An agonist-induced switch in G protein coupling of the gonadotropin-releasing hormone receptor regulates pulsatile neuropeptide secretion

Abstract

The pulsatile secretion of gonadotropin-releasing hormone (GnRH) from normal and immortalized hypothalamic GnRH neurons is highly calcium-dependent and is stimulated by cAMP. It is also influenced by agonist activation of the endogenous GnRH receptor (GnRH-R), which couples to Gq/11 as indicated by release of membrane-bound ␣q/11 subunits and increased inositol phosphate͞Ca 2؉ signaling. Conversely, GnRH antagonists increase membrane-associated ␣q/11 subunits and abolish pulsatile GnRH secretion. GnRH also stimulates cAMP production but at high concentrations has a pertussis toxinsensitive inhibitory effect, indicative of receptor coupling to Gi. Coupling of the agonist-activated GnRH-R to both Gs and Gi proteins was demonstrated by the ability of nanomolar GnRH concentrations to reduce membrane-associated ␣s and ␣i3 levels and of higher concentrations to diminish ␣i3 levels. Conversely, ␣i3 was increased during GnRH antagonist and pertussis toxin treatment, with concomitant loss of pulsatile GnRH secretion. In cholera toxin-treated GnRH neurons, decreases in ␣s immunoreactivity and increases in cAMP production paralleled the responses to nanomolar GnRH concentrations. Treatment with cholera toxin and 8-bromo-cAMP amplified episodic GnRH pulses but did not affect their frequency. These findings suggest that an agonist concentration-dependent switch in coupling of the GnRH-R between specific G proteins modulates neuronal Ca 2؉ signaling via Gs-cAMP stimulatory and Gi-cAMP inhibitory mechanisms. Activation of Gi may also inhibit GnRH neuronal function and episodic secretion by regulating membrane ion currents. This autocrine mechanism could serve as a timer to determine the frequency of pulsatile GnRH release by regulating Ca 2؉ -and cAMPdependent signaling and GnRH neuronal firing.

Expression and Function of Gonadotropin-releasing Hormone (GnRH) Receptor in Human Olfactory GnRH-secreting Neurons

“…Antiproliferative signaling of GnRH in human reproductive tract tumors is mediated through the PTX-sensitive G i protein (43,44). Moreover, recent research has demonstrated that, in GT1 cells, GnRH-modulated neuronal Ca 2ϩ signaling occurs also via G i inhibitory mechanisms (45). Our findings in human olfactory neurons suggest that GnRH, according to its concentration, is able to switch the coupling of GnRH-R to specific G proteins and thus to regulate distinct biological responses (46).…”

“…A specific GnRH receptor antagonist blocked the migratory promoting activity of GnRH. Moreover, the fact that PTX pretreatment was able to reduce GnRH-dependent migration in a dose-dependent manner suggests that these effects were coupled to PTX-sensitive G i/o proteins (43)(44)(45). Several factors, including anosmin-1 (48), NELF (3), the receptor tyrosine kinase Ark (11), and FGFR1 (20), have been described as playing a role in some aspects of GnRH cell migration.…”

G Proteins and Autocrine Signaling Differentially Regulate Gonadotropin Subunit Expression in Pituitary Gonadotrope

“…In various cell types, the GnRHR has been shown to regulate G␣ s , G␣ q/11 , and G␣ i/o family G proteins (17)(18)(19)(20)(21)(22)(23)(24). To guide the study of G protein coding of gonadotropin expression, we determined which G protein mRNAs were expressed in gonadotrope cells.…”

“…On the other hand, in reproductive tumor cells, G␣ i/o played an important role in GnRHR signaling by regulating MAPK activation and in inhibiting cell proliferation in response to GnRH stimulation (20 -23). In the GnRH-producing neurons of the hypothalamus, G␣ i and G␣ q were involved in the pulsatile GnRH release in response to autocrine/paracrine GnRH stimulation (24). G protein activation by GnRHR may vary, as the GnRHR-activated G protein shifted from G␣ s to G␣ i under increasing GnRH concentration in hypothalamic neurons (24).…”

“…In the GnRH-producing neurons of the hypothalamus, G␣ i and G␣ q were involved in the pulsatile GnRH release in response to autocrine/paracrine GnRH stimulation (24). G protein activation by GnRHR may vary, as the GnRHR-activated G protein shifted from G␣ s to G␣ i under increasing GnRH concentration in hypothalamic neurons (24). The involvement of G␣ 12/13 in GnRHR signaling has not been explored.…”