Calcium‐induced calcium release in rod photoreceptor terminals boosts synaptic transmission during maintained depolarization

Abstract

We examined the contribution of calcium-induced calcium release (CICR) to synaptic transmission from rod photoreceptor terminals. Whole-cell recording and confocal calcium imaging experiments were conducted on rods with intact synaptic terminals in a retinal slice preparation from salamander. Low concentrations of ryanodine stimulated calcium increases in rod terminals, consistent with the presence of ryanodine receptors. Application of strong depolarizing steps (−70 to −10 mV) exceeding 200 ms or longer in duration evoked a wave of calcium that spread across the synaptic terminals of voltage-clamped rods. This secondary calcium increase was blocked by high concentrations of ryanodine, indicating it was due to CICR. Ryanodine (50 μM) had no significant effect on rod calcium current (I ca ) although it slightly diminished rod light-evoked voltage responses. Bath application of 50 μM ryanodine strongly inhibited light-evoked currents in horizontal cells. Whether applied extracellularly or delivered into the rod cell through the patch pipette, ryanodine (50 μM) also inhibited excitatory post-synaptic currents (EPSCs) evoked in horizontal cells by depolarizing steps applied to rods. Ryanodine caused a preferential reduction in the later portions of EPSCs evoked by depolarizing steps of 200 ms or longer. These results indicate that CICR enhances calcium increases in rod terminals evoked by sustained depolarization, which in turn acts to boost synaptic exocytosis from rods.

Kinetics of Synaptic Transmission at Ribbon Synapses of Rods and Cones

“…Antibodies to L-type calcium channels produce punctate labeling at the base of the ribbon that co-localizes with synaptic proteins such as bassoon and ribeye [85,128,[133][134][135]. This is consistent with physiological results demonstrating highly localized sites of calcium influx at the base of the photoreceptor terminal [136]. Freeze fracture electron micrographs show ~500 transmembrane particles clustered beneath the arciform density of each ribbon [60,137].…”

“…In rods, lengthening the step duration to 200 ms or greater stimulated a secondary wave of calcium that spread across the synaptic terminal. This secondary wave was blocked by high concentrations of ryanodine introduced into the cell through the patch pipette, suggesting it was mediated by ryanodine receptors and thus resulted from CICR [136].…”

“…Given the presence of release mechanisms with an unusually high affinity for calcium in rod terminals, it is thus perhaps no surprise that the release of calcium from ryanodine-sensitive intracellular stores is also capable of stimulating glutamate release from rods [136,159,160]. In paired recordings from rods and post-synaptic neurons, blocking CICR with high concentrations of ryanodine preferentially reduced later portions of the depolarization-evoked EPSC [136,160]. Consistent with the finding that CICR significantly boosts intraterminal calcium levels when rods are depolarized for at least 200 ms, ryanodine detectably inhibited PSCs evoked by depolarizing steps maintained for at least 200 ms [136].…”

“…In paired recordings from rods and post-synaptic neurons, blocking CICR with high concentrations of ryanodine preferentially reduced later portions of the depolarization-evoked EPSC [136,160]. Consistent with the finding that CICR significantly boosts intraterminal calcium levels when rods are depolarized for at least 200 ms, ryanodine detectably inhibited PSCs evoked by depolarizing steps maintained for at least 200 ms [136]. CICR amplifies and sustains synaptic output from rods, and thereby, contributes to the slower decay of rod-driven EPSCs observed in paired recordings.…”

“…Activation of ryanodine receptors with caffeine or low concentrations of ryanodine stimulates the release of calcium from intracellular stores in the inner segments and synaptic terminals of rods ( Fig. 2; [136,158,159]). Brief depolarizing steps (<200 ms, −70 to −10 mV) applied to voltage clamped rods and cones produce highly localized calcium increases due to influx through calcium channels clustered close to the ribbons.…”