Diurnal changes in pineal melatonin content in four rodent species: Relationship to photoperiodism

Daily changes in pineal melatonin content were examined in four rodent species (rat, Syrian hamster, Turkish hamster, and Siberian hamster). All species had elevated pineal melatonin levels during the dark period of a lighting cycle with a long photoperiod of 16 h of light per day, and no obvious differences were observed between photoperiodic and nonphotoperiodic species. Pineal melatonin levels of Siberian hamsters maintained in either a short (10 h of light per day) or a long photoperiod were elevated for most of the dark period, so the duration of elevated levels of pineal melatonin was longer in a short photoperiod than in a long photoperiod. Treatment of pinealectomized Syrian hamsters with melatonin during the time of the night when pineal melatonin content would be elevated induced gonadal regression, while pinealectomized rats failed to respond to exogenous melatonin.