The effects of cholinergic agents on morphine-induced circling behavior in the intact mouse.

The effects of morphine on locomotor activity in mice was studied. It was shown that morphine in this species induces circling, a dose-dependent, stereotyped behavior. At high doses of morphine, mice engaged virtually exclusively in circling uninterrupted by other activities. The effect was modified by muscarinic agents. Blockade of muscarinic receptors with atropine potentiated circling while the muscarinic agonist, oxotremorine, attenuated it. The mixed muscarinic/nicotinic cholinergic agonist, physostigmine, had no effect on this behavior.