Ketamine-50 years in use : from anesthesia to rapid antidepressant effects and neurobiological mechanisms

Over the past 50 years, ketamine has solidified its position in both human and veterinary medicine as an important anesthetic with many uses. More recently, ketamine has been studied and used for several new indications, ranging from chronic pain to drug addiction and post-traumatic stress disorder. The discovery of the rapid-acting antidepressant effects of ketamine has resulted in a surge of interest towards understanding the precise mechanisms driving its effects. Indeed, ketamine may have had the largest impact for advancements in the research and treatment of psychiatric disorders in the past few decades. While intense research efforts have been aimed towards uncovering the molecular targets underlying ketamine's effects in treating depression, the underlying neurobiological mechanisms remain elusive. These efforts are made more difficult by ketamine's complex dose-dependent effects on molecular mechanisms, multiple pharmacologically active metabolites, and a mechanism of action associated with the facilitation of synaptic plasticity. This review aims to provide a brief overview of the different uses of ketamine, with an emphasis on examining ketamine's rapid antidepressant effects spanning molecular, cellular, and network levels. Another focus of the review is to offer a perspective on studies related to the different doses of ketamine used in antidepressant research. Finally, the review discusses some of the latest hypotheses concerning ketamine's action.Peer reviewe

Commentary : Commonly Used Anesthesia/Euthanasia Methods for Brain Collection Differentially Impact MAPK Activity in Male and Female C57BL/6 Mice

Non peer reviewe

Encoding, Consolidation, and Renormalization in Depression : Synaptic Homeostasis, Plasticity, and Sleep Integrate Rapid Antidepressant Effects

Recent studies have strived to find an association between rapid antidepressant effects and a specific subset of pharmacological targets and molecular pathways. Here, we propose a broader hypothesis of encoding, consolidation, and renormalization in depression (ENCORE-D), which suggests that, fundamentally, rapid and sustained antidepressant effects rely on intrinsic homeostatic mechanisms evoked as a response to the acute pharmacological or physiologic effects triggered by the treatment. We review evidence that supports the notion that various treatments with a rapid onset of action, such as ketamine, electroconvulsive therapy, and sleep deprivation, share the ability to acutely excite cortical networks, which increases synaptic potentiation, alters patterns of functional connectivity, and ameliorates depressive symptoms. We proceed to examine how the initial effects are short-lived and, as such, require both consolidation during wake and maintenance throughout sleep to remain sustained. Here, we incorporate elements from the synaptic homeostasis hypothesis and theorize that the fundamental mechanisms of synaptic plasticity and sleep, particularly the homeostatic emergence of slow-wave electroencephalogram activity and the renormalization of synaptic strength, are at the center of sustained antidepressant effects. We conclude by discussing the various implications of the ENCORE-D hypothesis and offer several considerations for future experimental and clinical research. Significance Statement-Proposed molecular perspectives of rapid antidepressant effects fail to appreciate the temporal distribution of the effects of ketamine on cortical excitation and plasticity as well as the prolonged influence on depressive symptoms. The encoding, consolidation, and renormalization in depression hypothesis proposes that the lasting clinical effects can be best explained by adaptive functional and structural alterations in neural circuitries set in motion in response to the acute pharmacological effects of ketamine (i.e., changes evoked during the engagement of receptor targets such as N-methyl-D-aspartate receptors) or other putative rapid-acting antidepressants. The present hypothesis opens a completely new avenue for conceptualizing and targeting brain mechanisms that are important for antidepressant effects wherein sleep and synaptic homeostasis are at the center stage.Peer reviewe

