Regenerative amacrine cell depolarization and formation of on‐off ganglion cell response.

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Abstract
Recordings from amacrine and ganglion cells in the mudpuppy [Necturus maculosus] retina suggested mechanisms whereby the relatively slow, sustained light responses measured in bipolar cells were converted to rapid, brief, transient activity in the on-off ganglion cells. Double-barrel electrodes were used to control the membrane potential under voltage clamp. The clamp revealed synaptic currents, but eliminated the otherwise obvious spike activity elicited by steps of illumination in both amacrine and ganglion cells, suggesting that the spikes are initiated near the somata. The synaptic current in the on-off ganglion cells was biphasic: a brief inward (depolarizing) membrane current preceded a transient outward (hyperpolarizing) membrane current by about 20 ms. Each component could be isolated by polarizing the membrane to a level near reversal potential for the other. Each was apparently due to a transient conductance increase of sawtooth shape with a 40 ms time to peak and a decay longer than 400 ms. Synaptic membrane current in amacrine cells was monophasic and inward (depolarizing) of similar sawtooth shape at all potential levels. It was apparently mediated by a conductance increase to ions with a reversal potential more positive than the dark level. When amacrine cells were depolarized in the dark under voltage clamp, a large transient inward membrane current with threshold within 4 mV of the dark level was generated. This regenerative event is capable of boosting a small, 4 mV EPSP [excitatory postsynaptic potential] to > 30 mV in a few milliseconds, thereby generating the leading edge of a rapid sawtooth response. The rapid transient on-off activity in ganglion cells is apparently mediated by opposing sawtooth shaped synaptic currents with different latencies. Each of these antagonistic inputs may be generated by a regenerative depolarization in amacrine cells which then form synaptic inputs to the ganglion cells.