Contralateral circling induced by tegmental morphine: anatomical localization, pharmacological specificity, and phenomenology.

Unilateral morphine application to sites in the ventral tegmentum caused contralateral circling. Positive sites were restricted to the region of the dopamine-containing cells in the ventral tegmental area and substantia nigra; rates were highest with ventral tegmental application. The opiate receptor antagonists naltrexone and naloxone blocked morphine-induced circling whether given before or during the sessions. Pimozide pretreatment also blocked morphine-induced circling at doses that spared muscimol-induced circling. Morphine induced little postural asymmetry and induced forward locomotion in all 4 limbs. The radius of morphine-induced circling was large or small depending on the size of the test environment; thus the behavior was controlled by environmental (sensory) and not central (motor) constraints. These studies suggest that morphine-induced circling results from activation of dopaminergic circuitry involved in sensory-motor integration, particularly that involved in forward locomotion.