Synaptic Cleft Acidification and Modulation of Short-Term Depression by Exocytosed Protons in Retinal Bipolar Cells

Abstract

The release of vesicular protons during exocytosis causes a feedback inhibition of Ca2+ channels in photoreceptor terminals; however, the effect of this inhibition on subsequent exocytosis has not been studied. Here we show that a similar L-type Ca2+ channel inhibition occurs in bipolar cell terminals in slices of goldfish retina, and we investigate the effect that this has on subsequent exocytosis with membrane capacitance measurements. We find that transient Ca2+ current inhibition is correlated with exocytosis and modulated by the concentration of extracellular pH buffer. Ca2+ current inhibition is negligible in acutely dissociated terminals, demonstrating the importance of an intact synaptic cleft. The sensitivity of bipolar cell Ca2+ currents to extracellular pH was assessed: channel conductance is reduced and activation is shifted to more positive potentials by acidification. The effect of Ca2+ current inhibition on subsequent exocytosis was investigated by measuring paired-pulse depression. Under conditions in which there is a large amount of inhibition of Ca2+ influx, the degree of paired-pulse depression is significantly reduced. Finally, we show that under physiological (bicarbonate) buffering conditions, pronounced Ca2+ current inhibition occurs after exocytosis ( approximately 60% peak inhibition), which can decrease subsequent exocytosis during single depolarizations. We estimate that exocytosis is accompanied by a transient change in synaptic cleft pH from 7.5 to approximately 6.9. We suggest that this effect serves as an activity-dependent modulator of exocytosis at ribbon-type synapses where a large and compact coterie of vesicles can fuse at each active zone.

Calmodulin enhances ribbon replenishment and shapes filtering of synaptic transmission by cone photoreceptors

“…These findings are also roughly consistent with calculations by Holt et al (2004), who predicted a rate of 1 collision per attachment site every 2.8 s. Slightly faster values for replenishment time constants at BCs have also been reported (Gomis et al, 1999;Palmer et al, 2003). In RBCs, measurements of  = 38 nm and  = 1933 v/µm 3 (Graydon et al, 2014), used with D = 0.015 µm 2 /s from goldfish BCs (Holt et al, 2004), give a predicted replenishment time constant of 908 ms, which is slower than the measured fast kinetic component of recovery from paired pulse protocols ( = 400 ms; Singer and Diamond, 2006).…”

“…In goldfish BCs, recovery from synaptic depression is Ca 2+ dependent (Gomis et al, 1999) and proceeds by two time constants with  fast = 0.6-1 s and  slow = 12-30 s (Gomis et al, 1999;Palmer et al, 2003), very similar to our findings in salamander cones. Recovery is no such mechanism appears to operate in cones as fluorescence recovery after photobleaching measurements indicate that Ca 2+ does not alter vesicle mobility (Rea et al, 2004).…”

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] .…”