APB increases apparent coupling between horizontal cells in mudpuppy retina.

2-Amino-4-phosphonobutyrate (APB), an agonist at a unique type of glutamate receptor on depolarizing bipolar cells, caused an apparent increase in coupling between horizontal cells as evidenced by a decrease in amplitude of responses to illumination of the receptive field center and an increase in responses to illumination of the peripheral part of the receptive field. APB also caused a hyperpolarization of horizontal cells in darkness and increased the amplitude of responses to full-field illumination, which cannot be explained by an increase in electrical coupling between horizontal cells. Possible mechanisms for these actions are discussed.