Activation of hypothalamic beta-endorphin pools by reward induced by highly palatable food

Experiments were performed to find biochemical evidence of an activation of endogenous opiate peptides in the brain by incentive reward. A method used to estimate specific in vivo opiate binding in rats using the labelled opiate agonist, 3H-etorphine, indicated a considerable reduction in opiate binding exclusively in the hypothalamus of non-deprived animals given a highly palatable food to eat for 20 min. Radioimmunoassay of the hypothalamus of rats under similar conditions found a pronounced drop in the concentration of beta-endorphin, but not in dynorphin, in the hypothalamus, indicating a release and breakdown of beta-endorphin. Therefore, the reduction in opiate binding in the hypothalamus may at least be partially explained by an occupation of opiate receptors by beta-endorphin, causing a reduced availability of receptors to etorphine. A possible role of hypothalamic beta-endorphin in the facilitation of reward pathways in the brain is discussed.