mediated event. In this regard, clozapine may also be considered a functionally selective agonist. The cognate neurotransmitter at the 5-HT2AR, serotonin, also induces 5-HT2AR internalization and Akt phosphorylation. Serotonin promotes interactions with the scaffolding and regulatory protein, βarrestin2, which results in the recruitment and activation of Akt. These interactions prove to be critical for serotonin-induced, 5-HT2AR-mediated behavioral responses in mice. Herein, we sought to determine whether clozapine also utilizes βarrestin2-mediated mechanisms to induce 5-HT2AR signaling, and whether this interaction contributes to its behavioral effects in mice. We demonstrate that unlike serotonin, clozapine-mediated 5-HT2AR internalization and Akt phosphorylation is independent of receptor interactions with βarrestin2. Moreover, clozapine-mediated suppression of MK-801 and phencyclidine (PCP)-induced hyperlocomotion is βarrestin2 independent, although it is dependent upon Akt. These results demonstrate that pharmacologically oppositional ligands, serotonin and clozapine, utilize differential mechanisms to achieve the same 5-HT2AR-meadiated downstream events: Akt phosphorylation and receptor internalization. Although βarrestin2 has no effect on clozapine's actions in vivo, Akt phosphorylation is required for clozapine's efficacy in blocking MK-801- and PCP-induced models of schizophrenic behaviors in mice.

Yokukansan and its ingredients as possible treatment options for schizophrenia

Schizophrenia is a debilitating psychotic mental disorder that affects almost the entire range of human mental function. The devastating effect of the illness is usually long-lasting and requires lifelong treatment. Despite an evolved psychopharmacological understanding, the overall therapeutic effect of antipsychotics is still not satisfactory. The choice of proper medication presents a clinical dilemma between efficacy and safety. As a result, searching for comparable treatment options with safer profiles is very important. Yokukansan (TJ-54), also called yi-gan san in Chinese, is a traditional herbal medicine with evident therapeutic effect for neuropsychiatric disorders. There are several open-label clinical studies upholding the possibility of using yokukansan to treat schizophrenia or schizophrenia-like psychosis. Evidence from animal studies and neurobiology also sheds light on the antipsychotic implications of yokukansan and its ingredients. Nevertheless, correlations between the experimental environment and clinical settings may be complicated by a number of confounders. Clinical trials with more sophisticated designs are required to fill the gap between the experimental environment and clinical settings.

Serotonergic mechanisms as targets for existing and novel antipsychotics

A variety of serotonin (5-HT) receptors, especially 5-HT(2A), 5-HT(1A), 5-HT(6), 5-HT(7), and 5-HT(2C), have been postulated to contribute to the mechanism of action of atypical antipsychotic drugs (APDs), i.e., APDs which cause fewer extrapyramidal side effects (EPS) at clinically optimal doses, in contrast with typical APDs, which are more likely to cause EPS. This advantage, rarely disputed, has made such drugs the preferred treatment for schizophrenia and other indications for APDs. These 5-HT receptors are still of interest as components of novel multireceptor or stand-alone APDs, and potentially to remediate cognitive deficits in schizophrenia. Almost all currently available atypical APDs are 5-HT(2A) receptor inverse agonists, as well as dopamine (DA) D(2) receptor antagonists or partial agonists. Amisulpride, an exceptional atypical APD, has 5-HT(7) antagonism to complement its DA D(2/3) antagonism. Some atypical APDs are also 5-HT(1A) partial agonists, 5-HT(6), or 5-HT(7) antagonists, or some combination of the above. 5-HT(2C) antagonism has been found to contribute to the metabolic side effects of some atypical APDs, whereas 5-HT(2C) agonists have potential as stand-alone APDs and/or cognitive enhancers. This review will provide an update of current preclinical and clinical evidence for the role of these five 5-HT receptors in the actions of current APDs and for the development of novel psychotropic drugs.

Olanzapine increases RGS7 protein expression via stimulation of the Janus tyrosine kinase-signal transducer and activator of transcription signaling cascade

Atypical antipsychotics such as olanzapine have high affinity for multiple monoamine neurotransmitter receptors and are the mainstay of pharmacological therapy for treatment of schizophrenia. In addition to blocking monoamine receptors, these drugs also affect intracellular signaling cascades. We now report that 24-h treatment with 300 nM olanzapine causes desensitization of serotonin (5-HT)(2A) receptors in A1A1v cells, a rat cortical cell line, as indicated by a reduction in inositol phosphate accumulation following stimulation with a 5-HT(2A/2C) receptor agonist (-)-1-(2,5-dimethoxy-4-lodophenyl)-2-aminopropane HCl. Olanzapine treatment for 24 h increased the levels of 5-HT(2A) receptors in both cytosol (234 +/- 34% of control level) and membrane fractions (206 +/- 14% of control levels) and RGS7 proteins in both cytosol (193 +/- 32% of control levels) and membrane fractions (160 +/- 18% of control levels) as measured on Western blots. Increased phosphorylation of Janus tyrosine kinase (JAK) 2 and increased phosphorylation and nuclear translocation of signal transducer and activator of transcription (STAT) 3 with 24-h olanzapine treatment demonstrate activation of the JAK-STAT signaling cascade. Pretreatment with a JAK inhibitor, AG490 [alpha-cyano-(3,4-dihydroxy)-N-benzylcinnamide], prevented the olanzapine-induced increase in membrane RGS7 protein levels; AG490 alone had no effect on RGS7 protein levels. We verified that treatment with AG490 reduced phosphorylation of JAK2 and inhibited the nuclear localization of phospho-STAT3. Interestingly, treatment with the JAK inhibitor had no effect on 5-HT(2A) receptor protein levels. These data suggest that olanzapine-induced activation of the JAK-STAT signaling cascade causes increased expression of RGS7 protein, which in turn could mediate desensitization of 5-HT(2A) receptor signaling caused by olanzapine because RGS7 binds to Galpha(q) protein and accelerates GTP hydrolysis.

Clotiapine compared with chlorpromazine in chronic schizophrenia

OBJECTIVES

Clotiapine is a classic neuroleptic with a chemical structure similar to clozapine. It was said that patients unresponsive to other neuroleptics respond to clotiapine although it causes extrapyramidal syndromes (EPS) like other typical neuroleptics. We conducted a study of clotiapine vs. chlorpromazine in severe chronic active psychotic hospitalized schizophrenia patients.

METHODS

The design was double-blind crossover of clotiapine vs chlorpromazine. No washout was necessary from previous neuroleptic treatment, and flexible overlap with the study medication was individualized for each patient. Patients were treated after reaching neuroleptic monotherapy for 3 months with clotiapine and 3 months with chlorpromazine, in random order. Medication was supplied in identical capsules of 100 mg chlorpromazine or 40 mg of clotiapine. Positive and Negative Syndrome Scale (PANSS) and Clinical Global Impression (CGI) were rated every 2 weeks and Nurse's Observation Scale for Inpatient Evaluation (NOSIE) every month.

