Brain region-specific action of ketamine as a rapid antidepressant

Lateral habenula induces cognitive and affective dysfunctions in mice with neuropathic pain via an indirect pathway to the ventral tegmental area

Neuropathic pain, which has become a major public health concern, is frequently accompanied by the deterioration of affective behavior and cognitive function. However, the brain circuitry underlying these changes is poorly understood. Therefore, we aimed to identify in a mouse model the converging circuit that influences the sensory, affective, and cognitive consequences of neuropathic pain. The lateral habenula (LHb) and ventral tegmental area (VTA) have been confirmed to play critical roles in the regulation of pain, cognition, and depression. Given the essential role of the LHb in depression and cognition, we attempted to clarify how neural circuitry involving the LHb integrates pain-related information. Our data confirmed that the VTA receives projections from the LHb, but our results suggest that inhibition of this direct pathway has no effect on the behavior of mice with chronic neuropathic pain. The rostromedial tegmental nucleus (RMTg), a GABAergic structure believed to underlie the transient inhibition of DAergic neurons in the VTA, received glutamatergic inputs from the LHb and projected strongly to the VTA. Furthermore, our data suggest that a projection from LHb glutamatergic neurons to RMTg GABAergic neurons in the VTA, constituting an indirect LHbGlu → RMTgGABA → VTADA pathway, participates in peripheral nerve injury-induced nociceptive hypersensitivity, depressive-like behavior, and cognitive dysfunction. Ex vivo extracellular recordings of LHb neurons showed that the proportion of burst-firing cells in the LHb was significantly increased in indirect projections rather than in direct projections. This may explain the functional discrepancies between direct and indirect projections of the LHb to the VTA. Collectively, our study identifies a pivotal role of the LHbGlu → RMTgGABA → VTADA pathway in processing pain. This pathway may offer new therapeutic targets to treat neuropathic pain and its associated depressive-like and cognitive impairments.

Pharmacokinetics and brain distribution of ketamine after nasal administration

Major depressive disorder is a severe mental condition characterized by abnormalities in the structure and function of the brain. Ketamine is a novel antidepressant that has rapid effects on depression. This drug is clinically used to treat depression in patients with acute suicidal ideation or behavior. It is typically administered at a dose of 84 mg. However, the distribution of ketamine in the body after intranasal administration, particularly in the brain, remains unknown. In the present study, we utilized a high-performance liquid chromatography-tandem mass spectrometry method to measure ketamine concentrations in rat plasma and several tissues. The measurement ranges were 5-8000 ng/mL for the plasma samples and 5-5000 ng/mL for the tissue samples. The pharmacokinetic profile revealed that the rat plasma ketamine concentration rapidly spiked to a peak of 8002 ng/mL within about 5 min, followed by a rapid decline, nearly reaching 0 ng/mL by about 3 h; the half-life was 27 min. Tissue distribution results revealed that ketamine concentrations in different tissues peaked at 5 min. The highest concentration was noted in the kidneys, followed by the liver. In the rat brain regions, ketamine was primarily concentrated in the hypothalamus and hippocampus, with lower concentrations in the striatum and prefrontal cortex. Our novel methodological approach and findings provide a significant theoretical foundation for using ketamine in clinical settings.

An integrated analysis revealing that Sirt1-mediated decreased autophagy in the hippocampus of animal models of depression

BACKGROUND

Depression is a complex and prevalent mental disorder. Numerous studies have reported there were significant metabolomic and proteomic changes in hippocampus of depressed patients. However, few researches have systematically integrated these two omics data to identify key molecular mechanisms underlying depression.

METHODS

Based on the data of Protein and Metabolite Network of Depression Database (ProMENDA), we integrate the significantly altered metabolites and proteins of hippocampus in animal models of depression. Pathway analysis was performed using IPA software to explore biological functional disturbance underlying these molecules. Finally, animal model construction, molecular biology experiments, and lentiviral transfection in vitro for gene knockout were performed to verify potential pathways.

RESULTS

A total of 682 altered metabolites and 2300 altered proteins were retrieved. Pathway enrichment analysis identified 394 significantly enriched pathways, with the sirtuin signaling mediated autophagy being of particular interest. Further biological validations revealed the decrease of Sirt1, the autophagy-related genes, and autophagy markers in hippocampus of both mouse and Macaca fascicularis models of depression. Lastly, Sirt1 knockdown in primary neurons inhibited autophagy.

