Calcium stores in hippocampal synaptic boutons mediate short-term plasticity, store-operated Ca2+ entry, and spontaneous transmitter release

Evoked transmitter release depends upon calcium influx into synaptic boutons, but mechanisms regulating bouton calcium levels and spontaneous transmitter release are obscure. To understand these processes better, we monitored calcium transients in axons and presynaptic terminals of pyramidal neurons in hippocampal slice cultures. Action potentials reliably evoke calcium transients in axons and boutons. Calcium-induced calcium release (CICR) from internal stores contributes to the transients in boutons and to paired-pulse facilitation of EPSPs. Store depletion activates store-operated calcium channels, influencing the frequency of spontaneous transmitter release. Boutons display spontaneous Ca2+ transients; blocking CICR reduces the frequency of these transients and of spontaneous miniature synaptic events. Thus, spontaneous transmitter release is largely calcium mediated, driven by Ca2+ release from internal stores. Bouton store release is important for short-term synaptic plasticity and may also contribute to long-term plasticity.     

Retrograde inhibition of presynaptic calcium influx by endogenous cannabinoids at excitatory synapses onto Purkinje cells

Brief depolarization of cerebellar Purkinje cells was found to inhibit parallel fiber and climbing fiber EPSCs for tens of seconds. This depolarization-induced suppression of excitation (DSE) is accompanied by altered paired-pulse plasticity, suggesting a presynaptic locus. Fluorometric imaging revealed that postsynaptic depolarization also reduces presynaptic calcium influx. The inhibition of both presynaptic calcium influx and EPSCs is eliminated by buffering postsynaptic calcium with BAPTA. The cannabinoid CB1 receptor antagonist AM251 prevents DSE, and the agonist WIN 55,212-2 occludes DSE. These findings suggest that Purkinje cells release endogenous cannabinoids in response to elevated calcium, thereby inhibiting presynaptic calcium entry and suppressing transmitter release. DSE may provide a way for cells to use their firing rate to dynamically regulate synaptic inputs. Together with previous studies, these findings suggest a widespread role for endogenous cannabinoids in retrograde synaptic inhibition.     

How does calcium trigger neurotransmitter release?

Recent work has established that different geometric arrangements of calcium channels are found at different presynaptic terminals, leading to a wide spectrum of calcium signals for triggering neurotransmitter release. These calcium signals are apparently transduced by synaptotagmins - calcium-binding proteins found in synaptic vesicles. New biochemical results indicate that all synaptotagmins undergo calcium-dependent interactions with membrane lipids and a number of other presynaptic proteins, but which of these interactions is responsible for calcium-triggered transmitter release remains unclear.     

A novel presynaptic inhibitory mechanism underlies paired pulse depression at a fast central synapse.

Several distinct mechanisms may cause synaptic depression, a common form of short-term synaptic plasticity. These include postsynaptic receptor desensitization, presynaptic depletion of releasable vesicles, or other presynaptic mechanisms depressing vesicle release. At the endbulb of Held, a fast central calyceal synapse in the auditory pathway, cyclothiazide (CTZ) abolished marked paired pulse depression (PPD) by acting presynaptically to enhance transmitter release, rather than by blocking postsynaptic receptor desensitization. PPD and its response to CTZ were not altered by prior depletion of the releasable vesicle pool but were blocked by lowering external calcium concentration, while raising external calcium enhanced PPD. We conclude that a major component of PPD at the endbulb is due to a novel, transient depression of release, which is dependent on the level of presynaptic calcium entry and is CTZ sensitive.     

Stores not just for storage. intracellular calcium release and synaptic plasticity.

Activation of most excitatory synapses of central neurons produces calcium release signals from intracellular stores. Synaptically evoked calcium release from stores is frequently triggered by the binding of glutamate to metabotropic receptors and the subsequent activation of IP(3) receptors in spines and dendrites. There is increasing evidence for the presence of local calcium signals caused by calcium-induced calcium release (CICR) through activation of ryanodine or IP(3) receptors. Recent work on mutant mice indicates that store signaling determines activity-dependent synaptic plasticity.     

The maintenance of LTP at hippocampal mossy fiber synapses is independent of sustained presynaptic calcium.

