Antidepressant drugs act by directly binding to TRKB neurotrophin receptors

Neuroplasticity and psychedelics: A comprehensive examination of classic and non-classic compounds in pre and clinical models

Neuroplasticity, the ability of the nervous system to adapt throughout an organism's lifespan, offers potential as both a biomarker and treatment target for neuropsychiatric conditions. Psychedelics, a burgeoning category of drugs, are increasingly prominent in psychiatric research, prompting inquiries into their mechanisms of action. Distinguishing themselves from traditional medications, psychedelics demonstrate rapid and enduring therapeutic effects after a single or few administrations, believed to stem from their neuroplasticity-enhancing properties. This review examines how classic psychedelics (e.g., LSD, psilocybin, N,N-DMT) and non-classic psychedelics (e.g., ketamine, MDMA) influence neuroplasticity. Drawing from preclinical and clinical studies, we explore the molecular, structural, and functional changes triggered by these agents. Animal studies suggest psychedelics induce heightened sensitivity of the nervous system to environmental stimuli (meta-plasticity), re-opening developmental windows for long-term structural changes (hyper-plasticity), with implications for mood and behavior. Translating these findings to humans faces challenges due to limitations in current imaging techniques. Nonetheless, promising new directions for human research are emerging, including the employment of novel positron-emission tomography (PET) radioligands, non-invasive brain stimulation methods, and multimodal approaches. By elucidating the interplay between psychedelics and neuroplasticity, this review informs the development of targeted interventions for neuropsychiatric disorders and advances understanding of psychedelics' therapeutic potential.

New directions in neurosteroid therapeutics in neuropsychiatry

In recent years three neuroactive steroids (NAS), brexanolone (allopregnanolone, AlloP), ganaxolone and zuranolone, have been approved for the treatment of neuropsychiatric illnesses including postpartum depression and seizures in a neurodevelopmental syndrome. The approved agents are pregnane steroids and strong positive allosteric modulators (PAMs) of gamma-aminobutyric acid type A receptors (GABAARs). Broad effects on GABAARs play important roles in therapeutic benefits. However, these NAS also have actions on non-GABAR targets that could be important for clinical outcomes. Thus, understanding the broader effects of NAS is potentially important for expanding the therapeutic landscape of these important modulators. The approved NAS as well as other structurally distinct NAS and oxysterols have effects on non-GABAAR receptors and ion channels, along with intracellular actions that could have therapeutic importance, including modulation of cellular stress mechanisms, neuroinflammation, mitochondrial function and autophagy, among others. In this review, we explore GABAergic and other cellular effects of pregnane steroids including novel molecules that have potential therapeutic importance. This work discusses the complex chemical nature of NAS and what is being learned at cellular, molecular, synaptic and brain network levels about key sites of action including GABAARs and other targets.

Tauroursodeoxycholic acid mitigates depression-like behavior and hippocampal neuronal damage in a corticosterone model of female mice

Depression, a complex mental disorder influenced by both psychological and physiological factors, predominantly affects females. Studies have indicated that elevated levels of cortisol/corticosterone (CORT) under stress conditions can lead to hippocampal neuronal damage, thereby contributing to depression. Tauroursodeoxycholic acid (TUDCA), a bile acid, possesses anti-apoptotic, antioxidant, and anti-inflammatory properties. This study aimed to investigate the protective mechanism of TUDCA against CORT-induced neuromolecular and behavioral phenotypes of depression in female mice, providing theoretical support for its use in treating female depression. The antidepressant effects of TUDCA were evaluated through a series of behavioral tests, measurement of serum neurotransmitter levels, Nissl staining of the hippocampal CA3 region, and assessment of hippocampal proteins. Behavioral results demonstrated that TUDCA exhibited antidepressant effects, as evidenced by increased sucrose preference and locomotor activity, as well as reduced immobility time in depressed mice. Furthermore, TUDCA ameliorated neurotransmitter imbalances. Nissl staining revealed that TUDCA reduced neuronal damage in depressed mice, while Western blotting results indicated that TUDCA activated the hippocampal BDNF/TrkB/CREB pathway and regulated the expression of GR-related proteins. These findings suggested that TUDCA exerted neuroprotective effects in CORT-induced neuronal damage in female depressed mice. The mechanism appeared to be related to the activation of the BDNF/TrkB/CREB signaling pathway and the modulation of GR-related protein expression.

Stress dynamically modulates neuronal autophagy to gate depression onset

Chronic stress remodels brain homeostasis, in which persistent change leads to depressive disorders1. As a key modulator of brain homeostasis2, it remains elusive whether and how brain autophagy is engaged in stress dynamics. Here we discover that acute stress activates, whereas chronic stress suppresses, autophagy mainly in the lateral habenula (LHb). Systemic administration of distinct antidepressant drugs similarly restores autophagy function in the LHb, suggesting LHb autophagy as a common antidepressant target. Genetic ablation of LHb neuronal autophagy promotes stress susceptibility, whereas enhancing LHb autophagy exerts rapid antidepressant-like effects. LHb autophagy controls neuronal excitability, synaptic transmission and plasticity by means of on-demand degradation of glutamate receptors. Collectively, this study shows a causal role of LHb autophagy in maintaining emotional homeostasis against stress. Disrupted LHb autophagy is implicated in the maladaptation to chronic stress, and its reversal by autophagy enhancers provides a new antidepressant strategy.

Effects of fluoxetine on the gene expression of hippocampus and gap inhibition in noise-induced hearing loss rats

OBJECTIVE

Fluoxetine was reported to restore critical period-like neural plasticity via alleviating perineuronal nets (PNNs). This study aimed to investigate the effect of fluoxetine on auditory processing and PNNs in auditory cortex and hippocampus.

METHODS

Sprague-Dawley rats were exposed 2-20 kHz, 115 dB sound pressure level noise for 3 h per day from postnatal day 1-3 to postnatal day 21. After completion of noise exposure, 10 mg/kg/day of fluoxetine was administered for 19 days. There were four groups of rats according to the presence of noise exposure and fluoxetine treatment, vehicle, noise + vehicle, fluoxetine, and noise + fluoxetine rats. The gene expression changes of hippocampus were analyzed using RNA sequencing.

RESULTS

In the auditory cortex, the expression of aggrecan (ACAN) was lower in noise-exposed rats than vehicle rats, while the noise + fluoxetine rats presented higher expression levels of ACAN which was comparable with that of the vehicle rats (p = 0.01 in Mann-Whitney U test; 146 ± 15 vs. 100 ± 11). In the hippocampus, the expression of brain-derived neurotrophic factor (BDNF) was lower in noise + vehicle rats while noise + fluoxetine rats presented higher expression of BDNF than noise + vehicle rats (p < 0.001 in Mann-Whitney U test; 389 ± 21 vs. 249 ± 16). The RNA sequencing of the hippocampus predicted the down regulation of genes involving extracellular matrix organization when compared noise + vehicle vs. noise + fluoxetine rats.

CONCLUSION

The fluoxetine administration in noise exposed rats improved the gap inhibition ability. The noise exposure decreased expression of BDNF and modulated the expression of genes related with extracellular matrix organization which was partially reversed after fluoxetine treatment.

Peptidomimetic inhibitors targeting TrkB/PSD-95 signaling improves cognition and seizure outcomes in an Angelman Syndrome mouse model

Angelman syndrome (AS) is a rare genetic neurodevelopmental disorder with profoundly debilitating symptoms with no FDA-approved cure or therapeutic. Brain-derived neurotrophic factor (BDNF), and its receptor tropomyosin receptor kinase B (TrkB), have a well-established role as regulators of synaptic plasticity, dendritic outgrowth and spine formation. Previously, we reported that the association of postsynaptic density protein 95 (PSD-95) with TrkB is critical for intact BDNF signaling in the AS mouse model, as illustrated by attenuated PLCγ and PI3K signaling and intact MAPK pathway signaling. These data suggest that drugs tailored to enhance the TrkB-PSD-95 interaction may provide a novel approach for the treatment of AS and a variety of neurodevelopmental disorders (NDDs). To evaluate this critical interaction, we synthesized a class of high-affinity PSD-95 ligands that bind specifically to the PDZ3 domain of PSD-95, denoted as Syn3 peptidomimetic ligands. We evaluated Syn3 and its analog D-Syn3 (engineered using dextrorotary (D)-amino acids) in vivo using the Ube3a exon 2 deletion mouse model of AS. Following systemic administration of Syn3 and D-Syn3, we demonstrate improvement in the seizure domain of AS. Learning and memory using the novel object recognition assay also illustrated improved cognition following Syn3 and D-Syn3, along with restored long-term potentiation. A pharmacokinetic analysis of D-Syn3 demonstrates that it crosses the blood-brain barrier (BBB), and the brain influx rate is in the range of CNS therapeutics. Finally, D-Syn3 treated mice showed a partial rescue in motor learning. Neither Syn3 nor D-Syn3 improved gross exploratory locomotion deficits, nor gait impairments that have been well documented in the AS rodent models. These findings highlight the need for further investigation of this compound class as a potential therapeutic for AS and other genetic NDDs.

