High calcium concentrations shift the mode of exocytosis to the kiss-and-run mechanism

The debate on the kiss-and-run fusion at synapses

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.

Dynamic regulation of the large exocytotic fusion pore in pancreatic acinar cells

Loss of granule content during exocytosis requires the opening of a fusion pore between the secretory granule and plasma membrane. In a variety of secretory cells, this fusion pore has now been shown to subsequently close. However, it is still unclear how pore closure is physiologically regulated and contentious as to how closure relates to granule content loss. Here, we examine the behavior of the fusion pore during zymogen granule exocytosis in pancreatic acinar cells. By using entry of high-molecular-weight dyes from the extracellular solution into the granule lumen, we show that the fusion pore has a diameter of 29-55 nm. We further show that by 5 min after granule fusion, many granules have a closed fusion pore with evidence indicating that pore closure is a prelude to endocytosis and that in granules with a closed fusion pore the chymotrypsinogen content is low. Finally, we show that latrunculin B treatment promotes pore closure, suggesting F-actin affects pore dynamics. Together, our data do not support the classical view in acinar cells that exocytosis ends with granule collapse. Instead, for many granules the fusion pore closes, probably as a transition to endocytosis, and likely involving an F-actin-dependent mechanism.

Vesicular exocytosis under hypotonic conditions shows two distinct populations of dense core vesicles in bovine chromaffin cells

Several previous reports have discussed the effects of external osmolarity on vesicular exocytotic processes. However, few of these studies considered hypotonic conditions on chromaffin cells. Herein, the exocytosis of catecholamines by chromaffin cells was investigated in a medium of low osmolarity (200 mOsm) by amperometry at carbon fiber microelectrodes. It is observed that the frequency of the exocytotic events is significantly higher under hypotonic conditions than under physiological conditions (315 mOsm). This further confirms that the swelling of the polyelectrolytic matrix (which follows ionic exchanges) contained in dense core vesicles is the energetic driving force of the exocytotic phenomenon, being favored by a lower osmolarity. The mean amount of catecholamines released during secretory events also increases importantly under the hypotonic condition. This may be rationalized by the coexistence of two distinct populations of dense core vesicles with a relative content ratio of 4.7. The larger content population is favored under hypotonic conditions but plays only a side role under isotonic conditions.

Exocytosis of norepinephrine at axon varicosities and neuronal cell bodies in the rat brain

Norepinephrine secretion from central neurons was widely assumed to occur by exocytosis, but the essential characteristics of this process remained unknown. We developed an approach to study it directly by amperometry using carbon fiber microelectrodes in organotypic rat brainstem slice cultures. Noradrenergic neurons from areas A1 and A2 were fluorescently labeled by an adenoviral vector with noradrenergic-specific promoter. Quantal events, consistent with exocytotic release of norepinephrine, were registered at noradrenergic axonal varicosities as well as at cell bodies. According to their charge integrals, events were grouped into two populations. The majority (approximately 40 fC) were compatible with full exocytotic fusion of small clear and dense core vesicles shown in previous morphometric studies. The quantal size distribution was modulated by treatment with reserpine and amitriptyline. In addition, much larger quantal events (>1 pC) occurred at predominantly axonal release sites. The time course of signals was severalfold faster than in adrenal chromaffin cells, suggesting profound differences in the release machinery between these cell types. Tetrodotoxin eliminated the majority of events, indicating that release was partially, but not entirely, action potential driven. In conclusion, central norepinephrine release has unique characteristics, distinguishing it from those of other monoaminergic cells in periphery and brain.

Resolution of fusion pore formation in a cell-attached patch

Exocytosis of neurotransmitter is initiated by formation of a fusion pore, a narrow channel connecting the vesicle lumen to the extracellular space. Opening of the fusion pore can produce a flow of capacitative current, which was used to resolve the exocytosis of mast cell giant secretory granules by Breckenridge and Almers [Breckenridge LJ, Almers W. Currents through the fusion pore that forms during exocytosis of a secretory vesicle. Nature 1987;328:814-7]. We present an extension of this method to resolve fusion pore formation initiating exocytosis of single vesicles in bovine chromaffin cells. Cell-attached patch recordings revealed a capacitative current that was evoked by an increase of intracellular calcium (Ca(2+)) from application of a Ca(2+) ionophore or by the opening of a co-localized Ca(2+) channel. Calculated values for fusion pore conductance and vesicular membrane potential were in accord with previous estimates. Finally, we show that a single opening of a co-localized calcium channel evoked fusion pore formation, with delay times between channel opening and exocytosis agreeing with other methods. This method can be applied to resolve exocytosis regardless of what the vesicle contains or where exocytosis occurs, giving the possibility of using this method to resolve the release of synaptic vesicles.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

Stimulus-dependent alterations in quantal neurotransmitter release

Neurotransmitter release is a steep function of the intracellular calcium ion concentration ([Ca(2+)](i)) at the release sites. Both the Ca(2+) amplitude and the time course appear to be important for specifying neurotransmitter release. Ca(2+) influx regulates the number of vesicles exocytosed as well as the amount of neurotransmitter each individual vesicle releases. In our study we stimulated mouse chromaffin cells in two different ways to alter Ca(2+) presentation at the release sites. One method, digitonin permeabilization followed by exposure to Ca(2+), allows for a large uniform global elevation of [Ca(2+)](i), whereas the second method, application of nicotine, depolarizes chromaffin cells and activates voltage-dependent Ca(2+) channels, thereby producing more phasic and localized changes in [Ca(2+)](i). Using amperometry to monitor catecholamine release, we show that both kinds of stimuli elicit the exocytosis of similar quantities of neurotransmitter per large dense core vesicles (LDCVs) released. Even so, the release process was quite different for each stimulus; nicotine-elicited events were small and slow, whereas digitonin events were, in comparison, large and fast. In addition, the transient opening of the fusion pore, called the "foot," was essentially absent in digitonin-stimulated cells, but was quite common in nicotine-stimulated cells. Thus even though both strong stimuli used in this study elicited the release of many vesicles it appears that the differences in the Ca(2+) levels at the release sites were key determinants for the fusion and release of individual vesicles.