RESULTS

Fifty-eight patients were randomized. Forty-three patients completed at least one phase of the study, and thirty-three completed both phases. Because of the small number of hostel patients and the very high dropout rate in the hostel patients, data analysis was done separately for inpatients and hostel patients. Clotiapine was significantly superior to chlorpromazine in 26 inpatients completing the crossover, on the PANSS, NOSIE and CGI. Clotiapine was also superior to chlorpromazine in an analysis of the parallel inpatient groups in the first three months before the crossover.

CONCLUSION

Some classic neuroleptic compounds may have superiority to chlorpromazine in a "clozapine-like" manner, despite a typical profile for EPS.

Effects of clozapine and 2,5-dimethoxy-4-methylamphetamine [DOM] on 5-HT2A receptor expression in discrete brain areas

Activation of 5-HT2A receptors has been shown to be an essential component of the discriminative stimulus effects of indoleamine and phenethylamine hallucinogens. The objective of the present study was to determine the neuroanatomical location of the 5HT2A receptors which may be responsible for the stimulus effects of the phenethylamine hallucinogen [-]2,5-dimethoxy-4-methylamphetamine (DOM). It was hypothesized that brain areas containing altered 5-HT2A receptor expression in the context of a similar alteration in DOM-induced stimulus control might be important in mediating the stimulus effects of DOM. Fisher 344 rats were treated with either clozapine (25 mg/kg/day) or DOM (2 mg/kg/day) for 7 days, and the consequences of these drug treatment regimens on DOM-induced stimulus control and on 5-HT2A receptor expression in several brain areas were determined. Chronic administration of clozapine was associated with a wide-spread decrease in levels of 5-HT2A/2C receptors. Conversely, treatment with DOM had varied effects including a neuroanatomically selective decrease in 5-HT2A/2C receptor levels that was restricted to the olfactory nucleus. Both chronic treatment with DOM and clozapine decreased the stimulus effects of DOM. The present findings suggest a role for the olfactory nucleus in producing the stimulus effects of DOM.

The atypical antipsychotic drug clozapine enhances chronic PCP-induced regulation of prefrontal cortex 5-HT2A receptors

The ability of antipsychotic drugs to affect 5-HT(2A) receptor function has been widely suggested to contribute to their therapeutic properties. We have compared the ability of the antipsychotic drugs clozapine and haloperidol, alone and in combination with chronic phencyclidine (PCP), to modulate 5-HT(2A) receptor binding and mRNA. Acute (i.p. 45 min) and chronic (21-day) clozapine (osmotic minipump (OMP); 20 mg/kg/day) produced widespread decreases in 5-HT(2A) receptor binding (-60%-80%), measured using [(3)H]ketanserin autoradiography. Conversely, 5-HT(2A) mRNA levels, determined using in-situ hybridisation, were modestly increased by chronic clozapine treatment (+10%-30%). Chronic PCP treatment, at a dose (2.58 mg/kg i.p. intermittently for 28 days) that reproduces many of the neurochemical deficits of schizophrenia, decreased 5-HT(2A) receptor binding in the prefrontal cortex (PFC; -16%), consistent with the changes in post-mortem brain tissue from schizophrenic patients. Combined chronic PCP (i.p.) and clozapine (OMP) treatment down-regulated 5-HT(2A) receptor binding in many areas, similar to the effects of clozapine treatment alone and clozapine further enhanced the effects of PCP in the prefrontal cortex. In contrast 5-HT(2A) mRNA was not altered. Haloperidol treatment alone (1 mg/kg/day; OMP) and in combination with PCP (i.p.), generally produced no changes in 5-HT(2A) receptor protein or mRNA. Hence chronic PCP treatment, as employed here, mimics the decreased 5-HT(2A) receptor binding observed in the PFC of schizophrenic patients. Clozapine's enhancement of the natural response of PCP to down-regulate PFC 5-HT(2A) receptors may contribute to it's improved therapeutic profile against negative symptoms and cognitive deficits.

Effects of clozapine and its metabolites on the 5-HT2 receptor system in cortical and hippocampal cells in vitro

PURPOSE

The goal of the present study was to determine the effects of clozapine (Cloz) and its metabolites norclozapine (Norcloz) and clozapine-N-oxide (Cloz-N-oxide) on the 5-HT(2) receptor system on the levels of protein and gene expression in in vitro systems of primary cortical cells of the rat and human hippocampal SHS5Y5 neuroblastoma cells.

METHODS

Clinically relevant concentrations of Cloz (200/400 ng/ml) and its metabolites (200 ng/ml) were used for the examination of the effects of Cloz and its metabolites on serotoninergic 5-HT(2) receptor parameters (density, affinity and mRNA levels) as well as on glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA levels in primary cortical cells of the rat after treatment for 24 h under in vitro conditions. To compare the results to human cells, we also measured treatment-induced changes in 5-HT(2) and GAPDH mRNA levels in human hippocampal SHS5Y5 cells.

RESULTS

A significant decrease was found in primary cortical cells for 5-HT(2) receptor density (Cloz 200/Cloz 400/Norcloz 200 and Cloz-N-oxide 200 vs. control) and 5-HT(2A) receptor mRNA levels (Cloz 200 vs. control). 5-HT(2A) receptor mRNA levels were also significantly reduced (Norcloz 200 vs. control) in SHS5Y5 cells. GAPDH mRNA levels were not affected.

CONCLUSIONS

The results of the present study show that Cloz and Norcloz induce significant alterations on the 5-HT(2) receptor system in primary cortical cells of the rat and in human hippocampal cells.

