Acute Footshock Stress Induces Time-Dependent Modifications of AMPA/NMDA Protein Expression and AMPA Phosphorylation

Antidepressive synergism between crocin and D-AP5 in acute restraint-stressed mice

Emerging evidence suggests that crocin rescues stress-induced depressive symptoms in mice via stimulation of hippocampal neurogenesis. Glutamate modulators mainly involving N-methyl- d -aspartate (NMDA) receptors (NMDARs) have highlighted a role in neural development, synaptic plasticity, and depression. The research presented here was designed to appraise the interaction between NMDAR agents and crocin on depressive-related behaviors in the NMRI male mice exposed to acute restraint stress (ARS) for a period of 4 h. The mice were submitted to the splash test, forced swimming test, and tail suspension test to evaluate depressive-like behavior. The ARS decreased the grooming duration in the splash test and increased immobility time in the forced swimming test and tail suspension test, suggesting a depressive-like phenotype. NMDA (0.25 and 0.5 μg/mouse, intracerebroventricular) did not alter depression-related profiles in both non-acute restraint stress (NARS) and ARS mice, while the same doses of NMDAR antagonist D-AP5 potentiated the antidepressive-like activities in the ARS mice compared with the NARS mice. Moreover, a low dose of NMDA did not change depression-related parameters in the crocin-treated NARS or ARS mice, while D-AP5 enhanced the crocin response in the NARS and ARS mice. Isobologram analysis noted a synergism between crocin and D-AP5 on antidepressive-like behavior in the NARS and ARS mice. Collectively, the combination of crocin and D-AP5 was shown to mitigate depression symptoms and can be potentially used for the treatment of depression disorders.

The Antidepressant- and Anxiolytic-Like Effects of the Phosphodiesterase Type-5 Inhibitor Tadalafil are Associated with the Modulation of the Gut-Brain Axis During CNS Autoimmunity

Multiple Sclerosis (MS) is a debilitating disease that severely affects the central nervous system (CNS). Apart from neurological symptoms, it is also characterized by neuropsychiatric comorbidities, such as anxiety and depression. Phosphodiesterase-5 inhibitors (PDE5Is) such as Sildenafil and Tadalafil have been shown to possess antidepressant-like effects, but the mechanisms underpinning such effects are not fully characterized. To address this question, we used the EAE model of MS, behavioral tests, immunofluorescence, immunohistochemistry, western blot, and 16 S rRNA sequencing. Here, we showed that depressive-like behavior in Experimental Autoimmune Encephalomyelitis (EAE) mice is due to neuroinflammation, reduced synaptic plasticity, dysfunction in glutamatergic neurotransmission, glucocorticoid receptor (GR) resistance, increased blood-brain barrier (BBB) permeability, and immune cell infiltration to the CNS, as well as inflammation, increased intestinal permeability, and immune cell infiltration in the distal colon. Furthermore, 16 S rRNA sequencing revealed that behavioral dysfunction in EAE mice is associated with changes in the gut microbiota, such as an increased abundance of Firmicutes and Saccharibacteria and a reduction in Proteobacteria, Parabacteroides, and Desulfovibrio. Moreover, we detected an increased abundance of Erysipelotrichaceae and Desulfovibrionaceae and a reduced abundance of Lactobacillus johnsonii. Surprisingly, we showed that Tadalafil likely exerts antidepressant-like effects by targeting all aforementioned disease aspects. In conclusion, our work demonstrated that anxiety- and depressive-like behavior in EAE is associated with a plethora of neuroimmune and gut microbiota-mediated mechanisms and that Tadalafil exerts antidepressant-like effects probably by targeting these mechanisms. Harnessing the knowledge of these mechanisms of action of Tadalafil is important to pave the way for future clinical trials with depressed patients.

Acute stress transiently activates macrophages and chemokines in cervical lymph nodes

