Changes in Feeding Behavior, Locomotor Activity, and Metabolism in Rats upon Modulation of Opioid Receptors in the Gastrointestinal Tract

We studied the role of μ-, δ-, and κ-opioid receptors of the stomach in the regulation of natural feeding behavior, metabolism, and locomotor activity of rats. Locomotor activity (number of crossed squares), food and water intake, oxygen consumption, and carbon dioxide release in animals were estimated in the standard home cage using a Phenomaster device (TSE) for 24 h at 40-min intervals. Administration of a μ-opioid receptor agonist DAMGO suppressed feeding behavior of animals in the light phase, but had little effect on locomotor activity and metabolism. Treatment with a δ-opioid receptor agonist DADLE was followed by the increase in metabolism over 24 h. These changes were accompanied by a decrease in locomotor activity during the light phase and activation of feeding behavior in the transition period. Intragastric administration of a κ-opioid receptor agonist ICI-204,448 inhibited feeding behavior, metabolism, and locomotor activity of rats only in the nighttime. These data suggest that opioid peptides produced in the stomach during food digestion play an important role in the regulation of food motivation and metabolism in rats. Various subtypes of opioid receptors probably regulate feeding behavior and metabolism of animals in different phases of vital activity.