Rapid-onset antidepressant action of ketamine: potential revolution in understanding and future pharmacologic treatment of depression

Pre-adolescence repeat exposure to sub-anesthetic doses of ketamine induces long-lasting behaviors and cognition impairment in male and female rat adults

In pre-adolescence, repeated anesthesia may be required for therapeutic interventions. Adult cognitive and neurobehavioral problems may result from preadolescent exposure to anesthetics. This study examined the long-term morphological and functional effects of repeated sub-anesthetic doses of ketamine exposure on male and female rat adults during pre-adolescence. Weaned 48 pre-adolescent rats from eight mothers and were randomly divided into four equal groups: control group and the ketamine group of males and females (20 mg/kg daily for 14 days); then animals received care for 20-30 days. Repeated exposure to sub-anesthetic doses of ketamine on cognitive functions was assayed using Social discrimination and novel object tests. Besides, an elevated plus maze and fear conditioning apparatus were utilized to determine exploratory and anxiety-like behavior in adults. Toluidine blue stain was used to evaluate the number of dead neurons in the hippocampus, and the effects of ketamine on synaptic plasticity were compared in the perforant pathway of the CA1 of the hippocampus. Our study indicates that repeated exposure to sub-anesthetic doses of ketamine during pre-adolescence can result in neurobehavioral impairment in male and female rat adulthood but does not affect anxiety-like behavior. We found a significant quantifiable increase in dark neurons. Recorded electrophysiologically, repeat sub-anesthetic doses of ketamine resulted in hampering long-term potentiation and pair pulse in male adult animals. Our results showed that repeated exposure to sub-anesthetic doses of ketamine during pre-adolescence can induce hippocampus and neuroplasticity changes later in adulthood. This study opens up a new line of inquiry into potential adverse outcomes of repeated anesthesia exposure in pre-adolescent rats.

Ketamine causes poor maternal care in rats with postpartum depression and leads to few behavioral and neurochemical alterations on male offspring

Ketamine is an anesthetic drug that also has antidepressant properties, with quick action. Despite the great number of studies showing its effectiveness as a treatment for major depression, there is little information about its effects on postpartum depression, as pharmacological treatments bring risks to the health of both mother and child. Thus, this study aimed to evaluate the effects of prolonged treatment with subanesthetic doses of ketamine in a rat model of postpartum depression. Female dams were induced to postpartum depression by the maternal separation model from lactating day (LD) 2-12. They were divided into four groups: one control and three experimental groups, which were treated with different doses of ketamine (5, 10 or 20 mg/kg) from LD 2-21 i.p. Maternal studies were conducted from LD5 to LD21 and the offspring studies from postnatal day 2 through 90. Ketamine causes poor maternal care, with few neurochemical alterations. However, the highest dose used in this study had an antidepressant effect. Regarding the male offspring, indirect exposure to ketamine through breast milk caused few behavioral changes during infancy, but they were not permanent, as they faded in adulthood. Nevertheless, this exposure was able to cause alterations in their monoaminergic neurotransmission systems that were found in both infancy and adulthood periods.

Ketamine and serotonergic psychedelics: An update on the mechanisms and biosignatures underlying rapid-acting antidepressant treatment

The discovery of ketamine as a rapid-acting antidepressant spurred significant research to understand its underlying mechanisms of action and to identify other novel compounds that may act similarly. Serotonergic psychedelics (SPs) have shown initial promise in treating depression, though the challenge of conducting randomized controlled trials with SPs and the necessity of long-term clinical observation are important limitations. This review summarizes the similarities and differences between the psychoactive effects associated with both ketamine and SPs and the mechanisms of action of these compounds, with a focus on the monoaminergic, glutamatergic, gamma-aminobutyric acid (GABA)-ergic, opioid, and inflammatory systems. Both molecular and neuroimaging aspects are considered. While their main mechanisms of action differ-SPs increase serotonergic signaling while ketamine is a glutamatergic modulator-evidence suggests that the downstream mechanisms of action of both ketamine and SPs include mechanistic target of rapamycin complex 1 (mTORC1) signaling and downstream GABAA receptor activity. The similarities in downstream mechanisms may explain why ketamine, and potentially SPs, exert rapid-acting antidepressant effects. However, research on SPs is still in its infancy compared to the ongoing research that has been conducted with ketamine. For both therapeutics, issues with regulation and proper controls should be addressed before more widespread implementation. This article is part of the Special Issue on "Ketamine and its Metabolites".

