Mating alters gonadotropin-releasing hormone cell number and content

Female musk shrews (Suncus murinus) are induced ovulators, which lack a behavioral and ovarian estrous cycle. Females mate the first time they are introduced to a male, but a second or third mating, at least 24 h later, is usually required to induce ovulation. Because GnRH-immunoreactive (GnRH-ir) cell numbers increase during and after exposure to a male, we hypothesized that mating promotes synthesis of this important peptide. To test this hypothesis, we examined changes in GnRH-ir cell number and GnRH-ir content at select time points after mating and ovulation. One hour after mating, GnRH-ir cell numbers in olfactory-related regions of the forebrain were increased. By 15 h after mating, just before ovulation, GnRH-ir cell number and content were increased. Twenty-four hours after mating, GnRH-ir cell numbers in the tenia tecta and medial septum/diagonal band were lower in females that ovulated compared with females that did not ovulate. By 40 h postmating, females that ovulated had fewer GnRH-ir neurons and lower GnRH content in the entire brain than females that did not ovulate. In addition, we found significant negative correlations between plasma estradiol concentrations and both GnRH-ir cell numbers and content in the preoptic area of animals killed around the time of ovulation. Interestingly, significantly more GnRH-ir neurons and a greater content of GnRH peptide were observed in several forebrain nuclei of females that did not ovulate 40, compared to 24, h after mating. In contrast, numbers of GnRH-ir neurons in the midbrain declined 40 h postmating in ovulated females. These results suggest that mating stimulates activity in GnRH-ir neurons, and that ovulation is correlated with a decline in GnRH-ir cell number and content. In this species, mating can be used as an external trigger to activate GnRH neurons and examine the regulation and production of GnRH in heterogenic neuronal populations.