Serotonin 2C receptor antagonists induce fast-onset antidepressant effects

Molecular and structural insights into the 5-HT2C receptor as a therapeutic target for substance use disorders

Substance use disorder (SUD) is a chronic condition, with maintained abuse of a substance leading to physiological and psychological alterations and often changes in cognitive and social behaviours. Current therapies include psychotherapy coupled with medication; however, high relapse rates reveal the shortcomings of these therapies. The signalling, expression profile, and neurological function of the serotonin 2C receptor (5-HT2C receptor) make it a candidate of interest for the treatment of SUD. Recently, psychedelics, which broadly act at 5-HT2 receptors, have indicated potential for the treatment of SUD, implicating the 5-HT2C receptor. The modern psychedelic movement has rekindled interest in the 5-HT2C receptor, resulting in many new studies, especially structural analyses. This review explores the structural, molecular and cellular mechanisms governing 5-HT2C receptor function in the context of SUD. This provides the basis of the preclinical and clinical evidence for their role in SUD and highlights the potential for future exploration.

Somatostatin: Linking Cognition and Alzheimer Disease to Therapeutic Targeting

Over 4 decades of research support the link between Alzheimer disease (AD) and somatostatin [somatotropin-releasing inhibitory factor (SRIF)]. SRIF and SRIF-expressing neurons play an essential role in brain function, modulating hippocampal activity and memory formation. Loss of SRIF and SRIF-expressing neurons in the brain rests at the center of a series of interdependent pathological events driven by amyloid-β peptide (Aβ), culminating in cognitive decline and dementia. The connection between the SRIF and AD further extends to the neuropsychiatric symptoms, seizure activity, and inflammation, whereas preclinical AD investigations show SRIF or SRIF receptor agonist administration capable of enhancing cognition. SRIF receptor subtype-4 activation in particular presents unique attributes, with the potential to mitigate learning and memory decline, reduce comorbid symptoms, and enhance enzymatic degradation of Aβ in the brain. Here, we review the links between SRIF and AD along with the therapeutic implications. SIGNIFICANCE STATEMENT: Somatostatin and somatostatin-expressing neurons in the brain are extensively involved in cognition. Loss of somatostatin and somatostatin-expressing neurons in Alzheimer disease rests at the center of a series of interdependent pathological events contributing to cognitive decline and dementia. Targeting somatostatin-mediated processes has significant therapeutic potential for the treatment of Alzheimer disease.

Major depressive disorder: hypothesis, mechanism, prevention and treatment

Worldwide, the incidence of major depressive disorder (MDD) is increasing annually, resulting in greater economic and social burdens. Moreover, the pathological mechanisms of MDD and the mechanisms underlying the effects of pharmacological treatments for MDD are complex and unclear, and additional diagnostic and therapeutic strategies for MDD still are needed. The currently widely accepted theories of MDD pathogenesis include the neurotransmitter and receptor hypothesis, hypothalamic-pituitary-adrenal (HPA) axis hypothesis, cytokine hypothesis, neuroplasticity hypothesis and systemic influence hypothesis, but these hypothesis cannot completely explain the pathological mechanism of MDD. Even it is still hard to adopt only one hypothesis to completely reveal the pathogenesis of MDD, thus in recent years, great progress has been made in elucidating the roles of multiple organ interactions in the pathogenesis MDD and identifying novel therapeutic approaches and multitarget modulatory strategies, further revealing the disease features of MDD. Furthermore, some newly discovered potential pharmacological targets and newly studied antidepressants have attracted widespread attention, some reagents have even been approved for clinical treatment and some novel therapeutic methods such as phototherapy and acupuncture have been discovered to have effective improvement for the depressive symptoms. In this work, we comprehensively summarize the latest research on the pathogenesis and diagnosis of MDD, preventive approaches and therapeutic medicines, as well as the related clinical trials.

A complex relation between levels of adult hippocampal neurogenesis and expression of the immature neuron marker doublecortin

We investigated the mechanisms underlying the effects of the antidepressant fluoxetine on behavior and adult hippocampal neurogenesis (AHN). After confirming our earlier report that the signaling molecule β-arrestin-2 (β-Arr2) is required for the antidepressant-like effects of fluoxetine, we found that the effects of fluoxetine on proliferation of neural progenitors and survival of adult-born granule cells are absent in the β-Arr2 knockout (KO) mice. To our surprise, fluoxetine induced a dramatic upregulation of the number of doublecortin (DCX)-expressing cells in the β-Arr2 KO mice, indicating that this marker can be increased even though AHN is not. We discovered two other conditions where a complex relationship occurs between the number of DCX-expressing cells compared to levels of AHN: a chronic antidepressant model where DCX is upregulated and an inflammation model where DCX is downregulated. We concluded that assessing the number of DCX-expressing cells alone to quantify levels of AHN can be complex and that caution should be applied when label retention techniques are unavailable.

Safety outcomes of selective serotonin reuptake inhibitors in adolescent attention-deficit/hyperactivity disorder with comorbid depression: the ASSURE study

BACKGROUND

Attention deficit-hyperactivity disorder (ADHD) is related to depressive disorder, and adolescents with both present poor outcomes. However, evidence for the safety of concomitantly using a methylphenidate (MPH) and a selective serotonin reuptake inhibitor (SSRI) among adolescent ADHD patients is limited, a literature gap aimed to address through this investigation.

METHODS

We conducted a new-user cohort study using a nationwide claims database in South Korea. We identified a study population as adolescents who were diagnosed both ADHD and depressive disorder. MPH-only users were compared with patients who prescribed both a SSRI and a MPH. Fluoxetine and escitalopram users were also compared to find a preferable treatment option. Thirteen outcomes including neuropsychiatric, gastrointestinal, and other events were assessed, taking respiratory tract infection as a negative control outcome. We matched the study groups using a propensity score and used the Cox proportional hazard model to calculate the hazard ratio. Subgroup and sensitivity analyses were conducted in various epidemiologic settings.

RESULTS

The risks of all the outcomes between the MPH-only and SSRI groups were not significantly different. Regarding SSRI ingredients, the risk of tic disorder was significantly lower in the fluoxetine group than the escitalopram group [HR 0.43 (0.25-0.71)]. However, there was no significant difference in other outcomes between the fluoxetine and escitalopram groups.

CONCLUSION

The concomitant use of MPHs and SSRIs showed generally safe profiles in adolescent ADHD patients with depression. Most of the differences between fluoxetine and escitalopram, except those concerning tic disorder, were not significant.

Antidepressant-like Effect of 1-(2-(4-(4-Ethylphenyl)-1H-1,2,3-triazol-1-yl)phenyl)ethan-1-one in Mice: Evidence of the Contribution of the Serotonergic System

Major depressive disorder (MDD) is a psychiatric disorder that affects a large portion of the population, with dysregulation of the serotonergic system, which is deeply involved in both the pathophysiology of MDD and mechanism of action of many antidepressants. Current pharmacological therapies do not meet the neurobiological needs of all depressed individuals, making the development of new antidepressants necessary. In recent decades, compounds containing triazoles have become promising due to their range of biological activities, including antidepressant activity. In this study, we evaluated the antidepressant-like effect of a hybrid containing triazole and acetophenone, 1-(2-(4-(4-ethylphenyl)-1H-1,2,3-triazol-1-yl)phenyl)ethan-1-one (ETAP) (0.5-5 mg/kg), in the forced swimming test (FST) and tail suspension test (TST) in mice, as well as the involvement of the serotonergic system in this effect. Our findings demonstrated that ETAP exhibited an antidepressant-like effect from the dose of 1 mg/kg and that this effect is modulated by 5-HT_2A/2C and 5-HT₄ receptors. We also demonstrated that this effect may be related to inhibition of monoamine oxidase A activity in the hippocampus. Additionally, we evaluated the in silico pharmacokinetic profile of ETAP, which predicted its penetration into the central nervous system. ETAP exhibited a low potential for toxicity at a high dose, making this molecule interesting for the development of a new therapeutic strategy for MDD.

