Interaction between opioid agonists or naloxone and 5-HTP on feeding behavior in food-deprived rats

Morphine and the enkephalin analogs DAME, DADLE and FK-33824, as well as the opioid antagonist naloxone, decrease feeding in food-deprived rats after intraventricular or subcutaneous administration, FK-33824 being by far the most potent drug tested. The administration of subeffective doses of either morphine or naloxone given by the subcutaneous route induces anorexia when given in combination with a subeffective dose of 5-HTP whereas the treatment with subeffective intraventricular doses of any of the opioids or naloxone fails to potentiate 5-HTP. Similarly, the anorexia induced by FK-33824 is blocked by either morphine or naloxone given subcutaneously but not by intraventricular administration of the same two drugs. The results appear to suggest that central or peripheral opioid receptors differentially affect feeding behavior in the rat and, on the other hand, that the interaction of opiates with the serotonergic system appears to occur preferentially in the periphery.