Photoperiod effects on hydroxyindole-O-methyltransferase activity in the pineal gland of chinook salmon (Oncorhynchus tshawytscha)

Juvenile chinook salmon (Oncorhynchus tshawytsch) were acclimated to photoperiods of 12L:12D, 0L:24D, or 24L:0D. Fish were sampled over a 24-hr period and pineal glands were analyzed for hydroxyindole-O-methyltransferase (HIOMT) activity and total protein. HIOMT specific activity (pmoles melatonin/hr/μg protein) exhibited a diel cycle in fish acclimated to 12L:12D and 0L:24D, with a peak at approximately 2400 hr and a low at approximately 1700 hr. The cycle was not apparent in fish acclimated to 24L:0D or in fish acclimated to 12L:12D but sampled with 18L:6D. In all cases where a cycle in HIOMT specific activity occurred, pineal protein content was found to cycle opposite in phase to that of specific activity. No significant change was seen in HIOMT activity (pmoles melatonin produced/hr/pineal gland) at any of the sampling times. Thus, we conclude that the cycles observed in HIOMT specific activity reflect changes in pineal gland protein content and not changes in HIOMT activity.