Olanzapine and risperidone disrupt conditioned avoidance responding in phencyclidine-pretreated or amphetamine-pretreated rats by selectively weakening motivational salience of conditioned stimulus

Cannabidiol reverses myeloperoxidase hyperactivity in the prefrontal cortex and striatum, and reduces protein carbonyls in the hippocampus in a ketamine-induced schizophrenia rat model

BACKGROUND

Schizophrenia (SCZ) has limited treatment options, often with significant side effects. Cannabidiol (CBD), a non-euphoric phytocannabinoid, has shown potential as a novel therapeutic option in SCZ due to antipsychotic-like, anti-inflammatory, and antioxidant properties. We compared the therapeutic effects of CBD and risperidone (RISP) in a rat model of SCZ induced by sub-chronic ketamine (KET), focusing on inflammatory and oxidative stress, and behavioral phenotypes.

METHODS

Rats were pre-treated with KET or saline (SAL) for 10 days followed by CBD or RISP for 8 days. Locomotion, anxiety- and anhedonia-like behavior, and recognition memory were assessed. Oxidative damage as measured by protein carbonyls, thiobarbituric acid reactive substances, and catalase activity, and the inflammation markers myeloperoxidase (MPO) activity and nitrite/nitrate (N/N) concentration ratio were assessed in the prefrontal cortex (PFC), hypothalamus (HYP), hippocampus (HPC), and striatum, brain areas relevant to SCZ.

RESULTS

CBD restored the KET-induced decreased rearing behavior in the OFT, while RISP further decreased rearing. RISP treatment in control rats decreased rearing and elicited an anhedonic-like phenotype, while CBD did not. CBD, but not RISP restored the KET-induced increased levels of MPO activity in the PFC and the striatum, and protein carbonyls in the HPC. Post-KET treatment with RISP but not CBD decreased protein carbonyls in the PFC, and decreased the N/N concentration ratio in the HYP.

CONCLUSION

CBD restored the KET-induced decrease in rearing behavior without inducing an anhedonic-like phenotype as observed with RISP. CBD, and to a lesser extent RISP restored the oxidative stress and neuroinflammation elicited by KET in the striatum, HPC, and PFC. These findings support the possibility that the antipsychotic effects of CBD might be mediated by its antioxidant and anti-inflammatory effects.

Reinforcement attenuation as a behavioral technique to suppress conditioned avoidance response in rats: A comparative study with olanzapine

BACKGROUND

Antipsychotic treatment is effective in the treatment of psychosis, although it also brings with it some unwanted side effects and is associated with low compliance. Finding a non-pharmacological alternative for antipsychotic treatment is highly desirable.

AIMS

This preclinical study examined the 'antipsychotic' efficacy of such a behavioral technique using a conditioned avoidance response model. This technique, termed reinforcement attenuation (RA), is to administer a brief footshock (0.1-2.0 s, 0.8 mA) at the end of each trial regardless of whether a well-trained rat makes an avoidance response or not.

RESULTS

RA achieved the same avoidance suppressing effect as olanzapine (an atypical antipsychotic drug), including both acute suppression and sensitized suppression of avoidance response in well-trained Sprague-Dawley adult male rats. Interestingly, the RA-induced sensitization (an enhanced disruption of avoidance responding) enhanced subsequent olanzapine sensitivity, whereas the olanzapine (1.0 mg/kg)-induced sensitization had little impact on later RA treatment. When RA and olanzapine (0.5 mg/kg, subcutaneously) were used together, the RA-induced sensitization was still detectable in the RA challenge test, although its magnitude was reduced by olanzapine. Finally, we showed that the RA-induced sensitization in avoidance suppression persisted from adolescence into adulthood, long after such a treatment was terminated.

CONCLUSIONS

These findings demonstrate that the RA is functionally equivalent (if not superior) to antipsychotic treatment in the avoidance suppression effect (both acute and sensitization effects) in both adolescent and adult animals. Behavioral therapies that specifically target the reinforcer of psychotic thoughts might be a viable strategy for the treatment of psychosis.

