Melatonin reduces body weight gain and increases nocturnal activity in male Wistar rats

•The melatonin-treated rats gained less weight than the control rats.•The melatonin-treated rats had a significant elevation in the nocturnal activity.•Melatonin treatment led to a significant increase in the MESOR and amplitude.•There were no differences in blood glucose results between the two groups.

AimThis study evaluated the effect of the administration of melatonin, the chief secretory product of the pineal gland, on the body weight in male Wistar rats.Main methodsThe animals were housed for 4 months in cages equipped to log horizontal activity within a thermostatically-controlled chamber, under a 12 h/12 h light/dark photoperiod (lights on at 08:00 h). After acclimatization, the animals were divided into two groups: (1) control animals, and (2) melatonin-treated animals. Melatonin was administered in tap water (20 μg/ml), and fresh drinking fluid was changed twice weekly. Rats were fed a standard diet ad libitum.Key findingsFood and water intake, body weight, the amplitude of the activity/rest rhythm (motor activity), and blood melatonin and glucose concentrations were measured. The administration of melatonin did not influence either food or water intake or glucose levels relative to those found in the control animals. However, melatonin administration reduced body weight gain and increased nocturnal locomotor activity. The peak concentration of melatonin was found at night coinciding with the increase in nocturnal activity.SignificanceThe results show that exogenous melatonin reduces body weight gain without having marked effects on metabolism. This may be due in part to the increased nocturnal activity shown by the animals treated with the indoleamine.