BDNF enhances spontaneous and activity-dependent neurotransmitter release at excitatory terminals but not at inhibitory terminals in hippocampal neurons

Brain-derived neurotrophic factor scales presynaptic calcium transients to modulate excitatory neurotransmission

Brain-derived neurotrophic factor (BDNF) plays a critical role in synaptic physiology, as well as mechanisms underlying various neuropsychiatric diseases and their treatment. Despite its clear physiological role and disease relevance, BDNF's function at the presynaptic terminal, a fundamental unit of neurotransmission, remains poorly understood. In this study, we evaluated single synapse dynamics using optical imaging techniques in hippocampal cell cultures. We find that exogenous BDNF selectively increases evoked excitatory neurotransmission without affecting spontaneous neurotransmission. However, acutely blocking endogenous BDNF has no effect on evoked or spontaneous release, demonstrating that different approaches to studying BDNF may yield different results. When we suppressed BDNF-Tropomyosin receptor kinase B (TrkB) activity chronically over a period of days to weeks using a mouse line enabling conditional knockout of TrkB, we found that evoked glutamate release was significantly decreased while spontaneous release remained unchanged. Moreover, chronic blockade of BDNF-TrkB activity selectively downscales evoked calcium transients without affecting spontaneous calcium events. Via pharmacological blockade by voltage-gated calcium channel (VGCC) selective blockers, we found that the changes in evoked calcium transients are mediated by the P/Q subtype of VGCCs. These results suggest that BDNF-TrkB activity increases presynaptic VGCC activity to selectively increase evoked glutamate release.

Adiponectin-Receptor Agonistic Dipeptide Tyr-Pro Stimulates the Acetylcholine Nervous System in NE-4C Cells

The dipeptide Tyr-Pro has physiological potential for intact transportability into the brain parenchyma, prevention of cognitive impairment, and an adiponectin receptor 1 (AdipoR1) agonistic effect. The present study aimed to understand the effect of Tyr-Pro on the acetylcholine (ACh) nervous system and its underlying mechanism in NE-4C nerve cells. Concentration-dependent ACh production was induced by stimulation with Tyr-Pro and AdipoRon (an AdipoR1 agonist), along with the expression of AdipoR1 and choline acetyltransferase (ChAT) in NE-4C cells. By knocking down AdipoR1 in the cells, Tyr-Pro promoted ChAT expression, along with the activations of AMPK and ERK 1/2. Tyr-Pro did not alter acetylcholinesterase or ACh receptors, indicating that the dipeptide might operate as an ACh accelerator in nerve cells. This study provides the first evidence that the AdipoR1 agonistic Tyr-Pro is a promising dipeptide responsible for the stimulation of the ACh nervous system by AdipoR1-induced ChAT activation.

Brain-derived neurotrophic factor is a regulator of synaptic transmission in the adult visual thalamus

Brain-derived neurotrophic factor (BDNF) is an important regulator of circuit development, neuronal survival, and plasticity throughout the nervous system. In the visual system, BDNF is produced by retinal ganglion cells (RGCs) and transported along their axons to central targets. Within the dorsolateral geniculate nucleus (dLGN), a key RGC projection target for conscious vision, the BDNF receptor tropomyosin receptor kinase B (TrkB) is present on RGC axon terminals and postsynaptic thalamocortical (TC) relay neuron dendrites. Based on this, the goal of this study was to determine how BDNF modulates the conveyance of signals through the retinogeniculate (RG) pathway of adult mice. Application of BDNF to dLGN brain slices increased TC neuron spiking evoked by optogenetic stimulation of RGC axons. There was a modest contribution to this effect from a BDNF-dependent enhancement of TC neuron intrinsic excitability including increased input resistance and membrane depolarization. BDNF also increased evoked vesicle release from RGC axon terminals, as evidenced by increased amplitude of evoked excitatory postsynaptic currents (EPSCs), which was blocked by inhibition of TrkB or phospholipase C. High-frequency stimulation revealed that BDNF increased synaptic vesicle pool size, release probability, and replenishment rate. There was no effect of BDNF on EPSC amplitude or short-term plasticity of corticothalamic feedback synapses. Thus, BDNF regulates RG synapses by both presynaptic and postsynaptic mechanisms. These findings suggest that BNDF influences the flow of visual information through the retinogeniculate pathway.NEW & NOTEWORTHY Brain-derived neurotrophic factor (BDNF) is an important regulator of neuronal development and plasticity. In the visual system, BDNF is transported along retinal ganglion cell (RGC) axons to the dorsolateral geniculate nucleus (dLGN), although it is not known how it influences mature dLGN function. Here, BDNF enhanced thalamocortical relay neuron responses to signals arising from RGC axons in the dLGN, pointing toward an important role for BDNF in processing signals en route to the visual cortex.

