Potentiation of olanzapine substitution in rats discriminating clozapine by the D2/3 agonist quinpirole

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.

Discriminative stimulus properties of the atypical antidepressant, mirtazapine, in rats: a pharmacological characterization

RATIONALE

Though interoceptive properties of antidepressants have been described, discriminative stimulus (DS) properties of mirtazapine, which does not affect monoamine reuptake, remain uncharacterized.

OBJECTIVES

The objectives of the study are to train rats to recognize a mirtazapine DS, then perform substitution studies with other antidepressants and drugs acting at sites occupied by mirtazapine.

MATERIALS AND METHODS

Using a two-lever, fixed-ratio 10 schedule, rats were trained to discriminate mirtazapine (2.5 mg/kg, i.p.) from saline.

RESULTS

Sessions, 63 +/- 8, were necessary to reach the criterion for 14 rats that all subsequently recognized (100%) mirtazapine at the training dose. Mirtazapine blocks serotonin (5-HT)(2C) receptors, and the 5-HT(2C) antagonists, SB242,084, SB243,213 and S32006, revealed dose-dependent and full (> or =80%) substitution at doses of 2.5, 2.5, and 0.63 mg/kg, respectively. By contrast, the 5-HT(2A) antagonists, MDL100,907 and SR46349-B, the 5-HT(2B) antagonist, SB204,741, and the 5-HT(3) antagonist, ondansetron, showed no significant substitution. Though mirtazapine indirectly recruits 5-HT(1A) receptors, the 5-HT(1A) agonists, buspirone and 8-OH-DPAT, did not substitute. Mirtazapine blocks alpha(2)-adrenoceptors, but several alpha(2)-adrenoceptor antagonists (yohimbine, RX821,002 and atipamezole) failed to substitute. Despite blockade by mirtazapine of histamine H(1) receptors, no substitution was seen with the selective H(1) antagonist, pyrilamine. Finally, the selective noradrenaline reuptake inhibitor, reboxetine (0.16), fully substituted for mirtazapine, whereas the 5-HT/noradrenaline reuptake inhibitors, duloxetine and S33005, several 5-HT reuptake inhibitors (citalopram, fluvoxamine, and paroxetine) and the dopamine reuptake inhibitors, bupropion and GBR12,935, did not substitute.

CONCLUSION

Mirtazapine elicits a DS in rats for which selective antagonists at 5-HT(2C) receptors display dose-dependent substitution, whereas drugs acting at other sites recognized by mirtazapine are ineffective.

Further characterization of the discriminative stimulus properties of the atypical antipsychotic drug clozapine in C57BL/6 mice: role of 5-HT(2A) serotonergic and alpha (1) adrenergic antagonism

RATIONALE

The discriminative stimulus properties of the atypical antipsychotic drug (APD) clozapine (CLZ) have recently been studied in C57BL/6 mice, a common background strain for genetic alterations. However, further evaluation is needed to fully characterize CLZ's discriminative cue in this strain of mice.

OBJECTIVES

The objectives of the study were to confirm the previous findings using a shorter pretreatment time and to further characterize the receptor mechanisms mediating the discriminative stimulus properties of CLZ by testing APDs, selective ligands, and N-desmethylclozapine (CLZ's major metabolite) in C57BL/6 mice.

MATERIALS AND METHODS

C57BL/6 male mice were trained to discriminate 2.5 mg/kg CLZ (s.c.) from vehicle in a two-lever drug discrimination task.

RESULTS

Generalization testing with CLZ yielded an ED(50) = 1.19 mg/kg. Substitution testing with APDs showed that the atypical APDs quetiapine, sertindole, zotepine, iloperidone, and melperone fully substituted for CLZ (> or =80% CLZ-appropriate responding), but aripiprazole did not. The typical APDs chlorpromazine and thioridazine substituted for CLZ (fluphenazine and perphenazine did not). The serotonin (5-HT) (2A) antagonist M100907 and the alpha(1)-adrenoceptor antagonist prazosin fully substituted for CLZ. The H(1) histaminergic antagonist pyrilamine, dopamine agonist amphetamine, and the selective serotonin reuptake inhibitor fluoxetine did not substitute for CLZ. While N-desmethylclozapine did not substitute for CLZ when tested alone, N-desmethylclozapine plus a low dose of CLZ combined in an additive manner produced full substitution.

CONCLUSIONS

CLZ's discriminative cue in C57BL/6 mice is a "compound" cue mediated in part by antagonism of 5-HT(2A) and alpha(1) receptors.

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.