Chronic social stress induces generalized hyper-sensitivity to aversion: A mouse model with translational validity for understanding and treating negative valence disorders

The RDoC framework focuses on neurobehavioral processes often dysfunctional in mental disorders and commensurate with translational research. Generalized hyper-sensitivity to aversion/threat is common in various stress-related emotional disorders; increased Pavlovian aversion learning-memory (PAL, PAM) provides a translational paradigm for its study. Here we present the development and application of a mouse model for the study of generalized hyper-sensitivity to aversion/threat. In male adult mice, chronic exposure to social aversion (chronic social stress, CSS) leads, relative to controls (CON), to increased acquisition and expression of tone-footshock conditioned freezing behavior. The altered neurobehavioral state of CSS mice is expected to involve structure-function changes in amygdala: in CSS mice, higher levels of both PAL and PAM freezing behavior co-occurred with fewer lateral/basal amygdala glutamate neurons expressing the immediate early-gene protein c-Fos. A current antidepressant, SSRI escitalopram, reversed excessive PAM freezing behavior in CSS mice with sub-chronic dosing. The model was applied to investigate 3 compounds with novel mechanisms of action: indoleamine dioxygenase 1 (IDO 1) inhibition, somatostatin receptor 4 (SSTR4) agonism, and transient receptor potential canonical channels 4 and 5 (TRPC4/5) inhibition. For each, there was evidence for attenuation of excessive PAL and/or PAM in CSS mice. Preclinical validation of TRPC4/5 channels inhibition contributed to the decision to investigate, and accurately predicted, clinical efficacy, measured as reduced amygdala and emotional reactivities to aversion in major depressive disorder. Future work will focus on (back-)translational studies that address stress-induced changes in amygdala reactivity and aversion processing, their underlying etio-pathophysiological causes, and neuropharmacological responsiveness.

Pharmacologically activating BDNF/TrkB signaling exerted rapid-acting antidepressant-like effects through improving synaptic plasticity and neuroinflammation

BDNF (Brain-derived neurotrophic factor)/TrkB (tropomyosin receptor kinase B) signaling has great therapeutic potential for depression, but the underlying mechanism remains unclear. This study aims to investigate the molecular mechanism underlying the BDNF/TrkB signaling-mediated antidepressant effects. Chronic Cort drinking for 4 weeks and a single injection of LPS for 24 h were used to induce depression-like behaviors; this study used 7,8-dihydroxyflavone (7,8-DHF, 10 mg/kg, i.p.), a selective TrkB receptor agonist, to activate the BDNF/TrkB signaling and examined its rapid-acting antidepressant-like effects; levels of pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α) in BV2 microglial cells and synapse-related factors (BDNF, GluA1, Synapsin-1, and PSD95) in HT22 cells were examined by ELISA. Our behavioral results suggested that 7,8-DHF (10 mg/kg, i.p.) exerted rapid-acting antidepressant-like effects in Cort/LPS-treated mice; our immunofluorescence staining results suggested that Cort/LPS reduced the number of NeuN + HT22 cells and increased the number of Iba1 + BV2 microglial cells, which were completely reversed by 7,8-DHF pre-treatment. Our ELISA results suggested that 7,8-DHF significantly normalized the release of synapse-related factors (BDNF, GluA1, and PSD95) in HT22 cells and suppressed the production of inflammatory cytokines (IL-1β, IL-6, and TNF-α) in BV2 microglial cells. Taken together, this study suggested that pharmacologically activating the BDNF/TrkB signaling pathway exerted rapid-acting antidepressant-like effects through improving synaptic plasticity and inhibiting neuroinflammation, which provided new insights for developing next-generation rapid-acting antidepressants.

Inhibition of TrkB-BDNF positive feedback loop attenuates intervertebral disc degeneration and low back pain in a composite mouse model

Intervertebral disc degeneration (IVDD) is a significant contributor to low back pain (LBP); however, the specific mechanisms involved remain unclear. Herein, a novel LBP mouse model was developed by integrating a bipedal standing model with a lumbar spine instability model (BS + LSI). This model effectively reproduced the behavioral characteristics of LBP and the pathological features of IVDD. Notably, a higher degree of degeneration and innervation in the endplates were observed in the BS + LSI mice. Transcriptome analysis revealed a significant upregulation of Ntrk2, the gene encoding TrkB, in the intervertebral discs of BS + LSI mice. Immunohistochemical staining further confirmed elevated expression of TrkB and its ligand BDNF in the endplates of these mice. Moreover, cyclic tensile strain (CTS) (20 %, 0.1 Hz, 24 h) upregulated TrkB expression and activated NF-κB signaling pathway to promote inflammatory responses in endplate chondrocytes. siBDNF transfection or treatment with the TrkB inhibitor ANA-12 effectively inhibited these pathological changes. Mechanistically, TrkB promoted BDNF expression by enhancing CREB phosphorylation, thereby establishing a TrkB-CREB-BDNF positive feedback loop. In vivo injection of ANA-12 significantly alleviated endplate inflammation and LBP-related behaviors in BS + LSI mice. Thus, an effective and replicable mouse model of LBP was established to identify TrkB as both the receptor for and an upstream regulator of BDNF, making it a crucial target for interventions to alleviate CEP inflammation and LBP.

Stimulus-Dependent Expression of Bdnf Is Mediated by ATF2, MYT1L, and EGR1 Transcription Factors

Neurotrophins like BDNF have a key role in the proper functioning of the central nervous system, influencing numerous processes like memory formation and behavior. An imbalance in BDNF levels can lead to a wide range of diseases, including depression and neurodevelopmental disorders. While the potential therapeutic effects of BDNF are well-recognized, there is a knowledge gap in understanding the mechanisms governing BDNF expression levels. Here, we focused on the regulation of Bdnf gene expression in response to different stimuli, specifically studying the effects of neuronal activity and BDNF-TrkB signaling on Bdnf transcription in cultured neurons from rats of either sex. We used in vitro DNA pulldown combined with mass spectrometry to determine transcription factors that interact with the Bdnf promoters upon different stimuli and validated numerous known regulators, such as USF and AP1 family, and novel candidate regulators using reporter assays. We show that the USF family of transcription factors is specifically recruited after membrane depolarization, whereas the AP1 family participates in Bdnf regulation only after BDNF-TrkB signaling. We further describe ATF2, MYT1L, and EGR family as novel regulators of Bdnf expression by demonstrating their direct binding to Bdnf promoters using chromatin immunoprecipitation assays both in vitro and in vivo, showing their functional role in Bdnf gene expression and ultimately identifying their regulatory cis-elements in Bdnf promoters. Furthermore, our results show competition between ATF2, CREB, and AP1 family in regulating Bdnf levels. Collectively, our results provide insight into the regulation of Bdnf expression upon different stimuli.

Therapeutic and Structural Dimensions in Psychiatric Prescribing: Bridging Psychedelics and Antidepressants

ABSTRACT

As practitioners seek more personalized approaches, exploring how patients' environments, relationship templates, and mindsets factor into symptom burden can help broaden understanding of how psychotropic medications facilitate recovery. Despite increasing focus on medications to provide relief, there is an important and undeniable influence the therapeutic environment has on shaping outcomes, particularly for the patient-clinician alliance. While environmental dimensions are relevant for informing possible placebo or nocebo responses, they also build upon the pharmacodynamic and neurobiological effects of medications. By heightening neuroplasticity, some antidepressants may amplify the effects of nonmedication factors in patients' lives, including the patient-prescriber therapeutic relationship. There are important parallels between antidepressants and psychedelics in emerging literature. For instance, the preparatory and integrative work with a provider can be crucial in determining outcomes. This paper will draw from the extant literature to discuss the therapeutic relationship in psychiatric practice, including in acute care settings and instances in which psychotropic prescribing is a key aspect of treatment.

