Facilitation of presynaptic calcium currents in the rat brainstem

A maximum of two readily releasable vesicles per docking site at a cerebellar single active zone synapse

Recent research suggests that in central mammalian synapses, active zones contain several docking sites acting in parallel. Before release, one or several synaptic vesicles (SVs) are thought to bind to each docking site, forming the readily releasable pool (RRP). Determining the RRP size per docking site has important implications for short-term synaptic plasticity. Here, using mouse cerebellar slices, we take advantage of recently developed methods to count the number of released SVs at single glutamatergic synapses in response to trains of action potentials (APs). In each recording, the number of docking sites was determined by fitting with a binomial model the number of released SVs in response to individual APs. After normalization with respect to the number of docking sites, the summed number of released SVs following a train of APs was used to estimate of the RRP size per docking site. To improve this estimate, various steps were taken to maximize the release probability of docked SVs, the occupancy of docking sites, as well as the extent of synaptic depression. Under these conditions, the RRP size reached a maximum value close to two SVs per docking site. The results indicate that each docking site contains two distinct SV-binding sites that can simultaneously accommodate up to one SV each. They further suggest that under special experimental conditions, as both sites are close to full occupancy, a maximal RRP size of two SVs per docking site can be reached. More generally, the results validate a sequential two-step docking model previously proposed at this preparation.

Interpretation of presynaptic phenotypes of synaptic plasticity in terms of a two-step priming process

Studies on synaptic proteins involved in neurotransmitter release often aim at distinguishing between their roles in vesicle priming (the docking of synaptic vesicles to the plasma membrane and the assembly of a release machinery) as opposed to the process of vesicle fusion. This has traditionally been done by estimating two parameters, the size of the pool of fusion-competent vesicles (the readily releasable pool, RRP) and the probability that such vesicles are released by an action potential, with the aim of determining how these parameters are affected by molecular perturbations. Here, it is argued that the assumption of a homogeneous RRP may be too simplistic and may blur the distinction between vesicle priming and fusion. Rather, considering priming as a dynamic and reversible multistep process allows alternative interpretations of mutagenesis-induced changes in synaptic transmission and suggests mechanisms for variability in synaptic strength and short-term plasticity among synapses, as well as for interactions between short- and long-term plasticity. In many cases, assigned roles of proteins or causes for observed phenotypes are shifted from fusion- to priming-related when considering multistep priming. Activity-dependent enhancement of priming is an essential element in this alternative view and its variation among synapse types can explain why some synapses show depression and others show facilitation at low to intermediate stimulation frequencies. Multistep priming also suggests a mechanism for frequency invariance of steady-state release, which can be observed in some synapses involved in sensory processing.

A sequential two-step priming scheme reproduces diversity in synaptic strength and short-term plasticity

Central nervous system synapses are diverse in strength and plasticity. Short-term plasticity has traditionally been evaluated with models postulating a single pool of functionally homogeneous fusion-competent synaptic vesicles. Many observations are not easily explainable by such simple models. We established and experimentally validated a scheme of synaptic vesicle priming consisting of two sequential and reversible steps of release–machinery assembly. This sequential two-step priming scheme faithfully reproduced plasticity at a glutamatergic model synapse. The proposed priming and fusion scheme was consistent with the measured mean responses and with the experimentally observed heterogeneity between synapses. Vesicle fusion probability was found to be relatively uniform among synapses, while the priming equilibrium at rest of mature versus immature vesicle priming states differed greatly.

Glutamatergic synapses display variable strength and diverse short-term plasticity (STP), even for a given type of connection. Using nonnegative tensor factorization and conventional state modeling, we demonstrate that a kinetic scheme consisting of two sequential and reversible steps of release–machinery assembly and a final step of synaptic vesicle (SV) fusion reproduces STP and its diversity among synapses. Analyzing transmission at the calyx of Held synapses reveals that differences in synaptic strength and STP are not primarily caused by variable fusion probability (p_fusion) but are determined by the fraction of docked synaptic vesicles equipped with a mature release machinery. Our simulations show that traditional quantal analysis methods do not necessarily report p_fusion of SVs with a mature release machinery but reflect both p_fusion and the distribution between mature and immature priming states at rest. Thus, the approach holds promise for a better mechanistic dissection of the roles of presynaptic proteins in the sequence of SV docking, two-step priming, and fusion. It suggests a mechanism for activity-induced redistribution of synaptic efficacy.

Population temporal structure supplements the rate code during sensorimotor transformations

Sensorimotor transformations are mediated by premotor brain networks where individual neurons represent sensory, cognitive, and movement-related information. Such multiplexing poses a conundrum-how does a decoder know precisely when to initiate a movement if its inputs are active at times when a movement is not desired (e.g., in response to sensory stimulation)? Here, we propose a novel hypothesis: movement is triggered not only by an increase in firing rate but, critically, also by a reliable temporal pattern in the population response. Laminar recordings in the macaque superior colliculus (SC), a midbrain hub of orienting control, and pseudo-population analyses in SC and cortical frontal eye fields (FEFs) corroborated this hypothesis. Specifically, using a measure that captures the fidelity of the population code-here called temporal stability-we show that the temporal structure fluctuates during the visual response but becomes increasingly stable during the movement command. Importantly, we used spatiotemporally patterned microstimulation to causally test the contribution of population temporal stability in gating movement initiation and found that stable stimulation patterns were more likely to evoke a movement. Finally, a spiking neuron model was able to discriminate between stable and unstable input patterns, providing a putative biophysical mechanism for decoding temporal structure. These findings offer new insights into the long-standing debate on motor preparation and generation by situating the movement gating signal in temporal features of activity in shared neural substrates, and they highlight the importance of short-term population history in neuronal communication and behavior.

Dynamics of Neuromuscular Transmission Reproduced by Calcium-Dependent and Reversible Serial Transitions in the Vesicle Fusion Complex

Neuromuscular transmission, from spontaneous release to facilitation and depression, was accurately reproduced by a mechanistic kinetic model of sequential maturation transitions in the molecular fusion complex. The model incorporates three predictions. First, calcium-dependent forward transitions take vesicles from docked to preprimed to primed states, followed by fusion. Second, prepriming and priming are reversible. Third, fusion and recycling are unidirectional. The model was fed with experimental data from previous studies, whereas the backward (β) and recycling (ρ) rate constant values were fitted. Classical experiments were successfully reproduced with four transition states in the model when every forward (α) rate constant had the same value, and both backward rate constants were 50–100 times larger. Such disproportion originated an abruptly decreasing gradient of resting vesicles from docked to primed states. By contrast, a three-state version of the model failed to reproduce the dynamics of transmission by using the same set of parameters. Simulations predict the following: (1) Spontaneous release reflects primed to fusion spontaneous transitions. (2) Calcium elevations synchronize the series of forward transitions that lead to fusion. (3) Facilitation reflects a transient increase of priming following the calcium-dependent maturation transitions. (4) The calcium sensors that produce facilitation are those that evoke the transitions form docked to primed states. (5) Backward transitions and recycling restore the resting state. (6) Depression reflects backward transitions and slow recycling after intense release. Altogether, our results predict that fusion is produced by one calcium sensor, whereas the modulation of the number of vesicles that fuse depends on the calcium sensors that promote the early transition states. Such finely tuned kinetics offers a mechanism for collective non-linear transitional adaptations of a homogeneous vesicle pool to the ever-changing pattern of electrical activity in the neuromuscular junction.

The mammalian rod synaptic ribbon is essential for Cav channel facilitation and ultrafast synaptic vesicle fusion

Rod photoreceptors (PRs) use ribbon synapses to transmit visual information. To signal ‘no light detected’ they release glutamate continually to activate post-synaptic receptors. When light is detected glutamate release pauses. How a rod’s individual ribbon enables this process was studied here by recording evoked changes in whole-cell membrane capacitance from wild-type and ribbonless (Ribeye-ko) mice. Wild-type rods filled with high (10 mM) or low (0.5 mM) concentrations of the Ca²⁺-buffer EGTA created a readily releasable pool (RRP) of 87 synaptic vesicles (SVs) that emptied as a single kinetic phase with a τ<0.4 ms. The lower concentration of EGTA accelerated Ca_v channel opening and facilitated release kinetics. In contrast, ribbonless rods created a much smaller RRP of 22 SVs, and they lacked Ca_v channel facilitation; however, Ca²⁺ channel-release coupling remained tight. These release deficits caused a sharp attenuation of rod-driven scotopic light responses. We conclude that the synaptic ribbon facilitates Ca²⁺-influx and establishes a large RRP of SVs.

