Phosphorylation-dependent low-frequency depression at phasic synapses of a crayfish motoneuron

Frequency-dependent mobilization of heterogeneous pools of synaptic vesicles shapes presynaptic plasticity

The segregation of the readily releasable pool of synaptic vesicles (RRP) in sub-pools that are differentially poised for exocytosis shapes short-term plasticity. However, the frequency-dependent mobilization of these sub-pools is poorly understood. Using slice recordings and modeling of synaptic activity at cerebellar granule cell to Purkinje cell synapses of mice, we describe two sub-pools in the RRP that can be differentially recruited upon ultrafast changes in the stimulation frequency. We show that at low-frequency stimulations, a first sub-pool is gradually silenced, leading to full blockage of synaptic transmission. Conversely, a second pool of synaptic vesicles that cannot be released by a single stimulus is recruited within milliseconds by high-frequency stimulation and support an ultrafast recovery of neurotransmitter release after low-frequency depression. This frequency-dependent mobilization or silencing of sub-pools in the RRP in terminals of granule cells may play a role in the filtering of sensorimotor information in the cerebellum.

Tonic 5nM DA stabilizes neuronal output by enabling bidirectional activity-dependent regulation of the hyperpolarization activated current via PKA and calcineurin

Volume transmission results in phasic and tonic modulatory signals. The actions of tonic dopamine (DA) at type 1 DA receptors (D1Rs) are largely undefined. Here we show that tonic 5nM DA acts at D1Rs to stabilize neuronal output over minutes by enabling activity-dependent regulation of the hyperpolarization activated current (I _h). In the presence but not absence of 5nM DA, I _h maximal conductance (G _max) was adjusted according to changes in slow wave activity in order to maintain spike timing. Our study on the lateral pyloric neuron (LP), which undergoes rhythmic oscillations in membrane potential with depolarized plateaus, demonstrated that incremental, bi-directional changes in plateau duration produced corresponding alterations in LP I _hG _max when preparations were superfused with saline containing 5nM DA. However, when preparations were superfused with saline alone there was no linear correlation between LP I _hG_max and duty cycle. Thus, tonic nM DA modulated the capacity for activity to modulate LP I _h G _max; this exemplifies metamodulation (modulation of modulation). Pretreatment with the Ca2+-chelator, BAPTA, or the specific PKA inhibitor, PKI, prevented all changes in LP I _h in 5nM DA. Calcineurin inhibitors blocked activity-dependent changes enabled by DA and revealed a PKA-mediated, activity-independent enhancement of LP I _hG _max. These data suggested that tonic 5nM DA produced two simultaneous, PKA-dependent effects: a direct increase in LP I _h G _max and a priming event that permitted calcineurin regulation of LP I _h. The latter produced graded reductions in LP I _hG _max with increasing duty cycles. We also demonstrated that this metamodulation preserved the timing of LP’s first spike when network output was perturbed with bath-applied 4AP. In sum, 5nM DA permits slow wave activity to provide feedback that maintains spike timing, suggesting that one function of low-level, tonic modulation is to stabilize specific features of a dynamic output.

The cytoplasmic domain of rat synaptotagmin I enhances synaptic transmission

Synaptotagmin, an integral membrane protein of synaptic vesicles, functions as a calcium sensor in the temporal control of neurotransmitter release. Although synaptotagmin facilitates lipid membrane fusion in biochemical experiments, overexpression of synaptotagmin inhibits neurotransmission. A facilitatory effect of synaptotagmin on synaptic transmission was never observed. To determine whether synaptotagmin may accelerate synaptic transmission in vivo, we injected the cytoplasmic domain of rat synaptotagmin I (CD-syt) into crayfish motor axons and tested the effect of CD-syt on synaptic response. We confirmed that CD-syt accelerates neuromuscular transmission. The injected preparation had larger synaptic potentials with shorter rise time. Experiments with varying calcium concentrations showed that CD-syt increased the maximum synaptic response of the neuromuscular synapses. Further tests on short-term plasticity of neuromuscular synapses revealed that CD-syt increases the release probability of the release-ready vesicles.

