Responses of rod bipolar cells isolated from the rat retina to the glutamate agonist 2-amino-4-phosphonobutyric acid (APB).

Isolated rod bipolar cells were obtained by enzymatic (papain) and mechanical dissociation of the adult rat retina. Virtually all intact bipolar cells in the dissociates expressed protein kinase C (PKC) immunoreactivity, a selective marker for rod bipolar cells in the in vivo retina. Whole-cell recordings were performed using nystatin in the patch pipette to minimize washout of those cytoplasmic components necessary for the maintenance of ionic currents. At holding potentials of -33 mV, a tonic inward current was observed. The glutamate agonist 2-amino-4-phosphonobutyrate (APB) reduced this current by closing ion channels. Under normal conditions, Na+ appeared to be the main charge carrier. Both the internal and the external Ca2+ concentrations were found to exert a powerful influence on the APB-sensitive current. We conclude that the rod bipolar cell in situ is depolarized at light onset.