TITLE: Melatonin increases brown adipose tissue mass and its functionality in young Zucker diabetic fatty (ZDF) rats.

A Jiménez-Aranda1, G Fernández-Vázquez0,2, R Ibán-Arias1, A Agil1. 1School of Medicine (Granada), Department of Pharmacology and Neuroscience Institute, 18012, Spain, 2Carlos III Hospital of Madrid, Service of Endocrinology, 28006, Spain

Previous data from our laboratory indicate that melatonin reduces body weight gain in young ZDF rats, without influencing food intake. Furthermore, several studies suggest that melatonin can influence mitochondrial biogenesis and function in brown adipose tissue (BAT). Therefore, in the present work we have investigated the role of melatonin on mitochondrial metabolism in ZDF rats. 6-week-old male ZDF rats (10 animals) were treated with oral melatonin in drinking water (10 mg/kg B.W./day) (M-ZDF) or with vehicle (V-ZDF) (control group, 10 rats). After 6 weeks of treatment (12-wk-old) animals were sacrificed, body weight gain and weight of main internal white adipose tissue (WAT) components and brown adipose tissue (BAT) were recorded. Also, mitochondrial fractions of these tissues were obtained and mitochondrial function was evaluated by measurement of oxygen consumption, Cytochrome C oxidation and the capacity of calcium retention. In addition, mRNA levels for thermogenic genes (uncoupling protein1 -UCP1-, peroxisome proliferator-activated receptor γ -PPAR γ - and PPAR γ coactivator 1α -PGC-1α-) were quantitated. Melatonin reduced total internal WAT mass of ZDF rats (from 57.70 ± 1.90 to 47.19 ± 2.10 g, p <0.001) and adiposity index (weight of total internal WAT/body weight x 100) (from 12.62 ± 0.06 to 11.71 ± 0.06, p<0.05). The reduction on internal WAT involved all major compartments: epiplon (from 26.78 ± 1.33 g to 22.31 ± 1.35 g, p <0.05), mesenteric (from 13.20 ± 0.45 to 10.34 ± 0.32 g, p <0.05) and epididimal (from 17.72 ± 0.45 to 14.54 ± 0.32 g, p <0.01). Moreover, M-ZDF rats showed a significant increase in BAT mass (from 4369 ± 107 to 6289 ± 94 mg, p<0.01) and BAT mitochondrial protein (from 4.7 ± 1.0 to 7.8 ± 0.9 mg/ml, p<0.001). Melatonin increased BAT UCP1 mRNA levels in ZDF (from 0.92 ± 0.10 to 2.35 ± 0.10 AU, p < 0.01) but no differences were noted in mRNA expression of PPARy and PGC-1α. Also, melatonin significantly increased oxygen consumption flux, Cytochrome C oxidation levels and the capacity of calcium retention.

These results indicate that chronic oral melatonin increases BAT mitochondrial biogenesis and its functionality, including the expression of the thermogenic gene UCP1. This contribute to explain the positive effects of melatonin on body weight reduction, that ultimately can lead to metabolic beneficial effects, such as glycemic control and amelioration of dyslipidemia.