Galanin and Neuropeptide Y Interaction Enhances Proliferation of Granule Precursor Cells and Expression of Neuroprotective Factors in the Rat Hippocampus with Consequent Augmented Spatial Memory

Increased galanin-galanin receptor 1 signaling, inflammation, and insulin resistance are associated with affective symptoms and chronic fatigue syndrome due to long COVID

BACKGROUND

Patients with Long COVID (LC) often experience neuropsychiatric symptoms such as depression, anxiety, and chronic fatigue syndrome (CFS), collectively referred to as the physio-affective phenome of LC. Activated immune-inflammatory pathways and insulin resistance significantly contribute to the physio-affective phenome associated with LC.

METHODS

In a cohort of 90 individuals, categorized into those with and without LC, we evaluated, 3-6 months following acute SARS-CoV-2 infection, the correlations between the Hamilton Depression (HAMD), Hamilton Anxiety (HAMA), and Fibro-Fatigue (FF) Rating Scale scores, and serum C-reactive protein (CRP), prostaglandin E2 (PGE2), galanin-galanin receptor 1 (GAL-GALR1) signaling, insulin resistance, insulin-like growth factor (IGF-1), plasminogen activator inhibitor-1 (PAI1), S100B and neuron-specific enolase (NSE).

RESULTS

HAMD, HAMA, FF scores, CRP, PGE2, GAL-GALR1 signaling, insulin resistance, PAI1, NSE, and S100B are all higher in people with LC compared to those without LC. The HAMD/HAMA/FF scores were significantly correlated with PGE, CRP, GAL, GALR1, insulin resistance, and PAI1 levels, and a composite score based on peak body temperature (PBT) - oxygen saturation (SpO2) (PBT/SpO2 index) during the acute infectious phase. A combination of biomarkers explained a large part of the variance in CFS and affective scores (33.6%-42.0%), with GAL-GALR1 signaling, PGE2, and CRP being the top 3 most important biomarkers. The inclusion of the PBT/SpO2 index increased the prediction (55.3%-67.1%). The PBT/SpO2 index predicted the increases in GAL-GALR1 signaling.

CONCLUSION

These results indicate that the CFS and affective symptoms that are linked to LC are the consequence of metabolic aberrations, activated immune-inflammatory pathways, and the severity of inflammation during the acute phase of SARS-CoV-2 infection.

Oxidative and Excitatory Neurotoxic Stresses in CRISPR/Cas9-Induced Kynurenine Aminotransferase Knockout Mice: A Novel Model for Despair-Based Depression and Post-Traumatic Stress Disorder

BACKGROUNDS

Memory and emotion are especially vulnerable to psychiatric disorders such as post-traumatic stress disorder (PTSD), which is linked to disruptions in serotonin (5-HT) metabolism. Over 90% of the 5-HT precursor tryptophan (Trp) is metabolized via the Trp-kynurenine (KYN) metabolic pathway, which generates a variety of bioactive molecules. Dysregulation of KYN metabolism, particularly low levels of kynurenic acid (KYNA), appears to be linked to neuropsychiatric disorders. The majority of KYNA is produced by the aadat (kat2) gene-encoded mitochondrial kynurenine aminotransferase (KAT) isotype 2. Little is known about the consequences of deleting the KYN enzyme gene.

METHODS

In CRISPR/Cas9-induced aadat knockout (kat2-/-) mice, we examined the effects on emotion, memory, motor function, Trp and its metabolite levels, enzyme activities in the plasma and urine of 8-week-old males compared to wild-type mice.

