No abstract

Key points Presynaptic mitochondria not only absorb but also release Ca2+ during high frequency stimulation (HFS) when presynaptic [Ca2+] is kept low (<500 nm) by high cytosolic Ca2+ buffer or strong plasma membrane calcium clearance mechanisms under physiological external [Ca2+]. Mitochondrial Ca2+ release (MCR) does not alter the global presynaptic Ca2+ transients. MCR during HFS enhances short‐term facilitation and steady state excitatory postsynaptic currents by increasing vesicular release probability. The intra‐train MCR may provide residual calcium at interspike intervals, and thus support high frequency neurotransmission at central glutamatergic synapses. Abstract Emerging evidence indicates that mitochondrial Ca2+ buffering contributes to local regulation of synaptic transmission. It is unknown, however, whether mitochondrial Ca2+ release (MCR) occurs during high frequency synaptic transmission. Confirming the previous notion that 2 μm tetraphenylphosphonium (TPP+) is a specific inhibitor of the mitochondrial Na+/Ca2+ exchanger (mNCX), we studied the role of MCR via mNCX in short‐term plasticity during high frequency stimulation (HFS) at the calyx of Held synapse of the rat. TPP+ reduced short‐term facilitation (STF) and steady state excitatory postsynaptic currents during HFS at mature calyx synapses under physiological extracellular [Ca2+] ([Ca2+]o = 1.2 mm), but not at immature calyx or at 2 mm [Ca2+]o. The inhibitory effects of TPP+ were stronger at synapses with morphologically complex calyces harbouring many swellings and at 32°C than at simple calyx synapses and at room temperature. These effects of TPP+ on STF were well correlated with those on the presynaptic mitochondrial [Ca2+] build‐up during HFS. Mitochondrial [Ca2+] during HFS was increased by TPP+ at mature calyces under 1.2 mm [Ca2+]o, and further enhanced at 32°C, but not under 2 mm [Ca2+]o or at immature calyces. The close correlation of the effects of TPP+ on mitochondrial [Ca2+] with those on STF suggests that mNCX contributes to STF at the calyx of Held synapses. The intra‐train MCR enhanced vesicular release probability without altering global presynaptic [Ca2+]. Our results suggest that MCR during HFS elevates local [Ca2+] near synaptic sites at interspike intervals to enhance STF and to support stable synaptic transmission under physiological [Ca2+]o.

Expression of neuregulin-2 (NRG2) is intense in a few regions of the adult brain where neurogenesis persists; however, little is understood about its role in developments of newborn neurons.To study the role of NRG2 in synaptogenesis at different developmental stages, newborn granule cells in rat hippocampal slice cultures were labeled with retrovirus encoding tetracycline-inducible microRNA targeting NRG2 and treated with doxycycline (Dox) at the fourth or seventh postinfection day (dpi). The developmental increase of GABAergic postsynaptic currents (GPSCs) was suppressed by the early Dox treatment (4 dpi), but not by late treatment (7 dpi). The late Dox treatment was used to study the effect of NRG2 depletion specific to excitatory synaptogenesis. The Dox effect on EPSCs emerged 4 d after the impairment in dendritic outgrowth became evident (10 dpi). Notably, Dox treatment abolished the developmental increases of AMPA-receptor mediated EPSCs and the AMPA/NMDA ratio, indicating impaired maturation of glutamatergic synapses. In contrast to GPSCs, Dox effects on EPSCs and dendritic growth were independent of ErbB4 and rescued by concurrent overexpression of NRG2 intracellular domain. These results suggest that forward signaling of NRG2 mediates GABAergic synaptogenesis and its reverse signaling contributes to dendritic outgrowth and maturation of glutamatergic synapses.

Although calyx of Held synapses undergo dramatic changes around the hearing onset, previous in vivo studies suggest that the calyx synapses undergo further post-hearing maturation process. While developmental changes over the hearing onset have been extensively studied, this post-hearing maturation process remained relatively little investigated. Because of post-hearing maturation, previous results from studies around hearing onset and studies of post-hearing calyx synapses are somewhat inconsistent. Here, we characterized the post-hearing maturation of calyx synapses with regard to in vitro electrophysiological properties in rats and mice. We found that parameters for residual glutamate in the cleft during a train, EPSC kinetics, and vesicle pool size became close to a full mature level by P14, but they further matured until P16 in the rats. Consistently, the phasic and slow EPSCs evoked by action potential trains at P16 calyx synapses were not different from those at P18 or P25 under physiological extracellular [Ca 2+ ] o (1.2 mM). In contrast, the parameters for residual current and EPSC kinetics displayed drastic changes until P16 in mice, and slow EPSCs during the train further decreased between P16 and P18, suggesting that maturation of calyx synapses progresses at least up to P16 in rats and P18 in mice.

Neurotransmitter release occurs either synchronously to action potentials or spontaneously, yet whether molecular machineries underlying evoked and spontaneous release are identical, especially whether voltage-gated Ca2+ channels (VGCCs) can trigger spontaneous events has been in debate. To elucidate this issue, we characterized Ca2+ dependency of miniature excitatory postsynaptic currents (mEPSCs) in autaptic cultured hippocampal neurons. We found that 58 % mEPSC frequency was dependent on extracellular Ca2+([Ca2+]o), and Ca2+cooperativity of spontaneous release was comparable to that of evoked release. Moreover, most (> 90 %) of [Ca2+]o-dependent mEPSCs was attributable to VGCCs. Coupling distance between VGCCs and Ca2+ sensors was estimated as tight for both spontaneous and evoked release (~22 nm). In hippocampal slices, VGCC-dependence on spontaneous release was also observed, but to a different extent, at different areas and ages. At the calyx of Held synapses, mEPSCs showed VGCC-dependence in type 1 mature synapses where VGCCs and Ca2+ sensors are tightly coupled, but not in immature synapses. These data strongly suggest that the distance between VGCCs and Ca2+ sensors is the key factor to determine VGCC dependence of spontaneous release.