Differential regulation at functionally divergent release sites along a common axon

Information transfer within neuronal networks requires the precise coordination of distinct neuronal populations within a given circuit. Evidence from a variety of central pathways indicates that such coordination is mediated in part by the ability of neurons to differentially regulate release properties at functionally divergent presynaptic elements along their individual axons according to the identity of the postsynaptic cell being innervated. Recent findings have revealed the cellular mechanisms by which central afferents modify release properties at individual presynaptic sites independent of neighboring terminals. Such autonomy of presynaptic regulation enables target-cell-dependent short-term and long-term synaptic plasticity and ensures that distinct features of afferent activity are relayed to divergent target-cell populations.     

Altered presynaptic vesicle release and cycling during mGluR-dependent LTD

The site of modification of synaptic transmission during long-term plasticity in the mammalian hippocampus remains controversial. Here we used a fluorescent marker of presynaptic activity, FM 1-43, to directly image presynaptic function during metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) at CA3-CA1 excitatory synapses in acute hippocampal slices. We found a significant decrease in the rate of FM 1-43 release in response to synaptic stimulation following induction of mGluR-LTD, providing direct evidence for altered presynaptic function. Moreover, we found that mGluR-LTD causes several changes in FM dye release properties that are consistent with a change in the mode of vesicle cycling, possibly involving a switch from a full fusion mode of release to a "kiss-and-run" mode of release through the transient opening of a fusion pore.     

Regulation of Synaptic Transmission by Presynaptic CaMKII and BK Channels

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and the BK channel are enriched at the presynaptic nerve terminal, where CaMKII associates with synaptic vesicles whereas the BK channel colocalizes with voltage-sensitive Ca²⁺ channels in the plasma membrane. Mounting evidence suggests that these two proteins play important roles in controlling neurotransmitter release. Presynaptic BK channels primarily serve as a negative regulator of neurotransmitter release. In contrast, presynaptic CaMKII either enhances or inhibits neurotransmitter release and synaptic plasticity depending on experimental or physiological conditions and properties of specific synapses. The different functions of presynaptic CaMKII appear to be mediated by distinct downstream proteins, including the BK channel.     

Calcium and the tonic release of transmitter at a non-impulsive synapse in the crab

Depolarization-transmitter release coupling was studied in the promotor stretch receptor/motoneuron synapse of the crab. Callinectes sapidus, a preparation in which presynaptic action potentials do not occur. Intracellular microelectrode recordings were made from the presynaptic terminal and from the somata of postsynaptic motoneurons while injecting current pulses into the peripheral stretch receptor dendrite with the aid of the sucrose-gap. 1. For short current pulses, the relationship between presynaptic potential and postsynaptic response was found to be similar to that demonstrated in the giant synapse of the squid stellate ganglion, indicating a common reliance on the properties of voltage-dependent calcium channels. 2. The crab synapse was found to be capable of continuous transmission in the range of seconds and minutes without the pronounced depletion of transmitter seen in the squid, and without inactivation of the release process (i.e., the calcium conductance is non-inactivating). 3. A graded, transient response to depolarising current in the presynaptic fibre was found to be calcium-dependent, and probably to reflect the presence of a separate, inactivating calcium conductance. 4. It was concluded that the graded response of the presynaptic membrane could function in helping to compensate for capacitative distortion of receptor potentials decrementally conducted in the sensory dendrite, and was therefore a specialisation for non-impulsive transmission.     

Further Study of the Relationship between Pre- and Postsynaptic Potentials in the Squid Giant Synapse

The minimal presynaptic depolarization (MPD) for producing a detectable postsynaptic potential (PSP) was lower than 25 mv in normal or tetrodotoxin (TTX)-containing seawater. The MPD was about 10 mv when a small amount of tetraethylammonium ions (TEA) was injected into the presynaptic terminal. Application of linearly increasing depolarizing current to the normal presynaptic terminal at times produced a PSP before a presynaptic spike was evoked; the rate of rise of the resulting PSP was much slower than that of a PSP triggered by the normal presynaptic spike. A brief depolarizing pulse that preceded the presynaptic spike in normal seawater or the initial transient presynaptic depolarization in TTX decreased the PSP. It increased the PSP when it was applied during the spike or initial transient depolarization. Hyperpolarizing pulses had the reverse effect. The Off-PSP was also modified by inserting pulses at an initial part of the recovery phase of the strong presynaptic depolarization. These results indicate further that increases in Na⁺ and K⁺ conductance during presynaptic spike activity are not a requirement for transmitter release; the rate of release of transmitter can be controlled by electrical manipulation of the presynaptic terminal; there is a superficial correspondence between the time courses of presynaptic depolarization and the resulting PSP. Thus presynaptic depolarization appears to be only the first step in the series of events constituting excitation-transmitter release coupling. It may not be a necessary step for the release mechanism.     

