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B. A. Szmajda, S. H. DeVries; Glutamate Mediates Cross-Talk Between Mammalian Cone Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2009;50(13):6148.
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Independent signaling by the tightly-packed foveal cones is important for high acuity and chromatic vision. Cone terminals are ensheathed by Müller glial cell processes, which presumably act to limit the lateral diffusion of glutamate. Cones, however, may potentially release up to 400 docked vesicles (20 ribbons, with 20 vesicles docked per ribbon) during repolarization after a light step. We used the cone glutamate transporter Cl- conductance to test whether the glutamate released by one cone could spill-over to neighboring cones.
Patch-clamp studies were done in the cone-dominant ground squirrel. Adjacent cones were voltage-clamped in a slice or wholemount preparation. One cone, the ‘donor’, was stepped from -70 mV to 0 mV to elicit glutamate release. We measured the current response in an adjacent ‘acceptor’ cone held at a constant voltage, typically -70 mV. Patch pipette solutions contained Gramicidin D during experiments in which it was necessary to maintain the intracellular Cl- concentration. Values are expressed as mean ± S.D.
Stimulating a cone to release glutamate elicited a two-component response in a neighboring, unstimulated cone: a fast response component, resulting from cone-cone electrical coupling, and a slow component. The slow component could be blocked in three ways: by Co2+ and Cd2+-containing external solution (n = 3), when Cs+ was substituted for Na+ in the external solution (n = 2), or by glutamate transporter blockers (TBOA: 76.0 ± 9.7% block, n = 11; THA: 95.6 ± 9.8%, n = 5). The results suggest that the slow component is mediated by glutamate ‘spillover’ from the donor cone, causing the activation of a glutamate transporter Cl- conductance in the acceptor cone. In experiments with matched internal and external Cl-, acceptor cone currents had mean amplitudes of -8.5 ± 7.2 pA (range = 0-31 pA, n = 39), 20-80 rise times of 5.9 ± 1.9 ms, and were often bidirectional. Acceptor cone currents were increased by four to eight-fold when SCN- was substituted for Cl- in the pipette solution. The transporter current reversed at -26.8 ± 14.0 mV (n = 10).
We have observed a cross-talk between cone photoreceptors that is mediated by a glutamate transporter Cl- conductance. A transporter reversal potential of approximately -30 mV is consistent with an excitatory role for cone-cone chemical transmission.
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