Abstract
Purpose: :
To elucidate the mechanisms underlying the fast "nose" component or n–wave (Arden (1976), J. Physiol. 256, 333–361) in the saturated mouse electroretinogram (ERG) photoresponse.
Methods: :
Photoresponses to flashes (20 ms) of 544 nm light were recorded with ERG technique across isolated mouse (Mus musculus) retinas at 25 oC. The retinas were perfused at the photoreceptor side. The b–wave and higher–order neuron components were blocked with 2 mM aspartate. The glial component (slow PIII) was removed by adding barium (10 mM BaCl2) into the electrode space in contact with the proximal side of the retina. To remove the cone component, two bright rod–saturating flashes were applied at 1.7 s interstimulus interval. At the time of the second flash rods were completely saturated, which yielded a pure cone response. This was subtracted from the first response, leaving a cone–free rod component.
Results: :
The pure cone response did not contain any fast component while the extracted cone–free rod response included the n–wave. Removal of external calcium completely abolished the n–wave and substitution of Ba2+ for Ca2+ recovered it while intracellular calcium buffer BAPTA AM had no effect on the n–wave. Introduction of 2 mM Cs+, a blocker of hyperpolarization activated h channels, abolished the n–wave almost completely. 10 mM Ba2+, known to block Kx channels at high concentrations, reduced the n–wave in normal [Ca2+]. Carbenoxolone (100 µM), which blocks gap junctions, had no effect on the n–wave.
Conclusions: :
Our data suggests that the n–wave is generated by rod photoreceptors. Ca2+ (and Ba2+) may act as a current carrier in the n–wave generating mechanism, most probably through the L–type calcium channels located in the synaptic region. h channels participate in generating the n–wave, either as a current sink or via modulation of the membrane potential. The most probable source of the n–wave are potassium channels in the inner segment. Gap junctions seem to have no role in the generation of the n–wave.
Keywords: electroretinography: non-clinical • photoreceptors • electrophysiology: non-clinical