By using degenerate primers designed from glutamate decarboxylase (GAD) sequences of mammals, Xenopus and Drosophila, a 270‐bp cDNA fragment was cloned by reverse transcriptase‐polymerase chain reaction (RT‐PCR) from cerebellum total RNA of rainbow trout. This partial cDNA shows 90% identity with mammalian GAD 65 and presents the Asn‐Pro‐His‐Lys (NPHK) sequence corresponding to the pyridoxal‐binding region of porcine DOPA decarboxylase or mammalian GAD. The distribution of GAD 65 mRNA‐expressing neurons in the forebrain of the trout was studied by in situ hybridization using either digoxigenin‐ or 35S‐labeled probes. The results demonstrate that gamma‐amino butyric acid (GABA) neurons are widely distributed throughout the forebrain, with a high density in the periventricular regions. In this study, we report their precise distribution in the telencephalon and diencephalon. GAD mRNA‐expressing cells were particularly abundant in the preoptic region and the mediobasal hypothalamus, two major neuroendocrine and estrogen‐sensitive regions in fish. The presence of GAD mRNA‐expressing neurons was observed in visually related structures such as the suprachiasmatic nucleus, the pretectal region, and the thalamus. Immunohistochemistry with antibodies directed against mouse GAD failed to demonstrate the presence of immunoreactive cell bodies, but showed a very high concentration of GAD‐immunoreactive fibers in many brain regions, notably in the preoptic area, hypothalamus, and neurohypophyseal digitations of the pituitary, in particular in the proximal pars distalis. These results indicate that GABA neurons are ideally placed to modulate neuroendocrine activities at the hypothalamic and pituitary levels and to participate in the processing of sensorial information. J. Comp. Neurol. 410:277–289, 1999. © 1999 Wiley‐Liss, Inc.

In teleost fish, GTH sécrétion is controiled by a large number of neuroendocrine factors at the central level and steroid feedback represents an efficient process to synchronize the activity of ail the Systems involved along the brain-pituitary-gonad axis. Estrogen effects are mediated by spécifie nuclear receptors that act as transcription factors to regulate the expression of target gènes. In order to understand the neuroendocrine mechanisms involved in the estrogen feedback on GTH sécrétion, 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.