CONCLUSION

This study expanded our understanding of multi-omics alterations in the hippocampus of depression by revealing that Sirt1 may mediate neuronal autophagy in the hippocampus of animal models of depression, which could further contribute to the pathophysiology of depression.

Stress dynamically modulates neuronal autophagy to gate depression onset

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

Rapid hippocampal synaptic potentiation induced by ketamine metabolite (2R,6R)-hydroxynorketamine persistently primes synaptic plasticity

The pharmacologically active (R,S)-ketamine (ketamine) metabolite (2 R,6 R)-hydroxynorketamine (HNK) maintains ketamine's preclinical antidepressant profile without adverse effects. While hypotheses have been proposed to explain how ketamine and its metabolites initiate their antidepressant-relevant effects, it remains unclear how sustained therapeutic actions arise following drug elimination. To distinguish the physiological mechanisms involved in the rapid from sustained actions of HNK, we utilized extracellular electrophysiology combined with pharmacology to develop an in vitro hippocampal slice incubation model that exhibited pharmacological fidelity to the 1) rapid synaptic potentiation induced by HNK at the Schaffer collateral-CA1 (SC-CA1) synapse during bath-application to slices collected from mice, and 2) maintenance of metaplastic (priming) activity that enhanced N-methyl-D-aspartate receptor (NMDAR) activation-dependent long-term potentiation (LTP) hours after in vivo dosing. We used this model to reveal novel mechanisms engaged in HNK's temporally-sensitive antidepressant-relevant synaptic actions, finding that the induction of synaptic potentiation by HNK did not require NMDAR activity, but NMDAR activity was necessary to maintain synaptic priming. HNK required protein kinase A (PKA) activity to rapidly potentiate SC-CA1 neurotransmission to facilitate synaptic priming that persistently promoted LTP formation. HNK's rapid actions were blocked by inhibitors of adenylyl cyclase 1 (AC1), but not an AC5 inhibitor. We conclude that HNK rapidly potentiates SC-CA1 synaptic efficacy, which then stimulates priming mechanisms that persistently favor plasticity. Targeting such priming mechanisms may be an effective antidepressant strategy, and our incubation model may aid in revealing novel pharmacological targets.

Neuron-astrocyte coupling in lateral habenula mediates depressive-like behaviors

The lateral habenula (LHb) neurons and astrocytes have been strongly implicated in depression etiology, but it was not clear how the two dynamically interact during depression onset. Here, using multi-brain-region calcium photometry recording in freely moving mice, we discover that stress induces a most rapid astrocytic calcium rise and a bimodal neuronal response in the LHb. LHb astrocytic calcium requires the α1A-adrenergic receptor and depends on a recurrent neural network between the LHb and locus coeruleus (LC). Through the gliotransmitter glutamate and ATP/adenosine, LHb astrocytes mediate the second-wave LHb neuronal activation and norepinephrine (NE) release. Activation or inhibition of LHb astrocytic calcium signaling facilitates or prevents stress-induced depressive-like behaviors, respectively. These results identify a stress-induced positive feedback loop in the LHb-LC axis, with astrocytes being a critical signaling relay. The identification of this prominent neuron-glia interaction may shed light on stress management and depression prevention.

Lateral habenula astroglia modulate the potentiating antidepressant-like effects of bright light stimulation in intractable depression

Background

Beside image vision, light plays a pivotal role in regulating diverse non-visual functions, including affective behaviors. Recently, bright light stimulation (BLS) was revealed to be beneficial for treating non-seasonal depression, although its mechanism of action is not fully understood.

Methods

We developed a novel mouse model of refractory depression, induced through social isolation and chronic despair during the active (dark) phase of the animal, and we have tested if antidepressant treatments, including BLS, could protect against anxio-depressive-like behavior.

Results

We report that anxiety- and depressive-like behaviors are resistant to BLS as well as to both conventional and new antidepressants, including ketamine. Remarkably, we unveil that BLS potentiates the effect of antidepressants, and this beneficial effect is mediated via rod retinal photoreceptors. Furthermore, we demonstrate that both chemogenetic activation of lateral habenula (LHb) astroglia and serotonin (5-HT) depletion prevent the potentiating effect of BLS on chronic despair.

Conclusion

These results reveal, for the first time, that BLS enhances the efficacy of antidepressants through an unexpectedly circuit involving rods, LHb astroglia and 5-HT.

