Effects of osmotic stimulation on expression of neurohypophysial hormone genes in pre-spawning chum salmon

Changes in expression of vasotocin (VT) and isotocin (IT) genes were analyzed in chum salmon during the last stages of spawning migration. Pre-spawning chum salmon were caught at following four locations in the Sanriku coast of the Pacific Ocean in Japan: 1) the off-coast area north to the Otsuchi Bay, 2) the mouth of the Otsuchi Bay, 3) inside of the Otsuchi Bay, and 4) the place 500 m upstream to the mouth of the Otsuchi River. In addition, effects of hypo-osmotic stimulation by transition from sea water (SW) to freshwater (FW) were examined in animals caught at the mouth of the Otsuchi Bay. The levels of VT and IT mRNAs in the forebrains were determined by Northern blot analysis. The plasma osmolality and the levels of Na+ and Cl- were also analyzed. Expression patterns of VT and IT genes were different between the males and the females. In the males, VT and IT gene expression were maintained essentially at the same levels from the off-coast area to the Otsuchi River. In contrast, in the females, the level of VT-I mRNA was significantly increased in the fish caught at the mouth of the bay. After entering the bay, the level of VT-I mRNA was decreased and maintained at a low level through the final stages of spawning migration. Such sexual difference in VT and IT gene expression found in the field fish was further analyzed by a SW to FW transition experiment, in which fish were divided into two groups, those retained in SW and others replaced with FW. In the FW-replaced fish, the levels of VT and IT mRNAs were decreased in both sexes, although much more conspicuous in the females. In the SW-retained animals, changes in the levels of VT and IT mRNAs were sexually different. The levels of VT and IT mRNAs were increased in the males, whereas they were decreased in the females, when compared to the initial levels just before the experimental treatments. These results suggest that regulation of VT and IT gene expression is sexually dimorphic in pre-spawning chum salmon.