Proton-Mediated Feedback Inhibition of Presynaptic Calcium Channels at the Cone Photoreceptor Synapse

Abstract

Generation of center-surround antagonistic receptive fields in the outer retina occurs via inhibitory feedback modulation of presynaptic voltage-gated calcium channels in cone photoreceptor synaptic terminals. Both conventional and unconventional neurotransmitters, as well as an ephaptic effect, have been proposed, but the intercellular messaging that mediates the inhibitory feedback signal from postsynaptic horizontal cells (HCs) to cones remains unknown. We examined the possibility that proton concentration in the synaptic cleft is regulated by HCs and that it carries the feedback signal to cones. In isolated, dark-adapted goldfish retina, we assessed feedback in the responses of HCs to light and found that strengthened pH buffering reduced both rollback and the depolarization to red light. In zebrafish retinal slices loaded with Fluo-4, depolarization with elevated K ϩ increased Ca signals in the synaptic terminals of cone photoreceptors. Kainic acid, which depolarizes HCs but has no direct effect on cones, depressed the K ϩ -induced Ca signal, whereas CNQX, which hyperpolarizes HCs, increased the Ca signals, suggesting that polarization of HCs alters inhibitory feedback to cones. We found that these feedback signals were blocked by elevated extracellular pH buffering, as well as amiloride and divalent cations. Voltage clamp of isolated HCs revealed an amiloride-sensitive conductance that could mediate modulation of cleft pH dependent on the membrane potential of these postsynaptic cells.

Carbonic anhydrase XIV deficiency produces a functional defect in the retinal light response

“…In both the intact rat retina studied in vivo and in isolated rat retina, acetazolamide inhibition of CA attenuates the rod and cone b-wave (22). Extracellular acidification suppresses photocurrents in rod photoreceptors (23) and synaptic transmission between photoreceptors and second-order retinal neurons (24,25). Our observations that the a-and b-waves of the ERG are reduced in the CA XIV KO mouse are consistent with the findings in these extracellular acidification studies and suggest a significant role for CA XIV in regulation of extracellular pH and pCO 2 in the retina.…”

Protons are a neurotransmitter that regulates synaptic plasticity in the lateral amygdala

“…Although Ca 2+ channel blockers could have multiple effects, these results are consistent with proton release from synaptic vesicles as a source of the reduced pH. However, it is possible that other transport processes make important contributions to acidification (4)(5)(6)(7)(8).…”

Proton production, regulation and pathophysiological roles in the mammalian brain

“…Synaptic Ca 2+ currents are always mediated by more than one kind of VGCC. In mammalian photoreceptor synapses from the cone cells [8] , bipolar cells [95] and horizontal cells [96] , protons released into the cleft mediate negative feedback to presynaptic Ca 2+ channel activity. Increasing the proton buffering capacity in the perfusion solution reduces the inhibitory effect, as does alkalization (Fig.…”

“…The authors changed the proton-buffering capacity simply by varying the concentrations of HEPES in the perfusion solution, a method that was validated in the study of endogenous proton modulation of presynaptic VGCCs [8,95,96] .…”

“…Interestingly, another amiloride-sensitive synaptic acidification mechanism has been reported to play an inhibitory role in VGCC activity [96] . These results provide evidence for endogenous synaptic acidification and its influence on synaptic function under pathophysiological conditions.…”