RESULTS

Transgenic mice showed more depressive-like behaviors in the forced swim test, but not in the tail suspension, anxiety, or memory tests. They also had fewer center field and corner entries, shorter walking distances, and fewer jumping counts in the open field test. Plasma metabolite levels are generally consistent with those of urine: antioxidant KYNs, 5-hydroxyindoleacetic acid, and indole-3-acetic acid levels were lower; enzyme activities in KATs, kynureninase, and monoamine oxidase/aldehyde dehydrogenase were lower, but kynurenine 3-monooxygenase was higher; and oxidative stress and excitotoxicity indices were higher. Transgenic mice displayed depression-like behavior in a learned helplessness model, emotional indifference, and motor deficits, coupled with a decrease in KYNA, a shift of Trp metabolism toward the KYN-3-hydroxykynurenine pathway, and a partial decrease in the gut microbial Trp-indole pathway metabolite.

CONCLUSIONS

This is the first evidence that deleting the aadat gene induces depression-like behaviors uniquely linked to experiences of despair, which appear to be associated with excitatory neurotoxic and oxidative stresses. This may lead to the development of a double-hit preclinical model in despair-based depression, a better understanding of these complex conditions, and more effective therapeutic strategies by elucidating the relationship between Trp metabolism and PTSD pathogenesis.

Prenatal ethanol and cannabis exposure have sex- and region-specific effects on somatostatin and neuropeptide Y interneurons in the rat hippocampus

BACKGROUND

Cannabis is increasingly being legalized and socially accepted around the world and is often used with alcohol in social settings. We recently showed that in utero exposure to both substances can alter the density of parvalbumin-expressing interneurons in the hippocampus. Here we investigate the effects of in utero alcohol and cannabis exposure, alone or in combination, on somatostatin- and neuropeptide Y-positive (NPY) interneurons. These are separate classes of interneurons important for network synchrony and inhibition in the hippocampus.

METHODS

A 2 (Ethanol, Air) × 2 (tetrahydrocannabinol [THC], Vehicle) design was used to expose pregnant Sprague-Dawley rats to either ethanol or air, in addition to either THC or the inhalant vehicle solution, during gestational days 5-20. Immunohistochemistry for somatostatin- and NPY-positive interneurons was performed in 50 μm tissue sections obtained at postnatal day 70.

RESULTS

Exposure to THC in utero had region-specific and sex-specific effects on the density of somatostatin-positive interneurons in the adult rat hippocampus. A female-specific decrease in NPY interneuron cell density was observed in the CA1 region following THC exposure. Combined exposure to alcohol and THC reduced NPY neurons selectively in the ventral dentate gyrus hippocampal subfield. However, overall, co-exposure to alcohol and cannabis had neither additive nor synergistic effects on interneuron populations in other areas of the hippocampus.

CONCLUSIONS

These results illustrate how alcohol and cannabis exposure in utero may affect hippocampal function by altering inhibitory processes in a sex-specific manner.

Enhancing Cognitive Functions and Neuronal Growth through NPY1R Agonist and Ketamine Co-Administration: Evidence for NPY1R-TrkB Heteroreceptor Complexes in Rats

This study investigates the combined effects of the neuropeptide Y Y1 receptor (NPY1R) agonist [Leu31-Pro34]NPY at a dose of 132 µg and Ketamine at 10 mg/Kg on cognitive functions and neuronal proliferation, against a backdrop where neurodegenerative diseases present an escalating challenge to global health systems. Utilizing male Sprague-Dawley rats in a physiological model, this research employed a single-dose administration of these compounds and assessed their impact 24 h after treatment on object-in-place memory tasks, alongside cellular proliferation within the dorsal hippocampus dentate gyrus. Methods such as the in situ proximity ligation assay and immunohistochemistry for proliferating a cell nuclear antigen (PCNA) and doublecortin (DCX) were utilized. The results demonstrated that co-administration significantly enhanced memory consolidation and increased neuronal proliferation, specifically neuroblasts, without affecting quiescent neural progenitors and astrocytes. These effects were mediated by the potential formation of NPY1R-TrkB heteroreceptor complexes, as suggested by receptor co-localization studies, although further investigation is required to conclusively prove this interaction. The findings also highlighted the pivotal role of brain-derived neurotrophic factor (BDNF) in mediating these effects. In conclusion, this study presents a promising avenue for enhancing cognitive functions and neuronal proliferation through the synergistic action of the NPY1R agonist and Ketamine, potentially via NPY1R-TrkB heteroreceptor complex formation, offering new insights into therapeutic strategies for neurodegenerative diseases.