The microbiota-gut-brain axis and its modulation in the therapy of depression: comparison of efficacy of conventional drugs and traditional Chinese medicine approaches

Depression is a common and severe mental disease that places a heavy burden on human society, which can lead to decreased cognitive function, energy loss, insomnia, and even suicide. Although medication plays an important role in improving the symptoms of depression, approximately one third of people with depression do not significantly benefit from medication and experience various adverse reactions. Recently, increasing evidence has shown that gut microbes play an important role in the occurrence and development of depression. There have been illuminating studies previously conducted on the relationship between antidepressant chemicals, traditional Chinese medicine, and the microbiota-gut-brain axis (MGBA). Therefore, in this review, we summarize the role of the MGBA in the occurrence and development of depression, especially the important role of the MGBA in the mechanism of action of antidepressants. Modulation of the MGBA is proposed to enhance the efficacy of antidepressant drugs and reduce their side effects and disease recurrence, so as to provide a new method for the treatment of depression.

Beating Pain with Psychedelics: Matter over Mind?

Basic pain research has shed light on key cellular and molecular mechanisms underlying nociceptive and phenomenological aspects of pain. Despite these advances, [[we still yearn for] the discovery of novel therapeutic strategies to address the unmet needs of about 70% of chronic neuropathic pain patients whose pain fails to respond to opioids as well as to other conventional analgesic agents. Importantly, a substantial body of clinical observations over the past decade cumulatively suggests that the psychedelic class of drugs may possess heuristic value for understanding and treating chronic pain conditions. The present review presents a theoretical framework for hitherto insufficiently understood neuroscience-based mechanisms of psychedelics' potential analgesic effects. To that end, searches of PubMed-indexed journals were performed using the following Medical Subject Headings' terms: pain, analgesia, inflammatory, brain connectivity, ketamine, psilocybin, functional imaging, and dendrites. Recursive sets of scientific and clinical evidence extracted from this literature review were summarized within the following key areas: (1) studies employing psychedelics for alleviation of physical and emotional pain; (2) potential neuro-restorative effects of psychedelics to remediate the impaired connectivity underlying the dissociation between pain-related conscious states/cognitions and the subcortical activity/function leading to the eventual chronicity through immediate and long-term effects on dentritic plasticity; (3) anti-neuroinflammatory and pro-immunomodulatory actions of psychedelics as the may pertain to the role of these factors in the pathogenesis of neuropathic pain; (4) safety, legal, and ethical consideration inherent in psychedelics' pharmacotherapy. In addition to direct beneficial effects in terms of reduction of pain and suffering, psychedelics' inclusion in the analgesic armamentarium will contribute to deeper and more sophisticated insights not only into pain syndromes but also into frequently comorbid psychiatric condition associated with emotional pain, e.g., depressive and anxiety disorders. Further inquiry is clearly warranted into the above areas that have potential to evolve into further elucidate the mechanisms of chronic pain and affective disorders, and lead to the development of innovative, safe, and more efficacious neurobiologically-based therapeutic approaches.

Predictive performance of parent-metabolite population pharmacokinetic models of (S)-ketamine in healthy volunteers

Purpose

The recent repurposing of ketamine as treatment for pain and depression has increased the need for accurate population pharmacokinetic (PK) models to inform the design of new clinical trials. Therefore, the objectives of this study were to externally validate available PK models on (S)-(nor)ketamine concentrations with in-house data and to improve the best performing model when necessary.

Methods

Based on predefined criteria, five models were selected from literature. Data of two previously performed clinical trials on (S)-ketamine administration in healthy volunteers were available for validation. The predictive performances of the selected models were compared through visual predictive checks (VPCs) and calculation of the (root) mean (square) prediction errors (ME and RMSE). The available data was used to adapt the best performing model through alterations to the model structure and re-estimation of inter-individual variability (IIV).

