(A) Salamander RGCs stained with Oregon Green-BAPTA-1-dextran 10 kDa, on a microelectrode array. The ITO electrode (200-μm) is centered in the field of view, with two 10-μm plated electrodes in the upper left. (B–D) Rat retina stained with Oregon Green-BAPTA-1-dextran 10 kDa, at age P6, eye opening, and P45, respectively. The inability of the mature mammalian retina to retain the indicator is illustrated by the paucity of stained somata in (C) versus (B). Probenecid (2.5 mM) was added to the superfusate in (B–D). Potassium probenecid was added to the retrograde dye solution in (C) (10 mM) and in (D) (60 mM). Scale bars 100 μm.