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L. Cadetti, W. B. Thoreson; Quantal and Non-Quantal Contributions of Glutamate to Shaping Responses at the Photoreceptor Synapse. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3225.
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Convolution of miniature excitatory post-synaptic currents (mEPSCs) with the cone release function matches the actual cone-driven EPSC while convolving mEPSCs with the rod release function predicts a faster response than the actual EPSC, suggesting the presence of a non-quantal component (Cadetti et al, 2005) . We tested the contributions of quantal mEPSCs and non-quantal residual glutamate to light evoked currents (LECs) in horizontal cells (HCs) and EPSCs evoked by direct depolarization of rods and cones.
LECs and depolarization-evoked EPSCs were recorded from HCs and bipolar cells while simultaneously recording from a rod or cone in the salamander retinal slice preparation.
Rod-driven EPSCs involve fast and slow components. A low affinity glutamate receptor antagonist, kynurenic acid (KynA, 0.25 mM), induced an outward DC shift and preferentially reduced the slow component of the EPSC. Conversely, the high affinity antagonist NBQX (0.1 µM) reduced both components equally. The effects of KynA suggest that glutamate levels attained late in the EPSC are lower than those attained during the initial faster component. A glutamate-scavenging enzyme, GPT (glutamic-pyruvic transaminase), also caused an outward current and accelerated decay of the rod-driven EPSC. Conversely, the glutamate uptake inhibitor, TBOA (10 µM), induced an inward current and slowed EPSC decay. KynA, GPT, and TBOA did not alter mEPSC kinetics, indicating that changes in EPSC kinetics reflect non-quantal contributions of residual synaptic glutamate. Consistent with the conclusion the cone-driven EPSCs are largely quantal, these same drugs did not affect cone-driven EPSC kinetics. Both rod and cone dominated LECs were more strongly inhibited by KynA than EPSCs suggesting lower glutamate levels are attained during LECs. LECs were also significantly reduced by GPT even though GPT had no significant effect on EPSC or mEPSC amplitude. Conversely, TBOA enhanced LECs far more than it enhanced the EPSC or mEPSC amplitude.
Comparisons of drug effects on EPSCs and mEPSCs suggest that cone-driven EPSCs and the initial component of rod-driven EPSCs are largely quantal but there is an increasing contribution of residual glutamate to later portions of the EPSC. Drugs that preferentially enhance (TBOA) or reduce (KynA and GPT) residual glutamate levels exert significantly stronger effects on LECs than EPSCs. These results indicate that LECs in horizontal cells are less quantal than synaptic currents evoked by direct depolarization of rods and cones.
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