Repeated administration of 5-hydroxytryptamine 2C agonist MK212 produces a sensitization effect of antipsychotic activity

5-Hydroxytryptamine 2C (5-HT_2C ) receptor agonists have been suggested to possess an antipsychotic activity in several acute preclinical tests of antipsychotic drugs with low extra-pyramidal side effect liability. However, little is known about the long-term effect associated with chronic use of 5-HT_2C receptor agonists. The present study examined whether repeated activation of 5-HT_2C receptor with a highly selective 5-HT_2C receptor agonist MK212 would induce a long-term change in its antipsychotic-like activity (either a sensitization or tolerance) in the conditioned avoidance response and MK801-induced hyperlocomotion tests. Sprague-Dawley rats were first tested under the intraperitoneal (i.p.) treatment of MK212 (0.25, 0.5, 1.0 mg/kg) for 5 consecutive days. Three days later, when all rats were injected with a low dose of MK 212 (0.25 mg/kg) and tested for avoidance responding, rats that had been pretreated with 1.0 and 0.5 mg/kg MK212 made significantly fewer avoidance responses than those that had been treated with vehicle (0.9% saline). However, this past drug exposure-induced group difference was not significant in the MK801-induced hyperlocomotion test. Overall, results from this study suggest that repeated treatment of MK212 is capable of inducing a dose-dependent sensitization of antipsychotic activity in conditioned avoidance response. The discrepancy in sensitization of MK212 in CAR and MK801-induce hyperlocomotion may be related to the different mechanism underlying the effect of MK212 in these two tests. © 2016 IUBMB Life, 68(12):985-993, 2016.

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 repeated quetiapine treatment on conditioned avoidance responding in rats

The present study characterized the behavioral mechanisms of avoidance-disruptive effect of quetiapine in the conditioned avoidance response test under two behavioral testing (2 warning signals vs. 1 warning signal) and two drug administration conditions (subcutaneous vs. intravenous). In Experiments 1 and 2, well-trained adult male Sprague-Dawley rats were tested under the subcutaneous (s.c.) quetiapine treatment (5.0, 15.0, 25.0, 50.0mg/kg) for 7 days in a novel procedure consisting of two conditioned stimuli (CS) (white noise serving as CS1 and pure tone as CS2). Only the highest dose (50.0mg/kg) produced a persistent suppression of the avoidance response without impairing the escape response. The magnitude of suppression of the CS1 avoidance was similar to that of CS2 avoidance. No significant group difference was found in the quetiapine (15.0mg/kg, s.c.) challenge test, indicating a lack of a long-term quetiapine effect. In Experiment 3, well-trained rats were tested under the intravenous (i.v.) quetiapine treatment (3.0, 9.0, 15.0mg/kg) for 5 days and challenged with quetiapine (6.0mg/kg, i.v. followed by 9.0mg/kg, s.c.). Only the white noise was used as the CS. Similar to what was being observed in Experiments 1 and 2, intravenously administered quetiapine dose-dependently suppressed avoidance responding during the drug test days, but did not alter drug sensitivity in the challenge days. Thus, quetiapine does not appear to show a preferential inhibition of the avoidance response to a less salient stimulus; and prior quetiapine treatment (s.c. and i.v.) does not cause a sensitization or tolerance to quetiapine.

Long-lasting sensitization induced by repeated risperidone treatment in adolescent Sprague-Dawley rats: a possible D2 receptor mediated phenomenon?

RATIONALE

Risperidone use in children and adolescents for the treatment of various neuropsychiatric disorders (e.g., schizophrenia, autism, disruptive behavior, etc.) has increased substantially in recent decades. However, its long-term effect on the brain and behavioral functions is not well understood.

OBJECTIVE

The present study investigated how a short-term risperidone treatment in adolescence impacts antipsychotic response in adulthood in the conditioned avoidance response and phencyclidine (PCP)-induced hyperlocomotion tests.

METHODS

Male adolescent Sprague-Dawley rats (postnatal days [P] 40-44 or 43-48) were first treated with risperidone (0.3, 0.5, or 1.0 mg/kg, subcutaneously (sc)) and tested in the conditioned avoidance or PCP (3.2 mg/kg, sc)-induced hyperlocomotion model daily for five consecutive days. After they became adults (~P 76-80), they were challenged with risperidone (0.3 mg/kg, sc) to assess their sensitivity to risperidone reexposure. A quinpirole (a D2/3 receptor agonist, 1.0 mg/kg, sc)-induced hyperlocomotion test was later conducted to assess the risperidone-induced functional changes in D2 receptor.

RESULTS

In the risperidone challenge test in adulthood, adult rats previously treated with risperidone in adolescence made significantly fewer avoidance responses and exhibited significantly lower PCP-induced hyperlocomotion than those previously treated with vehicle. They also appeared to be more hyperactive than the vehicle-pretreated ones in the quinpirole-induced hyperlocomotion test. Prepulse inhibition of acoustic startle or fear-induced 22 kHz ultrasonic vocalizations in adulthood was not altered by adolescence risperidone treatment.

