Modulation Of Horizontal And Bipolar Cell Activities Mediated By NMDA Receptors And Glycine Receptors

1. NMDA receptor mediated modulation of horizontal and bipolar cells.Glutamate works as a major excitatory neurotransmitter in the vertebrate retina. By immunocytochemistry, we showed that the NMDA receptor subunit 1 (NR1) was expressed abundantly on cone-driven (H1), but not rod-driven (H4), horizontal cells (HCs) in carp retina. Whole-cell recordings made from isolated carp H1 cells further showed that NMDA, co-applied with glycine, could induce inward currents that were blocked by the NMDA receptor competitive antagonist D-2-amino-5-phosphonopentanoate (D-AP5) and 5,7-dichlorokynurenic (DCKA), a selective NMDA receptor antagonist acting at the glycine site on the NMDA receptor complex. Moreover, calcium imaging showed that NMDA caused a significant elevation of intracellular calcium levels ([Ca²⁺]_i) of H1 cells, which was also blocked by D-AP5. In contrast, neither inward currents nor changes in [Ca²⁺]_i could be induced by NMDA in H4 cells. Intracellular recordings made from H1 cells in the isolated retina, superfused with Ringer's containing 1 mM Mg²⁺, in the dark demonstrated that NMDA reduced the light-off overshoot of H1 cells. We therefore conclude that the functional NMDA receptor is expressed in carp H1 cells, from which this receptor has been thought to be absent, and may play a role in modulating cone-driven light responses of horizontal cells in the dark.Effects of NMDA on the receptive field organization of cone-dominant bipolar cells were investigated in isolated, superfused carp retina, using intracellular recording techniques. For the ON-type bipolar cells, 50 μM NMDA decreased the depolarizing center response, but increased the hyperpolarizing surround response. On the other hand, NMDA of 50 μM did not much change the hyperpolarizing centerresponse of the OFF-type bipolar cells, but dramatically reduced the depolarizing surround response. Furthermore, NMDA enhanced and reduced light responses of ON- and OFF-type sustained amacrine cells respectively, but affected neither L- type nor C-type cone-driven horizontal cells. All these effects of NMDA could be reversed by D-AP5, indicative of the involvement of NMDA receptors. These results suggest that NMDA receptors can be involved in modulating the receptive field organization of bipolar cells by modifying the balance in strength between the center and the surround. Whereas the effects of NMDA on the surround responses of both types of bipolar cells may be a consequence of the action of NMDA on the amacrine cells, the suppression of the center response of the ON-type bipolar cells is thought to reflect a direct action of NMDA on these cells.2. Glycine receptor mediated modulation of horizontal and bipolar cells.Effects of glycine on ON type rod-dominant bipolar cells (RBCs) were studied in isolated, superfused carp retina by intracellular recording techniques and in carp retinal slice preparations by whole cell recording. Glycine of 4 mM hyperpolarized RBCs and potentiated their light responses to large light spots, which was reversed by co-application of 10 μM strychnine. It was further found that illumination of the receptive field surround did not affect the depolarizing center response of RBCs. The above result therefore suggests that glycine modulates the center response of RBCs. Focal application of glycine to either dendrites or axon terminals of RBCs failed to induce any currents in both isolated cell and retinal slice preparations. On the other hand, glycine of 4 mM increased the amplitude of the scotopic electroretinographic PIII component, which reflects the activity of rod photoreceptors. It seems likely that modulation by glycine of the RBC center response may be in part ascribed to a consequence of the potentiation of rod responses by glycine.Moreover, effects of glycine on cone-dominant bipolar cells (CBCs) were studied in isolated, superfused carp retina by intracellular recording techniques. Glycine of 4 mM depolarized ON-type CBCs and reduced their light responses, which was blocked by DCKA, but not by strychnine. Glycine of 4 mM hyperpolarized OFF-type CBCsand hyperpolarizing light responses effects that were reversed by strychnine. We conclude that glycine modulates ON- and OFF-type cone-dominant bipolar cells through activation of different subtypes of glycine receptors.