The Role of the GluR2 Subunit in AMPA Receptor Function and Synaptic Plasticity

p53 inhibition during audiogenic kindling in Krushinsky-Molodkina rats attenuates seizure severity and prevents neurodegeneration in the hippocampus

In the present study, we analyzed the effects of the p53 inhibitor pifithrin-α (PFT) on the expression of brainstem audiogenic seizures (AGS) and limbic seizures in Krushinsky-Molodkina (KM) rats genetically prone to AGS. To reproduce limbic/mesial temporal lobe epilepsy (TLE)-like condition in KM rats, we used repetitive AGS stimulations (audiogenic kindling) during 14 days. In parallel with AGS stimulations, KM rats received daily intraperitoneal injections of PFT. Our data demonstrated that PFT treatment significantly decreased the duration and severity of both brainstem AGS and limbic seizures. In addition, PFT partially prevented the kindling-induced neurodegeneration and activation of apoptotic mechanisms in the hippocampus of KM rats. Moreover, PFT treatment led to the persistent upregulation of anti-apoptotic Bcl-2, along with GluA2 and GluN2A, glutamate receptor subunits which are involved into the mechanisms supporting cell survival and preventing neuronal hyperexcitability. Altogether, our data confirm that p53 can be considered as a perspective target for the development of novel strategies to mitigate seizure activity and avert its deleterious consequences.

5-Aminolevulinic acid improves spatial recognition memory in mice

The production of 5-aminolevulinic acid (5-ALA) is the rate-limiting step in heme biosynthesis, and is thus essential for maintaining cellular respiration and the activities of various heme-containing enzymes. Recently, it was reported that exogenous 5-ALA can alleviate cognitive impairments in animal models. To elucidate the contributions of 5-ALA to cognition and investigate the underlying molecular mechanisms, we examined the impact of 5-ALA administration on both novel objective recognition (NOR) and spatial recognition memories in male ddY mice and on long-term potentiation (LTP) in hippocampal slices isolated from these mice. Both intracerebroventricular and oral administration of 5-ALA enhanced object recognition memory as evidenced by increased time spent investigating a novel object compared to a familiar object in the NOR test. Further, oral administration of 5-ALA improved the spontaneous alternation performance in the Y-maze test. Administration of 5-ALA also increased the glutamate/GABA ratio in dorsal hippocampus, ventral hippocampus, and entorhinal cortex, brain regions essential for recognition memory. Further, direct 5-ALA administration increased the LTP of excitatory postsynaptic potentials in hippocampal slices induced by theta-burst stimulation (TBS), and this LTP enhancement was completely mitigated by pretreatment with 1-naphthyl acetyl spermine, an antagonist of Ca2+-permeable AMPA receptors lacking the GluR2 subunit (CP-AMPARs). We suggest that 5-ALA improves spatial recognition memory by enhancing the TBS-induced expression or activity of postsynaptic CP-AMPARs, resulting in greater and longer-lasting LTP. Endogenous 5-ALA appears critical for maintaining cognitive function in the mammalian central nervous system, while exogenous supplementation could be a useful strategy for the treatment of cognitive dysfunction.

Olfactory dysfunction and altered cortical excitability in the mouse model of Fragile X Syndrome

Fragile X Syndrome (FXS) is the most common monogenetic cause of autism and inherited intellectual disability. A key feature of FXS symptomatology is altered sensory processing greatly affecting FXS individual's life quality. Here, we use a combination of behavioral tests and slice physiology tools to study the neurophysiological alterations underlying aberrant sensory processing in the olfactory system of the FXS mouse model (Fmr1 KO). We focused on the piriform cortex (PC), since it is in this brain region where olfactory information is integrated and ultimately decoded. Using a go-no go behavioral task we have found that Fmr1 KO learn to discriminate between a rewarded and a not rewarded odorant but cannot distinguish complex odor mixtures, akin to what is found in the environment. Moreover, Fmr1 KO long-term memory is impaired compared to control mice suggesting possibly cortical processing alterations. In addition, electrophysiological data from PC layer II neurons of Fmr1 KO mice showed a hyperexcitable phenotype manifested by differences in active membrane properties and altered network connectivity. Taken together, our data suggest a possible causal link between the observed olfactory discrimination deficiencies in the Fmr1 KO mouse and the altered physiology of PC.

Disuse plasticity limits spinal cord injury recovery

Use-dependent plasticity after spinal cord injury (SCI) enhances neuromotor function, however, the optimal timing to initiate rehabilitation remains controversial. To test impacts of early disuse, we established a rodent model of transient hindlimb suspension in acute phase SCI. Early disuse in the first 2-week after SCI undermined recovery on open-field locomotion, kinematics, and swim tests even after 6-week of normal gravity reloading. Early disuse produced chronic spinal circuit hyper-excitability in H-reflex and interlimb reflex tests. Quantitative synaptoneurosome analysis of lumboventral spinal cords revealed shifts in AMPA receptor (AMPAR) subunit GluA1 localization and serine 881 phosphorylation, reflecting enduring synaptic memories of early disuse stored in the spinal cord. Automated confocal analysis of motoneurons revealed persistent shifts toward GluA2-lacking, calcium-permeable AMPARs in disuse subjects. Unsupervised machine learning associated multidimensional synaptic changes with persistent recovery deficits in SCI. The results argue for early aggressive rehabilitation to prevent disuse plasticity that limits SCI recovery.

GluR2 overexpression in ACC glutamatergic neurons alleviates cancer-induced bone pain in rats

BACKGROUND

Cancer-induced bone pain (CIBP) is a complex chronic pain with poorly understood mechanisms. The anterior cingulate cortex (ACC) plays a critical role in processing and modulating chronic pain. This study investigates how the GluR2 receptors (calcium impermeable AMPA receptors) in ACC glutamatergic neurons regulate CIBP.

METHODS

The CIBP models were established by injecting Walker 256 cells into the tibia of SD rats. Paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) were used as indicators of hyperalgesia. The immunofluorescence staining was employed to detect the expression of c-Fos in ACC and identify the subtypes of co-labeled c-Fos+ neurons. Real-time monitoring of calcium activity in ACC glutamatergic neurons was achieved through the fiber photometry. The excitability of glutamatergic neurons in ACC was modulated using chemicalgenetics and optogenetics techniques. The expression of GluR2 at the mRNA and protein level in ACC were assessed using RT-qPCR and Western blotting.

RESULTS

There were significant reductions in PWT and PWL of CIBP rats after Walker 256 cell injection. The ACC of CIBP rats showed increased c-Fos expression compared to sham rats, with mainly activated c-Fos co-localized with glutamatergic neurons. Optogenetic or chemogenetic activation of ACC glutamatergic neurons led to increased hyperalgesia in sham rats, while suppression of their activity alleviated hyperalgesia in CIBP rats. Calcium activity in ACC glutamatergic neurons of CIBP rats was increased with suprathreshold stimulation of von Frey filament. Notably, surface GluR2 protein and mRNA were reduced in ACC of CIBP rats. Furthermore, overexpression of GluR2 by AAV-CaMKII-GluR2 injection was decreased c-Fos expression in ACC and alleviated hyperalgesia in CIBP rats.

CONCLUSIONS

These findings suggest that decreased surface GluR2 receptors in ACC glutamatergic neurons contribute to calcium activity and excessive excitability, thereby inducing CIBP in rats. Conversely, GluR2 overexpression in ACC glutamatergic neurons alleviates CIBP in rats. This study provides a new potential therapeutic approach for targeting the GluR2 receptor to alleviate CIBP for cancer patients.

Incubation of oxycodone craving is associated with CP-AMPAR upregulation in D1 and D2 receptor-expressing medium spiny neurons in nucleus accumbens core and shell

A major problem in treating opioid use disorder is persistence of craving after protracted abstinence. This has been modeled in rodents using the incubation of craving model, in which cue-induced drug seeking increases over the first weeks of abstinence from drug self-administration and then remains high for an extended period. Incubation has been reported for several opioids, including oxycodone, but little is known about underlying synaptic plasticity. In contrast, it is well established that incubation of cocaine and methamphetamine craving depends on strengthening of glutamate synapses in the nucleus accumbens (NAc) through incorporation of calcium-permeable AMPARs (CP-AMPARs). CP-AMPARs have higher conductance than the calcium-impermeable AMPARs that mediate NAc excitatory transmission in drug-naïve animals, as well as other distinct properties. Here we examined AMPAR transmission in medium spiny neurons (MSN) of NAc core and shell subregions in rats during forced abstinence from extended-access oxycodone self-administration. In early abstinence (prior to incubation), CP-AMPAR levels were low. After 17-33 days of abstinence (when incubation is stably plateaued), CP-AMPAR levels were significantly elevated in both subregions. These results explain the prior demonstration that infusion of a selective CP-AMPAR antagonist into NAc core or shell subregions prevents expression of oxycodone incubation. Then, using transgenic rats, we found CP-AMPAR upregulation on both D1 and D2 receptor-expressing MSN, which contrasts with selective upregulation on D1 MSN after cocaine and methamphetamine incubation. Overall, our results demonstrate a common role for CP-AMPAR upregulation in psychostimulant and oxycodone incubation, albeit with differences in MSN subtype-specificity.

