Tremor production by intracaudate injections of morphine

Pronounced resting tremor was produced in unanesthetized cats by injecting morphine (25-110 mug) into the caudate nucleus. The effects of morphine were antagonized by intracaudate (i.c.) injections of nalorphine (81-263 mug). Tremor activity was also inhibited by i.c. injections of dopamine (61-145 mug), Ca2+ (24-40 mug), scopolamine (88-121 mug) and hemicholinium-3 (HC-3; 73-129 mug) while serotonin (125 mug) was ineffective. Tremor inhibition by HC-3 was reversed by i.c. doses of acetylcholine (15-30 mug) which were subthreshold for tremor production in the absence of morphine. Morphine (55-110 mug) further increased the intensity of ongoing tremor activity induced by physostigmine (111 mug i.c.) I.c. injections of nalorphine antagonized the motor effects of morphine without affecting physostigmime tremor. Tremor production by morphine is attributed to a reduction in dopamine function which allows cholinergic activity in the caudate nucleus to predominate.