Local production of neurostradiol affects gonadotropin-releasing hormone (GnRH) secretion at mid-gestation inLagostomus maximus(Rodentia, Caviomorpha)

Abstract

Females of the South American plains vizcacha, Lagostomus maximus, show peculiar reproductive features such as massive polyovulation up to 800 oocytes per estrous cycle and an ovulatory process around mid‐gestation arising from the reactivation of the hypothalamic–hypophyseal–ovary (H.H.O.) axis. Estradiol (E2) regulates gonadotropin‐releasing hormone (GnRH) expression. Biosynthesis of estrogens results from the aromatization of androgens by aromatase, which mainly occurs in the gonads, but has also been described in the hypothalamus. The recently described correlation between GnRH and ER α expression patterns in the hypothalamus of the vizcacha during pregnancy, with coexpression in the same neurons of the medial preoptic area, suggests that hypothalamic synthesis of E2 may affect GnRH neurons and contribute with systemic E2 to modulate GnRH delivery during the gestation. To elucidate this hypothesis, hypothalamic expression and the action of aromatase on GnRH release were evaluated in female vizcachas throughout pregnancy. Aromatase and GnRH expression was increased significantly in mid‐pregnant and term‐pregnant vizcachas compared to early‐pregnant and nonpregnant females. In addition, aromatase and GnRH were colocalized in neurons of the medial preoptic area of the hypothalamus throughout gestation. The blockage of the negative feedback of E2 induced by the inhibition of aromatase resulted in a significant increment of GnRH‐secreted mass by hypothalamic explants. E2 produced in the same neurons as GnRH may drive intracellular E2 to higher levels than those obtained from systemic circulation alone. This may trigger for a prompt GnRH availability enabling H.H.O. activity at mid‐gestation with ovulation and formation of accessory corpora lutea with steroidogenic activity that produce the necessary progesterone to maintain gestation to term and guarantee the reproductive success.

Hypothalamic GnRH expression and pulsatility depends on a balance of prolactin receptors in the plains vizcacha, Lagostomus maximus

“…The anti-GnRH antibody (Millipore; MAB5456) staining was eliminated by preincubation of the diluted antiserum (1:400) with 10 μg/ml GnRH synthetic peptide (LHRH, L7134; Sigma Co.) as previously described (Dorfman et al, 2011). The anti-GnRH antibody staining cellular and distribution pattern in the hypothalamus is identical with previous reports in vizcacha (Charif et al, 2017;Dorfman et al, 2013;Inserra et al, 2017) and in mouse and sheep (Bergman et al, 2010;Rietema et al, 2019).…”

“…This fine regulation can be achieved by both direct and indirect modulation of GnRH neurons by estrogen receptors α and β (ERα and ERβ), progesterone receptors (PR) and aromatase, the estradiol-synthesizing enzyme, among others. Significantly, the maximum levels of hypothalamic expression of those factors were detected at mid-pregnancy, coincidently with LH surge (Charif et al, 2017;Dorfman et al, 2013;Inserra et al, 2017. In addition, a significant increase of PRL has been detected in pituitary glands of vizcachas from mid-pregnancy onward, with its maximum at term pregnancy (Corso et al, 2020;Proietto et al, 2018).…”

“…In order to gather experimental groups during gestation, captures were planned according to the natural reproductive cycle previously described in Llanos and Crespo (1952) and in our own previous works (Charif et al, 2017;Dorfman et al, 2013;Fraunhoffer et al, 2017;Inserra et al, 2017;Leopardo et al, 2011;Proietto et al, 2019). Charif et al, 2017;Dorfman et al, 2013;Fraunhoffer et al, 2017;Leopardo et al, 2011;Proietto et al, 2019).…”

“…In order to gather experimental groups during gestation, captures were planned according to the natural reproductive cycle previously described in Llanos and Crespo (1952) and in our own previous works (Charif et al, 2017;Dorfman et al, 2013;Fraunhoffer et al, 2017;Inserra et al, 2017;Leopardo et al, 2011;Proietto et al, 2019). Charif et al, 2017;Dorfman et al, 2013;Fraunhoffer et al, 2017;Leopardo et al, 2011;Proietto et al, 2019). The presence or absence of ovulatory stigmata was verified by macroscopic inspection of the ovaries (Table 1) and correlated with microscopic analysis for the abundance of corpora lutea in hematoxylin-eosinstained ovary sections.…”

The key action of estradiol and progesterone enables GnRH delivery during gestation in the South American plains vizcacha, Lagostomus maximus

“…In order to extract total hypothalamic RNA, tissues were homogenized with TRIzol (Invitrogen, California, USA), according to the manufacturer's instructions, as previously described (Charif et al 2017. Its concentration was quantified by absorption at 260nm Table 2.…”