Characterization of Two Populations of Retinal Cone Bipolar Cells That Express Large Voltage–Dependent Na+ Currents in the Rat

Abstract

Abstract: : Purpose: Our previous studies have reported the expression of voltage–dependent Na⁺ channels in a subset of retinal cone bipolar cells in the rat (Pan & Hu, 2000). To understand the function of voltage–dependent Na⁺ channels in bipolar cells, we investigated the physiological and morphological properties of cone bipolar cells that express voltage–dependent Na⁺ currents in the rat retina. Methods: Whole–cell patch clamp recordings were performed in retinal slices. The morphology of the recorded cells was examined by filling fluorescent dye, Alexa 488. Antibody staining against Alexa 488 and choline–acetyltransferase was performed to determine the terminal stratification of recorded cone bipolar cells. Results:Two distinct populations of cone bipolar cells were found to show large voltage–dependent Na⁺ currents. The axon terminals of the first population of cone bipolar cells ramified in the OFF sublamina of the inner plexiform layer (IPL), slightly proximal to or partially overlapping with the outer cholinergic band. These cells are OFF–type cone bipolar cells based on their light response property and are likely to be the type 3 cone bipolar cells classified by Euler & Wassle (1995). A prominent action potential could be evoked by current injection in these cells. The axon terminals of the second population of cone bipolar cells ramified in the middle of IPL, slightly distal to or partially overlapping with the inner cholinergic band. These bipolar cells are ON–type and could be the type 5 cone bipolar cells (Euler & Wassle, 1995). Besides a prominent action potential, spontaneous or current injection–evoked multiple spiking activities were observed in some cells of this second population. We also observed another population of ON–type cone bipolar cells that had a similar terminal stratification level, but displayed no apparent Na⁺ current, suggesting the existence of two physiologically distinct type 5 cone bipolar cells. Conclusions: We identified two populations of cone bipolar cells, an ON–type and an OFF–type, that express large voltage–dependent Na⁺ channels. The axon terminals of these two populations of cone bipolar cells stratify in close proximity to or within the processes of the ON– and OFF–cholinergic amacrine cells, respectively. These cone bipolar cells are capable of generating prominent action potentials.

Keywords: bipolar cells • ion channels • electrophysiology: non-clinical