Morphine-stimulated feeding: analysis of macronutrient selection and paraventricular nucleus lesions

The hypothalamic paraventricular nucleus (PVN) has been found to be sensitive to the feeding stimulatory effects of opiates. The present experiments investigated the effect of systemic morphine (2 mg/kg) on macronutrient selection in freely-feeding and food-restricted rats and assessed the impact of PVN electrolytic and 6-hydroxydopamine lesions on the rats' ability to respond to peripheral morphine injection. In satiated rats, maintained ad lib on pure macronutrient diets, morphine increased food intake. This effect was associated with a preferential increase in protein ingestion; carbohydrate consumption, compared with fat and protein intake, was least affected. In food-restricted rats, permitted to eat for 6 hr, morphine instead produced a particular preference for fat, with no significant enhancement of total calorie intake. While PVN 6-hydroxydopamine lesions, which depleted PVN catecholamine levels by 70%, failed to alter morphine-stimulated feeding, electrolytic lesions of the PVN significantly attenuated this response, particularly protein and fat ingestion. This suggests that opiate-induced feeding may, in part, be mediated through the PVN, which is known to have an important function in the control of food ingestion.