Transcriptional Profiles in Nucleus Accumbens of Antidepressant Resistance in Chronically Stressed Mice

Unsuccessful response to several courses of antidepressants is a core feature of treatment-resistant depression (TRD), a severe condition that affects a third of patients with depression treated with conventional pharmacotherapy. However, the molecular mechanisms underlying TRD remain poorly understood. Here, we assessed the successful vs. unsuccessful response to ketamine (KET) in chronically stressed mice that failed to respond to initial treatment with fluoxetine (FLX) as a rodent model of TRD and characterized the associated transcriptional profiles in the nucleus accumbens (NAc) using RNA-sequencing. We observed that failed treatment with FLX exerts a priming effect that promotes behavioral and transcriptional responses to subsequent ketamine treatment. We also identified specific gene networks that are linked to both susceptibility to stress and resistance to antidepressant response. Collectively, these findings offer valuable insights into the molecular mechanisms underlying antidepressant resistance and help address a critical gap in preclinical models of TRD.

Meta-analysis of high-intensity interval training effects on cognitive function in older adults and cognitively impaired patients

Background

Cognitive enhancement treatments are limited, and while High-Intensity Interval Training (HIIT) has been suggested to improve cognitive function, high-quality evidence remains scarce. This meta-analysis evaluates the effects of HIIT on cognitive performance compared to moderate-intensity continuous training (MICT) and control groups in older adults and cognitively Impaired Patients.

Methods

A systematic search of PubMed, Embase, and Cochrane Library databases was conducted for articles published until 10 October 2024. Eighteen studies were included, comparing cognitive outcomes across HIIT, MICT, and control groups. Cognitive tests evaluated included the Stroop test, Digit Span Test (DST), Trail Making Test (TMT), and the MOST test.

Results

HIIT significantly improved performance compared to MICT in the Stroop test (SMD = -0.8, 95% CI: -1.3 to -0.2) and DST (SMD = 0.3, 95% CI: -0.0-0.5). Compared to control groups, HIIT significantly enhanced performance in the TMT (SMD = -0.7, 95% CI: -1.3 to 0.0) and MOST test (SMD = -1.2, 95% CI: -1.8 to -0.7).

Conclusion

This meta-analysis supports the efficacy of HIIT in enhancing cognitive functions, particularly in cognitive flexibility, working memory, task switching, attention control, and inhibitory control. These findings suggest that HIIT can be an effective intervention for improving cognitive behavior in older adults and cognitively Impaired Patients.

Systematic Review Registration

https://www.crd.york.ac.uk/PROSPERO/, Identifier CRD42023413879.

Serotonin regulates in a cell-type specific manner light-evoked response and synaptic activity in mouse retinal ganglion cells

BACKGROUND

Serotonin (5-HT) is known to be synthesized and accumulated in the vertebrate retina through the 5-HT transporter, SERT. While manipulation of the serotonergic system has been shown to impact visual processing, the role of 5-HT and SERT as modulators of retinal synaptic function remains poorly understood.

RESULTS

Using mouse retinal slices, we show that acute application of 5-HT produces a cell-type specific reduction in light-evoked excitatory responses (L-EPSC) in ON-OFF retinal ganglion cells (RGCs), but not in ON RGCs. Similarly, increasing 5-HT tone by acute application of citalopram, a selective 5-HT reuptake inhibitor, also reduces L-EPSC in ON-OFF RGCs while not affecting ON RGCs. Importantly, citalopram-mediated reduction of L-EPSC was absent in ON-OFF RGCs recorded from SERT null retina, highlighting the role of SERT in regulating light-evoked responses in RGCs. The effects of both exogenous and endogenous 5-HT on L-EPSC in ON-OFF RGCs are likely due to a presynaptic reduction in excitatory synaptic strength as 5-HT and citalopram reduced the frequency but not the amplitude of spontaneous excitatory currents (sEPSCs) in ON-OFF RGCs. Moreover, 5-HT and citalopram had no effect on currents elicited by the direct activation of postsynaptic receptors in RGCs by brief application of glutamate in the inner retina.

CONCLUSIONS

Altogether these findings indicate that 5-HT modulates excitatory inputs onto RGCs in a cell-type specific manner and highlight that in the adult mouse retina, 5-HT-mediated effects onto RGCs are tightly controlled by the 5-HT transporter SERT.

Targeting TrkB-PSD-95 coupling to mitigate neurological disorders

Tropomyosin receptor kinase B (TrkB) signaling plays a pivotal role in dendritic growth and dendritic spine formation to promote learning and memory. The activity-dependent release of brain-derived neurotrophic factor at synapses binds to pre- or postsynaptic TrkB resulting in the strengthening of synapses, reflected by long-term potentiation. Postsynaptically, the association of postsynaptic density protein-95 with TrkB enhances phospholipase Cγ-Ca2+/calmodulin-dependent protein kinase II and phosphatidylinositol 3-kinase-mechanistic target of rapamycin signaling required for long-term potentiation. In this review, we discuss TrkB-postsynaptic density protein-95 coupling as a promising strategy to magnify brain-derived neurotrophic factor signaling towards the development of novel therapeutics for specific neurological disorders. A reduction of TrkB signaling has been observed in neurodegenerative disorders, such as Alzheimer's disease and Huntington's disease, and enhancement of postsynaptic density protein-95 association with TrkB signaling could mitigate the observed deficiency of neuronal connectivity in schizophrenia and depression. Treatment with brain-derived neurotrophic factor is problematic, due to poor pharmacokinetics, low brain penetration, and side effects resulting from activation of the p75 neurotrophin receptor or the truncated TrkB.T1 isoform. Although TrkB agonists and antibodies that activate TrkB are being intensively investigated, they cannot distinguish the multiple human TrkB splicing isoforms or cell type-specific functions. Targeting TrkB-postsynaptic density protein-95 coupling provides an alternative approach to specifically boost TrkB signaling at localized synaptic sites versus global stimulation that risks many adverse side effects.

Therapeutic Potential of Saffron Extract in Mild Depression: A Study of Its Role on Anhedonia in Rats and Humans

Drugs generally used in major depressive disorder are considered inappropriate for the more common milder forms. The efficacy of saffron extracts has been demonstrated in mild to moderate depression and in preclinical models of depression. However, evidence of saffron activity on reduced hedonic responsiveness and motivational anhedonia is limited. Since dopamine transmission dysfunctions are crucially involved in anhedonia and saffron seems to positively modulate dopamine release, we studied the potential antidepressant and anti-anhedonic effects of a standardized formulation of saffron extract in preclinical models of anhedonia-like behaviors, and patients diagnosed with unipolar or bipolar depression. We tested saffron activity in a rat model of stress-induced motivational anhedonia using sucrose self-administration protocols and investigated the molecular underpinnings of this effect focusing on DARPP-32 phosphorylation pattern in response to a reinforcer and BDNF-TrkB signaling, in the nucleus accumbens and medial prefrontal cortex. In parallel, with a pilot double-blind placebo-controlled study we investigated whether saffron add-on therapy reduced symptoms of depression and anhedonia, measured by the Montgomery-Åsberg Depression Rating Scale. Repeated saffron administration restored motivation and reactivity to reward-associated cues in anhedonic rats, likely modulating dopaminergic transmission and BDNF-TrkB signaling. In depressed patients, an 8-week saffron add-on therapy induced a global improvement in depressive symptoms and a significant reduction in anhedonia. The study supports a pro-motivational effect of saffron and suggests a potentially useful saffron-based augmentation strategy in anhedonic patients, albeit with limitations due to small sample size and short trial duration.

[Mechanisms of action of antidepressive pharmacotherapy: brain and mind-body and environment]

BACKGROUND

Novel antidepressive substances are challenging the explanations for the mechanisms of action of traditional psychopharmacology.

OBJECTIVE

What could be the shared effects of various antidepressants and in this context what role do extrapharmacological factors, such as the body and environment, play?

MATERIAL AND METHOD

The available literature on clinical and preclinical data for assumed combined active factors of serotonergic psychedelic drugs, (es)ketamine, monoaminergic antidepressants and zuranolone are presented and the influence of context factors on the individual mechanisms of action is discussed.

RESULTS

There are many indications that classical and novel pharmacological approaches could share similar mechanisms of action in the treatment of depression. These mechanisms favor long-term neuroplasticity, which can trigger subsequent molecular cascades and vice versa. Furthermore, an improvement in the negative bias in emotional processing could be detected for most antidepressive substances. The influence of extrapharmacological factors appears to be necessary so that the biopsychological alterations can have an antidepressive effect.

