γ-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 GABAA and GABACreceptors 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 GABAA receptor antagonist, and imidazole-4-acetic acid (I4AA), a GABAC 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 GABAA and GABACreceptors. Subcellular localization and complements of GABAA and GABAC receptors at the dendrites and axon terminals were highly related to the dichotomy of offand on BCs. In the case of off BCs, GABAA receptors were rather evenly distributed at the dendrites and axon terminals, but GABAC receptors were predominantly expressed at the axon terminals. Moreover, the relative contribution of GABAC receptors to the axon terminals was prevalent over that of GABAA receptors, while the situation was reversed at the dendrites. In the case of on BCs, GABAA and GABAC 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 GABAC receptors were much less expressed than GABAA receptors. GABAA, but not GABAC receptors, were expressed clusteringly at axons of a population of BCs. In a minority of BCs, I4AA suppressed the GABAC responses at the dendrites, but not at the axon terminal, implying that the GABAC receptors at these two sites may be heterogeneous. Taken together, these results suggest that GABAA and GABAC 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.
Functional Architecture of Synapses in the Inner Retina: Segregation of Visual Signals by Stratification of Bipolar Cell Axon Terminals
