PARENT SESSION

(M655) MALE-INDUCED RISE IN PROGESTERONE PRECEDES RECEPTIVITY AND OVULATION IN THE GRAY SHORT-TAILED OPOSSUM (Monodelphis domestica).

Harder, John¹, Tadros, Lisa¹, Rogier, Yimei¹, Norfolk, Jennifer¹, Fadem, Barbara², ¹ The Ohio State University, Columbus, OH² University of Medicine and Dentistry of New Jersey, Newark, NJ

ABSTRACT- In gray short-tailed opossums, pairing with a male is sufficient to stimulate ovulation, even in the absence of copulation. Progesterone (P) is elevated in estrous females 10 to 20 hours prior to copulation and this elevation is coincident with the preovulatory LH surge. The objectives of this study were to determine: 1) if the preovulatory rise in P is dependent upon the presence of a male, 2) whether there is a temporal and functional relationship between the rise in P and female receptivity, and 3) the secretory source of the preovulatory rise in P. Adult females were randomly scheduled for blood sampling in 4 groups: Anestrous (n=7), Isolated at estrus (n=7), Paired with a male at estrus (n=12), or Ovariectomized (OVX) (n=5). Serial blood samples were collected from the lateral tail vein of restrained females at 6-hr intervals for 36 hr (beginning at 1200 hr of 2400 on day 1) and on days 3, 6, and 12 of gestation. Females were either intact and induced to estrus (monitored by urogenital sinus cytology) by exposure to male pheromone or OVX and induced to estrus by estradiol (E) implant. Both groups of females were paired with a male during 1300-0800 hr of days 1 and 2. Plasma concentrations of P were measured by radioimmunoassay. Three females copulated on the first night (lights out for 10 hr at 1400 hr) of pairing, and 6 copulated of the second night at 1848±78 hr. Six females did not copulate on their first or subsequent (n=3) inductions of estrus. In females copulating on the second night of pairing, plasma P increased from basal levels (<0.5 ng/ml) prior to pairing to 1.7 ng/ml at 1800hr and to 9.4 ng/ml at 2400 hr of the first night (ANOVA, P<0.05), 18-24 hr prior to copulation. By contrast, P remained basal throughout the 36-hr sampling period in both Anestrous and Isolated estrous females (P>0.05) indicating that neither the stress of blood sampling nor a spontaneous (male-independent) increase in P secretion is responsible for the pericopulatory rise in P. P also remained basal throughout the sampling period in OVX+E females, none of which copulated. The results of this study indicate that in the opossum the pericopulatory, preovulatory rise in P in estrous females is dependent upon the presence of the male, is of ovarian origin, and plays an essential role in the initiation of behavioral estrus. This study was funded by NSF, IBN 9723043.

KEY WORDS: opossum, progesterone, estrus, male stimuli