Distribution of glutamic acid decarboxylase mRNA in the forebrain of the rainbow trout as studied by in situ hybridization

By using degenerate primers designed from glutamate decarboxylase (GAD) seq uences of mammals, Xenopus and Drosophila, a 270-bp cDNA fragment was clone d by reverse transcriptase-polymerase chain reaction (RT-PCR) from cerebell um total RNA of rainbow trout. This partial cDNA shows 90% identity with ma mmalian GAD 65 and presents the Asn-Pro-His-Lys (NPHK) sequence correspondi ng to the pyridoxal-binding region of porcine DOPA decarboxylase or mammali an 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 S-35-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 thei r precise distribution in the telencephalon and diencephalon. GAD mRNA-expr essing cells were particularly abundant in the preoptic region and the medi obasal hypothalamus, two major neuroendocrine and estrogen-sensitive region s in fish. The presence of GAD mRNA-expressing neurons was observed in visu ally related structures such as the suprachiasmatic nucleus, the pretectal region, and the thalamus. Immunohistochemistry with antibodies directed aga inst mouse GAD failed to demonstrate the presence of immunoreactive cell bo dies, but showed a very high concentration of GAD-immunoreactive fibers in many brain regions, notably in the preoptic area, hypothalamus, and neurohy pophyseal digitations of the pituitary, in particular in the proximal pars distalis. These results indicate that GABA neurons are ideally placed to mo dulate neuroendocrine activities at the hypothalamic and pituitary levels a nd to participate in the processing of sensorial information. (C) 1999 Wile y-Liss, Inc.