Inter-spike mitochondrial Ca2+ release enhances high frequency synaptic transmission

KEY POINTS

Presynaptic mitochondria not only absorb but also release Ca²⁺ during high frequency stimulation (HFS) when presynaptic [Ca²⁺ ] is kept low (<500 nm) by high cytosolic Ca²⁺ buffer or strong plasma membrane calcium clearance mechanisms under physiological external [Ca²⁺ ]. Mitochondrial Ca²⁺ release (MCR) does not alter the global presynaptic Ca²⁺ transients. MCR during HFS enhances short-term facilitation and steady state excitatory postsynaptic currents by increasing vesicular release probability. The intra-train MCR may provide residual calcium at interspike intervals, and thus support high frequency neurotransmission at central glutamatergic synapses.

ABSTRACT

Emerging evidence indicates that mitochondrial Ca²⁺ buffering contributes to local regulation of synaptic transmission. It is unknown, however, whether mitochondrial Ca²⁺ release (MCR) occurs during high frequency synaptic transmission. Confirming the previous notion that 2 μm tetraphenylphosphonium (TPP⁺ ) is a specific inhibitor of the mitochondrial Na⁺ /Ca²⁺ exchanger (mNCX), we studied the role of MCR via mNCX in short-term plasticity during high frequency stimulation (HFS) at the calyx of Held synapse of the rat. TPP⁺ reduced short-term facilitation (STF) and steady state excitatory postsynaptic currents during HFS at mature calyx synapses under physiological extracellular [Ca²⁺ ] ([Ca²⁺ ]_o  = 1.2 mm), but not at immature calyx or at 2 mm [Ca²⁺ ]_o . The inhibitory effects of TPP⁺ were stronger at synapses with morphologically complex calyces harbouring many swellings and at 32°C than at simple calyx synapses and at room temperature. These effects of TPP⁺ on STF were well correlated with those on the presynaptic mitochondrial [Ca²⁺ ] build-up during HFS. Mitochondrial [Ca²⁺ ] during HFS was increased by TPP⁺ at mature calyces under 1.2 mm [Ca²⁺ ]_o , and further enhanced at 32°C, but not under 2 mm [Ca²⁺ ]_o or at immature calyces. The close correlation of the effects of TPP⁺ on mitochondrial [Ca²⁺ ] with those on STF suggests that mNCX contributes to STF at the calyx of Held synapses. The intra-train MCR enhanced vesicular release probability without altering global presynaptic [Ca²⁺ ]. Our results suggest that MCR during HFS elevates local [Ca²⁺ ] near synaptic sites at interspike intervals to enhance STF and to support stable synaptic transmission under physiological [Ca²⁺ ]_o .

Impact of vesicular glutamate leakage on synaptic transmission at the calyx of Held

KEY POINTS

It is controversial whether glutamate can leak out of vesicles in the nerve terminal. To address this issue, we abolished vesicular glutamate uptake by washing out presynaptic cytosolic glutamate or by blocking vacuolar ATPase activity using bafilomycin A1. In the absence of vesicular glutamate uptake, both spontaneous and nerve-evoked EPSCs underwent a rundown, suggesting that vesicular glutamate can leak out of vesicles. However, the rundown of evoked EPSCs was caused mainly by accumulation of unfilled vesicles after exocytic release of glutamate, suggesting a minor influence of glutamate leakage on synaptic transmission.

