Presynaptic hyperpolarization induces a fast analogue modulation of spike-evoked transmission mediated by axonal sodium channels.
ABSTRACT: In the mammalian brain, synaptic transmission usually depends on presynaptic action potentials (APs) in an all-or-none (or digital) manner. Recent studies suggest, however, that subthreshold depolarization in the presynaptic cell facilitates spike-evoked transmission, thus creating an analogue modulation of a digital process (or analogue-digital (AD) modulation). At most synapses, this process is slow and not ideally suited for the fast dynamics of neural networks. We show here that transmission at CA3-CA3 and L5-L5 synapses can be enhanced by brief presynaptic hyperpolarization such as an inhibitory postsynaptic potential (IPSP). Using dual soma-axon patch recordings and live imaging, we find that this hyperpolarization-induced AD facilitation (h-ADF) is due to the recovery from inactivation of Nav channels controlling AP amplitude in the axon. Incorporated in a network model, h-ADF promotes both pyramidal cell synchrony and gamma oscillations. In conclusion, cortical excitatory synapses in local circuits display hyperpolarization-induced facilitation of spike-evoked synaptic transmission that promotes network synchrony.
Project description:Sensory processing requires mechanisms of fast coincidence detection to discriminate synchronous from asynchronous inputs. Spike threshold adaptation enables such a discrimination but is ineffective in transmitting this information to the network. We show here that presynaptic axonal sodium channels read and transmit precise levels of input synchrony to the postsynaptic cell by modulating the presynaptic action potential (AP) amplitude. As a consequence, synaptic transmission is facilitated at cortical synapses when the presynaptic spike is produced by synchronous inputs. Using dual soma-axon recordings, imaging, and modeling, we show that this facilitation results from enhanced AP amplitude in the axon due to minimized inactivation of axonal sodium channels. Quantifying local circuit activity and using network modeling, we found that spikes induced by synchronous inputs produced a larger effect on network activity than spikes induced by asynchronous inputs. Therefore, this input synchrony-dependent facilitation may constitute a powerful mechanism, regulating synaptic transmission at proximal synapses.
Project description:Presynaptic kainate receptors (KARs) modulate transmission between dentate granule cells and CA3 pyramidal neurons. Whether presynaptic KARs affect other synapses made by granule cell axons [mossy fibers (MFs)], on hilar mossy cells or interneurons, is not known. Nor is it known whether glutamate release from a single MF is sufficient to activate these receptors. Here, we monitor Ca(2+) in identified MF boutons traced from granule cell bodies. We show that a single action potential in a single MF activates both presynaptic KARs and Ca(2+) stores, contributing to use-dependent facilitation at MF-CA3 pyramidal cell synapses. Rapid local application of kainate to the giant MF bouton has no detectable effect on the resting Ca(2+) but facilitates action-potential-evoked Ca(2+) entry through a Ca(2+) store-dependent mechanism. Localized two-photon uncaging of the Ca(2+) store receptor ligand IP(3) directly confirms the presence of functional Ca(2+) stores at these boutons. In contrast, presynaptic Ca(2+) kinetics at MF synapses on interneurons or mossy cells are insensitive to KAR blockade, to local kainate application or to photolytic release of IP(3). Consistent with this, postsynaptic responses evoked by activation of a single MF show KAR-dependent paired-pulse facilitation in CA3 pyramidal cells, but not in interneurons or mossy cells. Thus, KAR-Ca(2+) store coupling acts as a synapse-specific, short-range autoreceptor mechanism.
Project description:Neurotransmitter release probability (P(r)) largely determines the dynamic properties of synapses. While much is known about the role of presynaptic proteins in transmitter release, their specific contribution to synaptic plasticity is unclear. One such protein, tomosyn, is believed to reduce P(r) by interfering with the SNARE complex formation. Tomosyn is enriched at hippocampal mossy fiber-to-CA3 pyramidal cell synapses (MF-CA3), which characteristically exhibit low P(r), strong synaptic facilitation, and pre-synaptic protein kinase A (PKA)-dependent long-term potentiation (LTP). To evaluate tomosyn's role in MF-CA3 function, we used a combined knockdown (KD)-optogenetic strategy whereby presynaptic neurons with reduced tomosyn levels were selectively activated by light. Using this approach in mouse hippocampal slices, we found that facilitation, LTP, and PKA-induced potentiation were significantly impaired at tomosyn-deficient synapses. These findings not only indicate that tomosyn is a key regulator of MF-CA3 plasticity but also highlight the power of a combined KD-optogenetic approach to determine the role of presynaptic proteins.
