Olanzapine and JL13 induce cross-tolerance to the clozapine discriminative stimulus in rats

Drug Discrimination: Historical Origins, Important Concepts, and Principles

Research on the stimulus properties of drugs began with studies on state dependent learning during the first half of the twentieth century. From that research, an entirely new approach evolved called drug discrimination. Animals (including humans) could discriminate the presence or absence of a drug; once learned, the drug could serve as a discriminative stimulus, signaling the availability or nonavailability of reinforcement. Early drug discrimination research involved the use of a T-maze task, which evolved in the 1970s into a two-lever operant drug discrimination task that is still used today. A number of important concepts and principles of drug discrimination are discussed. (1) The discriminative stimulus properties of drugs are believed in large part to reflect the subjective effects of drugs. While it has been impossible to directly measure subjective effects in nonhuman animals, drug discrimination studies in human subjects have generally supported the belief that discriminative stimulus properties of drugs in nonhuman animals correlate highly with subjective effects of drugs in humans. In addition to the ability of the drug discrimination procedure to measure the subjective effects of drugs, it has a number of other strengths that help make it a valuable preclinical assay. (2) Drug discrimination can be used for classification of drugs based on shared discriminative stimulus properties. (3) The phenomena of tolerance and cross-tolerance can be studied with drug discrimination. (4) Discriminative stimulus properties of drugs typically have been found to be stereospecific, if a drug is comprised of enantiomers. (5) Discriminative stimulus properties of drugs reflect specific CNS activity at neurotransmitter receptors. (6) Both human and nonhuman subjects display individual differences in their sensitivity to discriminative stimuli and drugs. (7) The drug discrimination procedure has been used extensively as a preclinical assay in drug development. This chapter is the first in the volume The Behavioural Neuroscience of Drug Discrimination, which includes chapters concerning the discriminative stimulus properties of various classes of psychoactive drugs as well as sections on the applications and approaches for using this procedure.

Prefrontal cortical glutathione-dependent defense and proinflammatory mediators in chronically isolated rats: Modulation by fluoxetine or clozapine

Chronic psychosocial stress modulates brain antioxidant systems and causes neuroinflammation that plays a role in the pathophysiology of depression. Although the antidepressant fluoxetine (FLX) represents the first-line treatment for depression and the atypical antipsychotic clozapine (CLZ) is considered as a second-line treatment for psychotic disorders, the downstream mechanisms of action of these treatments, beyond serotonergic or dopaminergic signaling, remain elusive. We examined behavioral changes, glutathione (GSH)-dependent defense and levels of proinflammatory mediators in the prefrontal cortex (PFC) of adult male Wistar rats exposed to 21days of chronic social isolation (CSIS). We also tested the ability of FLX (15mg/kg/day) or CLZ (20mg/kg/day), applied during CSIS, to prevent stress-induced changes. CSIS caused depressive- and anxiety-like behaviors, compromised GSH-dependent defense, and induced nuclear factor-kappa B (NF-κB) activation with a concomitant increase in cytosolic levels of proinflammatory mediators cyclooxigenase-2, interleukin-1beta and tumor necrosis factor-alpha in the PFC. NF-κB activation and proinflammatory response in the PFC were not found in CSIS rats treated with FLX or CLZ. In contrast, only FLX preserved GSH content in CSIS rats. CLZ not only failed to protect against CSIS-induced GSH depletion, but it diminished its levels when applied to non-stressed rats. In conclusion, prefrontal cortical GSH depletion and the proinflammatory response underlying depressive- and anxiety-like states induced by CSIS were prevented by FLX. The protective effect of CLZ, which was equally effective as FLX on the behavioral level, was limited to proinflammatory components. Hence, different mechanisms underlie the protective effects of these two drugs in CSIS rats.