CONCLUSIONS

Adolescent risperidone exposure induces a long-term increase in behavioral sensitivity to risperidone that persists into adulthood. This long-lasting change might be due to functional upregulation of D2-mediated neurotransmission.

Long-term impacts of adolescent risperidone treatment on behavioral responsiveness to olanzapine and clozapine in adulthood

This preclinical study investigated how a short-term risperidone treatment in adolescence impacts antipsychotic response to olanzapine and clozapine in adulthood. Antipsychotic effect was indexed by a drug's suppressive effect on avoidance responding in a rat conditioned avoidance response (CAR) model. Male adolescent Sprague-Dawley rats were first treated with risperidone (1.0mg/kg, sc) or sterile water and tested in the CAR model for 5 consecutive days from postnatal days P 40 to 44. After they became adults (~P 80-84), they were switched to olanzapine (0.5mg/kg, sc), clozapine (5.0mg/kg, sc) or vehicle treatment and tested for avoidance for 5days. During the adolescent period, repeated risperidone treatment produced a persistent inhibition of avoidance response. Throughout the 5days of adulthood drug testing, rats previously treated with risperidone in adolescence made significantly fewer avoidance responses than the vehicle ones when they all were switched to olanzapine, indicating a risperidone-induced enhancement of behavioral sensitivity to olanzapine. In contrast, when switched to clozapine, rats previously treated with risperidone made significantly more avoidance responses than the vehicle rats, indicating a risperidone-induced decrease of behavioral sensitivity to clozapine. Performance in the prepulse inhibition of acoustic startle response in adulthood was not altered by adolescent risperidone treatment. Collectively, adolescent risperidone exposure induced a long-term change in behavioral sensitivity to other atypical antipsychotic drugs, with the specific direction of change (i.e., increase or decrease) dependent on the drug to be switched to. These long-lasting changes are likely mediated by drug-induced neuroplastic changes and may also have significant clinical implications for antipsychotic treatment of chronic patients with an early onset of psychotic symptoms.

Time-dependence of risperidone and asenapine sensitization and associated D2 receptor mechanism

When an antipsychotic drug is given repeatedly and intermittently, there is often a long-term increase in its behavioral efficacy, termed antipsychotic sensitization. With the passage of time, the magnitude of antipsychotic sensitization may increase or decrease based on the principle of time-dependent sensitization (TDS) or memory decay, respectively. In the present study, we examined the time-dependent feature and possible dopamine D2 receptor mechanism of sensitization induced by risperidone and asenapine in the conditioned avoidance response test. Well-trained male adult Sprague-Dawley rats were first repeatedly treated with risperidone (1.0mg/kg) or asenapine (0.2mg/kg) and tested for avoidance response daily for 5 consecutive days. Eight, 18 or 38 days after the 5th drug treatment, all rats were retested drug-free to assess the long-term impact of prior risperidone or asenapine treatment. Drug-pretreated rats had significantly lower avoidance than vehicle-pretreated ones on this test, and the group differences increased with the passage of time. In the subsequent drug challenge test at 10, 20 or 40 days after the 5th drug treatment, all rats were injected with a low dose of risperidone (0.3mg/kg) or asenapine (0.1mg/kg). Drug-pretreated rats again made significantly fewer avoidances than controls, confirming the antipsychotic sensitization effect. Finally, in the quinpirole (a D2/3 receptor agonist, 1.0mg/kg, sc)-induced hyperlocomotion test, risperidone-pretreated rats exhibited a significantly higher level of motor activity than the vehicle-pretreated ones. These findings suggest that risperidone and asenapine sensitization is long-lasting, follows the TDS principle, and is likely mediated by D2 receptor supersensitivity.

Repeated asenapine treatment produces a sensitization effect in two preclinical tests of antipsychotic activity