Results

The model developed by Fanta et al. (Eur J Clin Pharmacol 71:441–447, 2015) performed best at predicting the (S)-ketamine concentration over time, but failed to capture the (S)-norketamine C_max correctly. Other models with similar population demographics and study designs had estimated relatively small distribution volumes of (S)-ketamine and thus overpredicted concentrations after start of infusion, most likely due to the influence of circulatory dynamics and sampling methodology. Model predictions were improved through a reduction in complexity of the (S)-(nor)ketamine model and re-estimation of IIV.

Conclusion

The modified model resulted in accurate predictions of both (S)-ketamine and (S)-norketamine and thereby provides a solid foundation for future simulation studies of (S)-(nor)ketamine PK in healthy volunteers after (S)-ketamine infusion.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00228-021-03104-1.

Ketamine and Serotonergic Psychedelics: Common Mechanisms Underlying the Effects of Rapid-Acting Antidepressants

BACKGROUND

The glutamatergic modulator ketamine has created a blueprint for studying novel pharmaceuticals in the field. Recent studies suggest that "classic" serotonergic psychedelics (SPs) may also have antidepressant efficacy. Both ketamine and SPs appear to produce rapid, sustained antidepressant effects after a transient psychoactive period.

METHODS

This review summarizes areas of overlap between SP and ketamine research and considers the possibility of a common, downstream mechanism of action. The therapeutic relevance of the psychoactive state, overlapping cellular and molecular effects, and overlapping electrophysiological and neuroimaging observations are all reviewed.

RESULTS

Taken together, the evidence suggests a potentially shared mechanism wherein both ketamine and SPs may engender rapid neuroplastic effects in a glutamatergic activity-dependent manner. It is postulated that, though distinct, both ketamine and SPs appear to produce acute alterations in cortical network activity that may initially produce psychoactive effects and later produce milder, sustained changes in network efficiency associated with therapeutic response. However, despite some commonalities between the psychoactive component of these pharmacologically distinct therapies-such as engagement of the downstream glutamatergic pathway-the connection between psychoactive impact and antidepressant efficacy remains unclear and requires more rigorous research.

CONCLUSIONS

Rapid-acting antidepressants currently under investigation may share some downstream pharmacological effects, suggesting that their antidepressant effects may come about via related mechanisms. Given the prototypic nature of ketamine research and recent progress in this area, this platform could be used to investigate entirely new classes of antidepressants with rapid and robust actions.

Role of the medial prefrontal cortex in the effects of rapid acting antidepressants on decision-making biases in rodents

Major depressive disorder is a significant and costly cause of global disability. Until the discovery of the rapid acting antidepressant (RAAD) effects of ketamine, treatments were limited to drugs that have delayed clinical benefits. The mechanism of action of ketamine is currently unclear but one hypothesis is that it may involve neuropsychological effects mediated through modulation of affective biases (where cognitive processes such as learning and memory and decision-making are modified by emotional state). Previous work has shown that affective biases in a rodent decision-making task are differentially altered by ketamine, compared to conventional, delayed onset antidepressants. This study sought to further investigate these effects by comparing ketamine with other NMDA antagonists using this decision-making task. We also investigated the subtype selective GluN2B antagonist, CP-101,606 and muscarinic antagonist scopolamine which have both been shown to have RAAD effects. Both CP-101,606 and scopolamine induced similar positive biases in decision-making to ketamine, but the same effects were not seen with other NMDA antagonists. Using targeted medial prefrontal cortex (mPFC) infusions, these effects were localised to the mPFC. In contrast, the GABA_A agonist, muscimol, induced general disruptions to behaviour. These data suggest that ketamine and other RAADs mediate a specific effect on affective bias which involves the mPFC. Non-ketamine NMDA antagonists lacked efficacy and we also found that temporary inactivation of the mPFC did not fully recapitulate the effects of ketamine, suggesting a specific mechanism.

