A circuit from lateral hypothalamic to dorsal hippocampal dentate gyrus modulates behavioral despair in mice.

Behavioral despair is one of the clinical manifestations of major depressive disorder and an important cause of disability and death. However, the neural circuit mechanisms underlying behavioral despair are poorly understood. In a well-established chronic behavioral despair (CBD) mouse model, using a combination of viral tracing, in vivo fiber photometry, chemogenetic and optogenetic manipulations, in vitro electrophysiology, pharmacological profiling techniques, and behavioral tests, we investigated the neural circuit mechanisms in regulating behavioral despair. Here, we found that CBD enhanced CaMKIIα neuronal excitability in the dorsal dentate gyrus (dDG) and dDGCaMKIIα neurons involved in regulating behavioral despair in CBD mice. Besides, dDGCaMKIIα neurons received 5-HT inputs from median raphe nucleus (MRN) and were mediated by 5-HT1A receptors, whereas MRN5-HT neurons received CaMKIIα inputs from lateral hypothalamic (LH) and were mediated by AMPA receptors to regulate behavioral despair. Furthermore, fluvoxamine exerted its role in resisting behavioral despair through the LH-MRN-dDG circuit. These findings suggest that a previously unidentified circuit of LHCaMKIIα-MRN5-HT-dDGCaMKIIα mediates behavioral despair induced by CBD. Furthermore, these support the important role of AMPA receptors in MRN and 5-HT1A receptors in dDG that might be the potential targets for treatment of behavioral despair, and explain the neural circuit mechanism of fluvoxamine-resistant behavioral despair.

LRRTM2 controls presynapse nano-organization and AMPA receptor sub-positioning through Neurexin-binding interface.

Synapses are organized into nanocolumns that control synaptic transmission efficacy through precise alignment of postsynaptic neurotransmitter receptors and presynaptic release sites. Recent evidence show that Leucine-Rich Repeat Transmembrane protein LRRTM2, highly enriched and confined at synapses, interacts with Neurexins through its C-terminal cap, but the role of this binding interface has not been explored in synapse formation and function. Here, we develop a conditional knock-out mouse model (cKO) to address the molecular mechanisms of LRRTM2 regulation, and its role in synapse organization and function. We show that LRRTM2 cKO specifically impairs excitatory synapse formation and function in mice. Surface expression, synaptic clustering, and membrane dynamics of LRRTM2 are tightly controlled by selective motifs in the C-terminal domain. Conversely, the N-terminal domain controls presynapse nano-organization and postsynapse AMPAR sub-positioning and stabilization through the recently identified Neurexin-binding interface. Thus, we identify LRRTM2 as a central organizer of pre- and post- excitatory synapse nanostructure through interaction with presynaptic Neurexins.

Treadmill Exercise Facilitates Synaptic Plasticity in APP/PS1 Mice by Regulating Hippocampal AMPAR Activity.

Accumulating evidence underscores exercise as a straightforward and cost-effective lifestyle intervention capable of mitigating the risk and slowing the emergence and progression of Alzheimer's disease (AD). However, the intricate cellular and molecular mechanisms mediating these exercise-induced benefits in AD remain elusive. The present study delved into the impact of treadmill exercise on memory retrieval performance, hippocampal synaptic plasticity, synaptic morphology, and the expression and activity of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptors (AMPARs) in 6-month-old APP/PS1 mice. APP/PS1 mice (4-month-old males) were randomly assigned to either a treadmill exercise group or a sedentary group, with C57BL/6J mice (4-month-old males) as the control group (both exercise and sedentary). The exercise regimen spanned 8 weeks. Our findings revealed that 8-week treadmill exercise reversed memory retrieval impairment in step-down fear conditioning in 6-month-old APP/PS1 mice. Additionally, treadmill exercise enhanced basic synaptic strength, short-term potentiation (STP), and long-term potentiation (LTP) of the hippocampus in these mice. Moreover, treadmill exercise correlated with an augmentation in synapse numbers, refinement of synaptic structures, and heightened expression and activity of AMPARs. Our findings suggest that treadmill exercise improves behavioral performance and facilitates synaptic transmission by increasing structural synaptic plasticity and the activity of AMPARs in the hippocampus of 6-month-old APP/PS1 mice, which is involved in pre- and postsynaptic processes.