Non-negative Matrix Factorization as a Tool to Distinguish Between Synaptic Vesicles in Different Functional States

Synaptic vesicles (SVs) undergo multiple steps of functional maturation (priming) before being fusion competent. We present an analysis technique, which decomposes the time course of quantal release during repetitive stimulation as a sum of contributions of SVs, which existed in distinct functional states prior to stimulation. Such states may represent different degrees of maturation in priming or relate to different molecular composition of the release apparatus. We apply the method to rat calyx of Held synapses. These synapses display a high degree of variability, both with respect to synaptic strength and short-term plasticity during high-frequency stimulus trains. The method successfully describes time courses of quantal release at individual synapses as linear combinations of three components, representing contributions from functionally distinct SV subpools, with variability among synapses largely covered by differences in subpool sizes. Assuming that SVs transit in sequence through at least two priming steps before being released by an action potential (AP) we interpret the components as representing SVs which had been 'fully primed', 'incompletely primed' or undocked prior to stimulation. Given these assumptions, the analysis reports an initial release probability of 0.43 for SVs that were fully primed prior to stimulation. Release probability of that component was found to increase during high-frequency stimulation, leading to rapid depletion of that subpool. SVs that were incompletely primed at rest rapidly obtain fusion-competence during repetitive stimulation and contribute the majority of release after 3-5 stimuli.

The Role of CaV2.1 Channel Facilitation in Synaptic Facilitation

Activation of Ca_V2.1 voltage-gated calcium channels is facilitated by preceding calcium entry. Such self-modulatory facilitation is thought to contribute to synaptic facilitation. Using knockin mice with mutated Ca_V2.1 channels that do not facilitate (Ca IM-AA mice), we surprisingly found that, under conditions of physiological calcium and near-physiological temperatures, synaptic facilitation at hippocampal CA3 to CA1 synapses was not attenuated in Ca IM-AA mice and facilitation was paradoxically more prominent at two cerebellar synapses. Enhanced facilitation at these synapses is consistent with a decrease in initial calcium entry, suggested by an action-potential-evoked Ca_V2.1 current reduction in Purkinje cells from Ca IM-AA mice. In wild-type mice, Ca_V2.1 facilitation during high-frequency action potential trains was very small. Thus, for the synapses studied, facilitation of calcium entry through Ca_V2.1 channels makes surprisingly little contribution to synaptic facilitation under physiological conditions. Instead, Ca_V2.1 facilitation offsets Ca_V2.1 inactivation to produce remarkably stable calcium influx during high-frequency activation.

Weyrer et al. use Ca IM-AA mice in which Ca_V2.1 calcium channel facilitation is eliminated to study synaptic facilitation at hippocampal and cerebellar synapses. Under conditions of physiological temperature, external calcium, and presynaptic waveforms, facilitation of Ca_V2.1 channels is small and does not contribute to synaptic facilitation at these synapses.

Diverse roles of Synaptotagmin-7 in regulating vesicle fusion

Synaptotagmin 7 (Syt7) is a multifunctional calcium sensor expressed throughout the body. Its high calcium affinity makes it well suited to act in processes triggered by modest calcium increases within cells. In synaptic transmission, Syt7 has been shown to mediate asynchronous neurotransmitter release, facilitation, and vesicle replenishment. In this review we provide an update on recent developments, and the newly emerging roles of Syt7 in frequency invariant synaptic transmission and in suppressing spontaneous release. Additionally, we discuss Syt7's regulation of membrane fusion in non-neuronal cells, and its involvement in disease. How such diversity of functions is regulated remains an open question. We discuss several potential factors including temperature, presynaptic calcium signals, the localization of Syt7, and its interaction with other Syt isoforms.

Rapid regulation of vesicle priming explains synaptic facilitation despite heterogeneous vesicle:Ca2+ channel distances

Chemical synaptic transmission relies on the Ca²⁺-induced fusion of transmitter-laden vesicles whose coupling distance to Ca²⁺ channels determines synaptic release probability and short-term plasticity, the facilitation or depression of repetitive responses. Here, using electron- and super-resolution microscopy at the Drosophila neuromuscular junction we quantitatively map vesicle:Ca²⁺ channel coupling distances. These are very heterogeneous, resulting in a broad spectrum of vesicular release probabilities within synapses. Stochastic simulations of transmitter release from vesicles placed according to this distribution revealed strong constraints on short-term plasticity; particularly facilitation was difficult to achieve. We show that postulated facilitation mechanisms operating via activity-dependent changes of vesicular release probability (e.g. by a facilitation fusion sensor) generate too little facilitation and too much variance. In contrast, Ca²⁺-dependent mechanisms rapidly increasing the number of releasable vesicles reliably reproduce short-term plasticity and variance of synaptic responses. We propose activity-dependent inhibition of vesicle un-priming or release site activation as novel facilitation mechanisms.

Cells in the nervous system of all animals communicate by releasing and sensing chemicals at contact points named synapses. The ‘talking’ (or pre-synaptic) cell stores the chemicals close to the synapse, in small spheres called vesicles. When the cell is activated, calcium ions flow in and interact with the release-ready vesicles, which then spill the chemicals into the synapse. In turn, the ‘listening’ (or post-synaptic) cell can detect the chemicals and react accordingly.

When the pre-synaptic cell is activated many times in a short period, it can release a greater quantity of chemicals, allowing a bigger reaction in the post-synaptic cell. This phenomenon is known as facilitation, but it is still unclear how exactly it can take place. This is especially the case when many of the vesicles are not ready to respond, for example when they are too far from where calcium flows into the cell. Computer simulations have been created to model facilitation but they have assumed that all vesicles are placed at the same distance to the calcium entry point: Kobbersmed et al. now provide evidence that this assumption is incorrect.

Two high-resolution imaging techniques were used to measure the actual distances between the vesicles and the calcium source in the pre-synaptic cells of fruit flies: this showed that these distances are quite variable – some vesicles sit much closer to the source than others.

This information was then used to create a new computer model to simulate facilitation. The results from this computing work led Kobbersmed et al. to suggest that facilitation may take place because a calcium-based mechanism in the cell increases the number of vesicles ready to release their chemicals.

This new model may help researchers to better understand how the cells in the nervous system work. Ultimately, this can guide experiments to investigate what happens when information processing at synapses breaks down, for example in diseases such as epilepsy.

Calcium Dynamics and Synaptic Plasticity

Synaptic plasticity is a fundamental property of neurons referring to the activity-dependent changes in the strength and efficacy of synaptic transmission at preexisting synapses. Such changes can last from milliseconds to hours, days, or even longer and are involved in learning and memory as well as in development and response of the brain to injuries. Several types of synaptic plasticity have been described across neuronal types, brain regions, and species, but all of them share in one way or another capital importance of Ca²⁺-mediated processes. In this chapter, we will focus on the Ca²⁺-dependent events necessary for the induction and expression of multiple forms of synaptic plasticity.

