Role of D1 and D2 dopamine receptors in the discriminative stimulus properties of the atypical antipsychotic clozapine in rats

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.

Discriminative stimulus properties of the atypical antipsychotic drug clozapine in rats trained to discriminate 1.25 mg/kg clozapine vs. 5.0 mg/kg clozapine vs. vehicle

Clozapine, the prototype for atypical antipsychotic drugs, is used in the drug discrimination paradigm as a model for screening atypical from typical antipsychotic drugs. Previous drug discrimination studies in rats have shown that a 1.25 mg/kg clozapine training dose provides full stimulus generalization (i.e.) >or=80% condition-appropriate responding) to most atypical antipsychotic drugs, although a 5.0 mg/kg clozapine training dose appears necessary to provide stimulus generalization to other atypical antipsychotic drugs. The present study sought to characterize the pharmacological mechanisms that mediate these clozapine training doses. In rats trained to discriminate 1.25 vs. 5.0 mg/kg clozapine vs. vehicle in a three-choice drug discrimination task, various receptor-selective compounds were tested for stimulus generalization. The antidepressant mianserin was also tested. Full stimulus generalization from the 1.25 mg/kg clozapine training dose occurred only to mianserin (98.8%). Partial substitution (i.e. >or=60% and <80% condition-appropriate responding) to the 5.0 mg/kg clozapine training dose occurred for the muscarinic receptor antagonist scopolamine. The combined total percentage of responding on the 1.25 and 5.0 mg/kg clozapine levers, however, was well above the full substitution criteria at the 0.25, 0.5, and 1.0 mg/kg scopolamine doses. The M1 agonist N-desmethylclozapine, the nicotinic antagonist mecamylamine, the D1 antagonist SCH 23390, the D4 antagonist LU 38-012, the 5-HT1A agonist (+)-8-OH-DPAT, the 5-HT1A antagonist WAY 100 635, the 5-HT2A/2B/2C antagonist ritanserin, the 5-HT6 antagonist RO4368554, the alpha1 antagonist prazosin, the alpha2 antagonist yohimbine, and the histamine H1 antagonist pyrilamine all failed to substitute for either the 1.25 or the 5.0 mg/kg clozapine training doses. These results are consistent with previous evidence that antidepressant drugs have a tendency to substitute for clozapine and that muscarinic receptor antagonism may mediate the discriminative stimulus properties of 5.0 mg/kg clozapine. The lack of stimulus generalization from either clozapine training dose to other receptor-selective compounds, however, fails to explain how this model screens atypical from typical antipsychotic drugs and suggests that the discriminative stimulus properties of clozapine consist of a compound cue.

Generalization to atypical antipsychotic drugs depends on training dose in rats trained to discriminate 1.25 mg/kg clozapine versus 5.0 mg/kg clozapine versus vehicle in a three-choice drug discrimination task

The prototypical atypical antipsychotic drug (APD) clozapine (CLZ) elicits a discriminative cue that appears to be similar to the stimulus properties elicited by atypical, but not typical, antipsychotic drugs in two-choice drug discrimination procedures. However, the ability of CLZ to generalize to atypical APDs depends on the training dose, since several atypical APDs (e.g. sertindole, risperidone) do not substitute for a 5.0 mg/kg CLZ training dose in rats, but do so for a 1.25 mg/kg CLZ training dose. Yet, a 1.25 mg/kg CLZ discriminative stimulus has not generalized to all atypical APDs either (e.g. quetiapine); thus, both 1.25 mg/kg and 5.0 mg/kg CLZ discriminative stimuli may be necessary to provide a better screen for atypical APDs. The present study sought to determine whether a three-choice 1.25 mg/kg CLZ versus 5.0 mg/kg CLZ versus vehicle drug discrimination task in rats might better distinguish atypical from typical APDs. Adult male Sprague-Dawley rats were trained in this three-choice drug discrimination task with a fixed ratio 30 reinforcement schedule for food. Clozapine produced full substitution (>or=80% condition-appropriate responding) for both the 1.25 mg/kg CLZ dose (ED50=0.09 mg/kg) and the 5.0 mg/kg CLZ dose (ED50=2.71 mg/kg). The atypical APD olanzapine produced full substitution for the 5.0 mg/kg CLZ dose, but not for the 1.25 mg/kg CLZ dose (ED50=1.55 mg/kg). In contrast, the atypical APD quetiapine produced full substitution for the 1.25 mg/kg CLZ dose (ED50=0.13 mg/kg), but not for the 5.0 mg/kg CLZ dose. Similarly, the atypical APD sertindole produced full substitution for only the 1.25 mg/kg CLZ dose (ED50=0.94 mg/kg). Risperidone, another atypical APD, produced partial substitution (>or=60% and <or=80% condition-appropriate responding) for the 1.25 mg/kg CLZ dose, and failed to substitute for the 5.0 mg/kg CLZ dose. The atypical APD ziprasidone and the typical APDs haloperidol and chlorpromazine failed to substitute for either CLZ training dose. These results demonstrated that the 1.25 mg/kg CLZ training dose provides partial or full stimulus generalization to more atypical APDs than does the 5.0 mg/kg CLZ training dose. Full substitution by olanzapine for only the 5.0 mg/kg CLZ dose suggests that this higher training dose is also important for screening atypical APDs.

