Effects of photoperiod on pituitary gonadotropin levels in masu salmon

We examined the effects of photoperiod on pituitary levels of two types of gonadotropin (GTH), GTH I and GTH II, in masu salmon Oncorhynchus masou to study their mechanism of synthesis. In Experiment 1, the effects of long or short photoperiod combined with castration were examined using 8-month-old precocious males. Castration was carried out in early August and then the fish were reared under a short (8L16D) or long (16L8D) photoperiod for 60 d ays. In Experiment 2, the effects of photoperiod combined with testosterone treatment were examined using 12-month-old immature females. Silastic tube s containing testosterone (500 mug/fish) or vehicle were implanted intra-pe ritoneally in early October. Fish were reared under 16L8D for 60 days, and then half of the fish were transferred to 8L16D, while the remaining fish w ere kept under 16L8D until Day 90. In Experiment 1, GTH I contents were hig her under 16L8D than under 8L16D in the castrated group on Day 30. Moreover , GTH I contents were higher in the castrated group than the control group under 16L8D on Day 30. GTH II contents increased with testicular maturation in the control groups, whereas they remained at low levels in the castrate d groups regardless of photoperiodic treatment. In Experiment 2, GTH I cont ents did not change remarkably in all the groups, while GTH II contents wer e remarkably increased by testosterone treatment regardless of photoperiodi c treatment. These results indicate that the synthesis of GTH I and GTH II are differently regulated by photoperiod and testosterone in masu salmon. J . Exp. Zool. 289:449-455, 2001. (C) 2001 Wiley-Liss, Inc.