Synaptotagmin 7 Mediates Both Facilitation and Asynchronous Release at Granule Cell Synapses

When an action potential invades a presynaptic terminal it evokes large, brief Ca²⁺ signals that trigger vesicle fusion within milliseconds that is followed by a small residual Ca²⁺ (Ca_res) signal. At many synapses Ca_res produces synaptic facilitation that lasts up to hundreds of milliseconds and, although less common, Ca_res can also evoke asynchronous release (AR) that persists for tens of milliseconds. The properties of facilitation and AR are very different, which suggests that they are mediated by distinct mechanisms. However, recently it has been shown that the slow calcium sensor synaptotagmin 7 (Syt7) mediates facilitation at many synapses where AR does not occur, and conversely Syt7 can mediate AR without mediating facilitation. Here we study cerebellar granule cell synapses onto stellate cells and Purkinje cells in mice of both sexes to assess the role of Syt7 in these phenomena at the same synapse. This is of particular interest at granule cell synapses where AR is much more calcium dependent and shorter-lived than facilitation. We find that Syt7 can mediate these two processes despite their divergent properties. In Syt7 knock-out animals, facilitation and AR are smaller and shorter lived than in wild-type animals, even though the initial probability of release and Ca_res signals are unchanged. Although there are short-lived Syt7-independent mechanisms that mediate facilitation and AR in Syt7 KO animals, we find that at granule cell synapses AR and facilitation are both mediated primarily by Syt7.SIGNIFICANCE STATEMENT At synapses made by cerebellar granule cells, presynaptic activity elevates calcium for tens of milliseconds, which in turn evokes both asynchronous release (AR) and synaptic facilitation. AR is more calcium sensitive and shorter-lived than facilitation at these synapses, suggesting that they are mediated by different mechanisms. However, we find that the slow calcium sensor synaptotagmin 7 mediates both of these phenomena. Small, rapidly decaying components of AR and facilitation are present in Syt7 KO animals, indicating that additional mechanisms can contribute to both AR and facilitation at these synapses.

Calcium Channels, Synaptic Plasticity, and Neuropsychiatric Disease

Voltage-gated calcium channels couple depolarization of the cell-surface membrane to entry of calcium, which triggers secretion, contraction, neurotransmission, gene expression, and other physiological responses. They are encoded by ten genes, which generate three voltage-gated calcium channel subfamilies: Ca_V1; Ca_V2; and Ca_V3. At synapses, Ca_V2 channels form large signaling complexes in the presynaptic nerve terminal, which are responsible for the calcium entry that triggers neurotransmitter release and short-term presynaptic plasticity. Ca_V1 channels form signaling complexes in postsynaptic dendrites and dendritic spines, where their calcium entry induces long-term potentiation. These calcium channels are the targets of mutations and polymorphisms that alter their function and/or regulation and cause neuropsychiatric diseases, including migraine headache, cerebellar ataxia, autism, schizophrenia, bipolar disorder, and depression. This article reviews the molecular properties of calcium channels, considers their multiple roles in synaptic plasticity, and discusses their potential involvement in this wide range of neuropsychiatric diseases.

Past and Future of Analog-Digital Modulation of Synaptic Transmission

Action potentials (APs) are generally produced in response to complex summation of excitatory and inhibitory synaptic inputs. While it is usually considered as a digital event, both the amplitude and width of the AP are significantly impacted by the context of its emission. In particular, the analog variations in subthreshold membrane potential determine the spike waveform and subsequently affect synaptic strength, leading to the so-called analog-digital modulation of synaptic transmission. We review here the numerous evidence suggesting context-dependent modulation of spike waveform, the discovery analog-digital modulation of synaptic transmission in invertebrates and its recent validation in mammals. We discuss the potential roles of analog-digital transmission in the physiology of neural networks.

Regulation of synaptic release-site Ca2+ channel coupling as a mechanism to control release probability and short-term plasticity

Synaptic transmission relies on the rapid fusion of neurotransmitter-containing synaptic vesicles (SVs), which happens in response to action potential (AP)-induced Ca²⁺ influx at active zones (AZs). A highly conserved molecular machinery cooperates at SV-release sites to mediate SV plasma membrane attachment and maturation, Ca²⁺ sensing, and membrane fusion. Despite this high degree of conservation, synapses - even within the same organism, organ or neuron - are highly diverse regarding the probability of APs to trigger SV fusion. Additionally, repetitive activation can lead to either strengthening or weakening of transmission. In this review, we discuss mechanisms controlling release probability and this short-term plasticity. We argue that an important layer of control is exerted by evolutionarily conserved AZ scaffolding proteins, which determine the coupling distance between SV fusion sites and voltage-gated Ca²⁺ channels (VGCC) and, thereby, shape synapse-specific input/output behaviors. We propose that AZ-scaffold modifications may occur to adapt the coupling distance during synapse maturation and plastic regulation of synapse strength.

Control of Excitation/Inhibition Balance in a Hippocampal Circuit by Calcium Sensor Protein Regulation of Presynaptic Calcium Channels

Activity-dependent regulation controls the balance of synaptic excitation to inhibition in neural circuits, and disruption of this regulation impairs learning and memory and causes many neurological disorders. The molecular mechanisms underlying short-term synaptic plasticity are incompletely understood, and their role in inhibitory synapses remains uncertain. Here we show that regulation of voltage-gated calcium (Ca²⁺) channel type 2.1 (Ca_V2.1) by neuronal Ca²⁺ sensor (CaS) proteins controls synaptic plasticity and excitation/inhibition balance in a hippocampal circuit. Prevention of CaS protein regulation by introducing the IM-AA mutation in Ca_V2.1 channels in male and female mice impairs short-term synaptic facilitation at excitatory synapses of CA3 pyramidal neurons onto parvalbumin (PV)-expressing basket cells. In sharp contrast, the IM-AA mutation abolishes rapid synaptic depression in the inhibitory synapses of PV basket cells onto CA1 pyramidal neurons. These results show that CaS protein regulation of facilitation and inactivation of Ca_V2.1 channels controls the direction of short-term plasticity at these two synapses. Deletion of the CaS protein CaBP1/caldendrin also blocks rapid depression at PV-CA1 synapses, implicating its upregulation of inactivation of Ca_V2.1 channels in control of short-term synaptic plasticity at this inhibitory synapse. Studies of local-circuit function revealed reduced inhibition of CA1 pyramidal neurons by the disynaptic pathway from CA3 pyramidal cells via PV basket cells and greatly increased excitation/inhibition ratio of the direct excitatory input versus indirect inhibitory input from CA3 pyramidal neurons to CA1 pyramidal neurons. This striking defect in local-circuit function may contribute to the dramatic impairment of spatial learning and memory in IM-AA mice.SIGNIFICANCE STATEMENT Many forms of short-term synaptic plasticity in neuronal circuits rely on regulation of presynaptic voltage-gated Ca²⁺ (Ca_V) channels. Regulation of Ca_V2.1 channels by neuronal calcium sensor (CaS) proteins controls short-term synaptic plasticity. Here we demonstrate a direct link between regulation of Ca_V2.1 channels and short-term synaptic plasticity in native hippocampal excitatory and inhibitory synapses. We also identify CaBP1/caldendrin as the calcium sensor interacting with Ca_V2.1 channels to mediate rapid synaptic depression in the inhibitory hippocampal synapses of parvalbumin-expressing basket cells to CA1 pyramidal cells. Disruption of this regulation causes altered short-term plasticity and impaired balance of hippocampal excitatory to inhibitory circuits.

The Mechanisms and Functions of Synaptic Facilitation

The ability of the brain to store and process information relies on changing the strength of connections between neurons. Synaptic facilitation is a form of short-term plasticity that enhances synaptic transmission for less than a second. Facilitation is a ubiquitous phenomenon thought to play critical roles in information transfer and neural processing. Yet our understanding of the function of facilitation remains largely theoretical. Here we review proposed roles for facilitation and discuss how recent progress in uncovering the underlying molecular mechanisms could enable experiments that elucidate how facilitation, and short-term plasticity in general, contributes to circuit function and animal behavior.

Dynamics of volume-averaged intracellular Ca2+ in a rat CNS nerve terminal during single and repetitive voltage-clamp depolarizations

KEY POINTS

The intracellular concentration of free calcium ions ([Ca²⁺ ]_i ) in a nerve terminal controls both transmitter release and synaptic plasticity. The rapid triggering of transmitter release depends on the local micro- or nanodomain of highly elevated [Ca²⁺ ]_i in the vicinity of open voltage-gated Ca²⁺ channels, whereas short-term synaptic plasticity is often controlled by global changes in residual [Ca²⁺ ]_i , averaged over the whole nerve terminal volume. Here we describe dynamic changes of such global [Ca²⁺ ]_i in the calyx of Held - a giant mammalian glutamatergic nerve terminal, which is particularly suited for biophysical studies. We provide quantitative data on Ca²⁺ inflow, Ca²⁺ buffering and Ca²⁺ clearance. These data allow us to predict changes in [Ca²⁺ ]_i in the nerve terminal in response to a wide range of stimulus protocols at high temporal resolution and provide a basis for the modelling of short-term plasticity of glutamatergic synapses.