Serotonin receptors: their key role in drugs to treat schizophrenia

Serotonin (5-HT)-receptor-based mechanisms have been postulated to play a critical role in the action of the new generation of antipsychotic drugs (APDs) that are usually referred to as atypical APDs because of their ability to achieve an antipsychotic effect with lower rates of extrapyramidal side effects (EPS) compared to first-generation APDs such as haloperidol. Specifically, it has been proposed by Meltzer et al. [J. Pharmacol. Exp. Ther. 251 (1989) 238] that potent 5-HT2A receptor antagonism together with weak dopamine (DA) D2 receptor antagonism are the principal pharmacologic features that differentiate clozapine and other apparent atypical APDs from first-generation typical APD. This hypothesis is consistent with the atypical features of quetiapine, olanzapine, risperidone, and ziprasidone, which are the most common treatments for schizophrenia in the United States and many other countries, as well as a large number of compounds in various stages of development. Subsequent research showed that 5-HT1A agonism may be an important consequence of 5-HT2A antagonism and that substitution of 5-HT1A agonism for 5-HT2A antagonism may also produce an atypical APD drug when coupled with weak D2 antagonism. Aripiprazole, the most recently introduced atypical APD, and a D2 receptor partial agonist, may also owe some of its atypical properties to its net effect of weak D2 antagonism, 5-HT2A antagonism and 5-HT1A agonism [Eur. J. Pharmacol. 441 (2002) 137]. By contrast, the alternative "fast-off" hypothesis of Kapur and Seeman [Am. J. Psychiatry 158 (2001) 360] applies only to clozapine and quetiapine and is inconsistent with the "slow" off rate of most atypical APDs, including olanzapine, risperidone and ziprasidone. 5-HT2A and 5-HT1A receptors located on glutamatergic pyramidal neurons in the cortex and hippocampus, 5-HT2A receptors on the cell bodies of DA neurons in the ventral tegmentum and substantia nigra and GABAergic interneurons in the cortex and hippocampus, and 5-HT1A receptors in the raphe nuclei are likely to be important sites of action of the atypical APDs. At the same time, evidence has accumulated for the important modulatory role of 5-HT2C and 5-HT6 receptors for some of the effects of some of the current APDs. Thus, 5-HT has joined DA as a critical target for developing effective APDs and led to the search for novel drugs with complex pharmacology, ending the exclusive search for single-receptor targets, e.g., the D3 or D4 receptor, and drugs that are selective for them.

5-HT2A and 5-HT2C receptors and their atypical regulation properties

The 5-HT(2A) and 5-HT(2C) receptors belong to the G-protein-coupled receptor (GPCR) superfamily. GPCRs transduce extracellular signals to the interior of cells through their interaction with G-proteins. The 5-HT(2A) and 5-HT(2C) receptors mediate effects of a large variety of compounds affecting depression, schizophrenia, anxiety, hallucinations, dysthymia, sleep patterns, feeding behaviour and neuro-endocrine functions. Binding of such compounds to either 5-HT(2) receptor subtype induces processes that regulate receptor sensitivity. In contrast to most other receptors, chronic blockade of 5-HT(2A) and 5-HT(2C) receptors leads not to an up- but to a (paradoxical) down-regulation. This review deals with published data involving such non-classical regulation of 5-HT(2A) and 5-HT(2C) receptors obtained from in vivo and in vitro studies. The underlying regulatory processes of the agonist-induced regulation of 5-HT(2A) and 5-HT(2C) receptors, commonly thought to be desensitisation and resensitisation, are discussed. The atypical down-regulation of both 5-HT(2) receptor subtypes by antidepressants, antipsychotics and 5-HT(2) antagonists is reviewed. The possible mechanisms of this paradoxical down-regulation are discussed, and a new hypothesis on possible heterologous regulation of 5-HT(2A) receptors is proposed.

Serotonergic effects of clozapine and its metabolites in hippocampal HT22 cells

In the hippocampal neuronal in vitro system of HT22 cells, we studied the effects of clozapine (Cloz) and its metabolites clozapine-N-oxide (Cloz-N-oxide) and norclozapine (Norcloz) on 5-HT transporter affinity (K(M)) and uptake (V(max)), MAO-B affinity (K(M)) and maximal velocity (V(max)), as well as on 5-HT(2) receptor affinity and density. Clinically relevant concentrations of Cloz (200 and 400 ng/ml) and its metabolites (100 and 200 ng/ml) were used for the examination of the effects after short-term (4 h) and long-term (24 h) incubation. Statistical evaluation revealed that a significantly lowered 5-HT transporter affinity (higher K(M)) was related to higher concentrations of Cloz and its metabolites. A significantly higher 5-HT transporter uptake was dependent on both high concentrations of drugs and an increased time of incubation. No significant influence of the investigated independent variables on MAO-B affinity could be demonstrated, whereas a significant drug-related increase of MAO-B velocity was detectable. Additionally, low and high concentrations of Cloz and its metabolites induced a higher 5-HT(2) receptor affinity (lower K(D)). No significant influences of the investigated independent variables on 5-HT(2) receptor density were detectable. The results of the present study show that Cloz and its metabolites induce significant alterations in serotoninergic parameters of hippocampal HT22 cells, validating the system of hippocampal HT22 cells for further examinations of the mechanisms of action of atypical neuroleptics.

Clozapine withdrawal symptoms in a Parkinson's disease patient

Abrupt clozapine withdrawal can cause rebound psychosis and severe somatic symptoms in psychiatric patients. We report on the case of an advanced Parkinson's disease patient who developed myoclonus, tremor, rigidity, hyperreflexia, and stupor after abrupt clozapine withdrawal. The patient's symptoms resolved with treatment with cyproheptadine. This clinical picture suggests serotonergic rebound as an explanation for the patient's symptoms, although other pharmacological mechanisms are possible. Clozapine should be gradually withdrawn over a period of 1 to 2 weeks when possible, and abruptly discontinued only when necessary.

Decreased [(3)H]spiperone binding in the anterior cingulate cortex of schizophrenia patients: an autoradiographic study

Abnormalities in the anterior cingulate cortex have been reported in patients with schizophrenia, and have been implicated in the pathophysiology of this disorder. In the present study, we have examined antipsychotic-sensitive binding sites in the left anterior cingulate cortex of schizophrenia patients and controls. Using quantitative autoradiography and [(3)H]spiperone as a ligand, both saturation and competition experiments were performed in post-mortem brain tissue obtained from six schizophrenia and six control cases. Saturation experiments revealed that the maximum number of [(3)H]spiperone binding sites was significantly reduced by 31% in the schizophrenia group as compared to the control group (65.3+/-5.6 fmol/mg tissue versus 94.2+/-7.3 fmol/mg tissue). Increased dissociation constant was also observed in the schizophrenia group (2.2+/-0.4 nM versus 1.3+/-0.2 nM), but was not statistically significant (P=0.07). Competition experiments were performed in order to examine the pharmacological profile of [(3)H]spiperone binding, and revealed that: (i) displacement of [(3)H]spiperone binding by clozapine and mianserin was significantly reduced in the schizophrenia group as compared to the control group (-26% and -16% respectively); (ii) the order of displacement potency of the drugs tested was: haloperidol>mianserin>butaclamol approximately risperidone>clozapine>2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene. Our results suggest a reduction of antipsychotic-sensitive binding sites in the anterior cingulate cortex of patients with schizophrenia. Such abnormality could lead to an imbalance in neurotransmitter regulation in the anterior cingulate cortex which may contribute to the emergence of some symptoms of schizophrenia.

