Abstract
Purpose :
To determine Muller cell (MC) contributions to the ERG and particularly to the b-wave.
Methods :
Experiments were performed on isolated mouse (C57/Bl6J) retinae. Light flashes evoked local changes in extracellular K+ and electrical responses that were simultaneously recorded with double-barreled K+-selective microelectrodes. Recordings at different retinal depths allowed creating a 3D matrix of K+ changes in the retina (K+ concentration vs retinal depth vs time). The matrix was analyzed with a custom made computer model of distributed K+ conductances to determine K+-dependent extracellular potentials generated by Muller cells. In one series of experiments all post-photoreceptor light-induced activity was suppressed by Co++; in another series MC responses were suppressed by Ba++.
Results :
The pattern of light-induced extracellular K+ changes in mouse retinae was similar to what was earlier shown in other vertebrates. A slowly developing decrease dominates in the distal half of the retina with the highest amplitude recorded near the tip of the photoreceptors. In a thin layer, about 1/3 of the retinal thickness from the receptor side, presumed to be the OPL, a small (less than 0.1 mM) and short (a fraction of second) increase of K+ at light onset preceded the decrease. Through the proximal 1/3 of the retina a much more prominent increase of K+ that peaked after 1 s and then slowly declined was observed. Analysis of the time-space matrix of retinal K+ changes with a computational model allows recreating K+-dependent responses of MC, which were compared to the ERG components dissected with Co++ and Ba++.
Conclusions :
The K+ conductance of the mouse MC is not concentrated in its endfoot, although it is expected to be distributed unequally along the MC due to its morphology. Such distribution of conductances makes possible MC-generated extracellular potentials which are responsible for most of the slow PIII (sPIII) component (a small part of which has neuronal origin). Two major sources of K+ changes – distal decrease and proximal increase - contribute to sPIII. The MC-generated part of sPIII overlaps with the trailing edge of the b-wave. Thus, MC participates in shaping b-wave. However, the MC has no noticeable influence on the amplitude of the b-wave. The fast distal increase of K+ is incapable to initiate detectable extracellular potentials due to its small amplitude and very restricted location.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.