The role of alpha2-adrenoceptor antagonism in the anti-cataleptic properties of the atypical neuroleptic agent, clozapine, in the rat

Clozapine-Induced Cardiovascular Side Effects and Autonomic Dysfunction: A Systematic Review

Background: Clozapine is the antipsychotic of choice for treatment-resistant schizophrenia and has minimal risk for extrapyramidal symptoms. Therapeutic benefits, however, are accompanied by a myriad of cardiometabolic side-effects. The specific reasons for clozapine's high propensity to cause adverse cardiometabolic events remain unknown, but it is believed that autonomic dysfunction may play a role in many of these.

Objective: This systematic review summarizes the literature on autonomic dysfunction and related cardiovascular side effects associated with clozapine treatment.

Method: A search of the EMBASE, MEDLINE, and EBM Cochrane databases was conducted using the search terms antipsychotic agents, antipsychotic drug^*, antipsychotic^*, schizophrenia, schizophren^*, psychos^*, psychotic^*, mental ill^*, mental disorder^*, neuroleptic^*, cardiovascular^*, cardiovascular diseases, clozapine^*, clozaril^*, autonomic^*, sympathetic^*, catecholamine^*, norepinephrine, noradrenaline, epinephrine, adrenaline.

Results: The search yielded 37 studies that were reviewed, of which only 16 studies have used interventions to manage cardiovascular side effects. Side effects reported in the studies include myocarditis, orthostatic hypotension and tachycardia. These were attributed to sympathetic hyperactivity, decreased vagal contribution, blockade of cholinergic and adrenergic receptors, reduced heart rate variability and elevated catecholamines with clozapine use. Autonomic neuropathy was identified by monitoring blood pressure and heart rate changes in response to stimuli and by spectral analysis of heart rate variability. Metoprolol, lorazepam, atenolol, propranolol, amlodipine, vasopressin and norepinephrine infusion were used to treat tachycardia and fluctuations in blood pressure, yet results were limited to case reports.

Conclusion: The results indicate there is a lack of clinical studies investigating autonomic dysfunction and a limited use of interventions to manage cardiovascular side effects associated with clozapine. As there is often no alternative treatment for refractory schizophrenia, the current review highlights the need for better designed studies, use of autonomic tests for prevention of cardiovascular disease and development of novel interventions for clozapine-induced side effects.

The Role of Inhaled Loxapine in the Treatment of Acute Agitation in Patients with Psychiatric Disorders: A Clinical Review

Loxapine is a first generation antipsychotic, belonging to the dibenzoxazepine class. Recently, loxapine has been reformulated at a lower dose, producing an inhaled powder that can be directly administered to the lungs to treat the agitation associated with psychiatric disorders, such as schizophrenia and bipolar disorder. Thus, the aim of this narrative and clinical mini-review was to evaluate the efficacy and tolerability of inhaled loxapine in the treatment of acute agitation in patients with psychiatric disorders. The efficacy of inhaled loxapine has been evaluated in one Phase II trial on patients with schizophrenia, and in two Phase III trials in patients with schizophrenia and bipolar disorder. Moreover, there are two published case series on patients with borderline personality disorder and dual diagnosis patients. Inhaled loxapine has proven to be effective and generally well tolerated when administered to agitated patients with schizophrenia and bipolar disorder. Two case series have suggested that inhaled loxapine may also be useful to treat agitation in patients with borderline personality disorder and with dual diagnosis, but further studies are needed to clarify this point. However, the administration of inhaled loxapine requires at least some kind of patient collaboration, and is not recommended in the treatment of severe agitation in totally uncooperative patients. Moreover, the drug-related risk of bronchospasm must always be kept in mind when planning to use inhaled loxapine, leading to a careful patient assessment prior to, and after, administration. Also, the higher costs of inhaled loxapine, when compared to oral and intramuscular medications, should be taken into account when selecting it for the treatment of agitation.

Comparative analysis of the treatment of chronic antipsychotic drugs on epileptic susceptibility in genetically epilepsy-prone rats

Antipsychotic drugs (APs) are of great benefit in several psychiatric disorders, but they can be associated with various adverse effects, including seizures. To investigate the effects of chronic antipsychotic treatment on seizure susceptibility in genetically epilepsy-prone rats, some APs were administered for 7 weeks, and seizure susceptibility (audiogenic seizures) was evaluated once a week during treatment and for 5 weeks after drug withdrawal. Furthermore, acute and subchronic (5-day treatment) effects were also measured. Rats received haloperidol (0.2-1.0 mg/kg), clozapine (1-5 mg/kg), risperidone (0.03-0.50 mg/kg), quetiapine (2-10 mg/kg), aripriprazole (0.2-1.0 mg/kg), and olanzapine (0.13-0.66 mg/kg), and tested according to treatment duration. Acute administration of APs had no effect on seizures, whereas, after regular treatment, aripiprazole reduced seizure severity; haloperidol had no effects and all other APs increased seizure severity. In chronically treated rats, clozapine showed the most marked proconvulsant effects, followed by risperidone and olanzapine. Quetiapine and haloperidol had only modest effects, and aripiprazole was anticonvulsant. Finally, the proconvulsant effects lasted at least 2-3 weeks after treatment suspension; for aripiprazole, a proconvulsant rebound effect was observed. Taken together, these results indicate and confirm that APs might have the potential to increase the severity of audiogenic seizures but that aripiprazole may exert anticonvulsant effects. The use of APs in patients, particularly in patients with epilepsy, should be monitored for seizure occurrence, including during the time after cessation of therapy. Further studies will determine whether aripiprazole really has a potential as an anticonvulsant drug and might also be clinically relevant for epileptic patients with psychiatric comorbidities.