Mesoscale brain-wide fluctuation analysis: revealing ketamine's rapid antidepressant across multiple brain regions

Depression has been linked to cortico-limbic brain regions, and ketamine is known for its rapid antidepressant effects. However, how these brain regions encode depression collaboratively and how ketamine regulates these regions to exert its prompt antidepressant effects through mesoscale brain-wide fluctuations remain elusive. In this study, we used a multidisciplinary approach, including multi-region in vivo recordings in mice, chronic social defeat stress (CSDS), and machine learning, to construct a Mesoscale Brain-Wide Fluctuation Analysis platform (MBFA-platform). This platform analyzes the mesoscale brain-wide fluctuations of multiple brain regions from the perspective of local field potential oscillations and network dynamics. The decoder results demonstrate that our MBFA platform can accurately classify the Control/CSDS and ketamine/saline-treated groups based on neural oscillation and network activities among the eight brain regions. We found that multiple-region LFPs patterns are disrupted in CSDS-induced social avoidance, with the basolateral amygdala playing a key role. Ketamine primarily exerts the compensatory effects through network dynamics, contributing to its rapid antidepressant effect. These findings highlight the MBFA platform as an interdisciplinary tool for revealing mesoscale brain-wide fluctuations underlying complex emotional pathologies, providing insights into the etiology of psychiatry. Furthermore, the platform's evaluation capabilities present a novel approach for psychiatric therapeutic interventions.

A sensory-motor-sensory circuit underlies antinociception ignited by primary motor cortex in mice

Sensory-motor integration is crucial in the processing of chronic pain. The primary motor cortex (M1) is emerging as a promising target for chronic pain treatment. However, it remains elusive how nociceptive sensory inputs influence M1 activity and how rectifying M1 defects, in turn, regulates pain processing at cellular and network levels. We show that injury/inflammation leads to hypoactivity of M1Glu pyramidal neurons by excitation-inhibition imbalance between the primary somatosensory cortex (S1) and the M1. The impaired M1 output further weakens inputs to excitatory parvalbumin neurons of the lateral hypothalamus (LHPV) and impairs the descending inhibitory system, hence exacerbating spinal nociceptive sensitivity. When rectifying M1 defects with repetitive transcranial magnetic stimulation (rTMS), the imbalance of the S1-M1 microcircuitry can be effectively reversed, which aids in restoring the ability of the M1 to trigger the descending inhibitory system, thereby alleviating nociceptive hypersensitivity. Thus, a sensory-motor-sensory loop is identified for pain-related interactions between the sensory and motor systems and can be potentially exploited for treating chronic pain.

Effect and safety of perioperative ketamine/esketamine administration on postoperative pain and depression after breast cancer surgery: a systematic review and meta-analysis

Background

Patients with breast cancer experience varying degrees of pain, depression, and anxiety after surgery, which affect their postoperative recovery. Although ketamine/esketamine exhibit potential for opioid-sparing and controlling postoperative pain and depression, their effects on postoperative pain and depression remain unclear. This meta-analysis aimed to evaluate whether perioperative administration of ketamine/esketamine could reduce postoperative pain and depression, improve postoperative recovery, and reduce the incidence of adverse events in patients after breast cancer surgery.

Material and methods

PubMed, Embase, Web of Science, Cochrane Library, and Clinical Trials were searched from inception until June 2, 2024 for randomized controlled trials in English language on the effect of perioperative ketamine/esketamine on postoperative pain in patients undergoing breast cancer surgery. The primary outcome was the postoperative pain score, and the secondary outcomes were the postoperative depression score, quality of postoperative recovery, incidence of adverse events, and extubation time. The standardized mean difference and 95% confidence interval (CI) were calculated for continuous outcomes, and the risk ratio and 95% CI were calculated for binary variables.

Results

Seven studies involving 748 patients were included in this meta-analysis. No significant differences were found in postoperative pain scores at 2 h, 4 h, 1 day, 3 days, 7 days, and 3 months after surgery. Postoperative depression scores at 3 and 7 days after surgery were lower in the ketamine/esketamine group. The incidence of dizziness was lower in ketamine/esketamine group. No statistically significant differences were observed in postoperative depression scores at 30 days after surgery, quality of postoperative recovery at 1 and 3 days after surgery, extubation time, or the incidence of nausea, vomiting, and nightmares.

Conclusion

Perioperative ketamine/esketamine administration did not significantly reduce postoperative pain in patients undergoing breast cancer surgery; however, it may reduce depression within a short period after the surgery.

Clinical Trial Registration

https://www.crd.york.ac.uk/PROSPERO/display_record.php?ID=CRD42024572414, identifier CRD42024572414.