CONCLUSION

Instead of attributing factors such as environment, body and social interaction to placebo effects, they should be tested as essential components of the antidepressive effect and considered in the clinical practice.

Molecular insights into the modulation of the 5HT2A receptor by serotonin, psilocin, and the G protein subunit Gqα

5HT2AR is a G-protein-coupled receptor that drives many neuronal functions and is a target for psychedelic drugs. Understanding ligand interactions and conformational transitions is essential for developing effective pharmaceuticals, but mechanistic details of 5HT2AR activation remain poorly understood. We utilized all-atom molecular dynamics simulations and free-energy calculations to investigate 5HT2AR's conformational dynamics upon binding to serotonin and psilocin. We show that the active state of 5HT2AR collapses to a closed state in the absence of Gqα, underscoring the importance of G-protein coupling. We discover an intermediate "partially-open" receptor conformation. Both ligands have higher binding affinities for the orthosteric than the extended binding pocket. These findings enhance our understanding of 5HT2AR's activation and may aid in developing novel therapeutics. Impact statement This study sheds light on 5HT2AR activation, revealing intermediate conformations and ligand dynamics. These insights could enhance drug development for neurological and psychiatric disorders, benefiting researchers and clinicians in pharmacology and neuroscience.

Redefining Ketamine Pharmacology for Antidepressant Action: Synergistic NMDA and Opioid Receptor Interactions?

Ketamine is a racemic compound and medication comprised of (S)-ketamine and (R)-ketamine enantiomers and its metabolites. It has been used for decades as a dissociative anesthetic, analgesic, and recreational drug. More recently, ketamine, its enantiomers, and its metabolites have been used or are being investigated for the treatment of refractory depression, as well as for comorbid disorders such as anxiety, obsessive-compulsive, and opioid use disorders. Despite its complex pharmacology, ketamine is referred to as an N-methyl-d-aspartate (NMDA) receptor antagonist. In this review, the authors argue that ketamine's pharmacology should be redefined to include opioid receptors and the endogenous opioid system. They also highlight a potential mechanism of action of ketamine for depression that is attributed to bifunctional, synergistic interactions involving NMDA and opioid receptors.

Insights from a methylome-wide association study of antidepressant exposure

This study tests the association of whole-blood DNA methylation and antidepressant exposure in 16,531 individuals from Generation Scotland (GS), using self-report and prescription-derived measures. We identify 8 associations and a high concordance of results between self-report and prescription-derived measures. Sex-stratified analyses observe nominally significant increased effect estimates in females for four CpGs. There is observed enrichment for genes expressed in the Amygdala and annotated to synaptic vesicle membrane ontology. Two CpGs (cg15071067; DGUOK-AS1 and cg26277237; KANK1) show correlation between DNA methylation with the time in treatment. There is a significant overlap in the top 1% of CpGs with another independent methylome-wide association study of antidepressant exposure. Finally, a methylation profile score trained on this sample shows a significant association with antidepressant exposure in a meta-analysis of eight independent external datasets. In this large investigation of antidepressant exposure and DNA methylation, we demonstrate robust associations which warrant further investigation to inform on the design of more effective and tolerated treatments for depression.

Pharmacological Mechanism and Drug Research Prospects of Ginsenoside Rb1 as an Antidepressant

This review explores the antidepressant effects of ginsenoside Rb1, a natural compound in traditional Chinese medicine, and its potential for treating major depressive disorder (MDD). The aetiology of depression was reviewed up to 2024, focusing on the pathways and mechanisms through which ginsenoside Rb1 may exert its effects. Notably, ginsenoside Rb1 regulates oxidative stress and inflammatory processes while enhancing neural plasticity by downregulating miR-134 expression and alleviating depressive symptoms. Unlike traditional antidepressants that act on a single target, ginsenoside Rb1 interacts with multiple pathways, reflecting its potential for broader therapeutic application. To compensate for the current deficiency in animal experiments, clinical data, and research on the side effects of ginsenoside Rb1 in the treatment of depression, we reviewed some clinical data on the use of this component in the treatment of other diseases to explore its relevance to depression. Ginsenoside Rb1 is expected to serve as a novel antidepressant or as a complementary component in combination with other antidepressant compounds. However, further clinical trials and molecular studies are necessary to confirm its efficacy and potential side effects.

The dopaminergic effects of esketamine are mediated by a dual mechanism involving glutamate and opioid receptors

Esketamine represents a new class of drugs for treating mood disorders. Unlike traditional monoaminergic-based therapies, esketamine primarily targets N-methyl-D-aspartate receptors (NMDAR). However, esketamine is a complex drug with low affinity for NMDAR and can also bind to other targets, such as opioid receptors. Its precise mechanism of action for its antidepressant properties remains debated, as does its potential for misuse. A key component at the intersection of mood and reward processing is the dopaminergic system. In this study, we evaluated the effects of esketamine in locomotion, anxiety tests and operant responding and we used in vivo fiber photometry to explore the neurochemical effects of esketamine in the nucleus accumbens of mice. Our findings demonstrated multifaceted effects of esketamine on neurotransmitter dynamics. In freely behaving mice, esketamine increased locomotion and increased extracellular dopamine tone -by impairing dopamine clearance rather than promoting dopamine release- while decreasing glutamatergic activity. However, it decreased dopamine spontaneous release event frequency and impaired reward-evoked dopamine release, leading to a reduction in operant responding rates. These dopaminergic effects were partially, and conditionally, blocked by the opioid antagonist naloxone and required glutamatergic input. In summary, our study reveals a complex interaction between neurotransmitter systems, suggesting that the neurochemical effects of esketamine are both circuit- and state-dependent.

Fluoxetine promotes IL-10-dependent metabolic defenses to protect from sepsis-induced lethality

Selective serotonin reuptake inhibitors (SSRIs) are some of the most prescribed drugs in the world. While they are used for their ability to increase serotonergic signaling in the brain, SSRIs are also known to have a broad range of effects beyond the brain, including immune and metabolic effects. Recent studies have demonstrated that SSRIs are protective in animal models and humans against several infections, including sepsis and COVID-19; however, the mechanisms underlying this protection are largely unknown. Here, we mechanistically link two previously described effects of the SSRI fluoxetine in mediating protection against sepsis. We show that fluoxetine-mediated protection is independent of peripheral serotonin and instead increases levels of circulating interleukin-10 (IL-10). IL-10 is necessary for protection from sepsis-induced hypertriglyceridemia, preventing cardiac effects including impairment of glucose oxidation, ectopic lipid accumulation, ventricular stretch and possibly cardiac failure. Our work reveals a beneficial "off-target" effect of fluoxetine, and reveals a protective immunometabolic defense mechanism with therapeutic potential.

Differential effects of statins on the anti-dyskinetic activity of sub-anesthetic ketamine

Sub-anesthetic ketamine has been demonstrated to reduce abnormal involuntary movements (AIMs) in preclinical models of L-DOPA-induced dyskinesia (LID) and retrospective Parkinson's disease (PD) case reports. In this study, we examined the effects on LID of two different statins alone and in combination with ketamine in unilateral 6-hydroxydopamine-lesioned male rats, the standard model for preclinical LID studies. Ketamine attenuated the development of AIMs, while the non-polar lovastatin only showed anti-dyskinetic activity early in the priming period but did not prevent the development of LID, and the polar pravastatin showed no anti-dyskinetic activity. Furthermore, our main result is that pravastatin blocked the long-term neuroplastic anti-dyskinetic effects of ketamine, while lovastatin did not. This study shows two different statins affect LID and the anti-dyskinetic activity of ketamine differentially, pointing to an important drug interaction. The results further inform and support ongoing clinical testing of sub-anesthetic ketamine to treat LID in individuals with PD.

Ketamine induces plasticity in a norepinephrine-astroglial circuit to promote behavioral perseverance

Transient exposure to ketamine can trigger lasting changes in behavior and mood. We found that brief ketamine exposure causes long-term suppression of futility-induced passivity in larval zebrafish, reversing the "giving-up" response that normally occurs when swimming fails to cause forward movement. Whole-brain imaging revealed that ketamine hyperactivates the norepinephrine-astroglia circuit responsible for passivity. After ketamine washout, this circuit exhibits hyposensitivity to futility, leading to long-term increased perseverance. Pharmacological, chemogenetic, and optogenetic manipulations show that norepinephrine and astrocytes are necessary and sufficient for ketamine's long-term perseverance-enhancing aftereffects. In vivo calcium imaging revealed that astrocytes in adult mouse cortex are similarly activated during futility in the tail suspension test and that acute ketamine exposure also induces astrocyte hyperactivation. The cross-species conservation of ketamine's modulation of noradrenergic-astroglial circuits and evidence that plasticity in this pathway can alter the behavioral response to futility hold promise for identifying new strategies to treat affective disorders.

