We report here that galanin inhibits spontaneous GnRH neuronal activity and kisspeptin-induced GnRH neuronal activity. neurons. Applied after kp-10 activation, galanin 1C16 (Gal1C16) rapidly suppressed kp-10 activation. Applied with kp-10, Gal1C16 prevented kp-10 activation until its removal. To determine the mechanism by which galanin inhibited kp-10 activation of GnRH neurons, Gal1C16 and galanin were applied to spontaneously active GnRH neurons. Both inhibited GnRH neuronal activity, independent of GnRH neuronal inputs. This inhibition was mimicked by a GalR1 agonist but not by GalR2 or GalR2/3 agonists. Although Gal1C16 inhibition relied on Gi/o signaling, it was independent of cAMP levels but sensitive to blockers of G protein-coupled inwardly rectifying potassium channels. A newly developed bioassay for GnRH detection showed Gal1C16 decreased the kp-10-evoked GnRH secretion below detection threshold. Together, this study shows that galanin is a potent regulator of GnRH neurons, possibly acting as a physiological break to kisspeptin excitation. Reproductive success relies upon the integration of physiological and environmental cues. GnRH neurons are the final output in the central nervous system, relaying signals to the pituitary that then act upon the ovaries. Estrogen (E2) feedback from the ovaries to the central nervous system Clofibrate is one of the most important signals coming from the periphery to keep the hypothalamic-pituitary-gonadal axis tuned. E2 feedback is critically dependent on E2 receptor (ER); however, GnRH neurons lack ER and receive E2 signals from upstream E2-sensitive cell populations. Galanin is a brain-gut neuropeptide widely distributed in the brain (rat [1], human [2], and mouse [3]). Galanin gene expression (4) and immunoreactivity (5) are regulated by E2. Many neuronal cell types producing classical neurotransmitters or neuropeptides coexpress galanin (6). GnRH neuronal population is one of them (7, 8). GnRH neurons also receive inputs from fibers immunoreactive for galanin (rat [7], human [9], mouse [10]). Clofibrate The number of galanin fibers onto GnRH neurons increases at puberty (11), with E2 treatment in ovariectomized female rats (12) or with preoptic area grafts restoring Clofibrate cycles in hypogonadal female mice (13). Supporting the putative integration of galanin inputs, GnRH neurons express the galanin receptor (GalR)1 (14,C16); however, how GnRH neurons process galanin signals remains unclear (16). Recently, galanin has been identified in a subpopulation of kisspeptin neurons, a critical ER expressing input to GnRH neurons (10, 17). Whether galanin impacts the kisspeptin-evoked activation of GnRH neurons is unknown. This report shows that primary GnRH neurons maintained in explants expressed GalR1, not GalR2 or GalR3, and that galanin 1C16 (Gal1C16) rapidly suppresses the kisspeptin-10 (kp-10)-induced calcium responses of GnRH neurons and prevents calcium responses during coapplication. Both the full-length galanin peptide and its Clofibrate truncated form, Gal1C16, inhibit spontaneous intracellular calcium ([Ca2+]i) oscillations. The inhibition was independent of excitatory inputs and could be mimicked with a GalR1-specific agonist but not GalR2- or GalR2/3-specific agonists. Although the downstream signaling pathway relies on the activation of Gi/o protein, intracellular levels of cAMP do not mediate the inhibition. Galanin inhibits GnRH neurons by activating G protein-coupled inwardly rectifying potassium (GIRK) channels. Using gonadotrophs as biosensors for GnRH showed that Gal1C16 also decreased kp-10-induced GnRH secretion. These data provide evidence for a physiological break, galanin, to the long-term excitation mediated by kisspeptin. Materials and Methods Nasal explants Explants were cultured as previously described (18, 19). Briefly, embryonic day 11.5 embryos (undetermined sex) were obtained from timed pregnant NIH Swiss mice. Nasal pits were dissected under aseptic conditions in Gey’s balanced salt solution (Life Technologies, Inc) supplemented with glucose (Sigma Chemical Co). One embryo generates one single explant. Explants were adhered onto coverslips by a plasma (Cocalico Biologicals)/thrombin (Sigma) clot and maintained at 37C in a defined serum-free medium (SFM) in a humidified atmosphere with 5% CO2. On culture day 3, SFM was replaced by fresh SFM Clofibrate containing fluorodeoxyuridine (80M; Sigma) for 3 days to inhibit proliferation of dividing olfactory neurons and nonneuronal explant tissue. On culture day 6, and every CD4 2 days afterward, the medium was changed.