Rapid-Acting Antidepressants

Ginsenoside Rg1, lights up the way for the potential prevention of Alzheimer's disease due to its therapeutic effects on the drug-controllable risk factors of Alzheimer's disease

ETHNOPHARMACOLOGICAL RELEVANCE

In traditional Chinese medicine, Shen Nong, BenCao Jing, and Compendium of Materia Medica (Bencao Gangmu), Panax ginseng, and its prescriptions have been used for the treatment of dementia, depression, weight loss, Xiaoke disease (similar to diabetes), and vertigo. All these diseases are associated with the drug-controllable risk factors for Alzheimer's disease (AD), including depression, obesity, diabetes, and hypertension. Ginsenoside Rg1, one of the main active ingredients of P. ginseng and its congener Panax notoginseng, possesses therapeutic potentials against AD and associated diseases. This suggests that ginsenoside Rg1 might have the potential for AD prevention and treatment. Although the anti-AD effects of ginsenoside Rg1 have received more attention, a systematic review of its effects on depression, obesity, diabetes, and hypertension is not available.

AIM OF THE REVIEW

This systematic literature review comprehensively summarized existing literature on the therapeutic potentials of ginsenoside Rg1 in AD prevention for the propose of providing a foundation of future research aimed at enabling the use of such drugs in clinical practice.

METHODS

Information on ginsenoside Rg1 was collected from relevant published articles identified through a literature search in electronic scientific databases (PubMed, Science Direct, and Google Scholar). The keywords used were "Ginsenoside Rg1," "Panax ginseng," "Source," "Alzheimer's disease," "Brain disorders," "Depression," "Obesity," "Diabetes," and "Hypertension."

RESULTS

The monomer ginsenoside Rg1 can be relatively easily obtained and has therapeutic potentials against AD. In vitro and in vivo experiments have demonstrated the therapeutic potentials of ginsenoside Rg1 against the drug-controllable risk factors of AD including depression, obesity, diabetes, and hypertension. Thus, ginsenoside Rg1 alleviates diseases resulting from AD risk factors by regulating multiple targets and pathways.

CONCLUSIONS

Ginsenoside Rg1 has the potentials to prevent AD by alleviating depression, obesity, diabetes, and hypertension.

Interaction of hallucinogenic rapid-acting antidepressants with mGlu2/3 receptor ligands as a window for more effective therapies

The desire to find a gold-standard therapy for depression is still ongoing. Developing one universal and effective pharmacotherapy remains troublesome due to the high complexity and variety of symptoms. Over the last decades, the understanding of the mechanism of pathophysiology of depression and its key consequences for brain functioning have undergone significant changes, referring to the monoaminergic theory of the disease. After the breakthrough discovery of ketamine, research began to focus on the modulation of glutamatergic transmission as a new pharmacological target. Glutamate is a crucial player in mechanisms of a novel class of antidepressants, including hallucinogens such as ketamine. The role of glutamatergic transmission is also suggested in the antidepressant (AD) action of scopolamine and psilocybin. Despite fast, robust, and sustained AD action hallucinogens belonging to a group of rapid-acting antidepressants (RAA) exert significant undesired effects, which hamper their use in the clinic. Thus, the synergistic action of more than one substance in lower doses instead of monotherapy may alleviate the likelihood of adverse effects while improving therapeutic outcomes. In this review, we explore AD-like behavioral, synaptic, and molecular action of RAAs such as ketamine, scopolamine, and psilocybin, in combination with mGlu2/3 receptor antagonists.

The 5-HT1A receptor biased agonists, NLX-204 and NLX-101, display ketamine-like RAAD and anti-TRD activities in rat CMS models

OBJECTIVES

NLX-101 and NLX-204 are highly selective serotonin 5-HT1A 'biased' agonists, displaying potent and efficacious antidepressant-like activity upon acute administration in models such as the forced swim test.

METHODS

we compared the effects of repeated administration of NLX-101, NLX-204 and ketamine in the chronic mild stress (CMS) model of depression, considered to have high translational potential, on sucrose consumption (anhedonia measure), novel object recognition (NOR; working memory measure) and elevated plus maze (EPM; anxiety measure) in male Wistar and Wistar-Kyoto rats (the latter being resistant to classical antidepressants).

RESULTS

in Wistar rats, NLX-204 and NLX-101 (0.08-0.16 mg/kg i.p.), like ketamine (10 mg/kg i.p.) dose-dependently reversed CMS-induced sucrose intake deficit from treatment Day 1, with nearly full reversal observed at the higher dose at Days 8 and 15. These effects persisted for 3 weeks following treatment cessation. In the NOR test, both doses of NLX-101/NLX-204, and ketamine, rescued the deficit in discrimination index caused by CMS on Days 3 and 17; all three compounds increased time spent in open arms (EPM) but only NLX-204 achieved statistical significance on Days 2 and 16. In Wistar-Kyoto rats, all 3 compounds were also active in the sucrose test and, to a lesser extent, in the NOR and EPM. In non-stressed rats (both strains), the three compounds produced no significant effects in all tests.