Acute restraint stress (RS) is routinely used to study the effects of psychological and/or physiological stress. We evaluated the impact of RS on cervical lymph nodes in rats at molecular and cellular levels. Male Sprague-Dawley rats were subjected to stress by immobilization for 30, 60, and 120 min (RS30, RS60, and RS120, respectively) and compared with rats of a no-stress control (C) group. The expression of genes encoding chemokines CXCL1/CXCL2 (Cxcl1 and Cxcl2) and their receptor CXCR2 (Cxcr2) was analyzed using reverse transcription-quantitative PCR (RT-qPCR) and microarray analyses. Immunohistochemistry and in situ hybridization were performed to determine the expression of these proteins and the macrophage biomarker CD68. Microarray analysis revealed that the expression of 514 and 496 genes was upregulated and downregulated, respectively, in the RS30 group. Compared with the C group, the RS30 group exhibited a 23.0-, 13.0-, and 1.6-fold increase in Cxcl1, Cxcl2, and Cxcr2 expression. Gene Ontology analysis revealed the involvement of these three upregulated genes in the cytokine network, inflammation, and leukocyte chemotaxis and migration. RT-qPCR analysis indicated that the mRNA levels of Cxcl1 and Cxcl2 were significantly increased in the RS30 group but were reverted to normal levels in the RS60 and RS120 groups. Cxcr2 mRNA level was significantly increased in the RS30 and RS120 groups compared with that in the C group. RS-induced CXCL1-immunopositive cells corresponded to B/plasma cells, whereas CXCL2-immunopositive cells corresponded to endothelial cells of the high endothelial venules. Stress-induced CXCR2-immunopositive cells corresponded to macrophages. Psychological and/or physiological stress induces an acute stress response and formation of an immunoreactive microenvironment in cervical lymph nodes, with the CXCL1/CXCL2-CXCR2 axis being pivotal in the acute stress response.

Dopamine-Dependent Ketamine Modulation of Glutamatergic Synaptic Plasticity in the Prelimbic Cortex of Adult Rats Exposed to Acute Stress

Traumatic stress is the main environmental risk factor for the development of psychiatric disorders. We have previously shown that acute footshock (FS) stress in male rats induces rapid and long-lasting functional and structural changes in the prefrontal cortex (PFC), which are partly reversed by acute subanesthetic ketamine. Here, we asked if acute FS may also induce any changes in glutamatergic synaptic plasticity in the PFC 24 h after stress exposure and whether ketamine administration 6 h after stress may have any effect. We found that the induction of long-term potentiation (LTP) in PFC slices of both control and FS animals is dependent on dopamine and that dopamine-dependent LTP is reduced by ketamine. We also found selective changes in ionotropic glutamate receptor subunit expression, phosphorylation, and localization at synaptic membranes induced by both acute stress and ketamine. Although more studies are needed to understand the effects of acute stress and ketamine on PFC glutamatergic plasticity, this first report suggests a restoring effect of acute ketamine, supporting the potential benefit of ketamine in limiting the impact of acute traumatic stress.

Transcriptional Profiling of Rat Prefrontal Cortex after Acute Inescapable Footshock Stress

Stress is a primary risk factor for psychiatric disorders such as Major Depressive Disorder (MDD) and Post Traumatic Stress Disorder (PTSD). The response to stress involves the regulation of transcriptional programs, which is supposed to play a role in coping with stress. To evaluate transcriptional processes implemented after exposure to unavoidable traumatic stress, we applied microarray expression analysis to the PFC of rats exposed to acute footshock (FS) stress that were sacrificed immediately after the 40 min session or 2 h or 24 h after. While no substantial changes were observed at the single gene level immediately after the stress session, gene set enrichment analysis showed alterations in neuronal pathways associated with glia development, glia-neuron networking, and synaptic function. Furthermore, we found alterations in the expression of gene sets regulated by specific transcription factors that could represent master regulators of the acute stress response. Of note, these pathways and transcriptional programs are activated during the early stress response (immediately after FS) and are already turned off after 2 h-while at 24 h, the transcriptional profile is largely unaffected. Overall, our analysis provided a transcriptional landscape of the early changes triggered by acute unavoidable FS stress in the PFC of rats, suggesting that the transcriptional wave is fast and mild, but probably enough to activate a cellular response to acute stress.

Changes at glutamate tripartite synapses in the prefrontal cortex of a new animal model of resilience/vulnerability to acute stress