GluA2-containing AMPA receptors form a continuum of Ca2+-permeable channels

Fast excitatory neurotransmission in the mammalian brain is mediated by cation-selective AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors (AMPARs)1. AMPARs are critical for the learning and memory mechanisms of Hebbian plasticity2 and glutamatergic synapse homeostasis3, with recent work establishing that AMPAR missense mutations can cause autism and intellectual disability4-7. AMPARs have been grouped into two functionally distinct tetrameric assemblies based on the inclusion or exclusion of the GluA2 subunit that determines Ca2+ permeability through RNA editing8,9. GluA2-containing AMPARs are the most abundant in the central nervous system and considered to be Ca2+ impermeable10. Here we show this is not the case. Contrary to conventional understanding, GluA2-containing AMPARs form a continuum of polyamine-insensitive ion channels with varying degrees of Ca2+ permeability. Their ability to transport Ca2+ is shaped by the subunit composition of AMPAR tetramers as well as the spatial orientation of transmembrane AMPAR regulatory proteins and cornichon auxiliary subunits. Ca2+ crosses the ion-conduction pathway by docking to an extracellular binding site that helps funnel divalent ions into the pore selectivity filter. The dynamic range in Ca2+ permeability, however, arises because auxiliary subunits primarily modify the selectivity filter. Taken together, our work proposes a broader role for AMPARs in Ca2+ signalling in the mammalian brain and offers mechanistic insight into the pathogenic nature of missense mutations.

Astroglial TNFR2 signaling regulates hippocampal synaptic function and plasticity in a sex dependent manner

Astrocytes participate in synaptic transmission and plasticity through tightly regulated, bidirectional communication with pre- and post-synaptic neurons, as well as microglia and oligodendrocytes. A key component of astrocyte-mediated synaptic regulation is the cytokine tumor necrosis factor (TNF). TNF signals via two cognate receptors, TNFR1 and TNFR2, both expressed in astrocytes. While TNFR1 signaling in astrocytes has been long demonstrated to be necessary for physiological synaptic function, the role of astroglial TNFR2 has never been explored. Here, we demonstrate that astroglial TNFR2 is essential for maintaining hippocampal synaptic function and plasticity in physiological conditions. Indeed, Gfap creERT2 :Tnfrsf1b fl/fl mice with selective ablation of TNFR2 in astrocytes exhibited dysregulated expression of neuronal and glial proteins (e.g., SNARE complex molecules, glutamate receptor subunits, glutamate transporters) essential for hippocampal synaptic transmission and plasticity. Hippocampal astrocytes sorted from Gfap creERT2 :Tnfrsf1b fl/fl mice displayed downregulation of genes and pathways implicated in synaptic plasticity, as well as astrocyte-neuron and astrocyte-oligodendrocyte communication. These alterations were accompanied by increased glial reactivity and impaired astrocyte calcium dynamics, and ultimately translated into functional deficits, specifically impaired long-term potentiation (LTP) and cognitive functions. Notably, male Gfap creERT2 :Tnfrsf1b fl/fl mice exhibited more pronounced hippocampal synaptic and cellular alterations, suggesting sex-dependent differences in astroglial TNFR2 regulation of synaptic function. Together, these findings indicate that TNFR2 signaling in astrocytes is essential for proper astrocyte-neuron communication at the basis of synaptic function, and that this is regulated in a sex-dependent manner.

AMPA Receptor Modulation in the Treatment of High-Grade Glioma: Translating Good Science into Better Outcomes

Glioblastoma (GB) treatment, despite consisting of surgical resection paired with radiation, temozolomide chemotherapy and tumor-treating fields, yields a median survival of 15-20 months. One of the more recently appreciated hallmarks of GB aggressiveness is the co-opting of neurotransmitter signaling mechanisms that normally sustain excitatory synaptic communication in the CNS. AMPA-glutamate receptor (AMPAR) signaling governs the majority of excitatory synaptic activity in the mammalian brain. AMPAR activation in glioma cells activates cellular pathways that enhance proliferation and invasion and confer resistance to approved GB therapeutics. In addition, this review places a specific emphasis on discussing the redefined GB cytoarchitecture that consists of neuron-to-glioma cell synapses, whose oncogenic activity is driven by AMPAR activation on glioma cells, and the discovery of tumor microtubes, which propagate calcium signals throughout the tumor network in order to enhance resistance to complete surgical resection and radiotherapy. These new discoveries notwithstanding, some evidence suggests that AMPAR activation can produce excitotoxicity in tumor cells. This disparity warrants a closer examination at how AMPAR modulation can be leveraged to produce more durable outcomes in the treatment of GB and tumors in peripheral organs that express AMPAR.

MiR-186-5p inhibition restores synaptic transmission and neuronal network activity in a model of chronic stress

Chronic stress exerts profound negative effects on cognitive and emotional behaviours and is a major risk factor for the development of neuropsychiatric disorders. However, the molecular links between chronic stress and its deleterious effects on neuronal and synaptic function remain elusive. Here, using a combination of in vitro and in vivo approaches, we demonstrate that the upregulation of miR-186-5p triggered by chronic stress may be a key mediator of such changes, leading to synaptic dysfunction. Our results show that the expression levels of miR-186-5p are increased both in the prefrontal cortex (PFC) of mice exposed to chronic stress and in cortical neurons chronically exposed to dexamethasone. Additionally, viral overexpression of miR-186-5p in the PFC of naïve mice induces anxiety- and depressive-like behaviours. The upregulation of miR-186-5p through prolonged glucocorticoid receptor activation in vitro, or in a mouse model of chronic stress, differentially affects glutamatergic and GABAergic synaptic transmission, causing an imbalance in excitation/inhibition that leads to altered neuronal network activity. At glutamatergic synapses, we observed both a reduction in synaptic AMPARs and synaptic transmission, whereas GABAergic synaptic transmission was strengthened. These changes could be rescued in vitro by a miR-186-5p inhibitor. Overall, our results establish a novel molecular link between chronic glucocorticoid receptor activation, the upregulation of miR-186-5p and the synaptic changes induced by chronic stress, that may be amenable to therapeutic intervention.

Epac2-mediated synaptic insertion of Ca2+-permeable AMPARs in the nucleus accumbens contributes to incubation of cocaine craving

The accumulation of GluA2-lacking Ca2+-permeable AMPARs (CP-AMPARs) in the medium spiny neurons (MSNs) of the nucleus accumbens (NAc) is required for the expression of incubation of cocaine craving. The exchange protein directly activated by cAMP (Epac) is an intracellular effector of cAMP and a guanine nucleotide exchange factor for the small GTPase Rap1. Epac2 has been implicated in the trafficking of AMPA receptors at central synapses. We tested the hypothesis that Epac2 activation contributes to the accumulation of CP-AMPARs in NAc MSNs and incubation of cocaine craving. Here we demonstrate that the selective Epac2 agonist S-220 facilitated the synaptic insertion of GluA2-lacking CP-AMPARs at excitatory synapses onto NAc MSNs. In addition, prolonged abstinence from cocaine self-administration in rats resulted in elevated Rap1-GTP levels in the NAc, implying that Epac2 is activated during incubation. Importantly, we show that AAV-mediated shRNA knockdown of Epac2 in the NAc core attenuated the accumulation of CP-AMPARs and cue-induced drug-seeking behavior after prolonged abstinence from cocaine self-administration. In contrast, acute pharmacological inhibition of Epac2 with the selective Epac2 inhibitor ESI-05 did not alter CP-AMPARs that had already accumulated during incubation, and intra-NAc application of ESI-05 did not significantly affect cue-induced drug seeking following prolonged abstinence. Taken together, these results suggest that Epac2 activation during the period of incubation, but not during cue-induced drug seeking, leads to the accumulation of CP-AMPARs in NAc MSNs, which in turn contributes to incubation of cocaine craving.

AMPA Receptors in Synaptic Plasticity, Memory Function, and Brain Diseases

Tetrameric AMPA-type ionotropic glutamate receptors are primary transducers of fast excitatory synaptic transmission in the central nervous system, and their properties and abundance at the synaptic surface are crucial determinants of synaptic efficacy in neuronal communication across the brain. The induction of long-term potentiation (LTP) leads to the insertion of GluA1-containing AMPA receptors at the synaptic surface, whereas during long-term depression (LTD), these receptors are internalized into the cytoplasm of the spine. Disruptions in the trafficking of AMPA receptors to and from the synaptic surface attenuate both forms of synaptic plasticity. Homeostatic scaling up and scaling down, which are additional types of plasticity similar to LTP and LTD, are also regulated by the insertion and removal of GluA1-containing AMPA receptors from the synaptic surface. The trafficking of AMPA receptors is an intricate process assisted by various proteins. Furthermore, AMPA receptors are critical for the formation and consolidation of various types of memory, and alterations in their function are intimately associated with cognitive dysfunction in aging and several neurological and psychiatric diseases. In this review, we will provide an overview of the current understanding of how AMPA receptors regulate various forms of synaptic plasticity, their contribution to memory functions, and their role in aging and brain diseases.

