Melatonin limits obesity in rodents without affecting food intake and activity, suggesting a thermogenic effect. Previously we demonstrated that melatonin browns subcutaneous fat in Zücker diabetic fatty (ZDF) rats. Other works pointed to melatonin as a signal that increases brown adipose tissue (BAT) mass and function in rodents. However, direct proof of thermogenic properties (uncoupled mitochondria) of the newly recruited BAT in response to melatonin is still lacking. Therefore, in this work, we investigated if melatonin recruits thermogenic BAT in ZDF rats. Zücker lean (ZL) and ZDF animals were subdivided into two groups, control (C) and treated with oral melatonin (M) for 6 weeks. Mitochondrial mass, activity of citrate synthase (CS), and respiratory chain complexes I and IV were lower in C-ZDF than in C-ZL animals (P <.001). Melatonin treatment increased BAT weight in ZDF rats (P <.001). Also, it rose mitochondrial mass (P <.01) and activities of CS and complexes I and IV (P <.001) in both, ZDF and ZL rats. Uncoupling protein 1 (UCP1) mRNA and protein were 50% lower in BAT from obese rats. Also, guanosine diphosphate (GDP) binding was lower in ZDF than in lean rats (P <.01). Melatonin treatment of obese rats restored the expression of UCP1 and GDP binding to levels of lean rats and sensitized the thermogenic response to cold exposure. These data demonstrated that melatonin recruits thermogenic BAT in ZDF rats. This may contribute to melatonin's control of body weight and its metabolic benefits.
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