Effects of testosterone on gonadotropins, testes, and plasma 17 alpha,2 beta-dihydroxy-4-pregnene-3-one levels in postbreeding mature Atlantic salmon, Salmo salar, male parr

Atlantic salmon, Salmo salar, mature male parr were implanted with testoste rone (T) in small (T3) or large (T10) Silastic capsules in the breeding sea son or at its end in November or December, in order to find out whether the postbreeding decline in 17 alpha,20 beta-dihydroxy-4-pregnene-3-one (17,20 beta-P) and milt production is a consequence of declining T levels. In the first of three experiments, fish were sampled the following January and Ma rch, whereas in the second and in the third they were sampled in April. Pit uitary and plasma concentrations of gonadotropic hormone (GTHs) I and II an d plasma levels of T, 11-ketotestosterone (11-KT) and 17,20 beta-P were mea sured by radioimmunoassays, and the testes were examined histologically. Ad ministration of T prolonged the period in which running milt was present, s uppressed Sertoli cells, and prevented the postbreeding decline in testes w eight in experiment two. The postbreeding decline in plasma 17,20 beta-P le vels was diminished by T10 in experiment one, and by both T3 and T10 in exp eriment two. The similar decline in 11-KT levels was not influenced by T tr eatment (only studied in experiment one). T treatment also prevented a decl ine in pituitary GTH II content tin experiments two and three) and in plasm a GTH II levels (only studied in experiment three). However, pituitary GTH I content was not influenced (experiment two and three), whereas plasma GTH I levels (only studied in experiment three) were suppressed by T To summar ize, T treatment prevents postbreeding decline in 17,20 beta-P levels, prob ably via a stimulation of GTH II secretion. J. Exp. Zool. 284:425-436, 1999 . (C) 1999 Wiley-Liss, Inc.