Subcortical serotonin 5HT2c receptor-containing neurons sex-specifically regulate binge-like alcohol consumption, social, and arousal behaviors in mice

Binge alcohol consumption induces discrete social and arousal disturbances in human populations that promote increased drinking and accelerate the progression of Alcohol Use Disorder. Here, we show in a mouse model that binge alcohol consumption disrupts social recognition in females and potentiates sensorimotor arousal in males. These negative behavioral outcomes were associated with sex-specific adaptations in serotonergic signaling systems within the lateral habenula (LHb) and the bed nucleus of the stria terminalis (BNST), particularly those related to the receptor 5HT2c. While both BNST and LHb neurons expressing this receptor display potentiated activation following binge alcohol consumption, the primary causal mechanism underlying the effects of alcohol on social and arousal behaviors appears to be excessive activation of LHb5HT2c neurons. These findings may have valuable implications for the development of sex-specific treatments for mood and alcohol use disorders targeting the brain's serotonin system.

Involvement of the serotonergic system in the antidepressant-like effect of 1-(phenylselanyl)-2-(p-tolyl)indolizine in mice

RATIONALE

Depression is a mental disorder that affects approximately 280 million people worldwide. In the search for new treatments for mood disorders, compounds containing selenium and indolizine derivatives show promising results.

OBJECTIVES AND METHODS

To evaluate the antidepressant-like effect of 1-(phenylselanyl)-2-(p-tolyl)indolizine (MeSeI) (0.5-50 mg/kg, intragastric-i.g.) on the tail suspension test (TST) and the forced swim test (FST) in adult male Swiss mice and to elucidate the role of the serotonergic system in this effect through pharmacological and in silico approaches, as well to evaluate acute oral toxicity at a high dose (300 mg/kg).

RESULTS

MeSeI administered 30 min before the FST and the TST reduced immobility time at doses from 1 mg/kg and at 50 mg/kg and increased the latency time for the first episode of immobility, demonstrating an antidepressant-like effect. In the open field test (OFT), MeSeI did not change the locomotor activity. The antidepressant-like effect of MeSeI (50 mg/kg, i.g.) was prevented by the pre-treatment with p-chlorophenylalanine (p-CPA), a selective tryptophan hydroxylase inhibitor (100 mg/kg, intraperitoneally-i.p. for 4 days), with ketanserin, a 5-HT_2A/2C receptor antagonist (1 mg/kg, i.p.), and with GR113808, a 5-HT₄ receptor antagonist (0.1 mg/kg, i.p.), but not with WAY100635, a selective 5-HT_1A receptor antagonist (0.1 mg/kg, subcutaneous-s.c.) and ondansetron, a 5-HT₃ receptor antagonist (1 mg/kg, i.p.). MeSeI showed a binding affinity with 5-HT_2A, 5 -HT_2C, and 5-HT₄ receptors by molecular docking. MeSeI (300 mg/kg, i.g.) demonstrated low potential to cause acute toxicity in adult female Swiss mice.

CONCLUSION

In summary, MeSeI exhibits an antidepressant-like effect mediated by the serotonergic system and could be considered for the development of new treatment strategies for depression.

Plasticity of synapses and reward circuit function in the genesis and treatment of depression

What changes in brain function cause the debilitating symptoms of depression? Can we use the answers to this question to invent more effective, faster acting antidepressant drug therapies? This review provides an overview and update of the converging human and preclinical evidence supporting the hypothesis that changes in the function of excitatory synapses impair the function of the circuits they are embedded in to give rise to the pathological changes in mood, hedonic state, and thought processes that characterize depression. The review also highlights complementary human and preclinical findings that classical and novel antidepressant drugs relieve the symptoms of depression by restoring the functions of these same synapses and circuits. These findings offer a useful path forward for designing better antidepressant compounds.

Chronic Trazodone and Citalopram Treatments Increase Trophic Factor and Circadian Rhythm Gene Expression in Rat Brain Regions Relevant for Antidepressant Efficacy

Trazodone is an efficacious atypical antidepressant acting both as an SSRI and a 5HT2A and 5HT2C antagonist. Antagonism to H1-histaminergic and alpha1-adrenergic receptors is responsible for a sleep-promoting action. We studied long-term gene expression modulations induced by chronic trazodone to investigate the molecular underpinning of trazodone efficacy. Rats received acute or chronic treatment with trazodone or citalopram. mRNA expression of growth factor and circadian rhythm genes was evaluated by qPCR in the prefrontal cortex (PFCx), hippocampus, Nucleus Accumbens (NAc), amygdala, and hypothalamus. CREB levels and phosphorylation state were evaluated using Western blotting. BDNF levels were significantly increased in PFCx and hippocampus by trazodone and in the NAc and hypothalamus by citalopram. Likewise, TrkB receptor levels augmented in the PFCx after trazodone and in the amygdala after citalopram. FGF-2 and FGFR2 levels were higher after trazodone in the PFCx. The CREB phosphorylation state was increased by chronic trazodone in the PFCx, hippocampus, and hypothalamus. Bmal1 and Per1 were increased by both antidepressants after acute and chronic treatments, while Per2 levels were specifically augmented by chronic trazodone in the PFCx and NAc, and by citalopram in the PFCx, amygdala, and NAc. These findings show that trazodone affects the expression of neurotrophic factors involved in antidepressant responses and alters circadian rhythm genes implicated in the pathophysiology of depression, thus shedding light on trazodone's molecular mechanism of action.

Decursinol-mediated antinociception and anti-allodynia in acute and neuropathic pain models in male mice: Tolerance and receptor profiling

Korean scientists have shown that oral administration of Angelica gigas Nakai (AGN) root alcoholic extract and the metabolite of its pyranocoumarins, decursinol, have antinociceptive properties across various thermal and acute inflammatory pain models. The objectives of this study were 1) to assess whether tolerance develops to the antinociceptive effects of once-daily intraperitoneally administered decursinol (50 mg/kg) in acute thermal pain models, 2) to establish its anti-allodynic efficacy and potential tolerance development in a model of chemotherapy-evoked neuropathic pain (CENP) and 3) to probe the involvement of select receptors in mediating the pain-relieving effects with antagonists. The results show that decursinol induced antinociception in both the hot plate and tail-flick assays and reversed mechanical allodynia in mice with cisplatin-evoked neuropathic pain. Tolerance was detected to the antinociceptive effects of decursinol in the hot plate and tail-flick assays and to the anti-allodynic effects of decursinol in neuropathic mice. Pretreatment with either the 5-HT2 antagonist methysergide, the 5-HT2A antagonist volinanserin, or the 5-HT2C antagonist SB-242084 failed to attenuate decursinol-induced antinociception in the tail-flick assay. While pretreatment with the cannabinoid inverse agonists rimonabant and SR144528 failed to modify decursinol-induced anti-allodynia, pretreatment with the opioid antagonist naloxone partially attenuated the anti-allodynic effects of decursinol. In conclusion, our data support decursinol as an active phytochemical of AGN having both antinociceptive and anti-allodynic properties. Future work warrants a more critical investigation of potential receptor mechanisms as they are likely more complicated than initially reported.

COVID-19 and olfactory dysfunction: a looming wave of dementia?

