Control of testis function in fish : in vitro studies of gonadotropic regulation in the trout ( Salmo gairdneri )

This paper briefly describes recent data we have obtained on gonadotropin (GtH) regulation of the testicular somatic cells in relation to spermatogenesis and sperm production (spermiation). Throughout the spermatogenetic cycle, the number of testicular GtH binding sites was maximum before and during spermiation. While testicular sensitivity to s-GtH for 11 -ketotestosterone (11 1 KT) secretion was maximum at the beginning of spermiation, for 17a’,20;8-dihydroprogesterone (17 cx,20fJ-O H P) secretion, sensitivity was maximum during full spermiation. On the other hand, production of androgen binding protein by testicular explants was not influenced by GtH. Similarly, no effect of GtH was seen on testicular aromatase activity. Testosterone and 11 KT production by Leydig cell populations in culture could be stimulated by s-GtH. Whether s-GtH was present or not, the ability of these cells to secrete androgens decreased throughout the culture period, while 17a’,20/!-OH-P production increased. This data shows that the responsiveness to GtH varies with the cell type concerned (Leydig cell or Sertoli cell) and with the stage of the reproductive cycle. The work also raises questions dealing with the hormonal and local regulation of testicular functions in salmonids. What controls the androgen to protestagen shift that occurs at spermiation ? Which cells are the testicular target for GtH ? Are other pituitary factors and/or paracrine regulation involved in the control of testis function ?