Melatonin modulates glucose homeostasis during winter dormancy in a vespertilionid bat, Scotophilus heathi

The role for melatonin in glucose homeostasis and insulin resistance is not very clear and has recently been an active area of investigation. The present study investigated the role of melatonin in seasonal accumulation of adipose tissue in Scotophilus heathi, with particular reference to its role in glucose homeostasis and development of insulin resistance. The circulating melatonin levels correlated positively (p < 0.05) with the changes in body mass due to fat accumulation and circulating insulin level, but correlated negatively with the blood glucose level in S. heathi. The bats showed high circulating blood glucose levels and impaired glucose tolerance during the period of fat deposition suggesting insulin resistance condition which improves after winter when most of the fat has been utilized as a metabolic fuel. The high circulating melatonin levels during the period of maximum body fat at the beginning of winter prepare the bats for winter dormancy by modulating the glucose homeostasis through affecting blood glucose levels, muscle and liver glycogen stores, insulin receptor and glucose transporter 4 (GLUT 4) expression. This is also confirmed by in vivo study in which melatonin injection improves the glucose tolerance, increases muscle insulin receptor and GLUT 4 expression, and enhances glucose clearance from the blood. The results of present study further showed that the effect of melatonin injection on the blood glucose levels is determined by the metabolic state of the bats and may protect from decrease in blood glucose level during extreme starvation, however, melatonin when injected during fed state increases glucose clearance from the blood. In summary, the present study suggested that melatonin interferes with the glucose homeostasis through modulating intracellular glucose transport and may protect bats from hypoglycemia during winter dormancy.