Increase in testosterone sensitivity induced by constant light in relation to melatonin injections in rats

Abstract

In this experiment we investigated whether the lack of the nocturnal melatonin peak under constant light would cause an increase in testosterone sensitivity. Castrated rats were kept under periodic or constant light for one week. They received a daily injection of vehicle, testosterone propionate (125 micrograms), melatonin (50 micrograms) or testosterone plus melatonin (125 micrograms + 50 micrograms). Serum and pituitary gonadotrophins and pineal melatonin were measured at the end of the experiment. Under constant light, testosterone injections reduced the serum luteinizing hormone concentration in castrated rats to that in intact rats, but, under periodic light, the decrease was smaller. Melatonin did not reverse the stronger effect of testosterone under constant light. The serum melatonin peak produced by the exogenous melatonin injection had a higher amplitude, shorter duration and earlier appearance than the physiological melatonin peak. Exogenous melatonin did not modify the physiological melatonin secretion, measured either as serum melatonin concentration or pineal melatonin content on the consecutive day. We conclude that the increase in testosterone negative feedback sensitivity of castrated rats under constant light was not due to the absence of the nocturnal melatonin pulse.

Melatonin inhibits cholangiocyte hyperplasia in cholestatic rats by interaction with MT1 but not MT2 melatonin receptors

“…The serum levels of melatonin in normal rats were similar to that of previous studies (30) and, in agreement with previous findings (e.g., in liver cirrhosis) (14), increased following BDL. The finding that administration of melatonin to normal and BDL rats increases its circulating levels is supported by studies in rats (52) and humans (17). A number of studies support the inhibitory effect of melatonin on cell mitosis.…”