Data-driven synapse classification reveals a logic of glutamate receptor diversity

The rich diversity of synapses facilitates the capacity of neural circuits to transmit, process and store information. We used multiplex super-resolution proteometric imaging through array tomography to define features of single synapses in mouse neocortex. We find that glutamatergic synapses cluster into subclasses that parallel the distinct biochemical and functional categories of receptor subunits: GluA1/4, GluA2/3 and GluN1/GluN2B. Two of these subclasses align with physiological expectations based on synaptic plasticity: large AMPAR-rich synapses may represent potentiated synapses, whereas small NMDAR-rich synapses suggest "silent" synapses. The NMDA receptor content of large synapses correlates with spine neck diameter, and thus the potential for coupling to the parent dendrite. Overall, ultrastructural features predict receptor content of synapses better than parent neuron identity does, suggesting synapse subclasses act as fundamental elements of neuronal circuits. No barriers prevent future generalization of this approach to other species, or to study of human disorders and therapeutics.

Central treatment of neuropeptide-S attenuates cognitive dysfunction and hippocampal synaptic plasticity impairment by increasing CaMKII/GluR1 in hemiparkinsonian rats

Neuropeptide-S (NPS) has been demonstrated to mitigate learning and memory deficits in experimental models of Parkinson's Disease (PD). Despite this, the precise mechanisms through which NPS exerts its influence on cognitive functions remain to be fully unknown. This study aims to elucidate the effects of central administration of NPS on learning and memory deficits associated with an experimental rat hemiparkinsonian model, examining both electrophysiological and molecular parameters. The hemiparkinsonian model was established via stereotactic injection of 6-hydroxydopamine (6-OHDA) into the right medial forebrain bundle. Central NPS (1 nmol, icv) was administered into the lateral ventricle via a cannula for seven consecutive days following the 6-OHDA lesion. The Morris water maze and object recognition tests were used to evaluate the rat's learning and memory abilities. Long-term potentiation (LTP) recordings were conducted to assess hippocampal synaptic plasticity. Immunohistochemistry was employed to determine the expression levels of phosphorylated CaMKII (pCaMKII), GluR1, and GluR2 in the hippocampus. The 6-OHDA-induced decline in cognitive performance was significantly (p < 0.05) improved in rats that received central NPS. In 6-OHDA-lesioned rats, NPS treatment significantly (p < 0.05) enhanced the amplitude of LTP at the dentate gyrus/perforant path synapses. Furthermore, NPS significantly (p < 0.05) increased the number of pCaMKII and GluR1 immunoreactive cells in the hippocampus, which had been diminished due to 6-OHDA, except for GluR2 levels. These findings provide insight into the mechanisms by which central NPS administration enhances cognitive functions in an experimental model of PD, highlighting its potential therapeutic benefits for addressing cognitive deficits in PD.

Targeting Neuroplasticity in Substance Use Disorders: Implications for Therapeutics

The last two decades have witnessed substantial advances in identifying synaptic plasticity responsible for behavioral changes in animal models of substance use disorder. We have learned the most about cocaine-induced plasticity in the nucleus accumbens and its relationship to cocaine seeking, so that is the focus in this review. Synaptic plasticity pointing to potential therapeutic targets has been identified mainly using two drug self-administration models: extinction-reinstatement and abstinence models. A relationship between cocaine seeking and potentiated AMPAR transmission in nucleus accumbens is indicated by both models. In particular, an atypical subpopulation-Ca2+-permeable or CP-AMPARs-mediates cue-induced seeking that persists even after long periods of abstinence, modeling the persistent vulnerability to relapse that represents a major challenge in treating substance use disorder. We review strategies to reverse CP-AMPAR plasticity; strategies targeting other components of excitatory synapses, including dysregulated glutamate uptake and release; and behavioral interventions that can be augmented by harnessing synaptic plasticity.

Impaired hippocampal plasticity associated with loss of recycling endosomal SLC9A6/NHE6 is ameliorated by the TrkB agonist 7,8-dihydroxyflavone

Proper maintenance of intracellular vesicular pH is essential for cargo trafficking during synaptic function and plasticity. Mutations in the SLC9A6 gene encoding the recycling endosomal pH regulator (Na+, K+)/H+ exchanger isoform 6 (NHE6) are causal for Christianson syndrome (CS), a severe form of X-linked intellectual disability. NHE6 expression is also downregulated in other neurodevelopmental and neurodegenerative disorders, such as autism spectrum disorder and Alzheimer's disease, suggesting its dysfunction could contribute more broadly to the pathophysiology of other neurological conditions. To understand how ablation of NHE6 function leads to severe learning impairments, we assessed synaptic structure, function, and cellular mechanisms of learning in a novel line of Nhe6 knockout (KO) mice expressing a plasma membrane-tethered green fluorescent protein within hippocampal neurons. We uncovered significant reductions in dendritic spines density, AMPA receptor (AMPAR) expression, and AMPAR-mediated neurotransmission in CA1 pyramidal neurons. The neurons also failed to undergo functional and structural enhancement during long-term potentiation (LTP). Significantly, the selective TrkB agonist 7,8-dihydroxyflavone restored spine density as well as functional and structural LTP in KO neurons. TrkB activation thus may act as a potential clinical intervention to ameliorate cognitive deficits in CS and other neurodegenerative disorders.

Regulation of Dendrite and Dendritic Spine Formation by TCF20

Mutations in the Transcription Factor 20 (TCF20) have been identified in patients with autism spectrum disorders (ASDs), intellectual disabilities (IDs), and other neurological issues. Recently, a new syndrome called TCF20-associated neurodevelopmental disorders (TAND) has been described, with specific clinical features. While TCF20's role in the neurogenesis of mouse embryos has been reported, little is known about its molecular function in neurons. In this study, we demonstrate that TCF20 is expressed in all analyzed brain regions in mice, and its expression increases during brain development but decreases in muscle tissue. Our findings suggest that TCF20 plays a central role in dendritic arborization and dendritic spine formation processes. RNA sequencing analysis revealed a downregulation of pre- and postsynaptic pathways in TCF20 knockdown neurons. We also found decreased levels of GABRA1, BDNF, PSD-95, and c-Fos in total homogenates and in synaptosomal preparations of knockdown TCF20 rat cortical cultures. Furthermore, synaptosomal preparations of knockdown TCF20 rat cortical cultures showed significant downregulation of GluN2B and GABRA5, while GluA2 was significantly upregulated. Overall, our data suggest that TCF20 plays an essential role in neuronal development and function by modulating the expression of proteins involved in dendrite and synapse formation and function.

Post-Critical Period Transcriptional and Physiological Adaptations of Thalamocortical Connections after Sensory Loss

Unilateral whisker denervation activates plasticity mechanisms and circuit adaptations in adults. Single nucleus RNA sequencing and multiplex fluorescence in situ hybridization revealed differentially expressed genes related to altered glutamate receptor distributions and synaptogenesis in thalamocortical (TC) recipient layer 4 (L4) neurons of the sensory cortex, specifically those receiving input from the intact whiskers after whisker denervation. Electrophysiology detected increased spontaneous excitatory events at L4 neurons, confirming an increase in synaptic connections. Elevated expression levels of Gria2 mRNA and functional GluA2 subunit of AMPA receptors at the TC synapse indicate the presence of stabilized and potentiated TC synapses to L4 excitatory neurons along the intact pathway after unilateral whisker denervation. These adaptations likely underlie the increased cortical activity observed in rodents during intact whisker sensation after unilateral whisker denervation. Our findings provide new insights into the mechanisms by which the adult brain supports recovery after unilateral sensory loss.

Exploring the Landscape of Pre- and Post-Synaptic Pediatric Disorders with Epilepsy: A Narrative Review on Molecular Mechanisms Involved

Neurodevelopmental disorders (NDDs) are a group of conditions affecting brain development, with variable degrees of severity and heterogeneous clinical features. They include intellectual disability (ID), autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), often coexisting with epilepsy, extra-neurological comorbidities, and multisystemic involvement. In recent years, next-generation sequencing (NGS) technologies allowed the identification of several gene pathogenic variants etiologically related to these disorders in a large cohort of affected children. These genes encode proteins involved in synaptic homeostasis, such as SNARE proteins, implicated in calcium-triggered pre-synaptic release of neurotransmitters, or channel subunit proteins, such as post-synaptic ionotropic glutamate receptors involved in the brain's fast excitatory neurotransmission. In this narrative review, we dissected emerged molecular mechanisms related to NDDs and epilepsy due to defects in pre- and post-synaptic transmission. We focused on the most recently discovered SNAREopathies and AMPA-related synaptopathies.