Cocaine seeking and taking: role of hippocampal dopamine D1-like receptors

Despite the well-documented involvement of dopamine D1-like receptor stimulation in cocaine-induced goal-directed behaviours, little is known about the specific contribution of D1-like receptor populations in the dorsal hippocampus (DH) to drug context-induced cocaine-seeking or drug-reinforced instrumental behaviours. To investigate this question, rats were trained to lever press for un-signalled cocaine infusions in a distinct context followed by extinction training in a different context. Cocaine-seeking behaviour (non-reinforced lever responding) was then assessed in the previously cocaine-paired and extinction contexts. SCH23390-induced D1-like receptor antagonism in the DH, but not the overlying trunk region of the somatosensory cortex, dose-dependently inhibited drug context-induced cocaine-seeking behaviour, without altering cocaine-reinforced instrumental responding, cocaine intake, food-reinforced instrumental responding, or general motor activity, relative to vehicle treatment. These findings suggest that D1-like receptor stimulation in the DH is critical for the incentive motivational effects and/or memory of cocaine-paired contextual stimuli that contribute to drug-seeking behaviour.

Contribution of a mesocorticolimbic subcircuit to drug context-induced reinstatement of cocaine-seeking behavior in rats

Cocaine-seeking behavior triggered by drug-paired environmental context exposure is dependent on orbitofrontal cortex (OFC)-basolateral amygdala (BLA) interactions. Here, we present evidence supporting the hypothesis that dopaminergic input from the ventral tegmental area (VTA) to the OFC critically regulates these interactions. In experiment 1, we employed site-specific pharmacological manipulations to show that dopamine D1-like receptor stimulation in the OFC is required for drug context-induced reinstatement of cocaine-seeking behavior following extinction training in an alternate context. Intra-OFC pretreatment with the dopamine D1-like receptor antagonist, SCH23390, dose-dependently attenuated cocaine-seeking behavior in an anatomically selective manner, without altering motor performance. Furthermore, the effects of SCH23390 could be surmounted by co-administration of a sub-threshold dose of the D1-like receptor agonist, SKF81297. In experiment 2, we examined effects of D1-like receptor antagonism in the OFC on OFC-BLA interactions using a functional disconnection manipulation. Unilateral SCH23390 administration into the OFC plus GABA agonist-induced neural inactivation of the contralateral or ipsilateral BLA disrupted drug context-induced cocaine-seeking behavior relative to vehicle, while independent unilateral manipulations of these brain regions were without effect. Finally, in experiment 3, we used fluorescent retrograde tracers to demonstrate that the VTA, but not the substantia nigra, sends dense intra- and interhemispheric projections to the OFC, which in turn has reciprocal bi-hemispheric connections with the BLA. These findings support that dopaminergic input from the VTA, via dopamine D1-like receptor stimulation in the OFC, is required for OFC-BLA functional interactions. Thus, a VTA-OFC-BLA neural circuit promotes drug context-induced motivated behavior.

Clozapine-induced locomotor suppression is mediated by 5-HT2A receptors in the forebrain

The need for safer, more effective therapeutics for the treatment of schizophrenia is widely acknowledged. To optimally target novel pharmacotherapies, in addition to establishing the mechanisms responsible for the beneficial effects of antipsychotics, the pathways underlying the most severe side effects must also be elucidated. Here we investigate the role of serotonin 2A (5-HT(2A)), serotonin 2C (5-HT(2C)), and dopamine 2 receptors (D₂) in mediating adverse effects associated with canonical first- and second-generation antipsychotic drugs in mice. Wild-type (WT) and 5-HT(2A) knockout (KO) mice treated with haloperidol, clozapine, and risperidone were assessed for locomotor activity and catalepsy. WT mice showed a marked reduction in locomotor activity following acute administration of haloperidol and high-dose risperidone, which was most likely secondary to the severe catalepsy caused by these compounds. Clozapine also dramatically reduced locomotor activity, but in the absence of catalepsy. Interestingly, 5-HT(2A) KO mice were cataleptic following haloperidol and risperidone, but did not respond to clozapine's locomotor-suppressing effects. Restoration of 5-HT(2A) expression to cortical glutamatergic neurons re-instated the locomotor-suppressing effects of clozapine in the open field. In sum, we confirm that haloperidol and risperidone caused catalepsy in rodents, driven by strong antagonism of D₂. We also demonstrate that clozapine decreases locomotor activity in a 5-HT(2A)-dependent manner, in the absence of catalepsy. Moreover, we show that it is the cortical population of 5-HT(2A) that mediate the locomotor-suppressing effects of clozapine.