A rapid exocytosis mode in chromaffin cells with a neuronal phenotype

We have used astrocyte-conditioned medium (ACM) to promote the transdifferentiation of bovine chromaffin cells and study modifications in the exocytotic process when these cells acquire a neuronal phenotype. In the ACM-promoted neuronal phenotype, secretory vesicles and intracellular Ca2+ rise were preferentially distributed in the neurite terminals. Using amperometry, we observed that the exocytotic events also occurred mainly in the neurite terminals, wherein the individual exocytotic events had smaller quantal size than in undifferentiated cells. Additionally, duration of pre-spike current was significantly shorter, suggesting that ACM also modifies the fusion pore stability. After long exposure (7-9 days) to ACM, the kinetics of catecholamine release from individual vesicles was markedly accelerated. The morphometric analysis of vesicle diameters suggests that the rapid exocytotic events observed in neurites of ACM-treated cells correspond to the exocytosis of large dense-core vesicles (LDCV). On the other hand, experiments performed in EGTA-loaded cells suggest that ACM treatment promotes a better coupling between voltage-gated calcium channels (VGCC) and LDCV. Thus, our findings reveal that ACM promotes a neuronal phenotype in chromaffin cells, wherein the exocytotic kinetics is accelerated. Such rapid exocytosis mode could be caused at least in part by a better coupling between secretory vesicles and VGCC.

Fusion pores and fusion machines in Ca2+-triggered exocytosis

Exocytosis is initiated within a highly localized region of contact between two biological membranes. Small areas of these membranes draw close, molecules on the two surfaces interact, and structural transformations take place. Membrane fusion requires the action of proteins specialized for this task, and these proteins act as a fusion machine. At a critical point in this process, a fusion pore forms within the membrane contact site and then expands as the spherical vesicle merges with the flat target membrane. Hence, the operation of a fusion machine must be realized through the formation and expansion of a fusion pore. Delineating the relation between the fusion machine and the fusion pore thus emerges as a central goal in elucidating the mechanisms of membrane fusion. We summarize present knowledge of fusion machines and fusion pores studied in vitro, in neurons, and in neuroendocrine cells, and synthesize this knowledge into some specific and detailed hypotheses for exocytosis.

Vesicle pools, docking, priming, and release

The release of neurotransmitter from synaptic vesicles represents the final event by which presynapses send their chemical signal to the receiving postsynapses. Prior to fusion, synaptic vesicles undergo a series of maturation events, most notably the membrane-delimited docking and priming steps. Physiological and optical experiments with high-time resolution have allowed the distinction of vesicles in different maturation states with respect to fusion, the so-called vesicle pools. In this review, we define the various vesicle pools and discuss pathways leading into and out of these pools. We also provide an overview of an array of proteins that have been identified or are speculated to play a role in the transition between the various vesicle pools.

Regulation of large dense-core vesicle volume and neurotransmitter content mediated by adaptor protein 3

Adaptor protein 3 (AP-3) is a vesicle-coat protein that forms a heterotetrameric complex. Two types of AP-3 subunits are found in mammalian cells. Ubiquitous AP-3 subunits are expressed in all tissues of the body, including the brain. In addition, there are neuronal AP-3 subunits that are thought to serve neuron-specific functions such as neurotransmitter release. In this study, we show that overexpression of neuronal AP-3 in mouse chromaffin cells results in a striking decrease in the neurotransmitter content of individual vesicles (quantal size), whereas deletion of all AP-3 produces a dramatic increase in quantal size; these changes were correlated with alterations in dense-core vesicle size. AP-3 appears to localize in the trans-Golgi network and possibly immature secretory vesicles, where it may be involved in the formation of neurosecretory vesicles.

High throughput analysis of endogenous glutamate release using a fluorescence plate reader

Recent discoveries of different modes of exocytosis and a plethora of molecules involved in neurotransmitter release has resulted in demand for more rapid and efficient methods for monitoring endogenous glutamate release from various tissue sources. In this article, we describe a high throughput microplate version of the enzyme-linked fluorescence detection method for the measurement of released glutamate, which utilises glutamate dehydrogenase, and the reduction of NADP to NADPH. Previous versions of this method rely upon cuvette-based fluorimeters for detection that are limited by large sample volumes and small numbers of samples that can be measured simultaneously. Comparison between the two methods shows that the microplate assay has comparable performance to the cuvette-based assay but has the capacity to analyse many times more samples in a given run. This increased capacity provides improved experimental design opportunities, higher experimental throughput and better comparison between experimental conditions.

Double patch clamp reveals that transient fusion (kiss-and-run) is a major mechanism of secretion in calf adrenal chromaffin cells: high calcium shifts the mechanism from kiss-and-run to complete fusion

Transient fusion ("kiss-and-run") is accepted as a mode of transmitter release both in central neurons and neuroendocrine cells, but the prevalence of this mechanism compared with full fusion is still in doubt. Using a novel double patch-clamp method (whole cell/cell attached), permitting the recording of unitary capacitance events while stimulating under a variety of conditions including action potentials, we show that transient fusion is the predominant (>90%) mode of secretion in calf adrenal chromaffin cells. Raising intracellular Ca2+ concentration ([Ca]i) from 10 to 200 microM increases the incidence of full fusion events at the expense of transient fusion. Blocking rapid endocytosis that normally terminates transient fusion events also promotes full fusion events. Thus, [Ca]i controls the transition between transient and full fusion, each of which is coupled to different modes of endocytosis.

