Intracellular cesium separates two glutamate conductances in retinal bipolar cells of goldfish

The responses of depolarizing bipolar cells to glutamate were investigated in the superfused isolated goldfish retina. In intracellular recordings with potassium-filled microelectrodes, glutamate hyperpolarized cells but did not alter the net input conductance. In recordings with cesium-filled microelectrodes, the glutamate-evoked hyperpolarization was associated with a net conductance decrease. In the presence of internal cesium, glutamate action had the same reversal potential as the actions of the glutamate analog 2-amino-4-phosphonobutyrate (APB) and the rod transmitter, suggesting that all three of these substances act at the same class of receptor. We propose that glutamate acts both at the APB-sensitive receptor that mediates rod inputs and at another receptor type that produces a conductance increase, is blocked by cesium, and may mimic the action of the cone transmitter.