Stress represents a main risk factor for psychiatric disorders. Whereas it is known that even a single trauma may induce psychiatric disorders in humans, the mechanisms of vulnerability to acute stressors have been little investigated. In this study, we generated a new animal model of resilience/vulnerability to acute footshock (FS) stress in rats and analyzed early functional, molecular, and morphological determinants of stress vulnerability at tripartite glutamate synapses in the prefrontal cortex (PFC). We found that adult male rats subjected to FS can be deemed resilient (FS-R) or vulnerable (FS-V), based on their anhedonic phenotype 24 h after stress exposure, and that these two populations are phenotypically distinguishable up to two weeks afterwards. Basal presynaptic glutamate release was increased in the PFC of FS-V rats, while depolarization-evoked glutamate release and synapsin I phosphorylation at Ser⁹ were increased in both FS-R and FS-V. In FS-R and FS-V rats the synaptic expression of GluN2A and apical dendritic length of prelimbic PFC layers II-III pyramidal neurons were decreased, while BDNF expression was selectively reduced in FS-V. Depolarization-evoked (carrier-mediated) glutamate release from astroglia perisynaptic processes (gliosomes) was selectively increased in the PFC of FS-V rats, while GLT1 and xCt levels were higher and GS expression reduced in purified PFC gliosomes from FS-R. Overall, we show for the first time that the application of the sucrose intake test to rats exposed to acute FS led to the generation of a novel animal model of resilience/vulnerability to acute stress, which we used to identify early determinants of maladaptive response related to behavioral vulnerability to stress.

Involvement of miR-135a-5p Downregulation in Acute and Chronic Stress Response in the Prefrontal Cortex of Rats

Stress is a key risk factor in the onset of neuropsychiatric disorders. The study of the mechanisms underlying stress response is important to understand the etiopathogenetic mechanisms and identify new putative therapeutic targets. In this context, microRNAs (miRNAs) have emerged as key regulators of the complex patterns of gene/protein expression changes in the brain, where they have a crucial role in the regulation of neuroplasticity, neurogenesis, and neuronal differentiation. Among them, miR-135a-5p has been associated with stress response, synaptic plasticity, and the antidepressant effect in different brain areas. Here, we used acute unavoidable foot-shock stress (FS) and chronic mild stress (CMS) on male rats to study whether miR-135a-5p was involved in stress-induced changes in the prefrontal cortex (PFC). Both acute and chronic stress decreased miR-135a-5p levels in the PFC, although after CMS the reduction was induced only in animals vulnerable to CMS, according to a sucrose preference test. MiR-135a-5p downregulation in the primary neurons reduced dendritic spine density, while its overexpression exerted the opposite effect. Two bioinformatically predicted target genes, Kif5c and Cplx1/2, were increased in FS rats 24 h after stress. Altogether, we found that miR-135a-5p might play a role in stress response in PFC involving synaptic mechanisms.

Individual behavioral profiling as a translational approach to assess treatment efficacy in an animal model of post-traumatic stress disorder

A major challenge in treating post-traumatic stress disorder (PTSD) continues to be the large variability in responsiveness to pharmacotherapy. Only 20-30% of patients experience total remission to a specific treatment, while others demonstrate either partial remission or no response. However, this heterogeneity in response to pharmacotherapy has not been adequately addressed in animal models, since these analyze the averaged group effects, ignoring the individual variability to treatment response, which seriously compromises the translation power of such models. Here we examined the possibility of employing an "individual behavioral profiling" approach, originally developed to differentiate between "affected" and "exposed-unaffected" individuals in an animal model of PTSD, to also enable dissociating "responders" or "non-responders" after SSRI (fluoxetine) treatment. Importantly, this approach does not rely on a group averaged response to a single behavioral parameter, but considers a cluster of behavioral parameters, to individually characterize an animal as either "responder" or "non-responder" to the treatment. The main variable to assess drug efficacy thus being the proportion of "responders" following treatment. Alteration in excitatory/inhibitory (E/I) balance has been proposed as being associated with stress-related psychopathology. Toward a functional proof of concept for our behaviorally-based characterization approach, we examined the expression patterns of α1 and α2 subunits of GABA_A receptor, and GluN1 and GluN2A subunits of the NMDAR receptor in the ventral hippocampus, as well as electrophysiologically local circuit activity in the dorsal dentate gyrus (DG). We demonstrate that with both parameters, treatment "responders" differed from treatment "non-responders," confirming the functional validity of the behavior-based categorization. The results suggest that the ability to respond to fluoxetine treatment may be linked to the ability to modulate excitation-inhibition balance in the hippocampus. We propose that employing the "individual behavioral profiling" approach, and the resultant novel variable of the proportion of "recovered" individuals following treatment, offers an effective translational tool to assess pharmacotherapy treatment efficacy in animal models of stress and trauma-related psychopathology.