Paradoxical trafficking and regulation of 5-HT(2A) receptors by agonists and antagonists

5-Hydroxytryptamine(2A) (serotonin(2A), 5-HT(2A)) receptors are important for many physiologic processes including platelet aggregation, smooth muscle contraction, and the modulation of mood and perception. A large number of pharmaceutical agents mediate their actions, at least in part, by modulating the number and/or activity of 5-HT(2A) receptors. Drugs with action at 5-HT(2A) receptors are used in the treatment of many disorders, including schizophrenia, depression, and anxiety disorders. This review summarizes over two decades of research on the regulation of 5-HT(2A) receptors and provides a comprehensive review of numerous in vivo studies describing the paradoxical phenomenon of 5-HT(2A) receptor down-regulation by chronic treatment with antidepressants and antipsychotics. In addition, studies reporting antagonist-induced internalization of 5-HT(2A) receptors and other G protein-coupled receptors will be highlighted as a possible mechanism to explain this paradoxical down-regulation. Finally, a review of the cellular and molecular mechanisms that may be responsible for agonist-mediated desensitization and internalization of 5-HT(2A) receptors will be presented.

Effects of systemic injections of dopaminergic agents on the habituation of rats submitted to an open field test

Many studies have reported that systemic injections of dopaminergic drugs result in profound changes in the behavior of animals in the open field. At the same time, it is hypothesized that increased brain dopaminergic activity prevents animals from habituating to a novel environment, and this effect would be responsible for the conditioned hyperactivity observed when rats are tested later in a drug-free state. However, little is known on the concurrent effects of these drugs, on learning processes and motor activity. To examine this issue, in the present study we assessed the effects of injections of the dopamine (DA) agonists amphetamine and apomorphine, the receptor antagonist chlorpromazine and the atypical neuroleptic clozapine on exploratory activity and habituation in rats exposed to the open field test. The observer scored the occurrence of animal behavior during three postinjection observation periods (0-5, 5-10 and 10-15 min). The results confirmed that amphetamine and apomorphine lead to a dose-dependent behavioral activation, while chlorpromazine produces an opposite effect, extending previous accounts of behavior of systemic-injected rats with these drugs. Independent of the level of motor activity, higher or lower than the control groups, habituation still occurs following DA agonists (amphetamine and apomorphine) and antagonist (chlorpromazine) injections in rats submitted to the open field test. Furthermore, the reduction of the exploratory activity over time in the open field after clozapine was less pronounced than in controls, in contrast to classical DA agents. The present results suggest that activation of DA mechanisms is involved in species-typical behaviors associated with exploratory behavior, but does not seem to be involved in non-associative learning processes, such as habituation. These effects could not be attributed to the decline in brain levels of the drugs during the session, as the behavioral activation remained high in rats previously habituated and injected with dopaminergic agonists within the same protocol. On the other hand, the atypical DA blocker clozapine reduced the expression of habituation, opening the possibility of involvement of alternative mechanisms, such as 5-HT mechanisms, in this non-associative learning process.

The effect of tandospirone, a serotonin(1A) agonist, on memory function in schizophrenia

BACKGROUND

The purpose of this study was to test the hypothesis that the addition of tandospirone, a 5-HT(1A) partial agonist, to ongoing treatment with typical antipsychotic drugs, would improve memory function in patients with schizophrenia.

METHODS

Eleven outpatients (male/female = 7/4) with schizophrenia who had been on stable doses of haloperidol and biperiden were given tandospirone, 30 mg/day, for 4 weeks. The Wechsler Memory Scale-Revised (WMS-R) was administered at baseline and 4 weeks after the addition of tandospirone. The Brief Psychiatric Rating Scale (BPRS; Total, Positive, and Negative subscale scores) and the Simpson-Angus Scale for Extrapyramidal Symptoms (SAS) were also completed on the two occasions. To exclude the possibility of a practice effect on the WMS-R test, 11 age-matched patients with schizophrenia (M/F = 7/4) were tested at baseline and after a 4-week interval.

RESULTS

Repeated measures analysis of variance revealed a significant time by group (patients with or without tandospirone) effect for the Verbal-, but not the Visual Memory composite scores of the WMS-R test; no significant change was observed in patients without tandospirone, whereas improvement in the Verbal Memory score was noted in patients receiving tandospirone. Moreover, there was improvement in the Inclusion score, an index of memory organization as measured by the Logical Memory subtest of WMS-R, only in patients with tandospirone. Scores on the BPRS and SAS were improved during treatment with tandospirone, but the effects did not reach statistical significance.

CONCLUSIONS

The results suggest that adjunctive treatment with 5-HT(1A) agonists may improve some types of memory function in schizophrenia.

A voxel-by-voxel analysis of [18F]setoperone PET data shows no substantial serotonin 5-HT(2A) receptor changes in schizophrenia

Several postmortem studies have reported regionally localized decreases in serotonin(2A) receptors (5-HT(2A)R) in schizophrenia. This was not confirmed by two recent [18F]setoperone positron emission tomography (PET) studies. In these two studies relatively large regions of interest (ROIs) were used; hence, 5-HT(2A)R changes may have been missed in some brain areas. Therefore, data from one study were analyzed on a voxel-by-voxel basis using Statistical Parametric Mapping (SPM). We also used this method to examine the relationship between 5-HT(2A)R binding potential (BP) and five PANSS-derived factors: negative, positive, activation, dysphoric and autistic preoccupation. Thirteen schizophrenic patients (10 antipsychotic-naïve, 3 antipsychotic-free; 11 M, 2 F; age 31+/-7 years) and 35 age-matched control subjects (15 M, 20 F; age 30+/-7 years) were scanned. The 5-HT(2A)R BP was determined for each voxel using the pseudoequilibrium ratio method on PET data obtained between 65 and 90 min after [18F]setoperone bolus injection. The resulting parametric 5-HT(2A)R BP images were spatially normalized using a ligand specific template. Analyses of covariance were done using SPM99 with age as covariate. In tests for the effect of schizophrenia and for partial correlations between 5-HT(2A)R BP and the five factors, corrected P values <0.05 at cluster or voxel level were considered significant. No significant differences were detected between patients and control subjects, and no significant correlations were observed between 5-HT(2A)R BP and any of the five factors. Thus, in agreement with the previous ROI studies, voxel-by-voxel analysis confirmed the lack of substantial 5-HT(2A)R BP differences between schizophrenic patients and control subjects.

Serotonin(2A) receptors are reduced in the planum temporale from subjects with schizophrenia

[(3)H]ketanserin binding to 5HT(2A) receptors was measured in the left planum temporale (sensory speech cortex) from schizophrenic and non-schizophrenic (control) subjects using both particulate membranes and tissue sections. There was a significant decrease in the affinity of [(3)H]ketanserin binding to particulate membranes from schizophrenic subjects who were treated with phenothiazines up to death. Adding 2nM chlorpromazine to brain tissue from control subjects caused a similar decrease in the affinity of [(3)H]ketanserin binding to particulate membranes. This suggests that the decrease in affinity observed in the phenothiazine-treated subjects was due to residual drugs. In addition, there was a significant decrease in the density of [(3)H]ketanserin binding in both particulate membranes and tissue sections from schizophrenic subjects which did not appear to be due to residual antipsychotic drugs. Analysis of the laminar distribution of 5HT(2A) receptors showed that this decrease was greatest in cortical layer III. The decrease in the density of 5HT(2A) receptors was significant whether schizophrenic subjects were receiving phenothiazines or haloperidol at the time of death, and there was no correlation between the last recorded dose of antipsychotic drug and 5HT(2A) receptor density. These data suggest that a decrease in the density of 5HT(2A) receptors in the planum temporale may be associated with the pathology of schizophrenia.

