Neural Substrates of Depression and Resilience

Glutamatergic Neurotransmission: Pathway to Developing Novel Rapid-Acting Antidepressant Treatments

The underlying neurobiological basis of major depressive disorder remains elusive due to the severity, complexity, and heterogeneity of the disorder. While the traditional monoaminergic hypothesis has largely fallen short in its ability to provide a complete picture of major depressive disorder, emerging preclinical and clinical findings suggest that dysfunctional glutamatergic neurotransmission may underlie the pathophysiology of both major depressive disorder and bipolar depression. In particular, recent studies showing that a single intravenous infusion of the glutamatergic modulator ketamine elicits fast-acting, robust, and relatively sustained antidepressant, antisuicidal, and antianhedonic effects in individuals with treatment-resistant depression have prompted tremendous interest in understanding the mechanisms responsible for ketamine's clinical efficacy. These results, coupled with new evidence of the mechanistic processes underlying ketamine's effects, have led to inventive ways of investigating, repurposing, and expanding research into novel glutamate-based therapeutic targets with superior antidepressant effects but devoid of dissociative side effects. Ketamine's targets include noncompetitive N-methyl-D-aspartate receptor inhibition, α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid throughput potentiation coupled with downstream signaling changes, and N-methyl-D-aspartate receptor targets localized on gamma-aminobutyric acid-ergic interneurons. Here, we review ketamine and other potentially novel glutamate-based treatments for treatment-resistant depression, including N-methyl-D-aspartate receptor antagonists, glycine binding site ligands, metabotropic glutamate receptor modulators, and other glutamatergic modulators. Both the putative mechanisms of action of these agents and clinically relevant studies are described.

Elimination of vesicular zinc alters the behavioural and neuroanatomical effects of social defeat stress in mice

Chronic stress can have deleterious effects on mental health, increasing the risk of developing depression or anxiety. But not all individuals are equally affected by stress; some are susceptible while others are more resilient. Understanding the mechanisms that lead to these differing outcomes has been a focus of considerable research. One unexplored mechanism is vesicular zinc – zinc that is released by neurons as a neuromodulator. We examined how chronic stress, induced by repeated social defeat, affects mice that lack vesicular zinc due to genetic deletion of zinc transporter 3 (ZnT3). These mice, unlike wild type mice, did not become socially avoidant of a novel conspecific, suggesting resilience to stress. However, they showed enhanced sensitivity to the potentiating effect of stress on cued fear memory. Thus, the contribution of vesicular zinc to stress susceptibility is not straightforward. Stress also increased anxiety-like behaviour but produced no deficits in a spatial Y-maze test. We found no evidence that microglial activation or hippocampal neurogenesis accounted for the differences in behavioural outcome. Volumetric analysis revealed that ZnT3 KO mice have larger corpus callosum and parietal cortex volumes, and that corpus callosum volume was decreased by stress in ZnT3 KO, but not wild type, mice.

New Perspectives on the Use of Cannabis in the Treatment of Psychiatric Disorders

Following the discovery of the endocannabinoid system and its potential as a therapeutic target for various pathological conditions, growing interest led researchers to investigate the role of cannabis and its derivatives for medical purposes. The compounds Δ9-tetrahydrocannabinol and cannabidiol are the most abundant phytocannabinoids found in cannabis extracts, as well as the most studied. The present review aims to provide an overview of the current evidence for their beneficial effects in treating psychiatric disorders, including schizophrenia, anxiety, and depression. Nevertheless, further investigations are required to clarify many pending issues, especially those relative to the assessment of benefits and risks when using cannabis for therapeutic purposes, thereby also helping national and federal jurisdictions to remain updated.

Parkinson's Disease-Linked LRRK2-G2019S Mutation Alters Synaptic Plasticity and Promotes Resilience to Chronic Social Stress in Young Adulthood