Among several commonly used atypical antipsychotic drugs, olanzapine and risperidone cause a sensitization effect in the conditioned avoidance response (CAR) and phencyclidine (PCP)-induced hyperlocomotion paradigms--two well established animal tests of antipsychotic drugs, whereas clozapine causes a tolerance effect. Asenapine is a novel antipsychotic drug recently approved for the treatment of schizophrenia and manic disorders. It shares several receptor binding sites and behavioral features with other atypical antipsychotic drugs. However, it is not clear what type of repeated effect (sensitization or tolerance) asenapine would induce, and whether such an effect is transferrable to other atypicals. In this study, male adult Sprague-Dawley rats were first repeatedly tested with asenapine (0.05, 0.10 or 0.20 mg/kg, sc) for avoidance response or PCP (3.20 mg/kg, sc)-induced hyperlocomotion daily for 5 consecutive days. After 2-3 days of retraining/drug-free recovery, they were then challenged with asenapine (0.10 mg/kg, sc), followed by olanzapine (0.50 mg/kg, sc) and clozapine (2.50 mg/kg, sc). During the 5-day drug test period (the induction phase), repeated asenapine treatment progressively increased its inhibition of avoidance response and PCP-induced hyperlocomotion in a dose-dependent fashion. On the asenapine and olanzapine challenge tests (the expression phase), rats previously treated with asenapine still showed significantly lower avoidance response and lower PCP-induced hyperlocomotion than those previously treated with vehicle. An increased reactivity to clozapine challenge in prior asenapine-treated rats was also found in the PCP-induced hyperlocomotion test. These findings suggest that asenapine is capable of inducing a sensitization effect and a cross-sensitization to olanzapine and clozapine (to a lesser extent). Because the behavioral profile of asenapine in both tests is similar to that of olanzapine, but different from that of clozapine, we suggest that asenapine resembles olanzapine to a greater extent than clozapine in its therapeutic and side effect profiles.

An automatic recording system for the study of escape from fear in rats

Escape from fear (EFF) is an active response to a conditioned stimulus (CS) previously paired with an unconditioned fearful stimulus (US), which typically leads to the termination of the CS. In this paradigm, animals acquire two distinct associations: S-S [CS-US] and R-O [response-outcome] through Pavlovian and instrumental conditioning, respectively. The present study describes a computer controlled automatic recording system that captures the development of EFF and allows the determination of the respective roles of S-S and R-O associations in this process. We validated this system by showing that only rats subjected to a simultaneous CS-US conditioning (i.e., CS and US occur together at the beginning of each trial) acquired EFF, not those subjected to an unpaired CS-US conditioning. Paired rats had a progressively increased number of EFF and significantly shorter escape latencies than unpaired rats across the 5-trial blocks on the test day. However, during the conditioning phase, the unpaired rats emitted more 22kHz ultrasonic vocalizations, a validated measure of conditioned reactive fear responses. Our results demonstrate that the acquisition of EFF is contingent upon pairing of the CS with the US, not simply the consequence of a high level of generalized fear. Because this commercially available system is capable of examining both conditioned active and reactive fear responses in a single setup, it could be used to determine the relative roles of S-S and R-O associations in EFF, the neurobiology of conditioned active fear response and neuropharmacology of psychotherapeutic drugs.

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.

Iptakalim Preferentially Decreases Nicotine-induced Hyperlocomotion in Phencyclidine-sensitized Rats: A Potential Dual Action against Nicotine Addiction and Psychosis

Objective

Iptakalim is a putative ATP-sensitive potassium (K_ATP) channel opener. It is also a novel nicotinic acetylcholine receptor (nAChR) blocker and can antagonize nicotine-induced increase in dopamine release in the nucleus accumbens. Our recent work also shows that iptakalim exhibits a clozapine-like atypical antipsychotic profile, indicating that iptakalim may possess a dual action against nicotine addiction and schizophrenia.

Methods

The present study examined the potential therapeutic effects of iptakalim on nicotine use in schizophrenia. We created an animal model of comorbidity of nicotine addiction and schizophrenia by injecting male Sprague-Dawley rats with nicotine (0.40 mg/kg, subcutaneously[sc]) or saline, in combination with phencyclidine (PCP, 3.0 mg/kg, sc) or saline daily for 14 consecutive days.

Results

During the PCP/nicotine sensitization phase, PCP and nicotine independently increased motor activity over time. PCP also disrupted prepulse inhibition (PPI) of acoustic startle response. Acute nicotine treatment attenuated the PCP-induced hyperlocomotion and PCP-induced disruption of PPI, whereas repeated nicotine treatment potentiated these effects. Importantly, pretreatment with iptakalim (10-20 mg/kg, intraperitoneally) reduced nicotine-induced hyperlocomotion in a dose-dependent fashion. This reduction effect was highly selective: it was more effective in rats previously sensitized to the combination of PCP and nicotine, but less effective in rats sensitized to saline, nicotine alone or PCP alone.

Conclusion

To the extent that the combined nicotine and PCP sensitization mimics comorbid nicotine addiction in schizophrenia, the preferential inhibitory effect of iptakalim on nicotine-induced hyperlocomotion suggests that iptakalim may be a potential useful drug for the treatment nicotine abuse in schizophrenia.