[AMPA Receptors and Neuronal Plasticity].

Interneuronal information transfer occurs at synapses, where AMPA receptors play a key role. With regard to physiological function, synaptic trafficking of AMPA receptors underlies memory, learning and experience. Analysis of animal models of disease and postmortem brains of patients has revealed that abnormal expression and functions of AMPA receptors may trigger various neuropsychiatric disorders. Such findings are currently being used for the development of therapeutic drugs through quantification of AMPA receptors in patients' brains in real-world practice.

High Impact AMPAkines Induce a Gq-Protein Coupled Endoplasmic Calcium Release in Cortical Neurons: A Possible Mechanism for Explaining the Toxicity of High Impact AMPAkines.

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) positive allosteric modulators (AMPAkines) have a multitude of promising therapeutic properties. The pharmaceutical development of high impact AMPAkines has, however, been limited by the appearance of calcium-dependent neuronal toxicity and convulsions in vivo. Such toxicity is not observed at exceptionally high concentrations of low impact AMPAkines. Because most AMPAR are somewhat impermeable to calcium, the current study sought to examine the extent to which different mechanisms contribute to the rise in intracellular calcium in the presence of high impact ampakines. In the presence of AMPA alone, cytosolic calcium elevation is shown to be sodium-dependent. In the presence of high impact AMPAkines such as cyclothiazide (CTZ) or CX614, however, AMPAR potentiation also activates an additional mechanism that induces calcium release from endoplasmic reticular (ER) stores. The pathway that connects AMPAR to the ER system involves a Gq-protein, phospholipase Cß-mediated inositol triphosphate (InsP3) formation, and ultimately stimulation of InsP3-receptors located on the ER. The same linkage was not observed using high concentrations of the low impact AMPAkines, CX516 (Ampalex), and CX717. We also demonstrate that CX614 produces neuronal hyper-excitability at therapeutic doses, whereas the newer generation low impact AMPAkine CX1739 is safe at exceedingly high doses. Although earlier studies have demonstrated a functional linkage between AMPAR and G-proteins, this report demonstrates that in the presence of high impact AMPAkines, AMPAR also couple to a Gq-protein, which triggers a secondary calcium release from the ER and provides insight into the disparate actions of high and low impact AMPAkines.

Decreased cold-inducible RNA-binding protein (CIRP) binding to GluRl on neuronal membranes mediates memory impairment resulting from prolonged hypobaric hypoxia exposure.

AIM: To investigate the molecular mechanisms underlying memory impairment induced by high-altitude (HA) hypoxia, specifically focusing on the role of cold-inducible RNA-binding protein (CIRP) in regulating the AMPA receptor subunit GluR1 and its potential as a therapeutic target. METHODS: A mouse model was exposed to 14 days of hypobaric hypoxia (HH), simulating conditions at an altitude of 6000 m. Behavioral tests were conducted to evaluate memory function. The expression, distribution, and interaction of CIRP with GluR1 in neuronal cells were analyzed. The binding of CIRP to GluR1 mRNA and its impact on GluR1 protein expression were examined. Additionally, the role of CIRP in GluR1 regulation was assessed using Cirp knockout mice. The efficacy of the Tat-C16 peptide, which consists of the Tat sequence combined with the CIRP 110-125 amino acid sequence, was also tested for its ability to mitigate HH-induced memory decline. RESULTS: CIRP was primarily localized in neurons, with its expression significantly reduced following HH exposure. This reduction was associated with decreased GluR1 protein expression on the cell membrane and increased localization in the cytoplasm. The interaction between CIRP and GluR1 was diminished under HH conditions, leading to reduced GluR1 stability on the cell membrane and increased cytoplasmic relocation. These changes resulted in a decreased number of synapses and dendritic spines, impairing learning and memory functions. Administration of the Tat-C16 peptide effectively ameliorated these impairments by modulating GluR1 expression and distribution in HH-exposed mice. CONCLUSION: CIRP plays a critical role in maintaining synaptic integrity under hypoxic conditions by regulating GluR1 expression and distribution. The Tat-C16 peptide shows promise as a therapeutic strategy for alleviating cognitive decline associated with HA hypoxia.

