Why do some individuals succumb to stress and develop debilitating psychiatric disorders, whereas others adapt well in the face of adversity? There is a gap in understanding the neural bases of individual differences in the responses to environmental factors on brain development and functions. Here, using a novel approach for screening an inbred population of laboratory animals, we identified two sub-populations of mice: susceptible mice that show mood-related abnormalities compared to resilient mice, which cope better with stress. This approach combined with molecular and behavioral analyses, led us to recognize, in hippocampus, presynaptic mGlu2 receptors, which inhibit glutamate release, as a stress sensitive marker of individual differences to stress-induced mood disorders. Indeed genetic mGlu2 deletion in mice results in a more severe susceptibility to stress, mimicking the susceptible mouse sub-population. Furthermore, we describe an underlying mechanism by which glucocorticoids, acting via mineralocorticoid receptors (MR), decrease resilience to stress via down regulation of mGlu2 receptors. We also provide a mechanistic link between MR and an epigenetic control of the glutamatergic synapse that underlies susceptibility to stressful experiences. The approach and the epigenetic allostasis concept introduced here serve as a model for identifying individual differences based upon biomarkers and underlying mechanisms and also provide molecular features that may be useful in translation to human behavior and psychopathology.

Excitatory amino acids play a key role in both adaptive and deleterious effects of stressors on the brain, and dysregulated glutamate homeostasis has been associated with psychiatric and neurological disorders. Here, we elucidate mechanisms of epigenetic plasticity in the hippocampus in the interactions between a history of chronic stress and familiar and novel acute stressors that alter expression of anxiety-and depressive-like behaviors. We demonstrate that acute restraint and acute forced swim stressors induce differential effects on these behaviors in naive mice and in mice with a history of chronic-restraint stress (CRS). They reveal a key role for epigenetic up-and down-regulation of the putative presynaptic type 2 metabotropic glutamate (mGlu2) receptors and the postsynaptic NR1/NMDA receptors in the hippocampus and particularly in the dentate gyrus (DG), a region of active neurogenesis and a target of antidepressant treatment. We show changes in DG long-term potentiation (LTP) that parallel behavioral responses, with habituation to the same acute restraint stressor and sensitization to a novel forced-swim stressor. In WT mice after CRS and in unstressed mice with a BDNF loss-of-function allele (BDNF Val66Met), we show that the epigenetic activator of histone acetylation, P300, plays a pivotal role in the dynamic up-and down-regulation of mGlu2 in hippocampus via histone-3-lysine-27-acetylation (H3K27Ac) when acute stressors are applied. These hippocampal responses reveal a window of epigenetic plasticity that may be useful for treatment of disorders in which glutamatergic transmission is dysregulated.S tress effects on higher brain regions, such as hippocampus, are known to involve actions of excitatory amino acids to induce structural and functional changes depending upon the type, intensity, and duration of the stressor (1). These differential responses, including determining susceptibility versus resilience to stress, contribute to the pathophysiology of debilitating stress-related disorders (2-5). The hippocampus is a brain region noted for its plasticity in response to stress and sensitivity to adrenal steroid hormones (6). Acute stress enhances synaptic plasticity that is associated with improved cognition and other adaptive functions whereas chronic stress produces opposite effects mediating, in the hippocampus, spine synapse turnover, dendritic shrinkage, impaired long-term potentiation (LTP), and suppression of adult neurogenesis in the dentate gyrus (DG) (1, 7). Importantly, neuroanatomical changes in response to repeated stress recover in young adult animals, based upon the restoration of dendritic length and branching and spine density (8). However, there are underlying changes that can be seen at the level of gene expression and epigenetic regulation that indicate that the brain is continually changing (9, 10). Epigenetic modifications, such as acetylation of histones, have also been involved in the consolidation of contextual memories that allow the brain to respond and adapt to changes in ...

We demonstrate that stress differentially regulates glutamate homeostasis in the dorsal and ventral hippocampus and identify a role for the astroglial xCT in ventral dentate gyrus (vDG) in stress and antidepressant responses. We provide an RNA-seq roadmap for the stress-sensitive vDG. The transcription factor REST binds to xCT promoter in co-occupancy with the epigenetic marker H3K27ac to regulate expression of xCT, which is also reduced in a genetic mouse model of inherent susceptibility to depressive-like behavior. Pharmacologically, modulating histone acetylation with acetyl-L-carnitine (LAC) or acetyl-N-cysteine (NAC) rapidly increases xCT and activates a network with mGlu2 receptors to prime an enhanced glutamate homeostasis that promotes both pro-resilient and antidepressant-like responses. Pharmacological xCT blockage counteracts NAC prophylactic effects. GFAP-Cre-dependent overexpression of xCT in vDG mimics pharmacological actions in promoting resilience. This work establishes a mechanism by which vDG protection leads to stress resilience and antidepressant responses via epigenetic programming of an xCT-mGlu2 network.