We have examined the role of presynaptic residual calcium in maintaining long-term changes in synaptic efficacy observed at mossy fiber synapses between hippocampal dentate granule cells and CA3 pyramidal cells. Calcium concentrations in individual mossy fiber terminals in hippocampal slice were optically measured with the calcium indicator fura-2 while stimulating the mossy fiber pathway and recording excitatory postsynaptic potentials extracellularly. Short-term synaptic enhancement was accompanied by increased presynaptic residual calcium concentration. A 2-fold enhancement of transmitter release was accompanied by a 10-30 nM increase in residual calcium. Following induction of mossy fiber LTP, transiently elevated presynaptic calcium decayed to prestimulus levels, whereas enhancement of synaptic transmission persisted. Our results demonstrate that, despite an apparent strong sensitivity of synaptic enhancement to presynaptic residual calcium levels, sustained increases in presynaptic residual calcium levels are not responsible for the maintained synaptic enhancement observed during mossy fiber LTP.     

BCL-xL regulates synaptic plasticity

Mitochondria are the predominant organelle within many presynaptic terminals. During times of high synaptic activity, they affect intracellular calcium homeostasis and provide the energy needed for synaptic vesicle recycling and for the continued operation of membrane ion pumps. Recent discoveries have altered our ideas about the role of mitochondria in the synapse. Mitochondrial localization, morphology, and docking at synaptic sites may indeed alter the kinetics of transmitter release and calcium homeostasis in the presynaptic terminal. In addition, the mitochondrial ion channel BCL-xL, known as a protector against programmed cell death, regulates mitochondrial membrane conductance and bioenergetics in the synapse and can thereby alter synaptic transmitter release and the recycling of pools of synaptic vesicles. BCL-xL, therefore, not only affects the life and death of the cell soma, but its actions in the synapse may underlie the regulation of basic synaptic processes that subtend learning, memory and synaptic development.     

神经元突触前可塑性的结构及分子基础

突触可塑性是神经元间信息传递的重要生理调控机制,它包括突触前可塑性和突触后可塑性.突触前可塑性是指通过对神经递质释放过程的干预、修饰,调节突触强度的过程.突触强度的变化,是通过影响量子的大小,活动区的个数和囊泡释放概率来实现的.而突触前囊泡活动尤为重要：从转运、搭靠、融合至内吞进入下一轮循环,每一步都是由一群互相作用的蛋白质共同完成的.     

Presenilins in synaptic function and disease

The presenilin genes harbor approximately 90% of mutations linked to early-onset familial Alzheimer's disease (FAD), but how these mutations cause the disease is still being debated. Genetic analysis in Drosophila and mice demonstrate that presenilin plays essential roles in synaptic function, learning and memory, as well as neuronal survival in the adult brain, and the FAD-linked mutations alter the normal function of presenilin in these processes. Presenilin has also been reported to regulate the calcium homeostasis of intracellular stores, and presynaptic presenilin controls neurotransmitter release and long-term potentiation through modulation of calcium release from intracellular stores. In this review, we highlight recent advances in deciphering the role of presenilin in synaptic function, calcium regulation and disease, and pose key questions for future studies.     

Glutamate uptake in synaptic plasticity: from mollusc to mammal

A great deal of research has been directed toward understanding the cellular mechanisms underlying synaptic plasticity and memory formation. To this point, most research has focused on the more "active" components of synaptic transmission: presynaptic transmitter release and postsynaptic transmitter receptors. Little work has been done characterizing the role neurotransmitter transporters might play during changes in synaptic efficacy. We review several new experiments that demonstrate glutamate transporters are regulated during changes in the efficacy of glutamatergic synapses. This regulation occurred during long-term facilitation of the sensorimotor synapse of Aplysia and long-term potentiation of the Schaffer-collateral synapse of the rat. We propose that glutamate transporters are "co-regulated" with other molecules/processes involved in synaptic plasticity, and that this process is phylogenetically conserved. These new findings indicate that glutamate transporters most likely play a more active role in neurotransmission than previously believed.     