Neurobiology of Depression: Chronic Stress Alters the Glutamatergic System in the Brain-Focusing on AMPA Receptor

Major depressive disorder (MDD) is a common neuropsychiatric disorder affecting the mood and mental well-being. Its pathophysiology remains elusive due to the complexity and heterogeneity of this disorder that affects millions of individuals worldwide. Chronic stress is frequently cited as the one of the risk factors for MDD. To date, the conventional monoaminergic theory (serotonin, norepinephrine, and/or dopamine dysregulation) has received the most attention in the treatment of MDD, and all available classes of antidepressants target these monoaminergic systems. However, the contributions of other neurotransmitter systems in MDD have been widely reported. Emerging preclinical and clinical findings reveal that maladaptive glutamatergic neurotransmission might underlie the pathophysiology of MDD, thus revealing its critical role in the neurobiology of MDD and as the therapeutic target. Aiming beyond the monoaminergic hypothesis, studies of the neurobiological mechanisms underlying the stress-induced impairment of AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-glutamatergic neurotransmission in the brain could provide novel insights for the development of a new generation of antidepressants without the detrimental side effects. Here, the authors reviewed the recent literature focusing on the role of AMPA-glutamatergic neurotransmission in stress-induced maladaptive responses in emotional and mood-associated brain regions, including the hippocampus, amygdala, prefrontal cortex, nucleus accumbens and periaqueductal gray.

Acute Ketamine Facilitates Fear Memory Extinction in a Rat Model of PTSD Along With Restoring Glutamatergic Alterations and Dendritic Atrophy in the Prefrontal Cortex

Stress represents a major risk factor for psychiatric disorders, including post-traumatic stress disorder (PTSD). Recently, we dissected the destabilizing effects of acute stress on the excitatory glutamate system in the prefrontal cortex (PFC). Here, we assessed the effects of single subanesthetic administration of ketamine (10 mg/kg) on glutamate transmission and dendritic arborization in the PFC of footshock (FS)-stressed rats, along with changes in depressive, anxious, and fear extinction behaviors. We found that ketamine, while inducing a mild increase of glutamate release in the PFC of naïve rats, blocked the acute stress-induced enhancement of glutamate release when administered 24 or 72 h before or 6 h after FS. Accordingly, the treatment with ketamine 6 h after FS also reduced the stress-dependent increase of spontaneous excitatory postsynaptic current (sEPSC) amplitude in prelimbic (PL)-PFC. At the same time, ketamine injection 6 h after FS was found to rescue apical dendritic retraction of pyramidal neurons induced by acute stress in PL-PFC and facilitated contextual fear extinction. These results show rapid effects of ketamine in animals subjected to acute FS, in line with previous studies suggesting a therapeutic action of the drug in PTSD models. Our data are consistent with a mechanism of ketamine involving re-establishment of synaptic homeostasis, through restoration of glutamate release, and structural remodeling of dendrites.

miR-9-5p is involved in the rescue of stress-dependent dendritic shortening of hippocampal pyramidal neurons induced by acute antidepressant treatment with ketamine

Converging clinical and preclinical evidence demonstrates that depressive phenotypes are associated with synaptic dysfunction and dendritic simplification in cortico-limbic glutamatergic areas. On the other hand, the rapid antidepressant effect of acute ketamine is consistently reported to occur together with the rescue of dendritic atrophy and reduction of spine number induced by chronic stress in the hippocampus and prefrontal cortex of animal models of depression. Nevertheless, the molecular mechanisms underlying these morphological alterations remain largely unknown. Here, we found that miR-9-5p levels were selectively reduced in the hippocampus of rats vulnerable to Chronic Mild Stress (CMS), while acute subanesthetic ketamine restored its levels to basal condition in just 24h; miR-9-5p expression inversely correlated with the anhedonic phenotype. A decrease of miR-9-5p was reproduced in an in vitro model of stress, based on primary hippocampal neurons incubated with the stress hormone corticosterone. In both CMS animals and primary neurons, decreased miR-9-5p levels were associated with dendritic simplification, while treatment with ketamine completely rescued the changes. In vitro modulation of miR-9-5p expression showed a direct role of miR-9-5p in regulating dendritic length and spine density in mature primary hippocampal neurons. Among the putative target genes tested, Rest and Sirt1 were validated as biological targets in primary neuronal cultures. Moreover, in line with miR-9-5p changes, REST protein expression levels were remarkably increased in both CMS vulnerable animals and corticosterone-treated neurons, while ketamine completely abolished this alteration. Finally, the shortening of dendritic length in corticosterone-treated neurons was shown to be partly rescued by miR-9-5p overexpression and dependent on REST protein expression. Overall, our data unveiled the functional role of miR-9-5p in the remodeling of dendritic arbor induced by stress/corticosterone in vulnerable animals and its rescue by acute antidepressant treatment with ketamine.