Calcium-permeable AMPA receptors govern PV neuron feature selectivity

The brain helps us survive by forming internal representations of the external world1,2. Excitatory cortical neurons are often precisely tuned to specific external stimuli3,4. However, inhibitory neurons, such as parvalbumin-positive (PV) interneurons, are generally less selective5. PV interneurons differ from excitatory neurons in their neurotransmitter receptor subtypes, including AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors (AMPARs)6,7. Excitatory neurons express calcium-impermeable AMPARs that contain the GluA2 subunit (encoded by GRIA2), whereas PV interneurons express receptors that lack the GluA2 subunit and are calcium-permeable (CP-AMPARs). Here we demonstrate a causal relationship between CP-AMPAR expression and the low feature selectivity of PV interneurons. We find low expression stoichiometry of GRIA2 mRNA relative to other subunits in PV interneurons that is conserved across ferrets, rodents, marmosets and humans, and causes abundant CP-AMPAR expression. Replacing CP-AMPARs in PV interneurons with calcium-impermeable AMPARs increased their orientation selectivity in the visual cortex. Manipulations to induce sparse CP-AMPAR expression demonstrated that this increase was cell-autonomous and could occur with changes beyond development. Notably, excitatory-PV interneuron connectivity rates and unitary synaptic strength were unaltered by CP-AMPAR removal, which suggested that the selectivity of PV interneurons can be altered without markedly changing connectivity. In Gria2-knockout mice, in which all AMPARs are calcium-permeable, excitatory neurons showed significantly degraded orientation selectivity, which suggested that CP-AMPARs are sufficient to drive lower selectivity regardless of cell type. Moreover, hippocampal PV interneurons, which usually exhibit low spatial tuning, became more spatially selective after removing CP-AMPARs, which indicated that CP-AMPARs suppress the feature selectivity of PV interneurons independent of modality. These results reveal a new role of CP-AMPARs in maintaining low-selectivity sensory representation in PV interneurons and implicate a conserved molecular mechanism that distinguishes this cell type in the neocortex.

Chronic Sleep Deprivation Altered the Expression of Memory-Related Genes and Caused Cognitive Memory Dysfunction in Mice

Lack of sleep, whether acute or chronic, is quite common and negatively affects an individual's memory and cognitive function. The question of whether chronic sleep deprivation (CSD) causes cognitive impairment to arise and progress is not well studied. To investigate the effects of CSD on memory and cognition, this study began by establishing a CSD mouse model. Behavioral experiments on animals revealed that CSD induced cognitive behavioral abnormalities reminiscent of Alzheimer's disease. Western blot experiments further demonstrated a considerable increase in amyloid-β (Aβ) expression in the mouse brain following CSD. Meanwhile, the hub gene Prkcg was searched for in the cerebellum using RNA-seq and bioinformatics analysis. PKCγ (Prkcg) expression was significantly reduced, as demonstrated by RT-qPCR and Western blot validations. Additionally, CSD was associated with downregulated CREB expression, decreased expression of the endothelin-converting enzyme (ECE1), and increased phosphorylation of ERK1/2 downstream of PKCγ. These findings suggested that CSD down-regulated PKCγ expression, decreased ECE1 expression, impaired Aβ degradation, and affected the PKCγ/ERK/CREB pathway and the synthesis of memory-related proteins. Overall, this study highlighted how CSD modulated PKCγ-related metabolism, impacting Aβ clearance and the production of memory-related proteins. Such insights are crucial for understanding and preventing sporadic Alzheimer's disease (sAD) associated with CSD.

Aging of Krushinsky-Molodkina audiogenic rats is accompanied with pronounced neurodegeneration and dysfunction of the glutamatergic system in the hippocampus

Advancing age strongly correlates with an increased risk of epilepsy development. On the other hand, epilepsy may exacerbate the negative effects of aging making it pathological. In turn, the possible link between aging and epileptogenesis is dysregulation of glutamatergic transmission. In the present study, we analyzed the functional state of the glutamatergic system in the hippocampus of aging (18-month-old) Krushinsky-Molodkina (KM) audiogenic rats to disclose alterations associated with aging on the background of inherited predisposition to audiogenic seizures (AGS). Naïve KM rats with no AGS experience were recruited in the experiments. Wistar rats of the corresponding age were used as a control. First of all, aging KM rats demonstrated a significant decrease in cell population and synaptopodin expression in the hippocampus indicating enhanced loss of cells and synapses. Meanwhile, elevated phosphorylation of ERK1/2 and CREB and increased glutamate in the neuronal perikarya were revealed indicating increased activity of the rest hippocampal cells and increased glutamate production. However, glutamate in the fibers and synapses was mainly unchanged, and the proteins regulating glutamate exocytosis showed variable changes which could compensate each other and maintain glutamate release at the unchanged level. In addition, we revealed downregulation of NMDA-receptor subunit GluN2B and upregulation of AMPA-receptor GluA2 subunit, which could also prevent overexcitation and support cell survival in the hippocampus of aging KM rats. Nevertheless, abnormally high glutamate production, observed in aging KM rats, may provide the basis for hyperexcitability of the hippocampus and increased seizure susceptibility in old age.

The distribution of neurotransmitters in the brain circuitry: Mesolimbic pathway and addiction

Understanding the central nervous system (CNS) circuitry and its different neurotransmitters that underlie reward is essential to improve treatment for many common health issues, such as addiction. Here, we concentrate on understanding how the mesolimbic circuitry and neurotransmitters are organized and function, and how drug exposure affects synaptic and structural changes in this circuitry. While the role of some reward circuits, like the cerebral dopamine (DA)/glutamate (Glu)/gamma aminobutyric acid (GABA)ergic pathways, in drug reward, is well known, new research using molecular-based methods has shown functional alterations throughout the reward circuitry that contribute to various aspects of addiction, including craving and relapse. A new understanding of the fundamental connections between brain regions as well as the molecular alterations within these particular microcircuits, such as neurotrophic factor and molecular signaling or distinct receptor function, that underlie synaptic and structural plasticity evoked by drugs of abuse has been made possible by the ability to observe and manipulate neuronal activity within specific cell types and circuits. It is exciting that these discoveries from preclinical animal research are now being applied in the clinic, where therapies for human drug dependence, such as deep brain stimulation and transcranial magnetic stimulation, are being tested. Therefore, this chapter seeks to summarize the current understanding of the important brain regions (especially, mesolimbic circuitry) and neurotransmitters implicated in drug-related behaviors and the molecular mechanisms that contribute to altered connectivity between these areas, with the postulation that increased knowledge of the plasticity within the drug reward circuit will lead to new and improved treatments for addiction.

Retinoic acid-mediated homeostatic plasticity in the nucleus accumbens core contributes to incubation of cocaine craving

RATIONALE

Incubation of cocaine craving refers to the progressive intensification of cue-induced craving during abstinence from cocaine self-administration. We showed previously that homomeric GluA1 Ca2+-permeable AMPARs (CP-AMPAR) accumulate in excitatory synapses of nucleus accumbens core (NAcc) medium spiny neurons (MSN) after ∼1 month of abstinence and thereafter their activation is required for expression of incubation. Therefore, it is important to understand mechanisms underlying CP-AMPAR plasticity.

OBJECTIVES

We hypothesize that CP-AMPAR upregulation represents a retinoic acid (RA)-dependent form of homeostatic plasticity, previously described in other brain regions, in which a reduction in neuronal activity disinhibits RA synthesis, leading to GluA1 translation and CP-AMPAR synaptic insertion. We tested this using viral vectors to bidirectionally manipulate RA signaling in NAcc during abstinence following extended-access cocaine self-administration.

RESULTS

We used shRNA targeted to the RA degradative enzyme Cyp26b1 to increase RA signaling. This treatment accelerated incubation; rats expressed incubation on abstinence day (AD) 15, when it is not yet detected in control rats. It also accelerated CP-AMPAR synaptic insertion measured with slice physiology. CP-AMPARs were detected in Cyp26b1 shRNA-expressing MSN, but not control MSN, on AD15-18. Next, we used shRNA targeted to the major RA synthetic enzyme Aldh1a1 to reduce RA signaling. In MSN expressing Aldh1a1 shRNA, synaptic CP-AMPARs were reduced in late withdrawal (AD42-60) compared to controls. However, we did not detect an effect of this manipulation on incubated cocaine seeking (AD40).

CONCLUSIONS

These findings support the hypothesis that increased RA signaling during abstinence contributes to CP-AMPAR accumulation and incubation of cocaine craving.

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.

Preclinical characterization of a water-soluble low-impact ampakine prodrug, CX1942 and its active moiety, CX1763

Aim: AMPA-glutamate receptor (AMPAR) dysfunction mediates multiple neurological/neuropsychiatric disorders. Ampakines bind AMPARs and allosterically enhance glutamate-elicited currents. This report describes the activity of the water-soluble ampakine CX1942 prodrug and the active moiety CX1763.Results: CX1763 and CX1942 enhance synaptic transmission in hippocampi of rats. CX1763 increases attention in the 5CSRTT in rats and reduces amphetamine-induced hyperactivity in mice. CX1942 potently reverses opioid-induced respiratory depression in rats. CX1942/CX1763 was effective at 2.5-10 mg/kg. CX1763 lacked epileptogenicity up to 1500 mg/kg in rats.Conclusion: These data document that CX1942 and CX1763 are active and without prominent side effects in multiple pre-clinical assays. CX1942 could serve as a prodrug for CX1763 with the advantage of high water solubility as in an intravenous formulation.

