NMDA receptors control vagal afferent excitability in the nucleus of the solitary tract

Combination of Acute and Maintenance Esketamine Treatment With Adjunctive Long-Term Vagus Nerve Stimulation in Difficult-to-Treat Depression

INTRODUCTION

The antidepressant effect of N-methyl-D-aspartate antagonists is often short lasting, raising the question of the best maintenance strategy, which has remained unanswered. Vagus nerve stimulation (VNS) as a treatment option for refractory and chronic major depression was shown to reduce the need for maintenance treatment sessions in patients receiving electroconvulsive therapy. To our knowledge, there are no published data on the combination of VNS and esketamine in the literature.

MATERIALS AND METHODS

This is a naturalistic prospective and retrospective observational study in patients treated with long-term VNS owing to difficult-to-treat depression. These patients also have received esketamine maintenance sessions in addition to short-term treatment. We have investigated the need for maintenance esketamine sessions per month after VNS implantation (number of sessions/number of months between visits), the change in depression severity (mean Montgomery-Asberg Depression Rating Scale [MADRS] score), and the number of hospitalizations per month (number of hospitalizations/number of postoperative observation months). Follow-up visits have been scheduled every three months after VNS implantation (follow-up period 12-24 months, mean 17).

RESULTS

All patients (n = 8, mean age 53.1 years) had severe difficult-to-treat depression (DTD) (mean MADRS at baseline 30.9). Mean number of hospitalizations per month decreased from 0.17 to 0.11 after VNS implantation (p = 0.041, T = 2.030, df = 7). Mean MADRS at 12 months was 18.3 (40.8% MADRS reduction, p = 0.008, T = 3.146, df = 7). Six of eight patients were offered maintenance esketamine treatment. Mean number of esketamine treatment sessions per month and case decreased from 2.3 at the six-month visit to 0.8 at 12 months (p = 0.076, T = 1,690, df = 5) after VNS implantation. Termination of maintenance esketamine was possible in four cases after a mean of 11.5 months.

CONCLUSIONS

Combination of esketamine and VNS was effective in patients with DTD to relieve disease severity and reduce hospitalizations. The need for esketamine treatment sessions decreased after 6 months of VNS. No safety concerns arose in this study regarding the combination treatment.

CLINICAL TRIAL REGISTRATION

The Clinicaltrials.gov registration number for the study is NCT03320304.

Spinal cord injury-induced neurogenic bowel: A role for host-microbiome interactions in bowel pain and dysfunction

Background and aims

Spinal cord injury (SCI) affects roughly 300,000 Americans with 17,000 new cases added annually. In addition to paralysis, 60% of people with SCI develop neurogenic bowel (NB), a syndrome characterized by slow colonic transit, constipation, and chronic abdominal pain. The knowledge gap surrounding NB mechanisms after SCI means that interventions are primarily symptom-focused and largely ineffective. The goal of the present studies was to identify mechanism(s) that initiate and maintain NB after SCI as a critical first step in the development of evidence-based, novel therapeutic treatment options.

Methods

Following spinal contusion injury at T9, we observed alterations in bowel structure and function reflecting key clinical features of NB. We then leveraged tissue-specific whole transcriptome analyses (RNAseq) and fecal 16S rRNA amplicon sequencing in combination with histological, molecular, and functional (Ca2+ imaging) approaches to identify potential mechanism(s) underlying the generation of the NB phenotype.

Results

In agreement with prior reports focused on SCI-induced changes in the skin, we observed a rapid and persistent increase in expression of calcitonin gene-related peptide (CGRP) expression in the colon. This is suggestive of a neurogenic inflammation-like process engaged by antidromic activity of below-level primary afferents following SCI. CGRP has been shown to disrupt colon homeostasis and negatively affect peristalsis and colon function. As predicted, contusion SCI resulted in increased colonic transit time, expansion of lymphatic nodules, colonic structural and genomic damage, and disruption of the inner, sterile intestinal mucus layer corresponding to increased CGRP expression in the colon. Gut microbiome colonization significantly shifted over 28 days leading to the increase in Anaeroplasma, a pathogenic, gram-negative microbe. Moreover, colon specific vagal afferents and enteric neurons were hyperresponsive after SCI to different agonists including fecal supernatants.

