Plasma and pineal melatonin levels in female ferrets housed under long or short photoperiods.

Ovohysterectomized female ferrets were housed in controlled environment rooms in which the daily lighting schedule was either 15L:9D (long days) or 9L:15D (short days). After 2 weeks some ferrets in each group were given an intrajugular catheter: beginning 1 week later, a blood sample was taken daily at one of eight different clock times over an 8 to 10 day period. One additional blood sample plus the pineal gland were collected from these animals and from uncathetarized animals in each group after decapitation at different clock times. Both plasma melatonin concentrations and pineal melatonin content were elevated in a square-wave pattern during the dark hours, with the duration of elevation being longer in ferrets kept under the short days. These results suggest that differences in the duration of nocturnal increments in melatonin secretion may mediate the stimulatory and inhibitory effects of long and short days, respectively, on ovarian activity in female ferrets.