Changes in sex steroids, gonadotropins, prolactin, and inhibin in pregnantand nonpregnant Japanese black bears (Ursus thibetanus japonicus)

We examined changes in the concentrations of serum progesterone (P-4), estr adiol-17 beta (E-2), FSH, LH, prolactin (PRL), and inhibin to determine the ir interaction and their effect on the reproductive endocrine controls of p regnant and nonpregnant female Japanese black bears. Fourteen female bears were used in this study over a 2-yr period. In the first year, six of the b ears were divided into two groups; a pseudopregnant group and a nonpregnant group. In the second year, the remaining eight bears were also divided int o two groups; a pregnant group and a nonpregnant group. Pregnant and pseudo pregnant bears had similar P-4 trends with both groups exhibiting a signifi cant increase in December, which is the suspected time of implantation in p regnant bears. These trends correlated with an increase in PRL levels, wher eas low levels of LH were maintained throughout the year. Nonpregnant bears maintained low concentrations of P-4 and compared with pregnant and pseudo pregnant bears, they also exhibited a delayed elevation in PRL. Luteinizing hormone activity varied among individual animals, but regardless of reprod uctive status, fluctuation patterns of E-2 FSH, and inhibin did not differ among bears. Our results suggest that PRL may play a luteotropic role in bo th pregnant and pseudopregnant bears, and is possibly responsible for induc ing reactivation of the dormant corpus luteum that precedes implantation in the Japanese black bear.