Cone ERG Changes during Light Adaptation in Two All-Cone Mutant Mice: Implications for Rod-Cone Pathway Interactions

PURPOSE. The b-wave of the cone ERG increases in amplitude and speed during the first few minutes of adaptation to a rod-suppressing background light. Earlier studies implicate rod pathway input to the cone pathway in these changes. METHODS. The timing and amplitude of the cone b-wave and isolated oscillatory potentials (OP) during the first 10 minutes of light adaptation in wild-type (WT) mice and two mutant lines without functional rods was examined: rhodopsin knockout (Rho^―/^―), lacking rod outer segments, and NRL knockout (Nrl^―/―), in which rods are replaced by S-cones. Expression of the immediate-early gene c-fos, which is increased in the inner retina by light-induced activity, was evaluated by immunohistochemistry in dark- and light-adapted retinas. RESULTS. WT b-wave and OP amplitudes increased, and implicit times decreased during light adaptation. Subtracting OP did not alter b-wave changes. Rho^―/― b-wave and OP amplitudes did not increase during adaptation. B-wave timing and amplitude and the timing of the major OP at 1 minute of adaptation were equivalent to WT at 10 minutes. The light-adapted ERG b-wave in Nrl^―/^― mice, which originates in both the rod and cone pathways, changed in absolute amplitude and timing similar to WT. C-fos expression was present in the inner retinas of dark-adapted Rho^―/^― but not WT or Nrl^―/^― mice. CONCLUSIONS. Activity in the distal rod pathway produces changes in the cone ERG during light adaptation. Rods in Rho^―/― mice constitutively activate this rod-cone pathway interaction. The rod pathway S-cones in Nrl^―/― mice may maintain the WT interaction.