Discriminative stimulus properties of the atypical antipsychotic clozapine and the typical antipsychotic chlorpromazine in a three-choice drug discrimination procedure in rats

RATIONALE

The atypical antipsychotic drug (APD) clozapine elicits a robust discriminative cue that is generally selective for other atypical APDS in two-choice drug discrimination (DD) procedures.

OBJECTIVES

The present study determined whether a three-choice DD procedure with the atypical APD clozapine (CLZ) versus the typical APD chlorpromazine (CPZ) versus vehicle (VEH) could provide greater selectivity between atypical and typical APDs.

METHODS

Sprague-Dawley rats were trained to discriminate 5.0 mg/kg CLZ from 1.0 mg/kg CPZ from VEH in a three-lever DD task with an FR30 food reinforcement schedule.

RESULTS

Generalization testing with CLZ produced CPZ-appropriate responding at lower doses (ED50=0.103 mg/kg) and CLZ-appropriate responding at higher doses (ED50=1.69 mg/kg). Generalization testing with the atypical APD olanzapine produced similar results. In contrast, the atypical APD risperidone and the typical APDs CPZ and haloperidol produced only CPZ-appropriate responding. The muscarinic antagonist scopolamine produced CPZ-appropriate responding at lower doses and CLZ-appropriate responding at higher doses in a manner similar to CLZ and olanzapine. The co-administration of haloperidol (0.00625 mg/kg) with scopolamine shifted the dose-response curve for CLZ-appropriate responding to the left. The 5-HT(2A/2C) antagonist ritanserin and the H1 histamine antagonist pyrilamine did not substitute for either CLZ or CPZ. The alpha1 adrenergic antagonist prazosin did not substitute for CLZ, but produced full substitution for CPZ.

CONCLUSIONS

The three-choice DD procedure clearly distinguished the atypical APDs CLZ and olanzapine from the typical APDs CPZ and haloperidol; however, the stimulus properties of the atypical APD risperidone were similar to CPZ, but not to CLZ. These findings further suggest that CLZ, as well as CPZ, elicits a compound cue.

Discriminative stimulus properties of 1.25 and 5.0 mg/kg doses of clozapine in rats: examination of the role of dopamine, serotonin, and muscarinic receptor mechanisms

Clozapine (CLZ), an atypical antipsychotic drug (APD), produces minimal extrapyramidal side effects (EPS) and has significant advantages for treating both positive and negative symptoms in schizophrenic patients. CLZ has been established as a discriminative cue in the drug discrimination paradigm and in generalization tests the CLZ cue is more selective for atypical, rather than typical, APDs. However, greater selectivity for atypical antipsychotics has been demonstrated with a lower (1.25 mg/kg) CLZ training dose in rats [Psychopharmacology, 149 (2000) 189], rather than the traditional, higher training dose (5.0 mg/kg). It is therefore of interest to evaluate the properties mediating the 1.25 mg/kg CLZ discriminative cue. In the present study, rats were trained to discriminate either 1.25 mg/kg (N=7) or 5.0 mg/kg (N=7) CLZ from vehicle in a two-lever drug discrimination task. The typical antipsychotic haloperidol (0.1-0.4 mg/kg) did not substitute for either CLZ cue, whereas the atypical antipsychotic melperone (0.37-3.0 mg/kg) provided full substitution in both groups (>80% CLZ-appropriate responding). The 5-HT(1A) receptor agonist (+)-8-OH-DPAT (0.04-0.16 mg/kg), and the selective 5-HT(2A) receptor antagonist M100907 (0.03-1.0 mg/kg) did not produce substitution in either group. (+)-8-OH-DPAT combined with haloperidol (0.05 mg/kg) engendered only partial substitution (>60% CLZ-appropriate responding) for both CLZ cues, and M100907 combined with haloperidol (0.05 and 0.1 mg/kg doses) failed to provide substitution in either group. Trihexyphenidyl (0.18-6.0 mg/kg), a muscarinic M(1)-preferring receptor antagonist, engendered full substitution for the 1.25 mg/kg CLZ cue, but only partial substitution for the 5.0 mg/kg CLZ cue. These results provide evidence that antagonism at the muscarinic M(1) receptor is sufficient to provide 1.25 mg/kg CLZ-like discriminative stimulus effects.

