Effect of Low-dose Chronic Melatonin Administration on Metabolic and Hormonal Variables of Young Laboratory Rats

Abstract

Markova M., E. Adamekova, B. Bojkova, P. Kubatka, M. Kassayova, E. Ahlersova, I. Ahlers: Effect of Low-dose Chronic Melatonin Administration on Metabolic and Hormonal Variables in Young Laboratory Rats. Acta Vet. Brno 2004, 73: 445-453. The introduction of melatonin (MEL) as a drug remains open due to insufficient data about the effects of its long-term administration on metabolism. We explored effects of MEL given for 26 weeks at low doses, 4 µg/ml of tap water, on selected metabolic and hormonal indices in young female and male Sprague-Dawley rats. Male and female rats aged 5 weeks were adapted to standard housing conditions and artificial light regimen L:D = 12:12 h. The animals were fed standard laboratory diet and drank tap water (controls) or MEL solution ad libitum. Body weight gain, food and water intake was regularly recorded. Oral glucose tolerance tests (OGTTs) were carried out before and 24 weeks after MEL administration. At the end of the experiment following an overnight fast the animals were sacrificed, selected organs and tissues were weighed, selected metabolic indices and hormone concentrations were determined in the serum, liver, heart muscle and bone marrow (femur). Chronic MEL administration increased in females serum corticosterone concentration, decreased glycogen concentration in the liver and heart muscle, increased triacylglycerol concentration in the bone marrow. An increased absolute weight of periovarial fat and body weight gain were found in last 5 weeks of the experiment. In males, MEL decreased insulin concentration in the serum and body weight gain from week 2 to week 17 of its administration; it decreased relative liver weight in males and females. MEL did not alter food or water intake in both sexes. MEL treatment decreased glucose tolerance (i.e. reduced area under curve values in OGTTs) in males and prolonged the return of afterload glycemia values to initial level in females. Alterations of carbohydrate metabolism regulation and of body weigth gain were found in both sexes of young rats after long term melatonin administration.