C - Michigan Technological University

concentrations of melatonin were below quantifiable levels in all samples. This findingwas independently confirmed in the serum of grizzly bears, with the anatomicalobservation that the pineal gland in grizzly bears is extremely small (personalcorrespondence, Heiko Jansen).Testosterone has a decreasing trend with time in the serum of female Americanblack bears, such that the levels during post-hibernation are significantly lower than thelevels during prehibernation (Table 4.2). In contrast, previous findings demonstrateincreased testosterone in late hibernation in black bears [254-256]; however, theseprevious studies used male bears, which probably have different circannual cycles ofandrogens than females. Testosterone’s effects on bone are not fully understood. Overactivationof the androgen receptor in osteoblasts of mice increases bone turnover andreduces bone matrix quality and whole bone strength [257]. In culture, testosterone haspro-apoptotic effects on osteoblasts [258]. These two studies suggest that testosterone’sdirect effects on bone are catabolic. However, testosterone has anabolic effects ontissues which positively influence bone metabolism (e.g. muscle) [259] and is a substratefor the synthesis of estrogen, a steroid hormone with anabolic effects on bone.Therefore, testosterone may have indirect anabolic effects on bone. Indeed,testosterone has a positive correlation with femoral BMD and with serum levels of OCNand BSALP in post-menopausal women [260, 261], suggesting that testosterone maypromote bone formation in vivo. The serum levels of testosterone are not significantlydifferent during hibernation from either active season; therefore there is no evidence thattestosterone levels promote an anti-apoptotic environment during hibernation.PTH has anti-apoptotic effects on osteoblasts [49, 55], and intermittent injectionsof PTH can inhibit bone loss [56]. The Donahue lab has previously reported an increasein PTH from prehibernation through post-hibernation in bears [34]; however, thisprevious study was incomplete and limited in size. The current study has a completesample set from November through April, but includes only two bears. In another studywhich quantified PTH in the serum of 14 active and 27 hibernating American blackbears, no seasonal change in PTH was observed in non-lactating bears, though PTHwas lower in lactating than non-lactating bears [262]. In that study, there was only onesampling point in each season, in contrast to the Donahue studies which had multiple66