Enhanced neuronal survival and BDNF elevation via long-term co-activation of galanin 2 (GALR2) and neuropeptide Y1 receptors (NPY1R): potential therapeutic targets for major depressive disorder

BACKGROUND

Major Depressive Disorder (MDD) is a prevalent and debilitating condition, necessitating novel therapeutic strategies due to the limited efficacy and adverse effects of current treatments. We explored how galanin receptor 2 (GALR2) and Neuropeptide Y1 Receptor (NPYY1R) agonists, working together, can boost brain cell growth and increase antidepressant-like effects in rats. This suggests new ways to treat Major Depressive Disorder (MDD).

RESEARCH DESIGN AND METHODS

In a controlled laboratory setting, adult naive Sprague-Dawley rats were administered directly into the brain's ventricles, a method known as intracerebroventricular (ICV) administration, with GALR2 agonist (M1145), NPYY1R agonist, both, or in combination with a GALR2 antagonist (M871). Main outcome measures included long-term neuronal survival, differentiation, and behavioral.

RESULTS

Co-administration of M1145 and NPYY1R agonist significantly enhanced neuronal survival and maturation in the ventral dentate gyrus, with a notable increase in Brain-Derived Neurotrophic Factor (BDNF) expression. This neurogenic effect was associated with an antidepressant-like effect, an outcome partially reversed by M871.

CONCLUSIONS

GALR2 and NPYY1R agonists jointly promote hippocampal neurogenesis and exert antidepressant-like effects in rats without adverse outcomes, highlighting their therapeutic potential for MDD. The study's reliance on an animal model and intracerebroventricular delivery warrants further clinical exploration to confirm these promising results.

Potentiation of antidepressant effects: NPY1R agonist and ketamine synergy enhances TrkB signaling and neurogenesis in the ventral hippocampus

BACKGROUND

Major Depressive Disorder (MDD) poses a significant challenge to global health, with current treatments often limited by efficacy and onset delays. This study explores the synergistic antidepressant-like effects of an NPY1R agonist and Ketamine, targeting their neurobiological interactions within the ventral hippocampus.

RESEARCH DESIGN AND METHODS

Utilizing a preclinical model, this study administered Neuropeptide Y receptor 1 (NPY1R) agonist and Ketamine, both separately and in combination, through intracerebroventricular (icv) and intranasal (i.n.) routes. The Forced Swimming Test (FST) was employed to assess antidepressant-like activity, while in situ Proximity Ligation Assay and immunohistochemistry were used to examine NPY1R/TrkB heteroreceptor complexes and BDNF expression in the ventral dentate gyrus (DG), along with neurogenesis markers.

RESULTS

The combined treatment significantly reduced immobility in the FST, indicative of enhanced antidepressant-like effects, correlated with increased formation of NPY1R/TrkB complex and brain-derived neurotrophic factor (BDNF) expression in the ventral DG. These molecular alterations were associated with increased neurogenesis.

CONCLUSIONS

The coadministration of an NPY1R agonist and Ketamine in a rodent model demonstrated potentiated antidepressant responses through synergistic neurobiological pathways, including TrkB signaling and hippocampal neurogenesis. This indicates a novel therapeutic strategy for MDD, warranting further clinical investigation to fully understand its implications.

