A TTX–Sensitive Na+–Current in One Type of On Bipolar Cell in the Ground Squirrel Retina

Abstract

Abstract: : Purpose: Bipolar cells use graded changes in membrane voltage to carry light signals from the outer to the inner retina. However, Pan and Hu (2000) reported that some cultured rat bipolar cells had large TTX–sensitive Na⁺ currents, but it was not clear if the population that demonstrated large Na⁺ currents corresponded to a discrete anatomical subtype. Our goal was to determine if a specific anatomical subtype of ground squirrel cone bipolar cell has an unusually large Na⁺ current. Methods: Bipolar cell membrane currents were recorded in the whole cell configuration. Puffer application of TTX (5 µM) was used to isolate Na⁺–currents. To minimize interference from K⁺–currents, Cs⁺ replaced K⁺ in the pipette solution and TEA (20 mM) was added to both pipette and bath solutions. 0.5 mM sulforhodamine 101 was added to the pipette solution to visualize cone bipolar cells under epifluorescence. Results: All recorded cone bipolar cells had transient TTX–sensitive Na⁺–currents, however, only one subtype of On bipolar cell, b5 (West, 1978), had Na⁺–currents that were very large. The TTX–sensitive Na⁺–currents in the b5 had a mean peak amplitude of 542.2 ±136 pA. A second type of On bipolar cell, anatomically similar to the b5, also had TTX–sensitive Na⁺–currents, however the mean peak amplitude was significantly smaller than the b5 (174.6 ± 62 pA, p<.0001). The remaining bipolar cells, which are anatomically distinct from the b5, had even smaller mean peak Na⁺–currents (cell type, mean peak Na⁺–current ± SD): b1, 62±47; b2, 78±49; b3, 14±10; b4, 67±31; b6, 69±37; and, b7, 35±28 pA (total n=60). Na⁺–currents in the b5s were further characterized by fitting a Boltzmann function to plots of activation and inactivation. Currents were half maximally activated at a voltage of –32.7±1.5 mV (z=5.5±1.4); currents were half maximally inactivated a –52.5±1.8 mV (z=20.9 ±1.6) (n=13). Conclusions: One subtype of On bipolar cell in the ground squirrel, the b5, has Na⁺ currents that are significantly larger than those in other bipolar cells. Depending upon the resting potential, the b5 cells may be able to generate Na⁺ action potentials during a depolarizing light pulse.

Keywords: bipolar cells • retinal connections, networks, circuitry • electrophysiology: non-clinical