Evaluation of a Synthetic Retinoid, Ellorarxine, in the NSC-34 Cell Model of Motor Neuron Disease.

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disease worldwide and is characterized by progressive muscle atrophy. There are currently two approved treatments, but they only relieve symptoms briefly and do not cure the disease. The main hindrance to research is the complex cause of ALS, with its pathogenesis not yet fully elucidated. Retinoids (vitamin A derivatives) appear to be essential in neuronal cells and have been implicated in ALS pathogenesis. This study explores 4-[2-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydroquinoxalin-2-yl)ethylnyl]benzoic acid (Ellorarxine, or DC645 or NVG0645), a leading synthetic retinoic acid, discussing its pharmacological mechanisms, neuroprotective properties, and relevance to ALS. The potential therapeutic effect of Ellorarxine was analyzed in vitro using the WT and SOD1G93A NSC-34 cell model of ALS at an administered concentration of 0.3-30 nM. Histological, functional, and biochemical analyses were performed. Elorarxine significantly increased MAP2 expression and neurite length, increased AMPA receptor GluA2 expression and raised intracellular Ca2+ baseline, increased level of excitability, and reduced Ca2+ spike during depolarization in neurites. Ellorarxine also displayed both antioxidant and anti-inflammatory effects. Overall, these results suggest Ellorarxine shows relevance and promise as a novel therapeutic strategy for treatment of ALS.

Genetic mechanisms for impaired synaptic plasticity in schizophrenia revealed by computational modeling.

Schizophrenia phenotypes are suggestive of impaired cortical plasticity in the disease, but the mechanisms of these deficits are unknown. Genomic association studies have implicated a large number of genes that regulate neuromodulation and plasticity, indicating that the plasticity deficits have a genetic origin. Here, we used biochemically detailed computational modeling of postsynaptic plasticity to investigate how schizophrenia-associated genes regulate long-term potentiation (LTP) and depression (LTD). We combined our model with data from postmortem RNA expression studies (CommonMind gene-expression datasets) to assess the consequences of altered expression of plasticity-regulating genes for the amplitude of LTP and LTD. Our results show that the expression alterations observed post mortem, especially those in the anterior cingulate cortex, lead to impaired protein kinase A (PKA)-pathway-mediated LTP in synapses containing GluR1 receptors. We validated these findings using a genotyped electroencephalogram (EEG) dataset where polygenic risk scores for synaptic and ion channel-encoding genes as well as modulation of visual evoked potentials were determined for 286 healthy controls. Our results provide a possible genetic mechanism for plasticity impairments in schizophrenia, which can lead to improved understanding and, ultimately, treatment of the disorder.

Sex-dependent changes in AMPAR expression and Na, K-ATPase activity in the cerebellum and hippocampus of α-Klotho-Hypomorphic mice.

Aging is characterized by a functional decline in several physiological systems. α-Klotho-hypomorphic mice (Kl-/-) exhibit accelerated aging and cognitive decline. We evaluated whether male and female α-Klotho-hypomorphic mice show changes in the expression of synaptic proteins, N-methyl-d-aspartate receptor (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunits, postsynaptic density protein 95 (PSD-95), synaptophysin and synapsin, and the activity of Na+, K+-ATPase (NaK) isoforms in the cerebellum and hippocampus. In this study, we demonstrated that in the cerebellum, Kl-/- male mice have reduced expression of GluA1 (AMPA) compared to wild-type (Kl+/+) males and Kl-/- females. Also, Kl-/- male and female mice show reduced ɑ2/ɑ3-NaK and Mg2+-ATPase activities in the cerebellum, respectively, and sex-based differences in NaK and Mg2+-ATPase activities in both the regions. Our findings suggest that α-Klotho could influence the expression of AMPAR and the activity of NaK isoforms in the cerebellum in a sex-dependent manner, and these changes may contribute, in part, to cognitive decline.

