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Silke Becker, Hari Jayaram, Christopher R. Hogg, Graham E. Holder, G Astrid Limb; Activity Of Voltage-gated Sodium Channels Contributes To The B-wave Of The Photopic ERG In The Rabbit. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2458.
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The electroretinogram (ERG) is widely used clinically and experimentally to assess the function of the neural retina. The different components of the scotopic and photopic Ganzfeld ERGs have been attributed to various retinal cell types, with the most prominent contribution to the b-wave coming from the rod On and cone On and Off bipolar cells, respectively. The rabbit eye is frequently used experimentally for ocular studies and for the development of ophthalmic surgical procedures. However, relatively little is known about the rabbit ERG, limiting its use for functional outcome measures. In the present study we have investigated the role of tetrodotoxin (TTX)-sensitive voltage-gated sodium (Nav) channels in the scotopic and photopic ERGs in the rabbit.
Female Chinchilla bastard rabbits were anaesthetized by i.m. injection of ketamine (50mg/kg) and xylazine (10mg/kg). Phenylephrine (2.5%) and tropicamide (1%) eye drops were applied for pupil dilation.Tetrodotoxin (TTX, 1μM) was injected into the vitreous cavity and electroretinograms were recorded using a Ganzfeld stimlator (ColorDome system, Diagnosys Systems Ltd.). Rabbits were dark adapted for at least 45-60 min for the scotopic recordings and light adapted for 10 min with a background of 35 cd s/m2 for the photopic recordings. ERG responses were evoked by flashes of white light in the range of 10-4-10 cd s/m2 for the scotopic responses and 0.03-25 cd s/m2 for the photopic intensities. Flicker ERGs were recorded at a frequency of 30Hz with blue, green and yellow stimuli at 2.34 cd s/m2.
The a- and b-waves of the scotopic responses were not affected by intravitreal injection of TTX. Conversely the amplitude of the b-wave of the photopic ERG was profoundly attenuated and the latency increased by this Nav blocker, while the a-wave remained unaltered. The flicker ERG was greatly reduced at all wavelengths, although the effect of TTX appeared greatest on the blue flicker ERG.
In the present study we have investigated the effect of the Nav inhibitor TTX, which is known to block the spiking of neuronal cells, on the photopic and scotopic ERG in the rabbit. Rod and cone photoreceptors and rod bipolar cells are not affected by TTX. TTX-sensitive Nav channels are functional in bipolar cells of the cone, but not the rod pathway of the rabbit retina. Bipolar cells of the blue cone pathway appear to have the greatest sensitivity to TTX.
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