Circadian Rhythm in Pineal N‐Acetyltransferase Activity: Phase Shifting by Dark Pulses (III)

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Abstract
N‐Acetyltransferase (NAT) is an enzyme whose rhythmic activity in the pineal gland and retina is thought to be responsible for melatonin circadian rhythms. The enzyme has circadian properties–its rhythm persists in constant conditions, and it is precisely controlled by light and dark. Experiments are reported in which 4‐h light or dark pulses were imposed on chicks (Gallus domesticus) over a 24‐h period. Pineal NAT profiles were measured during and subsequent to the pulses. The phase of the NAT cycle following pulses was plotted to obtain phase‐response curves. Light pulses produced a maximum phase shift (advance of 5 h) 8 h after the expected time of lights‐out; dark pulses produced a maximum phase shift (advance of 4 h) 3 h after the expected time of lights‐out. Maximum phase delays (−2 h) occurred 1–2 h after the expected lights‐out for light pulses and 8 h after expected lights‐on for dark pulses.