Structural basis for the inhibition of cystathionine-β-synthase by isoflurane and its role in anaesthesia-induced social dysfunction in mice

BACKGROUND

Anaesthesia has been shown to impair social functioning, but the underlying mechanisms remain largely unknown. The volatile anaesthetic isoflurane potentially disrupts the methionine cycle and trans-sulphuration pathway, contributing to social deficits. Cystathionine-β-synthase (CBS), a key enzyme in this pathway, might be targeted by isoflurane. We investigated the CBS-isoflurane interaction and its role in neuronal function and social behaviour.

METHODS

Mice aged 3-15 months were anaesthetised with 2 vol% isoflurane for 2 h, and social behaviours were tested 24 h after exposure. Alterations in neuronal activity were assessed using electrophysiological analysis in vivo. Pharmacological activators (S-adenosylmethionine [SAM]) or inhibitors (amino-oxyacetic acid [AOAA]), and adeno-associated virus (AAV) were used to modulate CBS activity. The binding site of isoflurane on CBS was determined using X-ray crystallography. A novel transgenic model with a point mutation knock-in was constructed to eliminate the CBS-isoflurane interaction.

RESULTS

Isoflurane inhibited CBS activity (by 0.35-fold [0.07] vs 1.00-fold [0.05]; P<0.001), leading to neuronal hypoactivity in the anterior cingulate cortex (ACC) and social impairments in adult and elderly mice. SAM, AOAA, and AAV interventions demonstrated a causal link. Structural and functional analysis identified the lysine 273 (K273) in CBS to be involved in isoflurane inhibition. CBS K273A knock-in mice exhibited increased CBS activity compared with wild-type littermates after isoflurane exposure (2.2-fold [0.22] vs 1.0-fold [0.28]; P<0.001), with successful alleviation of ACC neuronal hypoactivity and social dysfunction.

CONCLUSIONS

These findings reveal a crucial role for CBS inhibition by isoflurane in anaesthesia-induced social impairment.

Sigma-1 receptor activation produces faster antidepressant-like effect through enhancement of hippocampal neuroplasticity: Focus on sigma-1-5-HT1A heteroreceptor complex

The sigma-1 receptor (S1R) has garnered significant attention as a potential target for rapid-onset antidepressant-like effects, particularly owing to its ability to swiftly stimulate serotonergic neurons in the dorsal raphe nucleus (DRN). However, the precise mechanisms underlying its regulatory effects remain unclear. Therefore, this study aims to examine the interaction between SA-4503 (a selective S1R agonist) and 8-OH-DPAT (a serotonin1A (5-HT1A) receptor agonist) in mice with depressive-like behavior induced by chronic restraint stress (CRS). Preliminary studies were conducted to explore the potential mechanisms underlying the accelerated antidepressant-like effects resulting from the combined activation of S1R and 5-HT1A receptors. The results showed that the coadministration of SA4503 (1.0 mg/kg, orally) and 8-OH-DPAT (0.3 mg/kg, i. g.) produced antidepressant-like effects. However, the doses of 8-OH-DPAT used in this study did not exhibit intrinsic antidepressant-like activity in this model. Moreover, using an in-situ proximity ligation assay provided the first evidence of S1R-5-HT1A heteroreceptor complexes in the midbrain DRN and dentate gyrus (DG) of the forebrain in mice. The formation of these heterocomplexes was influenced by pharmacological agents and was closely associated with depressive-like behavior development in mice. Mechanistic analysis revealed that the combined activation of S1R and 5-HT1A receptors synergistically enhanced neurogenesis and plasticity in the dorsal DG region of the hippocampus in mice subjected to CRS. These findings significantly advance our understanding of S1R-mediated neuroplasticity, suggesting potential therapeutic strategies for developing rapid-acting antidepressants.

NMDA receptors regulate the firing rate set point of hippocampal circuits without altering single-cell dynamics

Understanding how neuronal circuits stabilize their activity is a fundamental yet poorly understood aspect of neuroscience. Here, we show that hippocampal network properties, such as firing rate distribution and dimensionality, are actively regulated, despite perturbations and single-cell drift. Continuous inhibition of N-methyl-D-aspartate receptors (NMDARs) ex vivo lowers the excitation/inhibition ratio and network firing rates while preserving resilience to perturbations. This establishes a new network firing rate set point via NMDAR-eEF2K signaling pathway. NMDARs' capacity to modulate and stabilize network firing is mediated by excitatory synapses and the intrinsic excitability of parvalbumin-positive neurons, respectively. In behaving mice, continuous NMDAR blockade in CA1 reduces network firing without altering single-neuron drift or triggering a compensatory response. These findings expand NMDAR function beyond their canonical role in synaptic plasticity and raise the possibility that some NMDAR-dependent behavioral effects are mediated by their unique regulation of population activity set points.