Amitriptyline protects afferent synapses in the cochlea against excitotoxic trauma in vitro.

Afferent synapses between inner hair cells (IHCs) and the type I spiral ganglion neurons (SGNs) in the cochlea provide over 95% of sensory signals for auditory perception in the brain. However, these afferent synapses are particularly vulnerable to damage, for example from excitotoxicity, and exposure to noise in the environment which often leads to noise-induced cochlear synaptopathy (NICS). In this study, we simulated excitotoxic trauma by incubating kainic acid, a non-desensitizing agonist for AMPA type glutamate receptors on cultured cochleae. The possible protective effects of amitriptyline against NICS were examined. We found that, in IHCs, amitriptyline reversed the decrease of Ca2+ current and exocytosis caused by excitotoxic trauma. In SGNs, amitriptyline promoted the recovery of neurite loss caused by excitotoxic trauma. Furthermore, we found that the protective effects of amitriptyline are likely mediated by suppressing apoptosis factors that were upregulated during excitotoxic trauma. In conclusion, our results suggest that amitriptyline could protect afferent synapses in the cochlea from NICS, making it a potential drug candidate for hearing protection.

Slow-wave sleep drives sleep-dependent renormalization of synaptic AMPA receptor levels in the hypothalamus.

According to the synaptic homeostasis hypothesis (SHY), sleep serves to renormalize synaptic connections that have been potentiated during the prior wake phase due to ongoing encoding of information. SHY focuses on glutamatergic synaptic strength and has been supported by numerous studies examining synaptic structure and function in neocortical and hippocampal networks. However, it is unknown whether synaptic down-regulation during sleep occurs in the hypothalamus, i.e., a pivotal center of homeostatic regulation of bodily functions including sleep itself. We show that sleep, in parallel with the synaptic down-regulation in neocortical networks, down-regulates the levels of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) in the hypothalamus of rats. Most robust decreases after sleep were observed at both sites for AMPARs containing the GluA1 subunit. Comparing the effects of selective rapid eye movement (REM) sleep and total sleep deprivation, we moreover provide experimental evidence that slow-wave sleep (SWS) is the driving force of the down-regulation of AMPARs in hypothalamus and neocortex, with no additional contributions of REM sleep or the circadian rhythm. SWS-dependent synaptic down-regulation was not linked to EEG slow-wave activity. However, spindle density during SWS predicted relatively increased GluA1 subunit levels in hypothalamic synapses, which is consistent with the role of spindles in the consolidation of memory. Our findings identify SWS as the main driver of the renormalization of synaptic strength during sleep and suggest that SWS-dependent synaptic renormalization is also implicated in homeostatic control processes in the hypothalamus.

Plasticity-induced actin polymerization in the dendritic shaft regulates intracellular AMPA receptor trafficking.

AMPA-type receptors (AMPARs) are rapidly inserted into synapses undergoing plasticity to increase synaptic transmission, but it is not fully understood if and how AMPAR-containing vesicles are selectively trafficked to these synapses. Here, we developed a strategy to label AMPAR GluA1 subunits expressed from their endogenous loci in cultured rat hippocampal neurons and characterized the motion of GluA1-containing vesicles using single-particle tracking and mathematical modeling. We find that GluA1-containing vesicles are confined and concentrated near sites of stimulation-induced structural plasticity. We show that confinement is mediated by actin polymerization, which hinders the active transport of GluA1-containing vesicles along the length of the dendritic shaft by modulating the rheological properties of the cytoplasm. Actin polymerization also facilitates myosin-mediated transport of GluA1-containing vesicles to exocytic sites. We conclude that neurons utilize F-actin to increase vesicular GluA1 reservoirs and promote exocytosis proximal to the sites of synaptic activity.

Long-term saturated fat-enriched diets impair hippocampal learning and memory processes in a sex-dependent manner.