Investigating novel pharmacological strategies for treatment-resistant depression: focus on new mechanisms and approaches

INTRODUCTION

A substantial number of patients exhibit treatment-resistant depression (TRD), posing significant challenges to clinicians. The discovery of novel molecules or mechanisms that may underlie TRD pathogenesis and antidepressant actions is highly needed.

AREAS COVERED

Using the PubMed database, the authors searched for emerging evidence of novel approaches for TRD based on experimental and human studies. Herein, the authors discuss the mechanisms underlying glutamatergic antagonists, modulators of the opioid system, and tryptamine-derivate psychedelics as well as the emerging platforms to investigate novel pharmacological targets for TRD. A search for clinical trials investigating novel agents and interventions for TRD was also conducted.

EXPERT OPINION

The understanding of the multiple pathophysiological mechanisms involved in TRD may add further value to the effective treatment, contributing to a more personalized approach. Esketamine was approved for the treatment of TRD and novel drugs with rapid antidepressant actions such as psilocybin and buprenorphine have also been investigated as potential therapeutic strategies. Over the past decades, technological advances such as omics approaches have broadened our knowledge regarding molecular and genetic underpinnings of complex conditions like TRD. Omics approaches could open new avenues for investigating glial-mediated mechanisms, including their crosstalk with neurons, as therapeutic targets in TRD.

Fluoxetine and Ketamine Enhance Extinction Memory and Brain Plasticity by Triggering the p75 Neurotrophin Receptor Proteolytic Pathway

BACKGROUND

Diverse antidepressants were recently described to bind to TrkB (tyrosine kinase B) and drive a positive allosteric modulation of endogenous BDNF (brain-derived neurotrophic factor). Although neurotrophins such as BDNF can bind to p75NTR (p75 neurotrophin receptor), their precursors are the high-affinity p75NTR ligands. While part of an unrelated receptor family capable of inducing completely opposite physiological changes, TrkB and p75NTR feature a crosslike conformation dimer and carry a cholesterol-recognition amino acid consensus in the transmembrane domain. As such qualities were found to be crucial for antidepressants to bind to TrkB and drive behavioral and neuroplasticity effects, we hypothesized that their effects might also depend on p75NTR.

METHODS

Enzyme-linked immunosorbent assay-based binding and nuclear magnetic resonance spectroscopy were performed to assess whether antidepressants would bind to p75NTR. HEK293T cells and a variety of in vitro assays were used to investigate whether fluoxetine (FLX) or ketamine (KET) would trigger any α- and γ-secretase-dependent p75NTR proteolysis and lead to p75NTR nuclear localization. Ocular dominance shift was performed with male and female p75NTR knockout mice to study the effects of KET and FLX on brain plasticity, in addition to pharmacological interventions to verify how p75NTR signaling is important for the effects of KET and FLX in enhancing extinction memory in male wild-type mice and rats.

RESULTS

Antidepressants were found to bind to p75NTR. FLX and KET triggered the p75NTR proteolytic pathway and induced p75NTR-dependent behavioral/neuroplasticity changes.

CONCLUSIONS

We hypothesize that antidepressants co-opt both BDNF/TrkB and proBDNF/p75NTR systems to induce a more efficient activity-dependent synaptic competition, thereby boosting the brain's ability for remodeling.

Stress-induced changes in the molecular processes underlying fear memories: implications for PTSD and relevant animal models

Most of the fear literature on humans and animals tests healthy individuals. However, fear memories can differ between healthy individuals and those previously exposed to traumatic stress, such as a car accident, sexual abuse, military combat and personal assault. Traumatic stress can lead to post-traumatic stress disorder (PTSD) which presents alterations in fear memories, such as an impairment of fear extinction and extinction recall. PTSD-like animal models are exposed to a single highly stressful experience in the laboratory, such as stress immobilization or single-prolonged stress. Some days later, animals exposed to a PTSD-like model can be tested in fear procedures that help uncover molecular mechanisms of fear memories. In this review, there are discussed the molecular mechanisms in stress-induced fear memories of patients with PTSD and PTSD-like animal models. The focus is on the effects of estradiol and cortisol/corticosterone hormones and of different genes, such as FKBP prolyl isomerase 5 gene (FKBP5) - FK506 binding protein 51 (FKBP51), pituitary adenylate cyclase-activating peptide (PACAP) - pituitary adenylate cyclase-activating polypeptide type I receptor (PAC1R), endocannabinoid (eCB) system and the tropomyosin receptor kinase B (TrkB) - brain-derived neurotrophic factor (BDNF). The conclusion is that greater emphasis should be placed on investigating the molecular mechanisms of fear memories in PTSD, through direct testing of patients with PTSD or the use of relevant PTSD-like models.

Mushrooms, Microdosing, and Mental Illness: The Effect of Psilocybin on Neurotransmitters, Neuroinflammation, and Neuroplasticity

The incidence of mental health disorders is increasing worldwide. While there are multiple factors contributing to this problem, neuroinflammation underlies a significant subset of psychiatric conditions, particularly major depressive and anxiety disorders. Anti-inflammatory interventions have demonstrated benefit in these conditions. Psilocin, the active ingredient of mushrooms in the Psilocybe genus, is both a potent serotonin agonist and anti-inflammatory agent, increases neuroplasticity, and decreases overactivity in the default mode network. Studies using hallucinogenic doses of psilocin under the supervision of a therapist/guide have consistently demonstrated benefits to individuals with depression and end-of-life anxiety. Microdosing psilocybin in sub-hallucinogenic doses has also demonstrated benefit in mood disorders, and may offer a safe, less expensive, and more available alternative to full doses of psilocybin for mood disorders, as well as for other medical conditions in which inflammation is the principal pathophysiology.

Trends in research on novel antidepressant treatments

Mood disorders, such as major depressive disorder and bipolar disorder, are among the most common mental illnesses and a leading cause of disability worldwide. Key symptoms of these conditions include a depressed mood or anhedonia, sleep and psychomotor disturbances, changes in appetite or weight, and fatigue or loss of energy. Prolonged cognitive disturbances further impair the ability to think or concentrate and are often accompanied by persistent feelings of worthlessness or excessive guilt. Collectively, these symptoms underscore depression as a serious, long-term global health issue. In addition, clinical studies indicate a growing number of patients experiencing difficulties in responding to treatment, even in the long term. This phenomenon poses significant challenges for healthcare professionals, families, and patients alike. As a result, there is an urgent need for therapies that are both rapid-acting and safe. This review aims to summarize the prevailing trends in research on novel antidepressants, emphasizing their diversity and multi-directional mechanisms of action. The development of rapid-acting drugs is increasingly focused on achieving high efficacy, particularly for treatment-resistant depression. Such advances offer the potential for rapid therapeutic effects without the prolonged and often tedious administration of older generation antidepressants. Findings from studies using animal models of depression continue to play a crucial role in predicting and designing new therapeutic strategies. These models remain indispensable for understanding the physiological effects of newly developed compounds, thereby guiding the creation of innovative treatments.

The immunological perspective of major depressive disorder: unveiling the interactions between central and peripheral immune mechanisms

Major depressive disorder is a prevalent mental disorder, yet its pathogenesis remains poorly understood. Accumulating evidence implicates dysregulated immune mechanisms as key contributors to depressive disorders. This review elucidates the complex interplay between peripheral and central immune components underlying depressive disorder pathology. Peripherally, systemic inflammation, gut immune dysregulation, and immune dysfunction in organs including gut, liver, spleen and adipose tissue influence brain function through neural and molecular pathways. Within the central nervous system, aberrant microglial and astrocytes activation, cytokine imbalances, and compromised blood-brain barrier integrity propagate neuroinflammation, disrupting neurotransmission, impairing neuroplasticity, and promoting neuronal injury. The crosstalk between peripheral and central immunity creates a vicious cycle exacerbating depressive neuropathology. Unraveling these multifaceted immune-mediated mechanisms provides insights into major depressive disorder's pathogenic basis and potential biomarkers and targets. Modulating both peripheral and central immune responses represent a promising multidimensional therapeutic strategy.

