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B. J. O'Brien, S. J. H. Park; Tetrodotoxin-Resistant Voltage Gated Sodium Currents Are Present in Retinal Ganglion Cells but Not Starburst Amacrine Cells. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1857.
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© ARVO (1962-2015); The Authors (2016-present)
Recently, our laboratory localized an unusual voltage gated sodium channel, Nav1.8, in the somas and dendrites of retinal ganglion and amacrine cells. The Nav1.8 channel has unique biophysical properties including: TTX-resistance (TTX-R), long lasting activation, and resistance to subthreshold inactivation which can significantly alter neuronal synaptic integration. In addition, Nav1.8 has not previously been observed in the central nervous system. We have therefore undertaken a study to characterize the voltage gated sodium currents of both retinal ganglion and starburst amacrine cells.
Whole-cell patch clamp recordings were made in retinal wholemounts using a Cs+ based internal solution including Lucifer Yellow and Neurobiotin. Sodium currents were isolated by bath perfusion of TEA (10mM) and CdCl2 (100 uM). After recording, retinas were fixed and cells labelled with Streptavidin-488 for morphological reconstruction (Zeiss PASCAL). Optomotor behavioural experiments of Nav1.8 knockout and wild type animals were also conducted to determine if lack of Nav1.8 led to visual deficits.
TTX-R currents (Mean = -340pA) were observed in a subset of large retinal ganglion cells (12/30). Ion substitution experiments (Na/Choline) for each cell demonstrated conclusively that these currents were sodium based. Confocal reconstruction demonstrated that TTX-R currents were observed in at least two morphologically defined ganglion cell types. In contrast, while all recordings of starburst cells ( n = 25) exhibited TTX-sensitive currents (Mean = -251pA) no TTX-R currents were observed. Similarly, behavioural experiments demonstrated that drifting sine-wave grating stimuli induced optomotor responses in both Nav1.8 ko and wt mice.
Our results have demonstrated for the first time, the presence of TTX-R currents in the retina, including at least two types of retinal ganglion cell. In contrast, neither electrophysiological nor behavioural experiments suggested that Nav1.8 plays a role in starburst amacrine cell physiology. We have now demonstrated the RNA transcription, anatomical localization and functional expression of Nav1.8 in the retina. Since novel pain therapies are being developed to target Nav1.8 in the peripheral nervous system, it is now necessary to examine whether Nav1.8 is expressed more broadly in the CNS and in species more closely related to humans.
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