Enhancement of neurogenesis and cognition through intranasal co-delivery of galanin receptor 2 (GALR2) and neuropeptide Y receptor 1 (NPY1R) agonists: a potential pharmacological strategy for cognitive dysfunctions

BACKGROUND

Spatial memory deficits and reduced neuronal survival contribute to cognitive decline seen in the aging process. Current treatments are limited, emphasizing the need for innovative therapeutic strategies. This research explored the combined effects of intranasally co-administered galanin receptor 2 (GALR2) and neuropeptide Y1 receptor (NPY1R) agonists, recognized for their neural benefits, on spatial memory, neuronal survival, and differentiation in adult rats. After intranasal co-delivery of the GALR2 agonist M1145 and a NPY1R agonist to adult rats, spatial memory was tested with the object-in-place task 3 weeks later. We examined neuronal survival and differentiation by assessing BrdU-IR profiles and doublecortin (DCX) labeled cells, respectively. We also used the GALR2 antagonist M871 to confirm GALR2's crucial role in promoting cell growth.

RESULTS

Co-administration improved spatial memory and increased the survival rate of mature neurons. The positive effect of GALR2 in cell proliferation was confirmed by the nullifying effects of its antagonist. The treatment boosted DCX-labeled newborn neurons and altered dendritic morphology, increasing cells with mature dendrites.

CONCLUSIONS

Our results show that intranasal co-delivery of GALR2 and NPY1R agonists improves spatial memory, boosts neuronal survival, and influences neuronal differentiation in adult rats. The significant role of GALR2 is emphasized, suggesting new potential therapeutic strategies for cognitive decline.

Neuropeptide Y receptor 1 and galanin receptor 2 (NPY1R-GALR2) interactions in the dentate gyrus and their relevance for neurogenesis and cognition

Introduction

This study may unveil novel insights into the interactions between neuropeptide Y receptor 1 (NPY1R) and galanin receptor 2 (GALR2), in the dentate gyrus of the dorsal hippocampus, shedding light on their role in neurogenesis and cognitive functions. Existing literature highlights the potential of these interactions in enhancing learning and memory, yet detailed mechanisms remain underexplored.

Methods

Utilizing intracerebroventricular injections of GALR2 and NPY1R agonists in Sprague-Dawley male rats, we examined neurogenesis via markers PCNA and DCX, and memory consolidation through the object-in-place task over a three-week period.

Results

Significant increases in NPY1R-GALR2 co-localization and neuroblast proliferation were observed, alongside enhanced memory consolidation. These findings suggest a synergistic effect of NPY1R and GALR2 activation on cognitive functions.

Discussion

Our findings may foster the development of novel heterobivalent or multitargeting drugs, affecting NPY1R-GALR2 interaction, and suggest a future pharmacogical strategy for improving learning and memory found in many brain diseases. Further research is encouraged to explore these mechanisms in pathological models.

Artemisinin reduces PTSD-like symptoms, improves synaptic plasticity, and inhibits apoptosis in rats subjected to single prolonged stress

Post-Traumatic Stress Disorder (PTSD) is a chronic mental disorder characterized by symptoms of panic and anxiety, depression, impaired cognitive functioning, and difficulty in social interactions. While the effect of the traditional Chinese medicine artemisinin (AR) on PTSD is unknown, its therapeutic benefits have been demonstrated by studies on models of multiple neurological disorders. This study aimed to extend such findings by investigating the effects of AR administration on a rat model of PTSD induced by a regimen of single prolonged stress (SPS). After rats were subjected to the SPS protocol, AR was administered and its impact on PTSD-like behaviors was evaluated. In the present study, rats were subjected to a multitude of behavioral tests to evaluate behaviors related to anxiety, memory function, and social interactions. The expression of hippocampal synaptic plasticity-related proteins was detected using Western blot and immunofluorescence. The ultrastructure of synapses was observed under transmission electron microscopy. The apoptosis of hippocampal neurons was examined with Western blot, TUNEL staining, and HE staining. The results showed that AR administration alleviated the PTSD-like phenotypes in SPS rats, including behavior indicative of anxiety, cognitive deficits, and diminished sociability. AR administration was further observed to improve synaptic plasticity and inhibit neuronal apoptosis in SPS rats. These findings suggest that administering AR after the onset of severe traumatic events may alleviate anxiety, cognitive deficits, and impaired social interaction, improve synaptic plasticity, and diminish neuronal apoptosis. Hence, the present study provides evidence for AR's potential as a multi-target agent in the treatment of PTSD.