l-tetrahydropalamatine: a potential new medication for the treatment of cocaine addiction

Levo-tetrahydropalmatine (l-THP) is an active constituent of herbal preparations containing plant species of the genera Stephania and Corydalis and has been approved and used in China for a number of clinical indications under the drug name Rotundine. The pharmacological profile of l-THP, which includes antagonism of dopamine D1 and D2 receptors and actions at dopamine D3, α adrenergic and serotonin receptors, suggests that it may have utility for treating cocaine addiction. In this review, we provide an overview of the pharmacological properties of l-THP and the evidence supporting its development as an anti-addiction medication. The results of preclinical work demonstrating that l-THP attenuates cocaine's reinforcing/rewarding effects and reinstatement in rat models of cocaine relapse are summarized, and the outcomes of studies demonstrating efficacy in human addicts are described. Finally, an overview of the safety profile of l-THP is provided and challenges associated with US FDA approval of l-THP are discussed.

Oral administration of levo-tetrahydropalmatine attenuates reinstatement of extinguished cocaine seeking by cocaine, stress or drug-associated cues in rats

Cocaine addiction is characterized by a persistently heightened susceptibility to drug relapse. For this reason, the identification of medications that prevent drug relapse is a critical goal of drug abuse research. Drug re-exposure, the onset of stressful life events, and exposure to cues previously associated with drug use have been identified as determinants of relapse in humans and have been found to reinstate extinguished cocaine seeking in rats. This study examined the effects of acute oral (gavage) administration of levo-tetrahydropalmatine (l-THP), a tetrahydroprotoberberine isoquinoline with a pharmacological profile that includes antagonism of D1, D2 and D3 dopamine receptors, on the reinstatement of extinguished cocaine seeking by a cocaine challenge (10mg/kg, ip), a stressor (uncontrollable electric footshock [EFS]) or response-contingent exposure to a stimulus (tone and light complex) previously associated with drug delivery in male Sprague-Dawley rats. Extinguished drug seeking was reinstated by ip cocaine, EFS, or response-contingent presentation of drug-associated cues in vehicle-pretreated rats following extinction of iv cocaine self-adminisration. Oral administration of either 3.0 or 10.0mg/kg l-THP 1h prior to reinstatement testing significantly attenuated reinstatement by each of the stimuli. Food-reinforced responding and baseline post-extinction responding were significantly attenuated at the 10.0, but not the 3.0mg/kg, l-THP dose, indicating that the effects of 3mg/kg l-THP on reinstatement were likely independent of non-specific motor impairment. These findings further suggest that l-THP may have utility for the treatment of cocaine addiction.

Therapeutic potential of alpha2 adrenoceptor antagonism for antipsychotic-induced extrapyramidal motor disorders

We examined the effects of JP-1302 (a selective alpha2C antagonist), BRL-44408 (a selective alpha2A antagonist) and yohimbine (a non-selective alpha2 antagonist) on haloperidol-induced bradykinesia and catalepsy in mice to elucidate the role of alpha2 adrenoceptor subtypes in modifying extrapyramidal motor disorders. JP-1302 (0.1-1 mg/kg, s.c.) dose-dependently ameliorated haloperidol-induced bradykinesia in the pole-test and reversed the catalepsy time increased by haloperidol. Antibradykinetic and anticataleptic actions of JP-1302 were statistically significant at 0.3 and 1 mg/kg, and these doses did not alter the ambulatory distance, rearing or center-perimeter residence time in the open-field test. BRL-44408 (1-10 mg/kg, s.c.) and yohimbine (0.3-3 mg/kg, i.p.) also ameliorated haloperidol-induced bradykinesia and catalepsy. However, both agents significantly decreased ambulatory distance and rearing in the open-field test, possibly reflecting their anxiogenic actions associated with alpha2A antagonism. The present study shows for the first time that blockade of alpha2C receptors can alleviate antipsychotic-induced extrapyramidal motor disorders without affecting gross behaviors.

Effects of clozapine on ethanol withdrawal syndrome in rats

AIMS

Co-morbid substance use in schizophrenic patients is common, and an important factor affects the outcome of disease. On the other hand, drug dependence is a predictive factor for psychosis. Alcohol is one of the most frequently abused psychoactive substances and may contribute psychotic symptoms in several conditions, such as withdrawal syndrome. The present study was designed to investigate the effects of clozapine on ethanol withdrawal syndrome (EWS) in rats.

METHODS

Adult male Wistar rats were used in the study. Ethanol (7.2%, v/v) was given to rats by a liquid diet for 14 days. An isocaloric liquid diet without containing ethanol was also given to control rats. Clozapine (2.5, 5 and 10 mg/kg) and its vehicle (0.1% acetic acid) were injected to rats subcutaneously at the 1.5th and 5.5th hours of ethanol withdrawal. At 2nd, 4th and 6th hours of ethanol withdrawal, rats were observed for 5 min and withdrawal signs that included locomotor hyperactivity, agitation, tremor, tail stiffness, stereotyped behaviour and wet dog shakes were recorded or rated. Following the observations at 6th hour, subjects were tested for audiogenic seizures.

