Na⁺/Ca²⁺ exchange and Ca²⁺ homeostasis in axon terminals of mammalian central neurons

We investigated Ca²⁺ clearance mechanisms (CCMs) at the axon terminals of mammalian central neurons: neurohypophysial (NHP) axon terminals and calyces of Held. Ca²⁺ transients were evoked by applying a short depolarization pulse via a patch pipette containing Ca²⁺ indicator dye. Quantitative analysis of the Ca²⁺ decay phases revealed that Na⁺/Ca²⁺ exchange (Na/CaX) is a major CCM at both axon terminals. In contrast, no Na/CaX activity was found in the somata of NHP axon terminals (supraoptic magnocellular neurons), indicating that the distribution of Na⁺/Ca²⁺ exchangers is polarized. Intracellular dialysis of axon terminals with a K⁺-free pipette solution attenuated the Na/CaX activities by 90% in the NHP axon terminals and by 60% at the calyx of Held, indicating that K⁺-dependent Na⁺/Ca²⁺ exchangers are involved. Studying the effects of specific inhibitors of smooth endoplasmic reticulum Ca²⁺-ATPase (SERCA) and plasma membrane Ca ²⁺-ATPase (PMCA) on the Ca²⁺ decay rate revealed thatPMCAcontributed 23% of total Ca²⁺ clearance, but that SERCA made no contribution at the calyx of Held. The contribution of mitochondria was negligible for small Ca²⁺ transients, but became apparent at peak Ca²⁺ levels higher than 2.5 μM. When mitochondrial function was inhibited, the dependence of CCMs on [Ca²⁺]_i at the calyx of Held showed saturation kinetics with K_1/2 = 1.7 μM, suggesting that the Na/CaX activity is saturated at high [Ca²⁺]_i. The presynaptic Na⁺/Ca²⁺ exchanger activity, which competes for cytosolic Ca²⁺ with mitochondria, may contribute to nonplastic synaptic transmission at these axon terminals.