ABSTRACT

Many aspects of short-term synaptic plasticity (STP) are controlled by relatively slow changes in the presynaptic intracellular concentration of free calcium ions ([Ca²⁺ ]_i ) that occur in the time range of a few milliseconds to several seconds. In nerve terminals, [Ca²⁺ ]_i equilibrates diffusionally during such slow changes, such that the globally measured, residual [Ca²⁺ ]_i that persists after the collapse of local domains is often the appropriate parameter governing STP. Here, we study activity-dependent dynamic changes in global [Ca²⁺ ]_i at the rat calyx of Held nerve terminal in acute brainstem slices using patch-clamp and microfluorimetry. We use low concentrations of a low-affinity Ca²⁺ indicator dye (100 μm Fura-6F) in order not to overwhelm endogenous Ca²⁺ buffers. We first study voltage-clamped terminals, dialysed with pipette solutions containing minimal amounts of Ca²⁺ buffers, to determine Ca²⁺ binding properties of endogenous fixed buffers as well as the mechanisms of Ca²⁺ clearance. Subsequently, we use pipette solutions including 500 μm EGTA to determine the Ca²⁺ binding kinetics of this chelator. We provide a formalism and parameters that allow us to predict [Ca²⁺ ]_i changes in calyx nerve terminals in response to a wide range of stimulus protocols. Unexpectedly, the Ca²⁺ affinity of EGTA under the conditions of our measurements was substantially lower (K_D  = 543 ± 51 nm) than measured in vitro, mainly as a consequence of a higher than previously assumed dissociation rate constant (2.38 ± 0.20 s^-1 ), which we need to postulate in order to model the measured presynaptic [Ca²⁺ ]_i transients.

Towards a Unified Theory of Calmodulin Regulation (Calmodulation) of Voltage-Gated Calcium and Sodium Channels

Voltage-gated Na and Ca(2+) channels represent two major ion channel families that enable myriad biological functions including the generation of action potentials and the coupling of electrical and chemical signaling in cells. Calmodulin regulation (calmodulation) of these ion channels comprises a vital feedback mechanism with distinct physiological implications. Though long-sought, a shared understanding of the channel families remained elusive for two decades as the functional manifestations and the structural underpinnings of this modulation often appeared to diverge. Here, we review recent advancements in the understanding of calmodulation of Ca(2+) and Na channels that suggest a remarkable similarity in their regulatory scheme. This interrelation between the two channel families now paves the way towards a unified mechanistic framework to understand vital calmodulin-dependent feedback and offers shared principles to approach related channelopathic diseases. An exciting era of synergistic study now looms.

The calyx of Held in the auditory system: Structure, function, and development

The calyx of Held synapse plays an important role in the auditory system, relaying information about sound localization via fast and precise synaptic transmission, which is achieved by its specialized structure and giant size. During development, the calyx of Held undergoes anatomical, morphological, and physiological changes necessary for performing its functions. The large dimensions of the calyx of Held nerve terminal are well suited for direct electrophysiological recording of many presynaptic events that are difficult, if not impossible to record at small conventional synapses. This unique accessibility has been used to investigate presynaptic ion channels, transmitter release, and short-term plasticity, providing invaluable information about basic presynaptic mechanisms of transmission at a central synapse. Here, we review anatomical and physiological specializations of the calyx of Held, summarize recent studies that provide new mechanisms important for calyx development and reliable synaptic transmission, and examine fundamental presynaptic mechanisms learned from studies using calyx as a model nerve terminal. This article is part of a Special Issue entitled <Annual Reviews 2016>.

Large Ca²⁺-dependent facilitation of Ca(V)2.1 channels revealed by Ca²⁺ photo-uncaging

KEY POINTS

CaV 2.1 channels constitute a dominant Ca(2+) entry pathway into brain neurons, triggering downstream Ca(2+) -dependent processes such as neurotransmitter release. CaV 2.1 is itself modulated by Ca(2+) , resulting in activity-dependent enhancement of channel opening termed Ca(2+) -dependent facilitation (CDF). Real-time Ca(2+) imaging and Ca(2+) uncaging here reveal that CDF turns out to be strikingly faster, more Ca(2+) sensitive, and larger than anticipated on previous grounds. Robust resolution of the quantitative profile of CDF enables deduction of a realistic biophysical model for this process. These results suggest that CaV 2.1 CDF would figure most prominently in short-term synaptic plasticity and cerebellar Purkinje cell rhythmicity.

ABSTRACT

CaV 2.1 (P-type) voltage-gated Ca(2+) channels constitute a major source of neuronal Ca(2+) current, strongly influencing rhythmicity and triggering neurotransmitter release throughout the central nervous system. Fitting with such stature among Ca(2+) entry pathways, CaV 2.1 is itself feedback regulated by intracellular Ca(2+) , acting through calmodulin to facilitate channel opening. The precise neurophysiological role of this calcium-dependent facilitation (CDF) remains uncertain, however, in large measure because the very magnitude, Ca(2+) dependence and kinetics of CDF have resisted quantification by conventional means. Here, we utilize the photo-uncaging of Ca(2+) with CaV 2.1 channels fluxing Li(+) currents, so that voltage-dependent activation of channel gating is no longer conflated with Ca(2+) entry, and CDF is then driven solely by light-induced increases in Ca(2+) . By using this strategy, we now find that CDF can be unexpectedly large, enhancing currents by as much as twofold at physiological voltages. CDF is steeply Ca(2+) dependent, with a Hill coefficient of approximately two, a half-maximal effect reached by nearly 500 nm Ca(2+) , and Ca(2+) on/off kinetics in the order of milliseconds to tens of milliseconds. These properties were established for both native P-type currents in cerebellar Purkinje neurons, as well as their recombinant channel counterparts under heterologous expression. Such features suggest that CDF of CaV 2.1 channels may substantially enhance the regularity of rhythmic firing in cerebellar Purkinje neurons, where regularity is believed crucial for motor coordination. In addition, this degree of extensive CDF would be poised to exert large order-of-magnitude effects on short-term synaptic plasticity via rapid modulation of presynaptic Ca(2+) entry.

Calcium sensor regulation of the CaV2.1 Ca2+ channel contributes to short-term synaptic plasticity in hippocampal neurons

Short-term synaptic plasticity is induced by calcium (Ca(2+)) accumulating in presynaptic nerve terminals during repetitive action potentials. Regulation of voltage-gated CaV2.1 Ca(2+) channels by Ca(2+) sensor proteins induces facilitation of Ca(2+) currents and synaptic facilitation in cultured neurons expressing exogenous CaV2.1 channels. However, it is unknown whether this mechanism contributes to facilitation in native synapses. We introduced the IM-AA mutation into the IQ-like motif (IM) of the Ca(2+) sensor binding site. This mutation does not alter voltage dependence or kinetics of CaV2.1 currents, or frequency or amplitude of spontaneous miniature excitatory postsynaptic currents (mEPSCs); however, synaptic facilitation is completely blocked in excitatory glutamatergic synapses in hippocampal autaptic cultures. In acutely prepared hippocampal slices, frequency and amplitude of mEPSCs and amplitudes of evoked EPSCs are unaltered. In contrast, short-term synaptic facilitation in response to paired stimuli is reduced by ∼ 50%. In the presence of EGTA-AM to prevent global increases in free Ca(2+), the IM-AA mutation completely blocks short-term synaptic facilitation, indicating that synaptic facilitation by brief, local increases in Ca(2+) is dependent upon regulation of CaV2.1 channels by Ca(2+) sensor proteins. In response to trains of action potentials, synaptic facilitation is reduced in IM-AA synapses in initial stimuli, consistent with results of paired-pulse experiments; however, synaptic depression is also delayed, resulting in sustained increases in amplitudes of later EPSCs during trains of 10 stimuli at 10-20 Hz. Evidently, regulation of CaV2.1 channels by CaS proteins is required for normal short-term plasticity and normal encoding of information in native hippocampal synapses.

