Changes in serum inhibin levels and immunolocalization of inhibin/activin subunits during the breeding season in the wild male Japanese black bear (Ursus thibetanus japonicus)

The objective of this study was to investigate the changes in secretion of inhibin and cellular localization of inhibin alpha and inhibin/activin (beta(A) and beta(B)) subunits during the breeding season in the wild male Japanese black bear. Histological observations of testes were performed and seminiferous tubule diameters were measured. The sections of the testes were immunostained by the avidin- biotin-peroxidase complex method (ABC) using polyclonal antisera raised against porcine inhibin alpha, inhibin/ activin beta(A), and inhibin/activin beta(B) during the breeding season. Serum concentrations of immunoreactive (ir-)inhibin, testosterone, and luteinizing hormone (LH) were measured by radioimmunoassay. Higher values of seminiferous tubule diameters and all types of spermatogenic cells including mature-phase spermatozoa were found during the breeding season. There were seasonal changes in serum concentrations of ir-inhibin, testosterone, and LH. Ir-inhibin was positively correlated with testosterone, and LH. In addition, immunoreactivity of inhibin alpha, beta(A), and beta(B) subunits were also detected in Sertoli and Leydig cells during the breeding season. These results suggest that Japanese black bear testes may secrete bioactive inhibins during the breeding season and that the circulating inhibin may be a useful indicator of the testicular function in wild male Japanese black bears.