Mechanisms underlying presynaptic Ca²⁺ transient and vesicular glutamate release at a CNS nerve terminal during in vitro ischaemia

Abstract

Key pointsr Here we demonstrate presynaptic responses and mechanisms of increased vesicular glutamate release during in vitro ischaemia in the calyx of Held terminal, an experimentally accessible presynaptic terminal in the CNS. Abstract An early consequence of brain ischaemia is an increase in vesicular glutamate release from presynaptic terminals. However, the mechanisms of this increased glutamate release are not fully understood. Here we studied presynaptic responses and mechanisms of increased glutamate release during in vitro ischaemia, using pre-and postsynaptic whole-cell recordings and presynaptic Ca 2+ imaging at the calyx of Held synapse in rat brainstem slices. Consistent with results from other brain regions, in vitro ischaemia significantly increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) without affecting their amplitude, suggesting that ischaemia enhances vesicular glutamate release from presynaptic terminals. We found that ischaemia-induced vesicular glutamate release was dependent on a rise in basal Ca 2+ at presynaptic terminals, which resulted from extracellular Ca 2+ influx and Ca 2+ release from intracellular stores. During early ischaemia, increased Ca 2+ influx into presynaptic terminals was due to reverse operation of the plasma membrane Na + /Ca 2+ exchanger (NCX) rather than presynaptic depolarization or voltage-activated Ca 2+ currents. KB-R7943, an inhibitor of NCX, prevented the ischaemia-induced increases in presynaptic Ca 2+ and vesicular glutamate release. In addition, the removal of extracellular Na + completely inhibited the ischaemia-induced Ca 2+ rise. It therefore appears that a link between Na + accumulation and Ca 2+ uptake via NCX underlies the ischaemia-induced Ca 2+ rise and the consequent increase in vesicular glutamate release from presynaptic terminals in the early phase of brain ischaemia.

Functional and structural properties of ion channels at the nerve terminal depends on compact myelin

“…However, we cannot rule out the possibility that demyelization also affects Na + extrusion mechanisms mediated by Na + /K + ATPase and NCX (Kim et al . , ; Lee and Kim, ) in the LES rat.…”

Ischemia-Triggered Glutamate Excitotoxicity From the Perspective of Glial Cells

“…The main route by which mature axons release glutamate during acute ischemia primarily comprises vesicular release (Figure 3), which has been shown in white matter (Doyle et al, 2018), in cerebral ischemia in vivo (Katayama et al, 1991;Jabaudon et al, 2000), in a slice model of hippocampal ischemia (Andrade and Rossi, 2010), and in an in vitro model of ischemia (Fujimoto et al, 2004;Lee and Kim, 2015). The second route of glutamate release from neurons was described in severe brain ischemic injury by Rossi et al (2000) who demonstrated that a reversed operation of neuronal glutamate transporters (Figure 3) plays a key role in generating the anoxic depolarization, that eliminates information processing in the CNS a few minutes after the onset of ischemia.…”

“…In a slice model of hippocampal ischemia, Andrade and Rossi (2010) identified several early cellular cascades triggered by ischemia, such as Ca 2+ entry and release from intracellular stores, actin filament depolymerization, and vesicular release of glutamate, which is dependent on actin dynamics but independent of [Ca 2+ ] i rise. Lee and Kim (2015) showed that the Ca 2+ uptake via plasma membrane Na + /Ca 2+ exchanger (NCX) underlies the ischemia-induced Ca 2+ rises and the consequent increase in vesicular glutamate release from presynaptic terminals in the early phase of brain ischemia. All of these processes precede the anoxic depolarization by a few minutes.…”

Activity and cytosolic Na+regulates synaptic vesicle endocytosis

“…It has been shown that plasma membrane Na + /Ca 2+ exchanger (NCX) is involved in maintaining presynaptic Ca 2+ homeostasis (Kim et al, 2005; Lee and Kim, 2015), while Ca 2+ plays specific roles in synaptic vesicle exocytosis (Leitz and Kavalali, 2016). To test whether presynaptic cytosolic Na + affects the NCX function, we lowered the extracellular Na + concentration to mimic the Na + concentration gradient change in elevated intracellular Na + conditions.…”

“…Another possible mechanism for Na + effects on endocytosis is through effects on [Ca 2+ ]. Na + has been shown to affect presynaptic Ca 2+ homeostasis through controlling the Na + /Ca 2+ exchanger activity (Kim et al, 2005; Lee and Kim, 2015), while Ca 2+ plays specific roles in synaptic vesicle exocytosis (Leitz and Kavalali, 2016). Our results showed that changes in extracellular Na + or blocking NCX activity did not affect the endocytosis rate (Fig.…”