Altered short-term synaptic plasticity and reduced muscle strength in mice with impaired regulation of presynaptic CaV2.1 Ca2+ channels

Facilitation and inactivation of P/Q-type calcium (Ca(2+)) currents through the regulation of voltage-gated Ca(2+) (CaV) 2.1 channels by Ca(2+) sensor (CaS) proteins contributes to the facilitation and rapid depression of synaptic transmission in cultured neurons that transiently express CaV2.1 channels. To examine the modulation of endogenous CaV2.1 channels by CaS proteins in native synapses, we introduced a mutation (IM-AA) into the CaS protein-binding site in the C-terminal domain of CaV2.1 channels in mice, and tested synaptic facilitation and depression in neuromuscular junction synapses that use exclusively CaV2.1 channels for Ca(2+) entry that triggers synaptic transmission. Even though basal synaptic transmission was unaltered in the neuromuscular synapses in IM-AA mice, we found reduced short-term facilitation in response to paired stimuli at short interstimulus intervals in IM-AA synapses. In response to trains of action potentials, we found increased facilitation at lower frequencies (10-30 Hz) in IM-AA synapses accompanied by slowed synaptic depression, whereas synaptic facilitation was reduced at high stimulus frequencies (50-100 Hz) that would induce strong muscle contraction. As a consequence of altered regulation of CaV2.1 channels, the hindlimb tibialis anterior muscle in IM-AA mice exhibited reduced peak force in response to 50 Hz stimulation and increased muscle fatigue. The IM-AA mice also had impaired motor control, exercise capacity, and grip strength. Taken together, our results indicate that regulation of CaV2.1 channels by CaS proteins is essential for normal synaptic plasticity at the neuromuscular junction and for muscle strength, endurance, and motor coordination in mice in vivo.

Ca(2+) current facilitation determines short-term facilitation at inhibitory synapses between cerebellar Purkinje cells

KEY POINTS

Short-term facilitation takes place at GABAergic synapses between cerebellar Purkinje cells (PCs). By directly patch clamp recording from a PC axon terminal, we studied the mechanism of short-term facilitation. We show that the Ca(2+) currents elicited by high-frequency action potentials were augmented in a [Ca(2+) ]i -dependent manner. The facilitation of synaptic transmission showed 4-5th power dependence on the Ca(2+) current facilitation, and was abolished when the Ca(2+) current amplitude was adjusted to be identical. Short-term facilitation of Ca(2+) currents predominantly mediates short-term facilitation at synapses between PCs.

ABSTRACT

Short-term synaptic facilitation is critical for information processing of neuronal circuits. Several Ca(2+) -dependent positive regulations of transmitter release have been suggested as candidate mechanisms underlying facilitation. However, the small sizes of presynaptic terminals have hindered the biophysical study of short-term facilitation. In the present study, by directly recording from the axon terminal of a rat cerebellar Purkinje cell (PC) in culture, we demonstrate a crucial role of [Ca(2+) ]i -dependent facilitation of Ca(2+) currents in short-term facilitation at inhibitory PC-PC synapses. Voltage clamp recording was performed from a PC axon terminal visualized by enhanced green fluorescent protein, and the Ca(2+) currents elicited by the voltage command consisting of action potential waveforms were recorded. The amplitude of presynaptic Ca(2+) current was augmented upon high-frequency paired-pulse stimulation in a [Ca(2+) ]i -dependent manner, leading to paired-pulse facilitation of Ca(2+) currents. Paired recordings from a presynaptic PC axon terminal and a postsynaptic PC soma demonstrated that the paired-pulse facilitation of inhibitory synaptic transmission between PCs showed 4-5th power dependence on that of Ca(2+) currents, and was completely abolished when the Ca(2+) current amplitude was adjusted to be identical. Thus, short-term facilitation of Ca(2+) currents predominantly mediates short-term synaptic facilitation at synapses between PCs.

Synaptic Multivesicular Release in the Cerebellar Cortex: Its Mechanism and Role in Neural Encoding and Processing

The number of synaptic vesicles released during fast release plays a major role in determining the strength of postsynaptic response. However, it remains unresolved how the number of vesicles released in response to action potentials is controlled at a single synapse. Recent findings suggest that the Cav2.1 subtype (P/Q-type) of voltage-gated calcium channels is responsible for inducing presynaptic multivesicular release (MVR) at rat cerebellar glutamatergic synapses from granule cells to molecular layer interneurons. The topographical distance from Cav2.1 channels to exocytotic Ca(2+) sensors is a critical determinant of MVR. In physiological trains of presynaptic neurons, MVR significantly impacts the excitability of postsynaptic neurons, not only by increasing peak amplitude but also by prolonging decay time of the postsynaptic currents. Therefore, MVR contributes additional complexity to neural encoding and processing in the cerebellar cortex.

Strength and precision of neurotransmission at mammalian presynaptic terminals

Classically, the basic concept of chemical synaptic transmission was established at the frog neuromuscular junction, and direct intracellular recordings from presynaptic terminals at the squid giant presynaptic terminal have further clarified principles of neurotransmitter release. More recently, whole-cell patch-camp recordings from the calyx of Held in rodent brainstem slices have extended the classical concept to mammalian synapses providing new insights into the mechanisms underlying strength and precision of neurotransmission and developmental changes therein. This review summarizes findings from our laboratory and others on these subjects, mainly at the calyx of Held, with a particular focus on precise, high-fidelity, fast neurotransmission. The mechanisms by which presynaptic terminals acquire strong, precise neurotransmission during postnatal development are also discussed.

Conservation of Ca2+/calmodulin regulation across Na and Ca2+ channels

Voltage-gated Na and Ca2+ channels comprise distinct ion channel superfamilies, yet the carboxy tails of these channels exhibit high homology, hinting at a long-shared and purposeful module. For different Ca2+ channels, carboxyl-tail interactions with calmodulin do elaborate robust and similar forms of Ca2+ regulation. However, Na channels have only shown subtler Ca2+ modulation that differs among reports, challenging attempts at unified understanding. Here, by rapid Ca2+ photorelease onto Na channels, we reset this view of Na channel regulation. For cardiac-muscle channels (NaV1.5), reported effects from which most mechanistic proposals derive, we observe no Ca2+ modulation. Conversely, for skeletal-muscle channels (NaV1.4), we uncover fast Ca2+ regulation eerily similar to that of Ca2+ channels. Channelopathic myotonia mutations halve NaV1.4 Ca2+ regulation, and transplanting the NaV1.4 carboxy tail onto Ca2+ channels recapitulates Ca2+ regulation. Thus, we argue for the persistence and physiological relevance of an ancient Ca2+ regulatory module across Na and Ca2+ channels.

Genetic, cellular, and functional evidence for Ca2+ inflow through Cav1.2 and Cav1.3 channels in murine spiral ganglion neurons

Spiral ganglion neurons (SGNs) of the eighth nerve serve as the bridge between hair cells and the cochlear nucleus. Hair cells use Cav1.3 as the primary channel for Ca(2+) inflow to mediate transmitter release. In contrast, SGNs are equipped with multiple Ca(2+) channels to mediate Ca(2+)-dependent functions. We examined directly the role of Cav1.3 channels in SGNs using Cav1.3-deficient mice (Cav1.3(-/-)). We revealed a surprising finding that SGNs functionally express the cardiac-specific Cav1.2, as well as neuronal Cav1.3 channels. We show that evoked action potentials recorded from SGNs show a significant decrease in the frequency of firing in Cav1.3(-/-) mice compared with wild-type (Cav1.3(+/+)) littermates. Although Cav1.3 is the designated L-type channel in neurons, whole-cell currents recorded in isolated SGNs from Cav1.3(-/-) mice showed a surprising remnant current with sensitivity toward the dihydropyridine (DHP) agonist and antagonist, and a depolarization shift in the voltage-dependent activation compared with that in the Cav1.3(+/+) mice. Indeed, direct measurement of the elementary properties of Ca(2+) channels, in Cav1.3(+/+) neurons, confirmed the existence of two DHP-sensitive single-channel currents, with distinct open probabilities and conductances. We demonstrate that the DHP-sensitive current in Cav1.3(-/-) mice is derived from Cav1.2 channel activity, providing for the first time, to our knowledge, functional data for the expression of Cav1.2 currents in neurons. Finally, using shRNA gene knockdown methodology, and histological analyses of SGNs from Cav1.2(+/-) and Cav1.3(+/-) mice, we were able to establish the differential roles of Cav1.2 and Cav1.3 in SGNs.

