Morphology and synaptic circuitry of recoverin-like immunoreactive bipolar cells in the rat retina

Purpose. It has been reported that a certain population of cone bipolar cells are labelled with antiserum against recoverin in mammalian retinae studied to date. This study was conducted to identify the cell types expressing recovein immunoreactivity and to clarify their morphology and synaptic connectivity in the rat retina. Methods. Immunocytochemical method using anti-recoverin antibody was applied to 50μm thick horizontal and vertical vibratome sections. Cell density and morphology of the labeled cells were identified in central and peripheral regions of the retina, and types of synapses made by labeled cells in the inner plexiform layer (IPL) were observed with electron microscope. Results. Recoverin labeled cells were two types of bipolar cells, type I and type II, in the rat retina. The cell density and mean diameter were 4192.1 ± 187.3/mm² and 4.9 ± 0.5 μm in the central and were 3580.1 ± 221.7/mm² and 5.8 ± 0.5 μm in the peripheral region, respectively. The axon of type I bipolar cells ended in sublamina a of the IPL. They received chemical synapses from All and other amacrine cells. Of profiles, posteynaptic to type I bipolar cell, 68% belonged to amacrine cells and 32% belonged to ganglion cells. Their main synaptic targets were postsynaptic dyads consisting of a amacrine/amacrine cell, or a amacrine/ganglion cell. The axon of type II bipolar cells terminated in sublamina b of the IPL. They received gap junction contacts from All amacrine cells and input synapses from amacrine cells. Of profiles that was postsynaptic to their axon terminals, 59% belonged to amacrine cells and 41% belonged to ganglion cell. Their most frequent output targets were a amacrine/ganglion cell dyad. A amacrine/amacrine and a ganglion/ganglion cell dyads were also observed. Conclusions. These results demonstrate that a certain populations of on- and off-bipolar cells were labelled by anti-recoverin antiserum. Though these cells are primarily involved in photopic pathways, they are also available to the scotopic pathways through receiving electrical and chemical synaptic inputs from All amacrine cells.