BDNF and its Role in the Alcohol Abuse Initiated During Early Adolescence: Evidence from Preclinical and Clinical Studies

Brain-derived neurotrophic factor (BDNF) is a crucial brain signaling protein that is integral to many signaling pathways. This neurotrophin has shown to be highly involved in brain plastic processes such as neurogenesis, synaptic plasticity, axonal growth, and neurotransmission, among others. In the first part of this review, we revise the role of BDNF in different neuroplastic processes within the central nervous system. On the other hand, its deficiency in key neural circuits is associated with the development of psychiatric disorders, including alcohol abuse disorder. Many people begin to drink alcohol during adolescence, and it seems that changes in BDNF are evident after the adolescent regularly consumes alcohol. Therefore, the second part of this manuscript addresses the involvement of BDNF during adolescent brain maturation and how this process can be negatively affected by alcohol abuse. Finally, we propose different BNDF enhancers, both behavioral and pharmacological, which should be considered in the treatment of problematic alcohol consumption initiated during the adolescence.

BDNF signaling in context: From synaptic regulation to psychiatric disorders

Brain-derived neurotrophic factor (BDNF) is a neuropeptide that plays numerous important roles in synaptic development and plasticity. While its importance in fundamental physiology is well established, studies of BDNF often produce conflicting and unclear results, and the scope of existing research makes the prospect of setting future directions daunting. In this review, we examine the importance of spatial and temporal factors on BDNF activity, particularly in processes such as synaptogenesis, Hebbian plasticity, homeostatic plasticity, and the treatment of psychiatric disorders. Understanding the fundamental physiology of when, where, and how BDNF acts and new approaches to control BDNF signaling in time and space can contribute to improved therapeutics and patient outcomes.

Neuroligin-1 mediates presynaptic maturation through brain-derived neurotrophic factor signaling

Background

Maturation is a process that allows synapses to acquire full functionality, optimizing their activity to diverse neural circuits, and defects in synaptic maturation may contribute to neurodevelopmental disorders. Neuroligin-1 (NL1) is a postsynaptic cell adhesion molecule essential for synapse maturation, a role typically attributed to binding to pre-synaptic ligands, the neurexins. However, the pathways underlying the action of NL1 in synaptic maturation are incompletely understood, and some of its previously observed effects seem reminiscent of those described for the neurotrophin brain-derived neurotrophic factor (BDNF). Here, we show that maturational increases in active zone stability and synaptic vesicle recycling rely on the joint action of NL1 and brain-derived neurotrophic factor (BDNF).

Results

Applying BDNF to hippocampal neurons in primary cultures or organotypical slice cultures mimicked the effects of overexpressing NL1 on both structural and functional maturation. Overexpressing a NL1 mutant deficient in neurexin binding still induced presynaptic maturation. Like NL1, BDNF increased synaptic vesicle recycling and the augmentation of transmitter release by phorbol esters, both hallmarks of presynaptic maturation. Mimicking the effects of NL1, BDNF also increased the half-life of the active zone marker bassoon at synapses, reflecting increased active zone stability. Overexpressing NL1 increased the expression and synaptic accumulation of BDNF. Inhibiting BDNF signaling pharmacologically or genetically prevented the effects of NL1 on presynaptic maturation. Applying BDNF to NL1-knockout mouse cultures rescued defective presynaptic maturation, indicating that BDNF acts downstream of NL1 and can restore presynaptic maturation at late stages of network development.

Conclusions

Our data introduce BDNF as a novel and essential component in a transsynaptic pathway linking NL1-mediated cell adhesion, neurotrophin action, and presynaptic maturation. Our findings connect synaptic cell adhesion and neurotrophin signaling and may provide a therapeutic approach to neurodevelopmental disorders by targeting synapse maturation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01145-7.

