Light activation of gonadal function restrained by continuous dark treatment in male rats.

The confinement of male rats in complete dankness from birth until 47 days of age resulted in severe retardation of reproductive function. The supply of light-on condition for the last one week caused a remarkable overshoot of testosterone secretion beyond the level of intact animals, associating with a significant weight increase in accessory reproductive organs. Eye-enucleation nullified these effects. This androgen overshoot was considered to be occurred as a result of the removal of inhibitory effect of dark condition rather than a result of positive stimulatory effect of light-on condition. The possibility to simulate the inhibitory effect of dark condition was examined. The intra-ocular injection of a serotonin precursor (5-hydroxytryptophan) at the initiation of 7 days of light-on treatment, lowered both androgen titer and weight of accessory reproductive organs to the light-off level. This might suggest a possible role of serotonin as an inhibitory dark substance involved in neuroendocrine mechanisms.