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F. Vinberg, A. Koskelainen; Origin of the Nose Component in Mouse ERG. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2175.
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The leading edge of the a-wave of the ERG is generally believed to reflect the photocurrent of photoreceptors. The aspartate-isolated mammalian ERG response to rod-saturating flash contains a peak component (nose) timely overlapping with the a-wave (Arden (1976), J. Physiol. 256, 333-361). The purpose of this study was to elucidate the mechanism generating the nose.
Photoresponses to flashes (2 - 20 ms) were recorded with the ERG technique across isolated mouse (Mus musculus) retinas or across retinal layers with microelectrodes (T = 25 oC). The b-wave and higher-order neuron components were blocked with 2 mM aspartate. The glial component (slow PIII) was removed by barium.
The nose component was not present in the outer segment layer but became apparent in the inner segment layer of the retina. A significant portion (up to 80-90 %) of the saturated rod ERG signal was derived in the inner segment layer. Pure cone responses did not contain any significant nose component while the cone-free rod responses included the nose. Lowering the free [Ca2+]out from 1 mM to ~25 nM removed the nose. The Ca2+-channel blockers Cd2+ and Co2+ or the intracellular Ca2+ buffer BAPTA(-AM) did not affect the nose component. The h-channel blockers Cs+ (2 - 5 mM) and ZD7288 (50 µM) removed the nose. Both blockers increased the saturated photoresponse amplitude when measured at plateau level following the nose. Potassium channel blockers TEA (20 mM) and Ba2+ (10 mM) decreased the nose amplitude.
The nose in the mouse ERG is generated in the inner segments of rod photoreceptors. We propose a following model for the nose mechanism: In darkness a "sodium loop" circulates between the Na+/K+ pumps located in the distal part of the inner segment and the cGMP channels in the outer segment, while a "potassium loop" circulates between the Na+/K+ pumps and potassium channels located more proximally along the inner segment. During the saturated response "the sodium loop" current goes to zero while the potassium loop current follows the "peak-plateau" process of membrane potential (Fain et al. (1978), Nature, 272, 466-9) generated by the hyperpolarization-activated h-channels. If the h-channels are located proximal to the Na+/K+ pumps, the h current can produce an additional contribution to the nose.
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