Calcium, calpain, and calcineurin in low-frequency depression of transmitter release

Low-frequency depression (LFD) of transmitter release occurs at phasic synapses with stimulation at 0.2 Hz in both isolated crayfish (Procambarus clarkii) neuromuscular junction (NMJ) preparations and in intact animals. LFD is regulated by presynaptic activity of the Ca(2+)-dependent phosphatase calcineurin (Silverman-Gavrila and Charlton, 2009). Since the fast Ca(2+) chelator BAPTA-AM inhibits LFD but the slow chelator EGTA-AM does not, the Ca(2+) sensor for LFD may be close to a Ca(2+) source at active zones. Calcineurin can be activated by the Ca(2+)-activated protease calpain, and immunostaining showed that both proteins are present at nerve terminals. Three calpain inhibitors, calpain inhibitor I, MDL-28170, and PD150606, but not the control compound PD145305, inhibit LFD both in the intact animal as shown by electromyograms and by intracellular recordings at neuromuscular junctions. Analysis of mini-EPSPs indicated that these inhibitors had minimal postsynaptic effects. Proteolytic activity in CNS extract, detected by a fluorescent calpain substrate, was modulated by Ca(2+) and calpain inhibitors. Western blot analysis of CNS extract showed that proteolysis of calcineurin to a fragment consistent with the constitutively active form required Ca(2+) and was blocked by calpain inhibitors. Inhibition of LFD by calpain inhibition blocks the reduction in phosphoactin and the depolymerization of tubulin that normally occurs in LFD, probably by blocking the dephosphorylation of cytoskeletal proteins by calcineurin. In contrast, high-frequency depression does not involve protein phosphorylation- or calpain-dependent mechanisms. LFD may involve a specific pathway in which local Ca(2+) signaling activates presynaptic calpain and calcineurin at active zones and causes changes of tubulin cytoskeleton.

Structure/function assessment of synapses at motor nerve terminals

The release of transmitter at neuromuscular junctions (NMJ) of the opener muscle in crayfish is quantal in nature. This NMJ offers the advantage of being able to record quantal events at specific visually identified release sites, thus allowing measurement of the physiological parameters of vesicle release and its response to be directly correlated with synaptic structure. These experiments take advantage of areas between the varicosities on the nerve terminal that we define as "stems." Stems were chosen as the region to study because of their low synaptic output due to fewer synaptic sites. Through 3D reconstruction from hundreds of serial sections, obtained by transmission electron microscopy (TEM), at a site in which focal macropatch recordings were obtained, the number of synapses and AZs are revealed. Thus, physiological profiles with various stimulation conditions can be assessed in regards to direct synaptic structure. Here, we used the properties of the quantal shape to determine if distinct subsets of quantal signatures existed and if differences in the distributions are present depending on the frequency of stimulation. Such a quantal signature could come about by parameters of area, rise time, peak amplitude, latency, and tau decay. In this study, it is shown that even at defined sites on the stem, with few active zones, synaptic transmission is still complex and the quantal responses appear to be variable even for a given synapse over time. In this study, we could not identify a quantal signature for the conditions utilized.

Synaptic transmission and plasticity are modulated by nonmuscle myosin II at the neuromuscular junction of Drosophila