Retinoic acid-mediated homeostatic plasticity drives cell type-specific CP-AMPAR accumulation in nucleus accumbens core and incubation of cocaine craving

Incubation of cocaine craving, a translationally relevant model for the persistence of drug craving during abstinence, ultimately depends on strengthening of nucleus accumbens core (NAcc) synapses through synaptic insertion of homomeric GluA1 Ca2+-permeable AMPA receptors (CP-AMPARs). Here we tested the hypothesis that CP-AMPAR upregulation results from a form of homeostatic plasticity, previously characterized in vitro and in other brain regions, that depends on retinoic acid (RA) signaling in dendrites. Under normal conditions, ongoing synaptic transmission maintains intracellular Ca2+ at levels sufficient to suppress RA synthesis. Prolonged blockade of neuronal activity results in disinhibition of RA synthesis, leading to increased GluA1 translation and synaptic insertion of homomeric GluA1 CP-AMPARs. Using slice recordings, we found that increasing RA signaling in NAcc medium spiny neurons (MSN) from drug-naïve rats rapidly upregulates CP-AMPARs, and that this pathway is operative only in MSN expressing the D1 dopamine receptor. In MSN recorded from rats that have undergone incubation of craving, this effect of RA is occluded; instead, interruption of RA signaling in the slice normalizes the incubation-associated elevation of synaptic CP-AMPARs. Paralleling this in vitro finding, interruption of RA signaling in the NAcc of 'incubated rats' normalizes the incubation-associated elevation of cue-induced cocaine seeking. These results suggest that RA signaling becomes tonically active in the NAcc during cocaine withdrawal and, by maintaining elevated CP-AMPAR levels, contributes to the incubation of cocaine craving.

Calcineurin and CK2 Reciprocally Regulate Synaptic AMPA Receptor Phenotypes via α2δ-1 in Spinal Excitatory Neurons

Calcineurin inhibitors, such as cyclosporine and tacrolimus (FK506), are commonly used immunosuppressants for preserving transplanted organs and tissues. However, these drugs can cause severe and persistent pain. GluA2-lacking, calcium-permeable AMPA receptors (CP-AMPARs) are implicated in various neurological disorders, including neuropathic pain. It is unclear whether and how constitutive calcineurin, a Ca2+/calmodulin protein phosphatase, controls synaptic CP-AMPARs. In this study, we found that blocking CP-AMPARs with IEM-1460 markedly reduced the amplitude of AMPAR-EPSCs in excitatory neurons expressing vesicular glutamate transporter-2 (VGluT2), but not in inhibitory neurons expressing vesicular GABA transporter, in the spinal cord of FK506-treated male and female mice. FK506 treatment also caused an inward rectification in the current-voltage relationship of AMPAR-EPSCs specifically in VGluT2 neurons. Intrathecal injection of IEM-1460 rapidly alleviated pain hypersensitivity in FK506-treated mice. Furthermore, FK506 treatment substantially increased physical interaction of α2δ-1 with GluA1 and GluA2 in the spinal cord and reduced GluA1/GluA2 heteromers in endoplasmic reticulum-enriched fractions of spinal cords. Correspondingly, inhibiting α2δ-1 with pregabalin, Cacna2d1 genetic knock-out, or disrupting α2δ-1-AMPAR interactions with an α2δ-1 C terminus peptide reversed inward rectification of AMPAR-EPSCs in spinal VGluT2 neurons caused by FK506 treatment. In addition, CK2 inhibition reversed FK506 treatment-induced pain hypersensitivity, α2δ-1 interactions with GluA1 and GluA2, and inward rectification of AMPAR-EPSCs in spinal VGluT2 neurons. Thus, the increased prevalence of synaptic CP-AMPARs in spinal excitatory neurons plays a major role in calcineurin inhibitor-induced pain hypersensitivity. Calcineurin and CK2 antagonistically regulate postsynaptic CP-AMPARs through α2δ-1-mediated GluA1/GluA2 heteromeric assembly in the spinal dorsal horn.

Obesity Alters POMC and Kisspeptin Neuron Cross Talk Leading to Reduced Luteinizing Hormone in Male Mice

Obesity is associated with hypogonadism in males, characterized by low testosterone and sperm number. Previous studies determined that these stem from dysregulation of hypothalamic circuitry that regulates reproduction, by unknown mechanisms. Herein, we used mice fed chronic high-fat diet, which mimics human obesity, to determine mechanisms of impairment at the level of the hypothalamus, in particular gonadotropin-releasing hormone (GnRH) neurons that regulate luteinizing hormone (LH), which then regulates testosterone. Consistent with obese humans, we demonstrated lower LH, and lower pulse frequency of LH secretion, but unchanged pituitary responsiveness to GnRH. LH pulse frequency is regulated by pulsatile GnRH secretion, which is controlled by kisspeptin. Peripheral and central kisspeptin injections, and DREADD-mediated activation of kisspeptin neurons, demonstrated that kisspeptin neurons were suppressed in obese mice. Thus, we investigated regulators of kisspeptin secretion. We determined that the LH response to NMDA was lower in obese mice, corresponding to fewer glutamate receptors in kisspeptin neurons, which may be critical for kisspeptin synchronization. Given that kisspeptin neurons also interact with anorexigenic POMC neurons, which are affected by obesity, we examined their cross talk, and determined that the LH response to either DREADD-mediated activation of POMC neurons or central injection of αMSH, a product of POMC, is abolished in obese mice. This was accompanied by diminished levels of αMSH receptor, MC4R, in kisspeptin neurons. Together, our studies determined that obesity leads to the downregulation of receptors that regulate kisspeptin neurons, which is associated with lower LH pulse frequency, leading to lower LH and hypogonadism.

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.

CD3ζ-Mediated Signaling Protects Retinal Ganglion Cells in Glutamate Excitotoxicity of the Retina

Excessive levels of glutamate activity could potentially damage and kill neurons. Glutamate excitotoxicity is thought to play a critical role in many CNS and retinal diseases. Accordingly, glutamate excitotoxicity has been used as a model to study neuronal diseases. Immune proteins, such as major histocompatibility complex (MHC) class I molecules and their receptors, play important roles in many neuronal diseases, while T-cell receptors (TCR) are the primary receptors of MHCI. We previously showed that a critical component of TCR, CD3ζ, is expressed by mouse retinal ganglion cells (RGCs). The mutation of CD3ζ or MHCI molecules compromises the development of RGC structure and function. In this study, we investigated whether CD3ζ-mediated molecular signaling regulates RGC death in glutamate excitotoxicity. We show that mutation of CD3ζ significantly increased RGC survival in NMDA-induced excitotoxicity. In addition, we found that several downstream molecules of TCR, including Src (proto-oncogene tyrosine-protein kinase) family kinases (SFKs) and spleen tyrosine kinase (Syk), are expressed by RGCs. Selective inhibition of an SFK member, Hck, or Syk members, Syk or Zap70, significantly increased RGC survival in NMDA-induced excitotoxicity. These results provide direct evidence to reveal the underlying molecular mechanisms that control RGC death under disease conditions.

Altered excitatory and inhibitory ionotropic receptor subunit expression in the cortical visuospatial working memory network in schizophrenia

Dysfunction of the cortical dorsal visual stream and visuospatial working memory (vsWM) network in individuals with schizophrenia (SZ) likely reflects alterations in both excitatory and inhibitory neurotransmission within nodes responsible for information transfer across the network, including primary visual (V1), visual association (V2), posterior parietal (PPC), and dorsolateral prefrontal (DLPFC) cortices. However, the expression patterns of ionotropic glutamatergic and GABAergic receptor subunits across these regions, and alterations of these patterns in SZ, have not been investigated. We quantified transcript levels of key subunits for excitatory N-methyl-D-aspartate receptors (NMDARs), excitatory alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs), and inhibitory GABAA receptors (GABAARs) in postmortem total gray matter from V1, V2, PPC, and DLPFC of unaffected comparison (UC) and matched SZ subjects. In UC subjects, levels of most AMPAR and NMDAR mRNAs exhibited opposite rostral-to-caudal gradients, with AMPAR GRIA1 and GRIA2 mRNA levels highest in DLPFC and NMDAR GRIN1 and GRIN2A mRNA levels highest in V1. GABRA5 and GABRA1 mRNA levels were highest in DLPFC and V1, respectively. In SZ, most transcript levels were lower relative to UC subjects, with these differences largest in V1, intermediate in V2 and PPC, and smallest in DLPFC. In UC subjects, these distinct patterns of receptor transcript levels across the cortical vsWM network suggest that the balance between excitation and inhibition is achieved in a region-specific manner. In SZ subjects, the large deficits in excitatory and inhibitory receptor transcript levels in caudal sensory regions suggest that abnormalities early in the vsWM pathway might contribute to altered information processing in rostral higher-order regions.

