Abstract
Purpose: :
The ERG b-wave measures light-induced response of inner retinal cells, most notably the depolarization of bipolar cells. The contribution of Müller glial cells to the b-wave is a subject of controversy. A negative ERG due to an abnormal b-wave is a hallmark of several disorders, including congenital stationary night blindness (CSNB), X-linked retinoschisis, and those forms of retinitis pigmentosa wherein defective bipolar cells are the likely origin. We used the mdxCv3 mouse model of Duchenne muscular dystrophy (DMD), which shows reduced b-wave amplitude, to assess how alteration of dystrophin affects ERG output.
Methods: :
Segments of dark-adapted isolated mouse retina were placed in a recording chamber on the stage of a Nikon FN-1 microscope. The epi-fluorescence pathway was used to illuminate the retina using neutral-density filters to control light intensity. Ringer’s solution containing (in mM) NaCl 110, KCl 5, Na2HPO4 0.8, NaH2PO4 0.1, NaHCO3 30, MgSO4 1, CaCl2 1.8, glucose 22 and glutamine 0.5 was superfused at 3 ml/min at 37ºC. Two transretinal electrodes (one Ag/AgCl macroelectrode and a glass microelectrode) were used. We added 200 µM barium to block the Müller cell inwardly-rectifying potassium channels and then isolated the barium sensitive component by mathematical subtraction.
Results: :
The negative scotopic ERG from the isolated retina of the mdxCv3 showed a) an increased b-wave implicit time, b) a reduced b-wave amplitude, followed by c) a large slow wave. The b-wave and slow wave amplitudes were 131 ± 16 and 64 ± 14 ∝V, respectively, in control retina compared to 92 ± 11 and 138 ± 20 ∝V in the mdxCv3 retina. Exposure of the retina to barium completely blocked the slow wave and increased the b-wave amplitude to 236.9 ± 50 and 212 ± 40 ∝V in control and mdxCv3 retina, respectively. The barium sensitive component, presumed to be the Müller cell potassium conductance through Kir4.1 channels, was larger in the mdxCv3 retina as compared to the normal retinal response (P < 0.05).
Conclusions: :
These analyses suggest that bipolar cell function is actually normal in the mdxCv3 with the abnormal b-wave being the end-result of a net negative ERG due to the masking of the b-wave by an abnormal slow wave (of opposite polarity). This integrated ERG output from Müller cells onto bipolar cells supports the complexity of the b-wave response and demonstrates the utility of genetically mutant mouse models, such as the mdxCv3, in understanding the biological processes involved in retinal electrophysiology.
Keywords: Muller cells • electroretinography: non-clinical • inner retina dysfunction: hereditary