Keeping track of time under ice and snow in a sub-arctic lake: plasma melatonin rhythms in Arctic charr overwintering under natural conditions.

Although photoperiod is considered as a major environmental cue for timing of seasonal events in fish, little is known about the photic information perceived by fish in different aquatic environments. The strongly seasonal Arctic charr, Salvelinus alpinus, reside in lakes covered by thick ice and snow throughout the dark winter in the north. In the present study, we have measured diel changes in their plasma melatonin concentrations from September to June in Lake Storvatnet (70 degrees N), northern Norway. In addition, we have measured the in vitro melatonin production of Arctic charr pineal glands held at experimental light conditions. From September to April a diel profile in plasma melatonin was seen in the charr in Lake Storvatn, with highest concentrations at night. This profile reflected the prevailing above-surface photoperiod, even in February when there were minimal changes in sub-surface irradiance between day and night. In June, plasma melatonin was low throughout the 24-hr cycle, despite there being a marked sub-surface difference in irradiance between night and day. At this time the irradiance in night probably remained above the threshold for suppression of melatonin production. The in vitro experiments revealed no endogenous rhythm in the pineal melatonin secretion, supporting the conclusion that the diel profile seen in the Arctic charr in their natural habitat was driven by ambient photoperiod. In conclusion, the Arctic charr appear to keep track of time even under the extreme conditions of high latitudes during winter, when lakes have thick ice and snow cover.

Melatonin reduces noradrenaline-induced vasoconstriction in the uterine artery of pregnant hooded seals (Cystophora cristata).

In pregnant seals the dive-associated constriction of the uterine artery is inhibited for unknown reasons. The seal fetus has an extremely large and active pineal gland, not found in any other mammals. We have investigated if the pineal hormone melatonin affects fetal blood supply during diving. Using isolated ring segments of the uterine artery from pregnant hooded seals (Cystophora cristata), we measured the change in isometric tension caused by noradrenaline (NA) with and without physiological concentrations of melatonin. Melatonin alone had no effects while NA increased the tension in a dose-dependent manner. The NA-induced tension was about 70% reduced by melatonin, but was completely recovered after washout of melatonin. These results indicate that the large and active pineal gland of the fetal seal may be involved in upholding maternal uterine blood flow during diving.

Quantitative differences in the pineal ultrastructure of perinatal and adult harp (Phoca groenlandica) and hooded seals (Cystophora cristata).

Seals are unique among mammals in that newborns have a large pineal gland and extremely high plasma levels of melatonin at birth. Melatonin levels are also high in the seal fetus but decline rapidly during the first few days of life. The aim of the present study was to provide quantitative information about the ultrastructure of the seal pineal gland using fetal, newborn, and adult hooded seals (Cystophora cristata), and newborn and adult harp seals (Phoca groenlandica). The relative and absolute volumes of pinealocytes (Pi), arteries and veins, nerves, connective tissue, capillaries and glial cells, as well as mitocondria and lipid droplets in Pi, were calculated by use of point count analysis. Whereas the pineal ultrastructure was similar in fetuses and newborns, both seal species showed a pronounced and particular reduction in the volume of Pi and a similar reduction in pinealocyte mitochondria. There was also a shift from unmyelinated to myelinated pineal nerves in adults compared with fetal/newborns. The selective and marked reduction of Pi may explain the zonated pineal structure typical of the adult seal. The results demonstrate that the fetal gland is as large and active as that of the newborn seal and support the notion that the large size and high activity of the pineal gland in the newborn seal is a fading consequence of its prenatal condition.