Neuronal Circuit Dysfunction in Amyotrophic Lateral Sclerosis

The primary neural circuit affected in Amyotrophic Lateral Sclerosis (ALS) patients is the corticospinal motor circuit, originating in upper motor neurons (UMNs) in the cerebral motor cortex which descend to synapse with the lower motor neurons (LMNs) in the spinal cord to ultimately innervate the skeletal muscle. Perturbation of these neural circuits and consequent loss of both UMNs and LMNs, leading to muscle wastage and impaired movement, is the key pathophysiology observed. Despite decades of research, we are still lacking in ALS disease-modifying treatments. In this review, we document the current research from patient studies, rodent models, and human stem cell models in understanding the mechanisms of corticomotor circuit dysfunction and its implication in ALS. We summarize the current knowledge about cortical UMN dysfunction and degeneration, altered excitability in LMNs, neuromuscular junction degeneration, and the non-cell autonomous role of glial cells in motor circuit dysfunction in relation to ALS. We further highlight the advances in human stem cell technology to model the complex neural circuitry and how these can aid in future studies to better understand the mechanisms of neural circuit dysfunction underpinning ALS.

InSpectro-Gadget: A Tool for Estimating Neurotransmitter and Neuromodulator Receptor Distributions for MRS Voxels

Magnetic resonance spectroscopy (MRS) is widely used to estimate concentrations of glutamate and γ -aminobutyric acid (GABA) in specific regions of the living human brain. As cytoarchitectural properties differ across the brain, interpreting these measurements can be assisted by having knowledge of such properties for the MRS region(s) studied. In particular, some knowledge of likely local neurotransmitter receptor patterns can potentially give insights into the mechanistic environment GABA- and glutamatergic neurons are functioning in. This may be of particular utility when comparing two or more regions, given that the receptor populations may differ substantially across them. At the same time, when studying MRS data from multiple participants or timepoints, the homogeneity of the sample becomes relevant, as measurements taken from areas with different cytoarchitecture may be difficult to compare. To provide insights into the likely cytoarchitectural environment of user-defined regions-of-interest, we produced an easy to use tool - InSpectro-Gadget - that interfaces with receptor mRNA expression information from the Allen Human Brain Atlas. This Python tool allows users to input masks and automatically obtain a graphical overview of the receptor population likely to be found within. This includes comparison between multiple masks or participants where relevant. The receptors and receptor subunit genes featured include GABA- and glutamatergic classes, along with a wide range of neuromodulators. The functionality of the tool is explained here and its use is demonstrated through a set of example analyses. The tool is available at https://github.com/lizmcmanus/Inspectro-Gadget .

Extrasynaptic distribution of NMDA receptors in cochlear inner hair cell afferent signaling complex

OBJECTIVE

The distribution and role of NMDA receptors is unclear in the afferent signaling complex of the cochlea. The present study aimed to examine the distribution of NMDA receptors in cochlear afferent signaling complex of the adult mouse, and their relationship with ribbon synapses of inner hair cells (IHCs) and GABAergic efferent terminals of the lateral olivocochlear (LOC).

METHODS

Immunofluorescence staining in combination with confocal microscopy was used to investigate the distribution of glutamatergic NMDA and AMPA receptors in afferent terminals of SGNs, and their relationship with ribbon synapses of IHCs and GABAergic efferent terminals of LOC.

RESULTS

Terminals with AMPA receptors along with Ribbons of IHC formed afferent synapses in the basal pole of IHCs, and those with NMDA receptors were mainly distributed longitudinally in the IHCs nuclei region. Significant difference was found in the distribution of NMDA and AMPA receptors in IHC afferent signaling complex (P<0.05). Some GABAergic terminals colocalized with NMDA receptors at the IHC nucleus region (P>0.05).

CONCLUSION

There is significant difference in the distribution of NMDA and AMPA receptors in cochlear afferent signaling complex. NMDA receptors are present in the extra-synaptic region of ribbon synapses of IHCs, and they are related to GABA efferent terminals of the afferent signaling complex.

Exposure to tungsten particles via inhalation triggers early toxicity marker expression in the rat brain

OBJECTIVE

Our work is focused on tungsten, considered as an emerging contaminant. Its environmental dispersion is partly due to mining and military activities. Exposure scenario can also be occupational, in areas such as the hard metal industry and specific nuclear facilities. Our study investigated the cerebral effects induced by the inhalation of tungsten particles.

METHODS

Inhalation exposure campaigns were carried out at two different concentrations (5 and 80 mg/m3) in single and repeated modes (4 consecutive days) in adult rats within a nose-only inhalation chamber. Processes involved in brain toxicity were investigated 24 h after exposure.

RESULTS AND DISCUSSION

Site-specific effects in terms of neuroanatomy and concentration-dependent changes in specific cellular actors were observed. Results obtained in the olfactory bulb suggest a potential early effect on the survival of microglial cells. Depending on the mode of exposure, these cells showed a decrease in density accompanied by an increase in an apoptotic marker. An abnormal phenotype of the nuclei of mature neurons, suggesting neuronal suffering, was also observed in the frontal cortex, and can be linked to the involvement of oxidative stress. The differential effects observed according to exposure patterns could involve two components: local (brain-specific) and/or systemic. Indeed, tungsten, in addition to being found in the lungs and kidneys, was present in the brain of animals exposed to the high concentration.

CONCLUSION

Our data question the perceived innocuity of tungsten relative to other metals and raise hypotheses regarding possible adaptive or neurotoxic mechanisms that could ultimately alter neuronal integrity.

Calcium-permeable AMPA and kainate receptors of GABAergic neurons

This Commentary presents a brief discussion of the action of glutamate calcium permeable receptors present with neurons on the release of the neurotransmitter gamma-aminobutyric acid (GABA). In particular, Glutamate sensitive Kainic Acid Receptors (KARs) and α-Amino-3-hydroxy-5-Methyl-4-isoxazole Propionic Acid Receptor (AMPARs) are Na+ channels that typically cause neuronal cells to depolarize and release GABA. Some of these receptors are also permeable to Ca2+ and are hence involved in the calcium-dependent release of GABA neurotransmitters. Calcium-permeable kainate and AMPA receptors (CP-KARs and CP-AMPARs) are predominantly located in GABAergic neurons in the mature brain and their primary role is to regulate GABA release. AMPARs which do not contain the GluA2 subunit are mainly localized in the postsynaptic membrane. CP-KAR receptors are located mainly in the presynapse. GABAergic neurons expressing CP-KARs and CP-AMPARs respond to excitation earlier and faster, suppressing hyperexcitation of other neurons by the advanced GABA release due to an early rapid [Ca2+]i increase. CP-AMPARs have demonstrated a more pronounced impact on plasticity compared to NMDARs because of their capacity to elevate intracellular Ca2+ levels independently of voltage. GABAergic neurons that express CP-AMPARs contribute to the disinhibition of glutamatergic neurons by suppressing GABAergic neurons that express CP-KARs. Hence, the presence of glutamate CP-KARs and CP-AMPARs is crucial in governing hyperexcitation and synaptic plasticity in GABAergic neurons.

Extension domain of amyloid processor protein inhibits amyloidogenic cleavage and balances neural activity in a traumatic brain injury mouse model

BACKGROUND

Mechanisms underlying cognitive dysfunction following traumatic brain injury (TBI) partially due to abnormal amyloid processor protein (APP) cleavage and neural hyperactivity. Binding of the extension domain of APP (ExD17) to the GABAbR1 receptor results in reduced neural activity, which might play a role in the mechanisms of cognitive dysfunction caused by TBI.

METHODS

Stretch-induced injury was utilized to establish a cell injury model in HT22 cells. The TBI model was created by striking the exposed brain tissue with a free-falling weight. Topical or intraperitoneal administration of ExD17 was performed. Cell viability was assessed through a cell counting kit-8 assay, while intracellular Ca2+ was measured using Fluo-4. Western blotting was used to investigate the expression of APP amyloidogenic cleavage proteins, GABAbR1, phospholipase C (PLC), PLCB3, and synaptic proteins. ELISA was performed to analyze the levels of Aβ42. Seizures were assessed using electroencephalography (EEG). Behaviors were evaluated through the novel object recognition test, open field test, elevated plus maze test, and nest-building test.

RESULTS

ExD17 improved cell viability and reduced intracellular calcium in the cell injury model. The treatment also suppressed the increased expression of APP amyloidogenic cleavage proteins and Aβ42 in both cell injury and TBI models. ExD17 treatment reversed the abnormal expression of GABAbR1, GRIA2, p-PLCG1/PLCG1 ratio, and p-PLCB3/PLCB3 ratio. In addition, ExD17 treatment reduced neural activity, seizure events, and their duration in TBI. Intraperitoneal injection of ExD17 improved behavioral outcomes in the TBI mouse model.

