Milt production by non-spermiating male Atlantic salmon ( Salmo salar) after injection of a commercial gonadotropin releasing hormone analog preparation, 17α-hydroxyprogesterone or 17α,20β-dihydroxy-4-pregnen-3-one, alone or in combination

Abstract

In three separate experiments, non-spermiating male Atlantic salmon ( Salmo salar) were injected with a commercial salmon gonadotropin releasing hormone analog (GnRHa) preparation or vehicle (propylene glycol). After 6 days, fish were also injected with 17α-hydroxyprogesterone (17P), 17α,20β-dihydroxy-4-pregnen-3-one (17,20βP) or vehicle. Injections were repeated on day 7, and after a further 24 h, milt was collected by catheterisation. Blood samples were also taken and both blood and seminal plasma were retained for analysis of steroid levels. Only those fish treated with GnRHa in combination with 17P displayed a consistent increase in milt volume relative to sham-injected controls (approximately 2–4 fold) whereas treatment with GnRHa alone, resulted in a significant increase in milt volume relative to controls (approximately two fold) on only one occasion. Treatment with GnRHa in combination with 17P tended to result in lower blood plasma levels of 11-ketotestosterone (11KT) and higher levels of 17,20βP relative to other treatments. 11KT levels in blood and seminal plasma tended to be negatively correlated with 17,20βP levels. Treatment with GnRHa in combination with 17,20βP resulted in similar blood plasma steroid profiles as combined GnRHa/17P treatment. Together, these results lend further support for the role of gonadal progestogens in mediating the processes that regulate milt volume in fish, but may also suggest additional roles for gonadotropin (GtH) and 17P, indicating that spermiation may not be simply 17,20βP dependent.