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L. Cadetti, K. Rabl, W.B. Thoreson; Release kinetics help make excitatory post–synaptic currents (EPSCs) evoked by cone inputs more transient than EPSCs evoked by rod inputs. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1336.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:We demonstrate that EPSCs in cone–driven second–order neurons exhibit faster kinetics than EPSCs in rod–driven neurons and that the faster kinetics are largely due to differences in kinetics of synaptic exocytosis. Methods:Miniature and depolarization–evoked EPSCs were recorded from horizontal cells (HCs) or OFF bipolar cells (OFF BPs) while simultaneously recording from a rod or cone using the salamander retinal slice preparation. Results:Application of a depolarizing step (–70 to –10 mV, 200 ms) to a rod evokes EPSCs in OFF BPs or HCs that rise quickly and then decay more slowly (τ = 57.7 + 4.1 ms, n=41 pairs). Applying the same step to a cone produces an EPSC that decays significantly faster (τ = 18.8 + 2.7; n=34 pairs). Differences in EPSC kinetics are not because rods and cones contact different cell types. In 3 experiments we recorded from both a rod and cone contacting the same second order cell and in every case found that the cone–driven EPSCs were much faster than rod–driven EPSCs. We tested whether kinetic differences reflected the clustering of different types of glutamate receptors (GluRs) adjacent to rod and cone terminals. However, both rod– and cone–driven EPSCs were blocked by an AMPA receptor antagonist, GYKI 52466 (3 µM ), and only slightly affected by a KA receptor agonist, SYM 2081 (10–20 µM), or KA antagonist, NS 102 (20 µM). Also, inhibiting AMPA receptor desensitization with cyclothiazide (0.1 mM) slowed EPSC decay in both rod– and cone–driven cells. Thus, rods and cones contact pharmacologically similar AMPA receptors. Using capacitance measurement techniques, we found two kinetic components to exocytosis from rods and cones. The slower time constant for exocytosis was similar in both cell types (∼ 5 s) but the fast component of release from cones was tenfold faster (τ=5.6 ms) than the fast component in rods (τ=45.6 ms). To test whether the faster release kinetics of cones might account for their faster EPSCs, we reconstructed the EPSC by summing a series of mEPSCs with the timing of individual mEPSCs determined by the exponential increase in the rate of release. Linear convolution successfully reconstructed the very different EPSC waveforms in both rod– and cone–driven OFF bipolar cells and in the presence or absence of cyclothiazide. Conclusions:The faster EPSC kinetics in cone–driven cells may allow them to follow higher temporal frequencies than rod–driven cells. The difference in EPSC kinetics does not appear to be due to differences in the types of GluRs contacted by rods and cones but can instead be explained by differences in the kinetics of release from rods and cones.
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