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Endocannabinoids are key modulators of synaptic function. By activating cannabinoid receptors expressed in the central nervous system, these lipid messengers can regulate several neural functions and behaviors. As experimental tools advance, the repertoire of known endocannabinoid-mediated effects at the synapse, and their underlying mechanism, continues to expand. Retrograde signaling is the principal mode by which endocannabinoids mediate short- and long-term forms of plasticity at both excitatory and inhibitory synapses. However, growing evidence suggests that endocannabinoids can also signal in a nonretrograde manner. In addition to mediating synaptic plasticity, the endocannabinoid system is itself subject to plastic changes. Multiple points of interaction with other neuromodulatory and signaling systems have now been identified. In this Review, we focus on new advances in synaptic endocannabinoid signaling in the mammalian brain. The emerging picture not only reinforces endocannabinoids as potent regulators of synaptic function but also reveals that endocannabinoid signaling is mechanistically more complex and diverse than originally thought. [Copyright &y& Elsevier]

Summary: Presynaptic autoreceptors modulate transmitter release at many synapses. At the mossy fiber to CA3 pyramidal cell (mf-CA3) synapse, two types of glutamatergic autoreceptors have been identified: transmitter release is reportedly suppressed by metabotropic glutamate receptors (mGluRs) and augmented by kainate receptors (KARs). However, the net effect of these autoreceptors when activated by endogenous glutamate is unknown. Here, we show that during low-frequency mossy fiber stimulation, glutamate acting through presynaptic mGluRs substantially suppresses transmitter release. However, using similar recording conditions, we find that presynaptic KARs are insufficient to facilitate transmitter release over a wide range of mossy fiber stimulus frequencies, indicating that the uniquely robust mf-CA3 short-term plasticity is KAR independent. Furthermore, we report that actions generally attributed to presynaptic KARs are likely due to activation of recurrent CA3 network activity. Thus, negative feedback via presynaptic mGluRs is the dominant mode of glutamatergic autoregulation at the mf-CA3 synapse. [Copyright &y& Elsevier]

Summary: The mossy fiber to CA3 pyramidal cell synapse (mf-CA3) provides a major source of excitation to the hippocampus. Thus far, these glutamatergic synapses are well recognized for showing a presynaptic, NMDA receptor-independent form of LTP that is expressed as a long-lasting increase of transmitter release. Here, we show that in addition to this “classical” LTP, mf-CA3 synapses can undergo a form of LTP characterized by a selective enhancement of NMDA receptor-mediated transmission. This potentiation requires coactivation of NMDA and mGlu5 receptors and a postsynaptic calcium rise. Unlike classical LTP, expression of this mossy fiber LTP is due to a PKC-dependent recruitment of NMDA receptors specifically to the mf-CA3 synapse via a SNARE-dependent process. Having two mechanistically different forms of LTP may allow mf-CA3 synapses to respond with more flexibility to the changing demands of the hippocampal network. [Copyright &y& Elsevier]

Repetitive activation of glutamatergic fibers that normally induces long-term potentiation (LTP) at excitatory synapses in the hippocampus also triggers long-term depression at inhibitory synapses (I-LTD) via retrograde endocannabinoid signaling. Little is known, however, about the physiological significance of I-LTD. Here, we show that synaptic-driven release of endocannabinoids is a highly localized and efficient process that strongly depresses cannabinoid-sensitive inhibitory inputs within the dendritic compartment of CA1 pyramidal cells. By removing synaptic inhibition in a restricted area of the dendritic tree, endocannabinoids selectively “primed” nearby excitatory synapses, thereby facilitating subsequent induction of LTP. This induction of local metaplasticity is a novel mechanism by which endocannabinoids can contribute to the storage of information in the brain. [Copyright &y& Elsevier]

Neuronal excitability and long-term synaptic plasticity at excitatory synapses are critically dependent on the level of inhibition, and accordingly, changes of inhibitory synaptic efficacy should have great impact on neuronal function and neural network processing. We describe here a form of activity-dependent long-term depression at hippocampal inhibitory synapses that is triggered postsynaptically via glutamate receptor activation but is expressed presynaptically. That is, glutamate released by repetitive activation of Schaffer collaterals activates group I metabotropic glutamate receptors at CA1 pyramidal cells, triggering a persistent reduction of GABA release that is mediated by endocannabinoids. This heterosynaptic form of plasticity is involved in changes of pyramidal cell excitability associated with long-term potentiation at excitatory synapses and could account for the effects of cannabinoids on learning and memory. [Copyright &y& Elsevier]