Synaptotagmin I and Ca(2+) promote half fusion more than full fusion in SNARE-mediated bilayer fusion

Synaptic membrane fusion, which is necessary for neurotransmitter release, may be mediated by SNAREs and regulated by synaptotagmin (Syt) and Ca(2+). Fusion of liposomes mediated by reconstituted SNAREs produces full fusion and hemifusion, a membrane structure in which outer leaflets are mixed but the inner leaflets remain intact. Here, using the liposome fusion assay, it is shown that Syt promoted both hemifusion and full fusion in a Ca(2+)-dependent manner. Syt.Ca(2+) increased hemifusion more than full fusion, modulating the ratio of hemifusion to full fusion. Unlike the case of neuronal SNAREs, stimulation of fusion by Syt.Ca(2+) was not seen for other SNAREs involved in trafficking in yeast, indicating that the Syt.Ca(2+) stimulation was SNARE-specific. We constructed hybrid SNAREs in which transmembrane domains were swapped between neuronal and yeast SNAREs. With these hybrid SNAREs, we demonstrated that the interaction between the SNARE motifs of neuronal proteins and Syt.Ca(2+) was required for the stimulation of fusion.

Src family tyrosine kinases differentially modulate exocytosis from rat brain nerve terminals

We have studied the role of src family tyrosine kinases in regulating synaptic transmitter release from rat brain synaptosomes by using two assays that measure different aspects of synaptic vesicle exocytosis: glutamate release (that directly measures exocytosis of vesicle contents) and release of FM 2-10 styryl dye (that is proportional to the time the synaptic vesicle is fused to the plasma membrane). Depolarisation was induced by KCl (30 mM) or 4-aminopyridine (4AP: 0.3mM) to induce release by full fusion (FF) exocytosis, or by 1 mM 4AP to induce release by both FF and kiss-and-run (KR)-like exocytosis. The src family selective inhibitor, PP1 (10 microM), increased KCl and 0.3 mM 4AP-evoked Ca2+ -dependent release of glutamate, but had little effect upon exocytosis evoked by 1mM 4AP. PP1 did not affect the release of FM 2-10 under any of the depolarisation conditions used. PP1 also had no effect on overall intracellular calcium levels, as measured by FURA2, suggesting that the effects of the inhibitor are downstream of calcium entry. At the same concentration the inactive analogue of this compound, PP3, had no effect on any measure. Immunoblotting with an antibody to phosphotyrosine revealed that phosphorylation of many synaptosomal proteins was reduced by PP1. The immunoreactivity of three protein bands increased upon depolarisation and this increase was blocked by PP1. Phosphorylation of src at tyrosine-416 was reduced by PP1 but changes in its phosphorylation did not correlate with the effects of PP1 on release. These results suggest one or more members of the src family of tyrosine kinases is a negative regulator of the KR mode of exocytosis in synaptosomes, perhaps by tonically inhibiting KR under normal stimulation conditions.

Frequency-dependent kinetics and prevalence of kiss-and-run and reuse at hippocampal synapses studied with novel quenching methods

The kinetics of exo-endocytotic recycling could restrict information transfer at central synapses if neurotransmission were entirely reliant on classical full-collapse fusion. Nonclassical fusion retrieval by kiss-and-run would be kinetically advantageous but remains controversial. We used a hydrophilic quencher, bromophenol blue (BPB), to help detect nonclassical events. Upon stimulation, extracellular BPB entered synaptic vesicles and quenched FM1-43 fluorescence, indicating retention of FM dye beyond first fusion. BPB also quenched fluorescence of VAMP (synaptobrevin-2)-EGFP, thus indicating the timing of first fusion of vesicles in the total recycling pool. Comparison with FM dye destaining revealed that kiss-and-run strongly prevailed over full-collapse fusion at low frequency, giving way to a near-even balance at high frequency. Quickening of kiss-and-run vesicle reuse was also observed at higher frequency in the average single vesicle fluorescence response. Kiss-and-run and reuse could enable hippocampal nerve terminals to conserve scarce vesicular resources when responding to widely varying input patterns.

Frequency selectivity of synaptic exocytosis in frog saccular hair cells

The ability to respond selectively to particular frequency components of sensory inputs is fundamental to signal processing in the ear. The frog (Rana pipiens) sacculus, which is used for social communication and escape behaviors, is an exquisitely sensitive detector of sounds and ground-borne vibrations in the 5- to 200-Hz range, with most afferent axons having best frequencies between 40 and 60 Hz. We monitored the synaptic output of saccular sensory receptors (hair cells) by measuring the increase in membrane capacitance (deltaC(m)) that occurs when synaptic vesicles fuse with the plasmalemma. Strong stepwise depolarization evoked an exocytic burst that lasted 10 ms and corresponded to the predicted capacitance of all docked vesicles at synapses, followed by a 20-ms delay before additional vesicle fusion. Experiments using weak stimuli, within the normal physiological range for these cells, revealed a sensitivity to the temporal pattern of membrane potential changes. Interrupting a weak depolarization with a properly timed hyperpolarization increased deltaC(m). Small sinusoidal voltage oscillations (+/-5 mV centered at -60 mV) evoked a deltaC(m) that corresponded to 95 vesicles per s at each synapse at 50 Hz but only 26 vesicles per s at 5 Hz and 27 vesicles per s at 200 Hz (perforated patch recordings). This frequency selectivity was absent for larger sinusoidal oscillations (+/-10 mV centered at -55 mV) and was largest for hair cells with the smallest sinusoidal-stimuli-evoked Ca2+ currents. We conclude that frog saccular hair cells possess an intrinsic synaptic frequency selectivity that is saturated by strong stimuli.

