Journal
In fish, most hormonal productions of the pituitary gland display daily and/or seasonal<br>
rhythmic patterns under control by upstream regulators, including internal biological<br>
clocks. The pineal hormone melatonin, one main output of the clocks, acts at different<br>
levels of the neuroendocrine axis. Melatonin rhythmic production is synchronized mainly<br>
by photoperiod and temperature. Here we aimed at better understanding the role<br>
melatonin plays in regulating the pituitary hormonal productions in a species of scientific<br>
and economical interest, the euryhaline European sea bass Dicentrarchus labrax. We<br>
investigated the seasonal variations in mRNA abundance of pituitary hormones in two<br>
groups of fish raised one in sea water (SW fish), and one in brackish water (BW fish). The<br>
mRNA abundance of three melatonin receptors was also studied in the SW fish. Finally,<br>
we investigated the in vitro effects of melatonin or analogs on the mRNA abundance of<br>
pituitary hormones at two times of the year and after adaptation to different salinities.<br>
We found that (1) the reproductive hormones displayed similar mRNA seasonal profiles<br>
regardless of the fish origin, while (2) the other hormones exhibited different patterns<br>
in the SW vs. the BW fish. (3) The melatonin receptors mRNA abundance displayed<br>
seasonal variations in the SW fish. (4) Melatonin affected mRNA abundance of most<br>
of the pituitary hormones in vitro; (5) the responses to melatonin depended on its<br>
concentration, the month investigated and the salinity at which the fish were previously<br>
adapted. Our results suggest that the productions of the pituitary are a response to<br>
multiple factors from internal and external origin including melatonin. The variety of the<br>
responses described might reflect a high plasticity of the pituitary in a fish that faces<br>
multiple external conditions along its life characterized by marked daily and seasonal<br>
changes in photoperiod, temperature and salinity.
