Expression of GABA transporters on bullfrog retinal Müller cells

gamma-Aminobutyric acid (GABA) transporters (GATs) are involved in regulating GABAergic transmission through the high-affinity uptake and release of GABA. In the present work, the expression of different subtypes of GATs (GAT-1, GAT-2, and GAT-3) on Müller cells was investigated both immunocytochemically and electrophysiologically in the bullfrog retina. Double-labeling experiments of glial fibrillary acidic protein (GFAP), a specific marker for Müller cells, and GATs showed that the somata, major processes, endfeet, and even branchlets of almost all Müller cells strongly expressed GAT-1 on the membrane, whereas the main trunks and endfeet of about 80-90% Müller cells were moderately GAT-2 labeled. No GAT-3 immunoreactivity was observed in Müller cells. Meanwhile, using the whole-cell recording technique, GAT-mediated currents were recorded from Müller cells in the retinal slice preparation, and they were reversibly blocked by removal of extracellular Na(+) and spread more or less evenly over the surface of these cells. Furthermore, the GATs on the Müller cells were sensitive to both nipecotic acid and beta-alanine, GABA uptake blockers. Taken together, the complementary immunocytochemical and electrophysiological results suggest that bullfrog Müller cells express functional GAT-1 and GAT-2, which may regulate GABAergic transmission by either taking up or releasing GABA, or both.