Circadian rhythm of tryptophan hydroxylase activity in chicken retina
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Retinal tryptophan hydroxylase activity in chickens (1–4 weeks old and embryos) was estimated by determination of levels of 5-hydroxytryptophan (5HTP) in retinas at defined intervals after inhibition of aromatic L-amino acid decarboxylase withm-hydroxybenzylhydrazine (NSD1015).
The relationship of tryptophan hydroxylase activity to photoperiod was explored. In chickens maintained on a 12-hr light:12-hr dark cycle, a diurnal cycle in tryptophan hydroxylase activity was observed. Activity during middark phase was 4.4 times that seen in midlight phase. Cyclic changes in tryptophan hydroxylase activity persisted in constant darkness with a period of approximately 1 day, indicating regulation of the enzyme by a circadian oscillator. The phase of the tryptophan hydroxylase rhythm was found to be determined by the phase of the light/dark cycle. The relationship of the tryptophan hydroxylase rhythm to the light/dark cycle mirrors previously described rhythms of melatonin synthesis and serotoninN-acetyltransferase (NAT) activity in the retina.
Light exposure for 1 hr during dark phase suppressed NAT activity by 82%, while tryptophan hydroxylase activity was suppressed by only 30%.
Based on the differential responses of retinal NAT activity and tryptophan hydroxylase activity to acute light exposure during dark phase, it was predicted that exposure to light during dark phase would divert serotonin in the retina from melatonin biosynthesis to oxidation by MAO. In support of this, levels of 5-hydroxyindole acetic acid (5HIAA) in retina were found to be elevated approximately two-fold in chickens exposed to 30 min of light during dark phase. In pargyline-treated chickens, 2 hr of light exposure during dark phase was found to increase retinal serotonin levels by 64% over pargyline-treated controls.
Cyclic changes in tryptophan hydroxylase activity and NAT activity persisted for 2–3 days in constant light. Tryptophan hydroxylase activity at mid-night gradually decreased on successive days in constant light; on the first day of constant light, tryptophan hydroxylase activity at mid-night was 70% of activity seen during middark phase of the normal light/dark cycle and decreased further on subsequent days. In contrast, on each of 3 days of constant light, NAT activity at mid-night was approximately 15% of normal middark phase activity.
Cycloheximide completely inhibited the nocturnal increase in tryptophan hydroxylase activity when given immediately before light offset. The nocturnal increase in NAT activity was inhibited in a similar fashion.
Like the development of the NAT rhythm, cyclic changes of tryptophan hydroxylase activity in the retinas of chickens began on or immediately before the day of hatching.
The results indicate that retinal tryptophan hydroxylase activity is controlled by a circadian oscillator. The similarity of the circadian rhythm of tryptophan hydroxylase activity in chicken retina to the rhythms of retinal NAT activity and melatonin levels raises the possibility of a common oscillator regulating NAT and tryptophan hydroxylase activities, as well as involvement of tryptophan hydroxylase as a regulatory component in the melatonin synthetic pathway.
Key wordsserotonin melatonin tryptophan hydroxylase 5-hydroxytryptophan retina circadian rhythms serotoninN-acetyltransferase
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