The synaptic vesicle population in a nerve terminal is traditionally divided into subpopulations according to physiological criteria; the readily releasable pool (RRP), the recycling pool, and the reserve pool. It is recognized that the RRP subserves synaptic transmission evoked by low-frequency neural activity and that the recycling and reserve populations are called on to supply vesicles as neural activity increases. Here we investigated the contribution of nonmuscle myosin II (NMMII) to synaptic transmission with emphasis on the role a motor protein could play in the supply of vesicles. We used Drosophila genetics to manipulate NMMII and assessed synaptic transmission at the larval neuromuscular junction. We observed a positive correlation between synaptic strength at low-frequency stimulation and NMMII expression: reducing NMMII reduced the evoked response, while increasing NMMII increased the evoked response. Further, we found that NMMII contributed to the spontaneous release of vesicles differentially from evoked release, suggesting differential contribution to these two release mechanisms. By measuring synaptic responses under conditions of differing external calcium concentration in saline, we found that NMMII is important for normal synaptic transmission under high-frequency stimulation. This research identifies diverse functions for NMMII in synaptic transmission and suggests that this motor protein is an active contributor to the physiology of synaptic vesicle recruitment.

A novel form of presynaptic plasticity based on the fast reactivation of release sites switched off during low-frequency depression

Repetitive firing of neurons at a low frequency often leads to a decrease in synaptic strength. The mechanism of this low-frequency depression (LFD) is poorly understood. Here, LFD was studied at Aplysia cholinergic synapses. The absence of a significant change in the paired-pulse ratio during LFD, together with the facts that neither the time course nor the extent of LFD were affected by the initial release probability, suggests that LFD is not related to a depletion of the ready-to-fuse synaptic vesicles (SVs) or to a decrease in the release probability, but results from the silencing of a subpopulation of release sites. A subset of SVs or release sites, which acquired a high release probability status during LFD, permits synapses to rapidly and temporarily recover the initial synaptic strength when the stimulation is stopped. However, the recovery of the full capacity of the synapse to sustain repetitive stimulations is slow and involves spontaneous reactivation of the silent release sites. Application of tetanic stimulations accelerates this recovery by immediately switching on the silent sites. This high-frequency-dependent phenomenon underlies a new form of synaptic plasticity that allows resetting of presynaptic efficiency independently of the recent history of the synapse. Microinjection of a mutated Aplysia synapsin that cannot be phosphorylated by cAMP-dependent protein kinase (PKA), or a PKA inhibitor both prevented high-frequency-dependent awakening of release sites. Changes in the firing pattern of neurons appear to be able to regulate the on-off status of release sites via a molecular cascade involving PKA-dependent phosphorylation of synapsin.

Two forms of long-term depression in a polysynaptic pathway in the leech CNS: one NMDA receptor-dependent and the other cannabinoid-dependent

Although long-term depression (LTD) is a well-studied form of synaptic plasticity, it is clear that multiple cellular mechanisms are involved in its induction. In the leech, LTD is observed in a polysynaptic connection between touch mechanosensory neurons (T cells) and the S interneuron following low frequency stimulation. LTD elicited by 450 s low frequency stimulation was blocked by N-methyl-D-aspartic acid (NMDA) receptor antagonists. However, LTD elicited by 900 s low frequency stimulation was insensitive to NMDA receptor antagonists and was instead dependent on cannabinoid signaling. This LTD was blocked by both a cannabinoid receptor antagonist and by inhibition of diacylglycerol lipase, which is necessary for the synthesis of the cannabinoid transmitter 2-arachidonyl glycerol (2-AG). Bath application of 2-AG or the cannabinoid receptor agonist CP55 940 also induced LTD at this synapse. These results indicate that two forms of LTD coexist at the leech T-to-S polysynaptic pathway: one that is NMDA receptor-dependent and another that is cannabinoid-dependent and that activation of either form of LTD is dependent on the level of activity in this circuit.