The synaptic basis underlying food intake is poorly understood. New research shows that an animal''s satiety state dictates the polarity of long-term inhibitory synaptic plasticity in the hypothalamus, which is mediated by an activity-dependent competition between endocannabinoid and nitric oxide signaling. [Copyright &y& Elsevier]

Neuroscientists have been looking for good examples linking neuronal activity to gene expression/regulation involved in synaptic plasticity and the formation of long-term memories. New findings from Park et al. and Waung et al. in this issue of Neuron show that fast dendritic translation of the immediate-early gene Arc/Arg3.1 is involved in hippocampal mGluR-LTD, a protein synthesis-dependent form of plasticity. [Copyright &y& Elsevier]

Activity-dependent expression of immediate early genes (IEGs) is critical for long-term synaptic remodeling and memory. It remains unknown how IEGs are maintained for memory despite rapid transcript and protein turnover. To address this conundrum, we monitored Arc , an IEG essential for memory consolidation. Using a knockin mouse where endogenous Arc alleles were fluorescently tagged, we performed real-time imaging of Arc mRNA dynamics in individual neurons in cultures and brain tissue. Unexpectedly, a single burst stimulation was sufficient to induce cycles of transcriptional reactivation in the same neuron. Subsequent transcription cycles required translation, whereby new Arc proteins engaged in autoregulatory positive feedback to reinduce transcription. The ensuing Arc mRNAs preferentially localized at sites marked by previous Arc protein, assembling a "hotspot" of translation, and consolidating "hubs" of dendritic Arc. These cycles of transcription-translation coupling sustain protein expression and provide a mechanism by which a short-lived event may support long-term memory. [Display omitted] • Reactivation of transcription drives cycling of the Arc gene in individual neurons • Feedback from new proteins reinduces Arc transcription in the next cycle • Arc mRNAs from later cycles localize to sites marked with previous Arc protein • Repetitive translation in hotspots consolidates dendritic Arc in selective hubs How are short-lived mRNAs and proteins maintained over time to impact long-term memory? Das et al. report that repetitive cycles of transcription and local translation of the immediate early gene Arc amplify a single burst stimulation over time to assemble Arc protein hubs, revealing a molecular mechanism that supports memory consolidation. [ABSTRACT FROM AUTHOR]

Summary Long-term changes of neurotransmitter release are critical for proper brain function. However, the molecular mechanisms underlying these changes are poorly understood. While protein synthesis is crucial for the consolidation of postsynaptic plasticity, whether and how protein synthesis regulates presynaptic plasticity in the mature mammalian brain remain unclear. Here, using paired whole-cell recordings in rodent hippocampal slices, we report that presynaptic protein synthesis is required for long-term, but not short-term, plasticity of GABA release from type 1 cannabinoid receptor (CB 1 )-expressing axons. This long-term depression of inhibitory transmission (iLTD) involves cap-dependent protein synthesis in presynaptic interneuron axons, but not somata. Translation is required during the induction, but not maintenance, of iLTD. Mechanistically, CB 1 activation enhances protein synthesis via the mTOR pathway. Furthermore, using super-resolution STORM microscopy, we revealed eukaryotic ribosomes in CB 1 -expressing axon terminals. These findings suggest that presynaptic local protein synthesis controls neurotransmitter release during long-term plasticity in the mature mammalian brain. [ABSTRACT FROM AUTHOR]

Summary Protein phosphorylation is an essential step for the expression of long-term potentiation (LTP), a long-lasting, activity-dependent strengthening of synaptic transmission widely regarded as a cellular mechanism underlying learning and memory. At the core of LTP is the synaptic insertion of AMPA receptors (AMPARs) triggered by the NMDA receptor-dependent activation of Ca 2+ /calmodulin-dependent protein kinase II (CaMKII). However, the CaMKII substrate that increases AMPAR-mediated transmission during LTP remains elusive. Here, we identify the hippocampus-enriched TARPγ-8, but not TARPγ-2/3/4, as a critical CaMKII substrate for LTP. We found that LTP induction increases TARPγ-8 phosphorylation, and that CaMKII-dependent enhancement of AMPAR-mediated transmission requires CaMKII phosphorylation sites of TARPγ-8. Moreover, LTP and memory formation, but not basal transmission, are significantly impaired in mice lacking CaMKII phosphorylation sites of TARPγ-8. Together, these findings demonstrate that TARPγ-8 is a crucial mediator of CaMKII-dependent LTP and therefore a molecular target that controls synaptic plasticity and associated cognitive functions. [ABSTRACT FROM AUTHOR]

