Seasonal variations of insulin sensitivity in edible dormouse (Glis glis) adipocytes

Seasonal variation in glucose and insulin is modulated by food and temperature conditions in a hibernating primate

Feast-fast cycles allow animals to live in seasonal environments by promoting fat storage when food is plentiful and lipolysis when food is scarce. Fat-storing hibernators have mastered this cycle over a circannual schedule, by undergoing extreme fattening to stockpile fuel for the ensuing hibernation season. Insulin is intrinsic to carbohydrate and lipid metabolism and is central to regulating feast-fast cycles in mammalian hibernators. Here, we examine glucose and insulin dynamics across the feast-fast cycle in fat-tailed dwarf lemurs, the only obligate hibernator among primates. Unlike cold-adapted hibernators, dwarf lemurs inhabit tropical forests in Madagascar and hibernate under various temperature conditions. Using the captive colony at the Duke Lemur Center, we determined fasting glucose and insulin, and glucose tolerance, in dwarf lemurs across seasons. During the lean season, we maintained dwarf lemurs under stable warm, stable cold, or fluctuating ambient temperatures that variably included food provisioning or deprivation. Overall, we find that dwarf lemurs can show signatures of reversible, lean-season insulin resistance. During the fattening season prior to hibernation, dwarf lemurs had low glucose, insulin, and HOMA-IR despite consuming high-sugar diets. In the active season after hibernation, glucose, insulin, HOMA-IR, and glucose tolerance all increased, highlighting the metabolic processes at play during periods of weight gain versus weight loss. During the lean season, glucose remained low, but insulin and HOMA-IR increased, particularly in animals kept under warm conditions with daily food. Moreover, these lemurs had the greatest glucose intolerance in our study and had average HOMA-IR values consistent with insulin resistance (5.49), while those without food under cold (1.95) or fluctuating (1.17) temperatures did not. Remarkably low insulin in dwarf lemurs under fluctuating temperatures raises new questions about lipid metabolism when animals can passively warm and cool rather than undergo sporadic arousals. Our results underscore that seasonal changes in insulin and glucose tolerance are likely hallmarks of hibernating mammals. Because dwarf lemurs can hibernate under a range of conditions in captivity, they are an emerging model for primate metabolic flexibility with implications for human health.

Hibernation depth influences the edible dormouse pancreatic B cell during the spring arousal

In order to determine the influence of hibernation depth upon the secretion and the effect of insulin, two groups of edible dormice were maintained in winter under different climatic and nutritional conditions, and their pancreatic B-cell function was tested during the spring arousal. The first group of animals was exposed to a moderate temperature and fed ad libitum. Their periods of hypothermia were short and irregular and the active periods sometimes lasted several days; their body weight increased during the winter months; in spring, the sensitivity of B cells to glucose was low, decreasing insulin secretion in vivo and in vitro, and the adipocytes were insulin resistant. The second group of fasting animals was exposed to a low and constant temperature (5 degrees). Their phases of lethargy were long and regular (about 15 days), separated by active periods (6-8 hr); their body weight decreased during the winter months; in spring the B-cell secretion was increased and the sensitivity of the tissues to insulin ensured a high peripheral glucose utilization. These data show that the winter climatic and nutritional conditions which influence the depth of hibernation modify the edible dormouse B-cell activity during the spring arousal.