May 2004
Volume 45, Issue 13
Free
ARVO Annual Meeting Abstract  |   May 2004
Tetrodotoxin–sensitive sodium currents in salamander ON bipolar cells.
Author Affiliations & Notes
  • T. Ichinose
    Ophthalmology, Washington Univ/School of Medici, St Louis, MO
  • P.D. Lukasiewicz
    Ophthalmology, Washington Univ/School of Medici, St Louis, MO
  • Footnotes
    Commercial Relationships  T. Ichinose, None; P.D. Lukasiewicz, None.
  • Footnotes
    Support  NIH grants EY08922, EY02687, Research to Prevent Blindness, The M. Bauer Foundation
Investigative Ophthalmology & Visual Science May 2004, Vol.45, 2194. doi:
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      T. Ichinose, P.D. Lukasiewicz; Tetrodotoxin–sensitive sodium currents in salamander ON bipolar cells. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):2194.

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Abstract

Abstract: : Purpose: We have shown that the sodium channel blocker, tetrodotoxin (TTX) reduced light–evoked EPSCs in a subset of ON–OFF ganglion cells and reduced light evoked EPSPs in a class of ON bipolar cells. The processes of the TTX–sensitive bipolar and ganglion cells ramified in the mid inner plexiform layer (IPL). Here, we show that TTX–sensitive inward currents, which boost excitatory signaling, are present in this class of ON bipolar cells. Methods: Using retinal slices, whole cell recordings were made from bipolar cells that were filled with Lucifer yellow to determine their morphology. Synaptic inputs were blocked by including strychnine, bicuculline, I4AA, picrotoxin, CNQX, AP5 and L–AP4 in the bath. Calcium currents were blocked by substituting cobalt for bath calcium. Potassium currents were partially blocked by TEA in both the pipette and the bath solutions. Results: TTX–sensitive inward currents were observed in response to voltage steps from –80 or –50 mV to –10 mV, after TTX responses were subtracted from control responses. The TTX–sensitive currents were observed only in ON bipolar cells with axon terminals ramifying in the mid–IPL, and never observed in ON bipolar cells with axon terminals ramifying the innermost IPL. Because voltage–gated calcium currents and synaptic inputs were blocked, we attributed the TTX–sensitive currents to sodium channels on bipolar cells. The inward sodium currents were relatively prolonged (>50 msec) with peak amplitudes of approximately –100 pA. The threshold for activation of the inward current was near –40 mV and minimal current inactivation was observed for prepulse voltages from –80 to –40 mV. Conclusions: TTX–sensitive inward currents were found in a class of ON bipolar cells whose axon terminals ramify in the mid–IPL. The bipolar cell sodium currents were more prolonged than those recorded from ganglion cells or amacrine cells. We propose that these sodium channels are responsible for the TTX induced reduction in light–evoked voltage responses of this class of bipolar cells. These channels boost light–evoked signaling between a subset of bipolar cells and their ganglion cell targets.

Keywords: bipolar cells • electrophysiology: non–clinical • retina: proximal (bipolar, amacrine, and ganglion cells) 
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