Munc18-1 Is Essential for Neuropeptide Secretion in Neurons

Neuropeptide secretion from dense-core vesicles (DCVs) controls many brain functions. Several components of the DCV exocytosis machinery have recently been identified, but the participation of a SEC1/MUNC18 (SM) protein has remained elusive. Here, we tested the ability of the three exocytic SM proteins expressed in the mammalian brain, MUNC18-1/2/3, to support neuropeptide secretion. We quantified DCV exocytosis at a single vesicle resolution on action potential (AP) train-stimulation in mouse CNS neurons (of unknown sex) using pHluorin-tagged and/or mCherry-tagged neuropeptide Y (NPY) or brain-derived neurotrophic factor (BDNF). Conditional inactivation of Munc18-1 abolished all DCV exocytosis. Expression of MUNC18-1, but not MUNC18-2 or MUNC18-3, supported DCV exocytosis in Munc18-1 null neurons. Heterozygous (HZ) inactivation of Munc18-1, as a model for reduced MUNC18-1 expression, impaired DCV exocytosis, especially during the initial phase of train-stimulation, when the release was maximal. These data show that neurons critically and selectively depend on MUNC18-1 for neuropeptide secretion. Impaired neuropeptide secretion may explain aspects of the behavioral and neurodevelopmental phenotypes that were observed in Munc18-1 HZ mice.SIGNIFICANCE STATEMENT Neuropeptide secretion from dense-core vesicles (DCVs) modulates synaptic transmission, sleep, appetite, cognition and mood. However, the mechanisms of DCV exocytosis are poorly characterized. Here, we identify MUNC18-1 as an essential component for neuropeptide secretion from DCVs. Paralogs MUNC18-2 or MUNC18-3 cannot compensate for MUNC18-1. MUNC18-1 is the first protein identified to be essential for both neuropeptide secretion and synaptic transmission. In heterozygous (HZ) Munc18-1 neurons, that have a 50% reduced MUNC18-1expression and model the human STXBP1 syndrome, DCV exocytosis is impaired, especially during the initial phase of train-stimulation, when the release is maximal. These data show that MUNC18-1 is essential for neuropeptide secretion and that impaired neuropeptide secretion on reduced MUNC18-1expression may contribute to the symptoms of STXBP1 syndrome.

Interaction between a Novel Oligopeptide Fragment of the Human Neurotrophin Receptor TrkB Ectodomain D5 and the C-Terminal Fragment of Tetanus Neurotoxin

This article presents experimental evidence and computed molecular models of a potential interaction between receptor domain D5 of TrkB with the carboxyl-terminal domain of tetanus neurotoxin (Hc-TeNT). Computational simulations of a novel small cyclic oligopeptide are designed, synthesized, and tested for possible tetanus neurotoxin-D5 interaction. A hot spot of this protein-protein interaction is identified in analogy to the hitherto known crystal structures of the complex between neurotrophin and D5. Hc-TeNT activates the neurotrophin receptors, as well as its downstream signaling pathways, inducing neuroprotection in different stress cellular models. Based on these premises, we propose the Trk receptor family as potential proteic affinity receptors for TeNT. In vitro, Hc-TeNT binds to a synthetic TrkB-derived peptide and acts similar to an agonist ligand for TrkB, resulting in phosphorylation of the receptor. These properties are weakened by the mutagenesis of three residues of the predicted interaction region in Hc-TeNT. It also competes with Brain-derived neurotrophic factor, a native binder to human TrkB, for the binding to neural membranes, and for uptake in TrkB-positive vesicles. In addition, both molecules are located together in vivo at neuromuscular junctions and in motor neurons.

The physiological role of Homer2a and its novel short isoform, Homer2e, in NMDA receptor-mediated apoptosis in cerebellar granule cells

Homer is a postsynaptic scaffold protein, which has long and short isoforms. The long form of Homer consists of an N-terminal target-binding domain and a C-terminal multimerization domain, linking multiple proteins within a complex. The short form of Homer only has the N-terminal domain and likely acts as a dominant negative regulator. Homer2a, one of the long form isoforms of the Homer family, expresses with a transient peak in the early postnatal stage of mouse cerebellar granule cells (CGCs); however, the functions of Homer2a in CGCs are not fully understood yet. In this study, we investigated the physiological roles of Homer2a in CGCs using recombinant adenovirus vectors. Overexpression of the Homer2a N-terminal domain construct, which was made structurally reminiscent with Homer1a, altered NMDAR1 localization, decreased NMDA currents, and promoted the survival of CGCs. These results suggest that the Homer2a N-terminal domain acts as a dominant negative protein to attenuate NMDAR-mediated excitotoxicity. Moreover, we identified a novel short form N-terminal domain-containing Homer2, named Homer2e, which was induced by apoptotic stimulation such as ischemic brain injury. Our study suggests that the long and short forms of Homer2 are involved in apoptosis of CGCs.

Activation of the glucocorticoid receptor rapidly triggers calcium-dependent serotonin release in vitro

Aims

Glucocorticoids rapidly provoke serotonin (5‐HT) release in vivo. We aimed to investigate molecular mechanisms of glucocorticoid receptor (GR)‐triggered 5‐HT release.

Methods

Employing 1C11 cells to model 5‐HT neurotransmission, immunofluorescence and Pearson's Correlation Coefficient were used to analyze colocalization of GR, 5‐HT, vesicle membrane protein synaptotagmin 1 and vesicle dye FM4‐64FX. FFN511 and FM4‐64FX dyes as well as calcium imaging were used to visualize vesicular 5‐HT release upon application of GR agonist dexamethasone, GR antagonist mifepristone and voltage‐gated calcium channel (VGCC) inhibitors.

Results

GR, 5‐HT, synaptotagmin 1 and FM4‐64FX showed overlapping staining patterns, with Pearson's Correlation Coefficient indicating colocalization. Similarly to potassium chloride, dexamethasone caused a release of FFN511 and uptake of FM4‐64FX, indicating vesicular 5‐HT release. Mifepristone, calcium depletion and inhibition of L‐type VGCC significantly diminished dexamethasone‐induced vesicular 5‐HT release.