CONCLUSIONS

these observations further strengthen the hypothesis that biased agonism at 5-HT1A receptors constitutes a promising strategy to achieve rapid-acting/sustained antidepressant effects combined with activity against TRD, in addition to providing beneficial effects against memory deficit and anxiety in depressed patients.

Role of trazodone in treatment of major depressive disorder: an update

Major depressive disorder (MDD) is the most common mood disorder and a leading cause of disability worldwide. Trazodone, a triazolopyridine serotonin receptor antagonist and reuptake inhibitor (SARI) antidepressant approved for major depressive disorder (MDD) in adults, has established efficacy that is comparable to other available antidepressants, and is effective for a range of depression symptoms, including insomnia, which is one of the most common and bothersome symptoms of depression. Also, trazodone's pharmacodynamic properties allow it to avoid the side effects of insomnia, anxiety and sexual dysfunction often associated with selective serotonin reuptake inhibitor antidepressants. In this narrative review, we have summarized recent clinical trials and real-world data on trazodone, including the recently introduced once-daily formulation, which has single dose pharmacokinetic properties that maintain effective blood trazodone levels for 24 h, while avoiding concentration peaks associated with side effects. This, combined with a low incidence of weight gain, and sexual dysfunction, may improve adherence to treatment. The most common adverse effects of trazodone are somnolence, headache, dizziness and xerostomia. It has minimal anticholinergic activity but may be associated infrequently with orthostatic hypotension (especially in patients with cardiovascular disease or older adults), QT interval prolongation, cardiac arrhythmias, and rare episodes of priapism. The low liability for activating side effects, the efficacy on symptoms such as insomnia and psychomotor agitation and the rapid onset of action make it useful for many depressed patients, both in monotherapy at nominal dosages of 150-300 mg/day, and in combination with other antidepressants at lower dosages.

Effects of ketamine optical isomers, fluoxetine and naloxone on timing in differential reinforcement of low-rate response (DRL) 72-s task in rats

(S)-ketamine-induced rapid-acting antidepressant effects have revolutionized the pharmacotherapy of major depression; however, this medication also produces psychotomimetic effects such as timing distortion. While (R)-ketamine produces fewer dissociative effects, its antidepressant actions are less studied. Depression is associated with time overestimation (i.e., subjectively, time passes slowly). Our recent report suggests that while (S)-ketamine induces an opposite effect, i.e., time underestimation, the (R)-isomer does not affect timing. It has been suggested that opioid receptors are involved in the antidepressant effect of ketamine. In the present study we tested (R)- and (S)-ketamine, and fluoxetine as a positive control in the differential-reinforcement-of-low-rate (DRL) 72-s schedule of reinforcement in male rats following naloxone pretreatment. DRL classic metrics as well as peak deviation analyses served to determine antidepressant-like actions and those associated with timing. We report antidepressant-like effects of (S)-ketamine (30-60 mg/kg) that resemble fluoxetine's (2.5-10 mg/kg), as both compounds increased reinforcement rate and peak location (suggesting increased performance), reduced premature responses (suggesting time underestimation) and decreased Weber's fraction (suggesting increased timing precision). (R)-ketamine (30, but not 60 mg/kg) increased only the reinforcement rate and peak location but did not affect timing. Only fluoxetine decreased burst responses, suggesting decreased impulsivity. Naloxone pretreatment did not block ketamine enantiomers' actions, but unexpectedly, increased fluoxetine' performance. Thus, while all three medications produced antidepressant-like effects in DRL 72-s, fluoxetine- and (S)- but not (R)- ketamine-induced time underestimation (the subject experiences the time as passing quickly). The potentiation of DRL performance of fluoxetine by naloxone was unexpected and warrants clinical studies.

The imidazodiazepine, KRM-II-81: An example of a newly emerging generation of GABAkines for neurological and psychiatric disorders

GABAkines, or positive allosteric modulators of γ-aminobutyric acid-A (GABA_A) receptors, are used for the treatment of anxiety, epilepsy, sleep, and other disorders. The search for improved GABAkines, with reduced safety liabilities (e.g., dependence) or side-effect profiles (e.g., sedation) constituted multiple discovery and development campaigns that involved a multitude of strategies over the past century. Due to the general lack of success in the development of new GABAkines, there had been a decades-long draught in bringing new GABAkines to market. Recently, however, there has been a resurgence of efforts to bring GABAkines to patients, the FDA approval of the neuroactive steroid brexanolone for post-partum depression in 2019 being the first. Other neuroactive steroids are in various stages of clinical development (ganaxolone, zuranolone, LYT-300, Sage-324, PRAX 114, and ETX-155). These GABAkines and non-steroid compounds (GRX-917, a TSPO binding site ligand), darigabat (CVL-865), an α2/3/5-preferring GABAkine, SAN711, an α3-preferring GABAkine, and the α2/3-preferring GABAkine, KRM-II-81, bring new therapeutic promise to this highly utilized medicinal target in neurology and psychiatry. Herein, we also discuss possible conditions that have enabled the transition to a new age of GABAkines. We highlight the pharmacology of KRM-II-81 that has the most preclinical data reported. KRM-II-81 is the lead compound in a new series of orally bioavailable imidazodiazepines entering IND-enabling safety studies. KRM-II-81 has a preclinical profile predicting efficacy against pharmacoresistant epilepsies, traumatic brain injury, and neuropathic pain. KRM-II-81 also produces anxiolytic- and antidepressant-like effects in rodent models. Other key features of the pharmacology of this compound are its low sedation rate, lack of tolerance development, and the ability to prevent the development of seizure sensitization.

