Potassium and retinal sensitivity

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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mM K+ Fig. 1. Voltage-intensity curves for the receptor potential (A) and b-wave (]3) in control elasmobranch Ringer's (K + = 12 raM) and in Ringer's containing 45 m M K +. C: increasing [K]o above control levels (dashed hne) results in a decrement in Vmax for both responses; decreasing [K]o produces an increase m Vmnx. Intensity scales here and in Fig. 2 give the logarithmic attenuation of the test flash (duration = 0.2 sec) produced by neutral density filters of the indicated values; at log I -- 0, the retinal radiant flux of the unattenuated stimulus (2 = 500 nm) was 10~ quanta]sq./~m/sec.

619 (1-2 ml/min) of elasmobranch Ringer's solution4 over the vitreal surface. The two sides of the tissue were electrically isolated, allowing transretinal potentials to be recorded between the perfusing fluid and a chlorided silver plate on which the tissue rested. The control Ringer's had a K + concentration of 12 mM; potassium levels were altered by adding (or subtracting) KCI from the solution and adjusting urea concentration to maintain osmolarity. For receptor potential recordings, the b-wave of the ERG was suppressed by substituting 50 mM sodium aspartate for 50 mM NaC1 in the elasmobranch Ringer's 6,1a. In each experiment complete voltage-intensity (V-log I) curves were determined; i.e. from below threshold to the maximum voltage (Vmax) that could be elicited photically. Fig. 1 shows the V-log I functions for the receptors (Fig. 1A) and b-wave (Fig. 1B) obtained with perfusates containing 12 mM and 45 mM concentrations of K +. It is immediately apparent that the increased level of [K]0 produced a marked decrease in Vmax; the percent change in Vmaxas a function of [K]0 concentration was approximately the same for both the b-wave and receptor potential, at least between [K]0 concentrations of 6 mM and 30 mM (Fig. 1C). At still higher levels of [K]0 (45 mM and 60 mM), potassium appeared to exert a more drastic effect on the b-wave than on the receptor potential. In addition, recovery of voltage amplitudes after exposure to the higher potassium levels was appreciably better for the receptor potential (80-100~o) than for the b-wave (~ 50~). Fig. 1B shows also that increasing [K]0 shifts the V-log I curve of the b-wave on the intensity axis; the V-log I curve of the receptor potential, on the other hand, does not appear to be displaced even though the response amplitudes have been depressed (Fig. 1A). This is shown more clearly in Fig. 2A and B where the V-log I curves for both responses have been normalized with respect to V max for a wide range of [K]0 levels. Note that the receptoral data now fall upon the same curve indicating that over this range of concentrations, [K]0 did not displace the V-log I function. However, a quite different situation is observed with the b-wave. Increasing [K]0 above the control values causes a shift of the normalized voltage curves on the intensity axis; i.e. there was a reduction in b-wave sensitivity as [K]0 was increased. Potassium levels of 30 mM altered b-wave sensitivity by approximately 0.5 log unit and levels of 45 mM [K]0 by almost 1 log unit. When the normal potassium levels were restored, the voltage curves shifted back to their initial position on the intensity axis, even when the maximum response (Vmax) remained depressed (see above). The fact that potassium affects Vmax for both the receptor potential and b-wave response is not surprising since K + is likely to be involved in determining the resting potentials of the cells underlying these responses3,10,12. However, the selective displacement of the b-wave voltage curves on the intensity axis produced by increased [K]0 is not easy to explain. This finding, reflecting a decrease in b-wave sensitivity, is similar to the effect of a weak background light on the b-wave voltage curve (Fig. 2 C). It is noteworthy that such a dim background light does not change the sensitivity of the photoreceptors 9. The parallels between the effects of light and of increased [K]0 on b-wave sensitivity are of particular interest since the b-wave, although arising from the Miiller (glial) cells, appears to reflect the activity of neurons in the inner

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Fig. 2. Normahzed voltage-intensity curves for varying [K]o concentrations (A and B), and in the presence of dim background dlumination (C). The receptor potential is unaffected by alterations in [K]o. Note, however, that the b-wave V-log I curves are displaced to the right on the intensity axis by increasing [K]o and by the steady light. The results in C are replotted from Green et aL 9 for a non-perfused preparation of larger dimensions than that used m the present study. For these data, the test stimulus was a 'white' light corresponding in intensity (at log I = 0) to a retinal irradiance of ~ l07 quanta (500 nm)/sq.#m/sec