Olfactory dysfunction is a hallmark symptom of COVID-19 disease resulting from the SARS-CoV-2 virus. The cause of the sudden and usually temporary anosmia that most people suffer from COVID-19 is likely entirely peripheral-inflammation and other damage caused by the virus in the sensory epithelium inside the upper recesses of the nasal cavity can damage or prevent chemicals from properly activating the olfactory sensory neurons. However, persistent olfactory dysfunction from COVID-19, in the form of hyposmia and parosmia (decreased or altered smell) may affect as many as 15 million people worldwide. This epidemic of olfactory dysfunction is thus a continuing public health concern. Mounting evidence suggests that the SARS-CoV-2 virus itself or inflammation from the immune response in the nasal sensory epithelium may invade the olfactory bulb, likely via non-neuronal transmission. COVID-19-related long-term olfactory dysfunction and early damage to olfactory and limbic brain regions suggest a pattern of degeneration similar to that seen in early stages of Alzheimer's disease, Parkinson's disease, and Lewy body dementia. Thus, long-term olfactory dysfunction coupled with cognitive and emotional disturbance from COVID-19 may be the first signs of delayed onset dementia from neurodegeneration. Few treatments are known to be effective to prevent further degeneration, but the first line of defense against degeneration may be olfactory and environmental enrichment. There is a pressing need for more research on treatments for olfactory dysfunction and longitudinal studies including cognitive and olfactory function from patients who have recovered from even mild COVID-19.NEW & NOTEWORTHY More than 15 million people worldwide experience persistent COVID-19 olfactory dysfunction, possibly caused by olfactory bulb damage. SARS-CoV-2 can cause inflammation and viral invasion of the olfactory bulb, initiating a cascade of degeneration similar to Alzheimer's disease and Lewy body disease. People who have had even mild cases of COVID-19 show signs of degeneration in cortical areas connected with the olfactory system. These data suggest a wave of post-COVID dementia in the coming decades.

Uncovering the Underlying Mechanisms of Ketamine as a Novel Antidepressant

Major depressive disorder (MDD) is a devastating psychiatric disorder which exacts enormous personal and social-economic burdens. Ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, has been discovered to exert rapid and sustained antidepressant-like actions on MDD patients and animal models. However, the dissociation and psychotomimetic propensities of ketamine have limited its use for psychiatric indications. Here, we review recently proposed mechanistic hypotheses regarding how ketamine exerts antidepressant-like actions. Ketamine may potentiate α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR)-mediated transmission in pyramidal neurons by disinhibition and/or blockade of spontaneous NMDAR-mediated neurotransmission. Ketamine may also activate neuroplasticity- and synaptogenesis-relevant signaling pathways, which may converge on key components like brain-derived neurotrophic factor (BDNF)/tropomyosin receptor kinase B (TrkB) and mechanistic target of rapamycin (mTOR). These processes may subsequently rebalance the excitatory/inhibitory transmission and restore neural network integrity that is compromised in depression. Understanding the mechanisms underpinning ketamine’s antidepressant-like actions at cellular and neural circuit level will drive the development of safe and effective pharmacological interventions for the treatment of MDD.

Translational control by ketamine and its implications for comorbid cognitive deficits in depressive disorders

Ketamine has shown antidepressant effects in patients with major depressive disorder (MDD) resistant to first-line treatments and approved for use in this patient population. Ketamine induces several forms of synaptic plasticity, which are proposed to underlie its antidepressant effects. However, the molecular mechanism of action directly responsible for ketamine's antidepressant effects remains under active investigation. It was recently demonstrated that the effectors of the mammalian target of rapamycin complex 1 (mTORC1) signalling pathway, namely, eukaryotic initiation factor 4E (eIF4E) binding proteins 1 and 2 (4E-BP1 and 4E-BP2), are central in mediating ketamine-induced synaptic plasticity and behavioural antidepressant-like effect. 4E-BPs are a family of messenger ribonucleic acid (mRNA) translation repressors inactivated by mTORC1. We observed that their expression in inhibitory interneurons mediates ketamine's effects in the forced swim and novelty suppressed feeding tests and the long-lasting inhibition of GABAergic neurotransmission in the hippocampus. In addition, another effector pathway that regulates translation elongation downstream of mTORC1, the eukaryotic elongation factor 2 kinase (eEF2K), has been implicated in ketamine's behavioural effects. We will discuss how ketamine's rapid antidepressant effect depends on the activation of neuronal mRNA translation through 4E-BP1/2 and eEF2K. Furthermore, given that these pathways also regulate cognitive functions, we will discuss the evidence of ketamine's effect on cognitive function in MDD. Overall, the data accrued from pre-clinical research have implicated the mRNA translation pathways in treating mood symptoms of MDD. However, it is yet unclear whether the pro-cognitive potential of subanesthetic ketamine in rodents also engages these pathways and whether such an effect is consistently observed in the treatment-resistant MDD population.

Disruption of 5-hydroxytryptamine 1A receptor and orexin receptor 1 heterodimer formation affects novel G protein-dependent signaling pathways and has antidepressant effects in vivo

G protein-coupled receptor (GPCR) heterodimers are new targets for the treatment of depression. Increasing evidence supports the importance of serotonergic and orexin-producing neurons in numerous physiological processes, possibly via a crucial interaction between 5-hydroxytryptamine 1A receptor (5-HT1AR) and orexin receptor 1 (OX1R). However, little is known about the function of 5-HT1AR/OX1R heterodimers. It is unclear how the transmembrane domains (TMs) of the dimer affect its function and whether its modulation mediates antidepressant-like effects. Here, we examined the mechanism of 5-HT1AR/OX1R dimerization and downstream G protein-dependent signaling. We found that 5-HT1AR and OX1R form constitutive heterodimers that induce novel G protein-dependent signaling, and that this heterodimerization does not affect recruitment of β-arrestins to the complex. In addition, we found that the structural interface of the active 5-HT1AR/OX1R dimer transforms from TM4/TM5 in the basal state to TM6 in the active conformation. We also used mutation analyses to identify key residues at the interface (5-HT1AR R151^4.40, 5-HT1AR Y198^5.41, and OX1R L230^5.54). Injection of chronic unpredictable mild stress (CUMS) rats with TM4/TM5 peptides improved their depression-like emotional status and decreased the number of endogenous 5-HT1AR/OX1R heterodimers in the rat brain. These antidepressant effects may be mediated by upregulation of BDNF levels and enhanced phosphorylation and activation of CREB in the hippocampus and medial prefrontal cortex. This study provides evidence that 5-HT1AR/OX1R heterodimers are involved in the pathological process of depression. Peptides including TMs of the 5-HT1AR/OX1R heterodimer interface are candidates for the development of compounds with fast-acting antidepressant-like effects.

Zebrafish as a Useful Tool in the Research of Natural Products With Potential Anxiolytic Effects

Zebrafish (Danio rerio) is a popular and valuable species used in many different biomedical research areas. The complex behavior that fish exhibit in response to different stimuli allows researchers to explore the biological and pharmacological basis of affective and mood disorders. In this sense, anxiety is commonly studied in preclinical research with animal models in rodents. During the last decade, those models have been successfully adapted to zebrafish. Stressful stimuli, such as novel environments, chemical substances, light conditions, and predator images, can trigger defensive behaviors considered indicators of an anxiety-like state. In the first stage, models were adapted and validated with different stressors and anxiolytic drugs with promising results and are now successfully used to generate scientific knowledge. In that sense, zebrafish allows several routes of administration and other methodological advantages to explore the anxiolytic effects of natural products in behavioral tests as novel tank, light-dark chamber, and black/white maze, among others. The present work will review the main findings on preclinical research using adult zebrafish to explore anxiolytics effects of natural products as plant secondary metabolites such as flavonoids, alkaloids and terpenes or standardized extracts of plants, among others. Scientific literature confirms the utility of zebrafish tests to explore anxiety-like states and anxiolytic-like effects of plant secondary metabolites, which represent a useful and ethical tool in the first stages of behavioral.

A selective serotonin reuptake inhibitor ameliorates obsessive-compulsive disorder-like perseverative behavior by attenuating 5-HT2C receptor signaling in the orbitofrontal cortex

Perseveration is a characteristic of patients with obsessive-compulsive disorder (OCD). Clinically, neuronal activity in the lateral orbitofrontal cortex (OFC) is increased in OCD patients. Successful treatment with selective serotonin reuptake inhibitors (SSRIs) reduces activity in the lateral OFC of OCD patients, but the precise mechanisms underlying this effect are unclear. Previously, we reported that repeated injection of the dopamine D₂ receptor agonist quinpirole (QNP) resulted in OCD-like deficits, including perseveration in a reversal learning task. QNP-treated mice showed hyperactivity in lateral OFC pyramidal neurons. The present study demonstrated that 4-week administration of an SSRI increased the rate of correct choice in a reversal learning task. Using the electrophysiological approach, we revealed that an SSRI decreased the activity of lateral OFC pyramidal neurons in QNP-treated mice by potentiating inhibitory inputs. The 4-week administration of an SSRI inhibited the potentiation of neuronal activity induced by a 5-HT_2C receptor agonist. Additionally, both 4-week administration of SSRI and acute application of 5-HT_2C receptor antagonist prevented the QNP-induced potentiation of inhibitory inputs to fast-spiking interneurons in the lateral OFC. Administration of a 5-HT_2C receptor antagonist to mice for 4 days increased the rate of correct choice in a reversal learning task. Collectively, these results indicate that chronic SSRI ameliorated perseverative behavior in QNP-treated mice by modulating inhibitory inputs in the lateral OFC. Short-term 5-HT_2C receptor blockade also ameliorated QNP-induced behavioral and neurological abnormalities by, at least in part, a common mechanism with chronic SSRI.

