Enhanced reproductive responses to melatonin in juvenile Siberian hamsters.

Reproductive responses to melatonin (Mel) exposure were assessed in male Siberian hamsters gestated in long (16 h light/day; 16L) or short (8 h light/day; 8L) day lengths (DLs). Hamsters were maintained in constant light beginning at 14 days of age (day 14) and were infused for 3 successive days beginning on day 18, 25, or 32 with long (12 h/day) or short (6 h/day) durations of Mel or saline. Two weeks later, testis and body weights were determined. Responsiveness to Mel was enhanced in hamsters beginning shortly after weaning and was influenced by photoperiodic history. Gonadal growth in 16L hamsters was inhibited by 12-h Mel infusions initiated on day 18 or day 25, but infusions initiated on day 32 were ineffective. Hamsters gestated in 8L were not responsive to Mel treatments initiated on day 18, but discriminated 12-h from 6-h infusions at day 25 and manifested reduced gonadal growth after 12-h Mel infusions on day 32. Hamsters born in 8L to dams judged as photoinsensitive to short DLs did not respond to 12-h infusions on day 32 as inhibitory signals. We propose that maternal Mel secretion provides juvenile males with a photoperiodic history, which allows them to compare gestational with early postweaning Mel signals and thereby program seasonally appropriate developmental trajectories.