Conclusions

In close proximity to 5‐HT releasing sites, activated GR rapidly triggers L‐type VGCC‐dependent vesicular 5‐HT release. These findings provide a better understanding of the interrelationship between glucocorticoids and 5‐HT release.

BDNF impact on synaptic dynamics: extra or intracellular long-term release differently regulates cultured hippocampal synapses

Brain Derived Neurotrophic Factor (BDNF) signalling contributes to the formation, maturation and plasticity of Central Nervous System (CNS) synapses. Acute exposure of cultured brain circuits to BDNF leads to up-regulation of glutamatergic neuro-transmission, by the accurate tuning of pre and post synaptic features, leading to structural and functional synaptic changes. Chronic BDNF treatment has been comparatively less investigated, besides it may represent a therapeutic option to obtain rescue of post-injury alterations of synaptic networks. In this study, we used a paradigm of BDNF long-term (4 days) incubation to assess in hippocampal neurons in culture, the ability of such a treatment to alter synapses. By patch clamp recordings we describe the augmented function of excitatory neurotransmission and we further explore by live imaging the presynaptic changes brought about by long-term BDNF. In our study, exogenous long-term BDNF exposure of post-natal neurons did not affect inhibitory neurotransmission. We further compare, by genetic manipulations of cultured neurons and BDNF release, intracellular overexpression of this neurotrophin at the same developmental age. We describe for the first-time differences in synaptic modulation by BDNF with respect to exogenous or intracellular release paradigms. Such a finding holds the potential of influencing the design of future therapeutic strategies.

Rb1, the Primary Active Ingredient in Panax ginseng C.A. Meyer, Exerts Antidepressant-Like Effects via the BDNF-Trkb-CREB Pathway

Panax ginseng C.A. Meyer (Araliaceae), a popular tonic and dietetic herbal medicine, has been traditionally prescribed in China and other countries to treat affective disorders. The medicinal parts of ginseng, the roots and flower buds, have become increasingly popular as dietary supplements due to the current holistic healthcare trend. We have investigated for the first time the antidepressive actions of the different medicinal parts, namely, the main roots, fibrous roots, and flower buds (in water extract and powder), of garden-cultivated ginseng through behavioral and drug-induced tests in mice. The water extracts, but not the powders of ginseng fibrous roots, flower buds, and main roots (1.5 g of crude drug per kilogram, p.o.), significantly reduced the immobility time in the forced swim test (FST) and tail suspension test (TST); moreover, the water extracts enhanced the 5-hydroxytryptophan (5-HTP)-induced head-twitch response and antagonized the action of reserpine in the mouse. We then explored the antidepressive mechanism of action of the ginsenoside Rb1 (Rb1) related to the brain-derived neurotrophic factor (BDNF) and its downstream proteins in mice exposed to chronic unpredictable mild stress (CUMS). Treatment with Rb1 (20 mg/kg, p.o.) for 21 days significantly attenuated the CUMS-induced decrease in the activities of BDNF, tropomyosin-related kinase B (TrkB), protein kinase B (AKT), extracellular regulatory protein kinase (ERK), and cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) in the mouse hippocampal CA3 region and prefrontal cortex (PFC). Interestingly, treatment with the novel TrkB antagonist ANA-12 (0.5 mg/kg, i.p.) did not alter the level of BDNF but significantly blocked the antidepressive effects of Rb1 on proteins downstream of BDNF in CUMS-treated mice. These results suggest that BDNF–TrkB–CREB signaling may be involved in the antidepressive mechanism of the action of Rb1.

Regulation of Acetylcholine Quantal Release by Coupled Thrombin/BDNF Signaling in Mouse Motor Synapses

The aim of this study was to compare the acute effects of thrombin and brain-derived neurotrophic factor (BDNF) on spontaneous miniature endplate potentials (MEPPs) and multiquantal evoked endplate potentials (EPPs) in mouse neuromuscular junctions (NMJs) of m. diaphragma and m. EDL. Intracellular microelectrode recordings of MEPPs and EPPs were used to evaluate the changes in acetylcholine (ACh) release in mature and newly-formed mouse NMJs. Thrombin (1 nM) increased the amplitude of MEPPs and EPPs by 25–30% in mature and newly-formed NMJs. This effect was due to an enhanced loading of synaptic vesicles with ACh and increase of ACh quantal size, since it was fully prevented by blocking of vesicular ACh transporter. It was also prevented by tropomyosin-related kinase B (TrkB) receptors inhibitor ANA12. Exogenous BDNF (1 nM) mimicked thrombin effect and increased the amplitude of MEPPs and EPPs by 25–30%. It required involvement of protein kinase A (PKA) and mitogen-activated protein kinase (MEK1/2)-mediated pathway, but not phospholipase C (PLC). Blocking A_2A adenosine receptors by ZM241385 abolished the effect of BDNF, whereas additional stimulation of A_2A receptors by CGS21680 increased MEPP amplitudes, which was prevented by MEK1/2 inhibitor U0126. At mature NMJs, BDNF enhanced MEPPs frequency by 30–40%. This effect was selectively prevented by inhibition of PLC, but not PKA or MEK1/2. It is suggested that interrelated effects of thrombin/BDNF in mature and newly-formed NMJs are realized via enhancement of vesicular ACh transport and quantal size increase. BDNF-induced potentiation of synaptic transmission involves the functional coupling between A_2A receptor-dependent active PKA and neurotrophin-triggered MAPK pathway, as well as PLC-dependent increase in frequency of MEPPs.

