Melatonin, but not somatostatin-14 influences in vitro steroidogenesis by ovarian follicles of rainbow trout

Ovarian follicles taken from sexually maturing rainbow trout at the mid-vit ellogenic stage of ovarian development were incubated in vitro in the prese nce or absence of melatonin or somatostatin-14 (SRIF-14) to determine wheth er there is evidence of a direct action of these factors on gonadal steroid ogenesis in fishes. The steroidogenic capacity of the ovarian follicles was assessed by measuring testosterone (T) and 17 beta-estradiol (E-2) release into the incubation medium, and by examining the steroid metabolites produ ced following incubation of follicles with radiolabelled steroid precursors . Melatonin appears to elicit a biphasic effect on steroidogenesis by in vitr o rainbow trout ovarian follicles; at a concentration of 1 x 10(-3) M, mela tonin stimulated basal T and E-2 production, but at a concentration of 1 x 10(-2) M there was an inhibition of basal and sGtH-stimulated T and E-2. Me latonin may act to reduce the activity of specific steroidogenic enzymes, s ince there was evidence of melatonin at 1 x 10(-2) M enhancing the accumula tion of [H-3]17 alpha-hydroxyprogesterone in the medium following incubatio n with [H-3]pregnenolone, possibly suggesting the inhibition of C-17,C-20-l yase activity. In contrast, SRIF-14, used at concentrations of 1 x 10(-8) M and 1 x 10(-6) M, had no effect on basal or sGtH-stimulated E-2 or T produ ction by ovarian follicles, incubated in vitro.