Clinical and preclinical evidence of psilocybin as antidepressant. A narrative review

In the rapidly growing field of psychedelic research, psilocybin (and active metabolite psilocin) has been proposed as a promising candidate in the search for novel treatments for neuropsychiatric disorders. Clinical trials have revealed that psilocybin has a large, rapid, and persistent effect in the improvement of symptoms of depression and anxiety. The safety profile is considered favourable, with low toxicity and good tolerance. Several preclinical studies have also been carried out to determine the long-term mechanism of action of this drug. In this sense, preclinical studies in naïve animals as well as in animal models of disease have shown somewhat discrepant results in conventional tests for assessment of depression- and anxiety-like phenotype in response to psilocybin, but overall suggest positive outcomes. Additionally, several valuable assays in rodent models have been developed over the years to elucidate the neurochemical correlates of serotonin 2A receptor (5HT2AR) activation in the brain, primary molecular target of psilocin. This review aims to provide a general overview of the current and most recent literature in the therapeutic potential of psilocybin through a description of clinical trials of psilocybin-assisted psychotherapy, and to showcase the scene in the up-to-date preclinical research. A detailed description of preclinical rodent models and experimental approaches that have been used to study the neurobiological and behavioural actions of psilocybin is provided, and potential therapeutic mechanisms of action are discussed.

Neurobiological mechanisms of antidepressant properties of psilocybin: A systematic review of blood biomarkers

Psilocybin represents a novel therapeutic approach for individuals with major depressive disorder (MDD) who do not respond to conventional antidepressant treatment. Investigating the influence of psilocybin on the pathophysiological processes involved in MDD could enhance our neurobiological understanding of the presumed antidepressant action mechanism. This systematic review aims to summarize the results of human studies investigating changes in blood-based biomarkers of MDD to guide future research on potentially relevant analytes that could be monitored in clinical trials. A systematic search was performed in MEDLINE, Embase, and Web of Science to retrieve studies investigating changes in serum and plasma levels of neurotrophic, immunologic, neuroendocrine, and metabolic markers. Nine studies were included, describing findings on 15 biomarkers, exclusively in healthy participants. Studies consistently reported a decrease in interleukin-6, C-reactive protein, and eosinophils, and an increase in cortisol, prolactin, oxytocin, thyroid-stimulating hormone, adrenocorticotropic hormone, brain-derived neurotrophic factor, and free fatty acids following psilocybin administration. Less consistent effects were observed on interleukin-1β, interleukin-8, tumour necrosis factor-alpha, soluble urokinase plasminogen activator receptor, and growth hormone. The results are in line with preclinical studies and provide initial support from human studies that psilocybin potentially leads to beneficial effects on biomarkers of MDD. However, given the limited number of studies, findings should be approached with caution prior to replication. Further research should include larger samples, clinical populations, longer-term assessment, rigorous experimental designs, and account for the potential confounding of psychological stress related to the psychedelic experience.

The immunomodulatory effects of psychedelics in Alzheimer's disease-related dementia

Dementia is an increasing disorder, and Alzheimer's disease (AD) is the cause of 60% of all dementia cases. Despite all efforts, there is no cure for stopping dementia progression. Recent studies reported potential effects of psychedelics on neuroinflammation during AD. Psychedelics by 5HT2AR activation can reduce proinflammatory cytokine levels (TNF-α, IL-6) and inhibit neuroinflammation. In addition to neuroinflammation suppression, psychedelics induce neuroplasticity by increasing Brain-derived neurotrophic factor (BDNF) levels through Sigma-1R stimulation. This review discussed the effects of psychedelics on AD from both neuroinflammatory and neuroplasticity standpoints.

Lipids shape brain function through ion channel and receptor modulations: physiological mechanisms and clinical perspectives

Lipids represent the most abundant molecular type in the brain, with a fat content of ∼60% of the dry brain weight in humans. Despite this fact, little attention has been paid to circumscribe the dynamic role of lipids in brain function and disease. Membrane lipids such as cholesterol, phosphoinositide, sphingolipids, arachidonic acid, and endocannabinoids finely regulate both synaptic receptors and ion channels that ensure critical neural functions. After a brief introduction on brain lipids and their respective properties, we review here their role in regulating synaptic function and ion channel activity, action potential propagation, neuronal development, and functional plasticity and their contribution in the development of neurological and neuropsychiatric diseases. We also provide possible directions for future research on lipid function in brain plasticity and diseases.

Indirect influence on the BDNF/TrkB receptor signaling pathway via GPCRs, an emerging strategy in the treatment of neurodegenerative disorders

Neuronal survival depends on neurotrophins and their receptors. There are two types of neurotrophin receptors: a nonenzymatic, trans-membrane protein of the tumor necrosis factor receptor (TNFR) family-p75 receptor and the tyrosine kinase receptors (TrkR) A, B, and C. Activation of the TrkBR by brain-derived neurotrophic factor (BDNF) or neurotrophin 4/5 (NT-4/5) promotes neuronal survival, differentiation, and synaptic function. It is shown that in the pathogenesis of several neurodegenerative conditions (Alzheimer's disease, Parkinson's disease, Huntington's disease) the BDNF/TrkBR signaling pathway is impaired. Since it is known that GPCRs and TrkR are regulating several cell functions by interacting with each other and generating a cross-communication in this review we have focused on the interaction between different GPCRs and their ligands on BDNF/TrkBR signaling pathway.

Brain region-specific roles of brain-derived neurotrophic factor in social stress-induced depressive-like behavior

Brain-derived neurotrophic factor is a key factor in stress adaptation and avoidance of a social stress behavioral response. Recent studies have shown that brain-derived neurotrophic factor expression in stressed mice is brain region-specific, particularly involving the corticolimbic system, including the ventral tegmental area, nucleus accumbens, prefrontal cortex, amygdala, and hippocampus. Determining how brain-derived neurotrophic factor participates in stress processing in different brain regions will deepen our understanding of social stress psychopathology. In this review, we discuss the expression and regulation of brain-derived neurotrophic factor in stress-sensitive brain regions closely related to the pathophysiology of depression. We focused on associated molecular pathways and neural circuits, with special attention to the brain-derived neurotrophic factor-tropomyosin receptor kinase B signaling pathway and the ventral tegmental area-nucleus accumbens dopamine circuit. We determined that stress-induced alterations in brain-derived neurotrophic factor levels are likely related to the nature, severity, and duration of stress, especially in the above-mentioned brain regions of the corticolimbic system. Therefore, BDNF might be a biological indicator regulating stress-related processes in various brain regions.

Modulation of the endocannabinoid system by (S)-ketamine in an animal model of depression

Ketamine (KET) is recognized as rapid-acting antidepressant, but its mechanisms of action remain elusive. Considering the role of endocannabinoids (eCB) in stress and depression, we investigated if S-KET antidepressant effects involve the regulation of the eCB system using an established rat model of depression based on selective breeding: the Flinders Sensitive Line (FSL) and their controls, the Flinders Resistant Line (FRL). S-KET (15 mg/kg) effects were assessed in rats exposed to the open field and forced swimming test (FST), followed by analysis of the eCB signaling in the rat prefrontal cortex (PFC), a brain region involved in depression neurobiology. Changes in eCB receptors and enzymes were assessed at mRNA and protein levels (qPCR and western blot), CB1 binding ([3H]SR141716A autoradiography) and endocannabinoid content (lipidomics). The results demonstrated that the depressive behavior in FSL was negatively correlated with 2-AG levels, which were restored upon acute S-KET treatment. Although S-KET decreased CB1 and FAAH gene expression in FSL, there were no significant changes at protein levels. [3H]SR141716A binding to CB1 receptors was increased by S-KET and in silico analysis suggested that it binds to CB1, CB2, GPR55 and FAAH. Overall, S-KET effects correlated with an increased endocannabinoid signaling in the PFC, but systemic treatment with rimonabant failed to block its behavioral effects. Altogether, our results indicate that S-KET facilitates eCB signaling in the PFC of FSL. The inability of rimonabant to block the antidepressant effect of S-KET highlights the complexity of its interaction with the ECS, warranting further investigation into the molecular pathways.

