The accumulation and extrusion of Ca 2+ in the pre-and postsynaptic compartments play a critical role in initiating plastic changes in biological synapses. To emulate this fundamental process in electronic devices, we developed diffusive Ag-in-oxide memristors with a temporal response during and after stimulation similar to that of the synaptic Ca 2+ dynamics. In situ high-resolution transmission electron microscopy and nanoparticle dynamics simulations both demonstrate that Ag atoms disperse under electrical bias and regroup spontaneously under zero bias because of interfacial energy 2 minimization, closely resembling synaptic influx and extrusion of Ca 2+ , respectively.The diffusive memristor and its dynamics enable a direct emulation of both short-and long-term plasticity of biological synapses and represent a major advancement in hardware implementation of neuromorphic functionalities.CMOS circuits have been employed to mimic synaptic Ca 2+ dynamics, but three-terminal devices bear limited resemblance to bio-counterparts at the mechanism level and require significant numbers and complex circuits to simulate synaptic behavior [1][2][3] . A substantial reduction in footprint, complexity and energy consumption can be achieved by building a two-terminal circuit element, such as a memristor directly incorporating Ca 2+ -like dynamics.Various types of memristors based on ionic drift (drift-type memristor) 4-8 have recently been utilized for this purpose in neuromorphic architectures [9][10][11][12][13][14][15] . Although qualitative synaptic functionality has been demonstrated, the fast switching and non-volatility of drift memristors optimized for memory applications do not faithfully replicate the nature of plasticity. Similar issues also exist in MOS-based memristor emulators [16][17][18] , although they are capable of simulating a variety of synaptic functions including spike-timing-dependent plasticity (STDP). Recently, Lu's group adopted second-order drift memristors to approximate the Ca 2+ dynamics of chemical synapses by utilizing thermal dissipation 19 or mobility decay 20 , which successfully demonstrated STDP with non-overlapping spikes and other synaptic functions, representing a significant step towards bio-realistic synaptic devices. This approach features repeatability and simplicity, but the significant differences of the dynamical response from actual synapses limit the fidelity and variety of desired synaptic functions. A device with similar physical behavior as the biological Ca 2+ dynamics would enable improved emulation of synaptic function and broad applications to neuromorphic computing. Here we report such an emulator, which is a memristor based on metal atom 3 diffusion and spontaneous nanoparticle formation, as determined by in situ high-resolution transmission electron microscopy (HRTEM) and nanoparticle dynamics simulations. The dynamical properties of the diffusive memristors were confirmed to be functionally equivalent to Ca 2+ in bio-synapses, and their operating characteri...

EPSCs at the synapses of sensory receptors and of some CNS neurons include large events thought to represent the synchronous release of the neurotransmitter contained in several synaptic vesicles by a process known as multiquantal release. However, determination of the unitary, quantal size underlying such putatively multiquantal events has proven difficult at hair cell synapses, hindering confirmation that large EPSCs are in fact multiquantal. Here, we address this issue by performing presynaptic membrane capacitance measurements together with paired recordings at the ribbon synapses of adult hair cells. These simultaneous presynaptic and postsynaptic assays of exocytosis, together with electron microscopic estimates of single vesicle capacitance, allow us to estimate a single vesicle EPSC charge of approximately Ϫ45 fC, a value in close agreement with the mean postsynaptic charge transfer of uniformly small EPSCs recorded during periods of presynaptic hyperpolarization. By thus establishing the magnitude of the fundamental quantal event at this peripheral sensory synapse, we provide evidence that the majority of spontaneous and evoked EPSCs are multiquantal. Furthermore, we show that the prevalence of uniquantal versus multiquantal events is Ca 2ϩ dependent. Paired recordings also reveal a tight correlation between membrane capacitance increase and evoked EPSC charge, indicating that glutamate release during prolonged hair cell depolarization does not significantly saturate or desensitize postsynaptic AMPA receptors. We propose that the large EPSCs reflect the highly synchronized release of multiple vesicles at single presynaptic ribbon-type active zones through a compound or coordinated vesicle fusion mechanism.

Hair cell ribbon synapses exhibit several distinguishing features. Structurally, a dense body, or ribbon, is anchored to the presynaptic membrane and tethers synaptic vesicles; functionally, neurotransmitter release is dominated by large EPSC events produced by seemingly synchronous multivesicular release. However, the specific role of the synaptic ribbon in promoting this form of release remains elusive. Using complete ultrastructural reconstructions and capacitance measurements of bullfrog amphibian papilla hair cells dialyzed with high concentrations of a slow Ca2+ buffer (10 mM EGTA), we found that the number of synaptic vesicles at the base of the ribbon correlated closely to those vesicles that released most rapidly and efficiently, while the rest of the ribbon-tethered vesicles correlated to a second, slower pool of vesicles. Combined with the persistence of multivesicular release in extreme Ca2+ buffering conditions (10 mM BAPTA), our data argues against the Ca2+-dependent compound fusion of ribbon-tethered vesicles at hair cell synapses. Moreover, during hair cell depolarization, our results suggest that elevated Ca2+ levels enhance vesicle pool replenishment rates. Finally, using Ca2+ diffusion simulations, we propose that the ribbon and its vesicles define a small cytoplasmic volume where Ca2+ buffer is saturated, despite 10 mM BAPTA conditions. This local buffer saturation permits fast and large Ca2+ rises near release sites beneath the synaptic ribbon that can trigger multiquantal EPSCs. We conclude that, by restricting the available presynaptic volume, the ribbon may be creating conditions for the synchronous release of a small cohort of docked vesicles.