Project description:Subthreshold somatic depolarization has been shown recently to modulate presynaptic neurotransmitter release in cortical neurons. To understand the mechanisms underlying this mode of signaling in the axons of dentate granule cells (hippocampal mossy fibers), we have combined two-photon Ca2+ imaging with dual-patch recordings from somata and giant boutons forming synapses on CA3 pyramidal cells. In intact axons, subthreshold depolarization propagates both orthodromically and antidromically, with an estimated length constant of 200-600 microm depending on the signal waveform. Surprisingly, presynaptic depolarization sufficient to enhance glutamate release at mossy fiber-CA3 pyramidal cell synapses has no detectable effect on either basal Ca2+-dependent fluorescence or action-potential-evoked fluorescence transients in giant boutons. We further estimate that neurotransmitter release varies with presynaptic Ca2+ entry with a 2.5-power relationship and that depolarization-induced synaptic facilitation remains intact in the presence of high-affinity presynaptic Ca2+ buffers or after blockade of local Ca2+ stores. We conclude that depolarization-dependent modulation of transmission at these boutons does not rely on changes in presynaptic Ca2+.
Project description:Analog-digital facilitations (ADFs) have been described in local excitatory brain circuits and correspond to a class of phenomena describing how subthreshold variations of the presynaptic membrane potential influence spike-evoked synaptic transmission. In many brain circuits, ADFs rely on the propagation of somatic membrane potential fluctuations to the presynaptic bouton where they modulate ion channels availability, inducing modifications of the presynaptic spike waveform, the spike-evoked Ca2+ entry, and the transmitter release. Therefore, one major requirement for ADFs to occur is the propagation of subthreshold membrane potential variations from the soma to the presynaptic bouton. To date, reported ADFs space constants are relatively short (250-500 ?m) which limits their action to proximal synapses. However, ADFs have been studied either in unmyelinated axons or in juvenile animals in which myelination is incomplete. We examined here the potential gain of ADFs spatial extent caused by myelination using a realistic model of L5 pyramidal cell. Myelination of the axon was found to induce a 3-fold increase in the axonal length constant. As a result, the different forms of ADF were found to display a much longer spatial extent (up to 3,000 ?m). In addition, while the internodal length displayed a mild effect, the number of myelin wraps ensheathing the internodes was found to play a critical role in the ADFs spatial extents. We conclude that axonal myelination induces an increase in ADFs spatial extent in our model, thus making ADFs plausible in long-distance connections.
Project description:Members of the Rab3 gene family are considered central to membrane trafficking of synaptic vesicles at mammalian central excitatory synapses. Recent evidence, however, indicates that the Rab27B-GTPase, which is highly homologous to the Rab3 family, is also enriched on SV membranes and co-localize with Rab3A and Synaptotagmin at presynaptic terminals. While functional roles of Rab3A have been well-established, little functional information exists on the role of Rab27B in synaptic transmission. Here we report on functional effects of Rab27B at SC-CA1 and DG-MF hippocampal synapses. The data establish distinct functional actions of Rab27B and demonstrate functions of Rab27B that differ between SC-CA1 and DG-MF synapses. Rab27B knockout reduced frequency facilitation compared to wild-type (WT) controls at the DG/MF-CA3 synaptic region, while increasing facilitation at the SC-CA1 synaptic region. Remarkably, Rab27B KO resulted in a complete elimination of LTP at the MF-CA3 synapse with no effect at the SC-CA1 synapse. These actions are similar to those previously reported for Rab3A KO. Specificity of action on LTP to Rab27B was confirmed as LTP was rescued in response to lentiviral infection and expression of human Rab27B, but not to GFP, in the DG in the Rab27B KO mice. Notably, the effect of Rab27B KO on MF-CA3 LTP occurred in spite of continued expression of Rab3A in the Rab27B KO. Overall, the results provide a novel perspective in suggesting that Rab27B and Rab3A act synergistically, perhaps via sequential effector recruitment or signaling for presynaptic LTP expression in this hippocampal synaptic region.
Project description:Recent data have provided evidence that microglia, the brain-resident macrophage-like cells, modulate neuronal activity in both physiological and pathophysiological conditions, and microglia are therefore now recognized as synaptic partners. Among different neuromodulators, purines, which are produced and released by microglia, have emerged as promising candidates to mediate interactions between microglia and synapses. The cellular effects of purines are mediated through a large family of receptors for adenosine and for ATP (P2 receptors). These receptors are present at brain synapses, but it is unknown whether they can respond to microglia-derived purines to modulate synaptic transmission and plasticity. Here, we used a simple model of adding immune-challenged microglia to mouse hippocampal slices to investigate their impact on synaptic transmission and plasticity at hippocampal mossy fibre (MF) synapses onto CA3 pyramidal neurons. MF-CA3 synapses show prominent forms of presynaptic plasticity that are involved in the encoding and retrieval of memory. We demonstrate that microglia-derived ATP differentially modulates synaptic transmission and short-term plasticity at MF-CA3 synapses by acting, respectively, on presynaptic P2X4 receptors and on adenosine A1 receptors after conversion of extracellular ATP to adenosine. We also report that P2X4 receptors are densely located in the mossy fibre tract in the dentate gyrus-CA3 circuitry. In conclusion, this study reveals an interplay between microglia-derived purines and MF-CA3 synapses, and highlights microglia as potent modulators of presynaptic plasticity.