Aspects of excitatory/inhibitory synapses in multiple brain regions are correlated with levels of brain-derived neurotrophic factor/neurotrophin-3

Appropriate synapse formation during development is necessary for normal brain function, and synapse impairment is often associated with brain dysfunction. Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are key factors in regulating synaptic development. We previously reported that BDNF/NT-3 secretion was enhanced by calcium-dependent activator protein for secretion 2 (CADPS2). Although BDNF/NT-3 and CADPS2 are co-expressed in various brain regions, the effect of Cadps2-deficiency on brain region-specific BDNF/NT-3 levels and synaptic development remains elusive. Here, we show developmental changes of BDNF/NT-3 levels and we assess disruption of excitatory/inhibitory synapses in multiple brain regions (cerebellum, hypothalamus, striatum, hippocampus, parietal cortex and prefrontal cortex) of Cadps2 knockout (KO) mice compared with wild-type (WT) mice. Compared with WT, BDNF levels in KO mice were reduced in young/adult hippocampus, but increased in young hypothalamus, while NT-3 levels were reduced in adult cerebellum and young hippocampus, but increased in adult parietal cortex. Immunofluorescence of vGluT1, an excitatory synapse marker, and vGAT, an inhibitory synapse marker, in adult KO showed that vGluT1 was higher in the cerebellum and parietal cortex but lower in the hippocampus, whereas vGAT was lower in the hippocampus and parietal cortex compared with WT. Immunolabeling for both vGluT1 and vGAT was increased in the parietal cortex but vGAT was decreased in the cerebellum in adult KO compared with WT. These data suggest that CADPS2-mediated secretion of BDNF/NT-3 may be involved in development and maturation of synapses and in the balance between inhibitory and excitatory synapses.

Autocrine signaling by an Aplysia neurotrophin forms a presynaptic positive feedback loop

Whereas short-term plasticity is often initiated on one side of the synapse, long-term plasticity involves coordinated changes on both sides, implying extracellular signaling. We have investigated the possible signaling role of an Aplysia neurotrophin (ApNT) in facilitation induced by serotonin (5HT) at sensory-to-motor neuron synapses in culture. ApNT is an ortholog of mammalian BDNF, which has been reported to act as either an anterograde, retrograde, or autocrine signal, so that its pre- and postsynaptic sources and targets remain unclear. We now report that ApNT acts as a presynaptic autocrine signal that forms part of a positive feedback loop with ApTrk and PKA. That loop stimulates spontaneous transmitter release, which recruits postsynaptic mechanisms, and presynaptic protein synthesis during the transition from short- to intermediate-term facilitation and may also initiate gene regulation to trigger the transition to long-term facilitation. These results suggest that a presynaptic ApNT feedback loop plays several key roles during consolidation of learning-related synaptic plasticity.

An Optical Assay for Synaptic Vesicle Recycling in Cultured Neurons Overexpressing Presynaptic Proteins

At active presynaptic nerve terminals, synaptic vesicles undergo cycles of exo- and endocytosis. During recycling, the luminal domains of SV transmembrane proteins become exposed at the cell surface. One of these proteins is Synaptotagmin-1 (Syt1). An antibody directed against the luminal domain of Syt1, once added to the culture medium, is taken up during the exo-endocytotic cycle. This uptake is proportional to the amount of SV recycling and can be quantified through immunofluorescence. Here, we combine Syt1 antibody uptake with double transfection of cultured hippocampal neurons. This allows us to (1) localize presynaptic sites based on expression of recombinant presynaptic marker Synaptophysin, (2) determine their functionality using Syt1 uptake, and (3) characterize the targeting and effects of a protein of interest, GFP-Rogdi.