Synaptic gain-of-function effects of mutant Cav2.1 channels in a mouse model of familial hemiplegic migraine are due to increased basal [Ca2+]i

Specific missense mutations in the CACNA1A gene, which encodes a subunit of voltage-gated CaV2.1 channels, are associated with familial hemiplegic migraine type 1 (FHM1), a rare monogenic subtype of common migraine with aura. We used transgenic knock-in (KI) mice harboring the human pathogenic FHM1 mutation S218L to study presynaptic Ca(2+) currents, EPSCs, and in vivo activity at the calyx of Held synapse. Whole-cell patch-clamp recordings of presynaptic terminals from S218L KI mice showed a strong shift of the calcium current I-V curve to more negative potentials, leading to an increase in basal [Ca(2+)]i, increased levels of spontaneous transmitter release, faster recovery from synaptic depression, and enhanced synaptic strength despite smaller action-potential-elicited Ca(2+) currents. The gain-of-function of transmitter release of the S218L mutant was reproduced in vivo, including evidence for an increased release probability, demonstrating its relevance for glutamatergic transmission. This synaptic phenotype may explain the misbalance between excitation and inhibition in neuronal circuits resulting in a persistent hyperexcitability state and other migraine-relevant mechanisms such as an increased susceptibility to cortical spreading depression.

Axonal noise as a source of synaptic variability

Post-synaptic potential (PSP) variability is typically attributed to mechanisms inside synapses, yet recent advances in experimental methods and biophysical understanding have led us to reconsider the role of axons as highly reliable transmission channels. We show that in many thin axons of our brain, the action potential (AP) waveform and thus the Ca++ signal controlling vesicle release at synapses will be significantly affected by the inherent variability of ion channel gating. We investigate how and to what extent fluctuations in the AP waveform explain observed PSP variability. Using both biophysical theory and stochastic simulations of central and peripheral nervous system axons from vertebrates and invertebrates, we show that channel noise in thin axons (<1 µm diameter) causes random fluctuations in AP waveforms. AP height and width, both experimentally characterised parameters of post-synaptic response amplitude, vary e.g. by up to 20 mV and 0.5 ms while a single AP propagates in C-fibre axons. We show how AP height and width variabilities increase with a ¾ power-law as diameter decreases and translate these fluctuations into post-synaptic response variability using biophysical data and models of synaptic transmission. We find for example that for mammalian unmyelinated axons with 0.2 µm diameter (matching cerebellar parallel fibres) axonal noise alone can explain half of the PSP variability in cerebellar synapses. We conclude that axonal variability may have considerable impact on synaptic response variability. Thus, in many experimental frameworks investigating synaptic transmission through paired-cell recordings or extracellular stimulation of presynaptic neurons, causes of variability may have been confounded. We thereby show how bottom-up aggregation of molecular noise sources contributes to our understanding of variability observed at higher levels of biological organisation.

Cav2.1 channels control multivesicular release by relying on their distance from exocytotic Ca2+ sensors at rat cerebellar granule cells

The concomitant release of multiple numbers of synaptic vesicles [multivesicular release (MVR)] in response to a single presynaptic action potential enhances the flexibility of synaptic transmission. However, the molecular mechanisms underlying MVR at a single CNS synapse remain unclear. Here, we show that the Cav2.1 subtype (P/Q-type) of the voltage-gated calcium channel is specifically responsible for the induction of MVR. In the rat cerebellar cortex, paired-pulse activation of granule cell (GC) ascending fibers leads not only to a facilitation of the peak amplitude (PPFamp) but also to a prolongation of the decay time (PPPdecay) of the EPSCs recorded from molecular layer interneurons. PPFamp is elicited by a transient increase in the number of released vesicles. PPPdecay is highly dependent on MVR and is caused by dual mechanisms: (1) a delayed release and (2) an extrasynaptic spillover of the GC transmitter glutamate and subsequent pooling of the glutamate among active synapses. PPPdecay was specifically suppressed by the Cav2.1 channel blocker ω-agatoxin IVA, while PPFamp responded to Cav2.2/Cav2.3 (N-type/R-type) channel blockers. The membrane-permeable slow Ca(2+) chelator EGTA-AM profoundly reduced the decay time constant (τdecay) of the second EPSC; however, it only had a negligible impact on that of the first, thereby eliminating PPPdecay. These results suggest that the distance between presynaptic Cav2.1 channels and exocytotic Ca(2+) sensors is a key determinant of MVR. By transducing presynaptic action potential firings into unique Ca(2+) signals and vesicle release profiles, Cav2.1 channels contribute to the encoding and processing of neural information.

A presynaptic ENaC channel drives homeostatic plasticity

An electrophysiology-based forward genetic screen has identified two genes, pickpocket11 (ppk11) and pickpocket16 (ppk16), as being necessary for the homeostatic modulation of presynaptic neurotransmitter release at the Drosophila neuromuscular junction (NMJ). Pickpocket genes encode Degenerin/Epithelial Sodium channel subunits (DEG/ENaC). We demonstrate that ppk11 and ppk16 are necessary in presynaptic motoneurons for both the acute induction and long-term maintenance of synaptic homeostasis. We show that ppk11 and ppk16 are cotranscribed as a single mRNA that is upregulated during homeostatic plasticity. Acute pharmacological inhibition of a PPK11- and PPK16-containing channel abolishes the expression of short- and long-term homeostatic plasticity without altering baseline presynaptic neurotransmitter release, indicating remarkable specificity for homeostatic plasticity rather than NMJ development. Finally, presynaptic calcium imaging experiments support a model in which a PPK11- and PPK16-containing DEG/ENaC channel modulates presynaptic membrane voltage and, thereby, controls calcium channel activity to homeostatically regulate neurotransmitter release.

Calcium channels and short-term synaptic plasticity

Voltage-gated Ca(2+) channels in presynaptic nerve terminals initiate neurotransmitter release in response to depolarization by action potentials from the nerve axon. The strength of synaptic transmission is dependent on the third to fourth power of Ca(2+) entry, placing the Ca(2+) channels in a unique position for regulation of synaptic strength. Short-term synaptic plasticity regulates the strength of neurotransmission through facilitation and depression on the millisecond time scale and plays a key role in encoding information in the nervous system. Ca(V)2.1 channels are the major source of Ca(2+) entry for neurotransmission in the central nervous system. They are tightly regulated by Ca(2+), calmodulin, and related Ca(2+) sensor proteins, which cause facilitation and inactivation of channel activity. Emerging evidence reviewed here points to this mode of regulation of Ca(V)2.1 channels as a major contributor to short-term synaptic plasticity of neurotransmission and its diversity among synapses.

Functional integration of calcium regulatory mechanisms at Purkinje neuron synapses

Cerebellar Purkinje neurons receive synaptic inputs from three different sources: the excitatory parallel fibre and climbing fibre synapses as well as the inhibitory synapses from molecular layer stellate and basket cells. These three synaptic systems use distinct mechanisms in order to generate Ca(2+) signals that are specialized for specific modes of neurotransmitter release and post-synaptic signal integration. In this review, we first describe the repertoire of Ca(2+) regulatory mechanisms that generate and regulate the amplitude and timing of Ca(2+) fluxes during synaptic transmission and then explore how these mechanisms interact to generate the unique functional properties of each of the Purkinje neuron synapses.

