GABAergic organization in the visual system of the leopard frog, Rana pipiens

Immunocytochemistry was used to study the distribution of gamma-aminobutyric acid (GABA) throughout the central visual nuclei and retina in Rana pipiens. In the diencephalon, intensely-labeled GABA immunoreactive neurons and nerve fibers were observed within the neuropil of Bellonci (nB) and corpus geniculatum (CG), while 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 were also found in the large-celled nucleus lentiformis mesencephali (nLM), as well as in the pretectal gray which contains neurons postsynaptic to the retinal terminal zones within nLM. In the optic tectum (OT), both immunoreactive perikarya and fibers were found within superficial layers 8 and 9; whereas only densely-packed immunoreactive perikarya were evident in the deep tectal layers (i.e. 2, 4, 6). The nucleus of the basal optic root (nBOR) contained a small number of lightly-labeled GABA immunoreactive perikarya mostly located in the dorsal half of the nucleus. A large number of perikarya within the nucleus isthmi (NI) were also lightly immunostained. In the retina, GABA immunoreactivity (both somata and fibers) was observed in all layers except the outer nuclear layer (ONL). Besides GABAergic putative horizontal and amacrine cells in the inner nuclear layer (INL), about 30% of total neurons within ganglion cell layer (GCL) expressed GABA immunoreactivity. Double-labeling studies indicated that about half of the GABA-containing perikarya in the GCL were retinal ganglion cells (RGCs). In addition, three GABAergic projection pathways existing in the visual system of Rana pipiens were demonstrated: (1) from RGCs to the contralateral OT; (2) from nBOR to the pretectal nLM; and (3) bilaterally from the NI to the OT. These results indicate that GABA is an important neurotransmitter in the frog visual system.