ABSTRACT

Glutamate leaks out of synaptic vesicles when the transvesicular proton gradient is dissipated in isolated vesicle preparations. In the nerve terminal, however, it is controversial whether glutamate can leak out of vesicles. To address this issue, we abolished vesicular glutamate uptake by washing out presynaptic cytosolic glutamate in whole-cell dialysis or by blocking vacuolar ATPase using bafilomycin A1 (Baf) at the calyx of Held in mouse brainstem slices. Presynaptic glutamate washout or Baf application reduced the mean amplitude and frequency of spontaneous miniature (m)EPSCs and the mean amplitude of EPSCs evoked every 10 min. The percentage reduction of mEPSC amplitude was much less than that of EPSC amplitude or mEPSC frequency, and tended to reach a plateau. The mean amplitude of mEPSCs after glutamate washout or Baf application remained high above the detection limit, deduced from the reduction of mEPSC amplitude by the AMPA receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione. Membrane capacitance measurements from presynaptic terminals indicated no effect of glutamate washout on exocytosis or endocytosis of synaptic vesicles. We conclude that glutamate can leak out of vesicles unless it is continuously taken up from presynaptic cytosol. However, the magnitude of glutamate leakage was small and had only a minor effect on synaptic responses. In contrast, prominent rundowns of EPSC amplitude and mEPSC frequency observed after glutamate washout or Baf application are likely to be caused by accumulation of unfilled vesicles in presynaptic terminals retrieved after spontaneous and evoked glutamate release.

Temporal patterns of gene expression during calyx of held development

Relating changes in gene expression to discrete developmental events remains an elusive challenge in neuroscience, in part because most neural territories are comprised of multiple cell types that mature over extended periods of time. The medial nucleus of the trapezoid body (MNTB) is an attractive vertebrate model system that contains a nearly homogeneous population of neurons, which are innervated by large glutamatergic nerve terminals called calyces of Held (CH). Key steps in maturation of CHs and MNTB neurons, including CH growth and competition, occur very quickly for most cells between postnatal days (P)2 and P6. Therefore, we characterized genome-wide changes in this system, with dense temporal sampling during the first postnatal week. We identified 541 genes whose expression changed significantly between P0-6 and clustered them into eight groups based on temporal expression profiles. Candidate genes from each of the eight profile groups were validated in separate samples by qPCR. Our tissue sample permitted comparison of known glial and neuronal transcripts and revealed that monotonically increasing or decreasing expression profiles tended to be associated with glia and neurons, respectively. Gene ontology revealed enrichment of genes involved in axon pathfinding, cell differentiation, cell adhesion and extracellular matrix. The latter category included elements of perineuronal nets, a prominent feature of MNTB neurons that is morphologically distinct by P6, when CH growth and competition are resolved onto nearly all MNTB neurons. These results provide a genetic framework for investigation of general mechanisms responsible for nerve terminal growth and maturation.

Yes, there is a medial nucleus of the trapezoid body in humans

The medial nucleus of the trapezoid body (MNTB) is a collection of brainstem neurons that function within the ascending auditory pathway. MNTB neurons are associated with a number of anatomical and physiological specializations which make these cells especially well-equipped to provide extremely fast and precise glycinergic inhibition to its target neurons in the superior olivary complex and ventral nucleus of the lateral lemniscus. The inhibitory influence of MNTB neurons plays essentials roles in the localization of sound sources and encoding temporal features of complex sounds. The morphology, afferent and efferent connections and physiological response properties of MNTB neurons have been well-characterized in a number of laboratory rodents and some carnivores. Furthermore, the MNTB has been positively identified in all mammals examined, ranging from opossum and mice to chimpanzees. From the early 1970s through 2009, a number of studies denied the existence of the MNTB in humans and consequentially, the existence of this nucleus in the human brain has been debated for nearly 50 years. The absence of the MNTB from the human brain would negate current principles of sound localization and would require a number of novel adaptations, entirely unique to humans. However, a number of recent studies of human post-mortem tissue have provided evidence supporting the existence of the MNTB in humans. It therefore seems timely to review the structure and function of the MNTB, critically review the literature which led to the denial of the human MNTB and then review recent investigations supporting the existence of the MNTB in the human brain.