Project description:CA1 pyramidal neurons receive two distinct excitatory inputs that are each capable of influencing hippocampal output and learning and memory. The Schaffer collateral (SC) input from CA3 axons onto the more proximal dendrites of CA1 is part of the trisynaptic circuit, which originates in Layer II of the entorhinal cortex (EC). The temporoammonic (TA) pathway to CA1 provides input directly from Layer III of the EC onto the most distal dendrites of CA1 pyramidal cells, and is involved in spatial memory and memory consolidation. We have previously described a developmental decrease in short-term facilitation from juvenile (P13-18) to young adult (P28-42) rats at SC synapses that is due to feedback inhibition via synaptically activated mGluR1 on CA1 interneurons. It is not known how short-term changes in synaptic strength are regulated at TA synapses, nor is it known how short-term plasticity is balanced at SC and TA inputs during development. Here we describe a novel developmental increase in short-term facilitation at TA synapses, which is the opposite of the decrease in facilitation occurring at SC synapses. Although short-term facilitation is much lower at TA synapses when compared with SC synapses in juveniles, short-term plasticity at SC and TA synapses converges at similar levels of paired-pulse facilitation in the young adult rat. However, in young adults CA3-CA1 synapses still exhibit more facilitation than TA-CA1 synapses during physiologically-relevant activity, suggesting that the two pathways are each poised to uniquely modulate CA1 output in an activity-dependent manner. Finally, we show that there is a developmental decrease in the initial release probability at TA synapses that underlies their developmental decrease in facilitation, but no developmental change in release probability at SC synapses. This represents a fundamental difference in the presynaptic function of the two major inputs to CA1, which could alter the flow of information in hippocampus during development.
Project description:Calcium-dependent activator protein for secretion 1 (CAPS1) regulates exocytosis of dense-core vesicles in neuroendocrine cells and of synaptic vesicles in neurons. However, the synaptic function of CAPS1 in the mature brain is unclear because Caps1 knockout (KO) results in neonatal death. Here, using forebrain-specific Caps1 conditional KO (cKO) mice, we demonstrate, for the first time, a critical role of CAPS1 in adult synapses. The amplitude of synaptic transmission at CA3-CA1 synapses was strongly reduced, and paired-pulse facilitation was significantly increased, in acute hippocampal slices from cKO mice compared with control mice, suggesting a perturbation in presynaptic function. Morphological analysis revealed an accumulation of synaptic vesicles in the presynapse without any overall morphological change. Interestingly, however, the percentage of docked vesicles was markedly decreased in the Caps1 cKO. Taken together, our findings suggest that CAPS1 stabilizes the state of readily releasable synaptic vesicles, thereby enhancing neurotransmitter release at hippocampal synapses.
Project description:Actin filaments (F-actin) are the major structural component of excitatory synapses. In excitatory synapses, F-actin is enriched in presynaptic terminals and in postsynaptic dendritic spines, and actin dynamics - the spatiotemporally controlled assembly and disassembly of F-actin - have been implicated in pre- and postsynaptic physiology, additionally to their function in synapse morphology. Hence, actin binding proteins that control actin dynamics have moved into the focus as regulators of synapse morphology and physiology. Actin depolymerizing proteins of the ADF/cofilin family are important regulators of actin dynamics, and several recent studies highlighted the relevance of cofilin 1 for dendritic spine morphology, trafficking of postsynaptic glutamate receptors, and synaptic plasticity. Conversely, almost nothing was known about the synaptic function of ADF, a second ADF/cofilin family member present at excitatory synapses, and it remained unknown whether ADF/cofilin is relevant for presynaptic physiology. To comprehensively characterize the synaptic function of ADF/cofilin we made use of mutant mice lacking either ADF or cofilin 1 or both proteins. Our analysis revealed presynaptic defects (altered distribution and enhanced exocytosis of synaptic vesicles) and behavioral abnormalities reminiscent of attention deficit-hyperactivity disorder in double mutants that were not present in single mutants. Hence, by exploiting gene-targeted mice, we demonstrated the relevance of ADF for excitatory synapses, and we unraveled novel functions for ADF/cofilin in presynaptic physiology and behavior.