The role of opioids in feeding and reward elicited by lateral hypothalamic electrical stimulation

We have previously shown that feeding induced by electrical stimulation in the lateral hypothalamus of rats is inhibited by naloxone but not its quaternary analogue. In the present study, effects of morphine and loperamide -an opiate that does not pass the blood-brain barrier- were examined. Loperamide inhibited stimulation-induced feeding; reversal of this effect by quaternary naloxone confirmed the peripheral site of action. A low dose of morphine (1.25 mg/kg) facilitated feeding but higher doses were inhibitory. An inhibitory dose of morphine became facilitory, however, when preceded by quaternary naloxone. It therefore appears that central opioid activity promotes ingestive behavior while peripheral activity inhibits ingestion. To evaluate the function served by the central facilitory process, we exploited the relation that exists between feeding and self-stimulation elicited through a common electrode. It was found that potentiation of self-stimulation by food deprivation is blocked by naloxone. It is concluded that endogenous opioid activity may promote feeding by enhancing the reward value of food as a function of hunger.