RESULTS

Clozapine significantly and dose-dependently inhibited the EWS-induced locomotor hyperactivity, wet dog shake, stereotyped behaviour, tremor and tail stiffness. However, it did not produce any significant effect on agitation and audiogenic seizures. Doses of clozapine used in the present study did not produce any significant change on locomotor activities of naïve rats.

CONCLUSIONS

Our results suggest that clozapine had some significant beneficial effects on EWS in rats. Thus, this drug may be helpful for controlling some withdrawal signs in ethanol-dependent patients.

Pharmacological characterization and CNS effects of a novel highly selective alpha2C-adrenoceptor antagonist JP-1302

BACKGROUND AND PURPOSE

Pharmacological validation of novel functions for the alpha2A-, alpha2B-, and alpha2C-adrenoceptor (AR) subtypes has been hampered by the limited specificity and subtype-selectivity of available ligands. The current study describes a novel highly selective alpha2C-adrenoceptor antagonist, JP-1302 (acridin-9-yl-[4-(4-methylpiperazin-1-yl)-phenyl]amine).

EXPERIMENTAL APPROACH

Standard in vitro binding and antagonism assays were employed to demonstrate the alpha2C-AR specificity of JP-1302. In addition, JP-1302 was tested in the forced swimming test (FST) and the prepulse-inhibition of startle reflex (PPI) model because mice with genetically altered alpha2C-adrenoceptors have previously been shown to exhibit different reactivity in these tests when compared to wild-type controls.

KEY RESULTS

JP-1302 displayed antagonism potencies (KB values) of 1,500, 2,200 and 16 nM at the human alpha2A-, alpha2B-, and alpha2C-adrenoceptor subtypes, respectively. JP-1302 produced antidepressant and antipsychotic-like effects, i.e. it effectively reduced immobility in the FST and reversed the phencyclidine-induced PPI deficit. Unlike the alpha2-subtype non-selective antagonist atipamezole, JP-1302 was not able to antagonize alpha2-agonist-induced sedation (measured as inhibition of spontaneous locomotor activity), hypothermia, alpha2-agonist-induced mydriasis or inhibition of vas deferens contractions, effects that have been generally attributed to the alpha2A-adrenoceptor subtype. In contrast to JP-1302, atipamezole did not antagonize the PCP-induced prepulse-inhibition deficit.

CONCLUSIONS AND IMPLICATIONS

The results provide further support for the hypothesis that specific antagonism of the alpha2C-adrenoceptor may have therapeutic potential as a novel mechanism for the treatment of neuropsychiatric disorders.

JP-1302: a new tool to shed light on the roles of alpha2C-adrenoceptors in brain

The discovery of JP-1302 as a selective, high affinity antagonist at the alpha2C-adrenoceptor will enable researchers to probe the functional role and address the therapeutic utility of this potentially highly important adrenoceptor subtype.

Hydrazides of clozapine: a new class of D1 dopamine receptor subtype selective antagonists

Acylated and aroylated hydrazinoclozapines are highly potent dopamine D(1) antagonists that show remarkable selectivity over other dopamine receptors. The most potent compound in this series is the 2,6-dimethoxybenzhydrazide 33 with a D(1)K(i) of 1.6 nM and 212-fold selectivity over D(2) receptor.

Contrasting loxapine to its isomer isoloxapine--the critical role of in vivo D2 blockade in determining atypicality

BACKGROUND

Loxapine is a typical antipsychotic while isoloxapine, its 8Cl-isomer, shows atypicality in some animal models. The basis for this difference is not well understood. The purpose of this study was to systematically compare the two drugs in in vitro and in vivo animal models, and to understand mechanisms underlying their differential typical/atypical profiles.

METHODS

The in vitro and in vivo receptor profiles as well as the action of loxapine and isoloxapine on rat conditioned avoidance response (CAR), catalepsy (CAT), striatal FOS expression and prolactin levels were determined. To understand loxapine's typical profile, we added MDL100,907, to provide loxapine+MDL the same in vivo 5-HT2/D2 ratio as isoloxapine, while holding its D2 component constant.

RESULTS

Isoloxapine behaved as an "atypical" antipsychotic demonstrating CAR inhibition, low CAT, no significant prolactin elevation, and minimal FOS expression in the dorsolateral striatum. Loxapine behaved like a typical antipsychotic, showing unexpectedly high in vivo D2 occupancy. Addition of MDL100,907, which resulted in a very high 5-HT2/D2 in vivo ratio, did not alter loxapine + MDL's typical profile.

CONCLUSIONS

Loxapine's behaviour as a typical antipsychotic is most likely due to its disproportionately high D2 occupancy. Appropriate action at D2 receptors in vivo, rather than the high 5-HT2/D2 ratio, seems to be critical in determining why isoloxapine behaves like an atypical antipsychotic.

Anticataleptic properties of alpha2 adrenergic antagonists in the crossed leg position and bar tests: differential mediation by 5-HT1A receptor activation

RATIONALE

Recent studies suggest that alpha(2) adrenoceptor blockade may improve the antipsychotic-like effects of neuroleptics and attenuate dopamine D(2) receptor antagonist-induced catalepsy. However, several alpha(2) adrenergic antagonists also display serotonin 5-HT(1A) receptor agonist activity, which may contribute to anticataleptic actions.

