Long-term effects of constant light or darkness on chicken pineal hydroxyindole-O-methyltransferase expression: biochemical and cellular aspects

Cloning and early expression pattern of two melatonin biosynthesis enzymes in the turbot (Scophthalmus maximus)

Melatonin biosynthesis from serotonin involves the sequential activation of the arylalkylamine N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase (HIOMT). Photoperiod synchronizes a daily rhythm in pineal and retinal melatonin secretion through controlling AANAT activity. Teleost fish possess two Aanat, one expressed in the retina (AANAT1) and the other expressed in the pineal gland (AANAT2). We report here the full-length cloning of Aanat1, Aanat2, SmHiomt and Otx5 (orthodenticle homeobox homolog 5) in the turbot (Scophthalmus maximus, Sm), a flatfish belonging to an evolutionary recent group of Teleost. The temporal expression pattern of the genes investigated is consistent with the idea that OTX5 is needed for photoreceptor specification, and that the pineal gland differentiates before the retina. SmAanat2 expression remained pineal specific during the period of time investigated, whereas SmOtx5 and SmHiomt expressions were seen in both the retina and pineal gland. Our results do not support the existence of a second SmHiomt, as is the case for SmAanat. Neither SmAanat2 nor SmHiomt mRNAs displayed cyclic accumulation in the pineal organ of embryos and larvae maintained under a light-dark cycle from fertilization onward. This is in marked contrast with the situation observed with zebrafish Aanat2, indicating that the molecular mechanisms controlling the development of the pineal melatonin system have been modified during the evolution of Teleost.

Hydroxyindole-O-methyltransferase gene expression in the pineal gland of chicken embryo: development of messenger RNA levels and regulation by serum

Hydroxyindole-O-methyltransferase (HIOMT), the enzyme which catalyzes the final step of melatonin biosynthesis, constitutes a marker of the functional differentiation of pineal cells. In addition, a day/night rhythm of HIOMT mRNA concentration, previously described in the chicken pineal gland [6], would suggest that HIOMT gene transcription is one output of the circadian system that controls pineal function. The study sought to monitor the developmental expression of HIOMT mRNA in the chick pineal gland and to investigate a possible role of instructive signals in this differentiation process. RT-PCR analysis indicated that HIOMT mRNA is expressed at embryonic day 8 (E8). At E12, HIOMT mRNA became detectable on northern blots and traces of HIOMT activity could be measured. HIOMT mRNA concentration increased 100-fold between E14 and day 10 post-hatch, then levelled off. A day/night rhythm of HIOMT mRNA concentration was readily observed in the pineal gland of 2-day-old chicks. Pineal glands isolated on minimum culture medium at E11 stopped developing HIOMT gene expression. However, the addition of serum to the culture medium restored HIOMT mRNA concentration to the levels observed in vivo. The data suggest that the functional differentiation of melatoninergic cells observed during the second week of embryonic life may be controlled [correction of controled] by serum factors.

Transcriptional regulation of hydroxyindole O-methyltransferase in the chicken pineal gland: day/night changes and long-term effects of light and darkness

The indolic hormone melatonin is produced by the pineal gland according to a daily rhythm. The terminal step of melatonin synthesis is catalysed by hydroxyindole O-methyltransferase (HIOMT, EC 2.1.1.4). Adaptation to constant light or darkness modifies HIOMT activity and concentration. Using a cDNA probe encoding HIOMT, we investigated the effect of environmental lighting on HIOMT gene expression in the chicken pineal gland. HIOMT mRNA levels increased by 100% in constant light as compared with constant darkness. In addition, the present study disclosed the existence of a day/night rhythm of HIOMT gene transcription, with 3-fold higher mRNA levels at midday than at midnight. This transcriptional rhythm was not accompanied by day/night changes in HIOMT concentration, probably due to a slow turnover of this protein. Unexpected darkness did not prevent the daytime rise in HIOMT mRNA levels, whereas unexpected light prevented the night-time fall in HIOMT mRNA levels. Together, the data would suggest that the day/night rhythm of HIOMT gene transcription in the chicken pineal gland involves both a response to light and the activity of a biological oscillator.