Antipsychotic-induced sensitization and tolerance: Behavioral characteristics, developmental impacts, and neurobiological mechanisms

Antipsychotic sensitization and tolerance refer to the increased and decreased drug effects due to past drug use, respectively. Both effects reflect the long-term impacts of antipsychotic treatment on the brain and result from the brain's adaptive response to the foreign property of the drug. In this review, clinical evidence of the behavioral aspect of antipsychotic sensitization and tolerance is selectively reviewed, followed by an overview of preclinical literature that examines these behavioral characteristics and the related pharmacological and nonpharmacological factors. Next, recent work on the developmental impacts of adolescent antipsychotic sensitization and tolerance is presented and recent research that delineates the neurobiological mechanisms of antipsychotic sensitization and tolerance is summarized. A theoretical framework based on "drug learning and memory" principles is proposed to account for the phenomena of antipsychotic sensitization and tolerance. It is maintained that antipsychotic sensitization and tolerance follow basic principles of learning or acquisition ("induction") and memory ("expression"). The induction and expression of both effects reflect the consequences of associative and nonassociative processing and are strongly influenced by various pharmacological, environmental, and behavioral factors. Drug-induced neuroplasticity, such as functional changes of striatal dopamine D2 and prefrontal serotonin (5-HT)2A receptors and their mediated signaling pathways, in principle, is responsible for antipsychotic sensitization and tolerance. Understanding the behavioral characteristics and neurobiological underpinnings of antipsychotic sensitization and tolerance has greatly enhanced our understanding of mechanisms of antipsychotic action, and may have important implications for future drug discovery and clinical practice.

Effects of central activation of serotonin 5-HT2A/2C or dopamine D 2/3 receptors on the acute and repeated effects of clozapine in the conditioned avoidance response test

RATIONALE

Acute administration of clozapine (a gold standard of atypical antipsychotics) disrupts avoidance response in rodents, while repeated administration often causes a tolerance effect.

OBJECTIVE

The present study investigated the neuroanatomical basis and receptor mechanisms of acute and repeated effects of clozapine treatment in the conditioned avoidance response test in male Sprague-Dawley rats.

METHODS

2,5-dimethoxy-4-iodo-amphetamine (DOI, a preferential 5-HT2A/2C agonist) or quinpirole (a preferential dopamine D2/3 agonist) was microinjected into the medial prefrontal cortex (mPFC) or nucleus accumbens shell (NAs), and their effects on the acute and long-term avoidance disruptive effect of clozapine were tested.

RESULTS

Intra-mPFC microinjection of quinpirole enhanced the acute avoidance disruptive effect of clozapine (10 mg/kg, sc), while DOI microinjections reduced it marginally. Repeated administration of clozapine (10 mg/kg, sc) daily for 5 days caused a progressive decrease in its inhibition of avoidance responding, indicating tolerance development. Intra-mPFC microinjection of DOI at 25.0 (but not 5.0) μg/side during this period completely abolished the expression of clozapine tolerance. This was indicated by the finding that clozapine-treated rats centrally infused with 25.0 μg/side DOI did not show higher levels of avoidance responses than the vehicle-treated rats in the clozapine challenge test. Microinjection of DOI into the mPFC immediately before the challenge test also decreased the expression of clozapine tolerance.

CONCLUSIONS

Acute behavioral effect of clozapine can be enhanced by activation of the D2/3 receptors in the mPFC. Clozapine tolerance expression relies on the neuroplasticity initiated by its antagonist action against 5-HT2A/2C receptors in the mPFC.

Base-regulated tunable synthesis of pyridobenzoxazepinones and pyridobenzoxazines

A base-regulated one-pot protocol for the tunable synthesis of pyridobenzoxazepinones and pyridobenzoxazines has been developed. Pyridobenzoxazepinones and pyridobenzoxazines were produced in good yields selectively.

Adult response to olanzapine or clozapine treatment is altered by adolescent antipsychotic exposure: a preclinical test in the phencyclidine hyperlocomotion model

This study examined how repeated olanzapine (OLZ) or clozapine (CLZ) treatment in adolescence alters sensitivity to the same drug in adulthood in the phencyclidine (PCP) hyperlocomotion model. Male adolescent Sprague-Dawley rats (postnatal day (P) 44-48) were first treated with OLZ (1.0 or 2.0 mg/kg, subcutaneously (sc)) or CLZ (10.0 or 20.0 mg/kg, sc) and tested in the PCP (3.2 mg/kg, sc)-induced hyperlocomotion model for five consecutive days. Then a challenge test with OLZ (0.5 mg/kg) or CLZ (5.0 mg/kg) was administered either during adolescence (~P 51) or after the rats matured into adults (~P 76 and 91). During adolescence, repeated OLZ or CLZ treatment produced a persistent inhibition of PCP-induced hyperlocomotion across the five test days. In the challenge test during adolescence, rats previously treated with OLZ did not show a significantly stronger inhibition of PCP-induced hyperlocomotion than those previously treated with vehicle (VEH). In contrast, those previously treated with CLZ showed a weaker inhibition than the VEH controls. When assessed in adulthood, the enhanced sensitivity to OLZ and the decreased sensitivity to CLZ were detected on ~P 76, even on ~P 91 in the case of OLZ. These findings suggest that adolescent OLZ or CLZ exposure can induce long-term alterations in antipsychotic response that persist into adulthood.