Natural Plant Materials as a Source of Neuroprotective Peptides

In many circumstances, some crucial elements of the neuronal defense system fail, slowly leading to neurodegenerative diseases. Activating this natural process by administering exogenous agents to counteract unfavourable changes seems promising. Therefore, looking for neuroprotective therapeutics, we have to focus on compounds that inhibit the primary mechanisms leading to neuronal injuries, e.g., apoptosis, excitotoxicity, oxidative stress, and inflammation. Among many compounds considered neuroprotective agents, protein hydrolysates and peptides derived from natural materials or their synthetic analogues are good candidates. They have several advantages, such as high selectivity and biological activity, a broad range of targets, and high safety profile. This review aims to provide biological activities, the mechanism of action and the functional properties of plant-derived protein hydrolysates and peptides. We focused on their significant role in human health by affecting the nervous system and having neuroprotective and brain-boosting properties, leading to memory and cognitive improving activities. We hope our observation may guide the evaluation of novel peptides with potential neuroprotective effects. Research into neuroprotective peptides may find application in different sectors as ingredients in functional foods or pharmaceuticals to improve human health and prevent diseases.

The Promising Role of a Zebrafish Model Employed in Neural Regeneration Following a Spinal Cord Injury

Spinal cord injury (SCI) is a devastating event that results in a wide range of physical impairments and disabilities. Despite the advances in our understanding of the biological response to injured tissue, no effective treatments are available for SCIs at present. Some studies have addressed this issue by exploring the potential of cell transplantation therapy. However, because of the abnormal microenvironment in injured tissue, the survival rate of transplanted cells is often low, thus limiting the efficacy of such treatments. Many studies have attempted to overcome these obstacles using a variety of cell types and animal models. Recent studies have shown the utility of zebrafish as a model of neural regeneration following SCIs, including the proliferation and migration of various cell types and the involvement of various progenitor cells. In this review, we discuss some of the current challenges in SCI research, including the accurate identification of cell types involved in neural regeneration, the adverse microenvironment created by SCIs, attenuated immune responses that inhibit nerve regeneration, and glial scar formation that prevents axonal regeneration. More in-depth studies are needed to fully understand the neural regeneration mechanisms, proteins, and signaling pathways involved in the complex interactions between the SCI microenvironment and transplanted cells in non-mammals, particularly in the zebrafish model, which could, in turn, lead to new therapeutic approaches to treat SCIs in humans and other mammals.

The role of SIRT2 inhibition on the aging process of brain in male rats

Background

Though the exact mechanisms regarding brain aging and its relation to neurodegenerative disorders are not precise, oxidative stress, the key regulators of apoptosis and autophagy, such as bcl-2 and beclin 1, seem to be the potential players in the aging of the cerebral cortex and hippocampus. As a type of nicotinamide adenine dinucleotide (NAD+)-dependent deacetylases, sirtuin 2 (SIRT2) has been associated to age-related diseases. However, the exact role of SIRT2 in brain aging is not well studied. The objective of the current study was to study the role of SIRT2 inhibition on brain aging through the neuroprotective mechanisms.