Sub-second cellular dynamics: time-resolved electron microscopy and functional correlation

Subcellular processes, from molecular events to organellar responses and cell movement, cover a broad scale in time and space. Clearly the extremes, such as ion channel activation are accessible only by electrophysiology, whereas numerous routine methods exist for relatively slow processes. However, many other processes, from a millisecond time scale on, can be "caught" only by methods providing appropriate time resolution. Fast freezing (cryofixation) is the method of choice in that case. In combination with follow-up methodologies appropriate for electron microscopic (EM) analysis, with all its variations, such technologies can also provide high spatial resolution. Such analyses may include, for example, freeze-fracturing for analyzing restructuring of membrane components, scanning EM and other standard EM techniques, as well as analytical EM analyses. The latter encompass energy-dispersive x-ray microanalysis and electron spectroscopic imaging, all applicable, for instance, to the second messenger, calcium. Most importantly, when conducted in parallel, such analyses can provide a structural background to the functional analyses, such as cyclic nucleotide formation or protein de- or rephosphorylation during cell stimulation. In sum, we discuss many examples of how it is practically possible to achieve strict function-structure correlations in the sub-second time range. We complement this review by discussing alternative methods currently available to analyze fast cellular phenomena occurring in the sub-second time range.

Functions of SNAREs in intracellular membrane fusion and lipid bilayer mixing

Intracellular membrane fusion occurs with exquisite coordination and specificity. Each fusion event requires three basic components: Rab-GTPases organize the fusion site; SNARE proteins act during fusion; and N-ethylmaleimide-sensitive factor (NSF) plus its cofactor alpha-SNAP are required for recycling or activation of the fusion machinery. Whereas Rab-GTPases seem to mediate the initial membrane contact, SNAREs appear to lie at the center of the fusion process. It is known that formation of complexes between SNAREs from apposed membranes is a prerequisite for lipid bilayer mixing; however, the biophysics and many details of SNARE function are still vague. Nevertheless, recent observations are shedding light on the role of SNAREs in membrane fusion. Structural studies are revealing the mechanisms by which SNARES form complexes and interact with other proteins. Furthermore, it is now apparent that the SNARE transmembrane segment not only anchors the protein but engages in SNARE-SNARE interactions and plays an active role in fusion. Recent work indicates that the fusion process itself may comprise two stages and proceed via a hemifusion intermediate.

"Kiss-and-run" glutamate secretion in cultured and freshly isolated rat hippocampal astrocytes

Under physiological conditions, astrocytes not only passively support and nourish adjacent neurons, but also actively modulate neuronal transmission by releasing "glial transmitters," such as glutamate, ATP, and D-serine. Unlike the case for neurons, the mechanisms by which glia release transmitters are essentially unknown. Here, by using electrochemical amperometry and frequency-modulated single-vesicle imaging, we discovered that hippocampal astrocytes exhibit two modes of exocytosis of glutamate in response to various stimuli. After physiological stimulation, a glial vesicle releases a quantal content that is only 10% of that induced by nonphysiological, mechanical stimulation. The small release event arises from a brief (approximately 2 ms) opening of the fusion pore. We conclude that, after physiological stimulation, astrocytes release glutamate via a vesicular "kiss-and-run" mechanism.

Synaptotagmin-Ca2+ triggers two sequential steps in regulated exocytosis in rat PC12 cells: fusion pore opening and fusion pore dilation

Synaptotagmin I (Syt I), the putative Ca(2+) sensor in regulated exocytosis, has two Ca(2+)-binding modules, the C2A and C2B domains, and a number of putative effectors to which Syt I binds in a Ca(2+)-dependent fashion. The role of Ca(2+) binding to these domains remains unclear, as efforts to address questions about Ca(2+)-triggered effector interactions have led to conflicting results. We have studied the effects of Ca(2+) on fusion pores using amperometry to follow the exocytosis of single vesicles in real time and analyse the kinetics of fusion pore transitions. Elevating [Ca(2+)] in permeabilized cells reduced the fusion pore lifetime, indicating an action of Ca(2+) during the actual fusion process. Analysing the Ca(2+) dependence of the fusion pore lifetime, together with the frequency of pore openings and the proportion of openings that close without dilating (kiss-and-run events) enabled us to resolve exocytosis into a sequence of kinetic steps representing functional transitions in the fusion pore. Fusion pore opening and dilation were both accelerated by Ca(2+), indicating separate Ca(2+) control over each of these steps. Ca(2+) ligand mutations in either the C2A or C2B domains of Syt I reduced fusion pore opening, but had opposite actions on the rate of fusion pore closure. These studies resolve two separate and distinct Ca(2+)-triggered steps during regulated exocytosis. The C2A and C2B domains of Syt I have different actions during these steps, and these actions may be linked to their distinctive effector interactions.

Electrochemical imaging of fusion pore openings by electrochemical detector arrays

Opening of individual exocytotic fusion pores in chromaffin cells was imaged electrochemically with high time resolution. Electrochemical detector arrays that consist of four platinum microelectrodes were microfabricated on a glass coverslip. Exocytosis of single vesicles containing catecholamines from a cell positioned on top of the array is detected by the individual electrodes as a time-resolved oxidation current, reflecting the time course of arrival of catecholamine molecules at the electrode surfaces. The signals exhibit low noise and reveal foot signals indicating fusion pore formation and expansion. The position of individual release events is determined from the fraction of catecholamines recorded by the individual electrodes. Simultaneous fluorescence imaging of release of acridine orange from individual vesicles confirmed the electrochemical position assignments. This electrochemical camera provides very high time resolution, spatiotemporal localization of individual fusion pore openings and quantitative data on the flux of transmitter from individual vesicles. Analysis of the amperometric currents employing random walk simulations indicates that the time course of amperometric spikes measured near the cell surface is due to a low apparent diffusion coefficient of catecholamines near the cell surface and not due to slow dissociation from the granular matrix.

Selective nucleotide-release from dense-core granules in insulin-secreting cells

Secretory granules of insulin-secreting cells are used to store and release peptide hormones as well as low-molecular-weight compounds such as nucleotides. Here we have compared the rate of exocytosis with the time courses of nucleotide and peptide release by a combination of capacitance measurements, electrophysiological detection of ATP release and single-granule imaging. We demonstrate that the release of nucleotides and peptides is delayed by approximately 0.1 and approximately 2 seconds with respect to membrane fusion, respectively. We further show that in up to 70% of the cases exocytosis does not result in significant release of the peptide cargo, likely because of a mechanism that leads to premature closure of the fusion pore. Release of nucleotides and protons occurred regardless of whether peptides were secreted or not. These observations suggest that insulin-secreting cells are able to use the same secretory vesicles to release small molecules either alone or together with the peptide hormone.