GluN2A/ERK/CREB Signaling Pathway Involved in Electroacupuncture Regulating Hypothalamic-Pituitary-Adrenal Axis Hyperactivity

The hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis caused by stress will inevitably disrupt the homeostasis of the neuroendocrine system and damage physiological functions. It has been demonstrated that electroacupuncture (EA) can modulate HPA axis hyperactivity during the perioperative period. As the initiating factor of the HPA axis, hypothalamic corticotrophin-releasing hormone (CRH) is the critical molecule affected by EA. However, the mechanism by which EA reduces CRH synthesis and secretion remains unclear. Activated N-methyl-D-aspartate receptor (NMDAR) has been linked to over-secretion of hypothalamic CRH induced by stress. To determine whether NMDAR is involved in EA regulating the over-expression of CRH, a surgical model of partial hepatectomy (HT) was established in our experiment. The effect of EA on hypothalamic NMDAR expression in HT mice was examined. Then, we investigated whether the extracellular regulated protein kinases (ERK)/cyclic adenosine monophosphate response element-binding protein (CREB) signaling pathway mediated by NMDAR was involved in EA regulating HPA axis hyperactivity. It was found that surgery enhanced the expression of hypothalamic CRH and caused HPA axis hyperactivity. Intriguingly, EA effectively suppressed the expression of CRH and decreased the activation of GluN2A (NMDAR subunit), ERK, and CREB in HT mice. GluN2A, ERK, and CREB antagonists had similar effects on normalizing the expression of CRH and HPA axis function compared with EA. Our findings suggested that surgery enhanced the activation of the hypothalamic GluN2A/ERK/CREB signaling pathway, thus promoting the synthesis and secretion of CRH. EA suppressed the phosphorylation of GluN2A, ERK, and CREB in mice that had undergone surgery, indicating that the GluN2A/ERK/CREB signaling pathway was involved in EA alleviating HPA axis hyperactivity.

Riluzole does not ameliorate disease caused by cytoplasmic TDP-43 in a mouse model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease commonly treated with riluzole, a small molecule that may act via modulation of glutamatergic neurotransmission. However, riluzole only modestly extends lifespan for people living with ALS, and its precise mechanisms of action remain unclear. Most ALS cases are characterised by accumulation of cytoplasmic TAR DNA binding protein of 43 kDa (TDP-43), and understanding the effects of riluzole in models that closely recapitulate TDP-43 pathology may provide insights for development of improved therapeutics. We therefore investigated the effects of riluzole in female transgenic mice that inducibly express nuclear localisation sequence (NLS)-deficient human TDP-43 in neurons (NEFH-tTA/tetO-hTDP-43ΔNLS, 'rNLS8', mice). Riluzole treatment from the first day of hTDP-43ΔNLS expression did not alter disease onset, weight loss or performance on multiple motor behavioural tasks. Riluzole treatment also did not alter TDP-43 protein levels, solubility or phosphorylation. Although we identified a significant decrease in GluA2 and GluA3 proteins in the cortex of rNLS8 mice, riluzole did not ameliorate this disease-associated molecular phenotype. Likewise, riluzole did not alter the disease-associated atrophy of hindlimb muscle in rNLS8 mice. Finally, riluzole treatment beginning after disease onset in rNLS8 mice similarly had no effect on progression of late-stage disease or animal survival. Together, we demonstrate specific glutamatergic receptor alterations and muscle fibre-type changes reminiscent of ALS in female rNLS8 mice, but riluzole had no effect on these or any other disease phenotypes. Future targeting of pathways related to accumulation of TDP-43 pathology may be needed to develop better treatments for ALS.

Modulation by chronic stress and ketamine of ionotropic AMPA/NMDA and metabotropic glutamate receptors in the rat hippocampus