Methods

We tested the effects of AGK-2, a SIRT2 inhibitor, on oxidative stress parameters, apoptosis and autophagy regulators including bcl-2, bax, beclin1 in young and old rats. 24 Wistar albino rats (3 months-old and 22 months-old) were divided into four groups; Young-Control (4% DMSO+PBS), Young-AGK-2 (10 µM/bw, ip), Aged-Control, and Aged-AGK-2. Following the 30 days of drug administration period the rats were sacrificed and the cerebral cortex, hippocampus, and cerebellum were isolated. Total antioxidant status (TAS) and total oxidant status (TOS) were measured as oxidative stress parameters in all three brain regions. SIRT2, bcl-2, and bax protein expression levels were measured by western blot and gene expression level of beclin 1, Atg5, and SIRT2 by real-time PCR.

Results

The bcl-2, bcl-2/bax ratio, beclin 1, and TAS in the cerebral cortex of the aged group were significantly decreased; however, the TOS, oxidative stress index (OSI), and SIRT2 expression in the cerebral cortex and hippocampus increased. SIRT2 inhibition by AGK-2 reduced TOS and OSI levels in all brain regions and increased bcl-2, bcl-2/bax ratio. In aged animals, AGK-2 also increased the beclin 1 levels in the cortex and hippocampus.

Conclusion

Our results indicate that SIRT2 has an essential role in brain aging. The inhibition of SIRT2 by AGK-2 may increase cell survival and decrease aging related processes in the cerebral cortex and hippocampus via decreasing oxidative stress, and increasing bcl-2 and beclin 1 expression.

Decreased medial prefrontal cortex activity related to impaired novel object preference task performance following GALR2 and Y1R agonists intranasal infusion

Different brain regions' interactions have been implicated in relevant neurological diseases, such as major depressive disorder (MDD), anxiety disorders, age-dependent cognitive decline, Alzheimer's disease (AD) and addiction. We aim to explore the role of the medial prefrontal cortex (mPFC) in the Neuropeptide Y (NPY) and Galanin (GAL) interaction since we have demonstrated specific NPY and GAL interactions in brain areas related to these brain diseases. We performed GALR2 and Y1R agonists intranasal infusion and analyzed the mPFC activation through c-Fos expression. To assess the associated cellular mechanism we studied the formation of Y1R-GALR2 heteroreceptor complexes with in situ proximity ligation assay (PLA) and the expression of the brain-derived neurotrophic factor (BDNF). Moreover, the functional outcome of the NPY and GAL interaction on the mPFC was evaluated in the novel object preference task. We demonstrated that the intranasal administration of both agonists decrease the medial prefrontal cortex activation as shown with the c-Fos expression. These effects were mediated by the decreased formation of Y1R-GALR2 heteroreceptor complexes without affecting the BDNF expression. The functional outcome of this interaction was related to an impaired performance on the novel object preference task. Our data may suggest the translational development of new heterobivalent agonist pharmacophores acting on Y1R-GALR2 heterocomplexes in the medial prefrontal cortex for the novel therapy on neurodegenerative and psychiatric diseases. DATA SHARING AND DATA ACCESSIBILITY: The data that support the findings of this study are openly available in Institutional repository of the University of Malaga (RIUMA) and from the corresponding author upon reasonable request.

Exploring Novel Therapeutic Targets in the Common Pathogenic Factors in Migraine and Neuropathic Pain

Migraine and neuropathic pain (NP) are both painful, disabling, chronic conditions which exhibit some symptom similarities and are thus considered to share a common etiology. The calcitonin gene-related peptide (CGRP) has gained credit as a target for migraine management; nevertheless, the efficacy and the applicability of CGRP modifiers warrant the search for more effective therapeutic targets for pain management. This scoping review focuses on human studies of common pathogenic factors in migraine and NP, with reference to available preclinical evidence to explore potential novel therapeutic targets. CGRP inhibitors and monoclonal antibodies alleviate inflammation in the meninges; targeting transient receptor potential (TRP) ion channels may help prevent the release of nociceptive substances, and modifying the endocannabinoid system may open a path toward discovery of novel analgesics. There may exist a potential target in the tryptophan-kynurenine (KYN) metabolic system, which is closely linked to glutamate-induced hyperexcitability; alleviating neuroinflammation may complement a pain-relieving armamentarium, and modifying microglial excitation, which is observed in both conditions, may be a possible approach. Those are several potential analgesic targets which deserve to be explored in search of novel analgesics; however, much evidence remains missing. This review highlights the need for more studies on CGRP modifiers for subtypes, the discovery of TRP and endocannabinoid modulators, knowledge of the status of KYN metabolites, the consensus on cytokines and sampling, and biomarkers for microglial function, in search of innovative pain management methods for migraine and NP.