Recycling and EH domain proteins at the synapse

In neurons, a network of endocytic proteins accomplishes highly regulated processes such as synaptic vesicle cycling and the timely internalization of intracellular signaling molecules. In this review, we discuss recent advances on molecular networks created through interactions between proteins bearing the Eps15 homology (EH) domain and partner proteins containing the Asn-Pro-Phe (NPF) motif, which participate in important aspects of neuronal function as the synaptic vesicle cycle, the internalization of nerve growth factor (NGF), the determination of neuronal cell fate, the development of synapses and the trafficking of postsynaptic receptors. We discuss novel functional findings on the role of intersectin and synaptojanin and then we focus on the features of an emerging family of EH domain proteins termed EHDs (EH domain proteins), which are important for endocytic recycling of membrane proteins.

Fusion pore regulation of transmitter release

During the last decade a wealth of new information about the properties of the exocytotic fusion pore is changing our current view of exocytosis. The exocytotic fusion pore, a necessary stage before the full merging of the vesicle membrane with the plasma membrane, is becoming a key cellular structure that might critically control the amount of neurotransmitter released into the synaptic cleft and that can be subjected to control by second messengers and phosphorylated proteins. Fusion pores form, expand to fully merge membranes, or can close leaving an intact and identical synaptic vesicle in place for a new round of exocytosis. Transient formation of fusion pores is the mechanistic representation of the "kiss-and-run" hypothesis of transmitter release and offers new alternatives for synaptic vesicle recycling besides to the classical mechanism mediated by clathrin coat endocytosis. For vesicle recycling transient fusion pores ensures a fast mechanism for maintaining an active pool of synaptic vesicles. The size reached by transient fusion pores and the time spent on the open state can determine the release of subquantal synaptic transmission, which could be a mechanism of synaptic potentiation. In this review we will described the electrophysiological and fluorescence methods that contribute to further explore the biophysical properties of the exocytotic fusion pore and the relevant experiments obtained by these methods.

Calcium-dependent regulation of exocytosis

A rapid increase in intracellular calcium directly triggers regulated exocytosis. In addition, changes in intracellular calcium concentration can adjust the extent of exocytosis (quantal content) or the magnitude of individual release events (quantal size) in both the short- and long-term. It is generally agreed that calcium achieves this regulation via an interaction with a number of different molecular targets located at or near to the site of membrane fusion. We review here the synaptic proteins with defined calcium-binding domains and protein kinases activated by calcium, summarize what is known about their function in membrane fusion and the experimental evidence in support of their involvement in synaptic plasticity.

Analysis of exocytotic events recorded by amperometry

Amperometry is widely used to study exocytosis of neurotransmitters and hormones in various cell types. Analysis of the shape of the amperometric spikes that originate from the oxidation of monoamine molecules released during the fusion of individual secretory vesicles provides information about molecular steps involved in stimulation-dependent transmitter release. Here we present an overview of the methodology of amperometric signal processing, including (i) amperometric signal acquisition and filtering, (ii) detection of exocytotic events and determining spike shape characteristics, and (iii) data manipulation and statistical analysis. The purpose of this review is to provide practical guidelines for performing amperometric recordings of exocytotic activity and interpreting the results based on shape characteristics of individual release events.

Activation of GPCRs modulates quantal size in chromaffin cells through G(betagamma) and PKC

Exocytosis proceeds by either full fusion or 'kiss-and-run' between vesicle and plasma membrane. Switching between these two modes permits the cell to regulate the kinetics and amount of secretion. Here we show that ATP receptor activation reduces secretion downstream from cytosolic Ca2+ elevation in rat adrenal chromaffin cells. This reduction is mediated by activation of a pertussis toxin-sensitive G(i/o) protein, leading to activation of G(betagamma) subunits, which promote the 'kiss-and-run' mode by reducing the total open time of the fusion pore during a vesicle fusion event. Furthermore, parallel activation of the muscarinic acetylcholine receptor removes the inhibitory effects of ATP on secretion. This is mediated by a G(q) pathway through protein kinase C activation. The inhibitory effects of ATP and its reversal by protein kinase C activation are also shared by opioids and somatostatin. Thus, a variety of G protein pathways exist to modulate Ca2+-evoked secretion at specific steps in fusion pore formation.

Ultrastructural evidence of piecemeal degranulation in large dense-core vesicles of brain neurons

Large dense-core vesicles (LDCV) are a group of neuronal secretory organelles with different size and characteristically condensed morphology. LDCV release their specific cargo by regulated exocytosis, either in the form of "full fusion" or "kiss-and-run" exocytosis. In this paper, we provide ultrastructural evidence indicative of a slow and particulate mode of secretion from LDCV, called piecemeal degranulation (PMD). A number of LDCV in the nerve boutons of mouse brain presented marked increase in their size accompanied by reduction and also disappearance of content material. Residual secretory constituents in altered LDCV displayed eroded marginated patterns, leading to eccentric "haloed" morphologies. Remarkably, altered LDCV never appeared to be fused with each other or with the nerve plasma membrane. Very small vesicles, empty or apparently loaded with the same material making-up the LDCV content, could be seen near or attached to LDCV and the plasma membrane. First described in basophils, mast cells and eosinophils, PMD has recently been recognized in various neuro-endocrine cells, like adrenal chromaffin cells and endocrine cells of the gastro-intestinal epithelia. Here we suggest that PMD may be a hitherto unrecognized pathway of neuron secretion. It would represent the morphological correlate of a long-lasting and low-level process of neuro-transmitter release. It extends the patterns of neuron secretion and possibly opens new perspectives in understanding neuron plasticity.