CAPS2 deficiency affects environmental enrichment-induced adult neurogenesis and differentiation/survival of newborn neurons in the hippocampal dentate gyrus

Hippocampal adult neurogenesis is observed in the subgranular zone of the dentate gyrus (DG), and is associated with hippocampal memory formation and several psychiatric disorders including autism spectrum disorder (ASD). Calcium-dependent activator protein for secretion 2 (CAPS2) is a candidate gene related to ASD, and is highly expressed in the hippocampal DG region, with Caps2 knockout (KO) mice exhibiting ASD-like behavior. Accordingly, CAPS2 is potentially associated with hippocampal adult neurogenesis, the relationship between CAPS2 and adult neurogenesis has not yet been investigated. Here, we determined whether deficit of the Caps2 gene affects hippocampal adult neurogenesis and maturation of newborn neurons. To induce adult neurogenesis, we used the environmental enrichment (EE) condition. Both wild-type (WT) and Caps2 KO mice were housed in control or EE conditions for 3 or 14days. Hippocampal levels of brain-derived neurotrophic factor (BDNF) can be used as a physiological EE conditioned marker, and were increased at 14days in the EE condition in both WT and KO mice. Newborn cells during control and EE conditions were labeled by BrdU, and the labeled cells co-immunostained with the immature and mature neuron markers, calretinin (CR) and NeuN. The ratio of CR/BrdU and NeuN/BrdU double positive cells to all of BrdU positive cells were significantly increased in WT mice housed in the EE condition for 14days compared with the control condition. Whereas KO mice in the EE condition showed no significant increase of newborn neurons. These findings suggest that CAPS2 deficiency strongly impairs hippocampal adult neurogenesis and maturation of newborn neurons.

TRPV1 regulates excitatory innervation of OLM neurons in the hippocampus

TRPV1 is an ion channel activated by heat and pungent agents including capsaicin, and has been extensively studied in nociception of sensory neurons. However, the location and function of TRPV1 in the hippocampus is debated. We found that TRPV1 is expressed in oriens-lacunosum-moleculare (OLM) interneurons in the hippocampus, and promotes excitatory innervation. TRPV1 knockout mice have reduced glutamatergic innervation of OLM neurons. When activated by capsaicin, TRPV1 recruits more glutamatergic, but not GABAergic, terminals to OLM neurons in vitro. When TRPV1 is blocked, glutamatergic input to OLM neurons is dramatically reduced. Heterologous expression of TRPV1 also increases excitatory innervation. Moreover, TRPV1 knockouts have reduced Schaffer collateral LTP, which is rescued by activating OLM neurons with nicotine—via α2β2-containing nicotinic receptors—to bypass innervation defects. Our results reveal a synaptogenic function of TRPV1 in a specific interneuron population in the hippocampus, where it is important for gating hippocampal plasticity.

The role of TRPV1 in the CNS is not fully understood. Here the authors show that TRPV1 is expressed specifically in somatostatin-positive OLM interneurons of the hippocampus, where it promotes excitatory innervation of these cells.

Inhibition of Calpains Protects Mn-Induced Neurotransmitter release disorders in Synaptosomes from Mice: Involvement of SNARE Complex and Synaptic Vesicle Fusion

Overexposure to manganese (Mn) could disrupt neurotransmitter release via influencing the formation of SNARE complex, but the underlying mechanisms are still unclear. A previous study demonstrated that SNAP-25 is one of substrate of calpains. The current study investigated whether calpains were involved in Mn-induced disorder of SNARE complex. After mice were treated with Mn for 24 days, Mn deposition increased significantly in basal nuclei in Mn-treated and calpeptin pre-treated groups. Behaviorally, less time spent in the center of the area and decreased average velocity significantly in an open field test after 24 days of Mn exposure. With the increase in MnCl₂ dosage, intracellular Ca²⁺ increased significantly, but pretreatment with calpeptin caused a dose-dependent decrease in calpains activity. There were fragments of N-terminal of SNAP-25 protein appearance in Mn-treated groups, but it is decreased with pretreatment of calpeptin. FM1-43-labeled synaptic vesicles also provided evidence that the treatment with Mn resulted in increasing first and then decreasing, which was consistent with Glu release and the 80 kDa protein levels of SNARE complexes. In summary, Mn induced the disorder of neurotransmitter release through influencing the formation of SNARE complex via cleaving SNAP-25 by overactivation of calpains in vivo.

Calcium-Dependent and Synapsin-Dependent Pathways for the Presynaptic Actions of BDNF

