Computational modeling predicts acidic microdomains in the glutamatergic synaptic cleft

Abstract

At chemical synapses, synaptic vesicles release their acidic contents into the cleft leading to the expectation that the cleft should acidify. However, fluorescent pH probes targeted to the cleft of conventional glutamatergic synapses in both fruit flies and mice reveal cleft alkalinization, rather than acidification. Here, using a reaction-diffusion scheme, we modeled pH dynamics at the Drosophila neuromuscular junction (NMJ) as glutamate, adenosine triphosphate (ATP) and protons (H+) are released into the cleft. The model incorporates bicarbonate and phosphate buffering systems as well as plasma membrane calcium-ATPase (PMCA) activity and predicts substantial cleft acidification but only for fractions of a millisecond following neurotransmitter release. Thereafter, the cleft rapidly alkalinizes and remains alkaline for over 100 milliseconds, as the PMCA removes H+ from the cleft in exchange for calcium ions (Ca2+) from adjacent pre- and post-synaptic compartments; thus recapitulating the empirical data. The extent of synaptic vesicle loading and time course of exocytosis has little influence on the magnitude of acidification. Phosphate, but not bicarbonate buffering is effective at ameliorating the magnitude and time course of the acid spike, while both buffering systems are effective at ameliorating cleft alkalinization. The small volume of the cleft levies a powerful influence on the magnitude of alkalinization and its time course. Structural features that open the cleft to adjacent spaces appear to be essential for alleviating the extent of pH disturbances accompanying neurotransmission. SIGNIFICANCE STATEMENTAcid-base imbalances have surprisingly potent neurological effects highlighting the acute pH sensitivity of many neural mechanisms. Acid-Sensing Ion-Channels (ASICs), which open in response to acid shifts in extracellular pH, are an example of such a mechanism. However, while ASICs open during neurotransmission at conventional glutamatergic synapses, pH-sensitive electrodes and fluorophores show no signs of acidification at these synapses, only alkalinization. To resolve this paradox, we built a computational model which allows a glimpse beyond the experimental limitations of pH-sensitive electrodes and fluorophores. Our model reveals 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, thus reconciling ASIC activation with direct measurements of extracellular pH.