Modification of haloperidol-induced pattern of c-fos expression by serotonin agonists

Acute challenge with clozapine and haloperidol produce different anatomical patterns of c-fos expression in the forebrain. The pharmacological profile of atypical antipsychotics suggests that serotonin might contribute to the unique therapeutic benefits of these drugs. In order to test this possibility, we examined the abilities of 5-HT1A and 5-HT2A/2c agonists to modify the pattern of c-fos expression induced by haloperidol and clozapine. Various groups of rats were pretreated with either saline, DOI, 8-OH-DPAT, and 8-OH-DPAT + DOI 30 min prior to haloperidol or clozapine administration. Rats were killed 90 min after antipsychotic administration. In saline-pretreated rats, haloperidol produced intense Fos-LI in all four striatal quadrants while the effect of clozapine was restricted to the medial part of the striatum. Prior administration of 8-OH-DPAT significantly reduced haloperidol-induced Fos-LI in all four striatal quadrants while DOI and 8-OHDPAT + DOI significantly reduced Fos-LI only in dorso- and ventrolateral quadrants. In the nucleus accumbens, haloperidol induced intense Fos-LI in the core and the shell regions whereas clozapine induced c-fos expression only in the shell. Pretreatment with 8-OHDPAT in haloperidol treated rats reduced Fos-LI in the core region yielding to a c-fos pattern similar to that induced by clozapine. In the prefrontal cortex of saline-pretreated rats, haloperidol produced a moderate c-fos expression compared with the intense expression produced by clozapine. Pretreatment with serotonin agonists before haloperidol brought the number of FOS-positive neurons to the same level as in clozapine treated rats. These results show the ability of 5-HT agonists to transform the typical pattern of c-fos expression induced by haloperidol into a pattern resembling that of clozapine.

Decreased serotonin2A receptors in Brodmann's area 9 from schizophrenic subjects. A pathological or pharmacological phenomenon?

There have been repeated reports of a decrease in serotonin2A receptors in the frontal cortex from subjects with schizophrenia. Similarly, in rats treated with antipsychotic drugs, it has been shown that many antipsychotic drugs decrease cortical serotonin2A receptors, an affect not seen with the antipsychotic drug haloperidol. We therefore compared the density of serotonin2A receptors in frontal cortex from schizophrenic subjects treated with haloperidol, schizophrenic subjects treated with other antipsychotic drugs, and nonschizophrenic subjects. Independent of antipsychotic drug treatment, serotonin2A receptors were decreased in the frontal cortex from schizophrenic subjects. Importantly, the density of serotonin2A receptors was not different in schizophrenic subjects whether or not they had been treated with haloperidol. This study suggests that data obtained from treating rats with antipsychotic drugs cannot be simplistically extrapolated to studies on tissue obtained postmortem from schizophrenic subjects treated with the same drugs.

Alterations in the cortical serotonergic system in schizophrenia: a postmortem study

Previous studies have suggested a disturbance in the cortical serotonergic (5-HT) system in schizophrenia; however, these studies have been confounded by suicide in the patients groups, which in itself is associated with alterations in the 5-HT system. In this study we characterized various components of the 5-HT system in 14 areas of the frontal and parietal cortex in tissue obtained at postmortem from aged chronically hospitalized nonsuicidal schizophrenics compared to age-matched controls. We found no differences between control and schizophrenic subjects in the density of 5-HT uptake sites or other markers of 5-HT innervation. In Brodmann areas 24 and 6 the concentration of 5-HT2A,C receptors was decreased in all schizophrenics regardless of their antipsychotic treatment history. In all other areas examined 5-HT2A,C receptor concentrations were dramatically decreased in schizophrenics patients on drugs at time of death, whereas those off drugs at death showed the same values as control subjects. The density of 5-HT1A receptors was increased in areas 24, 9a (caudal part of area 9), 44, and 6 in subjects with schizophrenia. Antipsychotic treatment did not appear to have a significant effect. Thus, the specific pattern of alterations in the 5-HT system in schizophrenia may depend on the patient population and on antemortem antipsychotic treatment. These data also provide evidence that regulation of the 5-HT2 receptor may be involved in antipsychotic action.

The effects of antipsychotic drugs on the mRNA levels of serotonin 5HT2A and 5HT2C receptors

We have looked for both global and regional changes in rat brain mRNA levels encoding serotonin 5HT2A and 5HT2C receptors following acute and chronic administration of antipsychotic drugs. In whole brain clozapine (30 mg/kg/day) elicited a transient rise in 5HT2C mRNA levels after 4 days and chlorpromazine (15 mg/kg/day) elicited a decrease of 20% in 5HT2A mRNA levels after 32 days. Regionally, 32 days treatment with haloperidol (3 mg/kg/day), sulpiride (100 mg/kg/day) or clozapine (10 mg/kg/day) resulted in a drop of approximately 30-40% in 5HT2C mRNA levels in both cortex and cerebellum, and decreases (or non-significant trends) of 15-40% in 5HT2A mRNA levels in hippocampus, brainstem and mid brain. 4 days treatment with clozapine resulted in a 40% rise of 5HT2C mRNA in the mid brain and a 24% rise of 5HT2A mRNA in the nucleus accumbens which were not found after 32 days of treatment. These results demonstrate common chronic effects of typical and atypical drugs but unique short term effects of clozapine.

Limbic circuits and monoamine receptors: dissecting the effects of antipsychotics from disease processes

There is considerable evidence for the involvement of brain dopaminergic and serotonergic systems in schizophrenia pathology. However, post-mortem studies have been limited by difficulties in separating the effects of chronic exposure to antipsychotics from that of the disease process. Our recent studies directly explored this by comparing groups that were free from antipsychotic treatment for up to a year prior to death and that were maintained on antipsychotics. We have used this approach to identify that there are prominent effects of both disease and of antipsychotic treatment. There appears to be a high association for schizophrenics between elevations of D3 receptors in target regions of the mesolimbic dopamine (DA) system and elevated numbers of 5-HT(1A) receptors in prefrontal cortex (PFc). Antipsychotic treatment was correlated with a reduction of D3 receptors in the ventral striatum and its output structures. It also led to a reduction in the number of 5-HT2 receptors in some regions of the PFc without modifying the concentration of 5-HT(1A) receptors. The limbic loop interconnecting the PFc and ventral striatum may be the site of antipsychotic regulation of certain symptoms in schizophrenia, particularly anhedonia and depression. The positive symptoms of schizophrenia are more likely to be associated with disturbances in the temporal lobe. However, dopaminergic systems in the temporal lobe have historically been thought to be underdeveloped compared to that in the basal ganglia and unlikely to be the target of antipsychotics. Our studies of the expression of the DA D2 receptor in the temporal lobe has shown a complex organization in the perirhinal and temporal cortices that is disrupted in schizophrenia. The disturbances, which might be of neurodevelopmental origin and are unrelated to antipsychotic treatment, include altered laminar distribution of the D2 receptor and modified modular organization of D2 receptors in the superior temporal gyrus. We hypothesize that modified expression of D2 receptors in these regions play a key role in the genesis of hallucinations. Treatment with antipsychotics leading to D2 receptor blockade in temporal cortex may reduce the presence of positive symptoms.