Secretion without membrane fusion: porocytosis

We have recently proposed a mechanism to describe secretion, a fundamental process in all cells. That hypothesis, called porocytosis, embodies all available data and encompasses both forms of secretion, i.e., vesicular and constitutive. The current accepted view of exocytotic secretion involves the physical fusion of vesicle and plasma membranes; however, that hypothesized mechanism does not fit all available physiological data. Energetics of apposed lipid bilayers do not favor unfacilitated fusion. We consider that calcium ions (e.g., 10(-4) to 10(-3) M calcium in microdomains when elevated for 1 ms or less), whose mobility is restricted in space and time, establish salt bridges among adjacent lipid molecules. This establishes transient pores that span both the vesicle and plasma membrane lipid bilayers; the diameter of this transient pore would be approximately 1 nm (the diameter of a single lipid molecule). The lifetime of the transient pore is completely dependent on the duration of sufficient calcium ion levels. This places the porocytosis hypothesis for secretion squarely in the realm of the physical and physical chemical interactions of calcium and phospholipids and places mass action as the driving force for release of secretory material. The porocytosis hypothesis that we propose satisfies all of the observations and provides a framework to integrate our combined knowledge of vesicular and constitutive secretion.

Mouse chromaffin cells have two populations of dense core vesicles

The quantal hypothesis states that neurotransmitter is released in discrete packages, quanta, thought to represent the neurotransmitter content of individual vesicles. If true, then vesicle size should influence quantal size. Although chromaffin cells are generally thought to have a single population of secretory vesicles, our electron microscopy analysis suggested two populations as the size distribution was best described as the sum of two Gaussians. The average volume difference was fivefold. To test whether this difference in volume affected quantal size, neurotransmitter release from permeabilized cells exposed to 100 microM Ca2+ was measured with amperometry. Quantal content was bimodally distributed with both large and small events; the distribution of vesicle sizes predicted by amperometry was extremely similar to those measured with electron microscopy. In addition, each population of events exhibited distinct release kinetics. These results suggest that chromaffin cells have two populations of dense core vesicles (DCV) with unique secretory properties and which may represent two distinct synthetic pathways for DCV biogenesis or alternatively they may represent different stages of biosynthesis.

Maintenance of quantal size and immediately releasable granules in rat chromaffin cells by glucocorticoid

Glucocorticoid is reported to regulate catecholamine synthesis and storage. However, it is not clear whether the actual amount of catecholamine released from individual granules (quantal size, Q) in mature chromaffin cells is affected by glucocorticoid. Using carbon fiber amperometry, we found that dexamethasone did not affect mean cellular Q or the proportional release from different populations of granules in rat chromaffin cells cultured for 1 day in a serum-free defined medium. After two extra days of culture in the defined medium, there was a rundown in mean cellular Q, and it was associated with a shift in the proportional release from the different granule populations. This phenomenon could not be rescued by serum supplementation but could be prevented by dexamethasone via an action that was independent of changes in voltage-gated Ca(2+) channel (VGCC) density. Using simultaneous measurements of membrane capacitance and cytosolic Ca(2+) concentration, we found that for cells cultured in defined medium dexamethasone enhanced the exocytotic response triggered by a brief depolarization (50 ms) without affecting the VGCC density or the fast exocytotic response triggered via flash photolysis of caged Ca(2+). Thus glucocorticoid may regulate the number of immediately releasable granules that are in close proximity to a subset of VGCC. Because chromaffin cells in vivo are exposed to high concentrations of glucocorticoid, our findings suggest that the paracrine actions of glucocorticoid maintain the mean catecholamine content in chromaffin cell granules as well as the colocalization of releasable granules with VGCCs.

Two modes of exocytosis at hippocampal synapses revealed by rate of FM1-43 efflux from individual vesicles

We have examined the kinetics by which FM1-43 escapes from individual synaptic vesicles during exocytosis at hippocampal boutons. Two populations of exocytic events were observed; small amplitude events that lose dye slowly, which made up more than half of all events, and faster, larger amplitude events with a fluorescence intensity equivalent to single stained synaptic vesicles. These populations of destaining events are distinct in both brightness and kinetics, suggesting that they result from two distinct modes of exocytosis. Small amplitude events show tightly clustered rate constants of dye release, whereas larger events have a more scattered distribution. Kinetic analysis of the association and dissociation of FM1-43 with membranes, in combination with a simple pore permeation model, indicates that the small, slowly destaining events may be mediated by a narrow ∼1-nm fusion pore.

Clathrin-mediated endocytosis in snake motor terminals is directly facilitated by intracellular Ca2+

At the snake neuromuscular junction, low temperature (LT, 5-7 degrees C) blocks clathrin-mediated endocytosis (CME) while exocytosis is largely unaffected. Thus compensatory endocytosis that normally follows transmitter release is inhibited, or 'delayed' until the preparation is warmed to room temperature (RT). This delay was exploited to observe how changes in bulk [Ca(2+)](i) directly affect CME. Motor terminals were loaded with fura-2 to monitor [Ca(2+)](i). With brief stimulation at LT, [Ca(2+)](i) transiently increased but returned to baseline ( approximately 63 nm) in < 8 min. After 15 min at LT, [Ca(2+)](i) was altered by incubating preparations in the Ca(2+) ionophore ionomyocin. Preparations were then warmed to RT to initiate delayed endocytosis, which was quantified as uptake of the fluorescent optical probe sulforhodamine 101. Endocytosis was more rapid when [Ca(2+)](i) increased; the rate at 300 nm Ca(2+) was approximately double that under basal conditions. Thus the rate of CME - isolated from stimulation, transmitter release, and other forms of endocytosis - is directly influenced by intraterminal Ca(2+).

Regulation of exocytosis in neurons and neuroendocrine cells

Neurons communicate with one another through the release of molecules from synaptic vesicles and large dense core granules through the process of exocytosis. During exocytosis, molecules are released to the extracellular space through a fusion pore, which can either dilate, resulting in full fusion, or close, resulting in incomplete exocytosis, often referred to as 'kiss and run' exocytosis. Recently, there has been much interest in the regulation of this process in both neurons and neuroendocrine cells. There has been much recent work that addresses the existence of incomplete exocytosis in neurons and neuroendocrine cells, as well as recent work probing the molecular components and modulation of the fusion pore.