The G2019S mutation in leucine-rich repeat kinase 2 (LRRK2) is a prevalent cause of late-onset Parkinson's disease, producing psychiatric and motor symptoms, including depression, that are indistinguishable from sporadic cases. Here we tested how this mutation impacts depression-related behaviors and associated synaptic responses and plasticity in mice expressing a Lrrk2-G2019S knock-in mutation. Young adult male G2019S knock-in and wild-type mice were subjected to chronic social defeat stress (CSDS), a validated depression model, and other tests of anhedonia, anxiety, and motor learning. We found that G2019S mice were highly resilient to CSDS, failing to exhibit social avoidance compared to wild-type mice, many of which exhibited prominent social avoidance and were thus susceptible to CSDS. In the absence of CSDS, no behavioral differences between genotypes were found. Whole-cell recordings of spiny projection neurons (SPNs) in the nucleus accumbens revealed that glutamatergic synapses in G2019S mice lacked functional calcium-permeable AMPARs, and following CSDS, failed to accumulate inwardly rectifying AMPAR responses characteristic of susceptible mice. Based on this abnormal AMPAR response profile, we asked whether long-term potentiation (LTP) of corticostriatal synaptic strength was affected. We found that both D₁ receptor (D₁R)- and D₂R-SPNs in G2019S mutants were unable to express LTP, with D₂R-SPNs abnormally expressing long-term depression following an LTP-induction protocol. Thus, G2019S promotes resilience to chronic social stress in young adulthood, likely reflecting synapses constrained in their ability to undergo experience-dependent plasticity. These unexpected findings may indicate early adaptive coping mechanisms imparted by the G2019S mutation.SIGNIFICANCE STATEMENT The G2019S mutation in LRRK2 causes late-onset Parkinson's disease (PD). LRRK2 is highly expressed in striatal neurons throughout life, but it is unclear how mutant LRRK2 affects striatal neuron function and behaviors in young adulthood. We addressed this question using Lrrk2-G2019S knock-in mice. The data show that young adult G2019S mice were unusually resilient to a depression-like syndrome resulting from chronic social stress. Further, mutant striatal synapses were incapable of forms of synaptic plasticity normally accompanying depression-like behavior and important for supporting the full range of cognitive function. These data suggest that in humans, LRRK2 mutation may affect striatal circuit function in ways that alter normal responses to stress and could be relevant for treatment strategies for non-motor PD symptoms.

Biological and Psychological Perspectives of Resilience: Is It Possible to Improve Stress Resistance?

The term “resilience” refers to the ability to adapt successfully to stress, trauma and adversity, enabling individuals to avoid stress-induced mental disorders such as depression, posttraumatic stress disorder (PTSD) and anxiety. Here, we review evidence from both animal models and humans that is increasingly revealing the neurophysiological and neuropsychological mechanisms that underlie stress susceptibility, as well as active mechanisms underlying the resilience phenotype. Ultimately, this growing understanding of the neurobiological mechanisms of resilience should result in the development of novel interventions that specifically target neural circuitry and brain areas that enhance resilience and lead to more effective treatments for stress-induced disorders. Stress resilience can be improved, but the outcomes and effects depend on the type of intervention and the species treated.

Genetic Control of Myelin Plasticity after Chronic Psychosocial Stress

Anxiety disorders often manifest in genetically susceptible individuals after psychosocial stress, but the mechanisms underlying these gene-environment interactions are largely unknown. We used the chronic social defeat stress (CSDS) mouse model to study resilience and susceptibility to chronic psychosocial stress. We identified a strong genetic background effect in CSDS-induced social avoidance (SA) using four inbred mouse strains: 69% of C57BL/6NCrl (B6), 23% of BALB/cAnNCrl, 19% of 129S2/SvPasCrl, and 5% of DBA/2NCrl (D2) mice were stress resilient. Furthermore, different inbred mouse strains responded differently to stress, suggesting they use distinct coping strategies. To identify biological pathways affected by CSDS, we used RNA-sequencing (RNA-seq) of three brain regions of two strains, B6 and D2: medial prefrontal cortex (mPFC), ventral hippocampus (vHPC), and bed nucleus of the stria terminalis (BNST). We discovered overrepresentation of oligodendrocyte (OLG)-related genes in the differentially expressed gene population. Because OLGs myelinate axons, we measured myelin thickness and found significant region and strain-specific differences. For example, in resilient D2 mice, mPFC axons had thinner myelin than controls, whereas susceptible B6 mice had thinner myelin than controls in the vHPC. Neither myelin-related gene expression in several other regions nor corpus callosum thickness differed between stressed and control animals. Our unbiased gene expression experiment suggests that myelin plasticity is a substantial response to chronic psychosocial stress, varies across brain regions, and is genetically controlled. Identification of genetic regulators of the myelin response will provide mechanistic insight into the molecular basis of stress-related diseases, such as anxiety disorders, a critical step in developing targeted therapy.

Stress-Induced Depression: Is Social Rank a Predictive Risk Factor?

