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Abstract
Arylalkylamine N‐acetyltransferase (AA‐NAT) is the rhythm‐generating enzyme for the synthesis of pineal melatonin. Molecular investigations have revealed two biological models for the activation of AA‐NAT. In rodent species, Aa‐nat gene transcription is turned off during the daytime and markedly activated at night. In primates, sheep, and cows, the Aa‐nat gene is constitutively transcripted with no visible daily variations. This inter‐species difference in Aa‐nat gene regulation leads to different daily profiles in melatonin synthesis and release. Thus, the nighttime onset of the rise in circulating melatonin is delayed and slow in rodents, whereas it is fast and sharp in sheep. In the camel (Camelus dromedarius), we have observed that circulating melatonin rises immediately after sunset, suggesting AA‐NAT activity is regulated at the post‐transcriptional level. In agreement with this hypothesis, we report herein the amount of Aa‐nat mRNA in the camel pineal gland is high, during both the day and night with no daily variations, while melatonin concentration in the same pineal tissue is five times higher during the night than daytime. (Author correspondence: [email protected])
