pH Dynamics within the Drosophila Synaptic Cleft During Activity

Acute pH sensitivity of many neural mechanisms highlights the vulnerability of neurotransmission to the pH of the extracellular milieu. The dogma is that the synaptic cleft will acidify upon neurotransmission because the synaptic vesicles corelease neurotransmitters and protons to the cleft, and the direct data from sensory ribbon-type synapses support the acidification of the cleft. However, ribbon synapses have a much higher release probability than conventional synapses, and it’s not established whether conventional synapses acidify as well. To test the acidification of the cleft in the conventional synapse, we used genetically encoded fluorescent pH reporters targeted to the synaptic cleft of Drosophila larvae. We observed alkalinization rather than acidification during activity, and this alkalinization was dependent on the exchange of protons for calcium at the postsynaptic membrane.
A reaction-diffusion computational model of the pH dynamics at the Drosophila larval neuromuscular junction was developed to leverage the experimental data. The model incorporates the release of glutamate, ATP, and protons from synaptic vesicles into the cleft, PMCA activity, bicarbonate, and phosphate buffering systems. By means of numerical simulations, we reveal a highly dynamic pH landscape within the synaptic cleft, harboring deep but exceedingly rapid acid transients that give way to a prolonged period of alkalinization.