Gonadotropin-inhibitory hormone (GnIH) was first identified in Japanese quail to be an inhibitor of gonadotropin synthesis and release. expressed in gonadotropes. Further GnIH inhibits gonadotropin-releasing hormone (GnRH)-induced gonadotropin subunit gene transcription by inhibiting the adenylate cyclase/cAMP/PKA-dependent ERK pathway in an immortalized mouse gonadotrope cell line (LβT2 cells). GnIH neurons also project to GnRH neurons that express GPR147 in the preoptic area (POA) in birds and mammals. Accordingly GnIH can inhibit gonadotropin synthesis and release by decreasing the BMS-354825 activity of GnRH neurons as well as by directly inhibiting pituitary gonadotrope activity. GnIH and GPR147 can thus centrally suppress testosterone secretion and spermatogenesis by acting in the hypothalamic-pituitary-gonadal axis. GnIH and GPR147 are also expressed in the testis of birds and mammals possibly acting in an autocrine/paracrine manner to suppress testosterone secretion and spermatogenesis. GnIH expression is also regulated by melatonin stress and social environment in birds and mammals. Accordingly the GnIH-GPR147 system may play a role in transducing physical and social environmental information to regulate optimal testicular activity in birds and mammals. This review discusses central and direct inhibitory effects of GnIH and GPR147 on testosterone secretion and spermatogenesis BMS-354825 in birds and mammals. was cloned using PCR primers designed from the sequence of the receptor for RFRPs. The crude membrane fraction of COS-7 cells transfected with the putative cDNA specifically bound GnIH GnIH-related peptides (-RPs) and RFRPs which Tmem33 have an LPXRFamide (X?=?L or Q) motif at their C-termini in a concentration-dependent manner (43). In contrast C-terminal non-amidated GnIH failed to bind the receptor. Accordingly the C-terminal LPXRFamide (X?=?L or Q) motif seems to be critical for its binding to GPR147 (43). It was suggested that there is no functional BMS-354825 difference among GnIH and GnIH-RPs because GPR147 bound GnIH and GnIH-RPs with similar affinities (43). Further studies are required to investigate if GnIH and GnIH-RPs work additively or synergistically to BMS-354825 achieve their effects on the target cells that express GnIH-R. Ikemoto and Park (29) cloned cDNAs in the chicken. cDNA was expressed only in the brain and pituitary where GnIH may act directly on gonadotropes. On the other hand cDNA was ubiquitously expressed in various tissue and organs where BMS-354825 GnIH action is unknown. Quail GnIH and putative chicken GnIH inhibited Gαi2 mRNA expression in COS-7 cells transiently transfected with chicken or than (29). These results further suggest that GPR147 is the principal receptor for GnIH in birds as in mammals. To further investigate the intracellular signaling pathway responsible for the actions of GnIH and its possible interaction with GnRH Son et al. (44) used a mouse gonadotrope cell line LβT2. Using this cell line this group established that mouse GnIHs (mRFRPs) effectively inhibit GnRH-induced cAMP signaling indicating that mouse GnIHs (mRFRPs) function as inhibitors of adenylate cyclase (AC). They further showed that mouse GnIHs (mRFRPs) inhibit GnRH-stimulated ERK phosphorylation and gonadotropin subunit gene transcription. The results indicated that mouse GnIHs (mRFRPs) inhibit GnRH-induced gonadotropin subunit gene transcriptions by inhibiting AC/cAMP/PKA-dependent ERK activation in LβT2 cells (44). Shimizu and Bédécarrats (45) showed that mRNA levels fluctuate in an opposite manner to GnRH-receptor-III a pituitary specific form of GnRH receptor (GnRH-R) in the BMS-354825 chicken (46 47 according to reproductive stages. They demonstrated that the chicken GPR147 inhibits cAMP production most likely by coupling to Gαi. This inhibition significantly reduces GnRH-induced cAMP responsive element activation in a dose-dependent manner and the ratio of GnRH/GnIH receptors was a significant modulatory factor. From these results they proposed that in avian species sexual maturation is characterized by a change in GnIH/GnRH receptor ratio changing pituitary sensitivity from GnIH inhibition of to GnRH stimulation of gonadotropin secretion (45). Suppression of Testicular Activity by GnIH Inhibition of Gonadotropin Secretion Gonadotropin-inhibitory hormone precursor mRNA was first localized by Southern blot analysis of the RT-PCR products in the quail brain. Within the samples from telencephalon diencephalon.