Rapid effects of S-ketamine on the morphology of hippocampal astrocytes and BDNF serum levels in a sex-dependent manner

The prevalence of major depressive disorder (MDD) is higher in women than men. Importantly, a differential behavioral response by sex to the antidepressant response to ketamine in rodents has been reported. Mechanistically, male depressed-like animals showed an increased spine density after ketamine treatment via restoration of synaptic protein levels while those proteins were not altered in female rats. In addition, preclinical studies indicate that the impairment of astrocytic plasticity is one of the contributing mechanisms in the pathophysiology of MDD. Accordingly, in this study, we determined the effect of sex on the rapid morphological alteration of hippocampal astrocytes and the serum level of BDNF one hour after S-ketamine injection. A single intraperitoneal dose of S-ketamine (15 mg/kg) or saline was injected to the male and female Flinders Sensitive Line (FSL) rats, a genetic animal model of depression and their brains were perfused one hour after treatment. The size of the GFAP positive astrocytes in the hippocampal subregions was measured. The volume of different hippocampal subregions was assessed using the Cavalieri estimator. Moreover, serum levels of BDNF were measured with enzyme-linked immunosorbent assay (ELISA) kits. The volume of hippocampal subregions significantly increased one hour after S-ketamine in both male and female FSL animals. However, a substantial alteration in the morphology of the hippocampal astrocytes was observed only in the female rats. Additionally, significantly increased serum BDNF levels in the female depressed rats were observed one hour after S-ketamine treatment. Our results indicate that the rapid effects of S-ketamine on the morphology of the hippocampal astrocytes and the serum level of BDNF are sex-dependent.

Preclinical toxicological study of prolonged exposure to ketamine as an antidepressant

BACKGROUND

Depression is one of the most common mentally debilitating diseases in the world. Ketamine has been recently identified as a potential novel antidepressant. Further animal model evaluations of the use of ketamine as an antidepressant are necessary to determine safety parameters for clinical use. Therefore, the objective of this study was to perform toxicological tests of prolonged treatment using three different doses of ketamine in adult male rats.

METHODS

The animals were divided into four groups: three treated with 5, 10 or 20 mg/kg of ketamine and a control group treated with saline solution. Intraperitoneal route of treatment was administered daily for 3 weeks. Body weight, water and food intake were measured once a week, as well as evaluation of the functional observational battery, which includes methodic monitoring of motor activity, motor coordination, behavioral changes, and sensory/motor reflex responses. Upon completion of treatment period, all animals were euthanized by decapitation followed by immediate collection of samples, which included brain structures and blood for neurochemical, hematological and biochemical analyses.

RESULTS

Rats treated with the highest tested dosage (20 mg/kg) of ketamine had lower weight gain in the 1st and 2nd weeks of treatment and all experimental groups had measurable alterations in the serotoninergic system.

CONCLUSIONS

Our data indicate that the alterations observed are minor and due to a predicted mechanism of action, which implies that ketamine is a promising drug for repurposing as an antidepressant.

The influence of subanaesthetic ketamine on regional cerebral blood flow in healthy dogs measured with 99mTc-HMPAO SPECT

Subanaesthetic ketamine has recently been proven to be a highly effective and fast acting alternative treatment for several psychiatric disorders. The mechanisms responsible for ketamine's antidepressant effects remain unclear, but a possible explanation could be that ketamine interacts with regional cerebral blood flow (rCBF). Therefore, the effects of two subanaesthetic ketamine doses on rCBF were evaluated. Twelve dogs were randomly assigned to one of the three treatment conditions (condition saline, condition 0.5 mg/kg ketamine or condition 2 mg/kg ketamine) and received in total five saline or ketamine infusions, with one week interval. Single Photon Emission Computed Tomography (SPECT) scans with the radiotracer 99mTc-hexamethylpropylene amine oxime were performed before the start of the infusions (baseline) and 24 hours after the first (single) and last (multiple) infusion. After a wash out period of 3 months, the animals were again assigned to one of the three treatment conditions described above and the infusion/scan protocol was repeated. During the infusions, cardiovascular parameters were evaluated every ten minutes. A one-way repeated measure ANOVA was set up to assess perfusion index for each ketamine dose for the left frontal cortex (alpha = 0.05). The remaining 11 brain regions were post hoc assessed. Perfusion index was significantly increased in the left frontal cortex and in the thalamus 24 hours after single and multiple ketamine infusions compared to baseline in the 2 mg/kg condition. No clinically relevant cardiovascular effects were observed during the ketamine infusions. This study shows that subanaesthetic ketamine can increase neuronal perfusion and therefore alter neuronal function in brain regions involved in depression and anxiety disorders. These perfusion increases may possibly contribute to ketamine's beneficial effects in these psychiatric disorders.