Regulated exocytosis and kiss-and-run of synaptic-like microvesicles in INS-1 and primary rat beta-cells

We have applied cell-attached capacitance measurements to investigate whether synaptic-like microvesicles (SLMVs) undergo regulated exocytosis in insulinoma and primary pancreatic beta-cells. SLMV and large dense-core vesicle (LDCV) exocytosis was increased 1.6- and 2.4-fold upon stimulation with 10 mmol/l glucose in INS-1 cells. Exocytosis of both types of vesicles was coupled to Ca(2+) entry through l-type channels. Thirty percent of SLMV exocytosis in INS-1 and rat beta-cells was associated with transient capacitance increases consistent with kiss-and-run. Elevation of intracellular cAMP (5 micromol/l forskolin) increased SLMV exocytosis 1.6-fold and lengthened the duration of kiss-and-run events in rat beta-cells. Experiments using isolated inside-out patches of INS-1 cells revealed that the readily releasable pool (RRP) of SLMVs preferentially undergoes kiss-and-run exocytosis (67%), is proportionally larger than the LDCV RRP, and is depleted more quickly upon Ca(2+) stimulation. We conclude that SLMVs undergo glucose-regulated exocytosis and are capable of high turnover. Following kiss-and-run exocytosis, the SLMV RRP may be reloaded with gamma-aminobutyric acid and undergo several cycles of exo- and endocytosis. Our observations support a role for beta-cell SLMVs in a synaptic-like function of rapid intra-islet signaling.

Calcium regulates the mode of exocytosis induced by hypotonic shock in isolated neuronal presynaptic endings

A decrease in the osmolarity of incubation medium is accompanied by calcium influx in neuronal presynaptic endings. We studied the influence of Ca2+ on exocytosis induced by hypotonic shock using the hydrophilic fluorescent dye acridine orange and the hydrophobic fluorescent dye FM2-10. It was shown using acridine orange that lowering of osmolarity to 230 mOsm/l induces exocytosis both in calcium-containing and calcium-free medium. By contrast, we were able to demonstrate calcium-dependence of exocytosis using styryl dye FM2-10. Lowering of osmolarity leads to increase of [3H]D-aspartate and [3H]GABA release in calcium-free medium. Addition of calcium inhibits hypotonic-induced neurotransmitter release. Decreasing of NaCl concentration to 92 mM in isotonic medium is able to induce d-aspartate and GABA release. Thus, our data suggest that hypotonic swelling induces calcium-independent exocytosis possibly by a "kiss and run" mechanism. Calcium influx mediated by stretch channels is able to provoke full fusion between plasma membrane and synaptic vesicles. [3H]D-aspartate and [3H]GABA released by hypotonic shock is determined by sodium lowering rather than by osmolarity decreasing itself.

Active zones for presynaptic plasticity in the brain

Some of the most abundant synapses in the brain such as the synapses formed by the hippocampal mossy fibers, cerebellar parallel fibers and several types of cortical afferents express presynaptic forms of long-term potentiation (LTP), a putative cellular model for spatial, motor and fear learning. Those synapses often display presynaptic mechanisms of LTP induction, which are either NMDA receptor independent of dependent of presynaptic NMDA receptors. Recent investigations on the molecular mechanisms of neurotransmitter release modulation in short- and long-term synaptic plasticity in central synapses give a preponderant role to active zone proteins as Munc-13 and RIM1-alpha, and point toward the maturation process of synaptic vesicles prior to Ca(2+)-dependent fusion as a key regulatory step of presynaptic plasticity.

Determination of cell capacitance using the exact empirical solution of partial differential Y/partial differential Cm and its phase angle

Measures of membrane capacitance offer insight into a variety of cellular processes. Unfortunately, popular methodologies rely on model simplifications that sensitize them to interference from inevitable changes in resistive components of the traditional cell-clamp model. Here I report on a novel method to measure membrane capacitance that disposes of the usual simplifications and assumptions, yet is immune to such interference and works on the millisecond timescale. It is based on the exact empirical determination of the elusive partial derivative, partial differential Y/partial differential C(m), which heretofore had been approximated. Furthermore, I illustrate how this method extends to the vesicle fusion problem by permitting the determination of partial differential Y(v)/partial differential C(v), thereby providing estimates of fusion pore conductance and vesicle capacitance. Finally, I provide simulation examples and physiological examples of how the method can be used to study processes that are routinely interrogated by measures of membrane capacitance.

A passionate kiss, then run: exocytosis and recycling of IgG by FcRn

The MHC-class-I-like Fcgamma receptor FcRn recycles immunoglobulin (Ig)G from most cells and transports it bi-directionally across epithelial barriers to affect systemic and mucosal immunity. Recent studies have shown that FcRn rescues IgG from intracellular lysosomal degradation by recycling it from the sorting endosome to the cell surface. Most recycling vesicles fuse completely with the plasma membrane in a classical pattern of exocytosis. Similar to the process seen for neurotransmitter release at synaptic junctions, other vesicles fuse only partially, releasing FcRn-IgG complexes to mix into the plasma membrane in cycles of 3-4s over prolonged periods of time.

