Electrophysiological properties of neurones following mild and acute retinal ischaemia

Early electrophysiological changes following acute retinal ischaemia were studied by recording single or multiunit retinal ganglion cells and the electroretinogram (ERG) in barbiturate anaesthetized cats. Retinal ischaemia was initiated photochemically by platelet aggregation in retinal vessels which had been irradiated with monochromatic green light following an intravenous injection of Rose Bengal dye. No physiologically active ganglion cells were found within, or close to, the irradiated sites with chorioretinal oedema. On the other hand, in the areas 5–20° away from the irradiation sites, ganglion cells had abnormally raised spontaneous (background) firing which obscured visually driven firing. The retinal areas where no physiologically active ganglion cells were found showed histopathological changes which are similar to those described for glutamate-induced retinal damage. Retinal areas where depolarized retinal ganglion cells were located, however, showed only minor vacuolation of the ganglion cell fibre layer. Early global electrophysiological changes following photochemically induced retinal vascular lesion were consistent with those predicted from the findings in the single cell study. Vascular lesions produced with high irradiation energy (10–30 J), which promote extensive chorioretinal oedema, resulted in gradual loss of visually responsive ganglion cells. Lesions produced by low-energy irradiation (2 J), causing slight narrowing of the blood columns in the vessels, on the other hand, resulted in significant increases in the amplitude and the implicit time of the ERG b-wave and the background firing of multiunit retinal ganglion cells. Electrophysiological changes associated with mild retinal ischaemia are analogous to physiological effects associated with exogenous glutamate or blockade of glutamate uptake. It is suggested that early ischaemic challenge causes extracellular glutamate accumulation in the retina.