Regulation of leptin synthesis in white adipose tissue of the female fruit bat, Cynopterus sphinx: Role of melatonin with or without insulin

Factors regulating leptin synthesis during adipogenesis in wild species are not well known. Studies in the female Cynopterus sphinx bat have shown that it undergoes seasonal changes in its fat deposition and serum leptin and melatonin levels. The aim of the present study was to investigate the hormonal regulation of leptin synthesis by the white adipose tissue during the period of fat deposition in female C. sphinx. This study showed a significant correlation between the seasonal changes in serum melatonin level with the circulating leptin level (r= 0.78; P < 0.05) and with the changes in body fat mass (r= 0.88; P < 0.05) in C. sphinx. A significant correlation between circulating insulin and leptin levels (r= 0.65; P < 0.05) was also found in this species. This in vivo finding suggests that melatonin together with insulin may enhance leptin synthesis by increasing adipose tissue accumulation. The in vitro study showed that melatonin interacts synergistically with insulin in stimulating leptin synthesis by adipose tissue in C. sphinx. The study showed MT₂ receptors in adipose tissue and a stimulatory effect of melatonin on leptin synthesis, which was blocked by treatment with an MT₂ receptor antagonist, suggesting that the effect of melatonin on leptin synthesis by adipose tissue is mediated through the MT₂ receptor in C. sphinx. The in vitro study showed that the synthesis of leptin is directly proportional to the amount of glucose uptake by the adipose tissue. It further showed that melatonin together with insulin synergistically enhanced the leptin synthesis by adipose tissue through phosphorylation of mitogen-activated protein kinase in C. sphinx.