Converging clinical and preclinical evidence has shown that dysfunction of the glutamate system is a core feature of major depressive disorder. In this context, the N-methyl-d-aspartate (NMDA) receptor antagonist ketamine has raised growing interest as fast acting antidepressant. Using the chronic mild stress (CMS) rat model of depression, performed in male rats, we aimed at analyzing whether hippocampal specific changes in subunit expression and regulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or NMDA ionotropic receptors and in metabotropic glutamate receptors could be associated with behavioral vulnerability/resilience to CMS. We also assessed whether acute ketamine (10 mg/kg) was able to dampen the alterations in CMS vulnerable animals. Although chronic stress and ketamine had no effect on ionotropic glutamate receptors mRNAs (expression, RNA editing and splicing), we found selective modulations in their protein expression, phosphorylation and localization at synaptic membranes. AMPA GluA2 expression at synaptic membranes was significantly increased only in CMS resilient rats (although a trend was found also in vulnerable animals), while its phosphorylation at Ser⁸⁸⁰ was higher in both CMS resilient and vulnerable rats, a change partially dampened by ketamine. In the hippocampus from all stressed groups, despite NMDA receptor expression levels were reduced in total extract, the levels of GluN2B-containing NMDA receptors were remarkably increased in synaptic membranes. Finally, mGlu2 underwent a selective downregulation in stress vulnerable animals, which was completely restored by acute ketamine. Overall, these results are in line with a hypofunction of activity-dependent glutamatergic synaptic transmission induced by chronic stress exposure in all the animals, as suggested by the alterations of ionotropic glutamate receptors expression and localization at synaptic level. At the same time, the selective modulation of mGlu2 receptor, confirms its previously hypothesized functional role in regulating stress vulnerability and, for the first time here, suggests a mGlu2 involvement in the fast antidepressant effect of ketamine.

Circadian Regulation of GluA2 mRNA Processing in the Rat Suprachiasmatic Nucleus and Other Brain Structures

The mammalian circadian system consists of a major circadian pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus and peripheral clocks in the body, including brain structures. The SCN depends on glutamatergic neurotransmission for transmitting signals from the retina, and it exhibits spontaneous 24-h rhythmicity in neural activity. The aim of this work was to evaluate the degree and circadian rhythmicity of AMPA receptor GluA2 subunit R/G editing and alternative flip/flop splicing in the SCN and other brain structures in Wistar rats. Our data show that the circadian rhythmicity in the SCN's GluA2 mRNA level was highest at dawn, while the circadian rhythm in R/G editing peaked at CT10 and the rhythmic flip varied with the acrophase at the late subjective night. The circadian rhythmicity was confirmed for R/G editing and splicing in the CA3 hippocampal area, and rhythmic variation of the flip isoform was also measured in the olfactory bulbs and cerebellum. The correlations between the R/G editing and alternative flip/flop splicing revealed a structure-dependent direction. In the hippocampus, the edited (G)-form level was positively correlated with the flip variant abundance, in accord with published data; by contrast, in the SCN, the flip variant was in associated more with the unedited (R) form. The edited (G) form and flop isoform also predominated in the retina and cerebellum.

RNA Editing and Modifications in Mood Disorders

Major depressive disorder (MDD) is a major health problem with significant limitations in functioning and well-being. The World Health Organization (WHO) evaluates MDD as one of the most disabling disorders in the world and with very high social cost. Great attention has been given to the study of the molecular mechanism underpinning MDD at the genetic, epigenetic and proteomic level. However, the importance of RNA modifications has attracted little attention until now in this field. RNA molecules are extensively and dynamically altered by a variety of mechanisms. Similar to "epigenomic" changes, which modify DNA structure or histones, RNA alterations are now termed "epitranscriptomic" changes and have been predicted to have profound consequences for gene expression and cellular functionality. Two of these modifications, adenosine to inosine (A-to-I) RNA editing and m6A methylations, have fascinated researchers over the last years, showing a new level of complexity in gene expression. In this review, we will summary the studies that focus on the role of RNA editing and m6A methylation in MDD, trying to underline their potential breakthroughs and pitfalls.

Ultrasound stress compromises the correlates of emotional-like states and brain AMPAR expression in mice: effects of antioxidant and anti-inflammatory herbal treatment

The modern lifestyle is associated with exposure to "psychological" or "emotional" stress. A growing portion of the population is exposed to emotional stress that results in a high incidence of anxiety disorders, a serious social problem. With this rise, there is a need for understanding the neurobiological causes of stress-induced anxiety and to offer safe remedies for this condition. Side effects of existing pharmaceuticals necessitate the search for alternatives. Having fewer adverse effects than classic remedies, natural extract-based therapies can be a promising solution. Here, we applied a model of emotional stress in BALB/c mice using ultrasound exposure to evoke the signs of anxiety-like behavior. We examined the behavioral and molecular impact of ultrasound and administration of herbal antioxidant/anti-inflammatory treatment (HAT) on AMPA receptor expression, markers of plasticity, inflammation and oxidative stress. A 3-week ultrasound exposure increased scores of anxiety-like behaviors in the standard tests and altered hippocampal expression as well as internalization of AMPA receptor subunits GluA1-A3. Concomitant treatment with HAT has prevented increases of anxiety-like behaviors and other behavioral changes, normalized hippocampal malondialdehyde content, GSK3β and pro-inflammatory cytokines Il-1β and Il-6, and the number of Ki67-positive cells. Levels of malondialdehyde, a common measure of oxidative stress, significantly correlated with the investigated end-points in stressed, but not in non-stressed animals. Our results emphasize the role of oxidative stress in neurobiological abnormalities associated with experimentally induced condition mimicking emotional stress in rodents and highlight the potential therapeutic use of anti-oxidants like herbal compositions for management of stress-related emotional disturbances within the community.

