ENHANCED REPRODUCTIVE RESPONSES TO MELATONIN IN JUVENILE SIBERIAN HAMSTERS

Reproductive responses to melatonin (Mel) exposure were assessed in ma le Siberian hamsters gestated in long (16 h light/day; 16L) or short ( 8 h light/day; 8L) day lengths (DLs). Hamsters were maintained in cons tant 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) o r short (6 h/day) durations of Mel or saline. Two weeks later, testis and body weights were determined. Responsiveness to Mel was enhanced i n hamsters beginning shortly after weaning and was influenced by photo periodic 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 responsiv e 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 photo insensitive to short DLs did not respond to 12-h infusions on day 32 a s inhibitory signals. We propose that maternal Mel secretion provides juvenile males with a photoperiodic history, which allows them to comp are gestational with early postweaning Mel signals and thereby program seasonally appropriate developmental trajectories.