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.

A preclinical assessment of d.l-govadine as a potential antipsychotic and cognitive enhancer

Tetrahydroprotoberberines (THPBs) are compounds derived from traditional Chinese medicine and increasing preclinical evidence suggests efficacy in treatment of a wide range of symptoms observed in schizophrenia. A receptor-binding profile of the THPB, d.l-govadine (d.l-Gov), reveals high affinity for dopamine and noradrenaline receptors, efficacy as a D2 receptor antagonist, brain penetrance in the 10-300 ng/g range, and thus motivated an assessment of the antipsychotic and pro-cognitive properties of this compound in the rat. Increased dopamine efflux in the prefrontal cortex and nucleus accumbens, measured by microdialysis, is observed following subcutaneous injection of the drug. d.l-Gov inhibits both conditioned avoidance responding (CAR) and amphetamine-induced locomotion (AIL) at lower doses than clozapine (CAR ED50: d.l-Gov 0.72 vs. clozapine 7.70 mg/kg; AIL ED50: d.l-Gov 1.70 vs. clozapine 4.27 mg/kg). Catalepsy is not detectable at low biologically relevant doses, but is observed at higher doses. Consistent with previous reports, acute d-amphetamine disrupts latent inhibition (LI) while a novel finding of enhanced LI is observed in sensitized animals. Treatment with d.l-Gov prior to conditioned stimulus (CS) pre-exposure restores LI to levels observed in controls in both sensitized animals and those treated acutely with d-amphetamine. Finally, possible pro-cognitive properties of d.l-Gov are assessed with the spatial delayed win-shift task. Subcutaneous injection of 1.0 mg/kg d.l-Gov failed to affect errors at a 30-min delay, but decreased errors observed at a 12-h delay. Collectively, these data provide the first evidence that d.l-Gov may have antipsychotic properties in conjunction with pro-cognitive effects, lending further support to the hypothesis that THPBs are a class of compounds which merit serious consideration as novel treatments for schizophrenia.

Avoidance disruptive effect of clozapine and olanzapine is potentiated by increasing the test trials: further test of the motivational salience hypothesis

Antipsychotic drugs suppress animals' ability to avoid an aversive stimulus in the conditioned avoidance response model (CAR). This behavioral effect is thought to reflect antipsychotic activity and is suggested to be mediated by a drug's action in attenuating the motivational salience of a conditioned stimulus (CS). In the present study, we tested whether atypical antipsychotic drugs clozapine and olanzapine act through this behavioral mechanism by manipulating the number of avoidance test trials. We reasoned that more CS trials in the presence of clozapine or olanzapine would afford the drug more opportunities to decrease the motivational salience of the CS, thus avoidance decline would be greater with the increase of CS trials in each test session. In two separate experiments, adult male Sprague-Dawley rats were tested under clozapine (5.0mg/kg, sc), olanzapine (0.5mg/kg, sc) or vehicle (sterile water) for 6 consecutive days in three CS trial conditions (i.e. 3, 10, and 40 CS trials per session). Two days later, all rats were tested under the same 40-trial session after receiving clozapine (5.0mg/kg, sc) or olanzapine (0.5mg/kg, sc). Results show that repeated clozapine and olanzapine treatment persistently decreased avoidance response, and this effect was potentiated by the increase of number of CS trials in the test sessions, as the clozapine-treated or olanzapine-treated rats tested under the 40-trial or 10-trial condition had significantly lower avoidance and faster decline across-sessions than those tested under the 3-trial condition. This potentiated effect was not only seen in the total avoidance percentage, but also observed in the within-session decline pattern in the last three drug test sessions and in the final 40-trial test session. These findings suggest that the clinical efficacy of a drug can be enhanced by increasing the exposure of symptoms in the presence of the drug.

