Difference in sensitivity of the mu and kappa systems in cafeteria rats

A highly palatable diet (cafeteria diet) provokes an hyperphagia. The effects of Mu and Kappa opiate antagonists (Mu : Naltrexone 0.5 mg/Kg IP; Kappa Mr2266 0.5; 2.5; 10) and agonists (Mu Morphine 0.05; 0.1; 0.5; 2.5; Kappa Mr2033 0.5; 2.5) were studied on the nocturnal food intake of cafeteria rats and chow rats fed with monotonous food. At low doses Mu as well as Kappa antagonists do not modify the food intake of chow rats, but suppress the hyperphagia induced by the cafeteria diet. Kappa agonist provokes a decrease in food intake in chow and cafeteria rats while the Mu agonist at low doses suppresses the hyperphagia induced by cafeteria diet. The involvement of these two opioid systems in this type of hyperphagia is discussed.