Two groups of retinal cone bipolar cells (CBCs) in rats were found to express voltage-gated Na+ channels. The axon terminals of the first group stratify in sublamina 2 of the inner plexiform layer (IPL) and partially overlap with the OFF-cholinergic band. This group was identified as type 3 CBCs. The axon terminals of the second group stratify in sublamina 3 of the IPL, slightly distal to the ON-cholinergic band. Cells of this second group resemble type 5 CBCs. In addition, we observed another group of ON-type CBCs with terminal stratification similar to that of the second group. However, this latter group did not show any Na+ current, instead exhibiting a large hyperpolarization-activated cyclic nucleotide-gated cation current, suggesting the existence of two subclasses of physiologically distinct type 5 CBCs. Both groups of Na+-expressing bipolar cells were capable of generating a rapid tetrodotoxin-sensitive action potential as revealed by current injection. Multiple spike-like potentials were also observed in some of these cells. Results of this study provide valuable insights into the function of voltage-gated Na+ channels of retinal bipolar cells in retinal processing.

Whole-cell voltage-clamp recordings were performed to investigate voltage-dependent K+ currents in acutely isolated retinal cone bipolar cells (CBCs) from the rat. The physiological and pharmacological properties of the currents were compared with those in rod bipolar cells (RBCs). The K+ currents were found to be much larger in CBC than in RBCs. In addition, the currents in CBCs were activated and inactivated at more negative potentials. Based on the apparent inactivation property of the currents, CBCs were found to fall into two groups of cells that differed in the inactivation kinetics of IK(V) but did not correlate to the ON- and OFF-type. The IK(V) for the group of CBCs showing faster inactivation, as well as for all RBCs, contained two components with decay time constants around 0.1 and 1 s. The IK(V) for the group of CBCs showing slower inactivation only contained the slower component. Furthermore, three components of IK(V) were observed based on tetraethylammonium (TEA) sensitivity: high-sensitive, low-sensitive, and resistant component. The IK(V) for a portion of CBCs showing faster inactivation, as well as for all RBCs, contained all three components. The IK(V) for the remaining CBCs, including all of those CBCs showing slower inactivation, only contained the latter two components. This study reveals a differential expression of K+ currents in rat retinal bipolar cells, suggesting that K+ channels may play an important role in bipolar cell processing in mammalian retinas.