OBJECTIVES

In this study, we examined a series of alpha(2) adrenergic antagonists to determine the role of activity at serotonin 5-HT(1A) receptors in their anticataleptic effects.

METHODS

Catalepsy in rats induced by the antipsychotic haloperidol (2.5 mg/kg, SC) was measured using the cross-legged position (CLP) and bar tests. The compounds examined in this study, in decreasing rank order of alpha(2) adrenergic versus 5-HT(1A) receptor selectivity, were atipamezole, methoxy-idazoxan (RX821002), efaroxan, idazoxan, and yohimbine. Antagonism studies were conducted using the selective 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide dihydrochloride (WAY100635).

RESULTS

Idazoxan, efaroxan, and yohimbine significantly attenuated the cataleptic effects of haloperidol (2.5 mg/kg, SC) in the CLP test and the actions of their highest doses were significantly blocked by pre-treatment with WAY100635 (0.63 mg/kg, SC). In contrast to the other compounds, methoxy-idazoxan was ineffective in the CLP test. Atipamezole exhibited anticataleptic effects in the bar and CLP tests which were not blocked by WAY100635. Similarly, the anticataleptic effects of methoxy-idazoxan and idazoxan in the bar test were not blocked by WAY100635.

CONCLUSIONS

Serotonin 5-HT(1A) receptors play a prominent role in anticataleptic effects of certain alpha(2) adrenergic antagonists in the CLP test, whereas alpha(2)-adrenergic mechanisms are likely to be primarily responsible for the anticataleptic effects of these ligands in the bar test.

Serotonergic influence on the potentiation of D-amphetamine and apomorphine-induced rotational behavior by the α2-adrenoceptor antagonist 2-methoxy idazoxan in hemiparkinsonian rats

The alpha(2)-adrenoceptor antagonists potentiate both ipsilateral and contralateral rotations induced by amphetamine and apomorphine respectively in hemiparkinsonian rats. The present study investigated the role of serotonergic transmission in this potentiation in unilaterally 6-hydroxydopamine nigral lesioned rats. D-amphetamine (0.5 mg/kg, i.p.) produced ipsilateral rotations, which were decreased by the dopamine receptor antagonist haloperidol (0.2 mg/kg, i.p.) and the alpha(1)-receptor antagonist prazosin (1 mg/kg, i.p.). The selective alpha(2)-antagonist 2-methoxy idazoxan (0.2 mg/kg, i.p.) potentiated the amphetamine-induced ipsilateral rotations, that were attenuated by haloperidol and prazosin. The selective serotonin re-uptake inhibitor citalopram (10 mg/kg, i.p.) and selective serotonin synthesis inhibitor p-chlorophenylalanine (150 mg/kg, i.p., 3 days) decreased and increased the observed potentiation respectively. Apomorphine (0.2 mg/kg, s.c.) produced contralateral rotations, which were decreased by haloperidol but not by prazosin. 2-methoxy idazoxan potentiated these rotations which were attenuated by haloperidol but not by prazosin. Citalopram and p-chlorophenylalanine increased and decreased the observed potentiation respectively. Citalopram and p-chlorophenylalanine had no effect by per se on D-amphetamine and apomorphine-induced rotations. 2-methoxy idazoxan alone increased both ipsilateral and contralateral spontaneous rotations. Taken together, these findings indicate that an increase in noradrenergic tone by 2-methoxy idazoxan potentiates both D-amphetamine-induced ipsilateral and apomorphine induced contralateral rotations. alpha(1)-Antagonism attenuates D-amphetamine induced ipsilateral rotations and its potentiation by 2-methoxy idazoxan but not apomorphine rotations or its potentiation. Increasing and decreasing the serotonergic transmission decreases and increases D-amphetamine potentiation, whereas increases and decreases apomorphine potentiation respectively. The possible mechanisms for these findings are discussed.

Receptor mechanisms in the treatment of schizophrenia

There remain many limitations to the treatment of schizophrenia. In addition to the poor response of negative and cognitive symptoms to antipsychotics, and the substantial proportion of poor- or non-responders, there are a variety of unpleasant and restricting side-effects of these drugs. The introduction of several 'atypical' drugs, with diminished propensity to cause extrapyramidal motor effects (EPS), has greatly improved the tolerability of antipsychotic treatments. The pharmacology of atypical antipsychotics is varied and, although dopamine D2 receptor antagonism is common to all antipsychotics, the mechanisms of a typicality are complex and not fully understood. Thus, antagonism at 5-HT2 and/or other receptors, weak dopamine receptor affinity and, most recently, partial agonism at dopamine D2 receptors, have been variously implicated. However, because EPS have diminished with improvements in drug treatment, drug-induced weight gain has emerged as a major concern, and the pharmacological basis of this problem, involving effects at 5-HT2c and perhaps other receptors, is yielding to investigation. Some drugs, notably the D2 partial agonists, can provide antipsychotic effects without the emergence of several of the seproblematic side-effects, which bodes well for future treatment.