Effects of ketamine optical isomers, psilocybin, psilocin and norpsilocin on time estimation and cognition in rats

RATIONALE

Ketamine and psilocybin belong to the rapid-acting antidepressants but they also produce psychotomimetic effects including timing distortion. It is currently debatable whether these are essential for their therapeutic actions. As depressed patients report that the "time is dragging," we hypothesized that ketamine and psilocybin-like compounds may produce an opposite effect, i.e., time underestimation, purportedly contributing to their therapeutic properties.

OBJECTIVES

Timing was tested following administration of (R)- and (S)-ketamine, and psilocybin, psilocin, and norpsilocin in the discrete-trial temporal discrimination task (TDT) in male rats. Timing related to premature responses, and cognitive and unspecific effects of compounds were tested in the 5-choice serial reaction time task (5-CSRTT) in the standard 1-s, and "easier" 2-s stimulus duration conditions, as well as in the vITI variant promoting impulsive responses.

RESULTS

(S)-ketamine (15 but not 3.75 or 7.5 mg/kg) shifted psychometric curve to the right in TDT and reduced premature responses in 5-CSRTT, suggesting expected time underestimation, but it also decreased the accuracy of temporal discrimination and increased response and reward latencies, decreased correct responses, and increased incorrect responses. While (R)-ketamine did not affect timing and produced no unspecific actions, it reduced incorrect responses in TDT and increased accuracy in 5-CSRTT, suggesting pro-cognitive effects. Psilocin and psilocybin produced mainly unspecific effects in both tasks, while norpsilocin showed no effects.

CONCLUSIONS

Time underestimation produced by (S)-ketamine could be associated with its antidepressant effects; however, it was accompanied with severe behavioral disruption. We also hypothesize that behavioral disruption produced by psychedelics objectively reflects their psychotomimetic-like actions.

Histamine and histamine receptors: Roles in major depressive disorder

Although the incidence of major depressive disorder (MDD) is high and its social impact is great, we still know very little about the pathophysiology of depression. The monoamine hypothesis of depression suggests that 5-HT, NE, and DA synergistically affect mood, which is the basis of current drug therapy for depression. However, histamine as a monoamine transmitter is rarely studied. Our review is the first time to illustrate the effect of histaminergic system on depression in order to find the way for the development of new antidepressant drugs. The brain neurotransmitter histamine is involved in MDD, and the brain histaminergic system operates through four receptors. Histamine and its receptors can also regulate the immune response to improve symptoms of depression. In addition, H3R can interact with other depression-related transmitters (including 5-HT, DA, GLU, and MCH); thus, histamine may participate in the occurrence of depression through other neural circuits. Notably, in rodent studies, several H3R and H1R antagonists were found to be safe and effective in alleviating depression-like behavior. To highlight the complex functions of histamine in depression, and reveals that histamine receptors can be used as new targets for antidepressant therapy.

Rapid tolerance to behavioral effects of ethanol in rats: Prevention by R-(-)-ketamine

R-(-)-ketamine has emerged as a potentially improved medication over that of the (S)-isomer (marketed as Spravato for depression). Recent data have suggested (R)-ketamine could have value in the treatment of substance use disorder. The present set of experiments was undertaken to examine whether (R)-ketamine might prevent tolerance development. Rapid ethanol (ETOH) tolerance was studied since racemic ketamine had previously been shown to block this tolerance development in rats. In the present study, male Sprague-Dawley rats were given two large doses of ETOH on Day 1 (2.3 + 1.7 g/kg) and 2.3 g/kg ETOH on Day 2. Animals were tested for effects of 2.3 g/kg ETOH on grip strength, inclined screen performance and rotarod performance on Day 1 with or without (R)-ketamine as a pretreatment. (R)-ketamine alone was tested at the highest dose studied (10 mg/kg) and did not significantly influence any dependent measure. (R)-ketamine (1-10 mg/kg) did not alter the acute effects of ETOH except for enhancing the effects of ETOH on the inclined screen test at 3 mg/kg. Between-subjects analysis documented that tolerance developed to the effects of ETOH only on the measure of grip strength. (R)-ketamine (3 mg/kg) given prior to ETOH on Day 1 exhibited a strong trend toward preventing tolerance development (p = 0.062). The present results extend prior findings on the potential value of (R)-ketamine in substance abuse disorder therapeutics and add to the literature on NMDA receptor blockade as a tolerance-regulating mechanism.

