Decline of blocking effect of cobalt ions on transmission from photoreceptors to horizontal cells during its prolonged application.

In high-Mg2+ medium, luminosity-type cone horizontal cells (L-cells) hyperpolarized and lost their photoresponses at a full membrane hyperpolarization of about -80 mV. These effects of high-Mg2+ are attributed to the blockade of transmitter release from photoreceptors. Application of Co2+ hyperpolarized the L-cells at first and the photoresponses disappeared at full membrane hyperpolarization. After a little while, however, the membrane potential began to depolarize and the light stimuli again evoked responses. The recovery of the membrane potential and photoresponses indicates that prolonged application of Co2+ weakens its ability to block transmission from photoreceptors to L-cells.