Gamma-aminobutyric acid (GABA) is the most prevalent inhibitory
neurotransmitter in the vertebrate brain. It can exert its
influence either as GABAergic projection pathways or as local
interneurons, which play an essential role in many visual
functions. However, no GABAergic visual pathways have been studied
in frogs so far. In the present study, GABAergic pathways in
the central visual system of Rana pipiens were
investigated with double-labeling techniques, combining
immunocytochemistry for GABA with Rhodamine microspheres for
retrograde tracing. Three GABAergic visual pathways were
identified: (1) a retino-tectal projection, from retina to the
contralateral optic tectum (OT); (2) an ipsilateral projection
from the nucleus of the basal optic root (nBOR) to the pretectal
nucleus lentiformis mesencephali (nLM); and (3) a second-order
pathway from the nucleus isthmi (NI), bilaterally, to the optic
tectum. These results indicate that GABA is involved in both
first-order (retina to optic tectum) as well as second-order
(nucleus isthmi to optic tectum) visual projections in Rana
pipiens, and may play a major role in mediating visuomotor
reflexs such as optokinetic nystagmus or other visually guided
behaviors.