N-Substituted-3-alkoxy-derivatives of dextromethorphan are functional NMDA receptor antagonists in vivo: Evidence from an NMDA-induced seizure model in rats

Interest in developing NMDA receptor antagonists with reduced side-effects for neurological and psychiatric disorders has been re-energized by the recent introduction of esketamine into clinical practice for treatment-resistant depression. Structural analogs of dextromethorphan bind with low affinity to the NMDA receptor ion channel, have functional effects in vivo, and generally display a lower propensity for side-effects than that of ketamine and other higher affinity antagonists. As such, the aim of the present study was to determine whether a series of N-substituted-3-alkoxy-substituted dextromethorphan analogs produce their anticonvulsant effects through NMDA receptor blockade. Compounds were studied against NMDA-induced seizures in rats. Compounds were administered intracerebroventricularly in order to mitigate confounds of drug metabolism that arise from systemic administration. Comparison of the anticonvulsant potencies to their affinities for NMDA, σ1, and σ2 binding sites were made in order to evaluate the contribution of these receptors to anticonvulsant efficacy. The potencies to block convulsions were positively associated with their affinities to bind to the NMDA receptor ion channel ([3H]-TCP binding) (r = 0.71, p < 0.05) but not to σ1 receptors ([3H]-SKF 10047 binding) (r = -0.31, p = 0.46) or to σ2 receptors ([3H]-DTG binding) (p = -0.38, p = 0.36). This is the first report demonstrating that these dextromethorphan analogs are functional NMDA receptor antagonists in vivo. Given their potential therapeutic utility and favorable side-effect profiles, such low affinity NMDA receptor antagonists could be considered for further development in neurological (e.g., anticonvulsant) and psychiatric (e.g., antidepressant) disorders.

Total Sleep Deprivation Followed by Bright Light Therapy as Rapid Relief for Depression: A Pragmatic Randomized Controlled Trial

Background: Total sleep deprivation (TSD) combined with bright light therapy (BLT) has been suggested as a valuable add-on to standard treatment for rapid relief of depression. However, there is a lack of randomized controlled trials in real-life clinical settings. The aim of this pragmatic randomized clinical trial was to investigate the effectiveness, acceptance, and feasibility of TSD combined with BLT as add-on to standard treatment for depression in a real-life clinical setting. Methods: Thirty-three inpatients were randomly assigned to either: a) an intervention group receiving a single-night TSD followed by 6 days BLT (10.000 lux, 30 min/day) as add-on to standard treatment; or b) a control group receiving a short sleep-hygiene consultation in addition to standard treatment. The follow-up period was 1 week. Results: No statistical differences were found in response rates, reduction of depressive and insomnia symptoms, length of stay, readmission rate, and clinical improvement. Both groups reported positive experiences toward the received treatment with low drop-out rates. Conclusions: One-night TSD followed by BLT was not effective as a rapid relief for depression at 1-week follow-up; however, the treatment was feasible and well-tolerated.

A New Paradigm for Achieving a Rapid Antidepressant Response

The substantive delay (often 4-6 weeks) between the commencement of an antidepressant and any discernible improvement in depressive symptoms is an ongoing concern in the management of depressive disorders. This delay incurs the risk of cessation of medication, self-harm/suicide and ongoing 'damage' to the brain caused by the illness. Both historically and now, off-label polypharmacy has been used in clinical practice in an attempt to facilitate both immediate- and long-term relief from symptoms. While somewhat effective, this strategy was unregulated and associated with severe adverse side effects for patients. In this article we proffer an alternative paradigm to achieve a more rapid antidepressant response by conceptualising the gap in terms of windows of response. The Windows of Antidepressant Response Paradigm (WARP) frames treatment response as windows of time in which a clinical response can be expected following initiation of an antidepressant. The paradigm defines three distinct windows-the immediate-response window (1-2 days), fast-response window (up to 1 week) and slow-response window (from 1 week onwards). Newer agents such as rapid-acting antidepressants are considered to act within the immediate-response window, whereas atypical antipsychotic augmentation strategies are captured within the fast-response window. The slow-response window represents the delay experienced with conventional antidepressant monotherapy. Novel agents such as esketamine and brexpiprazole are discussed as examples to better understand the clinical utility of WARP. This framework can be used to guide research in this field and aide the development of newer, more effective antidepressant agents as well as providing a strategy to guide the prescription of multiple agents in clinical practice.

[Ketamine, psilocybin, and rapid acting antidepressant: new promise for psychiatry?]

The hypothesis of monoaminergic deficiency has long dominated the conceptual framework for the development of new antidepressant strategies, but the limits of conventional antidepressant treatments targeting monoaminergic signaling have motivated the search for new antidepressant pathways. The success of ketamine in the management of depressive disorders has provoked a renewed interest in hallucinogenic substances such as psilocybin targeting the serotonergic signaling 5HT^2A and neurosteroid allosteric modulator of γ-aminobutyric acid (GABAA) receptors such as brexanolone. Unlike conventional treatments, these modulators of glutamatergic, serotonergic and GABAergic systems exert a rapid antidepressant effect ranging from 24hours to a week. Apart from their clinical interest and the fantasized search for a "miracle" molecule that jointly meets the expectations of patients and clinicians, these new targets could lead to the identification of potential new biomarkers for the development of rapid-acting antidepressants and redefine therapeutic strategies in mood disorders.

