| Summary: | The pineal gland is part of the endocrine system involved with rhythmic activity in e.g. fish. The main product of the pineal gland is the indole hormone melatonin, synthesised from the amino acid tryptophan. Melatonin is mainly synthesized when it is dark, as light inhibit the production. For this reason melatonin is thought to be strongly involved in biological rhythms. Arctic charr (Salvelinus alpinus) is a circumpolar species and anadromous in parts of its distribution area. Arctic charr experience strong seasonal changes in environmental factors, such as light regime, temperature and nutrient availability. This may influence for example smoltification and spawning. The pineal gland and melatonin are considered important in the adaptation to the shifting environment. On this background the present study was conducted in order to reveal seasonal differences in diel plasma melatonin rhythms and putative associated changes in pineal morphology, pinealocytes, glial cells, blood vessels, mitochondria, endoplasmatic reticulum and lumen. In order to do so Arctic charr were held in freshwater under natural light conditions from August 2006 to June 2007. Blood samples taken at 7 time points through four 24 hour periods (August, December, February and June) during the year were analysed for plasma melatonin levels. Further pineal glands were excised from Arctic charr at mid-day at the same dates for ultrastructure- and stereological analysis. Radioimmunoassay analysis of the plasma melatonin levels revealed higher peak values for the scotophase of December and February than was found during the photophase. August and June showed a consistent low level of plasma melatonin throughout the 24 hours period. No significant differences were found between the seasons with regard to ultrastructure and organelle volumes studied.
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