Atlantic salmon, Salmo salar, mature male parr were implanted with testosterone (T) in small (T3) or large (T10) Silastic capsules in the breeding season or at its end in November or December, in order to find out whether the postbreeding decline in 17alpha, 20beta-dihydroxy-4-pregnene-3-one (17,20beta-P) and milt production is a consequence of declining T levels. In the first of three experiments, fish were sampled the following January and March, whereas in the second and in the third they were sampled in April. Pituitary and plasma concentrations of gonadotropic hormone (GTHs) I and II and plasma levels of T, 11-ketotestosterone (11-KT) and 17, 20beta-P were measured by radioimmunoassays, and the testes were examined histologically. Administration of T prolonged the period in which running milt was present, suppressed Sertoli cells, and prevented the postbreeding decline in testes weight in experiment two. The postbreeding decline in plasma 17,20beta-P levels was diminished by T10 in experiment one, and by both T3 and T10 in experiment two. The similar decline in 11-KT levels was not influenced by T treatment (only studied in experiment one). T treatment also prevented a decline in pituitary GTH II content (in experiments two and three) and in plasma 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 summarize, T treatment prevents postbreeding decline in 17,20beta-P levels, probably via a stimulation of GTH II secretion. J. Exp. Zool. 284:425-436, 1999.
Copyright 1999 Wiley-Liss, Inc.