621 nuclear layer of the retinaT, t2. More important, under all adaptive conditions tested so far, ganglion cell sensitivity in skate closely follows changes in b-wave sensitivitys,9, and thus the b-wave provides a useful index of the retinal output to the central nervous system. There are a number of conflicting reports on the effect of [K]0 on the b-wave of other species. Therman 14 long ago observed that a 0.5 % solution of KCI instilled into a frog eyecup completely abolishes the b-wave. More recently, Miller 11 reported that an increase in the level of [K]0 depresses the amplitude of the b-wave of the isolated perfused frog retina more than it does the amplitude of the receptor potential or other neuronal responses in the retina. Winkler 15, on the other hand, claims that in the isolated perfused rat retina increased levels of [K]0 (up to 6 times normal) increase b-wave amplitudes, but higher levels produce a decrease in amplitude. Although Winkler's findings are difficult to reconcile either with earlier studies or our own, it should be noted that none of the earlier studies tested the V-log I relationship with various concentrations of [K]0. The observation reported here, that in skate increased levels of [K]0 shift b-wave voltage curves on the intensity axis, emphasizes the importance of constructing such functions when evaluating how altered ionic environments (or drugs) influence retinal responses. Part of the confusion concerning the effects of altered [K]0 levels on the b-wave may be due to the use of an inadequate range of flash intensities which can give misleading results. For example, if the effects of 45 m M [K]0 on the receptor potential and b-wave in the skate were evaluated with a single test flash (e.g. where the flash elicited 0.5 Vraax in 12 m M Ringer's), one would be led to the erroneous conclusion that the b-wave was completely abolished by the increased potassium whereas the receptor potential was reduced only by about 40% (see Fig. 1). We have on two occasions tested the effect of increased [K]0 levels on the isolated toad retina (Bufo marinus) and find that in this preparation also b-wave voltageintensity curves are shifted on the intensity axis with increased [K]0. Thus it is possible to influence the sensitivity of the retina to light in both toad and skate by manipulating potassium concentration in the solution bathing the retina. It will be of interest to determine whether significant alterations in [K]o levels accompany light- and darkadaptation, and to test whether such alterations form the basis of network adaptation. The differential effects of [K]0 on receptor and b-wave sensitivities, similar to the differential effects of dim background light on these sensitivitiesa, provides support for this notion. This study was supported by grants (EY-00824 and EY-00285) from the National Eye Institute, U.S. Public Health Service. 1 BLACKMAN,J. G., GINSBORG,B. L., AND RAY, C., Some effectsof changes in ionic concentration on the action potential of sympathetic ganglioncells in the frog, J. Physiol. (Lond.), 167 (1963) 374-388. 2 BRINLEY,F. S., JR., Ion fluxesin the central nervous system,Int. Rev. Neurobiol., 5 (1963) 185-242. 3 BROWN,J. E., ANDPINTO,L. H., Ionic mechanismsfor the photoreceptor potential of the retina of Bufo marinus, J. PkysioL (Lond.), 236 (1974) 575-592.

622 4 CAVANAUGH,G. N. (Ed), Formulae and Methods Manual, Vol. V, Marine Biological Laboratory, Woods Hole, Mass., 1956, p. 56. 5 DOWLING,J. E. AND Rtaps, H., Visual adaptation m the retina of the skate, J gen. Physiol, 56 (1970) 491-520. 6 DOWLING,J. E., AND RIPPS, H., Adaptation m skate photoreceptors, J. gen Physiol, 60 (1972) 698-719. 7 FABER, D. S., Analysis of the Slow Transretinal Potentials tn Response to Light, P h . D . Thesis, University of New York at Buffalo, 1969. 8 FRANKENHAEtJSER,B., AND HODOKIN, A. L., The after-effects of impulses m the giant nerve fibers of Loligo, J. Physiol. (Lond.), 131 (1956) 341-376. 9 GREEN,D. G., DOWLING,J. E., SEGAL, I. M., AND RIPPS, H., Retinal mechanisms of visual adaptation in the skate, J. gen. Physiol., 65 (1975) 483-502. 10 KUFFLER, S. W , AND NICHOLLS, J. G., The physiology of neuroghal cells, Ergebn. Physiol, 57 (1966) 1-90. 11 MILLER, R. F , Role of K + in generation of b-wave of electroretlnogram, J. Neurophysiol., 36 (1973) 28-38. 12 MILLER,R. F., AND DOWLtNG, J. E., Intracellular responses of the Mtiller (glial) cells of the mudpuppy retina: their relation to b-wave of the electroretlnogram, J. Neurophysiol., 33 (1970) 323-341. 13 SILLMAN,A. J., Ixo, H., AND TO~nXO, T., Studies on the mass receptor potentml of the ~solated frog retina, Vision Res., 9 (1969) 1435-1451. 14 THERMAN,P. O., The neurophys~ology of the retina in the light of chemical methods of modifyng its excitability, Acta Soc. ScL fenn. N.S.B., 2 (1938) 15 WINKLER, B. S., Dependence of fast components of the electroretinogram of the isolated rat retina on the ionic environment, V~sion Res., 13 (1973) 457-463.