An enzyme immunoassay for salmon gonadotropin-releasing hormone and its application to the study of the effects of diet on brain and pituitary GnRH in the sea bass, Dicentrarchus labrax.

The effects of two different diets [diet 1 (D1), high protein-low carbohydrate; diet 2 (D2), low protein-high carbohydrate] on brain and pituitary gonadotropin-releasing hormone (GnRH) contents, as well as circulating steroids and vitellogenin, were studied over the reproductive period of the sea bass. Salmon GnRH was measured using a newly developed competitive enzyme immunoassay with an enzymatic tracer made of sGnRH covalently coupled to acetylcholinesterase from the electric organ of the eel Electrophorus electricus. The pituitary GnRH content of animals of both sexes fed D1 was significantly reduced at the time of spawning compared with the pre- and postspawning stages, whereas fish fed D2 did not exhibit such changes. In the brain only minor differences in the GnRH content were observed between the two diets. It is concluded that GnRH release rather than synthesis is affected in fish fed a low protein-high carbohydrate regimen. Plasma sex steroids and vitellogenin were not greatly affected by the diet.