Linear coding of complex sound spectra by discharge rate in neurons of the medial nucleus of the trapezoid body (MNTB) and its inputs

The interaural level difference (ILD) cue to sound location is first encoded in the lateral superior olive (LSO). ILD sensitivity results because the LSO receives excitatory input from the ipsilateral cochlear nucleus and inhibitory input indirectly from the contralateral cochlear nucleus via glycinergic neurons of the ipsilateral medial nucleus of the trapezoid body (MNTB). It is hypothesized that in order for LSO neurons to encode ILDs, the sound spectra at both ears must be accurately encoded via spike rate by their afferents. This spectral-coding hypothesis has not been directly tested in MNTB, likely because MNTB neurons have been mostly described and studied recently in regards to their abilities to encode temporal aspects of sounds, not spectral. Here, we test the hypothesis that MNTB neurons and their inputs from the cochlear nucleus and auditory nerve code sound spectra via discharge rate. The Random Spectral Shape (RSS) method was used to estimate how the levels of 100-ms duration spectrally stationary stimuli were weighted, both linearly and non-linearly, across a wide band of frequencies. In general, MNTB neurons, and their globular bushy cell inputs, were found to be well-modeled by a linear weighting of spectra demonstrating that the pathways through the MNTB can accurately encode sound spectra including those resulting from the acoustical cues to sound location provided by head-related directional transfer functions (DTFs). Together with the anatomical and biophysical specializations for timing in the MNTB-LSO complex, these mechanisms may allow ILDs to be computed for complex stimuli with rapid spectrotemporally-modulated envelopes such as speech and animal vocalizations and moving sound sources.

Inhibitory projections from the ventral nucleus of the trapezoid body to the medial nucleus of the trapezoid body in the mouse

Neurons in the medial nucleus of the trapezoid body (MNTB) receive prominent excitatory input through the calyx of Held, a giant synapse that produces large and fast excitatory currents. MNTB neurons also receive inhibitory glycinergic inputs that are also large and fast, and match the calyceal excitation in terms of synaptic strength. GABAergic inputs provide additional inhibition to MNTB neurons. Inhibitory inputs to MNTB modify spiking of MNTB neurons both in-vitro and in-vivo, underscoring their importance. Surprisingly, the origin of the inhibitory inputs to MNTB has not been shown conclusively. We performed retrograde tracing, anterograde tracing, immunohistochemical experiments, and electrophysiological recordings to address this question. The results support the ventral nucleus of the trapezoid body (VNTB) as at least one major source of glycinergic input to MNTB. VNTB fibers enter the ipsilateral MNTB, travel along MNTB principal neurons and produce several bouton-like presynaptic terminals. Further, the contribution of GABA to the total inhibition declines during development, resulting in only a very minor fraction of GABAergic inhibition in adulthood, which is matched in time by a reduction in expression of a GABA synthetic enzyme in VNTB principal neurons.

Distinct roles of Kv1 and Kv3 potassium channels at the calyx of Held presynaptic terminal

Despite identification of >100 potassium channel subunits, relatively little is known about their roles in synaptic transmission. To address this issue we recorded presynaptic potassium currents (IPK) directly from the calyx of Held terminal in brainstem slices of rats. IPK was composed of a 4-aminopyridine (4-AP)-sensitive component and a smaller 4-AP-insensitive component composed of an iberiotoxin-sensitive current and an unidentified slowly activating potassium current. IPK could also be separated into a tetraethylammonium (TEA; 1 mm)-sensitive high-voltage-activated component and a margatoxin (10 nm)-sensitive low-voltage-activated component, which was also blocked by dendrotoxin-I (200 nm) and tityustoxin-Kalpha (100 nm). In outside-out patches excised from calyceal terminals, TEA (1 mm) consistently and to a large extent attenuated IPK, whereas margatoxin attenuated IPK only in a subset of patches (three of seven). Immunocytochemical examination using Kv subtype-specific antibodies indicated that multiple Kv1 and Kv3 subtypes were present at the calyceal terminal. In paired presynaptic and postsynaptic whole-cell recordings, TEA (1 mm) increased both the duration and peak amplitude of presynaptic action potentials and simultaneously potentiated EPSCs. Margatoxin alone had no such effect but reduced the amount of depolarization required for action potential generation, thereby inducing a burst of spikes when the nerve terminal was depolarized for a prolonged period. Thus, at the calyx of Held terminal, Kv3 channels directly regulate evoked transmitter release, whereas Kv1 channels reduce nerve terminal excitability, thereby preventing aberrant transmitter release. We conclude that both Kv3 and Kv1 channels contribute differentially to maintaining the fidelity of synaptic transmission at the calyx of Held.

