Altered GABA-mediated information processing and cognitive dysfunctions in depression and other brain disorders

Molecular Targets of Cannabinoids Associated with Depression

Novel therapeutic strategies are needed to address depression, a major neurological disorder affecting hundreds of millions of people worldwide. Cannabinoids and their synthetic derivatives have demonstrated numerous neurological activities and may potentially be developed into new treatments for depression. This review highlights cannabinoid (CB) receptors, monoamine oxidase (MAO), N-methyl-D-aspartate (NMDA) receptor, gamma-aminobutyric acid (GABA) receptor, and cholecystokinin (CCK) receptor as key molecular targets of cannabinoids that are associated with depression. The anti-depressant activity of cannabinoids and their binding modes with cannabinoid receptors are discussed, providing insights into rational design and discovery of new cannabinoids or cannabimimetic agents with improved druggable properties.

Amyloid Pathologies Modulate the Associations of Minimal Depressive Symptoms With Cognitive Impairments in Older Adults Without Dementia

BACKGROUND

The relationship between depression and Alzheimer's disease (AD) is complex and still not well understood. We aimed to examine the roles of the AD core pathologies in modulating the associations of minimal depressive symptoms (MDSs) with cognitive impairments.

METHODS

A total of 721 participants who had measures of cognition, depressive symptoms, and cerebrospinal fluid AD biomarkers were included from the CABLE (Chinese Alzheimer's Biomarker and LifestylE) study. Causal mediation analyses with 10,000 bootstrapped iterations were conducted to explore the mediation effects of AD pathologies on cognition. The ADNI (Alzheimer's Disease Neuroimaging Initiative) was used 1) to replicate the mediation effects and 2) to examine the longitudinal relationships of MDSs with amyloid pathology and incident AD risk.

RESULTS

In CABLE, MDSs were associated with poorer global cognition (p = .006) and higher amyloid burden as indicated by cerebrospinal fluid amyloid markers (p < .0001). The influence of MDSs on cognition was partially mediated by amyloid pathology (a maximum of 85%). The mediation effects were replicated in 725 elderly persons without dementia (age, mean ± SD = 73.5 ± 6.9 years; 301 female subjects [42%]) in ADNI, such that the mediation percentage varied from 10% to 30% for general cognition, memory, and executive functions. Longitudinal analyses revealed a bidirectional relationship between MDSs and amyloid pathology (p = .01). MDSs were associated with 83% increased risk of developing AD dementia (hazard ratio = 1.83, p < .01).

CONCLUSIONS

Overall, amyloid pathology might partially mediate and magnify the influences of MDSs on cognitive impairments and AD risk.

Behavioral Deficits Induced by Somatostatin-Positive GABA Neuron Silencing Are Rescued by Alpha 5 GABA-A Receptor Potentiation

INTRODUCTION

Deficits in somatostatin-positive gamma-aminobutyric acid interneurons (SST+ GABA cells) are commonly reported in human studies of mood and anxiety disorder patients. A causal link between SST+ cell dysfunction and symptom-related behaviors has been proposed based on rodent studies showing that chronic stress, a major risk factor for mood and anxiety disorders, induces a low SST+ GABA cellular phenotype across corticolimbic brain regions; that lowering Sst, SST+ cell, or GABA functions induces depressive-/anxiety-like behaviors (a rodent behavioral construct collectively defined as "behavioral emotionality"); and that disinhibiting SST+ cells has antidepressant-like effects. Recent studies found that compounds preferentially potentiating receptors mediating SST+ cell functions, α5-GABAA receptor positive allosteric modulators (α5-PAMs), achieved antidepressant-like effects. Together, the evidence suggests that SST+ cells regulate mood and cognitive functions that are disrupted in mood disorders and that rescuing SST+ cell function via α5-PAM may represent a targeted therapeutic strategy.

METHODS

We developed a mouse model allowing chemogenetic manipulation of brain-wide SST+ cells and employed behavioral characterization 30 minutes after repeated acute silencing to identify contributions to symptom-related behaviors. We then assessed whether an α5-PAM, GL-II-73, could rescue behavioral deficits.

RESULTS

Brain-wide SST+ cell silencing induced features of stress-related illnesses, including elevated neuronal activity and plasma corticosterone levels, increased anxiety- and anhedonia-like behaviors, and impaired short-term memory. GL-II-73 led to antidepressant- and anxiolytic-like improvements among behavioral deficits induced by brain-wide SST+ cell silencing.

CONCLUSION

Our data validate SST+ cells as regulators of mood and cognitive functions and demonstrate that bypassing low SST+ cell function via α5-PAM represents a targeted therapeutic strategy.

Exploring the Genetic Association of the ABAT Gene with Alzheimer's Disease

Accumulating evidence demonstrated that GABAergic dysfunction contributes to the pathogenesis of Alzheimer's disease (AD). The GABA aminotransferase (ABAT) gene encodes a mitochondrial GABA transaminase and plays key roles in the biogenesis and metabolism of gamma-aminobutyric acid (GABA), which is a major inhibitory neurotransmitter. In this study, we performed an integrative study at the genetic and expression levels to investigate the potential genetic association between the ABAT gene and AD. Through re-analyzing data from the currently largest meta-analysis of AD genome-wide association study (GWAS), we identified genetic variants in the 3'-UTR of ABAT as the top AD-associated SNPs (P < 1 × 10^-4) in this gene. Functional annotation of these AD-associated SNPs indicated that these SNPs are located in the regulatory regions of transcription factors or/and microRNAs. Expression quantitative trait loci (eQTL) analysis and luciferase reporter assay showed that the AD risk alleles of these SNPs were associated with a reduced expression level of ABAT. Further analysis of mRNA expression data and single-cell transcriptome data of AD patients showed that ABAT reduction in the neuron is an early event during AD development. Overall, our results indicated that ABAT genetic variants may be associated with AD through affecting its mRNA expression. An abnormal level of ABAT will lead to a disturbance of the GABAergic signal pathway in AD brains.