Neurological mechanism-based analysis of the role and characteristics of physical activity in the improvement of depressive symptoms

Depression is a common mental disorder characterized by a high prevalence and significant adverse effects, making the searching for effective interventions an urgent priority. In recent years, physical activity (PA) has increasingly been recognized as a standard adjunctive treatment for mental disorders owing to its low cost, easy application, and high efficiency. Epidemiological data shows positive preventive and therapeutic effects of PA on mental illnesses such as depression. This article systematically describes the prophylactic and therapeutic effects of PA on depression and its biological basis. A comprehensive literature analysis reveals that PA significantly improves depressive symptoms by upregulating the expression of "exerkines" such as irisin, adiponectin, and BDNF to positively impacting neuropsychiatric conditions. In particular, lactate could also play a critical role in the ameliorating effects of PA on depression due to the findings about protein lactylation as a novel protein post-transcriptional modification. The literature also suggests that in terms of brain structure, PA may improve hippocampal volume, basal ganglia (neostriatum, caudate-crustal nucleus) and PFC density in patients with MDD. In summary, this study elucidates the multifaceted positive effects of PA on depression and its potential biological mechanisms with a particular emphasis on the roles of various exerkines. Future research may further investigate the effects of different types, intensities, and durations of PA on depression, as well as how to better integrate PA interventions into existing treatment strategies to achieve optimal outcomes in mental health interventions.

The disappointment centre of the brain gets exciting: a systematic review of habenula dysfunction in depression

BACKGROUND

The habenula is an epithalamic brain structure that acts as a neuroanatomical hub connecting the limbic forebrain to the major monoamine centres. Abnormal habenula activity is increasingly implicated in depression, with a surge in publications on this topic in the last 5 years. Direct activation of the habenula is sufficient to induce a depressive phenotype in rodents, suggesting a causative role in depression. However, the molecular basis of habenula dysfunction in depression remains elusive and it is unclear how the preclinical advancements translate to the clinical field.

METHODS

A systematic literature search was conducted following the PRISMA guidelines. The two search terms depress* and habenula* were applied across Scopus, Web of Science and PubMed databases. Studies eligible for inclusion must have examined the habenula in clinical cases of depression or preclinical models of depression and compared their measures to an appropriate control.

RESULTS

Preclinical studies (n = 63) measured markers of habenula activity (n = 16) and neuronal firing (n = 22), largely implicating habenula hyperactivity in depression. Neurotransmission was briefly explored (n = 15), suggesting imbalances within excitatory and inhibitory habenula signalling. Additional preclinical studies reported neuroconnectivity (n = 1), inflammatory (n = 3), genomic (n = 3) and circadian rhythm (n = 3) abnormalities. Seven preclinical studies (11%) included both males and females. From these, 5 studies (71%) reported a significant difference between the sexes in at least one habenula measure taken. Clinical studies (n = 24) reported abnormalities in habenula connectivity (n = 15), volume (n = 6) and molecular markers (n = 3). Clinical studies generally included male and female subjects (n = 16), however, few of these studies examined sex as a biological variable (n = 6).

CONCLUSIONS

Both preclinical and clinical evidence suggest the habenula is disrupted in depression. However, there are opportunities for sex-specific analyses across both areas. Preclinical evidence consistently suggests habenula hyperactivity as a primary driver for the development of depressive symptoms. Clinical studies support gross habenula abnormalities such as altered activation, connectivity, and volume, with emerging evidence of blood brain barrier dysfunction, however, progress is limited by a lack of detailed molecular analyses and limited imaging resolution.

