HIGH POTASSIUM PROMOTES DIFFERENTIATION OF RETINAL NEURONS BUT DOES NOT FAVOR ROD DIFFERENTIATION

Neural retinal cells of newborn rats were cultured under dissociated c ulture conditions. Differentiation of several types of retinal cells w as confirmed by immunohistochemical detection of type-specific neural phenotypes. We used Thy-1.1 antigen as a ganglion cell marker, HPC-1 o r GABA as an amacrine cell marker and rhodopsin as a rod cell marker. With a high concentration of potassium (38 mM), expression of the resp ective neural phenotypes were differentially affected. High K+ increas ed the number of Thy-1.1 positive cells 6 to 8 fold, and drastically p romoted their neurite extension. The same culture conditions, however, reduced considerably the number of rhodopsin positive cells, possibly due to the unique membrane properties of photoreceptors. A high K+ co ncentration also promoted differentiation of HPC-1 positive and GABA p ositive cells, but to a lesser extent than the Thy-1.1 positive cells. Several possibilities were examined to understand the effect of a hig h K+ concentration on retinal neural cells. The total cell number in c ultures with a high Kf concentration was approximately half of that in control cultures at day 3 and slightly smaller at day 11, suggesting that high Kf did not have a positive general effect on the proliferati on or survival of retinal cells. Naturally occurring neuronal death (a poptosis) is a well-known phenomenon during retinal development. A his tochemical method for detecting DNA fragmentation, a step preceding ap optosis, showed that high K+ had no preventive effect. BrdU (bromodeox yuridine) immunohistochemistry showed that high K+ did not seem to enh ance proliferation of neural precursor cells. These results indicate t hat a high K+ concentration promotes the expression of neuronal phenot ypes but is not a favorable condition for rod differentiation. Since a high K+ concentration is considered to induce depolarization of nerve cells, the present results suggest an anterograde influence from surr ounding neuronal cells, through chronic depolarization by elevated K+, is essential for the differentiation and maturation of retinal cells.