A neurotrophin functioning with a Toll regulates structural plasticity in a dopaminergic circuit

Experience shapes the brain as neural circuits can be modified by neural stimulation or the lack of it. The molecular mechanisms underlying structural circuit plasticity and how plasticity modifies behaviour are poorly understood. Subjective experience requires dopamine, a neuromodulator that assigns a value to stimuli, and it also controls behaviour, including locomotion, learning, and memory. In Drosophila, Toll receptors are ideally placed to translate experience into structural brain change. Toll-6 is expressed in dopaminergic neurons (DANs), raising the intriguing possibility that Toll-6 could regulate structural plasticity in dopaminergic circuits. Drosophila neurotrophin-2 (DNT-2) is the ligand for Toll-6 and Kek-6, but whether it is required for circuit structural plasticity was unknown. Here, we show that DNT-2-expressing neurons connect with DANs, and they modulate each other. Loss of function for DNT-2 or its receptors Toll-6 and kinase-less Trk-like kek-6 caused DAN and synapse loss, impaired dendrite growth and connectivity, decreased synaptic sites, and caused locomotion deficits. In contrast, over-expressed DNT-2 increased DAN cell number, dendrite complexity, and promoted synaptogenesis. Neuronal activity modified DNT-2, increased synaptogenesis in DNT-2-positive neurons and DANs, and over-expression of DNT-2 did too. Altering the levels of DNT-2 or Toll-6 also modified dopamine-dependent behaviours, including locomotion and long-term memory. To conclude, a feedback loop involving dopamine and DNT-2 highlighted the circuits engaged, and DNT-2 with Toll-6 and Kek-6 induced structural plasticity in this circuit modifying brain function and behaviour.

Beyond NMDA Receptors: A Narrative Review of Ketamine's Rapid and Multifaceted Mechanisms in Depression Treatment

The rising prevalence of depression, with its associated suicide risk, demands effective fast-acting treatments. Ketamine has emerged as promising, demonstrating rapid antidepressant effects. While early studies show swift mood improvements, its precise mechanisms remain unclear. This article aims to compile and synthesize the literature on ketamine's molecular actions. Ketamine primarily works by antagonizing NMDA receptors, reducing GABAergic inhibition, and increasing glutamate release. This enhanced glutamate activates AMPA receptors, triggering crucial downstream cascades, including BDNF-TrkB and mTOR pathways, promoting synaptic proliferation and regeneration. Moreover, neuroimaging studies have demonstrated alterations in brain networks involved in emotional regulation, including the Default Mode Network (DMN), Central Executive Network (CEN), and Salience Network (SN), which are frequently disrupted in depression. Despite the promising findings, the literature reveals significant inaccuracies and gaps in understanding the full scope of ketamine's therapeutic potential. For instance, ketamine engages with opioid receptors, insinuating a permissive role of the opioid system in amplifying ketamine's antidepressant effects, albeit ketamine does not operate as a direct opioid agonist. Further exploration is requisite to comprehensively ascertain its safety profile, long-term efficacy, and the impact of genetic determinants, such as BDNF polymorphisms, on treatment responsiveness.

Nitrous oxide induces hypothermia and TrkB activation: Maintenance of body temperature abolishes antidepressant-like effects in mice

Recent studies indicate that nitrous oxide (N2O), a gaseous anesthetic and an NMDA (N-methyl-D-aspartate) receptor antagonist, produces rapid antidepressant effect in patients suffering from treatment-resistant depression. Our recent work implies that hypothermia and reduced energy expenditure are connected with antidepressant-induced activation of TrkB neurotrophin receptors - a key regulator of synaptic plasticity. In this study, we demonstrate that a brief exposure to N2O leads to a drop in body temperature following the treatment, which is linked to decreased locomotor activity; enhanced slow-wave electroencephalographic activity; reduced brain glucose utilization; and increased phosphorylation of TrkB, GSK3β (glycogen synthase kinase 3β), and p70S6K (a kinase downstream of mTor (mammalian target of rapamycin)) in the medial prefrontal cortex of adult male mice. Moreover, preventing the hypothermic response in a chronic corticosterone stress model of depression attenuated the antidepressant-like behavioral effects of N2O in the saccharin preference test. These findings indicate that N2O treatment modulates TrkB signaling and related neurotrophic signaling pathways in a temperature-dependent manner, suggesting that the phenomenon driving TrkB activation - altered thermoregulation and energy expenditure - is linked to antidepressant-like behavioral responses.

In Silico Tools to Score and Predict Cholesterol-Protein Interactions

Cholesterol is structurally distinct from other lipids, which confers it with singular roles in membrane organization and protein function. As a signaling molecule, cholesterol engages in discrete interactions with transmembrane, peripheral, and certain soluble proteins to control cellular responses. Accordingly, the cholesterol-protein interface is central to cholesterol-related diseases and is an essential consideration in drug design. However, cholesterol's hydrophobic, un-drug-like nature presents a unique challenge to traditional in silico analyses. In this Perspective, we survey a collection of tools designed to predict and evaluate cholesterol binding sites in proteins, including classical sequence motifs, molecular docking, template-based strategies, molecular dynamics simulations, and recent artificial intelligence approaches. We then comment on contemporary tools to evaluate ligand-protein interactions, their applicability to cholesterol, and the yet-untapped potential of cholesterol-protein interactions in human health and disease.

The effects of chronic desipramine treatment on neurotrophin-3 in the brain of mice with selective depletion of CREB and CREM in noradrenergic neurons

The disturbances in neurotrophic support are thought to be one of the main causes of depression, which depend not only on the neurotrophins themselves but also on the molecules regulating their synthesis and effector functions. One such molecule is cAMP responsive element binding protein (CREB), which role in depression and antidepressant drugs mechanism of action has been extensively studied. However, CREB's effects vary depending on brain structure, necessitating specific transgenic models for studying its function. Moreover, deletion of CREB enhances cAMP response element modulator (CREM) expression, suspected to compensate for CREB in its absence. Previously, mice lacking CREB in noradrenergic neurons and CREM (Creb1DbhCreCrem-/-) showed to be insensitive to acute desipramine, whereas mice lacking only CREB (Creb1DbhCre) showed similar effects as wild type animals (w/t). As neurotrophic changes require chronic antidepressant treatment, in current study mice (w/t, Creb1DbhCre and Creb1DbhCreCrem-/-; both males and females) were given desipramine for 21 days, to assess the effects of the drug on CREB, neurotrophins and their receptors in the hippocampus and prefrontal cortex. Interestingly, desipramine had no effect on CREB in neither of studied groups. However, both male and female mice lacking CREB and CREM displayed alterations in neurotrophin-3 (NTF3) expression or protein levels, modulated by desipramine. These findings suggest NTF3 is connected with inhibited response to acute and probably chronic desipramine administration in Creb1DbhCreCrem-/- mice, although in w/t chronic desipramine had no effect on NTF3. Nevertheless, our findings give insight into the role of non-BDNF neurotrophins in the mechanism of antidepressant drugs.

Myocardial Disorders in BDNF-Deficient Rats: Limited Recovery Post-Moderate Endurance Training

Introduction

The study aimed to determine whether heterozygous BDNF-deficient (BDNF-knockout, SD-BDNF) rats exhibit pathological changes in the myocardium and to assess whether a 5-week moderate-intensity endurance training program can reverse adverse changes in the heart muscle.

Methods

Experiments were conducted on four groups of rats: control wild-type, control BDNF knockout, trained wild-type and trained BDNF knockout. Knockout rats were selected due to the presence of symptoms resembling metabolic syndrome in serum and liver while 5-week moderate endurance training was used as an intervention targeted at restoring heart function. Measurements of BDNF/Trk-B concentrations and molecules levels and activities, such as cardiac specific enzymes like creatine kinase and creatine kinase myocardial band, lipids as total cholesterol, low-density lipoprotein and triglycerides, metabolic enzymes including alanine aminotransferase, aspartate aminotransferase, gamma-glutamyl transferase and lactate dehydrogenase and interleukin-1 were carried out in myocardium homogenates.

Results

In BDNF-deficient rats, the myocardium showed significantly reduced lipid concentrations, decreased metabolic and cardiac enzyme activity, and elevated Trk-B levels, all of which are indicative of myocardial ischemia or hypoxia. These changes in critical biomarkers were consistent with those earlier observed in the livers of BDNF-deficient rats, suggesting a link between the liver and cardiac function. Moderate endurance training led to an increase in creatine kinase activity in the myocardium of trained rats, suggesting increased production and utilization of energy required for myocardial contraction in trained wild-type and knockout populations of rats.

