Morphine-induced rotation in naive, nonlesioned rats.

In rats injected with morphine in the midbrain reticular formation, pronounced ipsilateral rotation behavior was elicited by mild auditory and visual stimuli. The frequency of occurrence and rate of rotation were dose-dependent. This effect was site specific and drug specific; other drugs (except heroin) failed to induce this behavior. Naloxone potentiated the morphine rotation. Pretreatment with drugs that either potentiated or attenuated the morphine rotation indicated involvement of the noradrenergic and cholinergic systems and excluded a role for the dopaminergic system. No analgesia was observed after morphine microinjection in this site; thus, the hyperresponsivity to mild auditory and visual stimuli and concurrent analgesia previously seen in animals with morphine microinjections in the periaqueductal gray matter appear to be dissociable effects of morphine, and site specific.