Astroglial Serotonin Receptors as the Central Target of Classic Antidepressants

Major depressive disorder (MDD) presents multiple clinical phenotypes and has complex underlying pathological mechanisms. Existing theories cannot completely explain the pathophysiological mechanism(s) of MDD, while the pharmacology of current antidepressants is far from being fully understood. Astrocytes, the homeostatic and defensive cells of the central nervous system, contribute to shaping behaviors, and regulating mood and emotions. A detailed introduction on the role of astrocytes in depressive disorders is thus required, to which this chapter is dedicated. We also focus on the interactions between classic antidepressants and serotonin receptors, overview the role of astrocytes in the pharmacological mechanisms of various antidepressants, and present astrocytes as targets for the treatment of bipolar disorder. We provide a foundation of knowledge on the role of astrocytes in depressive disorders and astroglial 5-HT_2B receptors as targets for selective serotonin reuptake inhibitors in vivo and in vitro.

Akkermansia muciniphila Protects Against Psychological Disorder-Induced Gut Microbiota-Mediated Colonic Mucosal Barrier Damage and Aggravation of Colitis

Psychological disorders are associated with increased risk of severe inflammatory bowel disease (IBD) by causing gut microbiota dysbiosis and colonic mucosal barrier damage. However, the interaction between chronic restraint stress (CRS), gut microbiota composition, and colonic mucus remains unclear. We demonstrated that mice under CRS conditions exhibited alterations in microbiota composition, disruption of colonic mucus, and aggravation of colitis. In addition, the abundance of Akkermansia muciniphila was significantly decreased in mice under CRS and UC patients with depression, and positively associated with the expression of MUC2. After antibiotic treatment, the recipient mice colonized with CRS microbiota showed barrier defects and severe colitis. Administration of Akkermansia muciniphila was found to restore colonic mucus and modify the gut microbiota. We confirm that CRS-mediated gut microbiota dysbiosis results in colonic mucosal barrier damage and aggravation of colitis. Our results suggest that A. muciniphila is expected to be a potential probiotic to protect and treat colonic mucus that is involved in IBD with psychological disorders.

Evidence of an antidepressant-like effect of xylopic acid mediated by serotonergic mechanisms

BACKGROUND

Depression causes significant debilitating symptoms and economic burden. Current management is challenged by slow onset of action and modest efficacies of antidepressants; thus, the search for newer antidepressants remains relevant. We evaluated the antidepressant effects of a kaurene diterpene, xylopic acid (XA), in zebrafish and mouse models.

METHODS

The chronic unpredictable stress (CUS) protocol in zebrafish and the tail suspension test (TST), forced swim test (FST), lipopolysaccharide-induced depression-like behaviour test (LID) and repeated open space swimming test (OSST) in mice were used. We further examined the impact of depleting monoamines on XA's antidepressant effects. The contribution of glutamatergic and nitrergic pathways on the antidepressant effect of XA in mice and XA's effects on 5-HT receptors and monoamine oxidase (MAO) enzymes were also evaluated. Finally, XA's influence on neuroprotection was evaluated by measuring BDNF and oxidative stress enzymes in whole brain. XA doses (1-10 μM) in zebrafish and (10, 30, 100 mg kg-1) in mice exerted potent antidepressant-like potential in FST, TST, LID and showed fast-onset antidepressant-like property in the OSST.

RESULTS

The antidepressant-like properties in mice were reversed by blocking synthesis/release of serotonin but not noradrenaline using p-chlorophenylalanine and α-methyl-p-tyrosine, respectively. This antidepressant-like effect was potentiated by D-cycloserine and Nω-Nitro-L-arginine methyl ester (L-NAME) but not by D-serine and L-arginine. XA also evoked partial agonist-like effects on 5-hydroxytrptamine receptors on the rat fundus but it did not have MAO inhibition effect. It also increased BDNF, glutathione and antioxidant enzymes.

CONCLUSION

Therefore, xylopic acid possesses antidepressant-like effects largely mediated by serotonergic and neuroprotective mechanisms.

Early response to trazodone once-a-day in major depressive disorder: review of the clinical data and putative mechanism for faster onset of action

BACKGROUND

Most antidepressants have a delayed onset of action and must be administered for several weeks to generate therapeutic effects. Trazodone is a serotonin antagonist and reuptake inhibitor approved for the treatment of major depressive disorder. The once-a-day (OAD) formulation of trazodone has an improved tolerability profile compared to its conventional formulations. In this study, we systematically reviewed the evidence available for the antidepressant efficacy and early improvement in depressive symptoms with trazodone OAD treatment.

METHOD

We conducted a PubMed database search for randomized controlled trials published from 2005 to 2020.

RESULTS

Two studies, a placebo-controlled and an active-comparator (venlafaxine extended-release or XR) study were found. Both the studies demonstrated that trazodone exhibits antidepressant activity at a starting dose of 150 mg/day and results in statistically significant greater reduction in Hamilton Depression Rating Scale (HAM-D17) scores within 1 week of starting treatment compared to placebo or venlafaxine XR (P < .05). Trazodone also resulted in significant early improvement in the HAM-D17 sleep disturbance factor compared to placebo or venlafaxine XR at day 7 (P < .05). This clinical effect is supported by in vitro proprietary data for the affinity of trazodone for different target receptors. Activity at these receptors may underlie trazodone's fast antidepressant action.

CONCLUSIONS

Trazodone, if properly dosed, can be an effective antidepressant with early onset of action and good tolerability. Future studies designed to specifically evaluate onset and timing of improvement of depressive symptoms remain necessary to confirm and extend these results.

EEG and Sleep Effects of Tramadol Suggest Potential Antidepressant Effects with Different Mechanisms of Action

Tramadol is a widely used, centrally acting, opioid analgesic compound, with additional inhibitory effects on the synaptic reuptake of serotonin and noradrenaline, as well as on the 5-HT₂ and NMDA receptors. Preclinical and clinical evidence also suggests its therapeutic potential in the treatment of depression and anxiety. The effects of most widely used antidepressants on sleep and quantitative electroencephalogram (qEEG) are well characterized; however, such studies of tramadol are scarce. Our aim was to characterize the effects of tramadol on sleep architecture and qEEG in different sleep–wake stages. EEG-equipped Wistar rats were treated with tramadol (0, 5, 15 and 45 mg/kg) at the beginning of the passive phase, and EEG, electromyogram and motor activity were recorded. Tramadol dose-dependently reduced the time spent in rapid eye movement (REM) sleep and increased the REM onset latency. Lower doses of tramadol had wake-promoting effects in the first hours, while 45 mg/kg of tramadol promoted sleep first, but induced wakefulness thereafter. During non-REM sleep, tramadol (15 and 45 mg/kg) increased delta and decreased alpha power, while all doses increased gamma power. In conclusion, the sleep-related and qEEG effects of tramadol suggest antidepressant-like properties, including specific beneficial effects in selected patient groups, and raise the possibility of a faster acting antidepressant action.

GW117: A novel serotonin (5-HT2C ) receptor antagonist and melatonin (MT1 /MT2 ) receptor agonist with potential antidepressant-like activity in rodents

Aims

To evaluate the antidepressant‐like effect of compound GW117 in rodents using in vitro binding and uptake assays as well in vivo behavioral tests.