Biological Mechanisms of Cancer-Induced Depression

Patients with cancer are more likely to develop depression than the general population, which negatively impacts their quality of life and prognosis. In order to identify effective antidepressants catered toward cancer patients, the biology of depression in the context of cancer must be well-understood. Many theories have emerged postulating the mechanisms underlying the development of depressive disorder. Here, we review the role inflammation, a hyperactive hypothalamic-pituitary-adrenal (HPA) axis, and glutamate excitotoxicity may play in cancer-induced depression. Hopefully, novel therapeutics targeting these dysregulated pathways may be potent in ameliorating depressive symptoms in the cancer population.

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.

Behavioural and computational methods reveal differential effects for how delayed and rapid onset antidepressants effect decision making in rats

Major depressive disorder (MDD) is one of the most prevalent psychiatric disorders. Until the recent discovery of the rapid onset antidepressant action of ketamine, pharmacological treatments for MDD were limited to conventional antidepressant drugs with delayed clinical efficacy. Using a judgement bias task, this study has investigated whether the temporal differences observed in patients would be reflected in affective biases and decision making behaviour in rodents. The diffusion model was also used to investigate the underlying decision making processes. Positive biases were induced in this task over timeframes that mirror the rapid versus delayed antidepressant efficacy of the drugs in clinical populations. Diffusion modelling revealed that the antidepressants tested also have different effects on decision making processes, suggesting they may act through different neurobiological substrates. This combination of behaviour and computational modelling may provide a useful approach to further investigate the mechanisms underlying rapid antidepressant effect and assess potential new treatments.

Rapid antidepressant effect of ketamine correlates with astroglial plasticity in the hippocampus

BACKGROUND AND PURPOSE

Astroglia contribute to the pathophysiology of major depression and antidepressant drugs act by modulating synaptic plasticity; therefore, the present study investigated whether the fast antidepressant action of ketamine is reflected in a rapid alteration of the astrocytes' morphology in a genetic animal model of depression.

EXPERIMENTAL APPROACH

S-Ketamine (15 mg·kg-1 ) or saline was administered as a single injection to Flinders Line (FSL/ FRL) rats. Twenty-four hours after the treatment, perfusion fixation was carried out and the morphology of glial fibrillary acid protein (GFAP)-positive astrocytes in the CA1 stratum radiatum (CA1.SR) and the molecular layer of the dentate gyrus (GCL) of the hippocampus was investigated by applying stereological techniques and analysis with Imaris software. The depressive-like behaviour of animals was also evaluated using forced swim test.

KEY RESULTS

FSL rats treated with ketamine exhibited a significant reduction in immobility time in comparison with the FSL-vehicle group. The volumes of the hippocampal CA1.SR and GCL regions were significantly increased 1 day after ketamine treatment in the FSL rats. The size of astrocytes in the ketamine-treated FSL rats was larger than those in the FSL-vehicle group. Additionally, the number and length of the astrocytic processes in the CA1.SR region were significantly increased 1 day following ketamine treatment.

CONCLUSIONS AND IMPLICATIONS

Our results support the hypothesis that astroglial atrophy contributes to the pathophysiology of depression and a morphological modification of astrocytes could be one mechanism by which ketamine rapidly improves depressive behaviour.

What is the mechanism of Ketamine's rapid-onset antidepressant effect? A concise overview of the surprisingly large number of possibilities

WHAT IS KNOWN AND OBJECTIVE

Abundant clinical data now confirm that ketamine produces a remarkable rapid-onset antidepressant effect - hours or days - in contrast to the delayed onset (typically weeks) of current antidepressant drugs. This surprising and revolutionary finding may lead to the development of life-saving pharmacotherapy for depressive illness by reducing the high suicide risk associated with the delayed onset of effect of current drugs. As ketamine has serious self-limiting drawbacks that restrict its widespread use for this purpose, a safer alternative is needed. Our objective is to review the proposed mechanism(s) of ketamine's rapid-onset antidepressant action for new insights into the physiological basis of depressive illness that may lead to new and novel targets for antidepressant drug discovery.

