Constant lighting conditions affect sexual behaviour and hormone levels in adult male rats.

Adult male Long-Evans rats (90 days of age) were housed in 12 h light: 12 h dark (Group LD), constant white light (Group LL), or constant dark (dim red light) (Group DD) for 60-90 days. The first of two sexual behaviour tests revealed that more (P less than 0.05) rats in Group LD achieved intromission than did those in Groups LL and DD. Only 33% of Group LL and 32% of Group DD rats ejaculated. In contrast, all Group LD rats that mounted eventually ejaculated. Males in Group LD achieved more (P less than 0.05) ejaculations (mean of both tests combined, 3.4) than did males in Groups LL (1.5) or DD (1.2). Although there were no notable differences amongst the groups in the weights of the accessory sex organs, males in Group DD had higher (P less than 0.05) serum androgen levels than did those in Groups LL and LD while Group LL males had higher (P less than 0.05) serum prolactin levels than did those in Groups LD and DD. There were no differences in LH or FSH levels. In addition, more rats in Group LL had disrupted activity rhythms than did Group DD rats. These results indicate that housing in constant white or dim red light disrupts sexual behaviour in male rats and may do so through different neuroendocrine mechanisms.