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The Journal of Neuroscience, August 31, 2005, 25(35):8097-8103; doi:10.1523/JNEUROSCI.1962-05.2005

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Behavioral/Systems/Cognitive
Voltage-Gated Sodium Channels Improve Contrast Sensitivity of a Retinal Ganglion Cell

Narender K. Dhingra, Michael A. Freed, and Robert G. Smith

Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6058

Voltage-gated channels in a retinal ganglion cell are necessary for spike generation. However, they also add noise to the graded potential and spike train of the ganglion cell, which may degrade its contrast sensitivity, and they may also amplify the graded potential signal. We studied the effect of blocking Na⁺ channels in a ganglion cell on its signal and noise amplitudes and its contrast sensitivity. A spot was flashed at 1–4 Hz over the receptive field center of a brisk transient ganglion cell in an intact mammalian retina maintained in vitro. We measured signal and noise amplitudes from its intracellularly recorded graded potential light response and measured its contrast detection thresholds with an "ideal observer." When Na⁺ channels in the ganglion cell were blocked with intracellular lidocaine N-ethyl bromide (QX-314), the signal-to-noise ratio (SNR) decreased (p < 0.05) at all tested contrasts (2–100%). Likewise, bath application of tetrodotoxin (TTX) reduced the SNR and contrast sensitivity but only at lower contrasts (50%), whereas at higher contrasts, it increased the SNR and sensitivity. The opposite effect of TTX at high contrasts suggested involvement of an inhibitory surround mechanism in the inner retina. To test this hypothesis, we blocked glycinergic and GABAergic inputs with strychnine and picrotoxin and found that TTX in this case had the same effect as QX-314: a reduction in the SNR at all contrasts. Noise analysis suggested that blocking Na⁺ channels with QX-314 or TTX attenuates the amplitude of quantal synaptic voltages. These results demonstrate that Na⁺ channels in a ganglion cell amplify the synaptic voltage, enhancing the SNR and contrast sensitivity.

Key words: retinal ganglion cell; contrast threshold; QX-314; tetrodotoxin; signal amplification; signal-to-noise ratio

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Received May 17, 2005; revised July 11, 2005; accepted July 25, 2005.

This article has been cited by other articles:

				G. B. Christianson and J. L. Pena Noise reduction of coincidence detector output by the inferior colliculus of the barn owl. J. Neurosci., May 31, 2006; 26(22): 5948 - 5954. [Abstract] [Full Text] [PDF]

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