Different Mechanisms of Phase Delays and Phase Advances of the Circadian Rhythm in Rat Pineal N-Acetyltransferase Activity

The circadian rhythm in rat pineal N-acetyltransferase (NAT) activity, which drives the rhythm in melatonin production, is controlled by a pacemaker located in the su prachiasmatic nucleus of the hypothalamus. As the NAT rhythm has two well-defined phase markers—namely, the time of the evening activity rise and of the morning decline—it is suitable for studies of the entrainment of the pacemaker by environmental light. Phase delays of the NAT rhythm proceed more rapidly than phase advances. One day after a brief light pulse applied before midnight, or after a delay in evening lights-off, or a delay of a light-dark (LD) cycle, phase delays of the evening NAT rise result in almost corresponding delays of the morning NAT decline. Consequently, the NAT rhythm is phase-shifted, but its pattern does not change. One day after a brief light pulse applied past midnight, or after bringing forward morning lights-on, or after an advance of an LD cycle, the morning NAT decline is phase- advanced, but the evening rise is not phase-advanced at all or may even be phase-delayed. Consequently, the phase relationship between the evening NAT activity onset and the mom ing offset may be compressed considerably, and it may take several transient cycles before phase advances of the morning NAT decline are followed by corresponding advances of the evening NAT rise. Due to the phase-delaying effect of evening light on the NAT rise and to the phase-advancing effect of morning light on the NAT decline, the phase relationship be tween the NAT rise and the decline is compressed on long days and decompressed on short days.