We used cultured hippocampal neurons to determine the signaling pathways mediating brain-derived neurotrophic factor (BDNF) regulation of spontaneous glutamate and GABA release. BDNF treatment elevated calcium concentration in presynaptic terminals; this calcium signal reached a peak within 1 min and declined in the sustained presence of BDNF. This BDNF-induced transient rise in presynaptic calcium was reduced by SKF96365, indicating that BDNF causes presynaptic calcium influx via TRPC channels. BDNF treatment increased the frequency of miniature excitatory postsynaptic currents (mEPSCs). This response consisted of two components: a transient component that peaked within 1 min of initiating BDNF application and a second component that was sustained, at a lower mEPSC frequency, for the duration of BDNF application. The initial transient component was greatly reduced by removing external calcium or by treatment with SKF96365, as well as by Pyr3, a selective blocker of TRPC3 channels. In contrast, the sustained component was unaffected in these conditions but was eliminated by U0126, an inhibitor of the MAP kinase (MAPK) pathway, as well as by genetic deletion of synapsins in neurons from a synapsin triple knock-out (TKO) mouse. Thus, two pathways mediate the ability of BDNF to enhance spontaneous glutamate release: the transient component arises from calcium influx through TRPC3 channels, while the sustained component is mediated by MAPK phosphorylation of synapsins. We also examined the ability of these two BDNF-dependent pathways to regulate spontaneous release of the inhibitory neurotransmitter, GABA. BDNF had no effect on the frequency of spontaneous miniature inhibitory postsynaptic currents (mIPSCs) in neurons from wild-type (WT) mice, but surprisingly did increase mIPSC frequency in synapsin TKO mice. This covert BDNF response was blocked by removal of external calcium or by treatment with SKF96365 or Pyr3, indicating that it results from calcium influx mediated by TRPC3 channels. Thus, the BDNF-activated calcium signaling pathway can also enhance spontaneous GABA release, though this effect is suppressed by synapsins under normal physiological conditions.

Manganese exposure disrupts SNARE protein complex-mediated vesicle fusion in primary cultured neurons

Overexposure to manganese (Mn) has been known to disrupt neurotransmitter release in the brain. However, the underlying mechanisms of Mn exposure on neurotransmitter vesicle release are still unclear. The current study investigated whether the protein expression and their interaction of SNARE complex associated proteins were the media between Mn exposure and neurotransmitter vesicle fusion disorders. After the neurons were respectively exposed to Mn (0-200 μM) for 0, 6, 12, 18, 24 h, there were different degrees of cell injury in neurons. According to the results, Mn exposures in subsequent experiments were restricted to concentrations of 100 μM for 0, 6, 12, 18, 24 h. Mn was found to down-regulate the expression of SNAP-25 and up-regulate the expression of VAMP-2 in cultured neurons. Moreover, the interaction of Munc 18 and Syntaxin increased significantly in response to Mn treatment for 18-24h, and the interaction of VAMP-2 and Synaptophysin increased first and then decreased. FM1-43-labeled synaptic vesicles also provided evidence that the treatment with Mn resulted in neurotransmitter vesicle fusion increasing first and then decreasing, which was consistent with the 80 kDa protein levels of SNARE complexes. The findings clearly demonstrated that Mn induced the disorders of neurotransmitter vesicle release via disturbing the protein expression and their interaction of SNARE complex associated proteins. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 705-716, 2017.

Low doses of the mycotoxin citrinin protect cortical neurons against glutamate-induced excitotoxicity

Citrinin, a natural mycotoxin that is found in fermented foods, is known as a cytotoxin and nephrotoxin. Exposure to high doses of citrinin result in apoptosis; however, the effects of low doses are not fully understood. Glutamate excitotoxicity is responsible for neuronal death in acute neurological disorders including stroke, trauma and other neurodegenerative diseases. Here, we show the neuroprotective effect of low doses of citrinin against glutamate-induced excitotoxicity. We examined the effect of citrinin exposure on glutamate-induced cell death in cultured rat cortical neurons under two conditions: simultaneous treatment with citrinin 0.1 to 1,000 nM and glutamate (30 μM) for 1, 3 hr; the same simultaneous treatment for 3 hr after pretreatment with citrinin for 21 hr. Both the MTT and immunocytochemical assay showed significant neuroprotective effects at several doses and exposure times tested. All concentrations of citrinin tested showed no remarkable cell death following 14-day exposure, and no marked alterations to synapses. These data suggest that low doses of citrinin can be used as a neuroprotective agent against glutamate-induced excitotoxicity without additional harmful cellular alterations.

Galacto-N-biose is neuroprotective against glutamate-induced excitotoxicity in vitro

Galacto-N-biose (GNB: Galβ1-3GalNAc) is an O-glycan disaccharide core moiety that is a core component of mucin in the gastrointestinal tract; however, the physiological properties of GNB are not well understood. Glutamate excitotoxicity causes neuronal death in acute neurological disorders including stroke, trauma, and neurodegenerative disease. Therefore the discovery of drugs to treat glutamate excitotoxicity is an important goal. Here, we report that GNB is neuroprotective against glutamate-induced excitotoxicity. We treated 14-15 days in vitro cultured rat cortical neurons with 0.1-1000nM GNB together with 30µm glutamate for various durations. Short-term (3h) GNB treatments showed a modest neuroprotective effect against glutamate neurotoxicity, however, long-term (24h) GNB treatment conferred significant neuroprotective effects, as shown by both MTT and immunocytochemical assays. Prolonged GNB treatment did not alter glutamate-induced calcium influx, but did induce antioxidant-related gene expression. Furthermore, GNB treatment did not induce cell death or alter synaptic connections. These data suggest that GNB is a potential candidate drug that protects against glutamate excitotoxicity without affecting cell viability and synaptic connections.