Neuroanatomical substrates of the disruptive effect of olanzapine on rat maternal behavior as revealed by c-Fos immunoreactivity

Olanzapine is one of the most widely prescribed atypical antipsychotic drugs in the treatment of schizophrenia. Besides its well-known side effect on weight gain, it may also impair human parental behavior. In this study, we took a preclinical approach to examine the behavioral effects of olanzapine on rat maternal behavior and investigated the associated neural basis using the c-Fos immunohistochemistry. On postpartum days 6-8, Sprague-Dawley mother rats were given a single injection of sterile water or olanzapine (1.0, 3.0 or 5.0mg/kg, sc). Maternal behavior was tested 2h later, after which rats were sacrificed and brain tissues were collected. Ten brain regions that were either implicated in the action of antipsychotic drugs and/or in the regulation of maternal behavior were examined for c-Fos immunoreactivity. Acute olanzapine treatment dose-dependently disrupted various components of maternal behavior (e.g., pup retrieval, pup licking, nest building, crouching) and increased c-Fos immunoreactivity in the medial prefrontal cortex (mPFC), nucleus accumbens shell and core (NAs and NAc), dorsolateral striatum (DLSt), ventral lateral septum (LSv), central amygdala (CeA) and ventral tegmental area (VTA), important brain areas generally implicated in the incentive motivation and reward processing. In contrast, olanzapine treatment did not alter c-Fos in the medial preoptic nucleus (MPN), ventral bed nucleus of the stria terminalis (vBST) and medial amygdala (MeA), the core brain areas directly involved in the mediation of rat maternal behavior. These findings suggest that olanzapine disrupts rat maternal behavior primarily by suppressing incentive motivation and reward processing via its action on the mesocorticolimbic dopamine systems, other limbic and striatal areas, but not by disrupting the core processes involved in the mediation of maternal behavior in particular.

Contextual and behavioral control of antipsychotic sensitization induced by haloperidol and olanzapine

Repeated administration of haloperidol (HAL) and olanzapine (OLZ) causes a progressively enhanced disruption of the conditioned avoidance response (CAR) and a progressively enhanced inhibition of phencyclidine (PCP)-induced hyperlocomotion in rats (termed antipsychotic sensitization). Both actions are thought to reflect intrinsic antipsychotic activity. The present study examined the extent to which antipsychotic-induced sensitization in one model (e.g. CAR) can be transferred or maintained in another (e.g. PCP hyperlocomotion) as a means of investigating the contextual and behavioral controls of antipsychotic sensitization. Well-trained male Sprague-Dawley rats were first repeatedly tested in the CAR or the PCP (3.2 mg/kg, subcutaneously) hyperlocomotion model under HAL or OLZ for 5 consecutive days. Then they were switched to the other model and tested for the expression of sensitization. Finally, all rats were switched back to the original model and retested for the expression of sensitization. Repeated HAL or OLZ treatment progressively disrupted avoidance responding and decreased PCP-induced hyperlocomotion, indicating a robust sensitization. When tested in a different model, rats previously treated with HAL or OLZ did not show a stronger inhibition of CAR-induced or PCP-induced hyperlocomotion than those treated with these drugs for the first time; however, they did show such an effect when tested in the original model in which they received repeated antipsychotic treatment. These findings suggest that the expression of antipsychotic sensitization is strongly influenced by the testing environment and/or selected behavioral response under certain experimental conditions. Distinct contextual cues and behavioral responses may develop an association with unconditional drug effects through a Pavlovian conditioning process. They may also serve as occasion setters to modulate the expression of sensitized responses. As antipsychotic sensitization mimics the clinical effects of antipsychotic treatment, understanding the neurobiological mechanisms of antipsychotic sensitization and its contextual control would greatly enhance our understanding of the psychological and neurochemical nature of antipsychotic treatment in the clinic.

Clozapine, but not olanzapine, disrupts conditioned avoidance response in rats by antagonizing 5-HT2A/2C receptors

The present study was designed to assess the role of 5-HT(2A/2C) receptors in the acute and repeated effect of clozapine and olanzapine in a rat conditioned avoidance response model, a validated model of antipsychotic activity. Male Sprague-Dawley rats that were previously treated with either phencyclidine (0.5-2.0 mg/kg, sc), amphetamine (1.25-5.0 mg/kg, sc), or saline and tested in a prepulse inhibition of acoustic startle study were used. They were first trained to acquire avoidance response to a white noise (CS1) and a pure tone (CS2) that differed in their ability to predict the occurrence of footshock. Those who acquired avoidance response were administered with clozapine (10.0 mg/kg, sc) or olanzapine (1.0 mg/kg, sc) together with either saline or 1-2,5-dimethoxy-4-iodo-amphetamine (DOI, a selective 5-HT(2A/2C) agonist, 1.0 or 2.5 mg/kg, sc), and their conditioned avoidance responses were tested for four consecutive days. After two drug-free retraining days, the long-term repeated effect was assessed in a challenge test during which all rats were injected with a low dose of clozapine (5 mg/kg, sc) or olanzapine (0.5 mg/kg). Results show that pretreatment of DOI dose-dependently reversed the acute disruptive effect of clozapine on both CS1 and CS2 avoidance responses, whereas it had little effect in reversing the acute effect of olanzapine. On the challenge test, pretreatment of DOI did not alter the clozapine-induced tolerance or the olanzapine-induced sensitization effect. These results confirmed our previous findings and suggest that clozapine, but not olanzapine, acts on through 5-HT(2A/2C) receptors to achieve its acute avoidance disruptive effect and likely its therapeutic effects. The long-term clozapine tolerance and olanzapine sensitization effects appear to be mediated by non-5-HT(2A/2C) receptors.

