Changes in brain aromatase and 5 alpha-reductase activities correlate significantly with seasonal reproductive cycles in goldfish (Carassius auratus).

Aromatization and 5 alpha-reduction are known to be required for the full expression of testosterone actions in neuroendocrine tissues. Although aromatase and 5 alpha-reductase activities in brain and pituitary can be experimentally manipulated by castration and steroid replacement, naturally occurring variations during seasonal reproductive cycles have not been examined in any species. Goldfish (Carassius auratus) were selected for study because they exhibit exceptionally high levels of aromatase in both brain and pituitary, although 5 alpha-reductase levels resemble the vertebrate norm. Four animals of each sex were tested monthly through three breeding seasons (2.5 yr). Using previously validated techniques, the enzymes were assayed by product formation from [3H]androstenedione in homogenates of anterior hypothalamus-preoptic area (AHPOA), remaining telencephalon (TEL), whole pituitary, ovary, and testis. Seasonal variations in aromatase were most dramatic in the AHPOA of female fish, exhibiting a peak in April and May that was 6-fold higher than the nadir in July. As judged by changes in the appearance and weight of the gonads, maximal aromatase coincided with the spawning season, whereas low enzyme levels corresponded to reproductive inactivity. Seasonal variations were similar but of a lesser magnitude in the TEL of females and in the AHPOA and TEL of males (2- to 3-fold, peak to nadir). Both ovarian and testicular aromatase showed cyclic changes; however, activity was much lower than that in brain at all times of the year (4.5, 1.2, and 47.0 pmol/mg protein, maximal values in ovaries, testes, and AHPOA, respectively). Pituitary aromatase varied from 5-22 pmol/mg protein, but was not consistently correlated with season. Cyclic changes in 5 alpha-reductase were distinctly different from those in aromatase, with maximal values in both brain and pituitary occurring when fish were reproductively inactive. In general, circulating sex steroids were high when aromatase was high and low when reductase was maximal; however, there was no apparent causal relationship suggested by temporal changes in a given steroid. Variations in testosterone metabolism, by regulating the quantity and quality of active hormone in close proximity to receptor sites, may be responsible for the changes in feedback sensitivity and behavioral responsiveness that are known to occur in seasonal breeders.