Consumption of saturated fat-enriched diets during adolescence has been closely associated with the reduction of hippocampal synaptic plasticity and the impairment of cognitive function. Nevertheless, the effect of long-term intake of these foods has not yet been studied. In the present study, we have investigated the effect of a treatment, lasting for 40 weeks, with a diet enriched in saturated fat (SOLF) on i) spatial learning and memory, ii) hippocampal synaptic transmission and plasticity, and iii) hippocampal gene expression levels in aged male and female mice. Our findings reveal that SOLF has a detrimental impact on spatial memory and synaptic plasticity mechanisms, such as long-term potentiation (LTP), and downregulates Gria1 expression specifically in males. In females, SOLF downregulates the gene expression of Gria1/2/3 and Grin1/2A/2B glutamate receptor subunits as well as some proinflammatory interleukins. These findings highlight the importance of considering sex-specific factors when assessing the long-term effects of high-fat diets on cognition and brain plasticity.

Inhibition of α-Amino-3-Hydroxy-5-Methyl-4-Isoxazolepropionic Acid Receptors Ameliorates Atrial Inflammation and Vulnerability to Atrial Fibrillation in Rats with Anxiety Disorders.

ABSTRACT: Previous studies have found that anxiety disorders may increase the incidence of atrial fibrillation (AF). More and more studies have shown that α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are involved in the occurrence and development of cardiovascular diseases. However, the role of AMPARs in AF associated with anxiety disorder remains unclear. The aim of this study was to investigate the effect of AMPARs on AF susceptibility in rats with anxiety disorder and its possible mechanism. The anxiety disorder rat model was established by unpredictable empty bottle stimulation and was treated with AMPARs agonist and antagonist. Our results showed that AMPARs antagonist treatment significantly reduced sympathetic activity, improved heart rate variability, shortened action potential duration, prolonged effective refractory period, reduced AF induction rate, and improved cardiac electrical remodeling and the expression of inflammatory factors. In addition, inhibition of AMPARs reduced the phosphorylation of IκBα and p65. Our experimental results suggest that inhibition of AMPARs can reduce autonomic remodeling, improve atrial electrical remodeling, and suppress myocardial inflammation, which provides a potential therapeutic strategy for the treatment of AF associated with anxiety disorder.

Prefrontal cortex molecular clock modulates development of depression-like phenotype and rapid antidepressant response in mice.

Depression is associated with dysregulated circadian rhythms, but the role of intrinsic clocks in mood-controlling brain regions remains poorly understood. We found increased circadian negative loop and decreased positive clock regulators expression in the medial prefrontal cortex (mPFC) of a mouse model of depression, and a subsequent clock countermodulation by the rapid antidepressant ketamine. Selective Bmal1KO in CaMK2a excitatory neurons revealed that the functional mPFC clock is an essential factor for the development of a depression-like phenotype and ketamine effects. Per2 silencing in mPFC produced antidepressant-like effects, while REV-ERB agonism enhanced the depression-like phenotype and suppressed ketamine action. Pharmacological potentiation of clock positive modulator ROR elicited antidepressant-like effects, upregulating plasticity protein Homer1a, synaptic AMPA receptors expression and plasticity-related slow wave activity specifically in the mPFC. Our data demonstrate a critical role for mPFC molecular clock in regulating depression-like behavior and the therapeutic potential of clock pharmacological manipulations influencing glutamatergic-dependent plasticity.

High-Speed Atomic Force Microscopy Reveals Fluctuations and Dimer Splitting of the N-Terminal Domain of GluA2 Ionotropic Glutamate Receptor-Auxiliary Subunit Complex.