Naϩ channel recovery from inactivation limits the maximal rate of neuronal firing. However, the properties of presynaptic Na ϩ channels are not well established because of the small size of most CNS boutons. Here we study the Na ϩ currents of the rat calyx of Held terminal and compare them with those of postsynaptic cells. We find that presynaptic Na ϩ currents recover from inactivation with a fast, single-exponential time constant (24°C, of 1.4 -1.8 ms; 35°C, of 0.5 ms), and their inactivation rate accelerates twofold during development, which may contribute to the shortening of the action potential as the terminal matures. In contrast, recordings from postsynaptic cells in brainstem slices, and acutely dissociated, reveal that their Na ϩ currents recover from inactivation with a doubleexponential time course ( fast of 1.2-1.6 ms; slow of 80 -125 ms; 24°C). Surprisingly, confocal immunofluorescence revealed that Na ϩ channels are mostly absent from the calyx terminal but are instead highly concentrated in an unusually long (Ϸ20 -40 m) unmyelinated axonal heminode. Outside-out patch recordings confirmed this segregation. Expression of Na v 1.6 ␣-subunit increased during development, whereas the Na v 1.2 ␣-subunit was not present. Serial EM reconstructions also revealed a long pre-calyx heminode, and biophysical modeling showed that exclusion of Na ϩ channels from the calyx terminal produces an action potential waveform with a shorter halfwidth. We propose that the high density and polarized locus of Na ϩ channels on a long heminode are critical design features that allow the mature calyx of Held terminal to fire reliably at frequencies near 1 kHz.

An obligatory role for the calcium sensor synaptotagmins in stimulus-coupled release of neurotransmitter is well established, but a role for synaptotagmin isoform involvement in asynchronous release remains conjecture. We show, at the zebrafish neuromuscular synapse, that two separate synaptotagmins underlie these processes. Specifically, knockdown of synaptotagmin 2 (syt2) reduces synchronous release, whereas knockdown of synaptotagmin 7 (syt7) reduces the asynchronous component of release. The zebrafish neuromuscular junction is unique in having a very small quantal content and a high release probability under conditions of either low-frequency stimulation or high-frequency augmentation. Through these features, we further determined that during the height of shared synchronous and asynchronous transmission these two modes compete for the same release sites.active zone | exocytosis | synapse | acetylcholine receptor A hallmark of synaptic transmission is the synchrony between the neuronal action potential and the evoked release of transmitter. However, an asynchronous release mode, first described at the nerve muscle junction (1, 2), also participates in neurotransmission at certain synapses (3-5). Although asynchronous release usually contributes less than 10% of the overall synaptic charge at low stimulus frequencies, it often plays a prominent role at higher frequencies (3, 6), prolonging both inhibitory (7) and excitatory (8, 9) postsynaptic responses through sustained release. Indeed, inhibitory deep cerebellar neurons, which are tuned for high-frequency signaling, rely exclusively on asynchronous synaptic transmission at contacts with inferior olive neurons (10). The mechanisms underlying synchronous and asynchronous release appear to be distinct but share a requirement for calcium (6,11). It is generally thought that local transient calcium signals govern synchronous release, whereas elevated residual calcium levels associated with highfrequency stimulation lead to asynchronous release (12, 13).A vesicular calcium sensor, synaptotagmin, couples synchronous release to presynaptic calcium entry in both mammals (14) and flies (15, 16). In hippocampus, the responsible isoform is syt1 (14, 17), whereas at neuromuscular synapses (18) and calyx of Held (11,19), it is syt2. The mechanisms underlying asynchronous release are not known. Our findings from zebrafish neuromuscular junction provide identification of a synaptotagmin isoform as a signaling component in asynchronous release. Results Paired Recordings Reveal a Small Quantal Content with ReleaseProbability Near "1." Paired whole-cell recordings between the caudal primary motor neuron (CaP) and ventral target skeletal muscle were performed on 72-to 96-h-postfertilization (hpf) zebrafish as previously described (20). The motor neuron was current clamped to −80 mV and stepped positive for 2 ms to elicit an action potential. The fast-type skeletal muscle was voltage clamped to −50 mV to inactivate sodium channels. The amplitude for spontaneous unitary events (...