Circling from intracranial morphine applied to the ventral tegmental area in rats

Crystalline morphine applied unilaterally to the ventral tegmental area in rats caused circling away from the side of application. This circling was reversed by naloxone (3 mg/kg) and blocked by pretreatment with pimozide (0.5 mg/kg). When tested in an open field the animals followed the perimeter of the enclosure; thus the radius of the circles described was determined by environmental rather than central factors. Morphine induced forward locomotion in all four limbs; there were no major signs of postural asymmetry noted in the longitudinal axis of the animal's body. This study suggests that morphine activates a population of A-10 dopamine cells known to be involved in locomotion.