In vertebrates three cognate receptors or receptor

In vertebrates, three cognate receptors or receptor-like sequences have been identified with distinct distributions and functions (Millar, 2005, Neill et al., 2001). Only two types of GnRHR occur in mammals, though (Morgan and Millar, 2004). The mammalian type I GnRHR shares over 80% amino FG4592 identity amongst rat, human, sheep and cow and pig (Millar et al., 2004). The type II GnRHR is fully functional in monkeys, pigs and dogs, but absent in mice, sheep, and cows, as well as silenced in human and chimpanzee genomes (Millar, 2003, Hapgood et al., 2005). The type I receptor is the type of receptor that is functional and predominant in the mammalian gonadotrope (in this review the term “GnRHR” refers to type I GnRHR). In some species, including humans, it is also expressed in reproductive tissues (e.g. breast, endometrium, ovary, prostate) and in tumors derived from these tissues (Cheung and Wong, 2008, Perrett and McArdle, 2013). What is structurally unique in the mammalian GnRHR, compared to other GPCRs, is the lack of an intracellular cytoplasmic carboxyl-terminal tail (Finch et al., 2004, Davidson et al., 1994). The C-terminal tail normally plays a key role in rapid desensitization and receptor internalization (Ferguson, 2001). It is an important phosphorylation target of the GPCR kinases (GRKs). GRK-mediated phosphorylation generates a docking site for β-arrestin scaffolding proteins, which upon binding mediate rapid desensitization through uncoupling from G proteins and dynamin-dependent internalization of the receptor (Bliss et al., 2010, Perrett and McArdle, 2013, Bockaert et al., 2003). It has been demonstrated experimentally that the mammalian GnRHR is resistant to rapid desensitization upon GnRH stimulation and instead undergoes relatively slow internalization (Vrecl et al., 2000, Finch et al., 2009, Pawson et al., 2008). Furthermore, experiments fusing the C-terminal tail of various GPCRs to the mammalian GnRHR demonstrated rapid desensitization and internalization, like other members of the GPCR family (Heding et al., 1998, Hanyaloglu et al., 2001). Therefore, these results together establish that the absence of the C-terminal tail in the mammalian GnRHR accounts for its resistance to rapid desensitization and ligand-induced internalization, and the absence of recruitment of β-arrestin (McArdle et al., 1999, Millar et al., 2008). The atypical lack of GnRHR desensitization, compared to the other members of the GPCR family, suggests that different mechanisms occur that potentially modulate the cellular response to pulsatile GnRH, such as changes in receptor abundance or changes in the signaling pathways that are activated upon binding GnRH to its receptor. Several factors are believed to affect Gnrhr gene expression based on in vitro studies, such as gonadal steroids, activins and inhibins, but the most notable factor is GnRH itself (Nathwani et al., 2000, Gregory and Kaiser, 2004). It has been demonstrated in rat pituitary cultures that Gnrhr gene expression is dependent on GnRH pulse frequency. Gnrhr mRNA levels are significantly increased at all pulse frequencies compared to untreated controls, with greater stimulation observed under conditions of high pulse frequency (e.g., every 30 min) (Kaiser et al., 1997a, Kaiser et al., 1997b). In addition, it has been shown that cell-surface GnRHR number in LβT2 cells follows similar pattern, with greater increases in number at higher frequencies of pulsatile GnRH (Bedecarrats and Kaiser, 2003). It has been shown that GnRH-regulated Gnrhr expression is PKC-dependent and involves the MAPK signaling pathway (Bjelobaba et al., 2016). Furthermore, at high cell surface densities of GnRHRs, Lhb expression is optimally stimulated, whereas Fshb expression is preferentially stimulated at lower cellular densities of the receptor (Kaiser et al., 1995). These studies support a model in which the number of cell surface GnRHRs plays a critical role in the differential responses to various GnRH pulse frequencies.