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F. Gao, J.-J. Pang, S. M. Wu; Sign-preserving and Sign-inverting Rod-cone Interactions in the Salamander Retina. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1047.
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© ARVO (1962-2015); The Authors (2016-present)
The objective is to determine the strength of electrical coupling between rods, between rods and single cones and between rods and double cones, and to characterize the sign-inverting signaling pathway from rods to cones.
Dual whole-cell voltage clamp recordings were made from pairs of photoreceptors in dark-adapted, flat-mounted salamander retinas. Simultaneous voltage clamp recordings were also made from pairs of rods and horizontal cells in living retinal slices.
A voltage step from -40 mV to -120 mV in a rod elicited a 120 pA sustained outward current in a next-neighbor rod, a 10 pA sustained outward current in a next-neighbor single cone, and a 5 pA sustained outward current in a next-neighbor double cone. A voltage step from -40 mV to +40 mV in the rod elicited sustained inward currents of the same amplitude. Reversal of driver/follower cells in these experiments generated identical currents, suggesting gap junctions between salamander photoreceptors are symmetrical and ohmic. In addition to the sign-preserving sustained currents, voltage steps in rods evoked sign-inverting transient currents at the step onset (with positive voltage steps) or offset (with negative voltage steps) in single and double cones, and sign-preserving transient currents of the same waveform in horizontal cells (HCs). Voltage steps in cones, however, did not evoke such transient currents in rods. This is presumably because the rod→cone sign-inverting transient current is thought to be mediated by the HC→cone feedback synapse (rod→HC→cone). The reversal potential of the rod→cone sign-inverting transient current shifted with ECl, and this current can be blocked by bath application of protons, DNQX and TBOA. Protons and DNQX blocked the rod→HC transient current by hyperpolarizing the HCs, and TBOA did not alter the rod→HC signal.
Our results suggest that in the salamander retina, rod-rod and rod-cone couplings are robust and symmetrical, and rod-rod coupling is about 10-25 times stronger than rod-cone coupling. The rod→cone sign-inverting transient signal is one way, consistent with the notion that HCs make negative feedback synapses on cones but not on rods. The HC→cone feedback signal is at least partially mediated by a TBOA-sensitive chloride conductance in cones.
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