Modulation Of γ-aminobutyric Acid Transporters By Activation Of Glutamate Receptors In Carp Retinal Horizontal Cell

Horizontal cells are interneurons in the outer retina, which receive glutamate input from photoreceptors, and release GABA via GABA transporters. Horizontal cells are responsible for the formation of the antagonistic surround receptive field property of the retinal bipolar cells and ganglion cells. Glutamate can activate glutamate receptors on horizontal cells, including AMPA receptors and NMDA receptors. It has been reported that activation of AMPA receptors inhibits GABA transporter currents on skate retinal horizontal cells. We have observed similar inhibition on carp horizontal cells using patch-clamp technique. Recently, it was reported that functional NMDA receptors are expressed in carp retinal horizontal cells, but the relevant functions of NMDA receptors in the horizontal cells are not fully understood. We consequently explored whether activation of NMDA receptors on horizontal cells could also alter the activity of GABA transport in these cells. Our results show that activation of NMDA receptors inhibited GABA transporter currents on carp retinal horizontal cells. Application of AP5 (100μM) eliminated the inhibition, which indicates NMDA receptors are necessary. When the extracellular Ca2+ was removed from the Ringer's solution, the NMDA inhibitory effect on the GABA transporter current was eliminated. The suppression effect could be attenuated when the Ca2+ release and Ca2+ uptake of intracellular Ca2+ store were blocked after the cell had been pre-incubated with 20μM ryanodine plus 2μM thapsigargin for 3 min. After achieving the whole-cell configuration, the retinal horizontal cell was loaded with 10 mM BAPTA by diffusion from the patch pipette for 5 min, and then the NMDA modulation of GABA transporters was suppressed. These results suggest that the activation of NMDA receptors inhibits GABA transporter-mediated current by affecting Ca2+ processes in the retinal horizontal cells. Furthermore, a model containing endoplasmic reticulum (ER) membrane processes and plasma membrane processes was constructed for quantitative analyses of the NMDA-triggered Ca2+ dynamics and its modulation of GABA transporters. The model results suggest that the intracellular Ca2+ store is involved in the NMDA modulation of GABA transporters.It is reported that NMDA receptors are expressed in retinal horizontal cells of carp, rat and human, but physiological function of the NMDA receptors is not clear. In present study, our results show that activation of NMDA receptors in carp retinal horizontal cells inhibit GABA transporter current. Under physiological conditions, photoreceptors release glutamate continually in the darkness. AMPA receptors and NMDA receptors on the retinal horizontal cells are activated, and extracellular calcium influx can occur through these glutamate receptors. Since AMPA receptors display very rapid and significant desensitization, and NMDA receptors do not desensitize as quickly or as fully, NMDA receptors might play a significant role in the regulation of GABA transporter activities under physiological conditions.