Olanzapine sensitization and clozapine tolerance: from adolescence to adulthood in the conditioned avoidance response model

Disruption of conditioned avoidance response (CAR) in rodents is one trademark feature of many antipsychotic drugs. In adult rats, repeated olanzapine (OLZ) treatment causes an enhanced disruption of avoidance response (sensitization), whereas repeated clozapine (CLZ) treatment causes a decreased disruption (tolerance). The present study addressed (1) whether OLZ sensitization and CLZ tolerance can be induced in adolescent rats, and (2) the extent to which OLZ sensitization and CLZ tolerance induced in adolescence persists into adulthood. Male adolescent Sprague-Dawley rats (approximate postnatal days (∼P) 43-47) were first treated with OLZ (1.0 or 2.0 mg/kg, subcutaneously (sc)) or CLZ (10 or 20 mg/kg, sc) daily for 5 consecutive days in the CAR model. They were then tested for the expression of OLZ sensitization or CLZ tolerance either in adolescence (∼P 50) or after they matured into adults (∼P 76 and 92) in a challenge test during which all rats were injected with either a lower dose of OLZ (0.5 mg/kg) or CLZ (5.0 mg/kg). When tested in adolescence, rats previously treated with OLZ showed a stronger inhibition of CAR than those previously treated with vehicle (ie, sensitization). In contrast, rats previously treated with CLZ showed a weaker inhibition of CAR than those previously treated with vehicle (ie, tolerance). When tested in adulthood, the OLZ sensitization was still detectable at both time points (∼P 76 and 92), whereas the CLZ tolerance was only detectable on ∼P 76, and only manifested in the intertrial crossing. Performance in the prepulse inhibition and fear-induced 22 kHz ultrasonic vocalizations in adulthood were not altered by adolescence drug treatment. Collectively, these findings suggest that atypical antipsychotic treatment during adolescence can induce a long-term specific alteration in antipsychotic effect that persists into adulthood despite the brain maturation. As antipsychotic drugs are being increasingly used in children and adolescents in the past two decades, findings from this study are important for understanding the impacts of adolescent antipsychotic treatment on the brain and behavioral developments. This work also has implications for clinical practice involving adolescence antipsychotic treatments in terms of drug choice, drug dose, and schedule.

Parametric studies of antipsychotic-induced sensitization in the conditioned avoidance response model: roles of number of drug exposure, drug dose, and test-retest interval

Repeated haloperidol and olanzapine treatment produces an enhanced disruption of avoidance responding, a validated measure of antipsychotic activity. Experimental parameters affecting this sensitization-like effect have not been thoroughly examined. The present study investigated the role of three parameters (number of injections, dose, and interval between initial exposure and challenge) in antipsychotic sensitization in the conditioned avoidance response paradigm. Well-trained Sprague-Dawley rats received different numbers of drug treatment (1-5 days) or different doses of haloperidol (0.025-0.10 mg/kg, subcutaneously) or olanzapine (0.5-2.0 mg/kg, subcutaneously). After certain time intervals (4, 10 or 17 days), they were tested for the expression of haloperidol or olanzapine sensitization in a challenge test in which all rats were injected with a lower dose of haloperidol (0.025 mg/kg) or olanzapine (0.5 mg/kg). Throughout the drug-treatment period, both haloperidol and olanzapine dose-dependently enhanced their disruption of avoidance responding. Three days later, the sensitization induced by a low dose of haloperidol (0.025 mg/kg) or olanzapine (0.5 mg/kg) was only apparent in rats that received treatment for 5 days, but not in those that received treatment for 1-4 days. The sensitization induced by the medium and high doses of haloperidol (0.05 and 0.10 mg/kg) or olanzapine (1.0 and 2.0 mg/kg) was still robust even with only 3 days of treatment. The sensitization induced by a 3-day haloperidol (0.10 mg/kg) and olanzapine (2.0 mg/kg) treatment was long-lasting, still detectable 17 days after the last drug treatment. This study suggests that antipsychotic sensitization is a robust behavioral phenomenon. Its induction and expression are strongly influenced by parameters such as number of drug exposures, drug dose, and test-retest interval. Given the importance of antipsychotic sensitization in the maintenance of antipsychotic effects in the clinic, this study introduces a paradigm that can be used to investigate the behavioral and neurobiological mechanisms underlying antipsychotic sensitization.

