Pharmacological modification of the light-induced responses of Müller (glial) cells in the amphibian retina.

The light-evoked responses of Müller (glial) cells were monitored by intracellular recording in the isolated, superfused retina of Xenopus laevis. Müller cells had dark resting potentials of -88.5 +/- 6.9 mV and small 1-2 mV light responses of variable waveform in normal Ringer's solution. Exposure to picrotoxin (0.5-1.0 mM) greatly enhanced the light response which then consisted of depolarizing transients (Vmax 5-15 mV) at stimulus onset and offset. GABA (5-10 mM) antagonized the picrotoxin effect and suppressed the light response, whereas 2-amino 4-phosphonobutyrate (0.10-0.15 mM) blocked selectively the 'on' transient. None of these agents appreciably modified the glial cells resting potential level. On the other hand, veratrine (6-9 micrograms/ml) depolarized the Müller cell by 4-13 mV and slowed and greatly reduced the light response. These effects were antagonized by tetrodotoxin (1-4 microM) which itself reduced the light response by 30-50% without altering its shape. On the basis of these findings, we suggest that alterations in the activity of the inner retinal neurons, i.e. amacrine and ganglion cells, are primarily responsible for the drug-induced changes in the membrane potential and light-evoked responses of the Müller cell.