Discriminative stimulus properties of haloperidol

Discriminative stimulus properties of the typical antipsychotic haloperidol compared to other antipsychotic drugs in C57BL/6 mice

Haloperidol (HAL) was developed in 1958 for the treatment of schizophrenia and is classified as a typical antipsychotic drug (APD). Effective in treating positive symptoms of schizophrenia, it does not treat negative symptoms and produces extrapyramidal motor side-effects. Atypical APDs like clozapine treat both positive and negative symptoms of schizophrenia, have reduced extrapyramidal motor side-effects and possess other clinical advantages. This study used a drug discrimination assay to allow a direct comparison between the subjective effects of HAL and other APDs. Eleven C57BL/6 mice were trained to discriminate 0.05 mg/kg HAL from the vehicle in a two-lever drug discrimination task. The HAL generalization curve (0.001563-0.2 mg/kg) yielded an ED50=0.0024 mg/kg (95% confidence interval: 0.0012-0.0048 mg/kg). The typical APD chlorpromazine produced full substitution at 4.0 mg/kg with 82.7% drug-lever responding (%DLR) with significant rate suppression and partial substitution (73.9% DLR) at 1.0 mg/kg with no rate suppression. The atypical APD clozapine produced partial substitution at 2.5 mg/kg (64.8% DLR) with significant rate suppression. The atypical APD amisulpride failed to substitute for HAL with a maximum %DLR of 57.9% at 40 mg/kg with no rate suppression. The atypical APD aripiprazole partially substituted with a maximum of 75.9% DLR at 1.25 mg/kg with significant rate suppression. These results demonstrate that HAL can be trained as a discriminative stimulus in C57BL/6 mice, and its discriminative cue appears to be unique and distinct from that of atypical APDs.

Translational Value of Drug Discrimination with Typical and Atypical Antipsychotic Drugs

This chapter focuses on the translational value of drug discrimination as a preclinical assay for drug development. In particular, the importance of two factors, i.e., training dose and species, for drug discrimination studies with the atypical antipsychotic clozapine is examined. Serotonin receptors appear to be an important pharmacological mechanism mediating clozapine's discriminative cue in both rats and mice, although differences are clearly evident as antagonism of cholinergic muscarinic receptors is important in rats at a higher training dose (5.0 mg/kg) of clozapine, but not at a lower training dose (1.25 mg/kg). Antagonism of α1 adrenoceptors is a sufficient mechanism in C57BL/6 and 129S2 mice to mimic clozapine's cue, but not in DBA/2 and B6129S mice, and only produces partial substitution in low-dose clozapine discrimination in rats. Dopamine antagonism produces partial substitution for clozapine in DBA/2, 129S2, and B6129S mice, but not in C57BL/6 mice, and partial substitution is seen with D4 antagonism in low-dose clozapine drug discrimination in rats. Thus, it is evident that clozapine has a complex mixture of receptor contributions towards its discriminative cue based on the data from the four mouse strains that have been tested that is similar to the results from rat studies. A further examination of antipsychotic stimulus properties in humans, particularly in patients with schizophrenia, would go far in evaluating the translational value of the drug discrimination paradigm for antipsychotic drugs.

Discriminative stimulus properties of the atypical antipsychotic amisulpride: comparison to its isomers and to other benzamide derivatives, antipsychotic, antidepressant, and antianxiety drugs in C57BL/6 mice

RATIONALE

Racemic (RS)-amisulpride (Solian^®) is an atypical antipsychotic drug used to treat schizophrenia and dysthymia. Blockade of dopamine D₂/D₃ and/or serotonin 5-HT₇ receptors is implicated in its pharmacological effects. While the (S)-amisulpride isomer possesses a robust discriminative cue, discriminative stimulus properties of (RS)-amisulpride have not been evaluated.

OBJECTIVES

The present study established (RS)-amisulpride as a discriminative stimulus and assessed amisulpride-like effects of amisulpride stereoisomers, other benzamide derivatives, and antipsychotic, antidepressant, and anxiolytic drugs.

METHODS

Adult, male C57BL/6 mice were trained to discriminate 10 mg/kg (RS)-amisulpride from vehicle in a two-lever food-reinforced operant conditioning task.

RESULTS

(RS)-Amisulpride's discriminative stimulus was dose-related, time-dependent, and stereoselective. (S)-Amisulpride (an effective dose of 50% (ED₅₀) = 0.21 mg/kg) was three times more potent than (RS)-amisulpride (ED₅₀ = 0.60 mg/kg) or (R)-amisulpride (ED₅₀ = 0.68 mg/kg). (RS)-Amisulpride generalized fully to the structurally related atypical antipsychotic/antidysthymia drug sulpiride (Sulpor^®; ED₅₀ = 7.29 mg/kg) and its (S)-enantiomer (ED₅₀ = 9.12 mg/kg); moderate to high partial generalization [60-75% drug lever responding (%DLR)] occurred to the benzamide analogs tiapride (Tiapridal^®) and raclopride, but less than 60% DLR to metoclopramide (Reglan^®), nemonapride (Emilace^®), and zacopride. Antipsychotic, antidepressant, and antianxiety drugs from other chemical classes (chlorpromazine, quetiapine, risperidone, and mianserin) produced 35-55% amisulpride lever responding. Lastly, less than 35% DLR occurred for clozapine, olanzapine, aripiprazole imipramine, chlordiazepoxide, and bupropion.