Environmental and behavioral controls of the expression of clozapine tolerance: evidence from a novel across-model transfer paradigm

Repeated administration of antipsychotic drugs induces a sensitization-like or tolerance-like effect in many behavioral tasks, including the conditioned avoidance response (CAR) and the phencyclidine (PCP)-induced hyperlocomotion, two rodent models with high predictive validity for antipsychotic activity. This study investigated the impacts of contextual and behavioral variables on the expression of clozapine tolerance using a recently validated across-model transfer paradigm (Zhang and Li, 2012 [1]). Male Sprague-Dawley rats were first repeatedly treated with clozapine (2.5-10.0 mg/kg, sc) in the CAR model or PCP (1.6 mg/kg, sc)-induced hyperlocomotion model for five consecutive days. They were then tested for the expression of clozapine tolerance in another model for another 5 days. Finally, all rats were switched back to the original model and tested again for the expression of clozapine tolerance. When tested in the PCP model, rats previously treated with clozapine in the CAR model did not show an immediate weaker inhibition of PCP-induced hyperlocomotion than those treated with clozapine for the first time, but showed a significantly weaker inhibition over time. In contrast, when tested in the CAR model, rats previously treated with clozapine in the PCP model showed an immediate weaker disruption of avoidance response than those treated with clozapine for the first time, but this weaker effect diminished over time. These results suggest that the expression of clozapine tolerance is strongly modulated by the test environment and/or selected behavioral response. Clozapine tolerance and its situational specificity may be related to the drug's low extrapyramidal motor side effect, its superior therapeutic efficacy and/or emergence of clozapine withdrawal syndrome.

Distinct neural mechanisms underlying acute and repeated administration of antipsychotic drugs in rat avoidance conditioning

RATIONALE

Acute antipsychotic treatment disrupts conditioned avoidance responding, and repeated treatment induces a sensitization- or tolerance-like effect. However, the neurochemical mechanisms underlying both acute and repeated antipsychotic effects remain to be determined.

OBJECTIVE

The present study examined the neuroreceptor mechanisms of haloperidol, clozapine, and olanzapine effect in a rat two-way conditioned avoidance model.

METHODS

Well-trained Sprague-Dawley rats were administered with haloperidol (0.05 mg/kg, sc), clozapine (10.0 mg/kg, sc), or olanzapine (1.0 mg/kg, sc) together with either saline, quinpirole (a selective dopamine D(2/3) agonist, 1.0 mg/kg, sc), or 2,5-dimethoxy-4-iodo-amphetamine (DOI; a selective 5-HT(2A/2C) agonist, 2.5 mg/kg, sc), and their conditioned avoidance responses were tested over 3 days. After 2 days of drug-free retraining, the repeated treatment effect was assessed in a challenge test.

RESULTS

Pretreatment of quinpirole, but not DOI, attenuated the acute haloperidol-induced disruption of avoidance responding and to a lesser extent, olanzapine-induced disruption. In contrast, pretreatment of DOI, but not quinpirole, attenuated the acute effect of clozapine. On the repeated effect, pretreatment of DOI, but not quinpirole, attenuated the potentiated disruption of haloperidol, whereas pretreatment of quinpirole attenuated the potentiated disruption of olanzapine but enhanced the tolerance-like effect of clozapine.

CONCLUSIONS

These findings suggest that acute haloperidol and olanzapine disrupt avoidance responding primarily by blocking dopamine D(2) receptors, whereas acute clozapine exerts its disruptive effect primarily by blocking the 5-HT(2A) receptors. The repeated haloperidol effect may be mediated by 5-HT(2A/2C) blockade-initiated neural processes, whereas the repeated clozapine and olanzapine effect may be mediated by D(2/3) blockade-initiated neural processes.

Discriminative stimulus properties of atypical and typical antipsychotic drugs: a review of preclinical studies

BACKGROUND

Drug discrimination is an increasingly valuable behavioral assay for the preclinical development of antipsychotic drugs. The majority of studies have used the atypical antipsychotic clozapine because it displays robust discriminative stimulus properties and is the "prototypical" or "gold standard" atypical antipsychotic against which other antipsychotics will undoubtedly be compared for many years.

