Non-linear, high-gain and sustained-to-transient signal transmission from rods to amacrine cells in dark-adapted retina of Ambystoma.

In darkness, On-Off amacrine cells (ACs) of the tiger salamander retina exhibited large spontaneous transient depolarizing potentials (sTDPs) with average peak amplitude of 5.05 +/- 2.5 mV and average frequency of 0.42 +/- 0.25 s(-1). Under voltage-clamp conditions the cell displayed large spontaneous postsynaptic currents (sPSCs) with average peak amplitude of 98 +/- 39 pA and average frequency of 0.45 +/- 0.22 s(-1). To a light step, ACs gave rise to a transient 'On' response at the light onset and a transient 'Off' response at light offset, followed by a train of TDPs ('After' response). Near the response threshold (0.3 activated rhodopsin molecules per rod per second), light-evoked TDPs (leTDPs) of similar amplitude and kinetics as the large sTDPs observed in darkness were seen, and about half of these leTDPs elicited a regenerative potential (RP). Brighter light steps gave rise to more leTDPs and higher rates of RPs in the On, Off and After responses. Within the linear response range of the rods, the AC response was non-linear, with the highest gain (676 +/- 429) near the dark potential. The amplitude of Off responses increased with the duration of the light step, and ACs may use this to encode speeds of moving stimuli: the faster the light object moves, the smaller the AC Off response. Moreover, the number of leTDPs in the AC After response increased with light intensity, and the onset of the After response coincides with bipolar cell tail response recovery. One possible origin of the large sTDPs and leTDPs is the spontaneous and depolarization-induced regenerative calcium potentials (RCaPs) in bipolar cell synaptic terminals. RCaPs in bipolar cell synaptic terminals cause transient glutamate release that results in the sTDPs in darkness, and leTDPs in On, Off and After responses in ACs.