Haloperidol, but not clozapine, produces dramatic catalepsy in delta9-THC-treated rats: possible clinical implications

The effect on rat catalepsy induced by Delta9-tetrahydrocannabinol (Delta9-THC) in association with haloperidol (HP) or clozapine (CLOZ) administration was investigated. Delta9-THC dose-dependently increased HP (0.05-1 mg kg-1, s.c.)-induced rat catalepsy, while no catalepsy was observed after CLOZ (1-20 mg kg-1, s.c.) or Delta9-THC+CLOZ administration. The CB1 antagonist SR141716A (0.5-5 mg kg-1, i.p.) reversed the increase mediated by Delta9-THC on HP-induced catalepsy. The D2 agonist quinpirole completely reversed the catalepsy induced by both HP and HP+Delta9-THC; however, higher doses of quinpirole were needed in the presence of Delta9-THC. The M1 antagonist scopolamine and alpha2 antagonist yohimbine were able to reduce the catalepsy induced by HP and HP+Delta9-THC in a similar manner. CLOZ and the 5-HT2A/2C antagonists ritanserin, RS102221 and SB242084 were more effective in antagonizing HP than HP+Delta9-THC-induced catalepsy.7 HP and CLOZ failed to inhibit in vitro [3H]CP-55,940 binding, while Delta9-THC and SR141716A did not show an appreciable affinity for the D2 receptor. It was suggested that the different effects on rat catalepsy induced by Delta9-THC following HP or CLOZ administration may depend on the receptor-binding profiles of the two antipsychotics. The preferential use of CLOZ rather than HP in the treatment of psychotic symptoms in cannabis abusers was discussed.

A comparison of the effects of loxapine with ziprasidone and thioridazine on the release of dopamine and acetylcholine in the prefrontal cortex and nucleus accumbens

RATIONALE

Atypical, but not typical, antipsychotic drugs (APDs), produce preferential increases in dopamine (DA) and acetylcholine (ACh) release in rat medial prefrontal cortex (mPFC) compared to the nucleus accumbens (NAC). The increase in DA release has been attributed, in part, to their greater serotonin (5-HT)(2A) relative to D(2) receptor occupancy, while the basis for the increase in ACh has not yet been determined. Loxapine, a dibenzoxazepine congener of clozapine, is generally considered to be a typical APD because it produces significant extrapyramidal symptoms (EPS) in humans, at generally recommended clinical doses (60-100 mg/day), and catalepsy in rodents, although several studies have found it to be effective at lower doses which do not produce significant EPS. Moreover, loxapine, like its congener clozapine, has higher affinity for serotonin (5-HT)(2A) than dopamine D(2) receptors, in vitro, suggesting the possibility it could be an atypical APD with clozapine-like potential.

OBJECTIVES

The purpose of this study was to compare the effects of loxapine on DA and ACh release in the mPFC and NAC with those of ziprasidone, a novel atypical APD, and thioridazine, which is generally classified as a typical APD.

RESULTS

Loxapine, 0.03-10 mg/kg, increased prefrontal dopamine release with the magnitude of this increase exceeding that in the NAC, at all doses, other than the 10 mg/kg dose. The effect of loxapine (0.3 mg/kg) on DA release in the prefrontal cortex was attenuated by WAY 100635 (0.2 mg/kg), a 5-HT(1A) antagonist, as is the case for other atypical APDs. Ziprasidone (0.1-3 mg/kg) also preferentially increased DA release in the mPFC compared to NAC. Thioridazine (5 and 20 mg/kg) did not increase DA release in either the mPFC or NAC. Loxapine (3 mg/kg) and ziprasidone (1 and 3 mg/kg), but not thioridazine (10 and 20 mg/kg), significantly increased cortical ACh release.

CONCLUSION

Loxapine has effects on cortical and NAC DA and ACh release which are comparable to those of known atypical APDs. Ziprasidone and thioridazine have effects on cortical DA and ACh characteristic of atypical and typical APDs, respectively. It is concluded that further clinical studies of the atypical APD properties of loxapine are indicated.

Anti-cataleptic effects of clozapine, but not olanzapine and quetiapine, on SCH 23390- or raclopride-induced catalepsy in rats

The present study investigated potential anti-cataleptic properties of the prototype atypical antipsychotic clozapine and two newly developed atypical antipsychotics, olanzapine and quetiapine, which are structurally related and display similar pharmacological profiles to clozapine. Clozapine (2.5 mg kg(-1), s.c.), but not olanzapine (2.0 mg kg(-1), s.c.) and quetiapine (20.0 mg kg(-1), s.c.), blocked catalepsy induced either by the dopamine D(1/5) receptor antagonist SCH 23390 (50.0 microg kg(-1), s.c) or the selective dopamine D(2/3) receptor antagonist raclopride (4.0 mg kg(-1), s.c.). Such findings are consistent with the beneficial effects of clozapine in the management of drug-induced psychosis in parkinsonian patients, and suggest that neither olanzapine nor quetiapine may be a safe alternative to clozapine in this field. Furthermore, the results indicate that clozapine has a unique pharmacological profile that distinguishes it from olanzapine and quetiapine. The mechanisms underlying anti-cataleptic or anti-parkinsonian properties of clozapine are unclear but may be related to dopamine D(1) receptor agonism of clozapine.