CONCLUSIONS

ExD17 treatment results in a reduction of amyloidogenic APP cleavage and neuroexcitotoxicity, ultimately leading to an improvement in the behavioral deficits observed in TBI mice.

Quantification of AMPA receptor subunits and RNA editing-related proteins in the J20 mouse model of Alzheimer's disease by capillary western blotting

Introduction

Accurate modelling of molecular changes in Alzheimer's disease (AD) dementia is crucial for understanding the mechanisms driving neuronal pathology and for developing treatments. Synaptic dysfunction has long been implicated as a mechanism underpinning memory dysfunction in AD and may result in part from changes in adenosine deaminase acting on RNA (ADAR) mediated RNA editing of the GluA2 subunit of AMPA receptors and changes in AMPA receptor function at the post synaptic cleft. However, few studies have investigated changes in proteins which influence RNA editing and notably, AD studies that focus on studying changes in protein expression, rather than changes in mRNA, often use traditional western blotting.

Methods

Here, we demonstrate the value of automated capillary western blotting to investigate the protein expression of AMPA receptor subunits (GluA1-4), the ADAR RNA editing proteins (ADAR1-3), and proteins known to regulate RNA editing (PIN1, WWP2, FXR1P, and CREB1), in the J20 AD mouse model. We describe extensive optimisation and validation of the automated capillary western blotting method, demonstrating the use of total protein to normalise protein load, in addition to characterising the optimal protein/antibody concentrations to ensure accurate protein quantification. Following this, we assessed changes in proteins of interest in the hippocampus of 44-week-old J20 AD mice.

Results

We observed an increase in the expression of ADAR1 p110 and GluA3 and a decrease in ADAR2 in the hippocampus of 44-week-old J20 mice. These changes signify a shift in the balance of proteins that play a critical role at the synapse. Regression analysis revealed unique J20-specific correlations between changes in AMPA receptor subunits, ADAR enzymes, and proteins that regulate ADAR stability in J20 mice, highlighting potential mechanisms mediating RNA-editing changes found in AD.

Discussion

Our findings in J20 mice generally reflect changes seen in the human AD brain. This study underlines the importance of novel techniques, like automated capillary western blotting, to assess protein expression in AD. It also provides further evidence to support the hypothesis that a dysregulation in RNA editing-related proteins may play a role in the initiation and/or progression of AD.

Proposed mechanisms of tau: relationships to traumatic brain injury, Alzheimer's disease, and epilepsy

Traumatic brain injury (TBI), Alzheimer's disease (AD), and epilepsy share proposed mechanisms of injury, including neuronal excitotoxicity, cascade signaling, and activation of protein biomarkers such as tau. Although tau is typically present intracellularly, in tauopathies, phosphorylated (p-) and hyper-phosphorylated (hp-) tau are released extracellularly, the latter leading to decreased neuronal stability and neurofibrillary tangles (NFTs). Tau cleavage at particular sites increases susceptibility to hyper-phosphorylation, NFT formation, and eventual cell death. The relationship between tau and inflammation, however, is unknown. In this review, we present evidence for an imbalanced endoplasmic reticulum (ER) stress response and inflammatory signaling pathways resulting in atypical p-tau, hp-tau and NFT formation. Further, we propose tau as a biomarker for neuronal injury severity in TBI, AD, and epilepsy. We present a hypothesis of tau phosphorylation as an initial acute neuroprotective response to seizures/TBI. However, if the underlying seizure pathology or TBI recurrence is not effectively treated, and the pathway becomes chronically activated, we propose a "tipping point" hypothesis that identifies a transition of tau phosphorylation from neuroprotective to injurious. We outline the role of amyloid beta (Aβ) as a "last ditch effort" to revert the cell to programmed death signaling, that, when fails, transitions the mechanism from injurious to neurodegenerative. Lastly, we discuss targets along these pathways for therapeutic intervention in AD, TBI, and epilepsy.

Mechanism of Analgesia by Gabapentinoid Drugs: Involvement of Modulation of Synaptogenesis and Trafficking of Glutamate-Gated Ion Channels

Gabapentinoids have clinically been used for treating epilepsy, neuropathic pain, and several other neurologic disorders for >30 years; however, the definitive molecular mechanism responsible for their therapeutic actions remained uncertain. The conventional pharmacological observation regarding their efficacy in chronic pain modulation is the weakening of glutamate release at presynaptic terminals in the spinal cord. While the α2/δ-1 subunit of voltage-gated calcium channels (VGCCs) has been identified as the primary drug receptor for gabapentinoids, the lack of consistent effect of this drug class on VGCC function is indicative of a minor role in regulating this ion channel's activity. The current review targets the efficacy and mechanism of gabapentinoids in treating chronic pain. The discovery of interaction of α2/δ-1 with thrombospondins established this protein as a major synaptogenic neuronal receptor for thrombospondins. Other findings identified α2/δ-1 as a powerful regulator of N-methyl-D-aspartate receptor (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) by potentiating the synaptic expression, a putative pathophysiological mechanism of neuropathic pain. Further, the interdependent interactions between thrombospondin and α2/δ-1 contribute to chronic pain states, while gabapentinoid ligands efficaciously reverse such pain conditions. Gabapentin normalizes and even blocks NMDAR and AMPAR synaptic targeting and activity elicited by nerve injury. SIGNIFICANCE STATEMENT: Gabapentinoid drugs are used to treat various neurological conditions including chronic pain. In chronic pain states, gene expression of cacnα2/δ-1 and thrombospondins are upregulated and promote aberrant excitatory synaptogenesis. The complex trait of protein associations that involve interdependent interactions between α2/δ-1 and thrombospondins, further, association of N-methyl-D-aspartate receptor and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor with the C-tail of α2/δ-1, constitutes a macromolecular signaling complex that forms the crucial elements for the pharmacological mode of action of gabapentinoids.

Temporal and quantitative analysis of the functional expression of Ca2+-permeable AMPA receptors during LTP

In the present study, we attempted to temporally and quantitatively analyze the functional contributions of Ca2+-permeable (CP) α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) during long-term potentiation (LTP) expression using electrophysiological and pharmacological approaches. In hippocampal CA1 neurons, using 1-naphthyl acetyl spermine (NASPM), a CP-AMPAR antagonist, we began by demonstrating that NASPM-sensitive components, probably including the GluA1 homomer, functionally contributed to about 15% of AMPAR-mediated EPSC amplitude in basal conditions. Then, when NASPM was treated at different time points (3-30 min) after LTP induction, it was found that LTP was almost completely impaired at 3 or 10 min but maintained at 20 or 30 min, although its potentiation was reduced. Further temporal and quantitative analysis revealed that the functional expression of CP-AMPARs began increasing approximately 20 min after LTP induction and reached more than twice the basal level at 30 min. These results suggest that CP-AMPARs in the first 3-10 min of LTP might play an important role in LTP maintenance. Moreover, their decay time was also significantly increased at 30 min, suggesting that CP-AMPARs changed not only quantitatively in LTP but also qualitatively.

Neuroplasticity in levodopa-induced dyskinesias: An overview on pathophysiology and therapeutic targets

Levodopa-induced dyskinesias (LIDs) are a common complication in patients with Parkinson's disease (PD). A complex cascade of electrophysiological and molecular events that induce aberrant plasticity in the cortico-basal ganglia system plays a key role in the pathophysiology of LIDs. In the striatum, multiple neurotransmitters regulate the different forms of physiological synaptic plasticity to provide it in a bidirectional and Hebbian manner. In PD, impairment of both long-term potentiation (LTP) and long-term depression (LTD) progresses with disease and dopaminergic denervation of striatum. The altered balance between LTP and LTD processes leads to unidirectional changes in plasticity that cause network dysregulation and the development of involuntary movements. These alterations have been documented, in both experimental models and PD patients, not only in deep brain structures but also at motor cortex. Invasive and non-invasive neuromodulation treatments, as deep brain stimulation, transcranial magnetic stimulation, or transcranial direct current stimulation, may provide strategies to modulate the aberrant plasticity in the cortico-basal ganglia network of patients affected by LIDs, thus restoring normal neurophysiological functioning and treating dyskinesias. In this review, we discuss the evidence for neuroplasticity impairment in experimental PD models and in patients affected by LIDs, and potential neuromodulation strategies that may modulate aberrant plasticity.

