Metabolic effects of prolonged melatonin administration and short-term fasting in laboratory rats

The aim of this work was to evaluate the effect of prolonged administration of the pineal hormone melatonin and short-term fasting on metabolic variables in male and female Wistar: Han rats. Melatonin (MEL, 4 mu g/ml of tap water) was administered daily since the 5th week of age. The control group drank tap water. Rats were fed a standard type of diet ad libitum and were kept in the light regimen L: D-12: 12 h. The experiment was terminated after 11 (variant B) or 12 (variant A) weeks of MEL administration. The animals were sacrificed by quick decapitation following overnight fasting (variant A) or 48-h fasting (variant B). Selected organs and tissues were removed and weighed and selected metabolic variables in the serum and tissues were determined. MEL decreased body mass independent of food and water intake in both sexes. In males (variant A) MEL increased the weight of the heart muscle, spleen and adrenals; it decreased the absolute weight of epididymal fat and increased serum corticosterone and phospholipids concentration in comparison with controls. In females, serum glucose decrease and liver triacylglycerols increase were found. After 48-h fasting (variant B) liver, spleen and adrenal weight increase in MEL-drinking females was found. In males MEL increased the thymus weight and decreased the epididymal fat weight. In both sexes MEL increased serum corticosterone and liver glycogen concentration; MEL increased serum glucose in males and serum cholesterol concentration in females. Changes in the evaluated variables were also related to fasting duration prior to decapitation. A 48-h fasting at the end of the prolonged MEL intake (variant B vs. A) decreased the absolute liver weight in both sexes and the epididymal/periovarial fat weight, and increased thymus weight in males. In females it decreased the absolute heart muscle weight and increased the spleen weight. In males, 48-h fasting increased serum corticosterone and phospholipids concentration; it decreased the liver triacylglycerols content in females and the liver cholesterol content in males and females. In both sexes 48-h fasting increased glucose concentration in the serum and glycogen concentration in the liver and heart muscle as well as triacylglycerols and cholesterol concentration in the serum, phospholipids concentration in the liver and bone marrow and decreased malondialdehyde concentration in the liver. Forty-eight hour fasting after prolonged MEL administration resulted in a wider range of carbohydrate and lipid metabolism alterations of young rats of both sexes.