Stimulus properties of the “atypical” antipsychotic zotepine in rats: comparisons with clozapine and quetiapine

The stimulus properties of the "atypical" antipsychotic zotepine were assessed in three studies in rats. In Study 1, the ability of zotepine to generalise to clozapine was studied. Two groups of rats were trained to discriminate clozapine at 2 and 5 mg/kg. Clozapine induced full generalisation in both groups, with the generalisation curves shifted significantly to the left in the low dose group. In generalisation tests clozapine did not suppress responding. Zotepine induced dose-related generalisation in both groups, with full generalisation in the low dose group and 50% maximal generalisation in the high dose group at the highest dose that could be tested. In contrast to clozapine, zotepine induced substantial (50% or more) substitution for clozapine only at doses which suppressed responding. In Study 2 zotepine was investigated in rats trained to discriminate quetiapine (10 mg/kg). Quetiapine induced full generalisation and zotepine only induced 54% generalisation at the highest dose that could be tested. Generalisation was accompanied by response suppression induced by both quetiapine and zotepine. In Study 3 an attempt was made to train a zotepine discrimination (1 mg/kg increased to 2 mg/kg). Even after 150 training sessions it proved impossible to obtain reliable discriminative responding with zotepine. These data suggest that: (i) The actions of zotepine in discrimination assays are similar to, but not identical with, those of clozapine and quetiapine; (ii) The differences among the actions of clozapine, quetiapine and zotepine may be related to either the unique ability of zotepine to block noradrenaline (NA) uptake, or to its more marked affinity for D(2) receptors; (iii) The finding that zotepine only mimicked quetiapine up to a level of 54% was unexpected, since quetiapine and clozapine generalise reciprocally and zotepine generalised fully to (low dose) clozapine. This finding may also be related either to zotepine's ability to inhibit NA uptake or its relatively high D(2) affinity; (iv) Although zotepine clearly possesses discriminative properties, it is not possible to train it as a reliable stimulus, in contrast to clozapine and quetiapine. This may be due to its more marked D(2) receptor affinity. Collectively, these data demonstrate both similarities and differences between zotepine and other novel atypical antipsychotics in drug discrimination assays.

Generalization of clozapine as compared to other antipsychotic agents to a discriminative stimulus elicited by the serotonin (5-HT)2A antagonist, MDL100,907

Employing a two-lever, food-reinforced FR10 procedure, rats were trained to recognize a discriminative stimulus (DS) elicited by the 5-HT(2A) receptor antagonist and potential antipsychotic agent, MDL100,907 (0.16 mg/kg, i.p.). In generalization tests, by analogy to MDL100,907 itself (Effective Dose(50) (ED(50)), 0.002 mg/kg, s.c.), the 'atypical' antipsychotic, clozapine, which displays high affinity for 5-HT(2A) as compared to D(2) receptors, dose-dependently and fully generalized to MDL100,907 (ED(50), 0.2 mg/kg, s.c.). S16924 (0.05 mg/kg, s.c.), S18327 (0.09 mg/kg, s.c.), quetiapine (1.8 mg/kg, s.c.), risperidone (0.02 mg/kg, s.c.) and ziprasidone (0.01 mg/kg, s.c.), antipsychotics which possess-like clozapine-marked affinity for 5-HT(2A) versus D(2) receptors, also generalized to MDL100,907. In distinction, raclopride, an antipsychotic which selectively interacts with D(2) versus 5-HT(2A) receptors, did not display significant generalization. Interestingly, haloperidol, which shows only modest affinity for 5-HT(2A) versus D(2) sites, generalized to MDL100,907 (ED(50), 0.02 mg/kg, s.c.). In light of the antagonist properties of haloperidol, clozapine and all other antipsychotics tested (except raclopride) at alpha(1)-adrenoceptors (ARs), the selective alpha(1)-AR antagonists, prazosin and WB4101, were examined. Both dose-dependently and fully generalized to MDL100,907 (ED(50)s, 0.07 and 0.11 mg/kg, s.c., respectively). At doses showing pronounced generalization to MDL100,907, the only drugs which significantly suppressed response rates were haloperidol and, weakly, quetiapine. Raclopride also markedly decreased response rates. In conclusion, the antipsychotic agents, clozapine, ziprasidone, risperidone, S16924, S18327, quetiapine and haloperidol, all generalized to a DS elicited by MDL100,907. While D(2) receptors are not implicated in their actions, in addition to antagonist properties at 5-HT(2A) receptors, blockade of alpha(1)-ARs and other, as yet unidentified, mechanisms may be involved. These data underpin interest in MDL100,907 as a potential antipsychotic agent.