Learning and memory rely on long-lasting, synapse-specific modifications. Although postsynaptic forms of plasticity typically require local protein synthesis, whether and how local protein synthesis contributes to presynaptic changes remain unclear. Here, we examined the mouse hippocampal mossy fiber (MF)-CA3 synapse, which expresses both structural and functional presynaptic plasticity and contains presynaptic fragile X messenger ribonucleoprotein (FMRP), an RNA-binding protein involved in postsynaptic protein-synthesis-dependent plasticity. We report that MF boutons contain ribosomes and synthesize protein locally. The long-term potentiation of MF-CA3 synaptic transmission (MF-LTP) was associated with the translation-dependent enlargement of MF boutons. Remarkably, increasing in vitro or in vivo MF activity enhanced the protein synthesis in MFs. Moreover, the deletion of presynaptic FMRP blocked structural and functional MF-LTP, suggesting that FMRP is a critical regulator of presynaptic MF plasticity. Thus, presynaptic FMRP and protein synthesis dynamically control presynaptic structure and function in the mature mammalian brain. [Display omitted] • Mossy fiber boutons (MFBs) contain ribosomes and synthesize protein locally • Local presynaptic translation is increased by in vitro and in vivo GC activity • MFB structural plasticity relies on de novo protein synthesis • Presynaptic FMRP is required for MF-CA3 structural and functional plasticity Monday, Kharod et al. report that neuronal activity regulates presynaptic FMRP function and the local synthesis of β-actin. In vitro activity and in vivo experience engage the FMRP-dependent spatiotemporal regulation of protein synthesis that is required for presynaptic remodeling and long-term changes in neurotransmitter release. [ABSTRACT FROM AUTHOR]

Summary: The presynaptic active zone is composed of a protein network that contains ELKS2α (a.k.a. CAST) as a central component. Here we demonstrate that in mice, deletion of ELKS2α caused a large increase in inhibitory, but not excitatory, neurotransmitter release, and potentiated the size, but not the properties, of the readily-releasable pool of vesicles at inhibitory synapses. Quantitative electron microscopy revealed that the ELKS2α deletion did not change the number of docked vesicles or other ultrastructural parameters of synapses, except for a small decrease in synaptic vesicle numbers. The ELKS2α deletion did, however, alter the excitatory/inhibitory balance and exploratory behaviors, possibly as a result of the increased synaptic inhibition. Thus, as opposed to previous studies indicating that ELKS2α is essential for mediating neurotransmitter release, our results suggest that ELKS2α normally restricts release and limits the size of the readily-releasable pool of synaptic vesicles at the active zone of inhibitory synapses. [Copyright &y& Elsevier]

Summary: Endocannabinoids (eCBs) have emerged as key activity-dependent signals that, by activating presynaptic cannabinoid receptors (i.e., CB1) coupled to Gi/o protein, can mediate short-term and long-term synaptic depression (LTD). While the presynaptic mechanisms underlying eCB-dependent short-term depression have been identified, the molecular events linking CB1 receptors to LTD are unknown. Here we show in the hippocampus that long-term, but not short-term, eCB-dependent depression of inhibitory transmission requires presynaptic cAMP/PKA signaling. We further identify the active zone protein RIM1α as a key mediator of both CB1 receptor effects on the release machinery and eCB-dependent LTD in the hippocampus. Moreover, we show that eCB-dependent LTD in the amygdala and hippocampus shares major mechanistic features. These findings reveal the signaling pathway by which CB1 receptors mediate long-term effects of eCBs in two crucial brain structures. Furthermore, our results highlight a conserved mechanism of presynaptic plasticity in the brain. [Copyright &y& Elsevier]

Summary Synaptic connections between hippocampal mossy fibers (MFs) and CA3 pyramidal neurons are essential for contextual memory encoding, but the molecular mechanisms regulating MF-CA3 synapses during memory formation and the exact nature of this regulation are poorly understood. Here we report that the activity-dependent transcription factor Npas4 selectively regulates the structure and strength of MF-CA3 synapses by restricting the number of their functional synaptic contacts without affecting the other synaptic inputs onto CA3 pyramidal neurons. Using an activity-dependent reporter, we identified CA3 pyramidal cells that were activated by contextual learning and found that MF inputs on these cells were selectively strengthened. Deletion of Npas4 prevented both contextual memory formation and this learning-induced synaptic modification. We further show that Npas4 regulates MF-CA3 synapses by controlling the expression of the polo-like kinase Plk2. Thus, Npas4 is a critical regulator of experience-dependent, structural, and functional plasticity at MF-CA3 synapses during contextual memory formation. [ABSTRACT FROM AUTHOR]