Conclusions

Our data suggest that SCI results in overexpression of colonic CGRP which could alter colon structure and function. Neurogenic inflammatory-like processes and gut microbiome dysbiosis can also sensitize vagal afferents, providing a mechanism for visceral pain despite the loss of normal sensation post-SCI. These data may shed light on novel therapeutic interventions targeting this process to prevent NB development in patients.

Correlations between GRIN2B and GRIN3A gene polymorphisms and postpartum depressive symptoms in Chinese parturients undergoing cesarean section: A prospective cohort study

OBJECTIVE

To investigate the association of postpartum depressive symptoms (PDS) and self-harm ideation with n-methyl-d-aspartate (NMDA) receptor GRIN2B and GRIN3A gene polymorphisms and other risk factors in women undergoing cesarean section.

METHODS

A total of 362 parturients undergoing cesarean section under lumbar anesthesia were selected and their postpartum depression level was assessed by the Edinburgh Postpartum Depression Scale (EPDS) at 42 days postpartum, with an EPDS score of 9/10 as the cut-off value. Three GRIN2B SNP loci (rs1805476, rs3026174, rs4522263) and five GRIN3A SNP loci (rs1983812, rs2050639, rs2050641, rs3739722, rs10989563) were selected for genotype detection. The role of each SNP, linkage disequilibrium and haplotypes in the development of postpartum depression was analyzed. Logistic regression analysis was performed for related risk factors.

RESULTS

PDS incidence was 16.85%, and self-harm ideation incidence was 13.54%. Univariate analysis showed that GRIN2B rs1805476, rs3026174 and rs4522263 gene polymorphisms were associated with PDS (p < 0.05), with GRIN2B rs4522263 gene also associated with maternal self-harm ideation. GRIN3A rs1983812, rs2050639, rest rs2050641, rs3739722 and rs10989563 alleles were not associated with PDS. Logistic regression analysis indicated that high pregnancy stress, as well as rs1805476 and rs4522263 alleles were PDS risk factors following cesarean delivery. GRIN2B (TTG p = 0.002) and GRIN3A (TGTTC p = 0.002) haplotypes were associated with the lower PDS incidence and higher PDS incidence respectively.

CONCLUSION

GRIN2B rs1805476 GG genotype, rs4522263 CC genotype and high stress during pregnancy were risk factors for PDS, whilst a significantly higher incidence of self-harm ideation was evident in parturients carrying GRIN2B rs4522263 CC genotype.

Investigating the possible mechanisms of autonomic dysfunction post-COVID-19

Patients with long COVID suffer from many neurological manifestations that persist for 3 months following infection by SARS-CoV-2. Autonomic dysfunction (AD) or dysautonomia is one complication of long COVID that causes patients to experience fatigue, dizziness, syncope, dyspnea, orthostatic intolerance, nausea, vomiting, and heart palpitations. The pathophysiology behind AD onset post-COVID is largely unknown. As such, this review aims to highlight the potential mechanisms by which AD occurs in patients with long COVID. The first proposed mechanism includes the direct invasion of the hypothalamus or the medulla by SARS-CoV-2. Entry to these autonomic centers may occur through the neuronal or hematogenous routes. However, evidence so far indicates that neurological manifestations such as AD are caused indirectly. Another mechanism is autoimmunity whereby autoantibodies against different receptors and glycoproteins expressed on cellular membranes are produced. Additionally, persistent inflammation and hypoxia can work separately or together to promote sympathetic overactivation in a bidirectional interaction. Renin-angiotensin system imbalance can also drive AD in long COVID through the downregulation of relevant receptors and formation of autoantibodies. Understanding the pathophysiology of AD post-COVID-19 may help provide early diagnosis and better therapy for patients.

Emerging effects of tryptophan pathway metabolites and intestinal microbiota on metabolism and intestinal function

