Role of lipid type on morphine-stimulated diet selection in rats

Administration of morphine is said to increase fat consumption among rats a llowed to self-select nutrients. However, fats represent a diverse group of molecules, differing in metabolic and sensory :properties. Despite this, l ipid has yet to be manipulated as a variable in drug-stimulated nutrient se lection studies. To determine whether lipid source can impact daily and mor phine-stimulated (1, 3, and 10 mg/kg) diet intake, rats were provided with a choice between a high-fat and high-carbohydrate diet in three regimens in which the source of fat was varied between vegetable shortening, lard, or corn oil. Daily and morphine-stimulated diet selections were determined und er all conditions. Under daily feeding conditions, rats ate more of the hig h-lipid diet compared with the high-carbohydrate diet when vegetable shorte ning or lard was the main lipid alternative, but lipid and carbohydrate int ake did not differ when corn oil was the main lipid alternative. When rats were stimulated with morphine, the percentage of lipid increased relative t o baseline intake only when the lipid diets were the preferred alternatives (i.e., vegetable shortening or lard). When preference between lipid and ca rbohydrate diets was neutral (i.e., corn oil condition), morphine did not e nhance lipid consumption. These results indicate that morphine increases co nsumption of total energy or preferred diets and not lipid per se.