OPIOID SYSTEMS IN THE LATERAL HYPOTHALAMUS REGULATE FEEDING BEHAVIOR THROUGH OREXIN AND GABA NEURONS

The hypothalamus controls feeding behavior. Since central opioid systems may regulate feeding behavior, we examined the role of mu-, delta- and kappa-opioid receptors in the lateral hypothalamus (LH), the hunger center, in feeding behavior of mice. Non-selective (naloxone; 3 mg/kg, s.c.) and selective mu- (beta-funaltrexamine, beta-FNA; 10 mg/kg, s.c.), delta- (naltrindole; 3 mg/kg, s.c.) and kappa- (norbinaltorphimine, norBNI; 20 mg/kg, s.c.) opioid receptor antagonists significantly decreased food intake in food-deprived mice. The injection of naloxone (20 mu g/side) into the LH significantly decreased food intake whereas the injection of naloxone (20 mu g/side) outside of the LH did not affect food intake. The injection of beta-FNA (2 mu g/side), naltrindole (1 mu g/side) or norBNI (2 mu g/side) into the LH significantly decreased food intake. Furthermore, all these antagonists significantly decreased the mRNA level of preproorexin, but not those of other hypothalamic neuropeptides. In addition, the injection of the GABA(A) receptor agonist muscimol (5 mu g/side) into the LH significantly decreased food intake, and this effect was abolished by the GABA(A) receptor antagonist bicuculline (50 mu g/side). Muscimol (1 mg/kg, i.p.) decreased the mRNA level of preproorexin in the hypothalamus. Naloxone (3 mg/kg, s.c.) significantly increased the GABA level in the LH and both bicuculline and the GABA release inhibitor 3-mercaptopropionic acid (3-MP, 5 mu g/side) attenuated the inhibitory effect of naloxone on feeding behavior. 3-MP also attenuated the effects of beta-FNA and norBNI, but not that of naltrindole. These results show that opioid systems in the LH regulate feeding behavior through orexin neurons. Moreover, mu- and kappa-, but not delta-, opioid receptor antagonists inhibit feeding behavior by activating GABA neurons in the LH. (C) 2016 IBRO. Published by Elsevier Ltd. All rights reserved.