DISTRIBUTION OF GABA-LIKE IMMUNOREACTIVE NEURONS AND FIBERS IN THE CENTRAL VISUAL NUCLEI AND RETINA OF FROG, RANA-PIPIENS

Immunocytochemistry was used to study the distribution of gamma-aminob utyric acid (GABA) throughout the central visual nuclei and retina in Rana pipiens. In the retina, GABA immunoreactivity (both somata and fi bers) was observed in all layers except the outer nuclear layer (ONL). Contrary to earlier reports, about 30% of total neurons within gangli on cell layer (GCL) expressed GABA immunoreactivity. Double-labeling s tudies indicated that about half of the GABA-containing perikarya in t he GCL were retinal ganglion cells (RGCs). In the diencephalon, intens ely labeled GABA-immunoreactive neurons and nerve fibers were observed within the neuropil of Bellonci (nB) and corpus geniculatum (CG), whi le only immunoreactive puncta were found in the rostral visual nucleus (RVN). In the pretectal region, the posterior thalamic nucleus (nPT) contained the most intensely labeled GABA immunoreactive perikarya and nerve fibers in the entire brain. Lightly immunoreactive perikarya we re also found in the large-celled nucleus lentiformis mesencephali (nL M), as well as in the pretectal gray, which contains neurons postsynap tic to the retinal terminal zones within nLM. In the optic tectum (OT) , both immunoreactive perikarya and fibers were found within superfici al layers 8 and 9, whereas only densely packed immunoreactive perikary a were evident in the deep tectal layers (i.e. 2, 4, 6). The nucleus o f the basal optic root (nBOR) contained a small number of lightly labe led GABA-immunoreactive perikarya, mostly located in the dorsal half o f the nucleus. A large number of perikarya within the nucleus isthmi ( NI) were also immunostained.