The alpha 2-adrenoceptor antagonist idazoxan reverses catalepsy induced by haloperidol in rats independent of striatal dopamine release: role of serotonergic mechanisms

The alpha(2)-adrenoceptor antagonist idazoxan may improve motor symptoms in Parkinson's disease and experimental Parkinsonism. We studied the effect of idazoxan on haloperidol-induced catalepsy in rats, an animal model of the drug-induced extrapyramidal side effects in man. Catalepsy was induced by a subcutaneous (s.c.) injection of haloperidol (1 mg/kg) and measured by the bar test for a maximum of 5 min. At 3 h after haloperidol, rats were given 0.16-5.0 mg/kg s.c. idazoxan, and descent latency was measured 1 h later. Idazoxan potently reversed haloperidol-induced catalepsy with an ED(50) of 0.25 mg/kg. This effect was mimicked by the selective alpha(2)-adrenoceptor antagonist RS-15385-197 (0.3 and 1 mg/kg orally). We assessed how dopaminergic mechanisms were involved in the anticataleptic effect of idazoxan by studying its effect on dopamine (DA) release in the striatum, with the microdialysis technique in conscious rats. Idazoxan (0.3 and 2.5 mg/kg) had no effect on extracellular DA and did not modify the rise of extracellular DA induced by haloperidol, indicating that changes of striatal DA release were not involved in the reversal of catalepsy. The anticataleptic effect of 2.5 mg/kg idazoxan (haloperidol+vehicle 288+/-8 s, haloperidol+idazoxan 47+/-22 s) was attenuated in rats given an intraventricular injection of 150 microg of the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (haloperidol+vehicle 275+/-25 s, haloperidol+idazoxan 137+/-28 s). The 5-HT(1A) receptor antagonist WAY100 635 (0.1 mg/kg s.c.) did not affect the anticataleptic effect of idazoxan. The results suggest that idazoxan reversed haloperidol-induced catalepsy by a mechanism involving blockade of alpha(2)-adrenoceptors and, at least in part, 5-HT neurons.

Synthesis and pharmacological evaluation of 1-[(1,2-diphenyl-1H-4-imidazolyl)methyl]-4-phenylpiperazines with clozapine-like mixed activities at dopamine D(2), serotonin, and GABA(A) receptors

A series of 18 1-[(1,2-diphenyl-1H-4-imidazolyl)methyl]-4-piperazines (1a-r) were designed and synthesized as possible ligands with mixed dopamine (DA) D(2)/serotonin 5-HT(1A) affinity, with the aim of identifying novel compounds with neurochemical and pharmacological properties similar to those of clozapine. The binding profile at D(2) like, 5-HT(1A), and 5-HT(2A) receptors of title compounds was determined. Modifications made in the phenyl rings of the parent compound (1a) produced congeners endowed with a broad range of binding affinities for DA D(2) like, serotonin 5-HT(1A), and 5-HT(2A) receptors, with IC(50) values ranging from 25 to >10,000 nM. As for the modification of the piperazine N(4)-phenyl ring, the affinities for both D(2) like and 5-HT(1A) receptors were progressively increased by introduction of ortho-methoxy and ethoxy groups (1b,o, respectively). Data revealed the presence of a para-chloro substituent in 1g to be associated with a relatively high affinity and substantial selectivity for D(2) like receptors, whereas the meta-chloro analogue 1f exhibited preferential affinity for 5-HT(1A) receptors. A quantitative structure-affinity relationship analysis of the measured binding data resulted in regression equations that highlighted substituent physicochemical properties modulating the binding to subtypes 1A and 2A of serotonin 5-HT receptors but not to D(2) like receptors. Thus, besides an electron-withdrawing field effect and ortho substitution, which both influence binding to serotonin 5-HT receptor subtypes, though to a different extent as revealed by regression coefficients in the multiparametric regression equations, the affinity of congeners 1a-r to 5-HT(1A) receptors proved to be linearly correlated with volume/polarizability descriptors, whereas their affinity to 5-HT(2A) receptors correlated with lipophilicity constants through a parabolic relationship. 1-[(1,2-Diphenyl-1H-4-imidazolyl)methyl]-4-(2-methoxyphenyl)piperazine (1b), with a D(2)/5-HT(1A) IC(50) ratio of approximately 1, was selected for a further pharmacological study. In rats, the intraperitoneal administration of compound 1b, like that of clozapine, induced an increase in the extracellular concentration of DA measured in the medial prefrontal cortex. Furthermore, 1b and clozapine each inhibited GABA-evoked Cl(-) currents at recombinant GABA(A) receptors expressed in Xenopus oocytes. These findings suggest that compound 1b may represent an interesting prototype of a novel class of drugs endowed with a neurochemical profile similar to that of atypical antipsychotics.

