Dopaminergic Regulation of Gonadotropin-II Secretion in Testosterone-treated Precocious Male and Immature Rainbow Trout Oncorhynchus mykiss

The present work examined the role of gonadotropin-releasing hormone (GnRH) and dopaminergic drugs on the secretion of maturational gonadotropin (GTH II) in relation to testosterone m treatment. This study provides evidence that the plasma GTH II levels are increased by T treatment in precocious males, but not in the immature animal. In addition, GnRH analogue (GnRHa) alone significantly increased the plasma GTH II secretion in immature rainbow trout treated with T, as well as in T-treated and T-untreated precocious males. However, injection with either dopamine (DA) or domperidone (DOM; DA D2 receptor antagonist) alone did not alter the basal plasma GTH 11 secretion in all experimental groups. The secretion of GTH II in the T-treated precocious males was remarkably influenced by GnRHa or combination of dopaminergic drugs. Notably, the effects of dopaminergic drugs on GnRHa-induced GTH II secretion w8s prolonged by T in precocious males. In T-treated immature animals, GnRHa-induced GTH II secretion was Increased only by a dose DOM (10$\mu$g/g body n) but not by higher dose DOM (100$\mu$/g body wt). In the T-untreated immature rainbow trout, however, plasma GTH 11 secretion was not influenced by the same treatments. Therefore, these results indicate that DA may be acting indirectly by blocking the effect of GnRH on GTH II secretion in vivo. T may act to modulate the relative contribution by the stimulatory (GnRH) and inhibitory (DA) neuroendocrine factors, which would ultimately determine the pattern of GTH II secretion.