Ca²⁺-dependent regulation of Ca²⁺ currents in rat primary afferent neurons: role of CaMKII and the effect of injury

Currents through voltage-gated Ca²⁺ channels (I(Ca)) may be regulated by cytoplasmic Ca²⁺ levels ([Ca²⁺](c)), producing Ca²⁺-dependent inactivation (CDI) or facilitation (CDF). Since I(Ca) regulates sensory neuron excitability, altered CDI or CDF could contribute to pain generation after peripheral nerve injury. We explored this by manipulating [Ca²⁺](c) while recording I(Ca) in rat sensory neurons. In uninjured neurons, elevating [Ca²⁺](c) with a conditioning prepulse (-15 mV, 2 s) inactivated I(Ca) measured during subsequent test pulses (-15 mV, 5 ms). This inactivation was Ca²⁺-dependent (CDI), since it was decreased with elimination of Ca²⁺ influx by depolarization to above the I(Ca) reversal potential, with high intracellular Ca²⁺ buffering (EGTA 10 mm or BAPTA 20 mm), and with substitution of Ba²⁺ for extracellular Ca²⁺, revealing a residual voltage-dependent inactivation. At longer latencies after conditioning (>6 s), I(Ca) recovered beyond baseline. This facilitation also proved to be Ca²⁺-dependent (CDF) using the protocols limiting cytoplasmic Ca²⁺ elevation. Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) blockers applied by bath (KN-93, myristoyl-AIP) or expressed selectively in the sensory neurons (AIP) reduced CDF, unlike their inactive analogues. Protein kinase C inhibition (chelerythrine) had no effect. Selective blockade of N-type Ca²⁺ channels eliminated CDF, whereas L-type channel blockade had no effect. Following nerve injury, CDI was unaffected, but CDF was eliminated in axotomized neurons. Excitability of sensory neurons in intact ganglia from control animals was diminished after a similar conditioning pulse, but this regulation was eliminated by injury. These findings indicate that I(Ca) in sensory neurons is subject to both CDI and CDF, and that hyperexcitability following injury-induced loss of CDF may result from diminished CaMKII activity.

Calretinin regulates Ca2+-dependent inactivation and facilitation of Ca(v)2.1 Ca2+ channels through a direct interaction with the α12.1 subunit

Voltage-gated Ca(v)2.1 Ca(2+) channels undergo dual modulation by Ca(2+), Ca(2+)-dependent inactivation (CDI), and Ca(2+)-dependent facilitation (CDF), which can influence synaptic plasticity in the nervous system. Although the molecular determinants controlling CDI and CDF have been the focus of intense research, little is known about the factors regulating these processes in neurons. Here, we show that calretinin (CR), a Ca(2+)-binding protein highly expressed in subpopulations of neurons in the brain, inhibits CDI and enhances CDF by binding directly to α(1)2.1. Screening of a phage display library with CR as bait revealed a highly basic CR-binding domain (CRB) present in multiple copies in the cytoplasmic linker between domains II and III of α(1)2.1. In pulldown assays, CR binding to fusion proteins containing these CRBs was largely Ca(2+)-dependent. α(1)2.1 coimmunoprecipitated with CR antibodies from transfected cells and mouse cerebellum, which confirmed the existence of CR-Ca(v)2.1 complexes in vitro and in vivo. In HEK293T cells, CR significantly decreased Ca(v)2.1 CDI and increased CDF. CR binding to α(1)2.1 was required for these effects, because they were not observed upon substitution of the II-III linker of α(1)2.1 with that from the Ca(v)1.2 α(1) subunit (α(1)1.2), which lacks the CRBs. In addition, coexpression of a protein containing the CRBs blocked the modulatory action of CR, most likely by competing with CR for interactions with α(1)2.1. Our findings highlight an unexpected role for CR in directly modulating effectors such as Ca(v)2.1, which may have major consequences for Ca(2+) signaling and neuronal excitability.

Fine-tuning synaptic plasticity by modulation of Ca(V)2.1 channels with Ca2+ sensor proteins

Modulation of P/Q-type Ca(2+) currents through presynaptic voltage-gated calcium channels (Ca(V)2.1) by binding of Ca(2+)/calmodulin contributes to short-term synaptic plasticity. Ca(2+)-binding protein-1 (CaBP1) and Visinin-like protein-2 (VILIP-2) are neurospecific calmodulin-like Ca(2+) sensor proteins that differentially modulate Ca(V)2.1 channels, but how they contribute to short-term synaptic plasticity is unknown. Here, we show that activity-dependent modulation of presynaptic Ca(V)2.1 channels by CaBP1 and VILIP-2 has opposing effects on short-term synaptic plasticity in superior cervical ganglion neurons. Expression of CaBP1, which blocks Ca(2+)-dependent facilitation of P/Q-type Ca(2+) current, markedly reduced facilitation of synaptic transmission. VILIP-2, which blocks Ca(2+)-dependent inactivation of P/Q-type Ca(2+) current, reduced synaptic depression and increased facilitation under conditions of high release probability. These results demonstrate that activity-dependent regulation of presynaptic Ca(V)2.1 channels by differentially expressed Ca(2+) sensor proteins can fine-tune synaptic responses to trains of action potentials and thereby contribute to the diversity of short-term synaptic plasticity.

Ca(2+) influx and neurotransmitter release at ribbon synapses

Ca(2+) influx through voltage-gated Ca(2+) channels triggers the release of neurotransmitters at presynaptic terminals. Some sensory receptor cells in the peripheral auditory and visual systems have specialized synapses that express an electron-dense organelle called a synaptic ribbon. Like conventional synapses, ribbon synapses exhibit SNARE-mediated exocytosis, clathrin-mediated endocytosis, and short-term plasticity. However, unlike non-ribbon synapses, voltage-gated L-type Ca(2+) channel opening at ribbon synapses triggers a form of multiquantal release that can be highly synchronous. Furthermore, ribbon synapses appear to be specialized for fast and high throughput exocytosis controlled by graded membrane potential changes. Here we will discuss some of the basic aspects of synaptic transmission at different types of ribbon synapses, and we will emphasize recent evidence that auditory and retinal ribbon synapses have marked differences. This will lead us to suggest that ribbon synapses are specialized for particular operating ranges and frequencies of stimulation. We propose that different types of ribbon synapses transfer diverse rates of sensory information by expressing a particular repertoire of critical components, and by placing them at precise and strategic locations, so that a continuous supply of primed vesicles and Ca(2+) influx leads to fast, accurate, and ongoing exocytosis.

Neuromodulatory changes in short-term synaptic dynamics may be mediated by two distinct mechanisms of presynaptic calcium entry

Although synaptic output is known to be modulated by changes in presynaptic calcium channels, additional pathways for calcium entry into the presynaptic terminal, such as non-selective channels, could contribute to modulation of short term synaptic dynamics. We address this issue using computational modeling. The neuropeptide proctolin modulates the inhibitory synapse from the lateral pyloric (LP) to the pyloric dilator (PD) neuron, two slow-wave bursting neurons in the pyloric network of the crab Cancer borealis. Proctolin enhances the strength of this synapse and also changes its dynamics. Whereas in control saline the synapse shows depression independent of the amplitude of the presynaptic LP signal, in proctolin, with high-amplitude presynaptic LP stimulation the synapse remains depressing while low-amplitude stimulation causes facilitation. We use simple calcium-dependent release models to explore two alternative mechanisms underlying these modulatory effects. In the first model, proctolin directly targets calcium channels by changing their activation kinetics which results in gradual accumulation of calcium with low-amplitude presynaptic stimulation, leading to facilitation. The second model uses the fact that proctolin is known to activate a non-specific cation current I ( MI ). In this model, we assume that the MI channels have some permeability to calcium, modeled to be a result of slow conformation change after binding calcium. This generates a gradual increase in calcium influx into the presynaptic terminals through the modulatory channel similar to that described in the first model. Each of these models can explain the modulation of the synapse by proctolin but with different consequences for network activity.

Ca(2+)-regulated ion channels

Due to its high external and low internal concentration the Ca(2+) ion is used ubiquitously as an intracellular signaling molecule, and a great many Ca(2+)-sensing proteins have evolved to receive and propagate Ca(2+) signals. Among them are ion channel proteins, whose Ca(2+) sensitivity allows internal Ca(2+) to influence the electrical activity of cell membranes and to feedback-inhibit further Ca(2+) entry into the cytoplasm. In this review I will describe what is understood about the Ca(2+) sensing mechanisms of the three best studied classes of Ca(2+)-sensitive ion channels: Large-conductance Ca(2+)-activated K(+) channels, small-conductance Ca(2+)-activated K(+) channels, and voltage- gated Ca(2+) channels. Great strides in mechanistic understanding have be made for each of these channel types in just the past few years.

