Differentiation of Recoverin Immunoreactive Cone Bipolar Cells and Their Timing of Synaptic Formation with GABAergic Amacrine Cells in the Rat Retina

Park, Dae Kyoon; Kim, In Beom; Chun, Myung Hoon; Chung, Jin Woong

Abstract

Recoverin is a member of the large family of EF-hand calcium binding proteins (Baimbridge et al., 1992), and it is thought to be involved in the regulation of phosphodiesterase in photoreceptors and in the phosphorylation of activated rhodopsin (Polans et al., 1996). Although the functional significance of recoverin in cone bipolar cells is not fully understood, the antiserum against recoverin has been widely used to identify a certain population of cone bipolar cells (Milam et al., 1993; Sasso's Pognetto et al., 1994; Euler & W sle, 1995). GABA is well known to act as major neurotransmitters in the mammalian central nervous system including retina. This study was conducted to identify the development process of recoverin-labeled cone bipolar cells, and the timing points of synaptic formation of the labeled bipolar cells and GABAergic amacrine cells in the rat retina. The results were as follows; In the adult rat retina, recoverin-labeled cone bipolar cells were subdivided into twotypes; type 2 cells with axon terminal stratified in sublamina a of the inner plexiform layer (IPL), and type 8 cells with axon terminals stratified in sublamina b of the IPL. Recoverin-labeled cone bipolar cells began to appear from postnatal day 5. The axon terminals of recoverin-labeled type 2 cone bipolar cells stratified at postnatal day 10, while those of type 8 cone bipolar cells stratified at postnatal day 13. The axon terminals of type 2 cone bipolar cells made ribbon synapses onto GABAergic amacrine cells in the IPL at postnatal day 10. These results demonstrate that recoverin-labeled type 2 cone bipolar cells differentiate earlier than recoverin-labeled type 8 cone bipolar cells, and suggest that GABAergic amacrine cells may play important roles in visual processing of recoverin-labeled type 2 cone bipolar cells by making synapse onto these cells at early stage. Synapses between type 2 cone bipolar cells and GABAergic amacrine cells are formed about the time of postnatal day 10 for visual processing.