Calcineurin and cytoskeleton in low-frequency depression

Transmitter release at high probability phasic synapses of crayfish neuromuscular junctions depresses by over 50% in 60 min when stimulated at 0.2 Hz. Inhibition of the protein phosphatase calcineurin by intracellular pre-synaptic injection of autoinhibitory peptide inhibited low-frequency depression (LFD) and resulted in facilitation of transmitter release. Since this inhibitor had no major effects when injected into the post-synaptic cell, only pre-synaptic calcineurin activity is necessary for LFD. To examine changes in phosphoproteins during LFD we performed a phosphoproteomic screen on proteins extracted from motor axons and nerve terminals after LFD induction or treatment with various drugs that affect kinase and phosphatase activity. Proteins separated by PAGE were stained with phospho-specific/total protein ratio stains (Pro-Q Diamond/SYPRO Ruby) to identify protein bands for analysis by mass spectrometry. Phosphorylation of actin and tubulin decreased during LFD, but increased when calcineurin was blocked. Tubulin and phosphoactin immunoreactivity in pre-synaptic terminals were also reduced after LFD. The actin depolymerizing drugs cytochalasin and latrunculin and the microtubule stabilizer taxol inhibited LFD. Therefore, dephosphorylation of pre-synaptic actin and tubulin and consequent changes in the cytoskeleton may regulate LFD. LFD is unlike long-term depression found in mammalian synapses because the latter requires in most instances post-synaptic calcineurin activity.Thus, this simpler invertebrate synapse discloses a novel pre-synaptic depression mechanism.

Okadaic acid-sensitive protein phosphatases constrain phrenic long-term facilitation after sustained hypoxia

Phrenic long-term facilitation (pLTF) is a serotonin-dependent form of pattern-sensitive respiratory plasticity induced by intermittent hypoxia (IH), but not sustained hypoxia (SH). The mechanism(s) underlying pLTF pattern sensitivity are unknown. SH and IH may differentially regulate serine/threonine protein phosphatase activity, thereby inhibiting relevant protein phosphatases uniquely during IH and conferring pattern sensitivity to pLTF. We hypothesized that spinal protein phosphatase inhibition would relieve this braking action of protein phosphatases, thereby revealing pLTF after SH. Anesthetized rats received intrathecal (C4) okadaic acid (25 nm) before SH (25 min, 11% O(2)). Unlike (vehicle) control rats, SH induced a significant pLTF in okadaic acid-treated rats that was indistinguishable from rats exposed to IH (three 5 min episodes, 11% O(2)). IH and SH with okadaic acid may elicit pLTF by similar, serotonin-dependent mechanisms, because intravenous methysergide blocks pLTF in rats receiving IH or okadaic acid plus SH. Okadaic acid did not alter IH-induced pLTF. In summary, pattern sensitivity in pLTF may reflect differential regulation of okadaic acid-sensitive serine/threonine phosphatases; presumably, these phosphatases are less active during/after IH versus SH. The specific okadaic acid-sensitive phosphatase(s) constraining pLTF and their spatiotemporal dynamics during and/or after IH and SH remain to be determined.

The pool of fast releasing vesicles is augmented by myosin light chain kinase inhibition at the calyx of Held synapse

Synaptic strength is determined by release probability and the size of the readily releasable pool of docked vesicles. Here we describe the effects of blocking myosin light chain kinase (MLCK), a cytoskeletal regulatory protein thought to be involved in myosin-mediated vesicle transport, on synaptic transmission at the mouse calyx of Held synapse. Application of three different MLCK inhibitors increased the amplitude of the early excitatory postsynaptic currents (EPSCs) in a stimulus train, without affecting the late steady-state EPSCs. A presynaptic locus of action for MLCK inhibitors was confirmed by an increase in the frequency of miniature EPSCs that left their average amplitude unchanged. MLCK inhibition did not affect presynaptic Ca(2+) currents or action potential waveform. Moreover, Ca(2+) imaging experiments showed that [Ca(2+)](i) transients elicited by 100-Hz stimulus trains were not altered by MLCK inhibition. Studies using high-frequency stimulus trains indicated that MLCK inhibitors increase vesicle pool size, but do not significantly alter release probability. Accordingly, when AMPA-receptor desensitization was minimized, EPSC paired-pulse ratios were unaltered by MLCK inhibition, suggesting that release probability remains unaltered. MLCK inhibition potentiated EPSCs even when presynaptic Ca(2+) buffering was greatly enhanced by treating slices with EGTA-AM. In addition, MLCK inhibition did not affect the rate of recovery from short-term depression. Finally, developmental studies revealed that EPSC potentiation by MLCK inhibition starts at postnatal day 5 (P5) and remains strong during synaptic maturation up to P18. Overall, our data suggest that MLCK plays a crucial role in determining the size of the pool of synaptic vesicles that undergo fast release at a CNS synapse.

