Diet and Lighting Conditions Modify the Effects of the Kappa Opioid Agonist U50,488H on Feeding Behavior in Rats

The effects of diet and lighting conditions on the actions of the kappa opioid receptor agonist, U50, 488H, on food and water intakes were examined in male Long—Evans rats. Animals were fed chow and water, or chow, water and either a 32% sucrose solution or a 0.15% saccharin solution. In the first part of the study, sucrose and saccharin solutions were removed for 24 h, and rats then injected with 0, 4.0 or 8.0 mg/kg U50, 488H at the beginning of the dark portion of the 24-h cycle. Chow and water intakes were measured 1, 2, 4 and 24 h later. This procedure was repeated weekly until all rats had received each dose of the drug. In the second part, the procedure was identical with the exception that injections were given at the beginning of the light. When injected in the dark, U50, 488H suppressed food intake in rats which had consumed sucrose, but had minimal effects on intakes in rats which had consumed saccharin or chow alone. When U50, 488H was injected in the light, food intakes were greater 1 and 2h after injections of 4.0 mg/kg, and lower after injections of 8.0 mg/kg U50, 488H than after saline. For rats given sucrose, but not saccharin or chow alone, intakes remained suppressed 4 and 24h after 8.0 mg/kg U50, 488H. In the light and dark, and across diets, water intakes were decreased 1 and 2h, and increased 24h after U50,488H injections.