Role of light in the mediation of acute effects of a single afternoon melatonin injection on steroidogenic activity of testis in the rat.

Young adult male rats, maintained either in an LD 12 : 12 or in continuous illumination (LL) for one week, were given a single injection of 25 microg melatonin/100 g body wt or ethanolic-saline (control) at 17.00 h. Animals from each group were sacrificed at 11.00 h on the following day. The activity of two important steroidogenic enzymes, 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) and delta(5)-3 beta-hydroxysteroid dehydrogenase (delta(5)-3 beta-HSD), and serum concentrations of testosterone, were measured following highly specific and sensitive spectrophotometric techniques and RIA, respectively. A significant decrease in the activity of both the steroidogenic enzymes was noted in the testes of melatonin-treated rats maintained under normal light-dark schedules, but this response was found to be lacking in the LL rats. However, no significant changes in the level of serum testosterone were noted in either group of melatonin-treated rats from the values in respective groups of ethanolic saline-administered LD and LL rats. Exposure of ethanolic saline-injected rats to continuous light also did not cause any change in the steroidogenic activity of the testis from those in LD rats. The study indicates that continuous light as such does not affect the endocrine function of testis but abolishes suppressive effects of melatonin on the steroidogenic activity of the testis in rat.