Effect of the amisulpride isomers on rat catalepsy

The substituted benzamide amisulpride is currently administered in its racemic form. In the present study, the biochemical and cataleptogenic profiles of the two enantiomers (R+ and S-) were compared with those of the racemic mixture. Displacement binding studies showed that the (S-)-isomer possesses an higher affinity for dopamine D2-like receptor (K(i) 5.2+/-0.4 nM) compared to (R+)-amisulpride (K(i) 244+/-12 nM) and to (RS)-amisulpride (K(i) 9.8+/-0.8 nM). In contrast, (S-)-amisulpride binds the alpha(2)-receptor with an affinity (K(i) 1528+/-45 nM) lower than that of the (R+)-isomer (K(i) 375+/-34 nM) and of (RS)-amisulpride (K(i) 783+/-27 nM). The bar test was used to evaluate the catalepsy induced by each drug. (RS)-amisulpride induced catalepsy only at very high doses (>100 mg/kg, s.c.) whereas, (S-)-amisulpride produced a catalepsy at a lower dose (30 mg/kg, s.c.) and (R+)-amisulpride did not produce any catalepsy up to the dose of 75 mg/kg. Interestingly, (R+)-amisulpride reduced the catalepsy induced by (S-)-amisulpride (50 mg/kg, s.c.) or haloperidol (0.3 mg/kg, s.c.), at the doses of 50 or 30 mg/kg, respectively. These results indicate that the weak cataleptic properties of (RS)-amisulpride might partially rely on its (R+)-isomer and provide a further explanation for the atypical properties of amisulpride as an antipsychotic.

Antipsychotic drug use in neurodegenerative disease in the elderly: problems and potential from a pharmacological perspective

The past decade has seen the introduction of several new antipsychotics for the treatment of schizophrenia. These drugs demonstrate substantially lower levels of extrapyramidal side effects (EPS) than the classical antipsychotics, as well as having (often poorly supported) claims of increased efficacy at ameliorating certain schizophrenic syndromes. Increasingly, these 'atypical' drugs are being used in the treatment of psychotic or related behavioural disturbances in patients with neurodegenerative disease. Thus, some newer antipsychotics are particularly valuable in ameliorating the L-dopa-induced psychosis in Parkinson's disease, while behavioural problems in dementing disorders, such as those occurring in Alzheimer's disease, are also frequently treated by antipsychotic drugs. The relationship between drug pharmacology and neurotransmitter pathology is essential to understanding the relative efficacy of individual antipsychotic drugs in treating the psychotic and behavioural disturbances of neurodegenerative disorders.

Monoamine pharmacology of the lobster cardiac ganglion

Monoamine agonists and antagonists were applied to the lobster cardiac ganglion in an attempt to clarify the different actions of 5-hydroxytryptamine (5HT) and dopamine (DA) on this rhythmic pattern generator. Experiments were designed to determine whether the similar responses to 5HT and DA applied to the anterior region of the ganglion could be separated by pharmacological approaches, and whether the different responses to 5HT applied to the anterior and posterior regions of the ganglion could be attributed to mediation by different receptors. A small number of the 5HT agonists which were tested mimic the effects of 5HT, in that they increase the frequency of bursting and decrease burst duration when applied to the whole ganglion, but decrease burst frequency and increase burst duration when applied only to the posterior half. Other 5HT agonists decrease frequency and prolong bursts when applied to the whole ganglion. Of the DA agonists tested, none acts as DA itself does. Rather, they mimic the effects of 5HT applied to the posterior ganglion, by slowing bursting and prolonging bursts. The actions of agonists do not correspond in any clear way to the receptor specificities as defined in vertebrates. Most antagonists tested do not show similar specificities to their effects in vertebrates. In particular, most of the DA antagonists tested are more effective in blocking exogenous 5HT than DA. One monoamine agonist directly alters the properties of endogenous burst-organizing potentials (driver potentials) in the motorneurons of the ganglion.

Low-dose clozapine pretreatment partially prevents haloperidol-induced deficits in conditioned active avoidance

The effectiveness of neuroleptics in disrupting conditioned active avoidance has led to the widespread use of this test as an index of antipsychotic efficacy, whereas the tendency for these drugs to induce catalepsy is believed to reflect their propensity to cause extrapyramidal motor side-effects. Although the typical neuroleptic haloperidol produces catalepsy as well as profound deficits in conditioned active avoidance, the atypical neuroleptic clozapine does not induce catalepsy and is less effective than haloperidol in disrupting active avoidance. Furthermore, clozapine pretreatment prevents haloperidol-induced catalepsy. We investigated whether clozapine pretreatment might also reduce the disruptive effects of haloperidol on two-way active avoidance. We assessed the avoidance acquisition of the following drug treatment groups in which all animals received two injections prior to testing: vehicle + vehicle, vehicle + haloperidol (0.1 mg/kg, i.p.), clozapine (2.5, 5.0 or 10 mg/kg, i.p.) + haloperidol (0.1 mg/kg, i.p.), or clozapine (2.5, 5.0 or 10 mg/kg, i.p.) + vehicle. Haloperidol-pretreated animals showed markedly impaired active avoidance, deficits which were improved by 2.5 and 5 mg/kg but not by 10 mg/kg clozapine pretreatment. These data suggest that the disruptive effects of haloperidol on conditioned active avoidance partially mirror its capacity to induce catalepsy and extrapyramidal motor symptoms. Furthermore, this study indicates that clozapine may be effective in reducing motor side-effects caused by typical neuroleptics.