Double-label study of relations between rod bipolar cells and GABAergic neurons in the rat retina

Purpose. The role of gamma aminobutyric acid (GABA) as an inhibitory transmitter is well established in the mammalian retinae. Immunocytochemical and electrophysiological studies revealed that rod bipolar cells are under inbitory control by GABA. This study employs a double-label immunocytochemical method to examine the synaptic relationships between rod bipolar cells and GABAergic amacrine cells in the rat retina. Methods. 50μm thick vertical vibratome sections were taken from rat retina. They were immunostained with a mixture of anti-protein kinase C(PKC) and anti-GAD antisera. Synaptic connectivity between rod bipolar cells and GAD immunoreactive neurons in the inner plexiform layer(IPL) was observed with electron microscope. Results. Rod bipolar cells were immunoreacrtive to anti-PKC antiserum. GAD labeled neurons were amacrine, displaced amacrine and interplexiform cells. Synaptic relationships of rod bipolar cells and GABAergic amacrine cells, both input and output, were mainly located in sublaminae 4 and 5 of the IPL. The majority of postsynaptic dyads at ribbon synapses were the processes of unlabeled All-amacrine cells and the process of reciprocal or conventional amacrine cells, all of which showed GAD immunoreactivity. Of amacrine cell processes which were presynaptic to rod bipolar axon and axon terminals, about 60% was immunoreactive against anti-GAD antiserum. In addition, descending axons of rod bipolar cells in sublamina a of the IPL receiving synaptic inputs from labeled amacrine cell processes observed in a few cases. Conclusions. These results provide morphological evidence that rod bipolar cells are mainly under direct GABA inhibitory control in the rat retina.