Ventral tegmental injections of a selective mu or delta opioid enhance feeding in food-deprived rats

Abstract

Food-deprived rats received injections of the selective mu opioid D-Ala2,N-Me-Phe4-Gly5-Ol-Enkephalin (DAMGO; 0.01, 0.1, 1.0 or 10 nmol) or the selective delta opioid D-Pen2,D-Pen5-enkephalin (DPDPE; 0.1, 1.0 or 10 nmol) into or dorsal to the ventral tegmental area (VTA). Repeated measures of the speed of eating and the latency to initiate eating were obtained by dividing meals into discrete segments. Intra-VTA injections of DAMGO or DPDPE produced dose-dependent increases in the speed of eating but had little effect on the latency to initiate eating. Injections into sites dorsal to the VTA were ineffective in the case of DPDPE and less than normally effective in the case of DAMGO. Intra-VTA DAMGO was at least 100 x more potent in enhancing feeding than was DPDPE. A similar difference in potency has been observed in the degree to which DAMGO or DPDPE activates the mesolimbic dopamine system. These data suggest the possibility that changes in the mesolimbic dopamine system underlie the potentiation of feeding by intra-VTA administration of mu or delta opioids.