Summary Excitatory hilar mossy cells (MCs) in the dentate gyrus receive inputs from dentate granule cells (GCs) and project back to GCs locally, contralaterally, and along the longitudinal axis of the hippocampus, thereby establishing an associative positive-feedback loop and connecting functionally diverse hippocampal areas. MCs also synapse with GABAergic interneurons that mediate feed-forward inhibition onto GCs. Surprisingly, although these circuits have been implicated in both memory formation (e.g., pattern separation) and temporal lobe epilepsy, little is known about activity-dependent plasticity of their synaptic connections. Here, we report that MC-GC synapses undergo a presynaptic, NMDA-receptor-independent form of long-term potentiation (LTP) that requires postsynaptic brain-derived neurotrophic factor (BDNF)/TrkB and presynaptic cyclic AMP (cAMP)/PKA signaling. This LTP is input specific and selectively expressed at MC-GC synapses, but not at the disynaptic inhibitory loop. By increasing the excitation/inhibition balance, MC-GC LTP enhances GC output at the associative MC-GC recurrent circuit and may contribute to dentate-dependent forms of learning and epilepsy. [ABSTRACT FROM AUTHOR]

Retrograde signaling at the synapse is a fundamental way by which neurons communicate and neuronal circuit function is fine-tuned upon activity. While long-term changes in neurotransmitter release commonly rely on retrograde signaling, the mechanisms remain poorly understood. Here, we identified adenosine/A 2A receptor (A 2A R) as a retrograde signaling pathway underlying presynaptic long-term potentiation (LTP) at a hippocampal excitatory circuit critically involved in memory and epilepsy. Transient burst activity of a single dentate granule cell induced LTP of mossy cell synaptic inputs, a BDNF/TrkB-dependent form of plasticity that facilitates seizures. Postsynaptic TrkB activation released adenosine from granule cells, uncovering a non-conventional BDNF/TrkB signaling mechanism. Moreover, presynaptic A 2A Rs were necessary and sufficient for LTP. Lastly, seizure induction released adenosine in a TrkB-dependent manner, while removing A 2A Rs or TrkB from the dentate gyrus had anti-convulsant effects. By mediating presynaptic LTP, adenosine/A 2A R retrograde signaling may modulate dentate gyrus-dependent learning and promote epileptic activity., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

Pannexins are large-pore ion channels expressed throughout the mammalian brain that participate in various neuropathologies; however, their physiological roles remain obscure. Here, we report that pannexin1 channels (Panx1) can be synaptically activated under physiological recording conditions in rodent acute hippocampal slices. Specifically, NMDA receptor (NMDAR)-mediated responses at the mossy fiber to CA3 pyramidal cell synapse were followed by a slow postsynaptic inward current that could activate CA3 pyramidal cells but was absent in Panx1 knockout mice. Immunoelectron microscopy revealed that Panx1 was localized near the postsynaptic density. Further, Panx1-mediated currents were potentiated by metabotropic receptors and bidirectionally modulated by burst-timing-dependent plasticity of NMDAR-mediated transmission. Lastly, Panx1 channels were preferentially recruited when NMDAR activation enters a supralinear regime, resulting in temporally delayed burst-firing. Thus, Panx1 can contribute to synaptic amplification and broadening the temporal associativity window for co-activated pyramidal cells, thereby supporting the auto-associative functions of the CA3 region., Competing Interests: The authors declare no competing financial interests., (© 2024 The Authors.)

Long-term potentiation (LTP), a well-characterized form of synaptic plasticity, is believed to underlie memory formation. Hebbian, postsynaptically expressed LTP requires TARPγ-8 phosphorylation for synaptic insertion of AMPA receptors (AMPARs). However, it is unknown whether TARP-mediated AMPAR insertion alone is sufficient to modify behavior. Here, we report the development of a chemogenetic tool, ExSYTE (Excitatory SYnaptic Transmission modulator by Engineered TARPγ-8), to mimic the cytoplasmic interaction of TARP with the plasma membrane in a doxycycline-dependent manner. We use this tool to examine the specific role of synaptic AMPAR potentiation in amygdala neurons that are activated by fear conditioning. Selective expression of active ExSYTE in these neurons potentiates AMPAR-mediated synaptic transmission in a doxycycline-dependent manner, occludes synaptically induced LTP, and mimics freezing triggered by cued fear conditioning. Thus, chemogenetic controlling of the TARP-membrane interaction is sufficient for LTP-like synaptic AMPAR insertion, which mimics fear conditioning., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