Stress-free cell aggregation by using the CEPT cocktail enhances embryoid body and organoid fitness

Embryoid bodies (EBs) and self-organizing organoids derived from human pluripotent stem cells (hPSCs) recapitulate tissue development in a dish and hold great promise for disease modeling and drug development. However, current protocols are hampered by cellular stress and apoptosis during cell aggregation, resulting in variability and impaired cell differentiation. Here, we demonstrate that EBs and various organoid models (e.g., brain, gut, kidney) can be optimized by using the small molecule cocktail named CEPT (chroman 1, emricasan, polyamines, trans-ISRIB), a polypharmacological approach that ensures cytoprotection and cell survival. Application of CEPT for just 24 h during cell aggregation has long-lasting consequences affecting morphogenesis, gene expression, cellular differentiation, and organoid function. Various qualification methods confirmed that CEPT treatment enhanced experimental reproducibility and consistently improved EB and organoid fitness as compared to the widely used ROCK inhibitor Y-27632. Collectively, we discovered that stress-free cell aggregation and superior cell survival in the presence of CEPT are critical quality control determinants that establish a robust foundation for bioengineering complex tissue and organ models.

Phosphorylation of the AMPARs regulated by protein kinase C (PKC) and protein interacting with C-kinase 1 (PICK1) contribute to orofacial neuropathic pain

The α-amino-3-hydroxy-5-methylisoxazole-4-isoxazolepropionic acid receptor (AMPAR) has been recognized to play a vital role in the development of neuropathic pain. Recent studies have indicated that protein kinase C (PKC) and protein interacting with C-kinase 1 (PICK1) are involved in the phosphorylation of AMPARs. However, whether PKC and PICK1 were involved in the AMPAR phosphorylation in the trigeminal ganglion (TG) to participate in orofacial neuropathic pain remains enigmatic. A behavioral test was utilized to evaluate the head withdrawal threshold (HWT) after chronic constriction injury of the infraorbital nerve (CCI-ION). The distribution and expression of GluA1, GluA2, PKC, and PICK1 were examined in the trigeminal ganglion (TG) by immunofluorescence, real-time reverse transcription-quantitative polymerase chain reaction, immunoblotting, and co-immunoprecipitation. Intra-ganglionic injections of drugs were performed to investigate the regulation mechanism. The present study demonstrated that CCI-ION-induced mechanical allodynia was maintained over at least 21 days. GluA1 and GluA2 were mainly expressed in the neurons. Trigeminal nerve injury potentiated the phosphorylation of GluA1, GluA2, and PKC in the TG, which was prevented by inhibiting PKC with chelerythrine chloride. Additionally, PICK1 colocalized and interacted with GluA2 in the TG. Following blocking PICK1 with FSC-231, the phosphorylation of GluA2 decreased. Finally, inhibition of PKC and PICK1 both alleviated mechanical allodynia in the whisker pad of CCI-ION mice. In conclusion, activation of PKC and PICK1 contribute to orofacial allodynia by regulating AMPAR phosphorylation in the TG of male mice, which provides potential therapeutic targets for alleviating orofacial neuropathic pain.

Acute inhibition of AMPA receptors by perampanel reduces amyloid β-protein levels by suppressing β-cleavage of APP in Alzheimer's disease models

Hippocampal hyperexcitability is a promising therapeutic target to prevent Aβ deposition in AD since enhanced neuronal activity promotes presynaptic Aβ production and release. This article highlights the potential application of perampanel (PER), an AMPA receptor (AMPAR) antagonist approved for partial seizures, as a therapeutic agent for AD. Using transgenic AD mice combined with in vivo brain microdialysis and primary neurons under oligomeric Aβ-evoked neuronal hyperexcitability, the acute effects of PER on Aβ metabolism were investigated. A single oral administration of PER rapidly decreased ISF Aβ40 and Aβ42 levels in the hippocampus of J20, APP transgenic mice, without affecting the Aβ40 /Aβ42 ratio; 5 mg/kg PER resulted in declines of 20% and 31%, respectively. Moreover, PER-treated J20 manifested a marked decrease in hippocampal APP βCTF levels with increased FL-APP levels. Consistently, acute treatment of PER reduced sAPPβ levels, a direct byproduct of β-cleavage of APP, released to the medium in primary neuronal cultures under oligomeric Aβ-induced neuronal hyperexcitability. To further evaluate the effect of PER on ISF Aβ clearance, a γ-secretase inhibitor was administered to J20 1 h after PER treatment. PER did not influence the elimination of ISF Aβ, indicating that the acute effect of PER is predominantly on Aβ production. In conclusion, acute treatment of PER reduces Aβ production by suppressing β-cleavage of amyloid-β precursor protein effectively, indicating a potential effect of PER against Aβ pathology in AD.

An amygdala-to-cingulate cortex circuit for conflicting choices in chronic pain

Chronic pain is a complex experience with multifaceted behavioral manifestations, often leading to pain avoidance at the expense of reward approach. How pain facilitates avoidance in situations with mixed outcomes is unknown. The anterior cingulate cortex (ACC) plays a key role in pain processing and in value-based decision-making. Distinct ACC inputs inform about the sensory and emotional quality of pain. However, whether specific ACC circuits underlie pathological conflict assessment in pain remains underexplored. Here, we demonstrate that mice with chronic pain favor cold avoidance rather than reward approach in a conflicting task. This occurs along with selective strengthening of basolateral amygdala inputs onto ACC layer 2/3 pyramidal neurons. The amygdala-cingulate projection is necessary and sufficient for the conflicting cold avoidance. Further, low-frequency stimulation of this pathway restores AMPA receptor function and reduces avoidance in pain mice. Our findings provide insights into the circuits and mechanisms underlying cognitive aspects of pain and offer potential targets for treatment.

The BDNF mimetic R-13 attenuates TBI pathogenesis using TrkB-related pathways and bioenergetics

Traumatic brain injury (TBI) is major neurological burden globally, and effective treatments are urgently needed. TBI is characterized by a reduction in energy metabolism and synaptic function that seems a primary cause of neuronal dysfunction. R13, a small drug and BDNF mimetic showed promising results in improving spatial memory and anxiety-like behavior after TBI. Additionally, R13 was found to counteract reductions in molecules associated with BDNF signaling (p-TrkB, p-PI3K, p-AKT), synaptic plasticity (GluR2, PSD95, Synapsin I) as well as bioenergetic components such as mitophagy (SOD, PGC-1α, PINK1, Parkin, BNIP3, and LC3) and real-time mitochondrial respiratory capacity. Behavioral and molecular changes were accompanied by adaptations in functional connectivity assessed using MRI. Results highlight the potential of R13 as a therapeutic agent for TBI and provide valuable insights into the molecular and functional changes associated with this condition.

Uncovering the Significance of STEP61 in Alzheimer's Disease: Structure, Substrates, and Interactome

STEP (STriatal-Enriched Protein Tyrosine Phosphatase) is a brain-specific phosphatase that plays an important role in controlling signaling molecules involved in neuronal activity and synaptic development. The striatum is the main location of the STEP enzyme. An imbalance in STEP61 activity is a risk factor for Alzheimer's disease (AD). It can contribute to the development of numerous neuropsychiatric diseases, including Parkinson's disease (PD), schizophrenia, fragile X syndrome (FXS), Huntington's disease (HD), alcoholism, cerebral ischemia, and stress-related diseases. The molecular structure, chemistry, and molecular mechanisms associated with STEP61's two major substrates, Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPAr) and N-methyl-D-aspartate receptors (NMDARs), are crucial in understanding the relationship between STEP61 and associated illnesses. STEP's interactions with its substrate proteins can alter the pathways of long-term potentiation and long-term depression. Therefore, understanding the role of STEP61 in neurological illnesses, particularly Alzheimer's disease-associated dementia, can provide valuable insights for possible therapeutic interventions. This review provides valuable insights into the molecular structure, chemistry, and molecular mechanisms associated with STEP61. This brain-specific phosphatase controls signaling molecules involved in neuronal activity and synaptic development. This review can aid researchers in gaining deep insights into the complex functions of STEP61.

A glutamatergic pathway between the medial habenula and the rostral ventrolateral medulla may regulate cardiovascular function in a rat model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a serious psychiatric disorder, and there is an association between it and the development of cardiovascular disease. The aim of this study was to explore whether there is a glutamatergic pathway connecting the medial habenula (MHb) with the rostral ventrolateral medulla (RVLM) that is involved in the regulation of cardiovascular function in a rat model of PTSD. Vesicular glutamate transporter 2 (VGLUT2)-positive neurons in the MHb region were retrogradely labeled with FluoroGold (FG) by the double-labeling technique of VGLUT2 immunofluorescence and FG retrograde tracing. Rats belonging to the PTSD model group were microinjected with artificial cerebrospinal fluid (ACSF) or kynurenic acid (KYN; a nonselective glutamate receptor blocker) into their RVLM. Subsequently, with electrical stimulation of MHb, the discharge frequency of the RVLM neurons, heart rate, and blood pressure were found to be significantly increased after microinjection of ACSF using an in vivo multichannel synchronous recording technology; however, this effect was inhibited by injection of KYN. The expression of N-methyl-D-aspartic acid (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits was significantly increased in RVLM of PTSD model rats analyzed by the Western blotting technique. These findings suggest that there may be a glutamatergic pathway connection between MHb and RVLM and that this pathway may be involved in the regulation of cardiovascular function in the PTSD model rats, by acting on NMDA and AMPA receptors in the RVLM.