CONCLUSIONS

(RS)-Amisulpride generalized to some, but not all benzamide derivatives, and it failed to generalize to any other antipsychotic, antidepressant, or antianxiety drugs tested. Interestingly, the (R)-isomer shared very strong stimulus properties with (RS)-amisulpride. This finding was in contrast to findings from Donahue et al. (Eur J Pharmacol 734:15-22, 2014), which found that the (R)-isomer did not share very strong stimulus properties when the (S)-isomer was the training drug.

The discriminative stimulus effects of the neurotensin NTS1 receptor agonist PD149163 in rats: stimulus generalization testing with dopamine D1 and D2 receptor ligands

Brain-penetrant neurotensin NTS1 receptor agonists produce antipsychotic drug-like effects in animal models, including inhibition of conditioned avoidance responding and reversal of psychostimulant-induced hyperactivity and stereotypy. Allosteric interactions between NTS1 receptors and dopamine D2 receptors may account for some of these antipsychotic effects. In order to determine the role that dopamine receptors may play in the behavioral effects produced by activation of NTS1 receptors, a drug discrimination approach was used in rats to evaluate the potential mediation of NTS1 receptor agonist stimulus effects by dopamine D1 and D2 receptors. Rats were trained to discriminate either the NTS1 receptor agonist PD149163, the D1 receptor agonist SKF81297, or the D2 receptor agonist quinpirole from vehicle in a two choice drug discrimination task. Full stimulus generalization occurred from PD149163 to the typical antipsychotic drug and D2 receptor-preferring antagonist haloperidol. However, stimulus generalization did not occur from SKF81297 or quinpirole to PD149163. The discriminative cue for SKF91297 and quinpirole was fully blocked the D1 receptor antagonist SCH23390 and the D2/3 receptor antagonist raclopride, respectively. Cross generalization did not occur between SKF91297 and quinpirole. Based on these findings, the stimulus effects of PD149163 may be mediated, in part, through D2 receptor antagonism, but this may only be evident when PD149163 is used as the training drug.

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.

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.

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.

The anxiolytic CRF(1) antagonist DMP696 fails to function as a discriminative stimulus and does not substitute for chlordiazepoxide in rats

RATIONALE

Compounds with a mechanism of action different from benzodiazepines may retain the anxiolytic effects of benzodiazepines with fewer side effects. CRF(1) antagonists have anxiolytic-like effects but may have different discriminative stimulus (DS) effects compared with benzodiazepines.

OBJECTIVE

The present study evaluated the similarity of DS effects of a CRF(1) antagonist DMP696 to the benzodiazepine chlordiazepoxide and the ability of DMP696 to produce DS effects on its own using drug discrimination procedures, as well as its anxiolytic-like effects after acute or chronic administration.

METHODS

Rats were trained to discriminate chlordiazepoxide (5.0 mg/kg, IP, 30 min prior to session) from vehicle under a fixed-ratio 10 schedule of food reinforcement and drug- or vehicle-lever selection following administration of DMP696 was determined. The effects of DMP696 on latency to exit a dark chamber (defensive withdrawal model of anxiety) were used as an index of anxiolytic-like activity.

RESULTS

In chlordiazepoxide-trained rats, DMP696 (1.0-100 mg/kg, PO) resulted in most of the animals selecting the vehicle lever, as did another anxiolytic, the 5-HT(1A) partial agonist buspirone (0.3-10 mg/kg, IP). DMP696 reduced exit latency in defensive withdrawal at 10 mg/kg administered either acutely or chronically for 14 days. Thus, the doses of DMP696 studied in drug discrimination were up to 10-fold higher than those active in the anxiety model. In addition, DMP696 (10-60 mg/kg, PO) could not be established as a DS under the conditions used in this study. In a subsequent study, chlordiazepoxide was established as a DS in these same animals.

CONCLUSIONS

Lack of substitution of DMP696 for the chlordiazepoxide DS in rats and its inability to acquire DS properties suggest that the DS effects of DMP696 differ from those of benzodiazepines.

Discriminative stimulus properties of antipsychotics

Drug discrimination methodology has been used in a number of ways to analyze the actions of novel and putative novel antipsychotics in vivo. Recent studies suggest (a) in contrast to earlier theorizing, antagonism of the low-dose d-amphetamine stimulus in rats may not be an effective screen for novel antipsychotics; (b) dopamine D2-like agonists and antagonists, some of which are putative antipsychotics, can be studied in vivo as discriminative cues, although there is a pressing need for more selective drugs that differentiate the various members of the D2 family. (c) antagonism of the cue induced by the noncompetitive NMDA antagonist MK-801, which has been proposed as a possible screen for clozapine-like compounds, may be an unreliable assay; and (d) the clozapine stimulus is probably a compound cue (a drug "mixture"), which can be used to screen for novel clozapine-like antipsychotics, although the precise receptor mechanisms involved in mediating the clozapine stimulus, and its direct relevance to the antipsychotic action of clozapine remains to be proven conclusively.