Discussion

BDNF-deficient rats exhibited numerous myocardial abnormalities, most of which were not reversible after moderate-intensity endurance training. These findings provide a basis for a deeper understanding of the mechanisms underlying myocardial disorders in BDNF-deficient rats, which appear to be a suitable model for studying various aspects of metabolic disorders.

Antidepressant-like and antistress effects of the ACTH(4-10) synthetic analogs Semax and Melanotan II on male rats in a model of chronic unpredictable stress

Current antidepressant therapy shows substantial limitations, and there is an urgent need for the development of new treatment strategies for depression. Stressful events and hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis play an important role in the pathogenesis of depression. HPA axis activity is self-regulated by negative feedback at several levels including adrenocorticotropic hormone (ACTH)-mediated feedback. Here, we investigated whether noncorticotropic synthetic analogs of the ACTH(4-10) fragment, ACTH(4-7)-Pro-Gly-Pro (Semax) and Ac-Nle4-cyclo[Asp5-His6-D-Phe7-Arg8-Trp9-Lys10]ACTH(4-10)-NH2 (Melanotan II (MTII), a potent agonist of melanocortin receptors), have potential antidepressant activity in a chronic unpredictable stress (CUS) rat model of depression. Stressed and control male adult Sprague-Dawley rats received daily intraperitoneal injections of saline or a low dose (60 nmol/kg of body weight (BW)) of Semax or MTII. Rats were monitored for BW and hedonic status, as measured in the sucrose preference test. We found that chronic treatment with Semax and MTII reversed or substantially attenuated CUS-induced anhedonia, BW gain suppression, adrenal hypertrophy and a decrease in the hippocampal levels of BDNF. In the forced swim test, no effects of the CUS procedure or peptides on the duration of rat immobility were detected. Our findings show that in the CUS paradigm, systemically administered ACTH(4-10) analogs Semax and MTII exert antidepressant-like effects on anhedonia and hippocampal BDNF levels, and attenuate markers of chronic stress load, at least in male rats. The results support the argument that ACTH(4-10) analogs and other noncorticotropic melanocortins may have promising therapeutic potential for the treatment and prevention of depression and other stress-related pathologies.

Escin ameliorates CUMS-induced depressive-like behavior via BDNF/TrkB/CREB and TLR4/MyD88/NF-κB signaling pathways in rats

Major depressive disorder (MDD) is a prevalent psychiatric disorder associated with brain inflammation and neuronal damage. Derived from the Aesculus chinensis Bunge fruit, escin has shown anti-inflammatory and neuroprotective effects. However, its potential as a treatment for MDD is unclear. This study investigates the antidepressant properties of escin using in vivo experimentation. The chronic unpredictable mild stress (CUMS) model was used to analyze the potential antidepressant effects and underlying mechanisms of escin. Wistar rats were exposed to CUMS for 35 consecutive days to induce MDD. The rats were then given either escin (1, 3, and 10 mg/kg) or fluoxetine (2 mg/kg) on a daily basis. Notably, escin significantly alleviated the depressive behaviors induced by CUMS, as evaluated through a series of behavioral assessments. Moreover, escin administration reduced TNF-α, IL-1β, and IL-6 levels in the hippocampus. It also decreased serum adrenal cortical hormone (ACTH) and corticosterone (CORT) levels while increasing 5-HT and Brain-derived neurotrophic factor (BDNF) levels in the CUMS rats, as measured by the enzyme-linked immunosorbent assay (ELISA). Pathological changes in the hippocampal regions were identified through Nissl staining, and Western blotting was used to quantify the protein levels of BDNF, TrkB, CREB, TLR4, MyD88, and NF-κB. Escin mitigated neuronal injury, elevated TrkB, BDNF, and CREB, and reduced TLR4, MyD88, and NF-κB protein levels in CUMS rats. The data from this study suggest that escin holds the potential for alleviating depression-like symptoms induced by CUMS. This effect may be mediated through the modulation of two signaling pathways, BDNF/TrkB/CREB and TLR4/MyD88/NF-κB.

Beyond the serotonin deficit hypothesis: communicating a neuroplasticity framework of major depressive disorder

The serotonin deficit hypothesis explanation for major depressive disorder (MDD) has persisted among clinicians and the general public alike despite insufficient supporting evidence. To combat rising mental health crises and eroding public trust in science and medicine, researchers and clinicians must be able to communicate to patients and the public an updated framework of MDD: one that is (1) accessible to a general audience, (2) accurately integrates current evidence about the efficacy of conventional serotonergic antidepressants with broader and deeper understandings of pathophysiology and treatment, and (3) capable of accommodating new evidence. In this article, we summarize a framework for the pathophysiology and treatment of MDD that is informed by clinical and preclinical research in psychiatry and neuroscience. First, we discuss how MDD can be understood as inflexibility in cognitive and emotional brain circuits that involves a persistent negativity bias. Second, we discuss how effective treatments for MDD enhance mechanisms of neuroplasticity-including via serotonergic interventions-to restore synaptic, network, and behavioral function in ways that facilitate adaptive cognitive and emotional processing. These treatments include typical monoaminergic antidepressants, novel antidepressants like ketamine and psychedelics, and psychotherapy and neuromodulation techniques. At the end of the article, we discuss this framework from the perspective of effective science communication and provide useful language and metaphors for researchers, clinicians, and other professionals discussing MDD with a general or patient audience.

(2R,6R)-hydroxynorketamine alleviates PTSD-like endophenotypes by regulating the PI3K/AKT signaling pathway in rats

BACKGROUND

Patients diagnosed with post-traumatic stress disorder (PTSD) mainly exhibit enduring adverse emotions, heightening susceptibility to suicidal thoughts and behaviors. Notably, metabolites of ketamine, particularly (2R,6R)-hydroxyketamine (HNK), have demonstrated favorable antidepressant properties. However, the precise mechanism through which HNK exerts its therapeutic effects on negative emotional symptoms in PTSD patients should be fully elucidated.

METHODS

In this investigation, a model involving a single prolonged stress and plantar shock (SPS&S) was utilized, followed by the administration of (2R, 6R)-HNK into the lateral ventricle subsequent to the recovery phase. The evaluation of PTSD-related behaviors was conducted through the open field test (OFT), elevated plus maze test (EMPT), and forced swim test (FST). The expression of phosphatidylinositol 3-kinase (PI3K)/phosphokinase B (AKT) signaling pathway in rat brain regions was analyzed using molecular biology experiments.

RESULTS

SPS&S rats displayed adverse emotional behaviors characterized by depression and anxiety. Treatment with (2R, 6R)-HNK enhanced exploratory behavior and reversed negative emotional behaviors. This intervention mitigated disruptions in the expression levels of PI3K/AKT signaling pathway-associated proteins in the HIP and PFC, without influencing PI3K/AKT signaling in the AMY of SPS&S rats.

CONCLUSION

Traumatic stress can trigger negative emotional reactions in rats, potentially involving the PI3K/AKT signaling pathway in the HIP, PFC, and AMY. The (2R, 6R)-HNK compounds have demonstrated the potential to mitigate adverse emotions in rats subjected to the SPS&S paradigm. This effect may be attributed to the modulation of the PI3K/AKT signaling pathway in the HIP, and PFC, with a particularly notable impact observed in the HIP region.

Modifications in the C-terminal tail of TrkC significantly alter neurotrophin-3-promoted outgrowth of neurite-like processes from PC12 cells

TrkB and TrkC are quite common neurotrophin receptors found on the same cells in CNS. In the C-terminal tail, TrkB and TrkC differ only in two amino acid residues at positions immediately preceding the tyrosine residue, which, upon phosphorylation, becomes the docking site for phospholipase Cγ1 (PLCγ1). The question arose whether such a difference near the PLCγ1 docking site might contribute to differential response to neurotrophin. PC12 clones with the following receptors were obtained: wild-type TrkC, TrkC-Y820F with a defective PLCγ1 binding site, TrkC-T817S-I819V with two amino acid residues replaced with those in the TrkB tail. The outgrowth of neurite-like processes from TrkC-Y820F-containing cells appeared to be impaired, while the TrkC-T817S-I819V variant appeared more effective than wild-type TrkC in promoting the outgrowth of neurite-like processes after neurotrophin stimulation, at least in the compared PC12 cell clones. Taken together, both the tyrosine residue at the PLCγ1 docking site and the amino acid residues immediately preceding it appear important for TrkC-supported outgrowth of neurite-like processes.