Second, retrograde tracing studies in goldfish, rainbow trout, and Atlantic salmon have clearly indicated that neurons in this area are hypophysiotropic. Furthermore, in Atlantic salmon, retrograde studies combined with TH immunohistochemistry have unambiguously demonstrated that these TH neurons are hypophysiotropia. In goldfish, rainbow trout, and Atlantic salmon, it is also known that these TH-positive neurons are dopaminergic. Extensive studies in goldfish and rainbow trout have demonstrated the inhibitory role of dopamine on GTH-2 secretion, and the role of estradiol in modulating this inhibitory tone is also well documented. In rainbow trout, there is evidence that the intensity of the dopaminergic inhibition is in direct relationship with the levels of circulating estradiol and that the drop in estradiol at the end of vitellogenesis is one of the signals triggering the preovulatory GTH-2 surge. proventil inhaler
As it has been shown that, in vitellogenic females, estrogen receptors are expressed in about 70% of the TH cells of this nucleus, it is believed that these cells represent the morphological substrate of the estrogen-dependent dopaminergic inhibition of GTH-2 secretion. After the cloning of a full-length cDNA of TH from rainbow trout, we have recently shown, by in situ hybridization, the presence of TH mRNA in this cell population (unpublished results and personal communication with T. Bailhache).