Variation in progesterone receptors and GnRH expression in the hypothalamus of the pregnant South American plains vizcacha, Lagostomus maximus (Mammalia, Rodentia).

In mammals, elevated levels of progesterone (P4) throughout gestation maintain a negative feedback over the hypothalamic-hypophyseal-gonadal (H-H-G) axis, avoiding preovulatory follicular growth and preventing ovulation. Recent studies showed that in the South American plains vizcacha (Lagostomus maximus) folliculogenesis progresses to preovulatory stages during gestation, and an ovulatory process seems to occur at midgestation. The aim of this work was to analyze hypothalamic gonadotropin-releasing hormone (GnRH) and P4 receptors (PR) expression and luteinizing hormone (LH) secretion and correlate these with the functional state of the ovary in nonovulating and ovulating females and gestating females with special emphasis in the supposedly ovulating females at midgestation. We investigated P4 and LH serum levels as well as the distribution, localization, and expression of PR and GnRH in the hypothalamus of L. maximus at different time points during gestation and in nongestating, ovulating and nonovulating, females. A significant increment in GnRH, P4, and LH was detected in midpregnant vizcachas with respect to early-pregnant and to ovulating females. PR was also significantly increased in midpregnant animals. PR was detected in neurons of the preoptic and hypothalamic areas. Coexistence of both PR and GnRH in neurons of medial preoptic area and supraoptic nucleus was detected. Midpregnant animals showed increased number of PR immunoreactive cells at median eminence, localized adjacently to GnRH immunoreactive fibers. High expression of hypothalamic GnRH and PR, despite an increased level of P4, was correlated with the presence of antral, preovulatory follicles, and luteinized unruptured follicles at midgestation that suggest a possible role of the H-H-G axis in the modulation of ovulation during gestation in L. maximus.