Control of plasma sex steroid-binding protein (SBP) in the little brown bat: effects of photoperiod and orchiectomy on the induction of SBP in immature males.

Immature male little brown bats (Myotis lucifugus lucifugus) exhibit striking increases in plasma levels of sex steroid-binding protein (SBP) when aroused prematurely from the latter half of hibernation. To identify some of the factors that control this induction, the effects of castration, photoperiod manipulation, and season (early vs. late hibernation) were studied. During the latter half of hibernation, the elevation of plasma SBP levels induced by premature arousal was found to be similar in intact and castrated males. Since castration completely abolished the postarousal rise in plasma testosterone levels, and estradiol concentrations remained low (less than 10 pg/ml) in both groups, changes in gonadal steroids are apparently not responsible for these increases in SBP. The induction of SBP in males aroused late in hibernation also appeared to be independent of whether animals were maintained under long [16 h of light, 8 h of darkness (16L:8D)] or short (8L:16D) photoperiods. Three weeks after arousal, plasma SBP concentrations in the intact groups were 228 +/- 17 nM (mean +/- SE; 16L:8D) and 271 +/- 5 (8L:16D), while in the castrated groups, SBP levels were 256 +/- 43 (16L:8D) and 231 +/- 19 (8L:16D). In addition, comparable degrees of gonadal activation, including increases in testicular weight, rises in circulating testosterone, and initiation of spermatogenesis, were observed in both the long and the short photoperiod groups. During the first half of hibernation, the induction of SBP was similarly unaffected by castration. However, at this time, photoperiod had a marked effect on SBP induction. Only males maintained under a long photoperiod after arousal from early hibernation exhibited increased levels of SBP (158 +/- 51 nM) and evidence of gonadal activity. The animals housed under a short photoperiod had lower levels of SBP (44 +/- 22 nM) and exhibited no gonadal activation. In general, responses seen in animals aroused during the first half of hibernation were reduced compared to those observed after arousal during the second half of hibernation. Thus, the postarousal increase in SBP appears to be independent of gonadal secretions, but can be influenced by photoperiod during early hibernation.