A genetic analysis of synaptic development: pre- and postsynaptic dCBP control transmitter release at the Drosophila NMJ

Postsynaptic dCBP (Drosophila homolog of the CREB binding protein) is required for presynaptic functional development. Viable, hypomorphic dCBP mutations have a approximately 50% reduction in presynaptic transmitter release without altering the Ca2+ cooperativity of release or synaptic ultrastructure (total bouton number is increased by 25%-30%). Exogenous expression of dCBP in muscle rescues impaired presynaptic release in the dCBP mutant background, while presynaptic dCBP expression does not. In addition, overexpression experiments indicate that elevated dCBP can also inhibit presynaptic functional development in a manner distinct from the effects of dCBP loss of function. Pre- or postsynaptic overexpression of dCBP (in wild type) reduces presynaptic release. However, we do not observe an increase in bouton number, and presynaptic overexpression impairs short-term facilitation. These data suggest that dCBP participates in a postsynaptic regulatory system that controls functional synaptic development.     

Spontaneous and Evoked Quantal Transmitter Release at a Vertebrate Central Synapse

THIS report deals with some of the statistical properties of quantal transmitter release at the Mauthner fibre-giant fibre synapse in the hatchetfish Gasteropelecus and the way in which these properties are modified by presynaptic intracellular depolarization. The experiments reported indicate that Poisson statistics describe transmitter release in the hatchetfish at low, but not at high, release levels.     

Correlation of Transmitter Release with Membrane Properties of the Presynaptic Fiber of the Squid Giant Synapse

Depolarization of the presynaptic terminal by current produced a postsynaptic potential (PSP) which increased with increasing presynaptic polarization and then reached a plateau. Iontophoretic injection of tetraethylammonium ions (TEA) into the presynaptic axon near the terminal produced a prolonged presynaptic spike. The resulting PSP is increased in size and its time course closely followed that of the presynaptic spike. The presynaptic fiber no longer exhibited rectification and strong depolarizations revealed that the PSP reached a maximum with about 110 mv depolarization. Further depolarization produced a decrease in PSP amplitude and finally transmission was blocked. However, a PSP then always appeared on withdrawal of the depolarizing current. Under the conditions of these experiments, the PSP could be considered a direct measure of transmitter release. Bathing the TEA-injected synapse with concentrations of tetrodotoxin (TTX) sufficient to block spike activity in both pre- and postsynaptic axons did not greatly modify postsynaptic electrogenesis. However, doubling TTX concentration reversibly blocked PSP. Thus the permeability changes to Na and K accompanying the spike do not appear necessary for transmitter release. Some other processes related to the level of presynaptic polarization must be involved to explain the data. The inhibition of transmitter release by strong depolarizations appears to be related to Ca action. A membrane Ca current may also be necessary for normal transmitter release.     

Differential pharmacological properties of GABAA receptors in axon terminals and soma of dentate gyrus granule cells

Although it has been well established that GABA_A receptors are molecular targets of a variety of allosteric modulators, such as benzodiazepines, the pharmacological properties of presynaptic GABA_A receptors are poorly understood. In this study, the effects of diazepam and Zn²⁺ on presynaptic GABA_A receptors have been investigated by measuring the GABA_A receptor-mediated facilitation of spontaneous glutamate release in mechanically dissociated rat CA3 pyramidal neurons. Diazepam significantly enhanced the muscimol-induced facilitation (particularly at submicromolar concentrations) of spontaneous glutamate release and shifted the concentration–response relationship for muscimol toward the left, whereas Zn²⁺ (≤ 100 μM) had little effect on the muscimol-induced facilitation of spontaneous glutamate release. In contrast, Zn²⁺ significantly suppressed the muscimol-induced currents mediated by GABA_A receptors expressed on dentate gyrus granule cells, which are parent neurons of mossy fibers, whereas the effect of diazepam on GABA_A receptors expressed on dentate gyrus granule cells was lesser than that on presynaptic GABA_A receptors. The results suggest that the pharmacological properties of GABA_A receptors differ considerably between presynaptic (axon terminals) and somatic regions in the same granule cell and that presynaptic GABA_A receptors should be considered as one of the important pharmacological targets of many drugs affecting GABA_A receptors.     

Functional maturation of CA1 synapses involves activity-dependent loss of tonic kainate receptor-mediated inhibition of glutamate release

Early in development, excitatory synapses transmit with low efficacy, one mechanism for which is a low probability of transmitter release (Pr). However, little is known about the developmental mechanisms that control activity-dependent maturation of the presynaptic release. Here, we show that during early development, transmission at CA3-CA1 synapses is regulated by a high-affinity, G protein-dependent kainate receptor (KAR), which is endogenously activated by ambient glutamate. By tonically depressing glutamate release, this mechanism sets the dynamic properties of neonatal inputs to favor transmission during high frequency bursts of activity, typical for developing neuronal networks. In response to induction of LTP, the tonic activation of KAR is rapidly down regulated, causing an increase in Pr and profoundly changing the dynamic properties of transmission. Early development of the glutamatergic connectivity thus involves an activity-dependent loss of presynaptic KAR function producing maturation in the mode of excitatory transmission from CA3 to CA1.     

