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M.A. Freed; Spike Bursts in Sluggish Ganglion Cells Persist During Visual Stimulation . Invest. Ophthalmol. Vis. Sci. 2003;44(13):5195.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: A ganglion cell’s spontaneous discharge exhibits bursts of 2 or 3 spikes lasting 10 – 40 ms that constitute a clear departure from Poisson statistics. Spontaneous EPSCs (sEPSCs) exhibit non-Poisson bursts of similar duration, which can be suppressed by the GABAC antagonist (1,2,5,6-Tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA). I investigated whether spike bursts persist during visual stimulation, and if so, whether TPMPA suppresses them. Methods: Mammalian ganglion cells were recorded extracellularly in the tight-seal mode and presented with sinusoidally modulated light [5 log(photons µm-2 s-1, 460 ± 20 nm, 60% contrast, 1 – 4 Hz). Afterwards, a cell was filled with Lucifer Yellow and identified as sluggish or brisk by morphology, frequency selectivity, and peak spike rate. An autocorrelogram was calculated as the lag time between any given spike and all other spikes. A Poisson prediction for the autocorrelogram was calculated from the autocorrelation of the post stimulus time histogram. Results: Autocorrelograms of all cells showed two major departures from the Poisson prediction: (1) all cells had a refractory period which was longer in sluggish than brisk (~20 vs. ~3 ms); (2) some sluggish (15/17) and all brisk cells (7/7) showed spike bursts, which were longer in sluggish (37 ± 12 vs. 12 ± 4 ms). During these intervals, bursts accounted for 22 ± 20% of all spikes in sluggish and 42 ± 26% in brisk. TPMPA (100 µM) suppressed bursts in all sluggish cells tested by 50 ± 30% (n = 7) but in none of the brisk cells tested (n = 4). Conclusions: Spike bursts occur during spontaneous discharge and during visual stimulation. Spike bursts and sEPSC bursts are both blocked by an antagonist to the GABAC receptor, which is predominantly localized on the bipolar cell's axon terminal at amacrine synapses. Thus for sluggish cells, spike bursts may be driven by EPSC bursts and both may result from feedback inhibition.
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