Mechanisms associated with the antidepressant-like effects of L-655,708

Previous research has demonstrated that selective modulation of hippocampal transmission by systemic administration of an α5-GABA_A receptor negative allosteric modulator, L-655,708, reproduces the sustained antidepressant-like (AD-like) effect of R,S-ketamine in the absence of any psychotomimetic or abuse-related effects. Pharmacological, electrophysiological (whole-cell patch clamp), and behavioral approaches were used to examine the mechanisms by which L-655,708 produces plasticity within the hippocampus that accounts for its sustained AD-like effect in rats. Inhibitors of either transcription or translation prevented the sustained AD-like effect of L-655,708. Unlike R,S-ketamine, L-655,708 did not cause an increase in the phosphorylation of the receptor for BDNF, TrkB, in the ventral hippocampus (vHipp) 30 or 60 min after its administration nor did administration of the TrkB inhibitor, K252a, directly into the vHipp, block the sustained AD-like effect of L-655,708. Similar to previous results with R,S-ketamine, administration of L-655,709 increased levels of GluA1 in the mPFC and, blockade of such receptors by direct administration of NBQX into the mPFC blocked the sustained AD-like effect of L-655,708. Patch-clamp recordings of ventral CA1 pyramidal cells 24 h after a single systemic administration of L-655,708 revealed a significant increase in input resistance, which resulted in an approximately two-fold increase in action potential frequency. These experiments indicate that the sustained AD-like effects of L-655,708 require protein synthesis and plasticity of GluA1 glutamate receptors in the mPFC. The drug also caused changes in GABA_A receptor gating properties in the vHipp with resultant changes in ventral CA1 that indirectly increases neuronal excitability. Such effects likely contribute to its sustained AD-like activity.

Blocking Astrocytic GABA Restores Synaptic Plasticity in Prefrontal Cortex of Rat Model of Depression

A decrease in synaptic plasticity and/or a change in excitation/inhibition balance have been suggested as mechanisms underlying major depression disorder. However, given the crucial role of astrocytes in balancing synaptic function, particular attention should be given to the contribution of astrocytes in these mechanisms, especially since previous findings show that astrocytes are affected and exhibit reactive-like features in depression. Moreover, it has been shown that reactive astrocytes increase the synthesis and release of GABA, contributing significantly to tonic GABA inhibition. In this study we found decreased plasticity and increased tonic GABA inhibition in the prelimbic area in acute slices from the medial prefrontal cortex in the Flinders Sensitive Line (FSL) rat model of depression. The tonic inhibition can be reduced by either blocking astrocytic intracellular Ca²⁺ signaling or by reducing astrocytic GABA through inhibition of the synthesizing enzyme MAO-B with Selegiline. Blocking GABA synthesis also restores the impaired synaptic plasticity in the FSL prefrontal cortex, providing a new antidepressant mechanism of Selegiline.

Connectivity of the Frontal Cortical Oscillatory Dynamics Underlying Inhibitory Control During a Go/No-Go Task as a Predictive Biomarker in Major Depression

BACKGROUND

Major depressive disorder (MDD) is characterized by core functional deficits in cognitive inhibition, which is crucial for emotion regulation. To assess the response to ruminative and negative mood states, it was hypothesized that MDD patients have prolonged disparities in the oscillatory dynamics of the frontal cortical regions across the life course of the disease.

METHOD

A "go/no-go" response inhibition paradigm was tested in 31 MDD patients and 19 age-matched healthy controls after magnetoencephalography (MEG) scanning. The use of minimum norm estimates (MNE) examined the changes of inhibitory control network which included the right inferior frontal gyrus (rIFG), pre-supplementary motor area (preSMA), and left primary motor cortex (lM1). The power spectrum (PS) within each node and the functional connectivity (FC) between nodes were compared between two groups. Furthermore, Pearson correlation was calculated to estimate the relationship between altered FC and clinical features.

RESULT

PS was significantly reduced in left motor and preSMA of MDD patients in both beta (13-30 Hz) and low gamma (30-50 Hz) bands. Compared to the HC group, the MDD group demonstrated higher connectivity between lM1 and preSMA in the beta band (t = 3.214, p = 0.002, FDR corrected) and showed reduced connectivity between preSMA and rIFG in the low gamma band (t = -2.612, p = 0.012, FDR corrected). The FC between lM1 and preSMA in the beta band was positively correlated with illness duration (r = 0.475, p = 0.005, FDR corrected), while the FC between preSMA and rIFG in the low gamma band was negatively correlated with illness duration (r = -0.509, p = 0.002, FDR corrected) and retardation factor scores (r = -0.288, p = 0.022, uncorrected).

CONCLUSION

In this study, a clinical neurophysiological signature of cognitive inhibition leading to sustained negative affect as well as functional non-recovery in MDD patients is highlighted. Duration of illness (DI) plays a key role in negative emotional processing, heighten rumination, impulsivity, and disinhibition.