Positive Allosteric Modulation of AMPAR by PF-4778574 Produced Rapid Onset Antidepressant Actions in Mice

It has been reported that fast-acting antidepressants enhance glutamatergic neurotransmission in the prefrontal cortex (PFC) regions via alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) activation. However, the precise mechanisms underlying the fast-acting antidepressants lead to an activation of AMPAR pathways remain largely unclear. To address this issue, a novel AMPAR positive allosteric agonist, PF-4778574, was used to test the rapid effects and the role of VGF (nonacronymic)/brain-derived neurotrophic factor (BDNF)/tropomyosin receptor kinase B (TrkB)/AKT signaling in these actions in mice. We found that PF-4778574 rapidly alleviated chronic unpredictable stress-induced depression-like behaviors in a concentration-dependent manner. In addition, knock down of vesicular glutamate transporter 1 (VGLUT1) in the PFC of mice induced depression-like behaviors, whereas treatment with PF-4778574 was sufficient to alleviate it, indicating a presynaptic VGLUT1 independent effect. Furthermore, we demonstrate that pharmacological inhibitors of AMPAR or of L-type voltage-dependent Ca2+ channel (L-VDCC) blocked the antidepressants' effect on behaviors and the upregulation on the AMPAR-mediated VGF/BDNF/TrkB/AKT signaling of PF-4778574. Together, our findings indicate that postsynaptic AMPAR activation followed by activation of L-VDCC and subsequent VGF/BDNF/TrkB/AKT signaling are required for the rapid antidepressant effects of PF-4778574. Our data support a promising therapeutic profile for PF-4778574 as a new fast-acting antidepressant.

Predicting Antidepressant Citalopram Treatment Response via Changes in Brain Functional Connectivity After Acute Intravenous Challenge

Introduction: The early and therapy-specific prediction of treatment success in major depressive disorder is of paramount importance due to high lifetime prevalence, and heterogeneity of response to standard medication and symptom expression. Hence, this study assessed the predictability of long-term antidepressant effects of escitalopram based on the short-term influence of citalopram on functional connectivity. Methods: Twenty nine subjects suffering from major depression were scanned twice with resting-state functional magnetic resonance imaging under the influence of intravenous citalopram and placebo in a randomized, double-blinded cross-over fashion. Symptom factors were identified for the Hamilton depression rating scale (HAM-D) and Beck's depression inventory (BDI) taken before and after a median of seven weeks of escitalopram therapy. Predictors were calculated from whole-brain functional connectivity, fed into robust regression models, and cross-validated. Results: Significant predictive power could be demonstrated for one HAM-D factor describing insomnia and the total score (r = 0.45-0.55). Remission and response could furthermore be predicted with an area under the receiver operating characteristic curve of 0.73 and 0.68, respectively. Functional regions with high influence on the predictor were located especially in the ventral attention, fronto-parietal, and default mode networks. Conclusion: It was shown that medication-specific antidepressant symptom improvements can be predicted using functional connectivity measured during acute pharmacological challenge as an easily assessable imaging marker. The regions with high influence have previously been related to major depression as well as the response to selective serotonin reuptake inhibitors, corroborating the advantages of the current approach of focusing on treatment-specific symptom improvements.

Molecular and cellular mechanisms underlying the antidepressant effects of ketamine enantiomers and its metabolites

Although the robust antidepressant effects of the N-methyl-D-aspartate receptor (NMDAR) antagonist ketamine in patients with treatment-resistant depression are beyond doubt, the precise molecular and cellular mechanisms underlying its antidepressant effects remain unknown. NMDAR inhibition and the subsequent α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) activation are suggested to play a role in the antidepressant effects of ketamine. Although (R)-ketamine is a less potent NMDAR antagonist than (S)-ketamine, (R)-ketamine has shown more marked and longer-lasting antidepressant-like effects than (S)-ketamine in several animal models of depression. Furthermore, non-ketamine NMDAR antagonists do not exhibit robust ketamine-like antidepressant effects in patients with depression. These findings suggest that mechanisms other than NMDAR inhibition play a key role in the antidepressant effects of ketamine. Duman's group demonstrated that the activation of mammalian target of rapamycin complex 1 (mTORC1) in the medial prefrontal cortex is reportedly involved in the antidepressant effects of ketamine. However, we reported that mTORC1 serves a role in the antidepressant effects of (S)-ketamine, but not of (R)-ketamine, and that extracellular signal-regulated kinase possibly underlie the antidepressant effects of (R)-ketamine. Several lines of evidence have demonstrated that brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), are crucial in the antidepressant effects of ketamine and its two enantiomers, (R)-ketamine and (S)-ketamine, in rodents. In addition, (2R,6R)-hydroxynormetamine [a metabolite of (R)-ketamine] and (S)-norketamine [a metabolite of (S)-ketamine] have been shown to exhibit antidepressant-like effects on rodents through the BDNF-TrkB cascade. In this review, we discuss recent findings on the molecular and cellular mechanisms underlying the antidepressant effects of enantiomers of ketamine and its metabolites. It may be time to reconsider the hypothesis of NMDAR inhibition and the subsequent AMPAR activation in the antidepressant effects of ketamine.