The molecular mechanisms of quantum mediator secretion in the synapse

The modern condition of knowledge about the molecular mechanisms underlying the quantal transmitter release in the central and the peripheric synapses is analysed. The data about the synaptic vesicles types, their forming, transporting to the sites of release at the nerve endings, exo- and endocytosis processes are presented. Ultrastructural and molecular organization of active zone of nerve ending and transmitter release morphofunctional unit--secretosome, which includes synaptic vesicle, exocytosis protein complex and calcium channels, are described. The basic proteins involved in the exo- and endocytosis and their interactions during transmitter release are examined. The role of the intracellular buffer systems, calcium micro- and macrodomains in the quantal transmitter secretion are considered. The reasons of the active zones functional non-uniformity and plasticity and factors reduced transmitter release in the active zone to the single quantum are analysed.     

Ion channels in nerve ending

The modern data about the structure and function of the nerve ending ion channels are generalized and systematized. Ion channels of nerve endings provide the forming of the rest membrane potential, excitability, generation of action potential, regulate the intracellular concentration of calcium ions, take part in exocytosis of synaptic vesicules, participate in short-term and long-term synaptic plasticity, ensure the modulation of presynaptic functions. Methods of investigation of ion channels and data about their localization in central and peripheral nerve systems are represented. The review gives the functional characteristics, molecular structure and mechanisms of regulation of the known voltage- and ligand-dependent ion channels, the role of the certain types of ion channels in the machinery of transmitter release.     

Single synaptic events evoke NMDA receptor-mediated release of calcium from internal stores in hippocampal dendritic spines

We have used confocal microscopy to monitor synaptically evoked Ca2+ transients in the dendritic spines of hippocampal pyramidal cells. Individual spines respond to single afferent stimuli (<0.1 Hz) with Ca2+ transients or failures, reflecting the probability of transmitter release at the activated synapse. Both AMPA and NMDA glutamate receptor antagonists block the synaptically evoked Ca2+ transients; the block by AMPA antagonists is relieved by low Mg2+. The Ca2+ transients are mainly due to the release of calcium from internal stores, since they are abolished by antagonists of calcium-induced calcium release (CICR); CICR antagonists, however, do not depress spine Ca2+ transients generated by backpropagating action potentials. These results have implications for synaptic plasticity, since they show that synaptic stimulation can activate NMDA receptors, evoking substantial Ca2+ release from the internal stores in spines without inducing long-term potentiation (LTP) or depression (LTD).     

Can presynaptic depolarization release transmitter without calcium influx?

Recent experimental evidence suggesting that presynaptic depolarization can evoke transmitter release without calcium influx has been re-examined. The presynaptic terminal of the squid giant synapse can be depolarized by variable amounts while recording presynaptic calcium current under voltage clamp and postsynaptic responses. Small depolarizations open few calcium channels with large single channel currents. Large depolarizations approaching the calcium equilibrium potential open many channels with small single channel currents. When responses to small and large depolarizations eliciting similar total macroscopic calcium currents are compared, the large pulses evoke more transmitter release. This apparent voltage-dependence of transmitter release may be explained by the greater overlap of calcium concentration domains surrounding single open calcium channels when many closely apposed channels open at large depolarizations. This channel domain overlap leads to higher calcium concentrations at transmitter release sites and more release for large depolarizations than for small depolarizations which open few widely dispersed channels. At neuromuscular junctions, a subthreshold depolarizing pulse to motor nerve terminals may release over a thousand times as much transmitter if it follows a brief train of presynaptic action potentials than if it occurs in isolation. This huge synaptic facilitation has been taken as indicative of a direct effect of voltage which is manifest only when prior activity raises presynaptic resting calcium levels. This large facilitation is actually due to a post-tetanic supernormal excitability in motor nerve terminals, causing the previously subthreshold test pulse to become suprathreshold and elicit a presynaptic action potential. When motor nerve terminals are depolarized by two pulses, as the first pulse increases above a certain level it evokes more transmitter release but less facilitation of the response to the second pulse.(ABSTRACT TRUNCATED AT 250 WORDS)     