The regulation of neurons by circadian clock genes is thought to contribute to the maintenance of neuronal functions that ultimately underlie animal behavior. However, the impact of specific circadian genes on cellular and molecular mechanisms controlling synaptic plasticity and cognitive function remains elusive. Here, we show that the expression of the circadian protein TIMELESS displays circadian rhythmicity in the mammalian hippocampus. We identify TIMELESS as a chromatin-bound protein that targets synaptic-plasticity-related genes such as phosphodiesterase 4B (Pde4b). By promoting Pde4b transcription, TIMELESS negatively regulates cAMP signaling to modulate AMPA receptor GluA1 function and influence synaptic plasticity. Conditional deletion of Timeless in the adult forebrain impairs working and contextual fear memory in mice. These cognitive phenotypes were accompanied by attenuation of hippocampal Schaffer-collateral synapse long-term potentiation. Together, these data establish a neuron-specific function of mammalian TIMELESS by defining a mechanism that regulates synaptic plasticity and cognitive function., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Presynaptic boutons in the mammalian brain are typically small and difficult to manipulate and study. Here, we present a protocol applying HaloTag self-labeling technology to detect de novo local protein synthesis in intact presynaptic mossy fiber boutons from acute mouse hippocampal slices. We describe stereotaxic injection of HaloTag-expressing virus into the brain region of interest, followed by brain slice preparation. We then detail the labeling of HaloTag-fused protein and image acquisition to visualize the labeled protein in an intact circuit. For complete details on the use and execution of this protocol, please refer to Monday et al. (2022). 1 ., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

Fragile X syndrome (FXS) is a leading cause of inherited intellectual disability and autism. Whereas dysregulated RNA translation in Fmr1 knockout (KO) mice, a model of FXS, is well studied, little is known about aberrant transcription. Using single-molecule mRNA detection, we show that mRNA encoding the AMPAR subunit GluA2 (but not GluA1) is elevated in dendrites and at transcription sites of hippocampal neurons of Fmr1 KO mice, indicating elevated GluA2 transcription. We identify CPEB3, a protein implicated in memory consolidation, as an upstream effector critical to GluA2 mRNA expression in FXS. Increased GluA2 mRNA is translated into an increase in GluA2 subunits, a switch in synaptic AMPAR phenotype from GluA2-lacking, Ca 2+ -permeable to GluA2-containing, Ca 2+ -impermeable, reduced inhibitory synaptic transmission, and loss of NMDAR-independent LTP at glutamatergic synapses onto CA1 inhibitory interneurons. These factors could contribute to an excitatory/inhibitory imbalance-a common theme in FXS and other autism spectrum disorders., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

The transport and translation of dendritic mRNAs by RNA-binding proteins (RBPs) allows for spatially restricted gene expression in neuronal processes. Although local translation in neuronal dendrites is now well documented, there is little evidence for corresponding effects on local synaptic function. Here, we report that the RBP Sam68 promotes the localization and translation of Arc mRNA preferentially in distal dendrites of rodent hippocampal CA1 pyramidal neurons. Consistent with Arc function in translation-dependent synaptic plasticity, we find that Sam68 knockout (KO) mice display impaired metabotropic glutamate-receptor-dependent long-term depression (mGluR-LTD) and impaired structural plasticity exclusively at distal Schaffer-collateral synapses. Moreover, by using quantitative proteomics, we find that the Sam68 interactome contains numerous regulators of mRNA translation and synaptic function. This work identifies an important player in Arc expression, provides a general framework for Sam68 regulation of protein synthesis, and uncovers a mechanism that enables the precise spatiotemporal expression of long-term plasticity throughout neurons., (Copyright © 2019 The Author(s). Published by Elsevier Inc. All rights reserved.)

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., (Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.)

Dendritic protein homeostasis is crucial for most forms of long-term synaptic plasticity, and its dysregulation is linked to a wide range of brain disorders. Current models of metabotropic glutamate receptor mediated long-term depression (mGluR-LTD) suggest that rapid, local synthesis of key proteins is necessary for the induction and expression of LTD. Here, we find that mGluR-LTD can be induced in the absence of translation if the proteasome is concurrently inhibited. We report that enhanced proteasomal degradation during the expression of mGluR-LTD depletes dendritic proteins and inhibits subsequent inductions of LTD. Moreover, proteasome inhibition can rescue mGluR-LTD in mice null for the RNA binding protein Sam68, which we show here lack mGluR-dependent translation and LTD. Our study provides mechanistic insights for how changes in dendritic protein abundance regulate mGluR-LTD induction. We propose that Sam68-mediated translation helps to counterbalance degradation, ensuring that protein levels briefly remain above a permissive threshold during LTD induction., (Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.)