C - Michigan Technological University

ears. Similarly, serum NPY increases in humans as renal function decreases [247] andis inversely correlated to BSALP (r = -0.28) and total ALP (r = -0.51) in hemodialysispatients [248]. These data suggest a relationship between high NPY levels and low boneturnover in human patients.Serum serotonin decreased during hibernation in black bears. This is the firstknown report of serum serotonin in a hibernator; however, serotonin levels increase inthe cerebral spinal fluid of hibernating yellow-bellied marmots [249]. Although there maybe species variations between different hibernators, it is possible that serotoninincreases in the cerebral spinal fluid and decreases in the serum during hibernation, asserotonin cannot pass through the blood-brain barrier [250]. Serotonin promotesproliferation of osteoblasts and differentiation of osteoclasts in vitro [219]; thus, reducedserotonin during hibernation could favor suppressed bone turnover. However, there iscontroversy about the effects of serotonin on bone in vivo (for review see: [251]), andserotonin levels are not correlated to bone turnover markers BSALP and TRACP inblack bears. Therefore, it is difficult to draw conclusions from this data. Although therewas no seasonal variation in serum NE of black bears, it is possible that tissue sensitivityto NE changes during hibernation. For instance, reduced temperatures can inhibit theNE-induced lipolysis of white adipose tissue from golden-mantled ground squirrels [252].Such an insensitivity to NE during hibernation, if exhibited in bone, could feasiblysuppress disuse-induced bone resorption since inhibiting NE with beta-blockers caninhibit bone loss in hind-limb unloaded animals [217].Despite these interesting findings, there were some limitations to this study. Forexample, body mass was not recorded at many data collection points, so leptin could notbe directly compared to changes in body mass in these bears. Another limitation is thatserum was only collected in the months immediately surrounding and encompassinghibernation. Thus, the bears were either preparing for hibernation, hibernating, orrecovering from hibernation throughout sample collection. It is possible that endocrinechanges between summer and hibernation would be more pronounced. Another pitfall isthat serum was collected under anesthesia, which could have an impact on endocrineconcentrations during the blood collection; however, the same dose of anesthesia wasgiven at each blood draw. There are possible effects of age and the birth of cubs on the55