OBJECTIVES

Pharmacological mechanisms mediating the discriminative stimulus properties of antipsychotics used as training drugs and the usefulness of drug discrimination for distinguishing typical and atypical antipsychotics were reviewed.

RESULTS

Clozapine appears to have a compound cue involving antagonism of two or more receptors. While muscarinic receptor antagonism is a prominent factor for mediation of clozapine's cue in rats with a 5.0-mg/kg training dose, there are differences in clozapine's cue with a low training dose and in pigeons and mice. With a low training dose, clozapine has consistently produced full or partial generalization to atypical but not to typical antipsychotics. Although not evaluated as extensively, the atypical antipsychotics quetiapine and ziprasidone also appear to generalize to atypical but not typical antipsychotics. This has not been the case for other antipsychotic drugs (olanzapine, chlorpromazine, haloperidol) used as training drugs.

CONCLUSIONS

There are important differences in discriminative stimulus properties both between and within atypical and typical antipsychotics and across species. While low-dose clozapine discrimination in rats appears to provide a more sensitive behavioral assay for distinguishing atypical from typical antipsychotics, the extent to which clozapine's discriminative stimulus properties are predictive of its antipsychotic effects remains to be determined.

A comparison of the discriminative stimulus properties of the atypical antipsychotic drug clozapine in DBA/2 and C57BL/6 inbred mice

Inbred mouse strain comparisons are an important aspect of pharmacogenetic research, especially in strains known to differ in regard to specific neurotransmitter systems. DBA/2 mice differ from C57BL/6 mice in terms of both functional and anatomical characteristics of dopamine systems. Given the importance of D2 antagonism in the action of antipsychotic drugs and in theories regarding schizophrenia (i.e. the dopamine hypothesis), this study compared the discriminative stimulus properties of the atypical antipsychotic drug clozapine (CLZ) in C57BL/6 and DBA/2 inbred mice. DBA/2 and C57BL/6 mice were trained to discriminate 2.5 mg/kg of CLZ from vehicle in a two-lever drug discrimination procedure and tested with a variety of antipsychotic drugs and selective ligands. Both strains of mice readily acquired the CLZ discrimination. The atypical antipsychotic drugs olanzapine and risperidone fully substituted for CLZ in both DBA/2 and C57BL/6 mice, but ziprasidone fully substituted only in the C57BL/6 mice. The typical antipsychotic drug haloperidol produced partial substitution for CLZ in the DBA/2 mice, and the dopamine agonist amphetamine required a higher dose to reduce response rates significantly in DBA/2 mice as compared with C57BL/6 mice. Antagonism of serotonergic (5-HT2A/2B/2C) receptors with ritanserin and alpha1-adrenergic receptors with prazosin engendered CLZ-appropriate responding only in the C57BL/6 mice. Thus, while serotonergic and alpha-adrenergic antagonism were shown to be important for CLZ's discriminative cue in C57BL/6 mice, none of the selective ligands produced CLZ-appropriate responding in DBA/2 mice. Differences in dopamine-mediated functions between the two strains of mice may explain some of the findings in this study.

Repeated antipsychotic treatment progressively potentiates inhibition on phencyclidine-induced hyperlocomotion, but attenuates inhibition on amphetamine-induced hyperlocomotion: relevance to animal models of antipsychotic drugs

Clinical observations indicate that antipsychotic action starts early and increases in magnitude with repeated treatment. Animal models that faithfully capture this time course of action are few. Inhibition of hyperlocomotion induced by amphetamine or phencyclidine has been widely used as a screening tool for the antipsychotic activity of a drug. We thus investigated whether repeated antipsychotic treatment could produce an early-onset and progressively increased antagonistic effect on amphetamine or phencyclidine-induced hyperlocomotion as a way of assessing the validity of such models in capturing time course of antipsychotic action. On each of the five consecutive test days, different groups of rats (n=6-7/group) received an initial injection of either haloperidol (0.01-0.10 mg/kg, sc), clozapine (5-20.0 mg/kg, sc), olanzapine (1.0 mg/kg, sc), chlordiazepoxide (10.0 mg/kg, ip) or vehicle (sterile water, sc) 30 min prior to a second injection of either amphetamine (1.5 mg/kg, sc) or phencyclidine (3.2 mg/kg, sc). Motor activity was subsequently monitored for 60 min after amphetamine or phencyclidine treatment. Repeated treatment of haloperidol, clozapine, or olanzapine progressively potentiated inhibition on repeated phencyclidine-induced hyperlocomotion and prolonged this action over the five consecutive days. In contrast, antipsychotic inhibition on repeated amphetamine-induced hyperlocomotion was gradually attenuated and shortened. Repeated treatment of chlordiazepoxide, a benzodiazepine anxiolytic, retained its inhibition on amphetamine-induced hyperlocomotion, but had no effect on phencyclidine-induced one. These results suggest that repeated phencyclidine-induced hyperlocomotion model based on repeated antipsychotic treatment regimen is capable of capturing the progressive increase pattern of antipsychotic treatment seen in the clinic and differentiating antipsychotics from anxiolytics; thus it may serve as a better model for the investigation of the neurobiological mechanisms of action of antipsychotic drugs and delineating the pathophysiology of schizophrenia.