Acute Inescapable Stress Rapidly Increases Synaptic Energy Metabolism in Prefrontal Cortex and Alters Working Memory Performance

Brain energy metabolism actively regulates synaptic transmission and activity. We have previously shown that acute footshock (FS)-stress induces fast and long-lasting functional and morphological changes at excitatory synapses in prefrontal cortex (PFC). Here, we asked whether FS-stress increased energy metabolism in PFC, and modified related cognitive functions. Using positron emission tomography (PET), we found that FS-stress induced a redistribution of glucose metabolism in the brain, with relative decrease of [18F]FDG uptake in ventro-caudal regions and increase in dorso-rostral ones. Absolute [18F]FDG uptake was inversely correlated with serum corticosterone. Increased specific hexokinase activity was also measured in purified PFC synaptosomes (but not in total extract) of FS-stressed rats, which positively correlated with 2-Deoxy [3H] glucose uptake by synaptosomes. In line with increased synaptic energy demand, using an electron microscopy-based stereological approach, we found that acute stress induced a redistribution of mitochondria at excitatory synapses, together with an increase in their volume. The fast functional and metabolic activation of PFC induced by acute stress, was accompanied by rapid and sustained alterations of working memory performance in delayed response to T-maze test. Taken together, the present data suggest that acute stress increases energy consumption at PFC synaptic terminals and alters working memory.

Pre-reproductive stress and fluoxetine treatment in rats affect offspring A-to-I RNA editing, gene expression and social behavior

Adenosine to inosine RNA editing is an epigenetic process that entails site-specific modifications in double-stranded RNA molecules, catalyzed by adenosine deaminases acting on RNA (ADARs). Using the multiplex microfluidic PCR and deep sequencing technique, we recently showed that exposing adolescent female rats to chronic unpredictable stress before reproduction affects editing in the prefrontal cortex and amygdala of their newborn offspring, particularly at the serotonin receptor 5-HT2c (encoded by Htr2c). Here, we used the same technique to determine whether post-stress, pre-reproductive maternal treatment with fluoxetine (5 mg/kg, 7 days) reverses the effects of stress on editing. We also examined the mRNA expression of ADAR enzymes in these regions, and asked whether social behavior in adult offspring would be altered by maternal exposure to stress and/or fluoxetine. Maternal treatment with fluoxetine altered Htr2c editing in offspring amygdala at birth, enhanced the expression of Htr2c mRNA and RNA editing enzymes in the prefrontal cortex, and reversed the effects of pre-reproductive stress on Htr2c editing in this region. Furthermore, maternal fluoxetine treatment enhanced differences in editing of glutamate receptors between offspring of control and stress-exposed rats, and led to enhanced social preference in adult offspring. Our findings indicate that pre-gestational fluoxetine treatment affects patterns of RNA editing and editing enzyme expression in neonatal offspring brain in a region-specific manner, in interaction with pre-reproductive stress. Overall, these findings imply that fluoxetine treatment affects serotonergic signaling in offspring brain even when treatment is discontinued before gestation, and its effects may depend upon prior exposure to stress.

Molecular and Epigenetic Mechanisms for the Complex Effects of Stress on Synaptic Physiology and Cognitive Functions

Evidence over the past decades has found that stress, particularly through the corticosterone stress hormones, produces complex changes in glutamatergic signaling in prefrontal cortex, which leads to the alteration of cognitive processes medicated by this brain region. Interestingly, the effects of stress on glutamatergic transmission appear to be "U-shaped," depending upon the duration and severity of the stressor. These biphasic effects of acute vs chronic stress represent the adaptive vs maladaptive responses to stressful stimuli. Animal studies suggest that the stress-induced modulation of excitatory synaptic transmission involves changes in presynaptic glutamate release, postsynaptic glutamate receptor membrane trafficking and degradation, spine structure and cytoskeleton network, and epigenetic control of gene expression. This review will discuss current findings on the key molecules involved in the stress-induced regulation of prefrontal cortex synaptic physiology and prefrontal cortex-mediated functions. Understanding the molecular and epigenetic mechanisms that underlie the complex effects of stress will help to develop novel strategies to cope with stress-related mental disorders.

