The beta Nerve Growth Factor triggers ovulation in the mouse by acting upstream of GnRH neurons
Résumé
The timed induction of ovulation requires a cascade of events in which GnRH neurons play a key role. In mammals, ovulation can take place either spontaneously or be induced by mating. For induced ovulators, β-Nerve Growth Factor (βNGF) was described as an important stimulatory factor. We tested whether βNGF could induce ovulation in a spontaneous ovulator, the mouse, and investigated if GnRH is involved in its mechanism of action. Fifty mice (21 days) received 5IU of PMSG and, 48 hours later, one of the following treatments: 0.9% NaCl, 5IU of hCG, or βNGF (0.1µg, 1µg, or 10µg/mouse). All three βNGF doses induced ovulation (p <0.001) with an efficacy similar to that of hCG: ovulation rates were 80% for hCG or βNGF (0.1µg and 1µg), 100% for βNGF (10µg), and 10% for NaCl. To check if the effect of βNGF on ovulation required GnRH receptor activation, we tested responses in mice pre-treated with a GnRH receptor antagonist (Cetrorelix). Prepubertal mice (128) were allocated to five groups of treatment: βNGF (1 µg), Cetrorelix (50 ng) + βNGF (1 µg), GnRH (50 ng), Cetrorelix (50 ng) + GnRH (50 ng), or 0.9% NaCl. Both βNGF and GnRH triggered ovulations and this effect was blocked by Cetrorelix co-administration (ovulation rate: Cetrorelix + βNGF vs βNGF, p<0.05). These results suggest that βNGF requires the action of GnRH to trigger ovulation. To search for the central site of βNGF action we performed a series of double immunohistochemical labelling reactions in the hypothalamus of adult OVX+E2 female mice to localize the βNGF receptor p75 NTR. We found that p75 NTR is expressed in organum vasculosum of the lamina terminalis (OVLT), hypothalamic arcuate nucleus (ARC) and Median Eminence (ME ). In these regions, p75 NTR was found in neurons and tanycytes, but was not expressed by GnRH or Kisspeptin immunoreactive neurons. In conclusion, we have demonstrated that βNGF induces ovulation in the mouse and that its action requires the release of GnRH. By using Kiss and Gpr54 KO mice, we are currently investigating if the Kisspeptin signalling is also involved in mediating the effect of βNGF on ovulation.