Evidence for the role of dopamine D1 receptors in morphine induced stereotypic behavior.

In the course of studying the effects of morphine on the escape from electrical stimulation to the rat medial forebrain bundle, we observed that a combination of low doses of morphine and stimulation, neither of which caused stereotypy in the rat, resulted in marked stereotypic behavior that was similar to that seen with high doses of morphine alone. This behavior could be blocked by either naloxone or by the D1 antagonist SCH 23390, but not by the D2 antagonist raclopride. Furthermore, the stereotypy caused by chronically administered high doses of morphine was blocked by naloxone and the D1 antagonist. These results strongly implicate the role of dopamine D1 activation in morphine-induced stereotypies.