Acute and long-term effectiveness of clozapine in treatment-resistant psychotic depression

The treatment of refractory major depression, including the psychotic subtype, is a therapeutic challenge. Three cases of resistant psychotic depression were treated with clozapine monotherapy, an atypical antipsychotic drug effective in treatment-resistant schizophrenia and mania. Both psychotic and mood symptoms responded well to clozapine monotherapy, although response was delayed in one case. Tardive dyskinesia improved markedly, and tardive dystonia improved moderately in one patient. No patient relapsed during a follow-up period of 4-6 years of clozapine treatment. Clozapine was well-tolerated with few side effects. These observations suggest controlled trials of clozapine in the treatment of psychotic depression that fails to respond to electroconvulsive therapy or typical neuroleptics plus tricyclic antidepressants are indicated. The same is true for the use of clozapine in maintenance treatment for psychotic depression in those cases in which typical neuroleptic drugs are required, in order to reduce the risk of tardive dyskinesia and dystonia.

The effects of clozapine and haloperidol on serotonin-1A, -2A and -2C receptor gene expression and serotonin metabolism in the rat forebrain

The therapeutic and side-effect profiles of clozapine differ from those of typical antipsychotic drugs such as haloperidol. Effects on the serotonin system, especially serotonin-2 receptors, may contribute to clozapine's atypicality. We injected rats for 14 days with clozapine (25 mg/kg/day) or haloperidol (2 mg/kg/day), and measured three aspects of the serotonin system in forebrain regions: abundance of serotonin-2A, -2C and -1A receptor messenger RNAs by in situ hybridization histochemistry; serotonin-2A and -1A binding sites using receptor autoradiography, and levels of serotonin and 5-hydroxyindoleacetic acid with high-performance liquid chromatography. Clozapine administration decreased serotonin-2A receptor messenger RNA and the density of [3H]ketanserin binding in cingulate and frontal cortex, but not in piriform cortex. Serotonin-1A receptor expression and serotonin-2C receptor messenger RNA were unchanged in all areas. The treatment markedly decreased serotonin and 5-hydroxyindoleacetic acid concentrations in striatum with similar trends in cortex and hippocampus. Haloperidol administration did not affect the expression of the three serotonin receptors, but was associated with a modest reduction of striatal and hippocampal 5-hydroxyindoleacetic acid. The selective reduction of serotonin-2A receptors confirms earlier findings and supports the view that this receptor may have relevance for the actions of clozapine. The fact that the encoding messenger RNA is decreased shows that the the effect is mediated at the level of gene expression. In contrast, the unchanged serotonin-2C receptor messenger RNA level indicates that the reported loss of serotonin-2C receptors after clozapine treatment is due to translational or post-translational events. The relationship between the reduction in serotonin-2A receptor expression and the altered serotonin metabolism remains unclear.

Serotonin-2 and dopamine-1 binding components of clozapine in frontal cortex and striatum in the human brain visualized by positron emission tomography

The specific binding of N-methyl-11C-clozapine in the human brain was studied in five healthy volunteers with positron emission tomography (PET). Four of the volunteers were reexamined after treatment with the dopamine D1 and D2 receptor antagonist flupenthixol, and all five volunteers were reexamined after pretreatment with the serotonin2 receptor antagonist ritanserin. The examinations after flupenthixol and ritanserin treatment were performed on different occasions. In the flupenthixol part of the study, two of the subjects were given an oral dose of 1 mg flupenthixol 2-3 h before the posttreatment study with PET. The other two subjects received 0.5 mg orally three times during the 24 h preceding the posttreatment PET study, with the last dose being administered < or = 4 h before the scan. All five ritanserin-treated subjects followed the same dosing regimen. During the 5 days preceding the posttreatment PET study, they were given a 10-mg tablet of ritanserin in the evening. The last dose was administered 2-1/2 hours before the study. Both flupenthixol and ritanserin pretreatment were associated with decreased binding of N-methyl-11C-clozapine in dorsolateral and medial frontal cortical regions. These results support previous findings that clozapine has affinity for both dopamine D1 and serotonin 5-HT2 receptors in the human frontal cortex. No consistent change of binding was observed in striatal regions following flupenthixol or ritanserin pretreatment. The clinical aspects of this feature are discussed, both with respect to efficacy and side effects.

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.

Relapse following clozapine withdrawal: effect of neuroleptic drugs and cyproheptadine

The objective of this study was to report the effect of the slow withdrawal of clozapine from 19 patients with neuroleptic-responsive schizophrenia at the end of a 2-year clinical trial of clozapine and to compare this with the results of naturalistic discontinuation of clozapine treatment in 64 neuroleptic-resistant schizophrenic patients. Nineteen neuroleptic-responsive schizophrenic patients who received clozapine were withdrawn from clozapine by tapering it over 3-week period with and without the addition of a typical neuroleptic. Fifteen of the 19 neuroleptic-responsive patients experienced the return of psychotic symptoms during or after the clozapine taper, which were most severe in the ten patients in whom the withdrawal of clozapine was carried out without prior addition of neuroleptic treatment. Addition of a neuroleptic prior to clozapine withdrawal prevented the emergence of positive symptoms during clozapine withdrawal in each of eight patients. Nevertheless, psychotic symptoms emerged, usually within a week after discontinuing clozapine, in six of the eight patients. Neuroleptic treatment, with or without an anticholingergic drug, was much less effective in treating positive symptoms in these patients immediately after the clozapine withdrawal than it had been 2 years previously. Cyproheptadine, a non-selective serotonin receptor antagonist, augmented the antipsychotic effect of neuroleptics in each of four patients who relapsed following withdrawal from clozapine and relieved extrapyramidal symptoms in a fifth patient. The frequency of relapse following withdrawal of clozapine in 64 neuroleptic-resistant patients was significantly lower (25/64, 39.1%) than in the neuroleptic-responsive patients.

