Effect of photoperiod on the pineal melatonin rhythm in neonatal rats.

Photoperiod already modulates pineal melatonin rhythm in neonatal rats. Pineal melatonin content was about 500 fmol during day and increased up to 2000 and 3000 fmol at night in 8- and 12-day-old rats, respectively. On long photoperiods (LD 14:10) melatonin was increased above 1000 fmol for about 8 h while on short photoperiods (LD 8:16) for 12 to 14 h. Melatonin pattern may thus transduce photoperiodic effects in neonatal rats. However, no differences in plasma LH were found in the rats kept on long and short photoperiods.