SEXUALLY DIFFERENT EXPRESSION OF NEUROHYPOPHYSEAL HORMONE GENES IN THE PREOPTIC NUCLEUS OF PRESPAWNING CHUM SALMON

Vasotocin (VT) and isotocin (IT) are teleost neurohypophysial hormones produced by neurosecretory neurons in the magnocellular part of preop tic nucleus (PM) of the hypothalamus. Several previous studies indicat ed that neurohypophysial hormones are involved in teleost reproductive behavior. The changes in the expression of VT and IT genes were thus studied by an in situ hybridization technique and an immunohistochemic al avidin-biotin-complex method in pre-spawning chum salmon (Oncorhync hus keta). Male and female fish were caught at Atsuta, the mouth of th e Ishikari Bay, and at Chitose, upstream to the Ishikari River in Octo ber, 1994. The former and the latter are referred to as bay fish and r iver fish, respectively. The intensity of autoradiographic hybridizati on signals were determined in individual neurosecretory neurons in the rostroventral, middle, and dorsocaudal parts of the PM. In the female s, the levels of VT and IT mRNAs in the river fish were significantly lower than those in the bay fish in all the three loci in the PM, wher eas VT and IT immunoreactivities in the river fish were higher than th ose in the bay fish. These results suggest that both the rates of tran scription and release of VT and IT were decreased in pre-spawning fema le chum salmon. Contrary, in the males, the levels of IT mRNA and IT i mmunoreactivity in the river fish were greater than those in the bay f ish particularly in the rostroventral part of the PM, whereas conspicu ous changes were not seen in the levels of VT mRNA and VT immunoreacti vity. The present results thus revealed sexually different expression of neurohypophysial hormone genes in the preoptic nucleus of pre-spawn ing chum salmon when compared between bay and river fish. The regulati on of neurohypophysial hormone gene expression may be different betwee n the male and the female during the last stages of spawning migration .