Progesterone receptor in the forebrain of female gray short-tailed opossums: Effects of exposure to male stimuli

Progesterone receptor immunoreactivity (PRir) in brain areas involved in reproductive behavior in eutherian species was examined for the first time in a female marsupial, the gray short-tailed opossum (Monodelphis domestica, hereinafter, opossum). PRir in nuclei of neurons, measured as area covered by stained nuclei, was seen in the arcuate nucleus (Arc); anteroventral periventricular nucleus (AVPv); bed nucleus of the stria terminalis (BST); medial preoptic area (MPOA), and ventromedial hypothalamus (VMH), but not in control areas adjacent to the hypothalamus or cortex. Female opossums are induced into cytological, urogenital sinus (UGS), estrus by male pheromones and into behavioral estrus, i.e., receptivity, by pairing with a male, and both estradiol (E) and progesterone (P) are involved in induction of receptivity in intact and ovariectomized females. PRir in the AVPv, MPOA, and VMH was very low in females that had never been exposed to males or their scent marks, i.e., naïve anestrous (NVA) females, and either previous or current exposure to males or their scent marks was associated with elevated PRir. PRir was significantly higher in the AVPv and MPOA of anestrous females with previous but no current exposure to males and their scent marks, i.e., experienced anestrous (EXPA) females, than in NVA females, but PRir was significantly lower in the MPOA and VMH of EXPA females than in females that were behaviorally receptive and had recently copulated, i.e., behavioral receptive estrous (BRE) females. PRir was higher in the VMH of both UGS estrous (UGSE) and BRE females compared to that in EXPA animals, but PRir did not differ between UGSE and BRE females in any of the 3 brain areas examined, including the MPOA These results provide evidence that pheromonal induction of estrus and sexual receptivity in opossums is associated with elevation of PRir in the VMH and MPOA and that prior exposure to males or their pheromones, even in the absence of current male stimuli, is associated with persistent elevation of PRir in the AVPv and MPOA.