RELATIONSHIP BETWEEN RECEPTIVE AND DENDRITIC FIELD SIZE OF AMACRINE CELLS IN THE RABBIT RETINA

1. Intracellular recordings were obtained from 40 amacrine cells in the isolated, superfused retina eyecup of the rabbit. Cells were subsequently labeled with horseradish peroxidase for morphological identification. Many of these cells displayed dendritic morphology consistent with that of amacrine cells described in prior anatomic studies, including starburst, A17, AII, and DAPI-3 cells. 2. The center receptive field of amacrine cells was measured with a 50- or 95-mum-wide, 6.0-mm-long rectangular slit of light that was displaced along its minor axis (parallel to the visual streak) in increments as small as 3 mum. The extent of the receptive field was calculated as the total distance over which the displaced slit could evoke a center response. Area summation of amacrine cells was measured with concentric spots of light with increasing diameters centered over the cell. 3. For a single amacrine cell, the receptive field size was comparable to the extent of its dendritic arbor. For the total population of amacrine cells, there was a strong, linear relationship between receptive field and dendritic field size. The receptive fields were, on average, 27% larger than the corresponding dendritic arbors, but this discrepancy can be accounted for entirely by tissue shrinkage associated with histological processing and a small imprecision of the light stimuli. Area summation measurements were consistent with those of receptive fields and were also related linearly to the dendritic field size of cells. 4. These findings indicate that even when the slit of light was placed at the distal edges of the dendritic arbor, synaptic inputs activated there were propagated effectively to the soma and recorded by microelectrodes placed there. In addition, amacrine cells were capable of summating synaptic inputs distributed throughout the entire arbor. 5. These results are inconsistent with the findings of prior computational modeling studies of passive, dendritic current flow in A17 and starburst amacrine cells that synaptic inputs on distal dendritic branches are isolated electrically from the soma and that these branches form autonomous, functional subunits. 6. The majority of amacrine cells encountered displayed light-evoked and/or spontaneous action potentials. These action potentials often took the form of high-amplitude somatic and low-amplitude dendritic spikes. On average, spiking amacrine cells showed considerably larger dendritic fields than nonspiking amacrine cells. In fact, all amacrine cells with arbors > 436 mum, which formed 45% of the total population, displayed spike activity. These data suggest that spike generation is a feature common to most medium- and large-field amacrine cells, permitting active propagation of synaptic currents over distances often > 1 mm. 7. Thus the discrepancy between the present electrophysiological data and those of prior modeling studies may be explained by the fact that synaptic inputs to most amacrine cells are not propagated passively within the arbor but rather by an active, regenerative process.