Methods

We investigated the target profile of GW117 using [³⁵S]‐GTPγS and [³H]PIP binding. Using the forced swimming test and chronic unpredictable stress in rats, tail suspension test in mice and rats, and learned helplessness model in mice, we further revealed the antidepressant‐like and anxiolytic‐like effects of GW117.

Results

The current study suggests that GW117 displays serotonin 2C (5‐HT_2C) receptor antagonist and melatonin type 1 and 2 (MT₁/MT₂) receptor agonist properties, as well as evident antidepressant and anxiolytic effects.

Conclusion

These data suggest that GW117 is probably a potent antidepressant.

Lysergic acid diethylamide (LSD) promotes social behavior through mTORC1 in the excitatory neurotransmission

Clinical studies have reported that the psychedelic lysergic acid diethylamide (LSD) enhances empathy and social behavior (SB) in humans, but its mechanism of action remains elusive. Using a multidisciplinary approach including in vivo electrophysiology, optogenetics, behavioral paradigms, and molecular biology, the effects of LSD on SB and glutamatergic neurotransmission in the medial prefrontal cortex (mPFC) were studied in male mice. Acute LSD (30 μg/kg) injection failed to increase SB. However, repeated LSD (30 μg/kg, once a day, for 7 days) administration promotes SB, without eliciting antidepressant/anxiolytic-like effects. Optogenetic inhibition of mPFC excitatory neurons dramatically inhibits social interaction and nullifies the prosocial effect of LSD. LSD potentiates the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and 5-HT2A, but not N-methyl-D-aspartate (NMDA) and 5-HT1A, synaptic responses in the mPFC and increases the phosphorylation of the serine-threonine protein kinases Akt and mTOR. In conditional knockout mice lacking Raptor (one of the structural components of the mTORC1 complex) in excitatory glutamatergic neurons (Raptorf/f:Camk2alpha-Cre), the prosocial effects of LSD and the potentiation of 5-HT2A/AMPA synaptic responses were nullified, demonstrating that LSD requires the integrity of mTORC1 in excitatory neurons to promote SB. Conversely, in knockout mice lacking Raptor in GABAergic neurons of the mPFC (Raptorf/f:Gad2-Cre), LSD promotes SB. These results indicate that LSD selectively enhances SB by potentiating mPFC excitatory transmission through 5-HT2A/AMPA receptors and mTOR signaling. The activation of 5-HT2A/AMPA/mTORC1 in the mPFC by psychedelic drugs should be explored for the treatment of mental diseases with SB impairments such as autism spectrum disorder and social anxiety disorder.

Disinhibition of somatostatin interneurons confers resilience to stress in male but not female mice

Chronic stress represents a vulnerability factor for anxiety and depressive disorders and has been widely used to model aspects of these disorders in rodents. Disinhibition of somatostatin (SST)-positive GABAergic interneurons in mice by deletion of γ2 GABAA receptors selectively from these cells (SSTCre:γ2f/f mice) has been shown to result in behavioral and biochemical changes that mimic the responses to antidepressant doses of ketamine. Here we explored the extent to which SSTCre:γ2f/f mice exhibit resilience to unpredictable chronic mild stress (UCMS). We found that male SSTCre:γ2f/f mice are resilient to UCMS-induced (i) reductions in weight gain, (ii) reductions in SST-immuno-positive cells in medial prefrontal cortex (mPFC), (iii) increases in phosphorylation of eukaryotic elongation factor 2 (eEF2) in mPFC, and (iv) increased anxiety in a novelty suppressed feeding test. Female SSTCre:γ2f/f mice were resilient to UCMS-induced reductions in SST-immuno-positive cells indistinguishably from males. However, in contrast to males, they showed no UCMS effects on weight gain independent of genotype. Moreover, in mPFC of female γ2f/f control mice, UCMS resulted in paradoxically reduced p-EF2 levels without stress effects in the SSTCre:γ2f/f mutants. Lastly, female SSTCre:γ2f/f mice showed increased rather than reduced UCMS induced anxiety compared to γ2f/f controls. Thus, disinhibition of SST interneurons results in behavioral resilience to UCMS selectively in male mice, along with cellular resilience of SST neurons to UCMS independent of sex. Thus, mechanisms underlying vulnerability and resilience to stress are sex specific and map to mPFC rather than hippocampus but appear unrelated to changes in expression of SST as a marker of corresponding interneurons.

Rapid and Prolonged Antidepressant-like Effect of Crocin Is Associated with GHSR-Mediated Hippocampal Plasticity-related Proteins in Mice Exposed to Prenatal Stress

Prenatal stress (PNS) has a prolonged and adverse effect on offspring, leading to a significantly increased vulnerability to developing depression in their later life. Traditional therapies have delayed onset and limited efficacy; thus, it remains an urgent need to find novel medications with fast-onset and high-efficacy potentials. Crocin, with its structure clearly examined, has shown antidepressant-like effects. However, few studies extensively investigated its effect especially in mice exposed to PNS. Using an established PNS model, we tested whether crocin could have a rapid and persistent antidepressant-like effect in PNS mice. Growth hormone secretagogue receptor (GHSR) and phosphoinositide 3-kinase (PI3K) inhibitors were used to test their effects in antidepressant-like effect of crocin. Hippocampal GHSR-PI3K signaling was examined both in PNS mice treated with a single dose of crocin and in combination of GHSR inhibitor. PNS mice showed depression-like behaviors at juvenile and adulthood, and crocin induced an instant and persistent antidepressant-like response in PNS mice in a dose-dependent manner. Moreover, crocin increased the expression of hippocampal synaptic plasticity-associated proteins through the restoration of GHSR-PI3K signaling. Inhibitions of both GHSR and PI3K abolished the effect of crocin in alleviating depressive-like behaviors. More importantly, GHSR inhibitor JMV2959 blocked the enhanced expression of hippocampal plasticity-related proteins induced by crocin. The present study demonstrated that crocin induced a fast-onset and prolonged antidepressant effect in PNS mice and suggested that GHSR-PI3K signaling may play a key role in crocin's effect at least partially by a restoration of hippocampal synaptic plasticity-associated proteins.

A Brain Signaling Framework for Stress-Induced Depression and Ketamine Treatment Elucidated by Phosphoproteomics

Depression is a common affective disorder characterized by significant and persistent low mood. Ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, is reported to have a rapid and durable antidepressant effect, but the mechanisms are unclear. Protein phosphorylation is a post-translational modification that plays a crucial role in cell signaling. Thus, we present a phosphoproteomics approach to investigate the mechanisms underlying stress-induced depression and the rapid antidepressant effect of ketamine in mice. We analyzed the phosphoprotein changes induced by chronic unpredictable mild stress (CUMS) and ketamine treatment in two known mood control centers, the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAc). We initially obtained >8,000 phosphorylation sites. Quantitation revealed 3,988 sites from the mPFC and 3,196 sites from the NAc. Further analysis revealed that changes in synaptic transmission-related signaling are a common feature. Notably, CUMS-induced changes were reversed by ketamine treatment, as shown by the analysis of commonly altered sites. Ketamine also induced specific changes, such as alterations in synapse organization, synaptic transmission, and enzyme binding. Collectively, our findings establish a signaling framework for stress-induced depression and the rapid antidepressant effect of ketamine.

Translocator protein-mediated fast-onset antidepressant-like and memory-enhancing effects in chronically stressed mice

BACKGROUND

Fast-acting and cognitive-enhancing antidepressants are desperately needed. Activation of translocator protein (18 kDa, TSPO) is a novel strategy for developing potential antidepressants, but there are no data available on the onset time of TSPO ligands. This study aimed to investigate the fast-onset antidepressant actions of AC-5216, a selective TSPO ligand, in TSPO knock-out (KO) mice.

METHODS

TSPO wild-type (WT) and KO mice were subjected to a six-week chronic unpredicted stress (CUS) paradigm. Then, the mice were treated with AC-5216 and tested with depressive and cognitive behaviours.

