Effects of neuroleptic drugs on the inhibition of exploratory behaviour induced by a low dose of apomorphine: implications for the identity of dopamine receptors.

Apomorphine in low doses inhibits spontaneous exploratory behaviour in rats. This effect is commonly referred to as an expression of selective stimulation of dopaminergic autoreceptors. The aim of the present study was to investigate the influence of neuroleptic drugs with different pharmacological profiles on this apomorphine induced inhibition of exploration using techniques for detailed recording of behaviour and multivariate statistical analysis of the results. By comparison with dose response analyses of apomorphine it was possible to determine whether a neuroleptic specifically antagonised the apomorphine effect or if the pattern of behaviour was qualitatively changed in some way. Apomorphine (0.05 mg/kg) was tested against cis-flupenthixol (0.01-0.5 mg/kg), haloperidol (0.01-0.1 mg/kg), metoclopramide (0.2-5 mg-kg), sulpiride (0.5-50 mg/kg) and SCH 23390 (0.005-0.05 mg/kg). Metoclopramide and haloperidol had weak antagonising effects against apomorphine while cis-flupenthixol and SCH 23390 was completely inefficient in this respect. The multivariate analysis indicated that the effects of haloperidol was restricted to only some aspects of the behavioural effects of apomorphine. Only sulpiride did selectively and dose-dependently antagonise the apomorphine induced behavioural suppression. The data provide evidence for a functional subdivision of dopamine receptors at the behavioural level.