Nociceptin/orphanin FQ (N/OFQ), an endogenous agonist of the opioid N/OFQ (NOP) receptor, increases food intake when administered centrally. As N/OFQ is part of a larger neural network that governs consummatory behavior, presumably its orexigenic properties stem from interplay with other neuropeptidergic components of the feeding-related circuitry. One such peptide may be the ligand of the melanocortin-3 and -4 receptors, α-melanocyte-stimulating hormone (α-MSH), which is known to inhibit food intake. The aim of the present study was to establish whether there is a functional "interaction" between N/OFQ and α-MSH in the regulation of feeding. By using double immunostaining for c-Fos and α-MSH, we found that intracerebroventricular (i.c.v.) injection of N/OFQ at a 10 nmol dose that moderately prolongs deprivation-induced food intake in rats, decreases activation of α-MSH neurons involved in feeding termination. However, i.c.v. injections of α-MSH at doses previously established to reduce deprivation-induced feeding, do not decrease hyperphagia generated by N/OFQ in ad libitum-fed animals. Our results suggest that while α-MSH does not appear to modify the orexigenic response to N/OFQ in sated rats, the NOP receptor ligand promotes a decrease in activation of neurons synthesizing the anorexigenic peptide, α-MSH, at the time of re-feeding. Thus, to some degree, the stimulatory effect of N/OFQ on consumption may arise from this peptide's inhibitory influence on activity of anorexigenic pathways containing α-MSH.
- Orphanin FQ
- α-Melanocyte-stimulating hormone