Decreased temporal precision of auditory signaling in Kcna1-null mice: an electrophysiological study in vivo

The voltage-gated potassium (Kv) channel subunit Kv1.1, encoded by the Kcna1 gene, is expressed strongly in the ventral cochlear nucleus (VCN) and the medial nucleus of the trapezoid body (MNTB) of the auditory pathway. To examine the contribution of the Kv1.1 subunit to the processing of auditory information, in vivo single-unit recordings were made from VCN neurons (bushy cells), axonal endings of bushy cells at MNTB cells (calyces of Held), and MNTB neurons of Kcna1-null (-/-) mice and littermate control (+/+) mice. Thresholds and spontaneous firing rates of VCN and MNTB neurons were not different between genotypes. At higher sound intensities, however, evoked firing rates of VCN and MNTB neurons were significantly lower in -/- mice than +/+ mice. The SD of the first-spike latency (jitter) was increased in VCN neurons, calyces, and MNTB neurons of -/- mice compared with +/+ controls. Comparison along the ascending pathway suggests that the increased jitter found in -/- MNTB responses arises mostly in the axons of VCN bushy cells and/or their calyceal terminals rather than in the MNTB neurons themselves. At high rates of sinusoidal amplitude modulations, -/- MNTB neurons maintained high vector strength values but discharged on significantly fewer cycles of the amplitude-modulated stimulus than +/+ MNTB neurons. These results indicate that in Kcna1-null mice the absence of the Kv1.1 subunit results in a loss of temporal fidelity (increased jitter) and the failure to follow high-frequency amplitude-modulated sound stimulation in vivo.

Distinguishing between presynaptic and postsynaptic mechanisms of short-term depression during action potential trains

Short-term facilitation and depression have a profound influence on transmission at many glutamatergic synapses, particularly during trains of stimuli. A major component of these processes is postsynaptic receptor desensitization. Both presynaptic and postsynaptic mechanisms can contribute to synaptic efficacy, but it is often difficult to define their respective contributions. Blockers of desensitization such as cyclothiazide (CTZ) can be used, but many of these drugs have nonspecific effects on transmitter release, complicating attempts to define synaptic effectiveness under physiological conditions. We describe and validate a new method to minimize desensitization during trains of synaptic stimuli that is based on the low-affinity competitive glutamate receptor antagonists gamma-D-glutamylglycine or kynurenic acid. A computational model of AMPA receptor kinetics shows that the mechanism can be accounted for by simple competitive antagonism of AMPA receptors, where the rapid off-rate of the antagonist permits re-equilibration between blocked and unblocked pools during the interstimulus interval. Our results at the calyx of Held show that desensitization makes little contribution to synaptic depression at frequencies below 10 Hz, but at higher frequencies it makes an important contribution, with accumulating desensitization masking short-term facilitation and causing an underestimation of quantal content. This novel method of protection from desensitization is compatible with physiological studies but cannot be used in conjunction with CTZ. Although presynaptic vesicle depletion makes the dominant contribution to short-term depression, our results show that AMPA receptor desensitization contributes to the depression at auditory synapses after hearing onset and in a frequency-dependent manner.

Synaptic activation of presynaptic glutamate transporter currents in nerve terminals

Glutamate uptake by high-affinity transporters is responsible for limiting the activation of postsynaptic receptors and maintaining low levels of ambient glutamate. The reuptake process generates membrane currents, which can be activated by synaptically released glutamate in glial cells and some postsynaptic neurons. However, less is known about presynaptic transporter currents because the small size of synaptic boutons precludes direct recordings. Here, we have recorded from two giant nerve terminals: bipolar cell synaptic terminals in goldfish retina and the calyx of Held in rat auditory brainstem. Exocytosis was evoked by brief depolarizations and measured as an increase in membrane capacitance. In isolated bipolar cell terminals, exocytosis was associated with an anion (NO3- or Cl-) current. The current peaked 2.8 msec after the start of the depolarization and decayed with a mean time constant of 8.5 msec. It was inhibited by the nontransportable glutamate transporter antagonist sc-threo-beta-benzyloxyaspartate (TBOA) but was insensitive to the GLT1/EAAT2 subtype-selective antagonist dihydrokainate and was affected by extracellular pH buffering. A TBOA-sensitive anion current was also evoked by application of exogenous glutamate to bipolar cell terminals. The large single-channel conductance, derived from noise analysis, and previous immunolocalization studies suggest that synaptically released glutamate activates EAAT5-type transporters in bipolar cell terminals. In contrast, neither exocytosis nor exogenous glutamate evoked a transporter current in the calyx of Held. Glutamate transporter currents with rapid kinetics are therefore identified and characterized in bipolar cell terminals, providing a valuable system for investigating the function and modulation of presynaptic glutamate transporters.