Time course of the attenuation effect of repeated antipsychotic treatment on prepulse inhibition disruption induced by repeated phencyclidine treatment

Antagonism of prepulse inhibition (PPI) deficits produced by psychotomimetic drugs has been widely used as an effective tool for the study of the mechanisms of antipsychotic action and identifying potential antipsychotic drugs. Many studies have relied on the acute effect of a single administration of antipsychotics, whereas patients with schizophrenia are treated chronically with antipsychotic drugs. The clinical relevance of acute antipsychotic effect in this model is still an open question. In this study, we investigated the time course of repeated antipsychotic treatment on persistent PPI deficit induced by repeated phencyclidine (PCP) treatment. After a baseline test with saline, male Sprague-Dawley rats were repeatedly injected with either vehicle, haloperidol (0.05mg/kg), clozapine (5.0 or 10.0mg/kg), olanzapine (2.0mg/kg), risperidone (1.0mg/kg) or quetiapine (10mg/kg), followed by PCP (1.5mg/kg, sc) and tested for PPI once daily for 6 consecutive days. A single injection of PCP disrupted PPI and this effect was maintained with repeated PCP injections throughout the testing period. Acute clozapine, but not other antipsychotic drugs, attenuated acute PCP-induced PPI disruption at both tested doses. With repeated treatment, clozapine and quetiapine maintained their attenuation, while risperidone enhanced its effect with a significant reduction of PCP-induced disruption toward the end of treatment period. In contrast, repeated haloperidol and olanzapine treatments were ineffective. The PPI effects of these drugs were more conspicuous at a higher prepulse level (e.g. 82dB) and were dissociable from their effects on startle response and general activity. Overall, the repeated PCP-PPI model appears to be a useful model for the study of the time-dependent antipsychotic effect, and may help identify potential treatments that have a quicker onset of action than current antipsychotics.

Multiple 'hits' during postnatal and early adulthood periods disrupt the normal development of sensorimotor gating ability in rats

In the present study, we evaluated a multiple-hit animal model of schizophrenia in an attempt to capture the complex interactions among various adverse developmental factors in schizophrenia. Sprague-Dawley rats were assigned to receive either repeated daily 3-h maternal separation for eight days (first 'hit') on postnatal days (PND) 3 to 10, and/or avoidance conditioning for six days (second 'hit') on PND 49-56, and/or repeated phencyclidine treatment (third 'hit', 3.0 mg/kg, sc) immediately after each daily avoidance conditioning. Prepulse inhibition (PPI) of acoustic startle reflex was assessed at late adolescence (PND 41-43) and early adulthood (PND 62-63). The change in %PPI from the adolescence phase to adulthood phase was used to index the maturation-related improvement of sensorimotor gating ability. Maternal separation, avoidance conditioning and PCP treatment had a complex three-way interaction on the functional improvement of sensorimotor gating. Maternal separation impaired PPI improvement preferentially in the saline rats that were not subjected to avoidance conditioning. Avoidance conditioning had no effect on PPI improvement in the non-maternally separated rats, but restored the maternal separation-induced disruption. However, this restoration effect was abolished by PCP treatment. The present study also identified a number of behavioral, emotional and learning abnormalities caused by these three developmental insults which may precede their interactive disruption of normal development of sensorimotor gating ability.

