Electrical coupling and passive membrane properties of AII amacrine cells

AII amacrine cells in the mammalian retina are connected via electrical synapses to on-cone bipolar cells and to other AII amacrine cells. To understand synaptic integration in these interneurons, we need information about the junctional conductance (g(j)), the membrane resistance (r(m)), the membrane capacitance (C(m)), and the cytoplasmic resistivity (R(i)). Due to the extensive electrical coupling, it is difficult to obtain estimates of r(m), as well as the relative contribution of the junctional and nonjunctional conductances to the total input resistance of an AII amacrine cell. Here we used dual voltage-clamp recording of pairs of electrically coupled AII amacrine cells in an in vitro slice preparation from rat retina and applied meclofenamic acid (MFA) to block the electrical coupling and isolate single AII amacrines electrically. In the control condition, the input resistance (R(in)) was approximately 620 Mohms and the apparent r(m) was approximately 760 Mohms. After block of electrical coupling, determined by estimating g(j) in the dual recordings, R(in) and r(m) were approximately 4,400 Mohms, suggesting that the nongap junctional conductance of an AII amacrine cell is approximately 16% of the total input conductance. Control experiments with nucleated patches from AII amacrine cells suggested that MFA had no effect on the nongap junctional membrane of these cells. From morphological reconstructions of AII amacrine cells filled with biocytin, we obtained a surface area of approximately 900 microm(2) which, with a standard value for C(m) of 0.01 pF/microm(2), corresponds to an average capacitance of approximately 9 pF and a specific membrane resistance of approximately 41 kohms cm(2). Together with information concerning synaptic connectivity, these data will be important for developing realistic compartmental models of the network of AII amacrine cells.