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Matthew J Van Hook, Wallace B Thoreson; Low endogenous calcium buffering conditions enhance tonic but not phasic synaptic transmission by distinct mechanisms in rod and cone photoreceptors. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):5513.
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© ARVO (1962-2015); The Authors (2016-present)
Ca2+ acts by several mechanisms to regulate fast and slow phases of synaptic transmission by rod and cone photoreceptors. Endogenous Ca2+ buffering is known to be relatively low in photoreceptor synaptic terminals yet the consequences for synaptic transmission are unknown. The goal of this study was to determine how these buffering conditions influence Ca2+ dynamics and synaptic transmission by rods and cones.
We performed whole-cell or perforated patch recordings of rod and cone photoreceptors and post-synaptic horizontal cells (HCs) in vertical slices tiger salamander retinas. Endogenous Ca2+ buffering was determined by comparing the V50 of the Ca2+-activated chloride current (ICl(Ca)) when photoreceptors were dialyzed with EGTA (5, 0.5, or 0.05 mM) or were recorded in perforated patch. Release efficiency - vesicles (v) released per Ca2+ channel opening - was measured using a deconvolution approach and the spatiotemporal dynamics of Ca2+ signals under different buffering conditions were measured using confocal Ca2+ imaging.
Measurements of ICl(Ca) V50 confirmed that Ca2+ buffering is equivalent to ~0.05 mM EGTA in rod and cone terminals. Efficiency of immediately releasable pool release (~0.3 v/channel in cones and ~0.2 v/channel in rods) was not significantly affected in different buffering conditions in either rods or cones. Confocal Ca2+ imaging showed that under conditions mimicking endogenous buffering, Ca2+ spreads throughout rod and cone terminals, causing a large, spatially unconstrained increase in [Ca2+]. Inhibiting Ca2+-induced Ca2+ release (CICR) with dantrolene (50 μM) reduced the amplitude of the Ca2+ signals in rods but not cones. In rod-HC paired recordings, low Ca2+ buffering boosted release during short depolarizations, an effect that was blocked by dantrolene. No similar enhancement or effect of dantrolene was seen in cone-HC recordings. However, stimulation with depolarizing pulse trains (13 Hz) indicated that the Ca2+-dependent replenishment rate (EPSC cumulative charge transfer slope) was enhanced under lower Ca2+ buffering conditions in cones.
The weak Ca2+ buffering in rods and cones contributes to slower kinetic components of exocytosis by enhancing both non-ribbon release triggered by CICR in rods and Ca2+-dependent replenishment of ribbons.
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