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Xianshi Zhang, Yongjie Li, Steven H DeVries; Light acts directly on S-cone terminals to regulate transmitter release. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5600.
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© ARVO (1962-2015); The Authors (2016-present)
Photoreceptors are functionally and morphologically polarized, with a light transducing mechanism in the outer segment (OS) that communicates by a change in membrane voltage with synaptic release mechanisms at the terminal. In S- or short wavelength light sensitive cones, the opsin is localized to both the OS and synaptic terminal. Based on the synaptic localization of S-opsin, we tested whether light could directly modulate transmitter release at S-cone terminals.
Experiments were performed on slices from the dark-adapted retinas of 13-lined ground squirrels (Ictidomys tridecemlineatus). Whole cell or perforated patch (b-escin) voltage clamp recordings (holding potential = -70 mV unless specified) were obtained from S- and M- (middle wavelength sensitive) cones. Recorded cones were identified as S- or M- prior to an experiment by the ratio of their responses to LED-generated stimuli at 468 and 574 nm, and after an experiment by an antibody to SW1-opsin. Synaptic release was evoked by briefly (1 ms) stepping a cone to -30 mV at 1 s intervals. Release was measured as the amplitude of the feedback current at the cone transporter anion conductance. The terminal Ca current was obtained as a difference current using a voltage ramp from -70 to 20 mV (1 mV/ms) in control and CoCl2 (2 mM) containing solution. For some experiments, the non-hydrolyzable GTP analog GTPγS was included in the pipette solution, and transporter, proton block, and horizontal cell feedback currents were suppressed by TBOA, 15 mM HEPES, and CNQX, respectively.
During whole cell and perforated patch recordings, 468 nm light significantly increased synaptic release in S- but not in M-cones (19.5 ± 2.2%, mean±SEM, n=15 S- and 39 M-cones, p<0.0001). Responses in the two recording conditions did not differ. Including 1 mM GTPγS in the recording pipette increased glutamate release in S- relative to M-cones (80.9 ± 9.3%,n=9 S- and 23 M-cones, p<0.0001). The increase was the same when 10 µM CNQX was added to block horizontal cell feedback and when the OS was ablated. In the presence of GTPγS, the 468 nm stimulus increased the S-cone Ca current without affecting the M-cone current (49.5 ± 2.9%, n=14 S- and 16 M-cones, p<0.0001).
Our results suggest that S-opsin at the S-cone terminal can activate a G protein which increases the terminal Ca current. The increase in Ca current produces an increase in synaptic release.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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