Developmental increase in vesicular glutamate content does not cause saturation of AMPA receptors at the calyx of Held synapse

Whether a quantal packet of transmitter saturates postsynaptic receptors is a fundamental question in central synaptic transmission. However, this question remains open with regard to saturation at mature synapses. The calyx of Held, a giant glutamatergic synapse in the auditory brainstem, becomes functionally mature during the fourth postnatal week in rats. During postnatal development, the mean amplitude of miniature (i.e., quantal) EPSCs (mEPSCs) becomes significantly larger. Experiments using the rapidly dissociating glutamate receptor antagonist kynurenate suggested that vesicular glutamate content increases with development. To test whether AMPA receptors are saturated by a packet of transmitter, we infused a high concentration of l-glutamate into mature calyceal terminals. This caused a marked increase in the mean amplitude of mEPSCs. We conclude that a single packet of transmitter glutamate does not saturate postsynaptic AMPA receptors even at the mature calyx of Held synapse with increased vesicular transmitter content.

Estimating transmitter release rates from postsynaptic current fluctuations

A method is presented that allows one to estimate transmitter release rates from fluctuations of postsynaptic current records under conditions of stationary or slowly varying release. For experimental applications, we used the calyx of Held, a glutamatergic synapse, in which "residual current," i.e., current attributable to residual glutamate in the synaptic cleft, is present. For a characterization of synaptic transmission, several postsynaptic parameters, such as the mean amplitude of the miniature postsynaptic current and an apparent single channel conductance, have to be known. These were obtained by evaluating variance and two more higher moments of the current fluctuations. In agreement with Fesce et al. (1986), we found both by simulations and by analyzing experimental records that high-pass filtering of postsynaptic currents renders the estimates remarkably tolerant against nonstationarities. We also found that release rates and postsynaptic parameters can be reliably obtained when release rates are low ( approximately 10 events/msec). Furthermore, during a long-lasting stimulus, the transmitter release at the calyx of Held was found to decay to a low, stationary rate of 10 events/msec after depletion of the "releasable pool" of synaptic vesicles. This stationary release rate is compatible with the expected rate of recruitment of new vesicles to the release-ready pool of vesicles. MiniatureEPSC (mEPSC) size is estimated to be similar to the value of spontaneously occurring mEPSC under this condition.

Protein kinase c increases the apparent affinity of the release machinery to Ca2+ by enhancing the release machinery downstream of the Ca2+ sensor

Modulation of the release probability of releasable vesicles in response to Ca(2+) influx (Prob(Ca)) is involved in mediating several forms of synaptic plasticity, including short-term depression, short-term augmentation, and potentiation induced by protein kinases. Given such an important role, however, the mechanism underlying modulation of the Prob(Ca) is unclear. We addressed this question by investigating how the activation of protein kinase C modulates the Prob(Ca) at a calyx-type nerve terminal in rat brainstem. Various lengths of step depolarization were applied to the nerve terminal to evoke different amounts of Ca(2+) currents and capacitance jumps, the latter of which reflect vesicle release. The relationship between the capacitance jump and the Ca(2+) current integral was sigmoidal and was fit well with a Hill function. The sigmoidal relationship was shifted significantly to the left during the application of the PKC activator 12-myristate 13-acetate (PMA), suggesting that PMA increases the apparent affinity of the release machinery to Ca(2+). This effect was blocked in large part by the application of the PKC inhibitor bisindolylmaleimide, suggesting that the effect is mediated mainly by the activation of PKC. We also found that PMA increased the rate of miniature EPSCs evoked by the application of hypertonic sucrose solution, which triggers release downstream of the Ca(2+) influx. Taken together, our results suggest that PKC enhances the apparent affinity of the release machinery to Ca(2+) by a mechanism downstream of the binding between Ca(2+) and its sensor. These results have provided the first example of the mechanisms underlying modulation of the Prob(Ca).

