Differential enhancement of behavioral sensitivity to apomorphine following chronic treatment of rats with (-)-sulpiride and haloperidol.

Rat exploratory activity as well as apomorphine-induced hypermotility and stereotyped behavior were assayed following acute (60 min before) or chronic (21 days) administration of sulpiride stereoisomers and haloperidol. Parallel groups of rats were assayed for their hypermotility and stereotyped responses to challenging doses of apomorphine 7 and 21 days after discontinuation of chronic treatments. Following its acute administration, (-)-sulpiride fully antagonized apomorphine-induced hypermotility without affecting the level of animal spontaneous activity and partially counteracted stereotyped behavior. Haloperidol completely suppressed both apomorphine responses and also depressed exploratory activity. Some tolerance to the anti-apomorphine effect of (-)-sulpiride groups exhibited enhanced behavioral sensitivity to apomorphine only with respect to hypermotility, whereas haloperidol groups were supersensitive with respect to both hypermotility and stereotyped responses. The results are discussed in terms of differential dopamine receptor supersensitivity arising from prolonged administration of butyrophenone and substituted benzamide.