Endogenous melatonin signal does not mediate the effect of photoperiod on the rat suprachiasmatic nucleus.

The duration of the nocturnal interval during which light can induce high Fos-like immunoreactivity in the suprachiasmatic nucleus is a function of the photoperiod to which rats are exposed. The effect of photoperiod is not altered by pinealectomy indicating that day length affects the functional state of the suprachiasmatic nucleus circadian pacemaking system in the rat directly, not via the pineal melatonin signal.