Method for determining the level of food motivation in rats

Involvement of Peripheral Opioid Receptors in the Realization of Food Motivation Into Eating Behavior

The involvement of peripheral opioid receptors in the mechanisms of eating behavior is still unclear. The aim of this work was to study the role of peripheral, predominantly gastric mu and delta opioid receptors in the realization of food motivation in conditions of different energy costs for eating behavior. Experiments were performed under a between-sessions progressive ratio schedule of reinforcement in food-deprived rats. The level of food motivation was calculated using a self-developed method. Food intake, motor activity, and metabolic rate were recorded in fed and hungry animals. Results showed that intragastric administration of the mu opioid receptor agonist DAMGO led to an increase in the level of food motivation in the light variant of operant feeding behaviors. Food consumption did not change. At high costs for feeding behavior, the administration of DAMGO did not alter food motivation; however, food consumption and motor activity were reduced. Intragastric administration of the delta opioid receptor agonist DADLE did not lead to changes in the level of food motivation and physical activity, but inhibition of feeding behavior was observed in all reinforcement schedules. Three regulatory pathways of eating behavior in difficult food conditions by peripheral, predominantly gastric opioid receptors are hypothesized: environmental-inhibitory afferentations and suppression of the realization of food motivation into behavior; homeostatic-inhibitory action on food motivation; and rewarding-suppression of the anticipatory reinforcement.

Effect of peptide agonists of peripheral opioid receptors on operant feeding behavior and food motivation in rats

We studied the effect of intragastric administration of peptide agonists of μ-opioid receptors (DAMGO) and δ-opioid receptors (DADLE) on food consumption and food motivation during operant feeding behavior of different intensity and effectiveness. To obtain one food granule, trained rats should press a lever 1 time (day 1), 2 times (day 2), 4 times (day 3), 8 times (day 4), 16 times (day 5), or 32 times (day 6). Activation of δ-opioid receptors in the stomach was followed by suppression of feeding behavior at low energy expenditure. The level of food motivation under these conditions practically did not differ from the control. Activation of μ-opioid receptors in the stomach suppressed energy-consuming feeding behavior, which was accompanied by an increase in the level of food motivation. It can be hypothesized that protein metabolites exhibiting μ-opioid activity probably provide afferent signals into CNS via the vagus nerve to terminate energy expenditure under adverse conditions (although food motivation is not satisfied). Food motivation under these conditions probably contributes to the behavior aimed towards the search for more available food.