Similar intracellular Ca2+ requirements for inactivation and facilitation of voltage-gated Ca2+ channels in a glutamatergic mammalian nerve terminal

Voltage-gated Ca2+ channels (VGCCs) of the P/Q-type, which are expressed at a majority of mammalian nerve terminals, show two types of Ca2+-dependent feedback regulation-inactivation (CDI) and facilitation (CDF). Because of the nonlinear relationship between Ca2+ influx and transmitter release, CDI and CDF are powerful regulators of synaptic strength. To what extent VGCCs inactivate or facilitate during spike trains depends on the dynamics of free Ca2+ ([Ca2+]i) and the Ca2+ sensitivity of CDI and CDF, which has not been determined in nerve terminals. In this report, we took advantage of the large size of a rat auditory glutamatergic synapse--the calyx of Held--and combined voltage-clamp recordings of presynaptic Ca2+ currents (ICa(V)) with UV-light flash-induced Ca2+ uncaging and presynaptic Ca2+ imaging to study the Ca2+ requirements for CDI and CDF. We find that nearly half of the presynaptic VGCCs inactivate during 100 ms voltage steps and require several seconds to recover. This inactivation is caused neither by depletion of Ca2+ ions from the synaptic cleft nor by metabotropic feedback inhibition, because it is resistant to blockade of metabotropic and ionotropic glutamate receptors. Facilitation of ICa(V) induced by repetitive depolarizations or preconditioning voltage steps decays within tens of milliseconds. Since Ca2+ buffers only weakly affect CDI and CDF, we conclude that the Ca2+ sensors are closely associated with the channel. CDI and CDF can be induced by intracellular photo release of Ca2+ resulting in [Ca2+]i elevations in the low micromolar range, implying a surprisingly high affinity of the Ca2+ sensors.

Ca2+-dependent modulation of voltage-gated Ca2+ channels

BACKGROUND

Voltage-gated (Cav) Ca2+ channels are multi-subunit complexes that play diverse roles in a wide variety of tissues. A fundamental mechanism controlling Cav channel function involves the Ca2+ ions that permeate the channel pore. Ca2+ influx through Cav channels mediates feedback regulation to the channel that is both negative (Ca2+-dependent inactivation, CDI) and positive (Ca2+-dependent facilitation, CDF).

SCOPE OF REVIEW

This review highlights general mechanisms of CDI and CDF with an emphasis on how these processes have been studied electrophysiologically in native and heterologous expression systems.

MAJOR CONCLUSIONS

Electrophysiological analyses have led to detailed insights into the mechanisms and prevalence of CDI and CDF as Cav channel regulatory mechanisms. All Cav channel family members undergo some form of Ca2+-dependent feedback that relies on CaM or a related Ca2+ binding protein. Tremendous progress has been made in characterizing the role of CaM in CDI and CDF. Yet, what contributes to the heterogeneity of CDI/CDF in various cell-types and how Ca2+-dependent regulation of Cav channels controls Ca2+ signaling remain largely unexplored.

GENERAL SIGNIFICANCE

Ca2+ influx through Cav channels regulates diverse physiological events including excitation-contraction coupling in muscle, neurotransmitter and hormone release, and Ca2+-dependent gene transcription. Therefore, the mechanisms that regulate channels, such as CDI and CDF, can have a large impact on the signaling potential of excitable cells in various physiological contexts. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

The calyx of Held synapse: from model synapse to auditory relay

The calyx of Held is an axosomatic terminal in the auditory brainstem that has attracted anatomists because of its giant size and physiologists because of its accessibility to patch-clamp recordings. The calyx allows the principal neurons in the medial nucleus of the trapezoid body (MNTB) to provide inhibition that is both well timed and sustained to many other auditory nuclei. The special adaptations that allow the calyx to drive its principal neuron even when frequencies are high include a large number of release sites with low release probability, a large readily releasable pool, fast presynaptic calcium clearance and little delayed release, a large quantal size, and fast AMPA-type glutamate receptors. The transformation from a synapse that is unremarkable except for its giant size into a fast and reliable auditory relay happens in just a few days. In rodents this transformation is essentially ready when hearing starts.

Short-term facilitation modulates size and timing of the synaptic response at the inner hair cell ribbon synapse

Inner hair cells (IHCs) in the mammalian cochlea are able to continuously release neurotransmitter in the presence of constant stimuli. Nonetheless, strong synaptic depression is observed over the first few milliseconds of stimulation. This process most likely underlies adaptation in the auditory nerve. In the present study we demonstrate that under certain conditions of stimulation, facilitation can occur at the IHC ribbon synapse. Using simultaneous whole-cell, voltage-clamp recordings from IHCs and afferent fiber endings in excised postnatal rat cochleae, we stimulated IHCs with 2 ms long test depolarizations from a holding potential of -89 mV. Synaptic currents in afferent fibers occurred with high failure rates of ∼ 50%. However, when a pre-depolarization to values of -55 to -49 mV was implemented before the test pulse, success rates of the synaptic response increased to 100%, the strength of the synaptic response increased ∼ 2.8-fold, and synaptic latency was reduced by ∼ 50%. When calcium influx was minimized during pre-depolarization, none of these effects were found, suggesting that calcium influx during pre-depolarizations is required for synaptic conditioning. Similarly, in response to paired-pulse protocols, short term facilitation occurred. The response to the second stimulus increased up to ∼ 5-fold, and its latency was reduced by up to 35% compared to the response to the first stimulus. We propose that at the IHC resting membrane potential, the ribbon synapse operates in a constantly facilitated mode caused by Ca(2+) influx, optimizing the size and timing of the postsynaptic response in auditory nerve fibers.

Voltage-gated calcium channels

Voltage-gated calcium (Ca(2+)) channels are key transducers of membrane potential changes into intracellular Ca(2+) transients that initiate many physiological events. There are ten members of the voltage-gated Ca(2+) channel family in mammals, and they serve distinct roles in cellular signal transduction. The Ca(V)1 subfamily initiates contraction, secretion, regulation of gene expression, integration of synaptic input in neurons, and synaptic transmission at ribbon synapses in specialized sensory cells. The Ca(V)2 subfamily is primarily responsible for initiation of synaptic transmission at fast synapses. The Ca(V)3 subfamily is important for repetitive firing of action potentials in rhythmically firing cells such as cardiac myocytes and thalamic neurons. This article presents the molecular relationships and physiological functions of these Ca(2+) channel proteins and provides information on their molecular, genetic, physiological, and pharmacological properties.

The number of components of enhancement contributing to short-term synaptic plasticity at the neuromuscular synapse during patterned nerve Stimulation progressively decreases as basal release probability is increased from low to normal levels by changing extracellular Ca2+

Presynaptic short-term plasticity (STP) dynamically modulates synaptic strength in a reversible manner on a timescale of milliseconds to minutes. For low basal vesicular release probability (prob0), four components of enhancement, F1 and F2 facilitation, augmentation (A), and potentiation (P), increase synaptic strength during repetitive nerve activity. For release rates that exceed the rate of replenishment of the readily releasable pool (RRP) of synaptic vesicles, depression of synaptic strength, observed as a rundown of postsynaptic potential amplitudes, can also develop. To understand the relationship between enhancement and depression at the frog (Rana pipiens) neuromuscular synapse, data obtained over a wide range of prob0 using patterned stimulation are analyzed with a hybrid model to reveal the components of STP. We find that F1, F2, A, P, and depletion of the RRP all contribute to STP during repetitive nerve activity at low prob0. As prob0 is increased by raising Ca(o)(2+) (extracellular Ca2+), specific components of enhancement no longer contribute, with first P, then A, and then F2 becoming undetectable, even though F1 continues to enhance release. For levels of prob0 that lead to appreciable depression, only F1 and depletion of the RRP contribute to STP during rundown, and for low stimulation rates, F2 can also contribute. These observations place prob0-dependent limitations on which components of enhancement contribute to STP and suggest some fundamental mechanistic differences among the components. The presented model can serve as a tool to readily characterize the components of STP over wide ranges of prob0.