Combining deconvolution and noise analysis for the estimation of transmitter release rates at the calyx of held

The deconvolution method has been used in the past to estimate release rates of synaptic vesicles, but it cannot be applied to synapses where nonlinear interactions of quanta occur. We have extended this method to take into account a nonlinear current component resulting from the delayed clearance of glutamate from the synaptic cleft. We applied it to the calyx of Held and verified the important assumption of constant miniature EPSC (mEPSC) size by combining deconvolution with a variant of nonstationary fluctuation analysis. We found that amplitudes of mEPSCs decreased strongly after extended synaptic activity. Cyclothiazide (CTZ), an inhibitor of glutamate receptor desensitization, eliminated this reduction, suggesting that postsynaptic receptor desensitization occurs during strong synaptic activity at the calyx of Held. Constant mEPSC sizes could be obtained in the presence of CTZ and kynurenic acid (Kyn), a low-affinity blocker of AMPA-receptor channels. CTZ and Kyn prevented postsynaptic receptor desensitization and saturation and also minimized voltage-clamp errors. Therefore, we conclude that in the presence of these drugs, release rates at the calyx of Held can be reliably estimated over a wide range of conditions. Moreover, the method presented should provide a convenient way to study the kinetics of transmitter release at other synapses.

Specialized synapse-associated structures within the calyx of Held

The calyx of Held exhibits fast glutamatergic neurotransmission at high rates with low temporal jitter and has adapted specialized synaptic mechanisms to support its functional demands. We report the presence in calyces of an atypical arrangement of subcellular organelles, called the mitochondria-associated adherens complex (MAC). We demonstrate that MACs are located adjacent to synapses and contain membranous elements linked with coated and uncoated vesicles. Mitochondria that form MACs have more complex geometries than other mitochondria within the calyx and can extend between clusters of synapses. We estimate that the calyx contains 1600 MACs, 2400 synapses, and 6200 readily releasable vesicles. We also identify synaptic vesicle endocytotic regions close to MACs and synapses and hypothesize that calyces are composed of multiple activity modules, each containing machinery for vesicle release and recycling.

Fine-tuning an auditory synapse for speed and fidelity: developmental changes in presynaptic waveform, EPSC kinetics, and synaptic plasticity

Fast, precise, and sustained synaptic transmission at high frequency is thought to be crucial for the task of sound localization in the auditory brainstem. However, recordings from the calyx of Held synapse have revealed severe frequency-dependent synaptic depression, which tends to degrade the exact timing of postsynaptic spikes. Here we investigate the functional changes occurring throughout the critical period of synapse refinement from immature calyx terminal [postnatal day 5 (P5)] to after the onset of hearing (P12-P14). Surprisingly, for recordings near physiological temperature (35 degrees C), we find that P14 synapses are already able to follow extremely high input rates of up to 800 Hz. This ability stems in part from a remarkable shortening of presynaptic action potentials, which may lead to a lowering of release probability and decrease in synaptic delays during development. In addition, AMPA receptor-mediated EPSCs as well as quantal synaptic currents acquired progressively faster kinetics, although their mean amplitudes did not change significantly. NMDA receptor-mediated EPSCs, however, diminished with age, as indicated by a 50% reduction in mean amplitude and faster decay kinetics. Finally, the degree of synaptic depression was greatly attenuated with age, presumably because of a 2.5-fold or larger increase in the releasable pool of vesicles, which together with a decreasing release probability produces a fairly constant EPSC amplitude. This finely tuned orchestra of developmental changes thus simultaneously promotes speed while preventing premature vesicle pool depletion during prolonged bouts of firing. A few critical days in postnatal development can thus have a large impact on synaptic function.

