Opioids in the medial nucleus of the solitary tract are not involved in feeding disorder in activity-based anorexia in rats

Abstract

<h2>Summary</h2><h3>Background & aims</h3> Activity-based anorexia (ABA) is an abnormal behavior caused by mealtime restriction and excessive running. A previous study revealed that running causes the secretion of opioids in rats, and excessive opioids are known to decrease food intake. Thus, we hypothesized that the suppression of food intake in ABA rats that experienced mealtime restriction and excessive wheel running was due to an increase in opioids caused by running. Our previous study showed that ABA rats consumed more food than the control group after an intraperitoneal injection of naloxone. This result indicates that opioids are involved in the regulation of food intake in ABA rats. Here, we queried the involvement of the medial nucleus of the solitary tract (mNST) of the brain that mediates satiety information from the internal organs in rats' ABA and also examined whether opioids in the mNST were involved in feeding regulation in them. <h3>Methods</h3> Each ABA rat was given a running wheel and received feeding time restriction, followed by microinjection of naloxone or saline into the mNST. Each control rat was subjected to only feeding time restriction and then microinjection of naloxone or saline into the mNST. <h3>Results</h3> The results showed that food intake of the ABA group was not increased by naloxone injection into the mNST. <h3>Conclusions</h3> Our results suggest that the mNST is not involved in the regulation of food intake by opioids. Regulation of food intake by opioids may occur in other parts of the brain, such as the parabrachial nucleus and/or hypothalamus.