The Journal of Neurophysiology Vol. 84 No. 2 August 2000, pp. 666-676
Copyright ©2000 by the American Physiological Society

Subcellular Localization and Complements of GABA_A and GABA_C Receptors on Bullfrog Retinal Bipolar Cells

Shanghai Institute of Physiology and Key Laboratory of Neurobiology, Chinese Academy of Sciences; and the Institute of Neurobiology, Fudan University, Shanghai 200031, China

Du, Jiu-Lin and Xiong-Li Yang. Subcellular Localization and Complements of GABA_A and GABA_C Receptors on Bullfrog Retinal Bipolar Cells. J. Neurophysiol. 84: 666-676, 2000. -Aminobutyric acid (GABA) receptors on retinal bipolar cells (BCs) are highly relevant to spatial and temporal integration of visual signals in the outer and inner retina. In the present work, subcellular localization and complements of GABA_A and GABA_C receptors on BCs were investigated by whole cell recordings and local drug application via multi-barreled puff pipettes in the bullfrog retinal slice preparation. Four types of the BCs (types 1-4) were identified morphologically by injection of Lucifer yellow. According to the ramification levels of the axon terminals and the responses of these cells to glutamate (or kainate) applied at their dendrites, types 1 and 2 of BCs were supposed to be OFF type, whereas types 3 and 4 of BCs might be ON type. Bicuculline (BIC), a GABA_A receptor antagonist, and imidazole-4-acetic acid (I4AA), a GABA_C receptor antagonist, were used to distinguish GABA receptor-mediated responses. In all BCs tested, not only the axon terminals but also the dendrites showed high GABA sensitivity mediated by both GABA_A and GABA_C receptors. Subcellular localization and complements of GABA_A and GABA_C receptors at the dendrites and axon terminals were highly related to the dichotomy of OFF and ON BCs. In the case of OFF BCs, GABA_A receptors were rather evenly distributed at the dendrites and axon terminals, but GABA_C receptors were predominantly expressed at the axon terminals. Moreover, the relative contribution of GABA_C receptors to the axon terminals was prevalent over that of GABA_A receptors, while the situation was reversed at the dendrites. In the case of ON BCs, GABA_A and GABA_C receptors both preferred to be expressed at the axon terminals; relative contributions of these two GABA receptor subtypes to both the sites were comparable, while GABA_C receptors were much less expressed than GABA_A receptors. GABA_A, but not GABA_C receptors, were expressed clusteringly at axons of a population of BCs. In a minority of BCs, I4AA suppressed the GABA_C responses at the dendrites, but not at the axon terminal, implying that the GABA_C receptors at these two sites may be heterogeneous. Taken together, these results suggest that GABA_A and GABA_C receptors may play different roles in the outer and inner retina and the differential complements of the two receptors on OFF and ON BCs may be closely related to physiological functions of these cells.