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J. Snellman, S. Nawy; Cyclic GMP Increases the Efficacy of Rod Bipolar Cell Release by Enhancing the Rate of Calcium Entry . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1512.
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We have previously shown that cGMP, as well as a nitric oxide donor, increases rod bipolar cell (RBC) sensitivity to small decreases in mGluR6 agonist, while sensitivity to larger decreases in transmitter remains constant. This increase in sensitivity is carried on to the post–synaptic AII amacrine cell (AII), resulting in enhanced amplitudes. Here, we present evidence that amplification of AII EPSCs results from an increase in the rate of RBC depolarization in the presence of cGMP.
Pairs of RBCs and AIIs were recorded in whole cell configuration from slices of mouse retina. RBCs were stimulated with ramps of physiological amplitudes (15 – 20 mV) and rates of rise (0.05 – 4 mV/ms), while simultaneously monitoring the RBC calcium current and the AII EPSC. When assaying the effects of cGMP, the mGluR6 agonist L–AP4 was included in the bath. RBCs were current–clamped and stimulated with focal puffs of the mGluR6 antagonist CPPG. AII responses were simultaneously monitored in voltage–clamp. cGMP was included in the RBC recording pipette or added in the bath solution.
Fast ramps (4 – 0.8 mV/ms) were as efficient as a voltage step in eliciting large amplitude AII EPSCs. Decreasing the ramp speed elongated EPSCs and incrementally reduced their amplitude despite an overall increase in the RBC calcium current. We found that fast ramps resulted in a significantly faster calcium influx than the slower ramps, and that there was a strong positive correlation between AII EPSC amplitude and the rate of calcium influx. In response to CPPG stimulation, the rising phase of current–clamped RBC responses was significantly sped up by intracellular addition of cGMP. Simultaneous recordings from synaptically connected AIIs showed that, as the RBC depolarization increased in speed, AII EPSCs grew in amplitude.
In addition to potentiating RBC responses, cGMP increases their rate of rise. We show an increase in the rising rate of RBC responses affects the speed of calcium entry into the synaptic terminal. Our experiments indicate that the amount of calcium entering the cell within the first 20 ms after response onset is an important indicator of release efficacy. Fast RBC depolarizations resulted in a larger initial influx of calcium than slow depolarizations and elicited larger AII EPSC amplitudes. These findings suggest that the speed of the RBC response is critical in determining efficacy of transmitter release, and that this is the mechanism by which cGMP–induced increases in RBC sensitivity are transferred to the inner retina.
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