Vesicular acidification modulates the synaptic current: a hybrid diffusion reaction model analysis

Abstract

Glutamate synaptic vesicles co-release protons, producing a brief acidification of the synaptic cleft that could modulate AMPA receptor (AMPARs) operation. To evaluate the extent of receptor acidification, we develop a diffusion-reaction model that couples vesicle-evoked proton and glutamate transients to AMPAR dynamics. Our simulations reveal that the rapid diffusion of protons and glutamate within the flat-cylindrical synaptic cleft leads to a mixture of protonated, singly glutamate-bound and doubly glutamate-bound AMPARs. We studied four postsynaptic AMPAR distributions - uniform disk, sub-disk, Gaussian cluster, and point-like cluster - and showed a [~] 50% increase in the number of acidified receptors when AMPARs are clustered on the postsynaptic cleft compared to a uniform arrangement. We further explored the impact of pH revealing that at acidic conditions (pH [~] 5), approximately 80-90% of open receptors are non-acidified, whereas under strongly acidic conditions (pH [~] 3), about 80-90% of open receptors exist in the protonated form. Finally, we explored how acidification modulates AMPARs during paired-pulse stimulation, a measure of short-term synaptic depression. While the presence of protons does not markedly alter the overall trends, acidified receptor states reduce the occupancies of their neutral counterparts by roughly 10-15%, indicating a mild redistribution toward protonated receptor conformations. To conclude, our model suggests that AMPAR protonation can influence the synaptic current, and we predict that this effect is determined primarily by the dissociation kinetics of glutamate and protons from AMPAR.