The GABAergic system:: A possible component of estrogenic feedback on gonadotropin secretion in rainbow trout (Oncorhynchus mykiss).

In teleost fish, GTH secretion is controlled by a large number of neuroendocrine factors at the central level and steroid feedback represents an efficient process to synchronize the activity of all the systems involved along the brain-pituitary-gonad axis. Estrogen effects are mediated by specific nuclear receptors that act as transcription factors to regulate the expression of target genes. In order to understand the neuroendocrine mechanisms involved in the estrogen feedback on GTH secretion, we need, as a first step, to know the nature of target cells at the pituitary and central levels. In rainbow trout, some estrogen receptors expressing cells are identified but the nature of a large number of them remains unknown. In this paper, we explain our strategy to identify the central target of estrogen using both in situ hybridization and immunohistochemistry. We focused our attention on GABA neurons of which the distribution, in some central areas, exactly overlaps with that of estrogen receptors expressing cells.