Response changes after repeated low apomorphine: dopamine autoreceptor desensitization or learning?

Repeated injection of rats with low doses of apomorphine (APO), which selectively interact with dopamine (DA) autoreceptors, caused a change in yawning responses that suggests initial low-APO-induced desensitization of DA autoreceptors, followed by a long-lasting rebound hypersensitivity. Repeated treatment with low APO followed by open-field testing, however, yielded totally different results. APO accelerated intrasession response decrement and upon repeated administration enhanced the intersession response decrement. Both for yawning and open-field behavior, the response change after the second dose of APO was only evident when the first as well as the second APO injection were followed by exposure of the rat to the same test situation. These results indicate that response changes after repeated treatment with low APO are not due to a simple DA-agonist-induced change in receptor sensitivity but that drug experience combined with environmental influences play a decisive role.