RESULTS

A single dose of AC-5216 (0.3 mg/kg) exerted anxiolytic- and antidepressant-like actions in TSPO WT mice. Moreover, in chronically stressed WT mice, two to four days of AC-5216 treatment (0.3 mg/kg, once per day) produced fast-onset antidepressant-like effects in the novelty-suppressed feeding and sucrose preference tests, as well as memory-enhancing effects in the novel object recognition test. In addition, a rapid (with five days of treatment) restoration of serum corticosterone levels and prefrontal cortex (PFC) allopregnanolone levels was found. Further studies showed that in these stress-exposed WT mice, AC-5216 significantly increased the levels of mTOR signalling-related proteins (mBDNF, p-mTOR, PSD-95, synapsin-1, GluR1), as well as the total dendritic length and branching points of pyramidal neurons in the PFC.

CONCLUSIONS

These results suggest that TSPO mediates the fast-onset antidepressant-like and memory-enhancing effects of AC-5216, possibly through the rapid activation of mTOR signalling and restoration of dendritic complexity in the PFC.

The effect of deoxyschizandrin on chronic unpredictable mild stress-induced depression

The purpose of the present study was to evaluate the antidepressant effect of deoxyschizandrin (DEO) in chronic unpredictable mild stress (CUMS)-induced mice. The mice were subjected to CUMS paradigm for 8 weeks. From the sixth week, the mice were intragastrically treated with DEO once daily for continuous 3 weeks. The behavior tests including sucrose preference test (SPT), forced swimming test (FST), tail suspension test (TST), and open field test were conducted. Additionally, the expressions of TLR4, MyD88, TRAF6, p-NF-κBp65, NLRP3, cleaved caspase-1, cleaved IL-1β, GluR, and PSD95 in hippocampus were detected by western blot. The concentrations of IL-6 and TNF-α in hippocampus were determined by enzyme linked immune sorbent assay (ELISA). The dendritic spine density was observed by Golgi-Cox staining. As a result, the treatment with DEO relieved anhedonia in SPT, and reduced immobile duration in FST and TST. DEO treatment effectively attenuated the CUMS-caused alterations of TLR4, MyD88, TRAF6, p-NF-κBp65, NLRP3, cleaved caspase-1, cleaved IL-1β, GluR, and PSD95. Furthermore, DEO could reduce the hippocampal inflammatory cytokine content and increase the density of dendritic spine. In conclusion, the present work indicated that DEO exhibited antidepressant effect on CUMS-induced depressive mice, which was possible due to the TLR4/NF-κB/NLRP3 pathway and the amelioration of dendritic spine density through GluR/PSD95 cascade.

Acute 5-HT2C Receptor Antagonist SB-242084 Treatment Affects EEG Gamma Band Activity Similarly to Chronic Escitalopram

Serotonin 2C receptors (5-HT_2CRs) are implicated in the pathomechanism and treatment of anxiety and depression. Recently, as a new biomarker of depression, alterations in the gamma power of the electroencephalogram (EEG) have been suggested. Chronic treatment with the selective serotonin reuptake inhibitor (SSRI) antidepressant escitalopram has been shown to cause sleep-wake stage-dependent alterations in gamma power. However, despite the antidepressant potency of 5-HT_2CR-antagonists, there is no data available regarding the effects of selective 5-HT_2CR-antagonists on gamma activity. Therefore, we investigate the acute effect of the 5-HT_2CR-antagonist SB-242084 on gamma power in different vigilance stages when given in monotherapy, or in combination with chronic escitalopram treatment. We administered SB-242084 (1 mg/kg, intraperitoneally) or vehicle to EEG-equipped rats after a 21-day-long pretreatment with escitalopram (10 mg/kg/day, via osmotic minipumps) or vehicle. Frontoparietal EEG, electromyogram, and motor activity were recorded during the first 3 h of passive phase, after the administration of SB-242084. Quantitative EEG analysis revealed that acute SB-242084 increased gamma power (30-60 Hz) in light and deep slow-wave sleep, and passive wakefulness. However, in active wakefulness, rapid eye movement sleep, and intermediate stage, no change was observed in gamma power. The profile of the effect of SB-242084 on gamma power was similar to that produced by chronic escitalopram. Moreover, SB-242084 did not alter chronic escitalopram-induced effects on gamma. In conclusion, the similarity in the effect of the 5-HT_2CR-antagonist and chronic SSRI on gamma power provides further evidence for the therapeutic potential of 5-HT_2CR-antagonists in the treatment of depression and/or anxiety.

5-HT2C receptor blockade reverses SSRI-associated basal ganglia dysfunction and potentiates therapeutic efficacy

Serotonin (5-HT) selective reuptake inhibitors (SSRIs) are widely used in the treatment of depression and anxiety disorders, but responsiveness is uncertain and side effects often lead to discontinuation. Side effect profiles suggest that SSRIs reduce dopaminergic (DAergic) activity, but specific mechanistic insight is missing. Here we show in mice that SSRIs impair motor function by acting on 5-HT2C receptors in the substantia nigra pars reticulata (SNr), which in turn inhibits nigra pars compacta (SNc) DAergic neurons. SSRI-induced motor deficits can be reversed by systemic or SNr-localized 5-HT2C receptor antagonism. SSRIs induce SNr hyperactivity and SNc hypoactivity that can also be reversed by systemic 5-HT2C receptor antagonism. Optogenetic inhibition of SNc DAergic neurons mimics the motor deficits due to chronic SSRI treatment, whereas local SNr 5-HT2C receptor antagonism or optogenetic activation of SNc DAergic neurons reverse SSRI-induced motor deficits. Lastly, we find that 5-HT2C receptor antagonism potentiates the antidepressant and anxiolytic effects of SSRIs. Together our findings demonstrate opposing roles for 5-HT2C receptors in the effects of SSRIs on motor function and affective behavior, highlighting the potential benefits of 5-HT2C receptor antagonists for both reduction of motor side effects of SSRIs and augmentation of therapeutic antidepressant and anxiolytic effects.

The influence of ketamine on drug discovery in depression

Recent research demonstrating that the glutamatergic modulator ketamine has rapid, robust, and sustained antidepressant effects has been a turning point in drug discovery for depression. The recent FDA approval of esketamine for adults with treatment-resistant major depressive disorder (MDD) has further underscored the relevance of this agent in spurring investigation into novel and mechanistically distinct agents for use in depression. Over the past two decades, ketamine research has ushered in a new wave of studies seeking to not only identify its mechanism of action but also to examine the antidepressant potential of novel or repurposed agents. This article reviews the approaches that have proven particularly fruitful for the field of neuropsychiatry.

Ketamine normalizes binge drinking-induced defects in glutamatergic synaptic transmission and ethanol drinking behavior in female but not male mice

Ketamine is a fast acting experimental antidepressant with significant therapeutic potential for emotional disorders such as major depressive disorder and alcohol use disorders. Of particular interest is binge alcohol use, which during intermittent withdrawal from drinking involves depressive-like symptoms reminiscent of major depressive disorder. Binge drinking has been successfully modeled in mice with the Drinking in the Dark (DID) paradigm, which involves daily access to 20% ethanol, for a limited duration and selectively during the dark phase of the circadian light cycle. Here we demonstrate that DID exposure reduces the cell surface expression of NMDA- and AMPA-type glutamate receptors in the prelimbic cortex (PLC) of female but not male mice, along with reduced activity of the mammalian target of rapamycin (mTOR) signaling pathway. Pretreatment with an acute subanesthetic dose of ketamine suppresses binge-like ethanol consumption in female but not male mice. Lastly, DID-exposure reduces spontaneous glutamatergic synaptic transmission in the PLC of both sexes, but synaptic transmission is rescued by ketamine selectively in female mice. Thus, ketamine may have therapeutic potential as an ethanol binge suppressing agent selectively in female subjects.