Olanzapine and risperidone disrupt conditioned avoidance responding by selectively weakening motivational salience of conditioned stimulus: further evidence

Suppression of conditioned avoidance response is a preclinical behavioral index of antipsychotic activity. Previous work shows that olanzapine and risperidone disrupt avoidance response elicited by a less salient conditioned stimulus (CS2) to a greater extent than avoidance elicited by a more salient stimulus (CS1), suggesting that antipsychotic drugs may have a weakening action on motivational salience of stimuli. In the present study, we further examined this mechanism of antipsychotic action, focusing on the possible impact of baseline difference of CS1 and CS2 response rates on the avoidance-disruptive effect of olanzapine and risperidone. Rats were first trained to acquire avoidance responding in a procedure in which the number of CS2 trials (i.e. 20) was twice the number of CS1 trials (i.e. 10), but the percentage of CS2-shock pairing was set at 25% lower (15 trials out of 20) than the percentage of CS1-shock pairing (20 trials out of 20). They were then tested daily under olanzapine (0.5 and 1.0 mg/kg, sc) or risperidone (0.33 and 1.0 mg/kg, sc) for 5 consecutive days. Repeated olanzapine and risperidone treatment dose-dependently disrupted avoidance responding to both CS1 and CS2. Both drugs at the high dose disrupted the CS2 avoidance to a greater extent than the CS1 avoidance. In the final challenge test, rats previously treated with olanzapine were tested under risperidone (0.33 mg/kg), whereas rats previously treated with risperidone were tested under olanzapine (0.5 mg/kg). Results show that rats previously treated with risperidone 1.0mg/kg group made significantly fewer avoidance responses than the vehicles under olanzapine at 0.5 mg/kg. These findings confirm that olanzapine and risperidone disrupt avoidance response primarily by selectively attenuating the motivational salience of the CS. The present study also suggests that there is a generality of antipsychotic drug experience that is mediated by a shared interoceptive drug state mechanism.

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.

Avoidance-suppressing effect of antipsychotic drugs is progressively potentiated after repeated administration: an interoceptive drug state mechanism

Antipsychotic drugs selectively suppress conditioned avoidance response. Using a two-way active avoidance response paradigm, we examined the role of drug-induced interoceptive state in the mediation of avoidance-suppressive effect. In Experiment 1, we found that rats intermittently treated with olanzapine (OLZ) (1.0 mg/kg, s.c.) or haloperidol (0.03 mg/kg, s.c.) on the 1st day of a 3-day cycle for seven cycles exhibited a progressive across-session decline in avoidance responding, despite the fact that they exhibited a comparable high level of avoidance responding on the 3rd day of each cycle during the drug-free retraining session. In Experiments 2 and 3, rats that were previously treated with OLZ (0.5-2.0 mg/kg, s.c.) or risperidone (0.2-1.0 mg/kg) during the acquisition phase of avoidance conditioning exhibited significantly fewer avoidance responses when they were retested 3 weeks later to the same drug in comparison to rats that were previously treated with nonantipsychotic drugs (chlordiazepoxide, 10 mg/kg, citalopram 10 mg/kg, or sterile water). Overall, these findings indicate a 'drug memory'-like mechanism that maintains the avoidance-suppressing effect of antipsychotics over time. This mechanism is likely driven by the interoceptive state caused by the antipsychotics, which may also be an important behavioral mechanism mediating the clinical effects of antipsychotic treatments.

An investigation of the behavioral mechanisms of antipsychotic action using a drug-drug conditioning paradigm

Antipsychotic drugs at noncataleptic doses selectively suppress conditioned avoidance response in rats. In our previous study, we had used a two-way active avoidance response paradigm to show that the antipsychotic-induced interoceptive state is one of the mechanisms underlying the avoidance-disruptive effect of antipsychotics. In this study, we sought to further examine this mechanism using a novel drug-drug conditioning procedure. We made use of the fact that both the typical neuroleptic haloperidol and the atypical neuroleptic olanzapine disrupt conditioned avoidance responding, whereas chlordiazepoxide (an anxiolytic) does not. We reasoned that if the antipsychotic interoceptive state is important in causing a disruption on avoidance responding (an index of antipsychotic efficacy), pairing chlordiazepoxide (a cueing drug conditional stimulus) with haloperidol or olanzapine (a cued drug unconditional stimulus) should engender chlordiazepoxide to exhibit this property and behave like an antipsychotic drug. Chlordiazepoxide exhibited an acquired antipsychotic-like property in disrupting avoidance responding after being repeatedly paired with haloperidol, but not with olanzapine. In contrast, it significantly attenuated the antiavoidance efficacy of olanzapine but not haloperidol after being repeatedly paired with these drugs. This study suggests that the haloperidol-induced interoceptive drug state is directly involved in its antiavoidance action, and chlordiazepoxide may attenuate the antiavoidance efficacy of antipsychotics (especially olanzapine). To the extent that the antiavoidance effect predicts clinical effects of antipsychotic treatment, this study suggests that the antipsychotic-induced interoceptive drug state may be an important behavioral mechanism mediating the clinical effects of antipsychotic treatments.