Apomorphine, in an intermediate dose (0.18 mg/kg s.c.) decreased
dopamine turnover and produced signs generally attributed to a decrease in dopaminergic neurotransmission, e.g. ptosis and yawning, as well as signs of an increased stimulation of
dopamine receptors in dopaminoceptive target neurones, e.g. stereotyped sniffing. In contrast, the former signs were exclusively observed after smaller doses and the latter after larger doses of
apomorphine. Since it had been shown in previous studies that these signs, except yawning, could be conditioned in association with discriminative stimuli in the environment, the present study using conditioning experiments with this intermediate dose aimed at determining, 1. the time course of each conditioned response, 2. the interaction of conditioned and unconditioned responses, and 3. the conditions under which
hypokinesia occurred. In each series, conditioned animals were compared with pseudoconditioned controls. Rats were conditioned for 8 days with
apomorphine, and on day 9, treated with saline in presence of the conditional stimuli (a test cage in combination with acoustic and olfactory stimuli). In contrast to pseudoconditioned controls, ptosis and stereotyped behaviour were observed in conditioned rats, sometimes occurring alternatingly. These signs closely resembled the direct, unconditioned pharmacological effects. In addition, akinesia occurred after conditioning, although it was never manifest as a pure
drug response, nor during the conditioning period. In contrast, yawning was observed in pseudoconditioned as well as in conditioned rats, although slightly more frequently in the former animals. Subsequently, the rats were again conditioned (or pseudoconditioned) on days 10-14 with
apomorphine and both groups tested with the same dose (0.18 mg/kg) of
apomorphine in the presence of the conditioned stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)