Concerning the specificity of the hypothalamic opiate receptor responsible for food intake in the rat

Direct application of small quantities of morphine (a mu-opiate receptor agonist) to the ventromedial hypothalamus (VMH) of rats can induce a stimulated food intake. Because this effect is only partly reduced by the opiate antagonist naloxone, given into the VMH, various other studies were undertaken to examine characteristics of the receptors at this site. The opiate agonist levorphanol but not its stereoisomer dextrorphan effectively increased feeding. Codeine, a weak opiate ligand, was also ineffective as were the kappa-opioid agonist ketocyclazocine and the sigma-opioid agonist phencyclidine. the hyperthermia which accompanies peripheral and central injections of morphine was not observed after hypothalamic application of a quantity of levorphanol sufficient to stimulate feeding. This leads us to propose that the opioid receptors in the VMH are: (1) stereoselective; (2) responsive to mu-, but not kappa- or sigma-agonists: and (3) different from the receptors that elicit hyperthermia.