An intriguing question in the field of stress is what makes an individual more likely to be susceptible or resilient to stress-induced depression. Predisposition to stress susceptibility is believed to be influenced by genetic factors and early adversity. However, beyond genetics and life experiences, recent evidence has highlighted social rank as a key determinant of susceptibility to stress, underscoring dominant individuals as the vulnerable ones. This evidence is in conflict with epidemiological, clinical, and animal work pointing at a link between social subordination and depression. Here, we review and analyze rodent protocols addressing the relevance of social rank to predict vulnerability to chronic social stress. We also discuss whether a specific social status (i.e., dominance or subordination) is the appropriate predictor of vulnerability to develop stress-induced depression or rather, the loss of social rank and resources.

Using Cognitive Neuroscience to Improve Mental Health Treatment: A Comprehensive Review

Mental health interventions do not yet offer complete, client-defined functional recovery, and novel directions in treatment research are needed to improve the efficacy of available interventions. One promising direction is the integration of social work and cognitive neuroscience methods, which provides new opportunities for clinical intervention research that will guide development of more effective mental health treatments that holistically attend to the biological, social, and environmental contributors to disability and recovery. This article reviews emerging trends in cognitive neuroscience and provides examples of how these advances can be used by social workers and allied professions to improve mental health treatment. We discuss neuroplasticity, which is the dynamic and malleable nature of the brain. We also review the use of risk and resiliency biomarkers and novel treatment targets based on neuroimaging findings to prevent disability, personalize treatment, and make interventions more targeted and effective. The potential of treatment research to contribute to neuroscience discoveries regarding brain change is considered from the experimental-medicine approach adopted by the National Institute of Mental Health. Finally, we provide resources and recommendations to facilitate the integration of cognitive neuroscience into mental health research in social work.

NPAS2 Regulation of Anxiety-Like Behavior and GABAA Receptors

Abnormal circadian rhythms and circadian genes are strongly associated with several psychiatric disorders. Neuronal PAS Domain Protein 2 (NPAS2) is a core component of the molecular clock that acts as a transcription factor and is highly expressed in reward- and stress-related brain regions such as the striatum. However, the mechanism by which NPAS2 is involved in mood-related behaviors is still unclear. We measured anxiety-like behaviors in mice with a global null mutation in Npas2 (Npas2 null mutant mice) and found that Npas2 null mutant mice exhibit less anxiety-like behavior than their wild-type (WT) littermates (in elevated plus maze, light/dark box and open field assay). We assessed the effects of acute or chronic stress on striatal Npas2 expression, and found that both stressors increased levels of Npas2. Moreover, knockdown of Npas2 in the ventral striatum resulted in a similar reduction of anxiety-like behaviors as seen in the Npas2 null mutant mouse. Additionally, we identified Gabra genes as transcriptional targets of NPAS2, found that Npas2 null mutant mice exhibit reduced sensitivity to the GABAa positive allosteric modulator, diazepam and that knockdown of Npas2 reduced Gabra1 expression and response to diazepam in the ventral striatum. These results: (1) implicate Npas2 in the response to stress and the development of anxiety; and (2) provide functional evidence for the regulation of GABAergic neurotransmission by NPAS2 in the ventral striatum.

HCN Channel Targets for Novel Antidepressant Treatment

Major depressive disorder (MDD) is a chronic and potentially life threatening illness that carries a staggering global burden. Characterized by depressed mood, MDD is often difficult to diagnose and treat owing to heterogeneity of syndrome and complex etiology. Contemporary antidepressant treatments are based on improved monoamine-based formulations from serendipitous discoveries made > 60 years ago. Novel antidepressant treatments are necessary, as roughly half of patients using available antidepressants do not see long-term remission of depressive symptoms. Current development of treatment options focuses on generating efficacious antidepressants, identifying depression-related neural substrates, and better understanding the pathophysiological mechanisms of depression. Recent insight into the brain's mesocorticolimbic circuitry from animal models of depression underscores the importance of ionic mechanisms in neuronal homeostasis and dysregulation, and substantial evidence highlights a potential role for ion channels in mediating depression-related excitability changes. In particular, hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are essential regulators of neuronal excitability. In this review, we describe seminal research on HCN channels in the prefrontal cortex and hippocampus in stress and depression-related behaviors, and highlight substantial evidence within the ventral tegmental area supporting the development of novel therapeutics targeting HCN channels in MDD. We argue that methods targeting the activity of reward-related brain areas have significant potential as superior treatments for depression.