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Author(s): Wei-Dong Zhao (corresponding author) [1]; Edaeni Hamid [1]; Wonchul Shin [1]; Peter J. Wen [1]; Evan S. Krystofiak [2]; Seth A. Villarreal [1]; Hsueh-Cheng Chiang [1]; Bechara Kachar [2]; [...]

Exocytosis at synapses involves fusion between vesicles and the plasmamembrane (1). Although compound fusion between vesicles (2,3) was proposed to occur at ribbon-type synapses (4,5), whether it exists, how it is mediated, and what role it plays at conventional synapses remain unclear. Here we report the existence of compound fusion, its underlying mechanism, and its role at a nerve terminal containing conventional active zones in rats and mice. We found that high potassium application and high frequency firing induced giant capacitance up-steps, reflecting exocytosis of vesicles larger than regular ones, followed by giant down-steps, reflecting bulk endocytosis. These intense stimuli also induced giant vesicle-like structures, as observed with electron microscopy, and giant miniature excitatory postsynaptic currents (mEPSCs), reflecting more transmitter release. Calcium and its sensor for vesicle fusion, synaptotagmin, were required for these giant events. After high frequency firing, calcium/synaptotagmin-dependent mEPSC size increase was paralleled by calcium/synaptotagmin-dependent post-tetanic potentiation. These results suggest a new route of exocytosis and endocytosis composed of three steps. First, calcium/synaptotagmin mediates compound fusion between vesicles. Second, exocytosis of compound vesicles increases quantal size, which increases synaptic strength and contributes to the generation of post-tetanic potentiation. Third, exocytosed compound vesicles are retrieved via bulk endocytosis. We suggest that this vesicle cycling route be included in models of synapses in which only vesicle fusion with the plasma membrane is considered (1)., We performed cell-attached capacitance recordings at the release face of calyces in brainstem (Fig. 1a) (6). Brainstem slices were from rats, unless otherwise mentioned. Before KCl application (control), spontaneous up-steps [...]

It has long been proposed that following vesicle fusion, a small pore might open and close rapidly without full dilation. Such 'kiss-and-run' vesicle fusion can in principle result in rapid vesicle recycling and influence the size and the kinetics of the resulting synaptic current. However, the existence of kiss-and-run remains highly controversial, as revealed by recent imaging and electrophysiological studies at several synapses, including hippocampal synapses, neuromuscular junctions and retinal bipolar synapses. Only a minor fraction of fusion events has been shown to be kiss-and-run, as determined using cell-attached capacitance recordings in endocrine cells, pituitary nerve terminals and calyx-type synapses. Further work is needed to determine whether kiss-and-run is a major mode of fusion and has a major role in controlling synaptic strength at synapses.

Studies from a variety of synapses indicate that the time course of endocytosis ranges from less than a second to hundreds of seconds. This raises questions about how the time course of endocytosis is regulated and why different rates of endocytosis are needed. Recent progress sheds light on these issues. Neuronal firing frequency and duration determine the time course of endocytosis. The dynamic nature of this time course could be a result of multiple endocytic pathways and/or of regulation by a variety of modulators. Because endocytosis is crucial for maintaining transmitter release during repetitive stimulation, regulation of endocytosis could thus provide a mechanism by which synaptic plasticity is achieved.

Author(s): Jian-Yuan Sun [1]; Xin-Sheng Wu [1]; Ling-Gang Wu (corresponding author) During synaptic transmission, neurotransmitter-laden vesicles fuse with the presynaptic membrane and discharge their contents into the synaptic cleft. After [...]

Voltage-dependent [Ca.sup.2+] currents evoke synaptic transmitter release. Of six types of [Ca.sup.2+] channels, L-, N-, P-, Q-, R-, and T-type, only N- and P/Q-type channels have been pharmacologically identified to mediate action-potential-evoked transmitter release in the mammalian central nervous system. We tested whether [Ca.sup.2+] channels other than N- and P/Q-type control transmitter release in a calyx-type synapse of the rat medial nucleus of the trapezoid body. Simultaneous recordings of presynaptic [Ca.sup.2+] influx and the excitatory postsynaptic current evoked by a single action potential were made at single synapses. The R-type channel, a high-voltage-activated [Ca.sup.2+] channel resistant to L-, N-, and P/Q-type channel blockers, contributed 26% of the total [Ca.sup.2+] influx during a presynaptic action potential. This [Ca.sup.2+] current evoked transmitter release sufficiently large to initiate an action potential in the postsynaptic neuron. The R-type current controlled release with a lower efficacy than other types of [Ca.sup.2+] currents. Activation of metabotropic glutamate receptors and [Gamma]-aminobutyric acid type B receptors inhibited the R-type current. Because a significant fraction of presynaptic [Ca.sup.2+] channels remains unidentified in many other central synapses, the R-type current also could contribute to evoked transmitter release in these synapses.

Activation of presynaptic receptors for a variety of neurotransmitters and neuromodulators inhibits transmitter release at many synapses. Such presynaptic inhibition might serve as a means of adjusting synaptic strength or preventing excessive transmitter release, or both. Previous evidence showed that presynaptic modulators inhibit [Ca.sup.2+] channels and activate [K.sup.+] channels at neuronal somata. These modulators also inhibit spontaneous transmitter release by mechanisms downstream of [Ca.sup.2+] entry. The relative contribution of the above mechanisms to the inhibition of elicited release has been debated for a long time. Recent evidence at synapses where the relationship between transmitter release and presynaptic [Ca.sup.2+] influx has been well characterized suggests that inhibition of presynaptic voltage-dependent [Ca.sup.2+] channels plays the major role in presynaptic inhibition of elicited neurotransmitter release. In addition, modulation of the release machinery might contribute to inhibition of elicited release.