METHODS

A search was conducted on published literature (e.g. PubMed) and Internet sources to identify information relevant to ketamine's rapid-acting antidepressant action and, specifically, to the possible mechanism(s) of this action. Key search words included 'ketamine', 'antidepressant', 'mechanism of action', 'depression' and 'rapid acting', either individually or in combination. Information was sought that would include less well-known, as well as well-known, basic pharmacologic properties of ketamine and that identified and evaluated the several hypotheses about ketamine's mechanism of antidepressant action.

RESULTS

Whether the mechanistic explanation for ketamine's rapid-onset antidepressant action is related to its well-known antagonism of the NMDA (N-Methyl-d-aspartate) subtype of glutamate receptor or to something else has not yet been fully elucidated. The evidence from pharmacologic, medicinal chemistry, animal model and drug-discovery sources reveals a wide variety of postulated mechanisms.

WHAT IS NEW AND CONCLUSION

The surprising discovery of ketamine's rapid-onset antidepressant effect is a game-changer for the understanding and treatment of depressive illness. There is some convergence on NMDA receptor antagonism as a likely, but to date unproven, common mechanism. The surprising number of other mechanisms, and the several novel biochemical aetiologies of depression proposed, suggests exciting new drug-discovery targets.

The kynurenine pathway and the brain: Challenges, controversies and promises

Research on the neurobiology of the kynurenine pathway has suffered years of relative obscurity because tryptophan degradation, and its involvement in both physiology and major brain diseases, was viewed almost exclusively through the lens of the well-established metabolite serotonin. With increasing recognition that kynurenine and its metabolites can affect and even control a variety of classic neurotransmitter systems directly and indirectly, interest is expanding rapidly. Moreover, kynurenine pathway metabolism itself is modulated in conditions such as infection and stress, which are known to induce major changes in well-being and behaviour, so that kynurenines may be instrumental in the etiology of psychiatric and neurological disorders. It is therefore likely that the near future will not only witness the discovery of additional physiological and pathological roles for brain kynurenines, but also ever-increasing interest in drug development based on these roles. In particular, targeting the kynurenine pathway with new specific agents may make it possible to prevent disease by appropriate pharmacological or genetic manipulations. The following overview focuses on areas of kynurenine research which are either controversial, of major potential therapeutic interest, or just beginning to receive the degree of attention which will clarify their relevance to neurobiology and medicine. It also highlights technical issues so that investigators entering the field, and new research initiatives, are not misdirected by inappropriate experimental approaches or incorrect interpretations at this time of skyrocketing interest in the subject matter. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.

S-Ketamine Rapidly Reverses Synaptic and Vascular Deficits of Hippocampus in Genetic Animal Model of Depression

BACKGROUND

The neurovascular plasticity of hippocampus is an important theory underlying major depression. Ketamine as a novel glutamatergic antidepressant drug can induce a rapid antidepressant effect within hours. In a mechanistic proof of this concept, we examined whether ketamine leads to an increase in synaptogenesis and vascularization within 24 hours after a single injection in a genetic rat model of depression.

METHODS

Flinders Sensitive Line and Flinders Resistant Line rats were given a single intraperitoneal injection of ketamine (15 mg/kg) or saline. One day later, their behavior was evaluated by a modified forced swim test. Microvessel length was evaluated with global spatial sampling and optical microscopy, whereas the number of asymmetric synapses was quantified through serial section electron microscopy by using physical disector method in the CA1.stratum radiatum area of hippocampus.

