ANNUAL CHANGES IN SERUM CONCENTRATIONS OF PROLACTIN IN CAPTIVE MALE BLACK BEARS (URSUS-AMERICANUS)

Prolactin may be involved in the regulation of reproduction in black b ears (Ursus americanus) as it is a mediator of photoperiodic changes i n a number of species. The objectives of this study were to validate a radioimmunoassay to measure prolactin in bear serum and to describe s easonal changes in serum prolactin concentrations in captive male bear s. Serum samples were obtained nine times during a year from three cap tive male black bears that were denning between November and March and active during the other months. The heterologous prolactin radioimmun oassay, using pig I-125-labelled prolactin and goat anti-pig prolactin as a primary antibody, was validated. Injection of thyrotrophin-relea sing hormone into the three male bears in June resulted in a rapid inc rease in serum concentrations of prolactin (t = 0, 11.4-14.8 ng ml(-1) ; t = 15-30 min, 18.4-28.7 ng ml(-1)). The sensitivity of the assay wa s 0.08 ng per tube. Intra- and interassay coefficients of variation we re 5.5% (n = 6) and 5.7% (n = 6), respectively. Serum concentrations o f prolactin changed seasonally, with the lowest concentrations in Dece mber (mean +/- SD = 1.1 +/- 0.1 ng ml(-1)); this was followed by a gra dual increase between January (2.6 +/- 0.6 ng ml(-1)) and April (6.4 /- 1.2 ng ml(-1)) and the highest concentrations in May (17.6 +/- 4.7 ng ml(-1)), preceding peak testosterone concentrations in June. The ob servation that prolactin secretion increased with increasing daylength suggests that photoperiod may be an external regulator. The presence of high concentrations of prolactin before peak testosterone concentra tions suggests that prolactin may play a role in regulating seasonal c hanges in the testes.