Cortical 5-hydroxytryptamine 1A receptor biased agonist, NLX-101, displays rapid-acting antidepressant-like properties in the rat chronic mild stress model

BACKGROUND

NLX-101 (also known as F15599) is a highly selective and efficacious 'biased' agonist at cortical 5-hydroxytryptamine 1A (5-HT_1A) heteroreceptors. In rodents, it possesses marked antidepressant-like activity, potently and completely abolishing immobility in the forced swim test (FST) with extended duration of action.

METHODS

We investigated the antidepressant-like activity of NLX-101 using the rat chronic mild stress (CMS) model of depression, considered to have a higher translational potential than the FST, as it possesses construct, face and predictive validity. The effects of CMS and repeated NLX-101 treatment were tested using sucrose consumption (a measure of anhedonia), novel object recognition (NOR; a measure of working memory) and elevated plus maze (EPM; a measure of anxiety) tests.

RESULTS

NLX-101 reversed the CMS-induced decrease of sucrose intake on day 1 of testing, with full reversal observed at the dose of 0.16 mg/kg and a less pronounced but still significant effect at 0.04 mg/kg, both given twice a day intraperitoneally. The effects of NLX-101 were maintained over the 2 week treatment period and persisted for four weeks following cessation of treatment. In the NOR test, both doses of NLX-101 rescued the deficit in discrimination index caused by CMS, without any effect on locomotor activity. However, NLX-101 had no effect on the reduction of open-arms entries produced by CMS in the EPM model. In control, non-stressed rats, NLX-101 produced non-significant effects in all three models.

CONCLUSIONS

NLX-101 displayed efficacious activity in the CMS test, with more rapid (1 day) antidepressant-like effects than pharmacological compounds tested previously under the same experimental conditions. These observations suggest that biased agonist targeting of cortical 5-HT_1A receptors constitutes a promising strategy to achieve rapid-acting and sustained antidepressant effects.

Rapid-acting antidepressant ketamine, its metabolites and other candidates: A historical overview and future perspective

Major depressive disorder (MDD) is one of the most disabling psychiatric disorders. Approximately one-third of the patients with MDD are treatment resistant to the current antidepressants. There is also a significant therapeutic time lag of weeks to months. Furthermore, depression in patients with bipolar disorder (BD) is typically poorly responsive to antidepressants. Therefore, there exists an unmet medical need for rapidly acting antidepressants with beneficial effects in treatment-resistant patients with MDD or BD. Accumulating evidence suggests that the N-methyl-D-aspartate receptor (NMDAR) antagonist ketamine produces rapid and sustained antidepressant effects in treatment-resistant patients with MDD or BD. Ketamine is a racemic mixture comprising equal parts of (R)-ketamine (or arketamine) and (S)-ketamine (or esketamine). Because (S)-ketamine has higher affinity for NMDAR than (R)-ketamine, esketamine was developed as an antidepressant. On 5 March 2019, esketamine nasal spray was approved by the US Food and Drug Administration. However, preclinical data suggest that (R)-ketamine exerts greater potency and longer-lasting antidepressant effects than (S)-ketamine in animal models of depression and that (R)-ketamine has less detrimental side-effects than (R,S)-ketamine or (S)-ketamine. In this article, the author reviews the historical overview of the antidepressant actions of enantiomers of ketamine and its major metabolites norketamine and hydroxynorketamine. Furthermore, the author discusses the other potential rapid-acting antidepressant candidates (i.e., NMDAR antagonists and modulators, low-voltage-sensitive T-type calcium channel inhibitor, potassium channel Kir4.1 inhibitor, negative modulators of γ-aminobutyric acid, and type A [GABA_A ] receptors) to compare them with ketamine. Moreover, the molecular and cellular mechanisms of ketamine's antidepressant effects are discussed.

Mitochondrial transplantation attenuates lipopolysaccharide- induced depression-like behaviors

The dysfunction of mitochondria plays important roles in the development of depression. Interestingly, increasing numbers of evidence show the therapeutic benefits of mitochondria transfer. Therefore, we hypothesized that injection of exogenous mitochondria would contribute to ameliorate depressive-like symptoms. In this study, the antidepressant-like effect of intravenous isolated mitochondria was evaluated on a lipopolysaccharide (LPS)- induced model of depression. The depressive-like behaviors were assessed using forced swim test (FST), tail suspension test (TST) and sucrose preference test. Besides, the neurogenesis, expression of brain-derived neurotrophic factor (BDNF), glial activation, neuroinflammation, oxidative stress and ATP production were determined in the hippocampus. The results showed that treatment of isolated mitochondria decreased the immobility time of mice in the FST and TST, and attenuated the decrease in sucrose preference test. Moreover, isolated mitochondria significantly reduced the activation of astrocyte and microglia as well as neuroinflammation (i.e. 1 L-1β, TNF-α and COX-2), increased BDNF expression and neurogenesis, restored the dysfunction of ATP production and oxidative stress in inflammation- induced depression. Taken together, the data suggested for the first time that injection of isolated mitochondria ameliorated LPS- induced depressive-like behaviors. The new discovery for the present study provides that mitochondrial transplantation might act as a new therapeutic strategy for MDD.