The role of 5-HT2A receptors in antipsychotic activity

The correlation between the clinical activity of antipsychotic agents and their affinity for the D2 dopamine receptor has been the mainstay of the hypothesis that schizophrenia is due to excessive dopaminergic function. More recently, the unique clinical profile of the atypical antipsychotic clozapine has been proposed to involve actions on additional receptor systems. In particular, the high affinity of clozapine for the 5HT2A receptor subtype has been suggested to contribute to its reduced side-effect liability, greater efficacy and its activity in therapy-resistant schizophrenia. We have used the highly selective 5-HT2A antagonist MDL 100,907 to explore the contribution of 5-HT2A receptor blockade to antipsychotic activity. Biochemical, electrophysiological and behavioral studies reveal that selective 5HT2A receptor antagonists have the preclinical profile of an atypical antipsychotic. The limited clinical evidence available also suggests that compounds producing 5-HT2A receptor blockade are effective, in particular, against the negative symptoms of schizophrenia.

The role of serotonin in schizophrenia

The hypothesis that the LSD psychosis and by inference schizophrenic psychoses are related to dysfunctions in central serotonergic systems, formulated by Woolley and Shaw in the early 1950s was the first testable theory of modern biological psychiatry. Initially, it did not get the scientific attention it deserved. First, because LSD fell into disrepute and was to all intents and purposes banned from human experimentation. Secondly, the antipsychotics were discovered in the same period, and it became clear that these compounds block dopaminergic transmission and hence for many years thereafter the dopaminergic system occupied center stage in biological schizophrenia research. Presently, interest in the relation between serotonin and schizophrenia has been revived, due to the development of serotonin-blocking agents that appear to exert therapeutic effects in schizophrenia. In this paper the evidence for and against a link between serotonergic defects and schizophrenia psychopathology is critically discussed. The conclusion to be reached is threefold. (1) Interruption of certain serotonergic circuits represents an antipsychotic principle. (2) Tentative evidence suggests the involvement of serotonergic dysfunctions in the pathogenesis of schizophrenic psychoses. (3) It is not yet known whether serotonergic lesions contribute directly to the occurrence of schizophrenic psychopathology or via alterations in the dopaminergic system.

Regulation of serotonin 5-HT2C receptors in the rat choroid plexus after acute clozapine treatment

We studied the effects of acute clozapine and haloperidol treatments on 5-HT2C receptor binding characteristics and 5-HT2C receptor-mediated phosphoinositide hydrolysis in the rat choroid plexus. Scatchard analysis (with [3H]mesulergine) showed that acute clozapine treatment (10 and 25 mg/kg) decreased the density (Bmax) of 5-HT2C receptors by 20-25% with no marked change in the affinity (Kd). Quantitative autoradiography was in accordance with homogenate binding studies showing that acute clozapine treatment, unlike haloperidol (0.5 mg/kg), decreased the number of both agonist ([125I](+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, [125I]DOI) and antagonist ([3H]mesulergine) labeled 5-HT2C receptor binding sites. The decrease was more robust with the higher dose of clozapine. For comparison, both doses of clozapine, unlike haloperidol, decreased equally the density of 5-HT2A receptors in the frontal cortex by about 45%, whereas none of the treatments altered dopamine D2 receptor characteristics in the striatum. The Kd value of 5-HT2A receptors was significantly increased after the dose of 25 mg/kg of clozapine. These clozapine treatments failed to decrease the maximal 5-HT2C receptor-mediated phosphoinositide hydrolysis response. The higher dose of clozapine increased 5-HT-induced phosphoinositide hydrolysis response, but also decreased significantly the basal levels of phosphoinositide hydrolysis. Haloperidol did not significantly affect the 5-HT2C receptor-mediated phosphoinositide hydrolysis. To summarize, the present data show that a single injection of clozapine is able to reduce the density of 5-HT2C receptors but fails to cause functional desensitization of 5-HT2C receptors in the rat choroid plexus.

Serotonin2 receptor-mediated excitation of interneurons in piriform cortex: antagonism by atypical antipsychotic drugs

Rat piriform cortex contains a subpopulation of presumed GABAergic interneurons located near the border of layers 2 and 3 that express excitatory serotonin2 receptors. These serotonin2-responsive interneurons send axons to layer 2 pyramidal cells. Using an in vitro brain slice preparation, serotonin2 receptor-mediated excitation can be assessed either by directly recording from the interneurons or by recording the increase in inhibitory postsynaptic potentials in the pyramidal cells. Intracellular recordings from the interneurons demonstrated that compared to pyramidal cells they had a more depolarized resting membrane potential, a higher input resistance and shorter action potential duration. The serotonin2 receptor-mediated excitation was associated with a strong depolarization (range 3-22 mV). We found that the atypical antipsychotic drugs, risperidone and clozapine, which have relatively high affinity for serotonin2 receptors, each dose-dependently inhibited the serotonin2-mediated excitation of the interneurons with IC50 values of 7 nM and 1.4 microM, respectively. This antagonism was specific to the extent that excitation mediated by agonists at excitatory amino acid receptors were not blocked at concentrations of risperidone and clozapine that completely antagonized the serotonin2 receptor-mediated excitation. The typical antipsychotic drug, chlorpromazine, inhibited the serotonin2-mediated excitation of the interneurons with an IC50 of 14 microM. Haloperidol, another typical antipsychotic drug, decreased the serotonin2 response to about half of baseline at a concentration of 10 microM (the exact IC50 could not be calculated because higher concentrations produced non-specific effects on cells). Both risperidone and clozapine blocked the serotonin-elicited inhibitory postsynaptic potentials in layer 2 pyramidal cells at concentrations that approximated the IC50 for antagonizing the serotonin2-mediated excitation of the interneurons. Chlorpromazine and haloperidol, in the concentration range that blocked serotonin2 receptor-mediated excitation of interneurons, also blocked the serotonin-elicited inhibitory postsynaptic potentials in the pyramidal cells. The IC50 values for risperidone and clozapine, but not for chlorpromazine or haloperidol, for blocking serotonin2 receptor-mediated actions in rodent piriform cortical slice are in the range of the plasma concentrations of the drug that are clinically efficacious. Our data suggest that a potential site of action of the atypical antipsychotic drugs risperidone and clozapine could be antagonism of serotonin acting through serotonin2 receptors on GABAergic interneurons in cerebral cortex.

Electroconvulsive shock decreases excitatory responses to serotonin in the caudate nucleus of the rat

1. The present study explored the changes on the firing rate of caudate neurons, and the response to serotonin locally applied in rats submitted to electroconvulsive shock. 2. Electroconvulsive shock diminished the firing rate of caudate neurons and blocked the excitatory response produced by serotonin in a small amount of serotonin-sensitive neurons. 3. Results are likely to be related with transient changes in the receptor's affinity and dissociation constants, which may impinge on immediate memory retrieval.

