DIFFERENTIAL-EFFECTS OF INTRAHYPOTHALAMIC ADMINISTRATION OF OPIOIDS ON FOOD-INTAKE IN NAIVE AND TOLERANT RATS

We investigated the effects of intrahypothalamic administrations of th e opioid agonists morphine (MOR) and ketocyclazocine (KCZ) and antagon ists naltrexone (NALTX) and Mr2266 on food intake (FI) during light an d dark phases of the diurnal cycle, after acute or chronic administrat ion in rats. Acute intralateral hypothalamic (LH) administration of MO R or KCZ (1 mu g/rat) enhanced FI during dark and light phases, respec tively, whereas intraventromedial hypothalamic (VMH) injections result ed in moderate hyperphagia during dark phases by both mu and kappa ago nists. The receptor specificity was evident from blockade of the respo nses to MOR or KCZ by the respective antagonists NALTX and Mr2266. Aft er repeated administrations of MOR and KCZ, FI responses to the test d ose of these agonists injected in LK were modulated in opposite direct ions. However, the adaptative changes in FI after intra-VMH injection of KCZ were similar to those seen with MOR. These results are discusse d in light of a differential opioid receptor involvement and their pos sible functional interactions within the hypothalamus during food inta ke.