The effect of K +-conductance-blocking substances on the occurrence of retinal spreading depression

Abstract

Retinal spreading depression (SD) has been thought to be generated by an abnormal increase in [K +] 0 in the inner plexiform layer. When a retina isolated from bullfrog was immersed in a medium conditioned for SD (low Cl − Ringer's solution), spontaneous SDs periodically occurred at a fixed interval in the dark. The effect of K +-conductance blocking substances (Ba 2+, Cs +, TEA and 4-AP) on the occurrence of SD was studied using the concomitant changes in the field potential (spreading depression potential; SDP) as an index. These substances increased the frequency of occurrence and decreased the amplitude of the potential. According to the Müller cell theory of SDP, an increase in [K +] 0 depolarizes the Müller cells and the resulting extracellular current generates the potential changes. Since the small amplitude of the SDP reflects the decreased K + current, it may be said that the K +-buffering capacity of the Müller cells was partially reduced by the substances and that the frequency of the SD occurrence consequently increased in order to clear accumulating [K +] 0 from neurons with the decreased buffering capacity. The present results lend a good support to the current Müller cell theory for the occurrence of SDP.