Chronic clozapine treatment in female rats does not induce weight gain or metabolic abnormalities but enhances adiposity: implications for animal models of antipsychotic-induced weight gain

The ability of clozapine to induce weight gain in female rats was investigated in three studies with progressively lowered doses of clozapine. In an initial preliminary high dose study, clozapine at 6 and 12 mg/kg (i.p., b.i.d.) was found to induce weight loss. In a subsequent intermediate dose study, we obtained no evidence for clozapine-induced weight gain despite using identical procedures and doses of clozapine (1-4 mg/kg, i.p., b.i.d.) with which we have observed olanzapine-induced weight gain, hyperphagia, enhanced adiposity and metabolic changes [Cooper G, Pickavance L, Wilding J, Halford J, Goudie A (2005). A parametric analysis of olanzapine-induced weight gain in female rats. Psychopharmacology; 181: 80-89.]. Instead, clozapine induced weight loss without alteration in food intake and muscle mass or changes in levels of glucose, insulin, leptin and prolactin. However, these intermediate doses of clozapine enhanced visceral adiposity and elevated levels of adiponectin. In a final study, low doses of clozapine (0.25-0.5 mg/kg, i.p, b.i.d.) induced weight loss. These data demonstrate that clozapine-induced weight gain can be much more difficult to observe in female rats than olanzapine-induced weight gain. Moreover, these findings contrast with clinical findings with clozapine, which induces substantial weight gain in humans. Clozapine-induced enhanced adiposity appears to be easier to observe in rats than weight gain. These findings, along with other preclinical studies, suggest that enhanced adiposity can be observed in the absence of antipsychotic-induced weight gain and hyperphagia, possibly reflecting a direct drug effect on adipocyte function independent of drug-induced hyperphagia [e.g. Minet-Ringuet J, Even P, Valet P, Carpene C, Visentin V, Prevot D, Daviaud D, Quignard-Boulange A, Tome D, de Beaurepaire R (2007). Alterations of lipid metabolism and gene expression in rat adipocytes during chronic olanzapine treatment. Molecular Psychiatry; 12: 562-571.]. These and other findings which show that the results of studies of antipsychotic treatment in animals do not always mimic clinical findings have important implications for the use of animal models of antipsychotic-induced weight gain. With regard to weight gain the results obtained appear to depend critically on the experimental procedures used and the specific drugs studied. Thus such models are not without limitations. However, they do consistently demonstrate the ability of various antipsychotics to enhance adiposity.

Olanzapine withdrawal/discontinuation-induced hyperthermia in rats

In female rats olanzapine (4 mg/kg b.i.d., i.p.) induced acute hypothermia, followed by very rapid full tolerance. With more prolonged treatment (over > 10 days) the hypothermic effect of olanzapine was reinstated. Subsequent withdrawal after 18 days of treatment induced very rapid onset (within 1 day) hyperthermia, which was time limited, dissipating completely over 3-4 days. These findings are similar to previous findings with clozapine [Goudie A Smith J Robertson A Cavanagh C (1999). Clozapine as a drug of dependence. Psychopharmacology; 142: 369-374.]. Although the mechanism(s) involved in the secondary hypothermic effect of olanzapine are, at present, unclear; the withdrawal hyperthermia observed represents the first report of a clear discontinuation effect of olanzapine. Such discontinuation effects are probably observed with many antipsychotic drugs. Since they have been suggested to facilitate relapse to psychosis and to interfere with subsequent clinical responses to antipsychotics, they merit further detailed analysis in both clinical and preclinical studies.