APB suppresses synaptic input to retinal horizontal cells in fish: a direct action on horizontal cells modulated by intracellular pH.

1. Membrane potentials and cone-driven light responses were recorded from the H1-type horizontal cells in isolated retinas. Membrane potentials and intracellular pH were recorded also in enzymatically dissociated solitary horizontal cells. 2. In isolated retinas the glutamate analogue 2-amino-4-phosphonobutyrate (APB) hyperpolarized horizontal cells and reduced their light responses in a dose-dependent manner (5-200 microM). 3. The action of APB depended on the formulation of the saline; APB was effective in L-15 saline buffered with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) but not in a commonly used, nominally CO2-free bicarbonate/Tris-buffered saline. 4. The major factor controlling the potency of APB was intracellular pH. APB was ineffective during retinal perfusion with NH4Cl-containing or CO2-free bicarbonate saline, both of which are known to alkalinize cells. In contrast, APB was effective in salines formulated to acidify the retinal neurons. These included both HEPES and Tris-buffered salines containing a weak acid and bicarbonate/Tris-buffered saline gassed with CO2. 5. APB reduced the size of glutamate-evoked depolarizations in solitary horizontal cells but had no independent action in the absence of glutamate. This reduction of glutamate-induced depolarization was observed in salines formulated to block voltage-dependent calcium and potassium currents. 6. The magnitude of APB's antagonistic action on solitary horizontal cells increased in a dose-dependent manner from 10 to 200 microM. The antagonism was increased by intracellular acidification and was reduced or eliminated by alkalinization. 7. We conclude that APB can reduce glutamate-evoked and, by inference, the photoreceptor neurotransmitter-evoked depolarization of horizontal cells by acting directly on the horizontal cells. This effect of APB is modulated by intracellular pH.