Phototransduction‐related circadian changes in indoleamine metabolism in the chick pineal gland in vivo

Abstract: The purpose of this study was to examine the day/night levels of pineal melatonin and its rate limiting enzyme N‐acetyltransferase (NAT) in relationship to the ratio of 11‐cis‐to all‐trans‐retinal. Three‐week‐old chicks were placed in 12:12 light: dark (LD 12:12) cycle for one week, pineals were collected during the light phase at 1500 (i.e., after 10 hr light), during the dark phase at 1900 (i.e., 2 hr after dark), at 2100 (i.e., 4 hr after dark), and at 2300 (i.e., 6 hr after dark) and after light extension to 1900. The results show that light‐sensitive 11‐cis‐retinal in the chick pineal has the same diurnal rhythm as NAT and melatonin; all constituents increased within 2 hr of darkness onset (at 1900) and reached their peak after 4 hr of dark. All values were lowest during the light phase at 1500. Low values for 11‐cis‐retinal, NAT, and melatonin were also seen in the group of chicks which experienced light extension to 1900. The data indicate that in vivo light plays a major role in triggering rhodopsin‐bound 11‐cis‐retinal production within 2–4 hr after darkness onset; this change likely serves as the signal for the subsequent formation of the hormonal product of the pineal gland, melatonin.