GALR2 and Y1R agonists intranasal infusion enhanced adult ventral hippocampal neurogenesis and antidepressant-like effects involving BDNF actions

Dysregulation of adult hippocampal neurogenesis is linked to major depressive disorder (MDD), with more than 300 million people diagnosed and worsened by the COVID-19 pandemic. Accumulating evidence for neuropeptide Y (NPY) and galanin (GAL) interaction was shown in various limbic system regions at molecular-, cellular-, and behavioral-specific levels. The purpose of the current work was to evaluate the proliferating role of GAL2 receptor (GALR2) and Y1R agonists interaction upon intranasal infusion in the ventral hippocampus. We studied their hippocampal proliferating actions using the proliferating cell nuclear antigen (PCNA) on neuroblasts or stem cells and the expression of the brain-derived neurothrophic factor (BDNF). Moreover, we studied the formation of Y1R-GALR2 heteroreceptor complexes and analyzed morphological changes in hippocampal neuronal cells. Finally, the functional outcome of the NPY and GAL interaction on the ventral hippocampus was evaluated in the forced swimming test. We demonstrated that the intranasal infusion of GALR2 and the Y1R agonists promotes neuroblasts proliferation in the dentate gyrus of the ventral hippocampus and the induction of the neurotrophic factor BDNF. These effects were mediated by the increased formation of Y1R-GALR2 heteroreceptor complexes, which may mediate the neurites outgrowth observed on neuronal hippocampal cells. Importantly, BDNF action was found necessary for the antidepressant-like effects after GALR2 and the Y1R agonists intranasal administration. Our data may suggest the translational development of new heterobivalent agonist pharmacophores acting on Y1R-GALR2 heterocomplexes in the ventral hippocampus for the novel therapy of MDD or depressive-affecting diseases.

Mitochondrial Impairment: A Common Motif in Neuropsychiatric Presentation? The Link to the Tryptophan-Kynurenine Metabolic System

Nearly half a century has passed since the discovery of cytoplasmic inheritance of human chloramphenicol resistance. The inheritance was then revealed to take place maternally by mitochondrial DNA (mtDNA). Later, a number of mutations in mtDNA were identified as a cause of severe inheritable metabolic diseases with neurological manifestation, and the impairment of mitochondrial functions has been probed in the pathogenesis of a wide range of illnesses including neurodegenerative diseases. Recently, a growing number of preclinical studies have revealed that animal behaviors are influenced by the impairment of mitochondrial functions and possibly by the loss of mitochondrial stress resilience. Indeed, as high as 54% of patients with one of the most common primary mitochondrial diseases, mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) syndrome, present psychiatric symptoms including cognitive impairment, mood disorder, anxiety, and psychosis. Mitochondria are multifunctional organelles which produce cellular energy and play a major role in other cellular functions including homeostasis, cellular signaling, and gene expression, among others. Mitochondrial functions are observed to be compromised and to become less resilient under continuous stress. Meanwhile, stress and inflammation have been linked to the activation of the tryptophan (Trp)-kynurenine (KYN) metabolic system, which observably contributes to the development of pathological conditions including neurological and psychiatric disorders. This review discusses the functions of mitochondria and the Trp-KYN system, the interaction of the Trp-KYN system with mitochondria, and the current understanding of the involvement of mitochondria and the Trp-KYN system in preclinical and clinical studies of major neurological and psychiatric diseases.