Distribution of proteins associated with synaptic vesicle endocytosis in the mouse and goldfish retina

Current models of synaptic transmission require retrieval of membrane from the presynaptic terminal following neurotransmitter exocytosis. Dynamin, a GTPase, is thought to be critical for this retrieval process. At ribbon synapses of retinal bipolar neurons, however, compensatory endocytosis does not require GTP hydrolysis, suggesting that endocytosis mechanisms may differ among synapses. To understand better the synaptic vesicle recycling at conventional and ribbon synapses, the distributions of dynamin and two associated proteins, amphiphysin and clathrin, were examined in the retinas of goldfish and mouse by using immunocytochemical methods. Labeling for dynamin, clathrin, and amphiphysin was distributed differentially among conventional and ribbon synapses in retinas of both species. Ribbon synapses of photoreceptors and most bipolar cells labeled only weakly for dynamin relative to conventional synapses. Amphyiphysin labeling was strong at many ribbon synapses, and labeling in rod terminals was stronger than in cone terminals in the mouse retina. Clathrin labeling was heterogeneous among ribbon synapses. Similarly to the case with amphiphysin, mouse rod terminals showed stronger clathrin labeling than cone terminals. Among conventional synapses, there was heterogeneous labeling for all three endocytic proteins. Some labeling for each protein might have been associated with postsynaptic terminals. The differential distribution of labeling for these proteins among identified synapses in the retina suggests considerable heterogeneity in the molecular mechanisms underlying synaptic membrane retrieval, even among synapses with similar active zone ultrastructure. Thus, as with exocytosis, mechanisms of synaptic membrane retrieval may be tuned by the precise complement of proteins expressed within the synaptic terminal.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Cycling of synaptic vesicles: how far? How fast!

Synaptic transmission is based on the regulated exocytotic fusion of synaptic vesicles filled with neurotransmitter. In order to sustain neurotransmitter release, these vesicles need to be recycled locally. Recent data suggest that two tracks for the cycling of synaptic vesicles coexist: a slow track in which vesicles fuse completely with the presynaptic plasma membrane, followed by clathrin-mediated recycling of the vesicular components, and a fast track that may correspond to the transient opening and closing of a fusion pore. In this review, we attempt to provide an overview of the components involved in both tracks of vesicle cycling, as well as to identify possible mechanistic links between these two pathways.

Recapture after exocytosis causes differential retention of protein in granules of bovine chromaffin cells

After exocytosis, chromaffin granules release essentially all their catecholamines in small fractions of a second, but it is unknown how fast they release stored peptides and proteins. Here we compare the exocytic release of fluorescently labelled neuropeptide Y (NPY) and tissue plasminogen activator from single granules. Exocytosis was tracked by measuring the membrane capacitance, and single granules in live cells were imaged by evanescent field microscopy. Neuropeptide Y left most granules in small fractions of a second, while tissue plasminogen activator remained in open granules for minutes. Taking advantage of the dependence on pH of the fluorescence of green fluorescent protein, we used rhythmic external acidification to determine whether and when granules re-sealed. One-third of them re-sealed within 100 s and retained significant levels of tissue plasminogen activator. Re-sealing accounts for only a fraction of the endocytosis monitored in capacitance measurements. When external [Ca2+] was raised, even neuropeptide Y remained in open granules until they re-sealed. It is concluded that a significant fraction of chromaffin granules re-seal after exocytosis, and retain those proteins that leave granules slowly. We suggest that granules vary the stoichiometry of release by varying both granule re-sealing and the association of proteins with the granule matrix.

Kinetic regulation of vesicle endocytosis 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.

Visualization of regulated exocytosis with a granule-membrane probe using total internal reflection microscopy

Secretory granules labeled with Vamp-green fluorescent protein (GFP) showed distinct signatures upon exocytosis when viewed by total internal reflection fluorescence microscopy. In approximately 90% of fusion events, we observed a large increase in fluorescence intensity coupled with a transition from a small punctate appearance to a larger, spreading cloud with free diffusion of the Vamp-GFP into the plasma membrane. Quantitation suggests that these events reflect the progression of an initially fused and spherical granule flattening into the plane of the plasma membrane as the Vamp-GFP simultaneously diffuses through the fusion junction. Approximately 10% of the events showed a transition from puncta to ring-like structures coupled with little or no spreading. The ring-like images correspond quantitatively to granules fusing and retaining concavity (recess of approximately 200 nm). A majority of fusion events involved granules that were present in the evanescent field for at least 12 s. However, approximately 20% of the events involved granules that were present in the evanescent field for no more than 0.3 s, indicating that the interaction of the granule with the plasma membrane that leads to exocytosis can occur within that time. In addition, approximately 10% of the exocytotic sites were much more likely to occur within a granule diameter of a previous event than can be accounted for by chance, suggestive of sequential (piggy-back) exocytosis that has been observed in other cells. Overall granule behavior before and during fusion is strikingly similar to exocytosis previously described in the constitutive secretory pathway.

Regulation of the fusion pore conductance during exocytosis by cyclin-dependent kinase 5

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase involved in synaptogenesis and brain development, and its enzymatic activity is essential for slow forms of synaptic vesicle endocytosis. Recent work also has implicated Cdk5 in exocytosis and synaptic plasticity. Pharmacological inhibition of Cdk5 modifies secretion in neuroendocrine cells, synaptosomes, and brain slices; however, the specific mechanisms involved remain unclear. Here we demonstrate that dominant-negative inhibition of Cdk5 increases quantal size and broadens the kinetics of individual exocytotic events measured by amperometry in adrenal chromaffin cells. Conversely, Cdk5 overexpression narrows the kinetics of fusion, consistent with an increase in the extent of kiss-and-run exocytosis. Cdk5 inhibition also increases the total charge and current of catecholamine released during the amperometric foot, representing a modification of the conductance of the initial fusion pore connecting the granule and plasma membrane. We suggest that these effects are not attributable to an alteration in catecholamine content of secretory granules and therefore represent an effect on the fusion mechanism itself. Finally, mutational silencing of the Cdk5 phosphorylation site in Munc18, an essential protein of the late stages of vesicle fusion, has identical effects on amperometric spikes as dominant-negative Cdk5 but does not affect the amperometric feet. Cells expressing Munc18 T574A have increased quantal size and broader kinetics of fusion. These results suggest that Cdk5 could, in part, control the kinetics of exocytosis through phosphorylation of Munc18, but Cdk5 also must have Munc18-independent effects that modify fusion pore conductance, which may underlie a role of Cdk5 in synaptic plasticity.