Intravenous vortioxetine to accelerate onset of effect in major depressive disorder: a 2-week, randomized, double-blind, placebo-controlled study

This 2-week randomized, double-blind, placebo-controlled fixed-dose study (NCT02919501) explored the potential of accelerating onset of antidepressant efficacy and plasma exposure with single-dose intravenous vortioxetine at oral vortioxetine treatment initiation. Outpatients (ages 18-65 years) with major depressive disorder and a current depressive episode (Montgomery Åsberg Depression Rating Scale total score ≥30) were randomized to an initial single dose of either intravenous vortioxetine 17 mg (n = 27) or intravenous placebo (n = 28), both treatments followed by 2 weeks of oral vortioxetine (10 mg/day). From baseline to day 7, both groups exhibited fast and substantial improvements by approximately 14 Montgomery Åsberg Depression Rating Scale points, with no statistically significant treatment difference for this primary endpoint. Improvements were substantial already within 24 hours, with numerical treatment differences of 1.3 and 1.6 points at days 1 and 3, respectively, in favour of intravenous vortioxetine + oral vortioxetine. Pharmacokinetic data confirmed that intravenous vortioxetine facilitated reaching steady-state plasma concentration within 24 hours. Intravenous vortioxetine + oral vortioxetine was safe and well-tolerated, with nausea as the most common adverse event. This study supported intravenous vortioxetine as a means of rapidly reaching therapeutic vortioxetine blood levels.

Antinociceptive and antidepressive efficacies of the combined ineffective doses of S-ketamine and URB597

Clinical studies have demonstrated that the NMDA receptor antagonist ketamine produces rapid antidepressant responses. There are safety concerns and adverse effects that limit the utilization of ketamine in psychiatry. Some studies have suggested combination therapy for optimal ketamine use. In this study, we evaluated the potential for combination therapy of ineffective doses of ketamine and fatty acid amide hydrolase inhibitor URB597 for the treatment of depression and pain in male NMRI mice. Intraperitoneal administration of ketamine (10 mg/kg) at the time intervals of 115, 145, and 160 min and ketamine (5 mg/kg) at the time interval of 160 min after administration increased the tail-flick latency, indicating an antinociceptive effect. The same doses of ketamine decreased immobility time in the forced swim test (FST), showing an antidepressant-like effect. Moreover, URB597 at the doses of 0.5 and 1 mg/kg induced an antinociceptive effect, while it at the dose of 1 mg/kg produced an antidepressant-like response. Furthermore, co-administration of the ineffective doses of ketamine (2.5 mg/kg) and URB597 (0.1 mg/kg) caused antinociceptive and antidepressant-like effects, while each one of them alone did not alter the performance of mice in the FST and tail-flick tests. It should be noted that none of the treatments alter animal locomotor activity compared to the control group. Therefore, the combined administration of ineffective doses of ketamine and URB597 might be an effective strategy in the therapy of depression and pain.

Decoding the Mechanism of Action of Rapid-Acting Antidepressant Treatment Strategies: Does Gender Matter?

Gender differences play a pivotal role in the pathophysiology and treatment of major depressive disorder. This is strongly supported by a mean 2:1 female-male ratio of depression consistently observed throughout studies in developed nations. Considering the urgent need to tailor individualized treatment strategies to fight depression more efficiently, a more precise understanding of gender-specific aspects in the pathophysiology and treatment of depressive disorders is fundamental. However, current treatment guidelines almost entirely neglect gender as a potentially relevant factor. Similarly, the vast majority of animal experiments analysing antidepressant treatment in rodent models exclusively uses male animals and does not consider gender-specific effects. Based on the growing interest in innovative and rapid-acting treatment approaches in depression, such as the administration of ketamine, its metabolites or electroconvulsive therapy, this review article summarizes the evidence supporting the importance of gender in modulating response to rapid acting antidepressant treatment. We provide an overview on the current state of knowledge and propose a framework for rodent experiments to ultimately decode gender-dependent differences in molecular and behavioural mechanisms involved in shaping treatment response.