The release of acetylcholine: from a cellular towards a molecular mechanism

The isolation of synaptic vesicles rich in acetylcholine (ACh) from the electric organ of Torpedo has indeed strengthened the hypothesis of transmitter exocytosis, but soon after it was found that non-vesicular free ACh was released and renewed upon stimulation. In contrast, vesicular ACh and the number of vesicles remained stable during physiological stimulations. In addition free ACh variations (representing the cytoplasmic pool) were correlated to the release kinetics as measured by the electroplaque discharge. Consequently, the mechanism releasing ACh from the cytoplasm in a packet form was searched at the presynaptic membrane itself. With synaptosomes isolated from the electric organ of Torpedo, it became possible to freeze them rapidly at the peak of ACh release and study their membrane and contents after cryofracture. A statistical analysis showed that the main structural change was the occurrence of large intramembrane particles at the peak of ACh release and under all release conditions. This impressive change contrasted with the stability in the number of vesicles. Another role for the vesicle was envisaged during intense stimulations when the cytoplasmic ACh and ATP pools become exhausted. The decrease in ATP leads to an increase in calcium and protons in the cytoplasm; this signals the depletion of vesicular ACh and ATP stores in the cytoplasm. Release can go on, while ATP promotes the uptake of calcium by vesicles. At the end of its cycle the vesicle will be full of calcium and will perhaps release it. As far as the mechanism of ACh release is concerned it probably depends on a membrane component (perhaps the large particles) activated by calcium and able to translocate ACh in a quantal or subquantal form. In most recent work we showed that if a lyophilized presynaptic membrane was used to make proteoliposomes filled with ACh, they released ACh upon calcium action.     

Calcium channel regulation and presynaptic plasticity

Voltage-gated calcium (Ca(2+)) channels initiate release of neurotransmitters at synapses, and regulation of presynaptic Ca(2+) channels has a powerful influence on synaptic strength. Presynaptic Ca(2+) channels form a large signaling complex, which targets synaptic vesicles to Ca(2+) channels for efficient release and mediates Ca(2+) channel regulation. Presynaptic plasticity regulates synaptic function on the timescale of milliseconds to minutes in response to neurotransmitters and the frequency of action potentials. This article reviews the regulation of presynaptic Ca(2+) channels by effectors and regulators of Ca(2+) signaling and describes the emerging evidence for a critical role of Ca(2+) channel regulation in control of neurotransmission and in presynaptic plasticity. Failure of function and regulation of presynaptic Ca(2+) channels leads to migraine, ataxia, and potentially other forms of neurological disease. We propose that presynaptic Ca(2+) channels serve as the regulatory node in a dynamic, multilayered signaling network that exerts short-term control of neurotransmission in response to synaptic activity.     

Role of Ca(2+) channels in short-term synaptic plasticity

Repetitive nerve activity induces various forms of short-term synaptic plasticity that have important computational roles in neuronal networks. Several forms of short-term plasticity are caused largely by changes in transmitter release, but the mechanisms that underlie these changes in the release process have been difficult to address. Recent studies of a giant synapse - the calyx of Held - have shed new light on this issue. Recordings of Ca(2+) currents or Ca(2+) concentrations at nerve terminals reveal that regulation of presynaptic Ca(2+) channels has a significant role in three important forms of short-term plasticity: short-term depression, facilitation and post-tetanic potentiation.     

Presynaptic N-type calcium channels regulate synaptic growth

Voltage-gated calcium channels couple changes in membrane potential to neuronal functions regulated by calcium, including neurotransmitter release. Here we report that presynaptic N-type calcium channels not only control neurotransmitter release but also regulate synaptic growth at Drosophila neuromuscular junctions. In a screen for behavioral mutants that disrupt synaptic transmission, an allele of the N-type calcium channel locus (Dmca1A) was identified that caused synaptic undergrowth. The underlying molecular defect was identified as a neutralization of a charged residue in the third S4 voltage sensor. RNA interference reduction of N-type calcium channel expression also reduced synaptic growth. Hypomorphic mutations in syntaxin-1A or n-synaptobrevin, which also disrupt neurotransmitter release, did not affect synapse proliferation at the neuromuscular junction, suggesting calcium entry through presynaptic N-type calcium channels, not neurotransmitter release per se, is important for synaptic growth. The reduced synapse proliferation in Dmca1A mutants is not due to increased synapse retraction but instead reflects a role for calcium influx in synaptic growth mechanisms. These results suggest N-type channels participate in synaptic growth through signaling pathways that are distinct from those that mediate neurotransmitter release. Linking presynaptic voltage-gated calcium entry to downstream calcium-sensitive synaptic growth regulators provides an efficient activity-dependent mechanism for modifying synaptic strength.