Glutamate-induced K+ flux in the photoreceptor layer of isolated retina of cyprinid fish: possible consequences for second-order neurones.

The effect of L-glutamate on extracellular K+ activity (aK0) in the micro-environment of the isolated retina of a cyprinid fish was investigated using double-barrelled K+-sensitive micro-electrodes. Glutamate was pulsed onto the retina from an atomizer, and aK0's recorded at different retinal depths. Glutamate induced a concentration-dependent rise in aK0 which was maximal in the first 100 microns of the retinal surface. The aK0 change was transient, with a time to peak of 20-30 s, and complete reversal within 4 min. However, if the retina was pre-treated with dinitrophenol or ouabain, the maximum rise in aK0 was greater than normal, and the effect was sustained. Application of equimolar glutamate transiently depolarized the membrane potentials of horizontal cells. The effects of glutamate on aK0 and horizontal cell membrane potential had some common characteristics. It is concluded that glutamate may have non-synaptic effects on post-receptoral retinal neurones, and possible mechanisms of this are considered.