Although regulation of energy metabolism has been linked with multiple disorders, its role in depression and responsiveness to antidepressants is less known. We found that an epigenetic and energetic agent, acetyl-L-carnitine (LAC, oral administration), rapidly rescued the depressiveand central and systemic metabolic-like phenotype of LAC-deficient Flinders Sensitive Line rats (FSL). After acute stress during LAC treatment, a subset of FSL continued to respond to LAC (rFSL), whereas the other subset did not (nrFSL). RNA sequencing of the ventral dentate gyrus, a mood-regulatory region, identified metabolic factors as key markers predisposing to depression (insulin receptors Insr, glucose transporters Glut-4 and Glut-12, and the regulator of appetite Cartpt) and to LAC responsiveness (leptin receptors Lepr, metabotropic glutamate receptors-2 mGlu2, neuropeptide-Y NPY, and mineralocorticoid receptors MR). Furthermore, we found that stress-induced treatment resistance in nrFSL shows a new gene profile, including the metabolic regulator factors elongation of long chain fatty acids 7 (Elovl7) and cytochrome B5 reductase 2 (Cyb5r2) and the synaptic regulator NPAS4. Finally, while improving central energy regulation and exerting rapid antidepressant-like effects, LAC corrected a systemic hyperinsulinemia and hyperglicemia in rFSL and failed to do that in nrFSL. These findings establish CNS energy regulation as a factor to be considered for the development of better therapeutics. Agents such as LAC that regulate metabolic factors and reduce glutamate overflow could rapidly ameliorate depression and could also be considered for treatment of insulin resistance in depressed subjects. The approach here serves as a model for identifying markers and underlying mechanisms of predisposition to diseases and treatment responsiveness that may be useful in translation to human behavior and psychopathology. metabolic factors | acetylcarnitine | dentate gyrus | insulin | RNAseq P revious research has shown that agents that influence energy homeostasis, such as the epigenetic molecule acetyl-L-carnitine (LAC), exert rapid antidepressant-like effects by correcting imbalance of the glutamate system in the hippocampus of a genetic rat model of depression, Flinders Sensitive Line rats (FSL), and in a mouse model of stress-induced depressive-like traits (1-3). LAC, which passes through the blood-brain barrier, is a nutritional supplement and is also synthetized in the brain, liver, and kidney (4). Among its beneficial effects on the brain and the body, LAC regulates mitochondrial metabolism by facilitating transfer of fatty acids from the cytosol to the mitochondrial matrix for subsequent β-oxidation (5), needed for energy metabolism, deficits of which have been associated with a variety of diseases, including psychiatric disorders (6). However, the role of energy regulation in depression and in the responsiveness and/or resistance to antidepressants is less known.Because the ventral dentate gyrus (vDG) has a key role in regulating resilie...

The adult brain is capable of adapting to internal and external stressors by undergoing structural plasticity, and failure to be resilient and preserve normal structure and function is likely to contribute to depression and anxiety disorders. While the hippocampus has provided the gateway for understanding stress effects on the brain, less is known about the amygdala, a key brain area involved in the neural circuitry of fear and anxiety. Here, in mice more vulnerable to stressors, we demonstrate structural plasticity within the medial and basolateral regions of the amygdala in response to prolonged 21day chronic restraint stress (CRS). Three days before the end of CRS, treatment with the putative, rapidly acting antidepressant, acetyl-L-carnitine (LAC) in the drinking water opposed the direction of these changes. Behaviorally, the LAC treatment during the last part of CRS enhanced resilience, opposing the effects of CRS, as shown by an increased social interaction and reduced passive behavior in a forced swim test. Furthermore, CRS mice treated with LAC show resilience of the CRS-induced structural remodeling of medial amygdala (MeA) stellate neurons. Within the basolateral (BLA) amygdala, LAC did not reduce, but slightly enhanced, the CRS-increased length and number of intersections of pyramidal neurons. No structural changes were observed in MeA bipolar neurons, BLA stellate neurons, or in lateral amygdala (LA) stellate neurons. Our findings identify MeA stellate neurons as an important component in the responses to stress and LAC action and show that LAC can promote structural plasticity of the MeA. This may be useful as a model for increasing resilience to stressors in at risk populations.