An antibody to synaptotagmin I facilitates synaptic transmission

Proper functioning of the nervous system requires precise control of neurotransmitter release. Synaptotagmin, a synaptic vesicle protein, is crucial for the temporal control of neurotransmitter release. The mechanism of synaptotagmin function is still under debate. To investigate the mechanism by which synaptotagmin controls neurotransmitter release, we injected an antibody of rat synaptotagmin I into a crayfish motor axon. We found that the antibody enhanced synaptic transmission at crayfish neuromuscular junctions by increasing the amplitude of the evoked synaptic response. This effect was antibody-dose dependent. The antibody also reduced the rise time of the synaptic potentials. These effects were accompanied by a reduction in the Hill coefficient for Ca(2+)-dependence of synaptic transmission. Our findings support the hypothesis that synaptotagmin inhibits neurotransmitter release in the absence of Ca(2+).

Automated classification of evoked quantal events

We provide both theoretical and computational improvements to the analysis of synaptic transmission data. Theoretically, we demonstrate the correlation structure of observations within evoked postsynaptic potentials (EPSP) are consistent with multiple random draws from a common autoregressive moving-average (ARMA) process of order (2, 2). We use this observation and standard time series results to construct a statistical hypothesis testing procedure for determining whether a given trace is an EPSP. Computationally, we implement this method in R, a freeware statistical language, which reduces the amount of time required for the investigator to classify traces into EPSPs or non-EPSPs and eliminates investigator subjectivity from this classification. In addition, we provide a computational method for calculating common functionals of EPSPs (peak amplitude, decay rate, etc.). The methodology is freely available over the internet. The automated procedure to index the quantal characteristics greatly facilitates determining if any one or multiple parameters are changing due to experimental conditions. In our experience, the software reduces the time required to perform these analyses from hours to minutes.

Cholesterol and synaptic transmitter release at crayfish neuromuscular junctions

During exocytosis of synaptic transmitters, the fusion of highly curved synaptic vesicle membranes with the relatively planar cell membrane requires the coordinated action of several proteins. The role of membrane lipids in the regulation of transmitter release is less well understood. Since it helps to control membrane fluidity, alteration of cholesterol content may alter the fusibility of membranes as well as the function of membrane proteins. We assayed the importance of cholesterol in transmitter release at crayfish neuromuscular junctions where action potentials can be measured in the preterminal axon. Methyl-beta-cyclodextrin (MbetaCD) depleted axons of cholesterol, as shown by reduced filipin labelling, and cholesterol was replenished by cholesterol-MbetaCD complex (Ch-MbetaCD). MbetaCD blocked evoked synaptic transmission. The lack of postsynaptic effects of MbetaCD on the time course and amplitude of spontaneous postsynaptic potentials or on muscle resting potential allowed us to focus on presynaptic mechanisms. Intracellular presynaptic axon recordings and focal extracellular recordings at individual boutons showed that failure of transmitter release was correlated with presynaptic hyperpolarization and failure of action potential propagation. All of these effects were reversed when cholesterol was replenished with Ch-MbetaCD. However, focal depolarization of presynaptic boutons and administration of a Ca2+ ionophore both triggered transmitter release after cholesterol depletion. Therefore, both presynaptic Ca2+ channels and Ca2+-dependent exocytosis functioned after cholesterol depletion. The frequency of spontaneous quantal transmitter release was increased by MbetaCD but recovered when cholesterol was reintroduced. The increase in spontaneous release was not through a calcium-dependent mechanism because it persisted with intense intracellular calcium chelation. In conclusion, cholesterol levels in the presynaptic membrane modulate several key properties of synaptic transmitter release.