PHYSIOLOGY OF THE A1 AMACRINE - A SPIKING, AXON-BEARING INTERNEURON OF THE MACAQUE MONKEY RETINA

We characterized the light response, morphology, and receptive-field s tructure of a distinctive amacrine cell type (Dacey, 1989), termed her e the A1 amacrine, by applying intracellular recording and staining me thods to the macaque monkey retina in vitro. A1 cells show two morphol ogically distinct components: a highly branched and spiny dendritic tr ee, and a more sparsely branched axon-like tree that arises from one o r more hillock-like structures near the soma and extends for several m illimeters beyond the dendritic tree. Intracellular injection of Neuro biotin reveals an extensive and complex pattern of tracer coupling to neighboring A1 amacrine cells, to two other amacrine cell types, and t o a single ganglion cell type. The A1 amacrine is an ON-OFF cell, show ing a large (10-20 mV) transient depolarization at both onset and offs et of a photopic, luminance modulated stimulus. A burst of fast, large -amplitude (similar to 60 mV) action potentials is associated with the depolarizations at both the ON and OFF phase of the response. No evid ence was found for an inhibitory receptive-field surround. The spatial extent of the ON-OFF response was mapped by measuring the strength of the spike discharge and/or the amplitude of the depolarizing slow pot ential as a function of the position of a bar or spot of light within the receptive field. Receptive fields derived from the slow potential and associated spike discharge corresponded in size and shape. Thus, t he amplitude of the slow potential above spike threshold was well enco ded as spike frequency. The diameter of the receptive field determined from the spike discharge was similar to 10% larger than the spiny den dritic field. The correspondence in size between the spiking receptive field and the spiny dendritic tree suggests that light driven signals are conducted to the soma from the dendritic tree but not from the ax on-like arbor. The function of the axon-like component is unknown but we speculate that it serves a classical output function, transmitting spikes distally from initiation sites near the soma.