Spike initiation and propagation in wide field transient amacrine cells of the salamander retina

Our results suggest that the prominent spike, commonly recorded in wide field amacrine cells, is actively propagated along its processes. Current was passed through a patch pipette at the soma to elicit spike activity in the cell. The field potentials accompanying this spike activity were then measured with an extracellular electrode positioned at different sites along the cell and its processes, which had been made visible with Lucifer yellow. Different extracellular waveforms were measured at the soma, stalk, and cell processes: A monophasic negative-going extracellular voltage waveform, typically found at the site of action potential initiation, was recorded along the stalk between the soma and the radial processes. A biphasic, positive- negative waveform, typically associated with the truncated propagation of an action potential, was measured at the soma. A triphasic, positive- negative-positive extracellular waveform, typically associated with a fully propagated action potential, was recorded along the peripheral processes. The time to peak of this triphasic waveform increased with distance from the soma such that the calculated propagation velocity ranged from 0.5 to 2.5 cm/sec. The membrane regions carrying potassium, sodium, and calcium currents were examined by depolarizing the soma and eliminating different ionic currents in the cell. With only sodium present, extracellular potentials were measured at the stalk and processes, but rarely at the soma. When only potassium was present, extracellular potentials were measured at the soma and processes, but not the stalk. When only calcium channels carried the membrane current, extracellular potentials were measured only at the processes. The sites of different ligand-gated receptors were identified by puffing various transmitter substances at different positions radially along the processes and measuring their effects at the soma. In all cells tested, glutamate puffs elicited currents only when applied at processes within 200 microns of the soma. In some cells, GABA and glycine elicited currents up to 300 microns from the soma. As a control for the measurement of electrotonic spread, potassium puffs elicited depolarizations along a broader region of the processes. These results suggest that the excitatory glutamate-elicited synaptic input to these cells is confined to a narrow area of the processes near the soma. The spike then appears to be initiated at the stalk and propagated along the processes. Calcium currents at these processes suggest the presence of possible transmitter release sites. Thus, each wide field amacrine cell seems to be functionally and concentrically polarized, receiving input centrally near the soma and broadcasting its output by propagating spikes along its extensive processes.