Nociceptin/Orphanin-FQ inhibits gonadotropin releasing hormone neurons via G-protein gated inwardly rectifying potassium channels

Abstract The pulsatile release of gonadotrophin releasing hormone (GnRH) is a key feature of the hypothalamic-pituitary-gonadal axis. Kisspeptin neurons in the arcuate nucleus (ARC) trigger GnRH neuronal activity, but how GnRH neurons return to baseline electrical activity is unknown. Nociceptin/orphanin-FQ (OFQ) is an inhibitory neuromodulator. ARC Proopiomelanocortin (POMC) neurons, known to receive inputs from ARC kisspeptin neurons, contact GnRH neurons and coexpress OFQ in rat. In the present study, the effect of OFQ(1-13) on GnRH neurons was determined in mouse. We identified transcripts for the OFQ receptor (opioid receptor like 1, ORL1) in GnRH neurons and, using two model systems (explants and slices), found that OFQ exerted a potent inhibition on GnRH neurons, with or without excitatory inputs. We confirmed the inhibition was mediated by ORL1 via G i/o protein coupling. The inhibition, occurring independently of levels of intracellular cyclic adenosine monophosphate, was sensitive to inwardly rectifying potassium channels. The only specific blocker of G i/o protein-coupled inwardly-rectifying potassium (GIRK) channels, tertiapin-Q (TPNQ), was ineffective on the OFQ inhibition. Two GIRK activators, one sharing the binding site of TPNQ and one active only on GIRK1-containing GIRK channels, failed to trigger an inhibition. In contrast, protein kinase C phosphorylation activation known to inhibit GIRK2-mediated currents prevented the OFQ inhibition. These results indicate a specific combination of GIRK subunits, GIRK2/3 in GnRH neurons. In vivo , double labeled OFQ/POMC fibers were found in the vicinity of GnRH neurons and OFQ fibers apposed GnRH neurons. Together, this study brings into light a potent neuromodulator of GnRH neurons. Significance Statement Fertility is controlled centrally by neurons secreting gonadotrophin releasing hormone (GnRH) and critically relies on their pulsatile secretory profile. GnRH pulses depend on kisspeptin neurons located in the arcuate nucleus (ARC). However, kisspeptin provides a long-lasting stimulation and how GnRH neurons return to baseline electrical activity is unknown. Here, we show nociceptin/orphanin-FQ (OFQ) potently inhibits GnRH neurons. The signaling pathway involves the receptor, opioid receptor like 1, and downstream effectors G i/o protein and G i/o protein-coupled inwardly-rectifying potassium (GIRK) channels. Notably, the GIRK channels in GnRH neurons exhibit a specific subunit composition, GIRK2/3. Together, these data identify a new messenger in modulating reproductive function.