Day/night variation of tryptophan hydroxylase and serotonin N-acetyltransferase mRNA levels in the ovine pineal gland and retina.

In mammals, the photoperiodic information, received by the retina, is transmitted to the pineal gland. In both organs, melatonin is produced and functions as a neurohormone giving temporal information to the organism. A four-step enzymatic pathway, involving in particular the tryptophan hydroxylase (TPOH), the rate-limiting enzyme in serotonin synthesis, and the serotonin N-acetyltransferase (NAT) that converts serotonin to N-acetylserotonin, allows the synthesis of melatonin. Many studies on melatonin synthesis modulation have focused on the enzyme NAT, but the regulation of TPOH is less well understood. We report here a quantitative study, using a reverse transcription polymerase chain reaction (RT-PCR) analysis, of the nycthemeral expression of TPOH and NAT mRNAs in the ovine retina and pineal gland. In both organs, we show a nocturnal increase in mRNA levels of the two enzymes. suggesting a role of transcriptional mechanisms in the regulation of melatonin synthesis. However, the amplitude of the observed increase in TPOH and NAT mRNAs expression can not entirely explain the 7-fold nocturnal increase in the plasma melatonin level. Our results suggest that, in the sheep, post-transcriptional mechanisms might also be involved in the day/night modulation of melatonin production.