EFFECTS OF AROMATASE INHIBITORS ON SEXUAL-MATURATION IN ATLANTIC SALMON, SALMO-SALAR, MALE PARR

Atlantic salmon, Salmo salar, male parr were implanted with Silastic c apsules filled with different aromatase inhibitors: 1,4,6-androstatrie ne-3,17-dione (ATD), 4-hydroxy-4-androstene-3,17-dione (4OH), and the non-steroidal CGS 16949 A, 4-benzonitrile monohydrochloride (CGS). Aro matization in brain homogenates were lower in salmon implanted with CG S and ATD than in controls. This was not the case for 4OH, but adminis tration of 4OH to brain homogenates reduced the aromatase activity. Al l three aromatase inhibitors had effected gonadal weights in fish samp led in the summer, but the effects were markedly different among inhib itors. Plasma levels of the androgen 11-ketotestosterone (11KT) and th e progestin 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20P) were measured by means of radioimmunoassay. CGS and ATD, but not 4OH, signi ficantly decreased the plasma 17,20P levels in the autumn. Plasma leve ls of 11 KT were not influenced by ATD or CGS treatment, but 4OH had a lowering effect in one autumn sampling. ATD and 4OH (CGS not tested) increased the proportion of maturing males. These findings suggest tha t aromatization is of physiological importance in different mechanisms controlling reproduction in salmon.