In vitro production of ovarian steroids in yellow perch (Perca flavescens): effects of photothermal manipulation, gonadotropin and phorbol ester.

We investigated the capacity of ovaries of yellow perch to produce steroid hormones in vitro and the ovarian response to gonadotropin and phorbol ester during the annual reproductive cycle. The effects of photothermal manipulation on perch gonadal steroidogenesis and its regulation have also been examined. Initially, all females kept indoors were exposed to the same water temperature and photoperiod. By the end of August, following the first sampling, fish were submitted to different photothermal regimes. Group A was maintained under photothermal conditions characteristic for southern Ohio. Group B was submitted to a condensed light/temperature regime designed to accelerate physiological changes that depend on photothermal stimuli. In group A, basal in vitro production of ovarian estradiol (E2) was the highest in October and November, and hCG significantly stimulated E2 secretion during the entire period of vitellogenesis (October-January). In this group, the highest production of basal testosterone (T) was observed before spawning. hCG-stimulated production of T was highest at the beginning of vitellogenesis. Gonadotropin stimulated T production before spawning, a time when gonadotropin was unable to stimulate E2 production. Phorbol ester (PDBu) stimulated E2 and T production during vitellogenesis at the same time points as hCG did (E2: December, January; T: December). hCG-stimulated T production was not mimicked by PDBu in April. Condensing of the photothermal cycle resulted in diminished ovarian production of E2 during vitellogenesis. Moreover, the fish submitted to a condensed photothermal cycle demonstrated augmented T production during the postvitellogenic stage of ovarian development. Ovaries of group B did not respond to PDBu. Generally, the seasonal fluctuations in ovarian capacity to produce E2 and T as well as in gonadal responsiveness to gonadotropin observed in female yellow perch illustrate the dynamic nature of ovarian endocrine function. The lack of response to gonadotropin with regard to E2 production prior to spawning is not due to insensitivity to gonadotropin, but rather due to some deficiency in steroidogenesis (e.g. reduced aromatase activity). It appears also that ovarian steroidogenesis and its regulation are dependent on annual changes of photothermal conditions.