SHORT PHOTOPERIOD ACCELERATES PREOPTIC AND VENTRAL TELENCEPHALIC SALMON GNRH SYNTHESIS AND PRECOCIOUS MATURATION IN UNDERYEARLING MALE MASUSALMON

The temporal relationship between testicular maturation and salmon gon adotropin-releasing hormone (sGnRH) mRNA expression was investigated i n underyearling precocious male masu salmon, Oncorhynchus masou. Testi cular maturation could be experimentally manipulated by changing the l ength of the light-dark photoperiod; maturation was accelerated in the short photoperiod group (8L-16D) and delayed in the long photoperiod group (16L-8D). sGnRH synthetic activity in the preoptic area and the ventral telencephalon, i.e., the number of neurons expressing sGnRH mR NA and total silver grains in these loci in individual fish, increased with advancing testicular maturation. They were maximal in the short photoperiod group in August and in the long photoperiod group in Septe mber, when spermiation occurred. In contrast, marked changes in sGnRH synthetic activity in relation to testicular maturation were not obser ved in the terminal nerve ganglion or in the olfactory bulbs. sGnRH ne urons in the preoptic area and the ventral telencephalon are clearly i nfluenced by photoperiod and are involved in the control of gonadal ma turation probably via gonadotropin secretion. (C) 1995 Academic Press, Inc.