Presynaptic mechanisms of zinc effects on the neuromuscular transmission]

The effect of zinc ions on presynaptic currents and transmitter release was studied at the neuromuscular junction of the frog cutaneous pectoris muscle preparation with using an extracellular microelectrode. It has been shown that zinc (100 mkM) amplified MEPP frequency at first, but suppressed it later. Zinc affected the presynaptic spike waveform and transmitter release in a concentration-dependent manner. Depending on concentration and time of exposure zinc increased or suppressed transmitter release. Increase of transmitter release was shown to be resulted by blockade voltage gated and calcium activated potassium channels in nerve ending, leading to broad of both presynaptic spike and action potential. Strong change of presynaptic spike waveform after high concentration zinc treatment supposed that under this condition zinc depressed voltage gated calcium and sodium channel leading to decrease of transmitter release. It was concluded that the final and irreversible depression of acetylcholine release by zinc was due to alteration of whole ion conductances in nerve ending and to change of configuration of proteins included in structure of ion channels. It is discussed possible mechanisms of various effects of zinc ions at the neuromuscular synapse.     

Unwebbing the presynaptic web

The release of neurotransmitter from nerve terminals occurs at a specialized region of the presynaptic plasma membrane called the active zone. A dense matrix of proteins associated with the active zone, called the presynaptic web, is thought to play a fundamental role in defining these neurotransmitter release sites. In this issue of Neuron, Phillips et al. have identified conditions for the biochemical purification of the presynaptic web and show that the web is comprised of proteins involved in the docking, fusion, and recycling of synaptic vesicles.     

The effect of repetitive stimulation on the passive electrical properties of the presynaptic terminal of the squid giant synapse.

The resting electrical properties of the presynaptic terminal of the squid giant synapse have been determined by using constant current pulses. After short periods of repetitive stimulation, the terminal resistance, time constant and capacitance are found to be increased. These changes are absent in terminals bathed in artificial sea water containing no calcium, and sea water containing 5 mM cobalt. It seems likely that these changes are associated with transmitter release.     

Presynaptic alpha-receptor control of adrenergic transmitter release in blood vessels

The presynaptic alpha-adrenergic receptor control of transmitter release in vascular tissues is discussed. A model of adrenergic innervation of the vascular bed is proposed based on ultrastructural and histochemical evidence. Evidence is presented to support the concept of intermittent or periodic release of norepinephrine (NE) from the varicosity. Intermittency combined with a mechanism such as presynaptic control to ensure spatial distribution of release sites, along with a slow effector response and recovery, results in a smooth, generalized change in tone and an overall economy of transmitter. The effective concentration of NE around the presynaptic membrane is maintained for considerably less than 0.1 s. It is argued that the transient presence of transmitter in the synapse combined with intermittency of release does not favor accumulation of transmitter at the cleft at physiological frequencies or desensitization of presynaptic receptors. In addition, intermittency provides an explanation for why exogenous NE is more effective presynaptically in influencing release than endogenous NE. The importance of cleft width in presynaptic control of transmitter release, the possible complications caused by facilitation, and resolution of some apparent problems with the presynaptic hypothesis are also discussed.     

Transsynaptic control of presynaptic Ca²⁺ influx achieves homeostatic potentiation of neurotransmitter release

Given the complexity of the nervous system and its capacity for change, it is remarkable that robust, reproducible neural function and animal behavior can be achieved. It is now apparent that homeostatic signaling systems have evolved to stabilize neural function. At the neuromuscular junction (NMJ) of organisms ranging from Drosophila to human, inhibition of postsynaptic neurotransmitter receptor function causes a homeostatic increase in presynaptic release that precisely restores postsynaptic excitation. Here we address what occurs within the presynaptic terminal to achieve homeostatic potentiation of release at the Drosophila NMJ. By imaging presynaptic Ca(2+) transients evoked by single action potentials, we reveal a retrograde, transsynaptic modulation of presynaptic Ca(2+) influx that is sufficient to account for the rapid induction and sustained expression of the homeostatic change in vesicle release. We show that the homeostatic increase in Ca(2+) influx and release is blocked by a point mutation in the presynaptic CaV2.1 channel, demonstrating that the modulation of presynaptic Ca(2+) influx through this channel is causally required for homeostatic potentiation of release. Together with additional analyses, we establish that retrograde, transsynaptic modulation of presynaptic Ca(2+) influx through CaV2.1 channels is a key factor underlying the homeostatic regulation of neurotransmitter release.     

