Effect of glutamate on extracellular potassium activity in isolated retina of cyprinid fish.

The effect of (l-)glutamate on extracellular K(+) activity of the isolated retina of the cyprinid fish, roach, was investigated using double-barrelled K(+)-sensitive micro-electrodes. Application of ?M-mM glutamate to the retina as a "puff" from an atomizer induced a transient rise in extracellular K(+) activity, which was maximal 50-100 ?m below the photoreceptor surface. The effect was concentration-dependent, but not related to the state of light adaptation of the retina. In the presence of dinitrophenol or ouabain, the glutamate-induced increase in extracellular K(+) activity was maintained. The following conclusions are made. (1) The most likely cellular origin(s) of the glutamate-induced rise in extracellular K(+) activity are the photoreceptors and/or the Muller (glial) cells. (2) The mechanism responsible for returning the extracellular K(+) activity to normal depends strongly on metabolic, Na(+), K(+) pump activity. (3) The effect parallels the action of endogenous glutamate, and may be important for modulation of neurotransmission in the intact retina.