Influence of brain-derived neurotrophic factor-tyrosine receptor kinase B signalling in the nucleus tractus solitarius on baroreflex sensitivity in rats with chronic heart failure

KEY POINTS

Impairment of baroreflex function is associated with the progression of chronic heart failure (CHF) and a poor prognosis. The baroreflex desensitization in CHF is at least partly the result of central neuronal network dysfunction. The dorsal medial nucleus tractus solitarius (dmNTS) has long been appreciated as a primary site of baroreceptor afferent termination in the central nervous system. However, the influence of neurotransmitters and neuromodulators in the dmNTS on baroreflex function both in normal and CHF states is not fully understood. The present study provides the first evidence showing a tonic sympatho-inhibitory role for brain-derived neurotrophic factor (BDNF) neurotransmission in the dmNTS. Most importantly, BDNF- tyrosine receptor kinase B (TrkB) signalling in the dmNTS is integral for normal baroreflex function as indicated by the blunting of baroreflex sensitivity (BRS) following the antagonization of TrkB, which inhibited baroreflex gain and range. Furthermore, we found that the tonic sympatho-inhibition of BDNF was withdrawn in the CHF state, thus contributing to the increased sympathetic tone associated with CHF. Consistent with this finding, BDNF/TrkB antagonism had little effect on reducing BRS in CHF animals, which is corroborated by the observation of decreased TrkB expression in the dmNTS during CHF. Taken together, these results implicate a reduction in BDNF-TrkB signalling in the dmNTS during CHF that contributes to sympatho-excitation and baroreflex desensitization. The observation that the BDNF/TrkB pathway is impaired in the dmNTS during CHF provides a novel mechanism for understanding the central alterations that contribute to baroreflex desensitization during CHF.

ABSTRACT

Chronic heart failure (CHF) results in blunting of arterial baroreflex sensitivity (BRS), which arises from alterations to both peripheral baroreceptors and central autonomic nuclei such as the nucleus tractus solitarius (NTS). Although glutamate is known to be an important neurotransmitter released from baroreceptor afferent synapses in the NTS, the influence of other neurotransmitters and neuromodulators remains unclear. Alterations to NTS signalling in CHF remain particularly undefined. The present study aimed to evaluate the role of brain-derived neurotrophic factor (BDNF) and tyrosine receptor kinase B (TrkB) receptor signalling in the NTS on baroreflex control both in healthy and CHF rats. To this end, we microinjected BDNF or the highly selective TrkB receptor antagonist [N2-2-2-oxoazepan-3-yl amino] carbonyl phenyl benzo (b)thiophene-2-carboxamide (ANA-12) into the dorsal medial NTS (dmNTS) of male Sprague-Dawley rats with coronary artery ligation-induced CHF and sham operated controls and recorded blood pressure and renal sympathetic nerve activity responses. We subsequently measured BRS before and after bilateral dmNTS microinjections of ANA-12. In sham rats, BDNF evoked a dose-dependent depressor and sympatho-inhibitory effect and ANA-12 produced the opposite response. Both of these responses were significantly blunted in CHF rats. Furthermore, bilateral microinjection of ANA-12 into the dmNTS greatly diminished baroreflex sensitivity in sham rats, whereas it had less of an effect in CHF rats. We observed decreased levels of TrkB protein and mRNA in the dmNTS of CHF rats. These data indicate that endogenous BDNF signalling in the NTS is integral for the maintenance of BRS and that BDNF/TrkB signalling is impaired in the NTS in the CHF state.

Wnt signalling tunes neurotransmitter release by directly targeting Synaptotagmin-1

The functional assembly of the synaptic release machinery is well understood; however, how signalling factors modulate this process remains unknown. Recent studies suggest that Wnts play a role in presynaptic function. To examine the mechanisms involved, we investigated the interaction of release machinery proteins with Dishevelled-1 (Dvl1), a scaffold protein that determines the cellular locale of Wnt action. Here we show that Dvl1 directly interacts with Synaptotagmin-1 (Syt-1) and indirectly with the SNARE proteins SNAP25 and Syntaxin (Stx-1). Importantly, the interaction of Dvl1 with Syt-1, which is regulated by Wnts, modulates neurotransmitter release. Moreover, presynaptic terminals from Wnt signalling-deficient mice exhibit reduced release probability and are unable to sustain high-frequency release. Consistently, the readily releasable pool size and formation of SNARE complexes are reduced. Our studies demonstrate that Wnt signalling tunes neurotransmitter release and identify Syt-1 as a target for modulation by secreted signalling proteins.