RESULTS

The immobility time in the forced swim test among Flinders Sensitive Line rats with ketamine treatment was significantly lower compared with Flinders Sensitive Line rats without treatment. The number of nonperforated and perforated synapses was significantly higher in the Flinders Sensitive Line-ketamine vs the Flinders Sensitive Line-vehicle group; however, ketamine did not induce a significant increase in the number of shaft synapses. Additionally, total length of microvessels was significantly increased 1 day after ketamine treatment in Flinders Sensitive Line rats in the hippocampal subregions, including the CA1.stratum radiatum.

CONCLUSION

Our findings indicate that hippocampal vascularization and synaptogenesis is co-regulated rapidly after ketamine, and microvascular elongation may be a supportive factor for synaptic plasticity and neuronal activity. These findings go hand-in-hand with the behavioral observations, where ketamine acts as a potent antidepressant.

The Gut-Brain Axis, BDNF, NMDA and CNS Disorders

Gastro-intestinal (GI) microbiota and the 'gut-brain axis' are proving to be increasingly relevant to early brain development and the emergence of psychiatric disorders. This review focuses on the influence of the GI tract on Brain-Derived Neurotrophic Factor (BDNF) and its relationship with receptors for N-methyl-D-aspartate (NMDAR), as these are believed to be involved in synaptic plasticity and cognitive function. NMDAR may be associated with the development of schizophrenia and a range of other psychopathologies including neurodegenerative disorders, depression and dementias. An analysis of the routes and mechanisms by which the GI microbiota contribute to the pathophysiology of BDNF-induced NMDAR dysfunction could yield new insights relevant to developing novel therapeutics for schizophrenia and related disorders. In the absence of GI microbes, central BDNF levels are reduced and this inhibits the maintenance of NMDAR production. A reduction of NMDAR input onto GABA inhibitory interneurons causes disinhibition of glutamatergic output which disrupts the central signal-to-noise ratio and leads to aberrant synaptic behaviour and cognitive deficits. Gut microbiota can modulate BDNF function in the CNS, via changes in neurotransmitter function by affecting modulatory mechanisms such as the kynurenine pathway, or by changes in the availability and actions of short chain fatty acids (SCFAs) in the brain. Interrupting these cycles by inducing changes in the gut microbiota using probiotics, prebiotics or antimicrobial drugs has been found promising as a preventative or therapeutic measure to counteract behavioural deficits and these may be useful to supplement the actions of drugs in the treatment of CNS disorders.

Xylopia aethiopica fruit extract exhibits antidepressant-like effect via interaction with serotonergic neurotransmission in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Xylopia aethiopica has been used traditionally to treat some central nervous system disorders including epilepsy.

AIM OF THE STUDY

Despite the central analgesic and sedative effects, there is little evidence for its traditional use for CNS disorders. This study thus assessed the antidepressant potential of Xylopia aethiopica ethanolic fruit extract (XAE).

MATERIAL AND METHODS

Antidepressant effect was assessed in the forced swim test (FST) and tail suspension test (TST) models in mice. The role of monoamines in the antidepressant effects of XAE was evaluated by selective depletion of serotonin and noradrenaline, whereas involvement of NMDA/nitric oxide was assessed with NMDA receptor co-modulators; d-serine and d-cycloserine and NOS inhibitor, l-NAME.

RESULTS

Xylopia aethiopica (30, 100, 300mgkg(-1)) dose dependently reduced immobility in both FST and TST. The reduced immobility was reversed after 5-hydroxytryptamine (5-HT) depletion with tryptophan hydroxylase inhibitor-p-chlorophenylalanine (pCPA) and after monoamine depletion with vesicular monoamine transporter inhibitor-reserpine. The observed antidepressant effect was not affected by catecholamine depletion with the tyrosine hydroxylase inhibitor, α-methyl-p-tyrosine (AMPT). Similarly XAE did not potentiate the toxicity of a sub-lethal dose of noradrenaline. XAE had a synergistic effect with the glycineB receptor partial agonist, d-cycloserine and nitric oxide synthase inhibitor, l-NAME. However established antidepressant effects of XAE were abolished by NMDA and NOS activation with d-serine and l-arginine.

CONCLUSION

This study shows that Xylopia aethiopica has antidepressant potential largely due to effects on 5-HT neurotransmission with possible glutamatergic effect through the glycineB co-binding site and nitric oxide synthase inhibition.