The influence of long-term melatonin administration on basic physiological and metabolic variables of young Wistar:Han rats

The question of effects of long-term melatonin (MEL) administration have not yet been explained sufficiently, especially its metabolic consequences in young persons and animals. The aim of the present study was to analyze the effects of MEL given during prolonged time (for 3 months) and chronically (for 6 months) at the dose of 4 µg/mL of tap water, on the selected metabolic and hormonal parameters in young female and male Wistar:Han (WH) rats. The weights of selected organs, tissues, body weight gains and food and water intake were registered. Six weeks aged rats were adapted to standard housing conditions and light regimen L:D=12:12 h, fed standard laboratory diet and drank tap water (controls) or MEL solution ad libitum; finally they were sacrificed after overnight fasting. Prolonged MEL administration decreased serum glucose concentration and increased triacylglycerol and malondialdehyde concentration/content in the liver in females. In males MEL increased concentrations of serum phospholipids, corticosterone and liver malondialdehyde. MEL treatment reduced the body weight in both sexes and weight of epididymal fat in males, without any alterations of food and water intake. Chronic MEL administration reduced serum glucose concentration and increased concentration/content of glycogen, triacylglycerol and cholesterol in the liver and glycogen concentration/content in heart muscle in males. In females, the significant rise of serum corticosterone concentration and liver malondialdehyde content was recorded. MEL significantly increased liver weight and decreased thymus weight in males. MEL administration increased temporarily water intake in males, body and epididymal fat weights were similar to that in controls. Body weight of MEL drinking females was reduced in the 1st half of experiment only; the food and water intake did not differ from control group. The response in WH rats on MEL was more prominent as in the Sprague-Dawley strain (our previous studies). Male rats were generally more affected, probably due to higher daily and total consumption of melatonin.