CHANGES IN LEVELS OF GNRH IN THE BRAIN AND PITUITARY AND GTH IN THE PITUITARY IN MALE MASU SALMON, ONCORHYNCHUS-MASOU, FROM HATCHING TO MATURATION

Levels of two types of gonadotropin-releasing hormone (salmon GnRH and chicken GnRH-11) in the brain and pituitary, and content of gonadotro pin (GTHIbeta and IIbeta) in the pituitary were measured in male masu salmon from hatching to gonadal maturation for three years in order to clarify the involvement of GnRHs in precocious maturation. Underyearl ing precocious males were distinguishable in summer of year 1 and were marked by an increased GSI. Spermiation was observed among these indi viduals thereafter every autumn. Pituitary GTHIbeta content in both pr ecocious and immature males, and GTHIIbeta content in precocious males showed seasonal fluctuations-high in autumn and low in winter. Pituit ary GTHIIbeta content was low in immature males. Pituitary sGnRH conte nt in precocious males increased from spring to autumn during the thre e-year period. sGnRH concentrations in discrete brain areas showed sea sonal changes - high during autumn to winter and low in summer. Concen trations in the olfactory bulbs and hypothalamus increased significant ly in association with testicular maturation during year 3. sGnRH conc entrations in the hypothalamus were significantly higher in precocious males than in immature males; this was possibly due to positive feedb ack of steroid hormones. cGnRH-II was undetectable in the pituitary an d no distinct changes were observed in its concentration in the brain in relation to maturation. The phenomenon of underyearling precocious maturation is considered to be triggered before the onset of early sum mer. It is suggested that males which mature precociously are larger i n size and contain much sGnRH in the pituitary before the outward sign s of precocity appear; sGnRH may stimulate GTH 11 synthesis and induce precocious maturation.