Effect of photoperiod on the growth of reproductive organs and on pineal N-acetyltransferase rhythm in male rats treated neonatally with testosterone propionate.

Neonatal administration of testosterone propionate (TP) sensitized male Wistar rats to photoperiodic regulation of reproduction. Testicular, seminal vesicle and prostate weights in 105-day-old TP-treated rats housed under natural photoperiodic conditions were lower during autumn and winter than during spring and summer. The reproductive organ weights in 75-day-old TP-treated rats kept under artificial lighting regimens were smaller under photoperiods shorter than 10 h per day than under photoperiods of 10 h per day or longer. In TP-treated males, pinealectomy prevented the decrease in reproductive organ weights induced by short days. The pineal involvement in the photoperiodic regulation of reproduction might be mediated by the rhythm in melatonin. Under a 8L:16D regimen, when reproductive organ weights were decreased, the period of high night N-acetyltransferase activity, which determines the period of the increased melatonin production, lasted about 1 h longer than under 12L:12D.