Hypidone Hydrochloride (YL-0919) Produces a Fast-Onset Reversal of the Behavioral and Synaptic Deficits Caused by Chronic Stress Exposure

Our previous study showed that hypidone hydrochloride (YL-0919), a partial serotonin 1A (5-HT1A) receptor agonist and 5-HT reuptake inhibitor, exerts a significant antidepressant effect in various animal models. The aim of the present study was to further investigate the underlying mechanisms and whether it could act as a fast-onset antidepressant. In the current study, depressive-like behavior was induced in rats by a chronic unpredictable stress (CUS) model and assessed with the Sucrose Preference Test (SPT). Treatment with YL-0919 (2.5 mg/kg, i.g.), but not with fluoxetine (Flx; 10 mg/kg, i.g.), caused a fast improvement in the SPT scores. In CUS-exposed rats, YL-0919 treatment for 5 days decreased the immobility time in a forced swimming test (FST), and a 10-day treatment decreased the latency to feed in a Novelty-Suppressed Feeding Test (NSFT). In addition to the behavioral tests, the effects of YL-0919 on synaptic protein expression were also evaluated. Western blotting showed that YL-0919 significantly enhanced the expression levels of synaptic proteins such as synapsin I, postsynaptic density protein 95 (PSD95), phosphorylated mammalian targeting of rapamycin (pmTOR) and brain-derived neurotrophic factor (BDNF) in the hippocampus. To determine how the mTOR signaling is involved in the fast-onset antidepressant-like effects of YL-0919, the mTOR-specific inhibitor rapamycin was administered intracerebroventricularly (i.c.v.) together with the YL-0919 treatment. The observed changes in behavioral tests and protein expression could be reversed by rapamycin treatment. This suggests that the fast-onset antidepressant effects of YL-0919 were partially caused by changes in synaptogenesis mediated by activation of mTOR pathways. Our data suggest that YL-0919 may be a powerful/effective antidepressant with fast-onset.

Intravenous infusion of xenon-containing liposomes generates rapid antidepressant-like effects

AIM

Similar to ketamine, xenon gas acts as a glutamatergic N-methyl-d-aspartate receptor antagonist, but devoid of propensity to cause untoward effects. Herein, we loaded xenon gas into a liposomal carrier called xenon-containing liposomes (Xe-liposome) for systemic delivery, and investigated its effect as an antidepressant and also analyzed synaptic biomarkers including brain-derived neurotrophic factor (BDNF), protein kinase B (AKT), mammalian target of rapamycin (mTOR), protein kinase C (PKC) and extracellular signal-regulated kinase-1/2 (ERK1/2) in blood and brain.

METHODS

Xe-liposomes (15 μl/mg) were prepared by a pressurized freeze-thaw method, and injected via the lateral tail vein (0.6 mL/rat) in male Wistar rats. The uncaging of xenon gas from circulating Xe-liposome was facilitated by continuous ultrasound application externally on the neck over the internal common carotid artery. One-hour after Xe-liposome infusion, animals were assessed for depression-like behaviors using a forced swimming test (FST), and spontaneous locomotor activity. Blood, as well as frontal cortex and hippocampal samples were obtained for immunoblotting and/or enzyme-linked immune sorbent assays.

RESULTS

Acute intravenous infusion of Xe-liposome, at 6 mg/kg, showed an increase in swimming time in the FST (p < 0.006), indicating antidepressant-like phenotypes. Higher doses of Xe-liposomes (9 mg/kg) failed to improve swimming duration. This behavioral discrepancy was not associated with locomotion aberrations, as gross activity of rats remained similar for both doses. In biochemical analyses of frontal cortex, protein levels of BDNF increased by 64%, and enhanced phosphorylation of AKT (43%) and mTOR (93%) was observed at the 6 mg/kg dose level of Xe-liposomes, while these biomarkers and phosphorylated PKC and ERK1/2 levels remained unchanged at the higher dose. Moreover, Xe-liposomal treatment did not change the plasma and protein levels of BDNF, and phosphorylated AKT, mTOR, PKC and ERK1/2 hippocampal expressions.

CONCLUSION

Xe-liposomes mediate a rapid antidepressant-like effect through activation of AKT/mTOR/BDNF signaling pathway.

Total Saikosaponins of Bupleurum yinchowense reduces depressive, anxiety-like behavior and increases synaptic proteins expression in chronic corticosterine-treated mice

Background

Bupleurum yinchowense Shan et Y. Li is widely used to treat depressive and anxiety disorders for hundreds of years in China. Total saikosaponins (TSS) is the major ingredient of Bupleurum yinchowense. A-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor and subsequent mammalian target of rapamycin (mTOR) signaling is responsible for synaptic maturation and may contribution to the synaptic alteration underlying depression. The aim of the study was to investigate the antidepressant-like and anxiolytic effect of TSS in chronic corticosterone-treated mice. The effect of TSS on synaptic proteins expression and AMPA receptor-mTOR signaling pathway alteration was also evaluated.

Methods

Dose-response effect of TSS (12.5, 25, 50 mg/kg) was investigated in forced swim test (FST) in ICR male mice. In the chronic corticosterine-treated model, TSS was given intragastrically once a day for 2 weeks and continued through the behavior testing period. Behavior tests and AMPA receptor related signaling pathway were investigated.

Results

TSS (25 and 50 mg/kg) decreased the immobility time in the FST when compared with the control group. TSS (25 mg/kg) showed antidepressant-like and anxiolytic effects in the chronic corticosterone treatment model in mice. TSS increased hippocampal synaptic proteins (synapsin-1 and postsynaptic density protein 95) expression. Immunohistochemistry analysis showed that TSS significantly increased the synapsin-1 expression in CA3 of hippocampus. TSS also increased hippocampal phosphorylation expression of GluR1 Ser 845 (AMPA receptor subunit) and its downstream regulators extracellular signaling-regulated kinase (ERK), protein kinase B (Akt) and mTOR.

Conclusion

TSS produces antidepressant-like and anxiolytic effects and increases synaptic proteins expression which may be mediated by induction of AMPA receptor and subsequent mTOR signaling pathway.

Convergent Mechanisms Underlying Rapid Antidepressant Action

Traditional pharmacological treatments for depression have a delayed therapeutic onset, ranging from several weeks to months, and there is a high percentage of individuals who never respond to treatment. In contrast, ketamine produces rapid-onset antidepressant, anti-suicidal, and anti-anhedonic actions following a single administration to patients with depression. Proposed mechanisms of the antidepressant action of ketamine include N-methyl-D-aspartate receptor (NMDAR) modulation, gamma aminobutyric acid (GABA)-ergic interneuron disinhibition, and direct actions of its hydroxynorketamine (HNK) metabolites. Downstream actions include activation of the mechanistic target of rapamycin (mTOR), deactivation of glycogen synthase kinase-3 and eukaryotic elongation factor 2 (eEF2), enhanced brain-derived neurotrophic factor (BDNF) signaling, and activation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs). These putative mechanisms of ketamine action are not mutually exclusive and may complement each other to induce potentiation of excitatory synapses in affective-regulating brain circuits, which results in amelioration of depression symptoms. We review these proposed mechanisms of ketamine action in the context of how such mechanisms are informing the development of novel putative rapid-acting antidepressant drugs. Such drugs that have undergone pre-clinical, and in some cases clinical, testing include the muscarinic acetylcholine receptor antagonist scopolamine, GluN2B-NMDAR antagonists (i.e., CP-101,606, MK-0657), (2R,6R)-HNK, NMDAR glycine site modulators (i.e., 4-chlorokynurenine, pro-drug of the glycineB NMDAR antagonist 7-chlorokynurenic acid), NMDAR agonists [i.e., GLYX-13 (rapastinel)], metabotropic glutamate receptor 2/3 (mGluR2/3) antagonists, GABAA receptor modulators, and drugs acting on various serotonin receptor subtypes. These ongoing studies suggest that the future acute treatment of depression will typically occur within hours, rather than months, of treatment initiation.