α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid glutamate receptors (AMPARs) enable rapid excitatory synaptic transmission by localizing to the postsynaptic density of glutamatergic spines. AMPARs possess large extracellular N-terminal domains (NTDs), which are crucial for AMPAR clustering at synaptic sites. However, the dynamics of NTDs and the molecular mechanism governing their synaptic clustering remain elusive. Here, we employed high-speed atomic force microscopy (HS-AFM) to directly visualize the conformational dynamics of NTDs in the GluA2 subunit complexed with TARP γ2 in lipid environments. HS-AFM videos of GluA2-γ2 in the resting and activated/open states revealed fluctuations in NTD dimers. Conversely, in the desensitized/closed state, the two NTD dimers adopted a separated conformation with less fluctuation. Notably, we observed individual NTD dimers transitioning into monomers, with extended monomeric states in the activated/open state. Molecular dynamics simulations provided further support, confirming the energetic stability of the monomeric NTD states within lipids. This NTD-dimer splitting resulted in subunit exchange between the receptors and increased the number of interaction sites with synaptic protein neuronal pentraxin 1 (NP1). Moreover, our HS-AFM studies revealed that NP1 forms a ring-shaped octamer through N-terminal disulfide bonds and binds to the tip of the NTD. These findings suggest a molecular mechanism in which NP1, upon forming an octamer, is secreted into the synaptic region and binds to the tip of the GluA2 NTD, thereby bridging and clustering multiple AMPARs. Thus, our findings illuminate the critical role of NTD dynamics in the synaptic clustering of AMPARs and contribute valuable insights into the fundamental processes of synaptic transmission.

Association of medullary reticular formation ventral part with spasticity in mice suffering from photothrombotic stroke.

Strokes cause spasticity via stretch reflex hyperexcitability in the spinal cord, and spastic paralysis due to involuntary muscle contraction in the hands and fingers can severely restrict skilled hand movements. However, the underlying neurological mechanisms remain unknown. Using a mouse model of spasticity after stroke, we demonstrate changes in neuronal activity with and without electrostimulation of the afferent nerve to induce the stretch reflex, measured using quantitative activation-induced manganese-enhanced magnetic resonance imaging. Neuronal activity increased within the ventral medullary reticular formation (MdV) in the contralesional brainstem during the acute post-stroke phase, and this increase was characterised by activation of circuits involved in spasticity. Interestingly, ascending electrostimulation inhibited the MdV activity on the stimulation side in normal conditions. Moreover, immunohistochemical staining showed that, in the acute phase, the density of GluA1, one of the α-amino-3 hydroxy­5 methyl -4 isoxazolepropionic acid receptor (AMPAR) subunits, at the synapses of MdV neurons was significantly increased. In addition, the GluA1/GluA2 ratio in these receptors was altered at 2 weeks post-stroke, confirming homeostatic plasticity as the underlying mechanisms of spasticity. These results provide new insights into the relationship between impaired skilled movements and spasticity at the acute post-stroke phase.

Computational Investigation of BMAA and Its Carbamate Adducts as Potential GluR2 Modulators.

Beta-N-methylamino-l-alanine (BMAA) is a potential neurotoxic nonprotein amino acid, which can reach the human body through the food chain. When BMAA interacts with bicarbonate in the human body, carbamate adducts are produced, which share a high structural similarity with the neurotransmitter glutamate. It is believed that BMAA and its l-carbamate adducts bind in the glutamate binding site of ionotropic glutamate receptor 2 (GluR2). Chronic exposure to BMAA and its adducts could cause neurological illness such as neurodegenerative diseases. However, the mechanism of BMAA action and its carbamate adducts bound to GluR2 has not yet been elucidated. Here, we investigate the binding modes and the affinity of BMAA and its carbamate adducts to GluR2 in comparison to the natural agonist, glutamate, to understand whether these can act as GluR2 modulators. Initially, we perform molecular dynamics simulations of BMAA and its carbamate adducts bound to GluR2 to examine the stability of the ligands in the S1/S2 ligand-binding core of the receptor. In addition, we utilize alchemical free energy calculations to compute the difference in the free energy of binding of the beta-carbamate adduct of BMAA to GluR2 compared to that of glutamate. Our findings indicate that carbamate adducts of BMAA and glutamate remain stable in the binding site of the GluR2 compared to BMAA. Additionally, alchemical free energy results reveal that glutamate and the beta-carbamate adduct of BMAA have comparable binding affinity to the GluR2. These results provide a rationale that BMAA carbamate adducts may be, in fact, the modulators of GluR2 and not BMAA itself.