Disruption of GluA2 phosphorylation potentiates stress responsivity

Cocaine addiction is characterized by persistent craving and addicts frequently relapse even after long periods of abstinence. Exposure to stress can precipitate relapse in humans and rodents. Stress and drug use can lead to common alterations in synaptic plasticity and these commonalities may contribute to the ability of stress to elicit relapse. These common changes in synaptic plasticity are mediated, in part, by alterations in the trafficking and stabilization of AMPA receptors. Exposure to both cocaine and stress can lead to alterations in protein kinase C-mediated phosphorylation of GluA2 AMPA subunits and thus alter the trafficking of GluA2-containing AMPARs. However, it is not clear what role AMPAR trafficking plays in the interactions between stress and cocaine. The current study utilized a mouse with a point mutation within the GluA2 subunit c-terminus resulting in a disruption of PKC-mediated GluA2 phosphorylation to examine stress responsivity. Although no differences were seen in the response to a forced swim stress in naïve mice, GluA2 K882A knock-in mice exhibited an increased stress response following cocaine self-administration. Furthermore, we demonstrated that disrupting GluA2 phosphorylation increases vulnerability to stress-induced reinstatement of both cocaine seeking and cocaine-conditioned reward. Finally, GluA2 K882A knock-in mice exhibit an increased vulnerability to social defeat as indicated by increased social avoidance. Taken together these results indicate that disrupting GluA2 phosphorylation leads to increased responsivity to acute stress following cocaine exposure and increased vulnerability to chronic stress. These results highlight the GluA2 phosphorylation site as a novel target for the stress-related disorders.

The expression of plasticity-related genes in an acute model of stress is modulated by chronic desipramine in a time-dependent manner within medial prefrontal cortex

It is well established that stress plays a major role in the pathogenesis of neuropsychiatric diseases. Stress-induced alteration of synaptic plasticity has been hypothesized to underlie the morphological changes observed by neuroimaging in psychiatric patients in key regions such as hippocampus and prefrontal cortex (PFC). We have recently shown that a single acute stress exposure produces significant short-term alterations of structural plasticity within medial PFC. These alterations were partially prevented by previous treatment with chronic desipramine (DMI). In the present study we evaluated the effects of acute Foot-shock (FS)-stress and pre-treatment with the traditional antidepressant DMI on the gene expression of key regulators of synaptic plasticity and structure. Expression of Homer, Shank, Spinophilin, Densin-180, and the small RhoGTPase related gene Rac1 and downstream target genes, Limk1, Cofilin1 and Rock1 were investigated 1 day (1d), 7 d and 14d after FS-stress exposure. We found that DMI specifically increases the short-term expression of Spinophilin, as well as Homer and Shank family genes, and that both acute stress and DMI exert significant long-term effects on mRNA levels of genes involved in spine plasticity. These findings support the knowledge that acute FS stress and antidepressant treatment induce both rapid and sustained time-dependent alterations in structural components of synaptic plasticity in rodent medial PFC.

Ca2+-permeable AMPA receptor: A new perspective on amyloid-beta mediated pathophysiology of Alzheimer's disease

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are the primary conduits of excitatory synaptic transmission. AMPARs are predominantly Ca²⁺-impermeable in the matured excitatory synapse, except under certain circumstances. Growing evidence implicates the Ca²⁺ permeability of AMPARs in the regulation of long-term synaptic plasticity and in the pathophysiology of several neurological disorders. Therefore, the Ca²⁺ conductance of AMPARs may have both physiological and pathological roles at synapses. However, our understanding of the role of Ca²⁺ permeable AMPARs (CP-AMPARs) in Alzheimer's disease is limited. Here we discuss insights into the potential CP-AMPAR mediated pathophysiology of Alzheimer's disease, including: 1. Ca²⁺-mediated aberrant regulation of synapse weakening mechanisms, and 2. neuronal network dysfunction in the brain. Consideration of CP-AMPARs as primary drivers of pathophysiology could help in understanding synaptopathologies, and highlights the potential of CP-AMPARs as therapeutic targets in Alzheimer's disease. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.