The neurotoxin histrionicotoxin interacts with the putative ion channel of the nicotinic acetylcholine receptors in the central nervous system

Perhydrohistrionicotoxin at micromolar concentrations blocked the nicotine-evoked transmitter release from perfused striatal (dopaminergic) and hippocampal (cholinergic) nerve terminals. Perhydrohistrionicotoxin failed to compete with [3H]nicotine for its high-affinity binding site in rat brain, suggesting that the action of this toxin on central nicotinic receptors is noncompetitive. From the dose-response curve, 50% inhibition of nicotine-evoked striatal dopamine release occurred at 5 microM perhydrohistrionicotoxin, a value similar to that obtained in frog sartorius muscle and Electrophorus electroplax. This close agreement may suggest that the ionic channel of the presynaptic nicotinic acetylcholine receptor of brain neurons has similar properties to those of the peripheral receptor.     

Mechanisms of facilitation of synaptic glutamate release by nicotinic agonists in the nucleus of the solitary tract

The nucleus of the solitary tract (NTS) is the principal integrating relay in the processing of visceral sensory information. Functional nicotinic acetylcholine receptors (nAChRs) have been found on presynaptic glutamatergic terminals in subsets of caudal NTS neurons. Activation of these receptors has been shown to enhance synaptic release of glutamate and thus may modulate autonomic sensory-motor integration and visceral reflexes. However, the mechanisms of nAChR-mediated facilitation of synaptic glutamate release in the caudal NTS remain elusive. This study uses rat horizontal brainstem slices, patch-clamp electrophysiology, and fluorescent Ca(2+) imaging to test the hypothesis that a direct Ca(2+) entrance into glutamatergic terminals through active presynaptic non-α7- or α7-nAChR-mediated ion channels is sufficient to trigger synaptic glutamate release in subsets of caudal NTS neurons. The results of this study demonstrate that, in the continuous presence of 0.3 μM tetrodotoxin, a selective blocker of voltage-activated Na(+) ion channels, facilitation of synaptic glutamate release by activation of presynaptic nAChRs (detected as an increase in the frequency of miniature excitatory postsynaptic currents) requires external Ca(2+) but does not require activation of presynaptic Ca(2+) stores and presynaptic high- and low-threshold voltage-activated Ca(2+) ion channels. Expanding the knowledge of mechanisms and pharmacology of nAChRs in the caudal NTS should benefit therapeutic approaches aimed at restoring impaired autonomic homeostasis.     

Stochastic and Reduced Biophysical Models of Synaptic Transmission

Stochastic and reduced biophysical models of synaptictransmission are formulated and evaluated. Thesynaptic transmission involves presynapticfacilitation of neurotransmitter release, depletionand recovery of the presynaptic pool of readilyreleasable vesicles containing neurotransmittermolecules and saturation of postsynaptic receptors ofboth fast non-NMDA and slow NMDA types. The models areshown to display the principal dynamicalcharacteristics experimentally observed of synaptictransmission. The two main types of neural coding,i.e. rate and temporal coding, can be distinguished bymeans of different dynamical properties of synaptictransmission determined by initial neurotransmitterrelease probability and presynaptic firing rate. Fromthe temporal evolution of the postsynaptic membranepotential response to a train of presynaptic actionpotentials at a sustained firing rate, in particularthe steady-state amplitude and steady-state averagelevel of postsynaptic membrane potentials aredetermined as functions of both initial releaseprobability and presynaptic firing rate. The modelsare applicable to studies of the primary stages oflearning processes and can be extended to incorporateshort-term and long-term potentiation in memoryconsolidation processes.     

Direct interaction between the ARF-specific guanine nucleotide exchange factor msec7-1 and presynaptic Munc13-1.

Msec7-1, a mammalian homologue of yeast sec7p, is a specific GDP/GTP exchange factor for small G-proteins of the ARF family. Overexpression of msec7-1 in Xenopus neuromuscular junctions leads to an increase in synaptic transmitter release that is most likely caused by an increase in the pool of readily releasable vesicles. However, the molecular mechanisms by which msec7-1 is targeted to presynaptic compartments and enhances neurotransmitter release are not known. In the present study, we demonstrate that msec7-1 interacts directly with Munc13-1, a phorbol ester-dependent enhancer of neurotransmitter release that is specifically localized to presynaptic transmitter release zones. Given that Munc13-1 and msec7-1 participate in very similar presynaptic processes and because Munc13-1 is specifically targeted to presynaptic active zones, we suggest that the msec7-1/Munc13-1 interaction serves to colocalize the two proteins at the active zone, a subcellular compartment with extremely high membrane turnover.