Effect of LH-releasing hormone on gonadal development in a salmonoid fish, the ayu.

Abstract

Maturing ayu fish at early vitellogenic or early spermatogenic stages received a single intraperitoneal (IP) injection of emulsified synthetic LH-RH (0, 50 or 500μg) at the start of the experiment and were reared under a gonad-suppressive photoperiod (16L-8D) for 20 days. Gonadal atrophy occurred in the control and 50μg LH-RH treated groups. However, 500μg LH-RH remarkably accelerated gonadal development despite the suppressive photoperiod. All males discharged a large amount of seminal fluid, and 3 of 4 females ovulated. The gonadotrophs of the group treated with 500μg LH-RH were hypertrophied and degranulated. In the control and 50μg LH-RH treated groups, the gonadotrophs were small and irregular and contained numberous small granules along with some larger ones. Immature ayu at pre-vitellogenic or pre-spermatogenic stages received a single IP injection of emulsified LH-RH (0, 50 or 500μg) on day 1 and 20 of a 40-day exposure to a gonad-sup-pressive photoperiod (16L-8D). Gonadal development failed to occur in any of the groups. The gonadotrophs from all groups were similarly inactive. These results indicate that synthetic mammalian LH-Rh cannot intiate gonadotropin synthesis in undeveloped gonadotrophs of immature fish, whereas large amounts are able to stimulate not only the release but also the synthesis of gonadotropin in the developed gonadotrophs of maturing fish.