Depression as a Neuroendocrine Disorder: Emerging Neuropsychopharmacological Approaches beyond Monoamines

Depression is currently recognized as a crucial problem in everyday clinical practice, in light of ever-increasing rates of prevalence, as well as disability, morbidity, and mortality related to this disorder. Currently available antidepressant drugs are notoriously problematic, with suboptimal remission rates and troubling side-effect profiles. Their mechanisms of action focus on the monoamine hypothesis for depression, which centers on the disruption of serotonergic, noradrenergic, and dopaminergic neurotransmission in the brain. Nevertheless, views on the pathophysiology of depression have evolved notably, and the comprehension of depression as a complex neuroendocrine disorder with important systemic implications has sparked interest in a myriad of novel neuropsychopharmacological approaches. Innovative pharmacological targets beyond monoamines include glutamatergic and GABAergic neurotransmission, brain-derived neurotrophic factor, various endocrine axes, as well as several neurosteroids, neuropeptides, opioids, endocannabinoids and endovanilloids. This review summarizes current knowledge on these pharmacological targets and their potential utility in the clinical management of depression.

Therapeutic Approaches for NOP Receptor Antagonists in Neurobehavioral Disorders: Clinical Studies in Major Depressive Disorder and Alcohol Use Disorder with BTRX-246040 (LY2940094)

Conventional antidepressants increase the efflux of biogenic amine neurotransmitters (the monoamine hypothesis of depression) in the central nervous system (CNS) and are the principle drugs used to treat major depressive disorder (MDD). However, the lack of efficacy in some patients, the slow onset of action, and the side effect profiles of existing antidepressants necessitate the exploration of additional treatment options. The discovery of the nociceptin/orphanin FQ peptide NOP receptor (N/OFQ-NOP receptor) system and its characterization in preclinical biological and pharmacological stress-related conditions supports the potential antidepressant and anti-stress properties of a NOP receptor antagonist for the treatment of neurobehavioral disorders. BTRX-246040 (formerly LY2940094) was designed to test this hypothesis in the clinic. A small clinical proof of concept study demonstrated efficacy of BTRX-246040 in MDD patients. In this study, BTRX-246040 (40 mg, p.o.) significantly reduced negative bias as assessed by the facial recognition test within 1 week of treatment and decreased depression symptoms after 8 weeks. BTRX-246040 also reduced depression symptoms in a second trial with heavy alcohol drinkers. Given the comorbidity of MDD and alcohol use disorder, a compound with such effects in patients could be a valuable addition to the medications available. A proof of concept study showed efficacy of BTRX-246040 in reducing heavy drinking and increasing the probability of abstinence in individuals diagnosed with alcohol dependence. In addition, plasma levels of gamma-glutamyl transferase were decreased by BTRX-246040 compared to placebo control implying improvement in liver function. Collectively, the clinical data reviewed within this chapter suggest that BTRX-264040 functions to normalize dysfunction in reward circuits. The overall efficacy and safety of this compound with a novel mechanism of action are encouraging of further clinical development. BTRX-246040 is currently under development for MDD by BlackThorn Therapeutics.

Animal models of fatigue in major depressive disorder

Fatigue is common in a host of neurological and psychiatric disorders including depression and often continues unabated even after primary symptoms of disease are treated. Its high estimated prevalence combined with the lack of effective medicines has engaged the preclinical research community to search for fatigue models. The present review briefly summarizes the most common animal models that have been brought forward along with some of the associated pharmacological validation data. Like all preclinical models, these models have issues that need to be appreciated in the generation and interpretation of data for the purposes of translation to human disease; specifically, there are deficiencies in construct validity, a lack of medicines that effectively address residual fatigue symptoms, and difficulties in defining specificity with respect to drug effects on fatigue per se. Nonetheless, existing animal models of fatigue arguably serve the valuable purpose of encouraging research in this large area of unmet medical need. Data from these models are predicted to engender human experimentation and the further development of improved model systems.

Comparison of rapid and long-lasting antidepressant effects of negative modulators of α5-containing GABAA receptors and (R)‑ketamine in a chronic social defeat stress model

The negative allosteric modulators (NAMs: L-655,708 and MRK-016) of α5 subunit-containing GABA_A receptors are reported to show rapid-acting antidepressant effects in rodents. However, there are no reports comparing these NAMs and (R)‑ketamine, (R)-enantiomer of the rapid-acting antidepressant ketamine, in a chronic social defeat stress (CSDS) model. Here we measured expression of α5 GABA_A receptor in the brain regions from CSDS susceptible mice and postmortem brain samples from depressed patients. Expression of α5 GABA_A receptor in the prefrontal cortex and hippocampus from CSDS susceptible mice was significantly higher than that of control mice. Furthermore, expression of α5 GABA_A receptor in the parietal cortex from depressed patients was also higher than that of control subjects. In the tail suspension and forced swimming tests, (R)‑ketamine and MRK-016 significantly attenuated the increased immobility time in the susceptible mice, compared with the vehicle-treated group. In the sucrose preference test, (R)‑ketamine and MRK-016 significantly enhanced the reduced preference in CSDS susceptible mice two days after a single injection. Unlike (R)‑ketamine, MRK-016 did not attenuate the reduced sucrose preference in susceptible mice 7 days after a single injection. In contrast, L-655,708 did not show antidepressant effects in the same model. In conclusion, this study shows that increased levels of α5 GABA_A receptors in the PFC and hippocampus may play a role in depression-like phenotype after CSDS. It is unlikely that MRK-016 has long-lasting antidepressant effects although it elicits rapid-acting antidepressant effects.