The metabolism of dietary tryptophan occurs locally in the gut primarily via host enzymes, with ~ 5% metabolized by gut microbes. Three major tryptophan metabolic pathways are serotonin (beyond the scope of this review), indole, kynurenine and related derivatives. We introduce the gut microbiome, dietary tryptophan and the potential interplay of host and bacterial enzymes in tryptophan metabolism. Examples of bacterial transformation to indole and its derivative indole-3 propionic acid demonstrate associations with human metabolic disease and gut permeability, although causality remains to be determined. This review will focus on less well-known data, suggestive of local generation and functional significance in the gut, where kynurenine is converted to kynurenic acid and xanthurenic acid via enzymatic action present in both host and bacteria. Our functional data demonstrate a limited effect on intestinal epithelial cell monolayer permeability and on healthy mouse ileum. Other data suggest a modulatory effect on the microbiome, potentially in pathophysiology. Supportive of this, we found that the expression of mRNA for three kynurenine pathway enzymes were increased in colon from high-fat-fed mice, suggesting that this host pathway is perturbed in metabolic disease. These data, along with that from bacterial genomic analysis and germ-free mice, confirms expression and functional machinery of enzymes in this pathway. Therefore, the host and microbiota may play a significant dual role in either the production or regulation of these kynurenine metabolites which, in turn, can influence both host and microbiome, especially in the context of obesity and intestinal permeability.

Glutamate-dependent regulation of food intake is altered with age through changes in NMDA receptor phenotypes on vagal afferent neurons

Compared to younger individuals, older human subjects have significantly lower food intakes and an increased satiety response. N-methyl-d-aspartate (NMDA) receptors expressed by vagal afferent neurons originating from nodose ganglia (NG) are involved in modulating the satiety response. The present study investigated how NMDA receptor subunit phenotypes in NG neurons change with age and how these age-related alterations in food intake are modulated by presynaptic NMDA receptors in the NG of male Sprague Dawley rats (six week-old and sixty week-old). Food intake was measured at 30-, 60-, and 120-min following intraperitoneal administration of cholecystokinin (CCK) or the non-competitive NMDA receptor antagonist MK-801. Immunofluorescence was used to determine NMDA receptor subunit expression (NR1, NR2B, NR2C, and NR2D) in the NG. The results showed that, CCK reduced food intake at 30-, 60-, and 120-min post injection in both young and the middle-age animals, with no statistical difference between the groups at 30- and 60-min. In contrast, MK-801 produced an increase in food intake that was significantly higher in middle-age rats compared to young animals at all time points studied. NR1 subunit was expressed by almost all NG neurons in both age groups. In young rats, NR2B, NR2C, and NR2D subunits were expressed in 56.1%, 49.3%, and 13.9% of NG neurons, respectively. In contrast, only 30.3% of the neuronal population in middle-aged rats expressed NR2B subunit immunoreactivity, NR2C was present in 34.1%, and only 10.6% of total neurons expressed the NR2D subunit. In conclusion, glutamate-dependent regulation of food intake is altered with age and one of the potential mechanisms through which this age-related changes in intake occur is changes in NMDA receptor phenotypes on vagal afferent neurons located in NG.

A Novel Negative Allosteric Modulator Selective for GluN2C/2D-Containing NMDA Receptors Inhibits Synaptic Transmission in Hippocampal Interneurons

N-Methyl-d-aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate excitatory synaptic transmission and have been implicated in numerous neurological disorders. NMDARs typically comprise two GluN1 and two GluN2 subunits. The four GluN2 subtypes (GluN2A-GluN2D) have distinct functional properties and gene expression patterns, which contribute to diverse functional roles for NMDARs in the brain. Here, we present a series of GluN2C/2D-selective negative allosteric modulators built around a N-aryl benzamide (NAB) core. The prototypical compound, NAB-14, is >800-fold selective for recombinant GluN2C/GluN2D over GluN2A/GluN2B in Xenopus oocytes and has an IC50 value of 580 nM at recombinant GluN2D-containing receptors expressed in mammalian cells. NAB-14 inhibits triheteromeric (GluN1/GluN2A/GluN2C) NMDARs with modestly reduced potency and efficacy compared to diheteromeric (GluN1/GluN2C/GluN2C) receptors. Site-directed mutagenesis suggests that structural determinants for NAB-14 inhibition reside in the GluN2D M1 transmembrane helix. NAB-14 inhibits GluN2D-mediated synaptic currents in rat subthalamic neurons and mouse hippocampal interneurons, but has no effect on synaptic transmission in hippocampal pyramidal neurons, which do not express GluN2C or GluN2D. This series possesses some druglike physical properties and modest brain permeability in rat and mouse. Altogether, this work identifies a new series of negative allosteric modulators that are valuable tools for studying GluN2C- and GluN2D-containing NMDAR function in brain circuits, and suggests that the series has the potential to be developed into therapies for selectively modulating brain circuits involving the GluN2C and GluN2D subunits.