Identification of a novel, fast-acting GABAergic antidepressant

Current pharmacotherapies for depression exhibit slow onset, side effects and limited efficacy. Therefore, identification of novel fast-onset antidepressants is desirable. GLO1 is a ubiquitous cellular enzyme responsible for the detoxification of the glycolytic byproduct methylglyoxal (MG). We have previously shown that MG is a competitive partial agonist at GABA-A receptors. We examined the effects of genetic and pharmacological inhibition of GLO1 in two antidepressant assay models: the tail suspension test (TST) and the forced swim test (FST). We also examined the effects of GLO1 inhibition in three models of antidepressant onset: the chronic FST (cFST), chronic mild stress (CMS) paradigm and olfactory bulbectomy (OBX). Genetic knockdown of Glo1 or pharmacological inhibition using two structurally distinct GLO1 inhibitors (S-bromobenzylglutathione cyclopentyl diester (pBBG) or methyl-gerfelin (MeGFN)) reduced immobility in the TST and acute FST. Both GLO1 inhibitors also reduced immobility in the cFST after 5 days of treatment. In contrast, the serotonin reuptake inhibitor fluoxetine (FLX) reduced immobility after 14, but not 5 days of treatment. Furthermore, 5 days of treatment with either GLO1 inhibitor blocked the depression-like effects induced by CMS on the FST and coat state, and attenuated OBX-induced locomotor hyperactivity. Finally, 5 days of treatment with a GLO1 inhibitor (pBBG), but not FLX, induced molecular markers of the antidepressant response including brain-derived neurotrophic factor (BDNF) induction and increased phosphorylated cyclic-AMP response-binding protein (pCREB) to CREB ratio in the hippocampus and medial prefrontal cortex (mPFC). Our findings indicate that GLO1 inhibitors may provide a novel and fast-acting pharmacotherapy for depression.

An Interaction between Serotonin Receptor Signaling and Dopamine Enhances Goal-Directed Vigor and Persistence in Mice

The functionally selective 5-HT2C receptor ligand SB242084 can increase motivation and have rapid onset anti-depressant-like effects. We sought to identify the specific behavioral effects of SB242084 treatment and elucidate the mechanism in female and male mice. Using a quantitative behavioral approach, we determined that SB242084 increases the vigor and persistence of goal-directed activity across different types of physical work, particularly when work requirements are demanding. We found this influence of SB242084 on effort, rather than reward to be reflected in striatal DA measured during behavior. Using in vivo fast scan cyclic voltammetry, we found that SB242084 has no effect on reward-related phasic DA release in the NAc. Using in vivo microdialysis to measure tonic changes in extracellular DA, we also found no changes in the NAc. In contrast, SB242084 treatment increases extracellular DA in the dorsomedial striatum, an area that plays a key role in response vigor. These findings have several implications. At the behavioral level, this work shows that the capacity to work in demanding situations can be increased, without a generalized increase in motor activity or reward value. At the circuit level, we identified a pathway restricted potentiation of DA release and showed that this was the reason for the increased response vigor. At the cellular level, we show that a specific serotonin receptor cross talks to the DA system. Together, this information provides promise for the development of treatments for apathy, a serious clinical condition that can afflict patients with psychiatric and neurological disorders.SIGNIFICANCE STATEMENT Motivated behaviors are modulated by reward value, effort demands, and cost-benefit computations. This information drives the decision to act, which action to select, and the intensity with which the selected action is performed. Because these behavioral processes are all regulated by DA signaling, it is very difficult to influence selected aspects of motivated behavior without affecting others. Here we identify a pharmacological treatment that increases the vigor and persistence of responding in mice, without increasing generalized activity or changing reactions to rewards. We show that the 5-HT2C-selective ligand boosts motivation by potentiating activity-dependent DA release in the dorsomedial striatum. These results reveal a novel strategy for treating patients with motivational deficits, avolition, or apathy.

The Aqueous Crude Extracts of Montanoa frutescens and Montanoa grandiflora Reduce Immobility Faster Than Fluoxetine Through GABAA Receptors in Rats Forced to Swim

BACKGROUND

Montanoa frutescens and Montanoa grandiflora have been indistinctly used for centuries in traditional Mexican medicine for reproductive impairments, anxiety, and mood disorders. Preclinical studies support their aphrodisiac and anxiolytic properties, but their effects on mood are still unexplored.

METHODS

The effects of 25 and 50 mg/kg of M frutescens and M grandiflora extracts were evaluated on days 1, 7, 14, 21, and 28 of treatment, and compared with fluoxetine (1 mg/kg) and Remotiv (7.14 mg/kg) in Wistar rats. The participation of GABAA receptor in the effects produced by the treatments was explored.

RESULTS

Montanoa extracts reduced immobility since day 1 of treatment, while fluoxetine and Remotiv required 14 days. The GABAA antagonism blocked the effects of Montanoa extracts, but not of fluoxetine or Remotiv.

CONCLUSIONS

Montanoa extracts prevented quickly the stress-induced behaviors in the swimming test through action at the GABAA receptor, exerting a protective effect different to the typical antidepressants drugs.

Selective effects of 5-HT2C receptor modulation on performance of a novel valence-probe visual discrimination task and probabilistic reversal learning in mice

RATIONALE

Dysregulation of the serotonin (5-HT) system is a pathophysiological component in major depressive disorder (MDD), a condition closely associated with abnormal emotional responsivity to positive and negative feedback. However, the precise mechanism through which 5-HT tone biases feedback responsivity remains unclear. 5-HT2C receptors (5-HT2CRs) are closely linked with aspects of depressive symptomatology, including abnormalities in reinforcement processes and response to stress. Thus, we aimed to determine the impact of 5-HT2CR function on response to feedback in biased reinforcement learning.

METHODS

We used two touchscreen assays designed to assess the impact of positive and negative feedback on probabilistic reinforcement in mice, including a novel valence-probe visual discrimination (VPVD) and a probabilistic reversal learning procedure (PRL). Systemic administration of a 5-HT2CR agonist and antagonist resulted in selective changes in the balance of feedback sensitivity bias on these tasks.

RESULTS

Specifically, on VPVD, SB 242084, the 5-HT2CR antagonist, impaired acquisition of a discrimination dependent on appropriate integration of positive and negative feedback. On PRL, SB 242084 at 1 mg/kg resulted in changes in behaviour consistent with reduced sensitivity to positive feedback. In contrast, WAY 163909, the 5-HT2CR agonist, resulted in changes associated with increased sensitivity to positive feedback and decreased sensitivity to negative feedback.

CONCLUSIONS

These results suggest that 5-HT2CRs tightly regulate feedback sensitivity bias in mice with consequent effects on learning and cognitive flexibility and specify a framework for the influence of 5-HT2CRs on sensitivity to reinforcement.

The modulation of adult neuroplasticity is involved in the mood-improving actions of atypical antipsychotics in an animal model of depression

Depression is a prevalent psychiatric disorder with an increasing impact in global public health. However, a large proportion of patients treated with currently available antidepressant drugs fail to achieve remission. Recently, antipsychotic drugs have received approval for the treatment of antidepressant-resistant forms of major depression. The modulation of adult neuroplasticity, namely hippocampal neurogenesis and neuronal remodeling, has been considered to have a key role in the therapeutic effects of antidepressants. However, the impact of antipsychotic drugs on these neuroplastic mechanisms remains largely unexplored. In this study, an unpredictable chronic mild stress protocol was used to induce a depressive-like phenotype in rats. In the last 3 weeks of stress exposure, animals were treated with two different antipsychotics: haloperidol (a classical antipsychotic) and clozapine (an atypical antipsychotic). We demonstrated that clozapine improved both measures of depressive-like behavior (behavior despair and anhedonia), whereas haloperidol aggravated learned helplessness in the forced-swimming test and behavior flexibility in a cognitive task. Importantly, an upregulation of adult neurogenesis and neuronal survival was observed in animals treated with clozapine, whereas haloperidol promoted a downregulation of these processes. Furthermore, clozapine was able to re-establish the stress-induced impairments in neuronal structure and gene expression in the hippocampus and prefrontal cortex. These results demonstrate the modulation of adult neuroplasticity by antipsychotics in an animal model of depression, revealing that the atypical antipsychotic drug clozapine reverts the behavioral effects of chronic stress by improving adult neurogenesis, cell survival and neuronal reorganization.