Calcium channel types with distinct presynaptic localization couple differentially to transmitter release in single calyx-type synapses

We studied how Ca2+ influx through different subtypes of Ca2+ channels couples to release at a calyx-type terminal in the rat medial nucleus of the trapezoid body by simultaneously measuring the presynaptic Ca2+ influx evoked by a single action potential and the EPSC. Application of subtype-specific toxins showed that Ca2+ channels of the P/Q-, N-, and R-type controlled glutamate release at a single terminal. The Ca2+ influx through the P/Q-type channels triggered release more effectively than Ca2+ influx through N- or R-type channels. We investigated mechanisms that contributed to these differences in effectiveness. Electrophysiological experiments suggested that individual release sites were controlled by all three subtypes of Ca2+ channels. Immunocytochemical staining indicated, however, that a substantial fraction of N- and R-type channels was located distant from release sites. Although these distant channels contributed to the Ca2+ influx into the terminal, they may not contribute to release. Taken together, the results suggest that the Ca2+ influx into the calyx via N- and R-type channels triggers release less effectively than that via P/Q-type because a substantial fraction of the N- and R-type channels in the calyx is localized distant from release sites.

Postsynaptic Ca2+ influx mediated by three different pathways during synaptic transmission at a calyx-type synapse

Whole-cell recordings and Ca2+ flux measurements were made at a giant calyx-type synapse in rat brainstem slices to determine the contribution of glutamate receptor (GluR) channels and voltage-dependent Ca2+ channels (VDCCs) to postsynaptic Ca2+ influx during synaptic transmission. A single presynaptic action potential (AP) evoked an EPSP, followed by a single AP. The EPSP-AP sequence caused a postsynaptic Ca2+ influx of approximately 3.0 pC, primarily through VDCCs ( approximately 70%) and NMDA-type (up to 30%) channels but also through AMPA-type (<5%) GluR channels. At -80 mV, the fractional Ca2+ current (Pf) mediated by AMPA receptor (AMPAR) and NMDA receptor (NMDAR) channels was 1.3 and 11-12%, respectively. Simulations of the time course of Ca2+ influx through GluR channels suggested that the small contribution of AMPAR channels occurred only during the first few milliseconds of an EPSP, whereas influx through NMDAR channels dominated later. The NMDAR-mediated Ca2+ influx was localized in regions covered by the presynaptic terminal, whereas the Ca2+ influx mediated by VDCCs was more homogeneously distributed. Because of the temporal and spatial differences, calcium ions entering through the three different pathways are likely to activate different intracellular targets in the postsynaptic cell.

Postnatal development of phase-locked high-fidelity synaptic transmission in the medial nucleus of the trapezoid body of the rat

Synaptic transmission in the medial nucleus of the trapezoid body of rats was analyzed in postnatal days 4-13 (P4-P13) by applying the whole-cell patch-recording technique to brain slices. In P4-P6 animals, evoked EPSCs fluctuated extensively in amplitude and occurred in marked asynchrony, followed by spontaneous EPSCs. With development of animals, the evoked EPSCs increased in amplitude, and the rise time became faster. In addition, the synaptic transmission became phase-locked. The coefficient of variation (CV) of EPSC amplitude decreased with development (0.32 +/- 0.03 for P4-P5 and 0. 05 +/- 0.01 for P9-P11). The amplitude of miniature EPSCs did not change throughout the postnatal days investigated (-30.2 +/- 0.3 pA at -70 mV). The CV was dependent on extracellular Ca2+ concentration ([Ca2+]o) and was reduced with the increase of [Ca2+]o, and this [Ca2+]o dependence was shifted toward lower [Ca2+]o with development. Direct patch recording of the presynaptic terminals demonstrated an increase in Ca2+ currents during these postnatal days. The phase-locked high-fidelity transmission in this synapse is achieved with development likely through the increase of Ca2+ currents and Ca2+ sensitivity of transmitter release mechanisms in the presynaptic terminal.