Irreversible suppression of alcohol drinking in cyanamide-treated rats after sustained delivery of the 5-HT2 antagonist amperozide

The purpose of this study was to evaluate the long-term effect of sustained treatment with amperozide, which has been shown to attenuate the volitional drinking of ethyl alcohol in the rat without side effects. Preference for alcohol first was induced pharmacologically in Sprague-Dawley rats by the inhibitor of aldehyde dehydrogenase, cyanamide, administered in a dose of 10 mg/kg twice daily for 3 days. Then following a standard preference test, each rat was offered water and its maximally preferred concentration of alcohol which ranged from 7% to 15%. Following a 4-day pre-drug test, saline control vehicle or amperozide was administered for 7 days by an osmotic minipump implanted in the intrascapular space. A single dose of 208 micrograms/kg/h (i.e., 5.0 mg/kg/day) was selected on the basis of a prior dose response study of amperozide. During the interval of sustained release of amperozide, the consumption of alcohol declined significantly in terms of both absolute g/kg intake and proportion of alcohol to water. When the preference of the rats was retested at 4, 30, 70, 110, and 140 day intervals after the pump had exhausted amperozide, the absolute g/kg consumption of alcohol continued to decline significantly. Unlike other drugs, amperozide did not produce any side effects, particularly on the intake of food or water or on body weight, which suggests a pharmacological specificity of its action.(ABSTRACT TRUNCATED AT 250 WORDS)

Time course of dopamine-D2 and serotonin-5-HT2 receptor occupancy rates by haloperidol and clozapine in vivo

In vivo occupancy of dopamine-D1, D2 and serotonin-5-HT2 receptors by haloperidol 10 mg/kg and clozapine 20 mg/kg were studied. Rats were injected intravenously with [3H]-YM-09151-2, [3H]-SCH23390, or [3H]-ketanserin 10 min after the administration of the tested drugs. Fifteen to 240 min after the ligand injection, the receptor occupancy rates of the drugs in the striatum and frontal cortex were calculated. Clozapine demonstrated the higher 5-HT2 and lower D2 occupancies in the respective regions. A dose-response analysis of D2 and 5-HT2 receptor occupancy by the drugs consolidated the higher 5-HT2 binding affinity of clozapine in comparison with haloperidol. The present methodology may serve as an accurate tool to evaluate the peculiarity of various antipsychotics.

Seroquel: biochemical profile of a potential atypical antipsychotic

Seroquel and the atypical antipsychotic clozapine were compared using a number of biochemical measures in rats which are indicative of potential antipsychotic activity and possible extrapyramidal side effect liability. Both in vitro and in vivo, these compounds are low potency D-2 dopamine (DA) receptor antagonists and are relatively more potent 5-HT2 antagonists than typical antipsychotic drugs. Seroquel also exhibited low affinity for D-1 DA receptors in vitro, but D-1 receptor occupancy was not detectable in vivo. Unlike clozapine, Seroquel lacks appreciable activity at either D-1 DA or muscarinic receptors. Following IP administration, both compounds produce similar elevations in DA metabolite concentrations. Following 1 month of daily administration, at doses which produce large increases in striatal DA metabolite concentrations, both Seroquel and clozapine fail, unlike typical antipsychotics, to increase the number of striatal D-2 receptors, but do decrease the number of 5-HT2 receptors in frontal cortex. ICI 204,636 produces a short-lasting increase in plasma prolactin levels, but these increases are much greater than those that are produced by clozapine. One day after 3 weeks of daily administration, tolerance, to the ability of Seroquel to elevate DA metabolite and plasma PRL concentrations is not observed. These biochemical observations are discussed with regard to the atypical profile of Seroquel in behavioral and electrophysiological studies.

Selective reduction by the 5-HT antagonist amperozide of alcohol preference induced in rats by systemic cyanamide

This investigation was undertaken to determine the effect of a unique psychotropic agent on the volitional drinking of alcohol induced pharmacologically in the rat by an inhibitor of aldehyde dehydrogenase. Following administration of cyanamide in a dose of 10 mg/kg twice daily for 3 days, the pattern of drinking of ethyl alcohol was determined in each of 12 Sprague-Dawley rats by means of a standard preference test for 3-30% alcohol vs. water. Then, each rat was offered water and its maximally preferred concentration of alcohol, which ranged from 7-15%. After a 4-day predrug test, either the saline control vehicle or the diphenylbutylpiperazinecarboxamide derivative, amperozide, was administered subcutaneously. The injections of amperozide were given b.i.d. at 1600 and 2200 h over 3 days in a dose of 0.5, 1.0, or 2.5 mg/kg. The intake of alcohol during the sequence of amperozide injections was significantly reduced in a dose-dependent manner in terms of both absolute g/kg and proportion of alcohol to water intake, whereas the saline control vehicle was without any effect on alcohol consumption. Although the highest dose of amperozide reduced the total intake of fluid due to the sharp decline in alcohol drinking, neither the consumption of food nor level of body weight was affected by any dose of the drug either during or after its administration. Because amperozide acts centrally on the synaptic activity of dopaminergic and serotonergic neurons in limbic system structures, it is envisaged that the drug ameliorates the aberrant drinking of alcohol by virtue of a direct effect on either one or both of these classes of neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

In rat brain amoxapine enhances dopamine metabolism: pharmacokinetic variations of the effect

When rats were given i.p. amoxapine, the drug was biotransformed to 7-hydroxyamoxapine, but not to 8-hydroxyamoxapine. The maximal concentrations of amoxapine and 7-hydroxyamoxapine in the serum and brain were found 30 min after the single injection, and the concentration of the former in the brain was higher than that of the latter. During the chronic treatment the concentration of amoxapine in the brain was much higher than that of 7-hydroxyamoxapine. A single administration of amoxapine increased the brain levels of dihydroxyphenylacetic acid and homovanillic acid. Their highest levels were observed 6 h after the injection. Repeated administration reduced the increases; chronic treatment caused tolerance to the enhancing effects on dopamine (DA) metabolism. Tolerance was observed in both striatum and hippocampus, but not in frontal cortex and hypothalamus. Single or chronic injection did not appear to change the level of DA in the brain. Amoxapine itself could be chiefly responsible for the enhancement of DA metabolism. In addition, the level of 3-methoxy-4-hydroxyphenylethyleneglycol in brain decreased transiently right after the injection of amoxapine, but the norepinephrine level did not seem to change following single or chronic administration of amoxapine.

Behavioral pharmacology of antagonists at 5-HT2/5-HT1C receptors

The possible implication of 5-HT2 receptors in CNS disorders such as schizophrenia, anxiety and depression suggests that 5-HT2 antagonists may be useful in the treatment of these disorders. The present review examines behavioral procedures used to characterize 5-HT2 antagonist properties of compounds and behavioral models of clinical activity in which 5-HT2 antagonists have been reported to be active. The pharmacological profile of 5-HT2 receptors in part resembles that of 5-HT1C receptors. Responses that have been proposed to involve the activation of 5-HT1C receptors are examined for their usefulness to detect 5-HT1C antagonist properties of compounds; these responses would help to differentiate 5-HT2 from 5-HT1C antagonist activity.