Neuronal TCF7L2 in Lateral Habenula Is Involved in Stress-Induced Depression

Depression is a complex psychiatric disorder that has substantial implications for public health. The lateral habenula (LHb), a vital brain structure involved in mood regulation, and the N-methyl-D-aspartate receptor (NMDAR) within this structure are known to be associated with depressive behaviors. Recent research has identified transcription factor 7-like 2 (TCF7L2) as a crucial transcription factor in the Wnt signaling pathway, influencing diverse neuropsychiatric processes. In this study, we explore the role of TCF7L2 in the LHb and its effect on depressive-like behaviors in mice. By using behavioral tests, AAV-mediated gene knockdown or overexpression, and pharmacological interventions, we investigated the effects of alterations in TCF7L2 expression in the LHb. Our results indicate that TCF7L2 expression is reduced in neurons within the LHb of male ICR mice exposed to chronic mild stress (CMS), and neuron-specific knockdown of TCF7L2 in LHb neurons leads to notable antidepressant activity, as evidenced by reduced immobility time in the tail suspension test (TST) and forced swimming test (FST). Conversely, the overexpression of TCF7L2 in LHb neurons induces depressive behaviors. Furthermore, the administration of the NMDAR agonist NMDA reversed the antidepressant activity of TCF7L2 knockdown, and the NMDAR antagonist memantine alleviated the depressive behaviors induced by TCF7L2 overexpression, indicating the involvement of NMDAR. These findings offer novel insights into the molecular mechanisms of depression, highlighting the potential of TCF7L2 as both a biomarker and a therapeutic target for depression. Exploring the relationship between TCF7L2 signaling and LHb function may lead to innovative therapeutic approaches for alleviating depressive symptoms.

KETAMINE: Neural- and network-level changes

Ketamine is a widely used clinical drug that has several functional and clinical applications, including its use as an anaesthetic, analgesic, anti-depressive, anti-suicidal agent, among others. Among its diverse behavioral effects, it influences short-term memory and induces psychedelic effects. At the neural level across different brain areas, it modulates neural firing rates, neural tuning, brain oscillations, and modularity, while promoting hypersynchrony and random connectivity between neurons. In our recent studies we demonstrated that topical application of ketamine on the visual cortex alters neural tuning and promotes vigorous connectivity between neurons by decreasing their firing variability. Here, we begin with a brief review of the literature, followed by results from our lab, where we synthesize a dendritic model of neural tuning and network changes following ketamine application. This model has potential implications for focused modulation of cortical networks in clinical settings. Finally, we identify current gaps in research and suggest directions for future studies, particularly emphasizing the need for more animal experiments to establish a platform for effective translation and synergistic therapies combining ketamine with other protocols such as training and adaptation. In summary, investigating ketamine's broader systemic effects, not only provides deeper insight into cognitive functions and consciousness but also paves the way to advance therapies for neuropsychiatric disorders.

Rapid Hippocampal Synaptic Potentiation Induced by Ketamine Metabolite ( 2R , 6R )-Hydroxynorketamine Persistently Primes Synaptic Plasticity

The pharmacologically active ( R , S )-ketamine (ketamine) metabolite ( 2R , 6R )-hydroxynorketamine (HNK) maintains ketamine's preclinical antidepressant profile without adverse effects. While hypotheses have been proposed to explain how ketamine and its metabolites initiate their antidepressant-relevant effects, it remains unclear how sustained therapeutic actions arise following drug elimination. To distinguish the physiological mechanisms involved in the rapid from sustained actions of HNK, we utilized extracellular electrophysiology combined with pharmacology to develop an in vitro hippocampal slice incubation model that exhibited pharmacological fidelity to the 1) rapid synaptic potentiation induced by HNK at the Schaffer collateral-CA1 (SC-CA1) synapse during bath-application to slices collected from mice, and 2) maintenance of metaplastic (priming) activity that lowered the threshold for N- methyl-D-aspartate receptor (NMDAR) activation-dependent long-term potentiation (LTP) hours after in vivo dosing. We then used this model to reveal novel druggable mechanisms engaged in HNK's temporally-sensitive antidepressant synaptic actions, finding that the induction of synaptic potentiation by HNK did not require NMDAR activity, but NMDAR activity was necessary to maintain synaptic priming. HNK required protein kinase A (PKA) activity to rapidly potentiate SC-CA1 neurotransmission to facilitate synaptic priming that persistently promoted LTP formation. HNK's rapid actions were blocked by inhibitors of adenylyl cyclase 1 (AC1), but not an AC5 inhibitor. We conclude that HNK rapidly potentiates SC-CA1 synaptic efficacy, which then stimulates priming mechanisms that persistently favor antidepressant-relevant plasticity. Targeting such priming mechanisms may be an effective antidepressant strategy, and using approaches such as our incubation model may aid in revealing novel pharmacological targets.

Locking away depression

The antidepressant ketamine blocks neuroreceptors in hyperactive brain regions.