Brain Research, 107 (1976) 617--622 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

617

Potassium and retinal sensitivity

JOHN E. DOWLING AND HARRIS RIPPS The Marine Biological Laboratory, Woods Hole, Mass. 02543, The Biological Laboratories, Harvard University, Cambridge, Mass. 02138 and the Departments o/ Ophthalmology and Physiology, New York University School of Medicine, New York, N.Y. 10016 (U.S.A.)

(Accepted February 2nd, 1976)

The ability of the visual system to adjust its sensitivity to the broad range of light intensities it encounters is largely a retinal phenomenon. Recent studies in the all-rod retina of the skate have shown that this adaptation process is subserved both by the photoreceptors 6 and by a post-receptoral mechanism which we have termed the 'network'L Depending upon the experimental conditions, one or the other seems to be primarily responsible for establishing visual thresholds; i.e. the minimal energy required to elicit threshold spike activity in ganglion cells. Thus, in the presence of a bright background field, or during dark adaptation after exposure to a strong preadapting light, receptoral adaptation appears to be the rate-limiting process. On the other hand, it is possible to reduce markedly the sensitivity of both the b-wave and ganglion cell discharge with steady background illumination too weak to have a detectable effect on receptoral thresholds 9. Clearly the alterations in visual sensitivity under such conditions are governed by the network mechanism. An interesting feature of the network mechanism is its remarkably slow time course; several minutes are often required for b-wave or ganglion cell thresholds to reach final levels during either light- or dark-adaptation. This has led to the speculation that the changes in sensitivity may reflect changes in the concentration of a chemical agent at certain loci within the retina 9. One of the substances that needs to be considered in this regard is potassium, for it is well known that the concentration of extracellular potassium [K]0 increases in the vicinity of active neurons and that this ionic change can affect the excitability of neighboring cells 1,~,8. Furthermore, glial cells, which are known to behave like K + electrodes10,11, have been implicated as the source of the b-wave of the electroretinogram (ERG)7,12. We have attempted, therefore, to determine whether alterations in [K]0 produce effects on the b-wave similar to those exerted by light adaptation. We report here that increasing the concentration of [K]0 desensitizes the b-wave in a manner similar to the desensitizing effect of light. The change cannot be attributed to a direct effect of potassium on the photoreceptors since the sensitivity of the receptor potential was not significantly altered by changes in [K]0. Pieces of skate retina (Raja oscellata or R. erinacea) with pigment epithelium and some choroid attached were mounted in a chamber which permitted a rapid flow

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