Inhibition of drinking by intrahypothalamic administration of morphine.

Abstract

MORPHINE inhibits the release of acetylcholine (ACh) within both the peripheral1—4 and central5—9 nervous systems. This would suggest that a major effect of acute morphine treatment should be inhibition of behaviour mediated by central cholin-ergic transmission, but few behavioural demonstrations of this antagonism exist. We report here that intrahypothalamic injections of equimolar concentrations of morphine effectively antagonise the muscarinic-cholinergic drinking response10—13 elicited by injection of 4.0 nmol of carbachol into the hypo-thalamus. In addition, intracranial (i.e.) injections of morphine also blocked drinking elicited by several other dipsogenic stimuli, while not affecting eating. The results suggest that morphine not only inhibits the release of ACh but also may block the postsynaptic muscarinic receptor. This inhibitory effect of i.e. morphine may, furthermore, provide a simple behavioural screening test for central opiate activity.