Pineal serotonin N-acetyltransferase activity in the rat during the estrous cycle and in response to light.

Abstract

Pineal serotonin N-acetyltransferase (SNAT; E.C.2.3.1.5) activity was determined 3 times/day throughout the estrous cycle in rats exposed to 6L:18D or 14L:1OD lighting conditions. The magnitude of increased SNAT activity measured during the dark phase of a photoperiod did not vary significantly throughout the estrous cycle in either photoperiod examined suggesting that SNAT does not regulate changes found in pineal melatonin content during the estrous cycle. In a second experiment, SNAT activity was determined 6 times/day in rats exposed to lighting conditions of continuous darkness (DD), 2L:22D, 6L:18D, 14L:1OD, 2OLs4D or continuous light (LL). Although no alterations in SNAT activity were measured in LL, SNAT activity was elevated for ‘\�8 h during the dark phase of the photoperiod throughout a broad range of other lighting conditions. Peaks in SNAT activity showed a parabolic relationship to increasing proportions of light/day and were related as well to the midpoint of the dark phase of each photoperiod. If SNAT activity is an index of melatonin production and release, these findings do not support a proposal that melatonin levels provide the rat with a measure of the duration of darkness. Instead, the results suggest that melatonin may act as an internal synchronizer cuing the animal to the midpoint of the dark phase of a photoperiod.