Rapid-acting antidepressants : Shared neuropharmacological mechanisms

Major depressive disorder is a common and devastating psychiatric disorder. While pharmacotherapy and psychotherapy can be effective, a significant proportion of patients remain treatment resistant. Traditional antidepressants need to be taken for several weeks or months before the therapeutic effects become evident. For treatment-resistant patients, electroconvulsive therapy (ECT) is still the most effective treatment. Postictal slowing of electroencephalogram (EEG) activity has been associated with the therapeutic effects of ECT, but the mechanistic basis of this remains poorly studied. For decades this has encouraged researchers to investigate the antidepressant effects of isoflurane anesthesia with promising, but inconsistent, results. More recently, evidence of the rapid-acting antidepressant effects of subanesthetic doses of ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist and a dissociative anesthetic, has sparked a renewed interest in the development of novel antidepressant therapies. Another treatment to show positive results is nitrous oxide (N2O), a gaseous anesthetic with NMDAR antagonist properties. One of the proposed mechanisms of ketamine’s action is related to its ability to increase glutamatergic signaling, leading to further changes in synaptic potentiation and in the function of neuronal networks. These changes have been suggested to involve the actions of brain-derived neurotrophic factor (BDNF) signaling via its receptor TrkB. Downstream of TrkB, the inhibition of glycogen synthase kinase 3β (GSK3β), the induction of mammalian target of rapamycin (mTOR) mediated protein synthesis, and the consolidation of synaptic changes have been implicated in ketamine´s actions. The first aim of this study is to investigate the molecular changes induced by isoflurane anesthesia in the adult mouse hippocampus using phosphoproteomics in the absence of a priori information. We find that brief isoflurane anesthesia induces 318 phosphorylation changes in a total of 237 proteins. While confirming the phosphorylation alterations on selected proteins, we also discover that various anesthetics, including urethane and ketamine, regulate these targets in a similar manner. In the second part, we investigate the effects of N2O on molecular signatures implicated in ketamine’s action. Findings reveal that N2O produces cortical excitation, followed by the rebound emergence of slow EEG activity following gas cessation, which coincide with the phosphorylation of TrkB, GSK3β and p70S6k (a kinase downstream of mTor). Moreover, we demonstrate that these pathways become regulated during the postictal period after flurothyl-induced seizures or during slow EEG activity induced by hypnotic agent medetomidine. Notably, medetomidine is not effective in the learned helplessness test. Finally, we investigate the dose-dependent changes induced by ketamine in TrkB signaling. An acute administration of sedative-anesthetic doses of ketamine, accompanied by increases in slow EEG activity, is found to increase the phosphorylation of the investigated pathways. These changes appear independent of ketamine’s metabolite hydroxynorketamine, an agent shown to have antidepressant-like behavioral effects in rodents.Masennus on yleinen psykiatrinen sairaus, joka aiheuttaa mittavaa inhimillistä kärsimystä ja merkittäviä yhteiskunnallisia kustannuksia. Masennusta hoidetaan farmakoterapialla, mutta moni potilaista ei saa merkittävää hyötyä lääkkeistä useiden viikkojen tai kuukausienkaan käytön jälkeen. Näille hoitoresistenteille masennuspotilaille psykiatrinen sähköhoito on yhä tehokkain hoitomuoto. Yhdeksi sähköhoidon tehoa ennustavaksi tekijäksi on esitetty kouristuksen jälkeisen aivojen hidasaaltotoiminnan lisääntymistä aivosähkökäyrässä. Ajatus kannusti tutkijoita selvittämään isofluraanilla aikaansaadun anestesian masennuslääkevaikutuksia jo vuosikymmeniä sitten, mutta tulokset jäivät epäselviksi. Tuoreemmissa tutkimuksissa on puolestaan toistettavasti havaittu N-metyyli-D-aspartaatti (NMDA) -reseptoreja salpaavan ketamiinin nopea masennusoireita lievittävä vaikutus. Hiljattain kliinisessä tutkimuksessa on havaittu myös NMDA-reseptoreja salpaavan typpioksiduulin (N2O) masennusoireita nopeasti lievittävä vaikutus. Isofluraanin ja typpioksiduulin vaikutusten taustalla olevat neurobiologiset mekanismit ovat kuitenkin yhä pääosin tuntemattomia. Ketamiinin vaikutusmekanismiksi on sen sijaan esitetty glutamatergisen hermovälityksen ja aivokuoren ärtyvyyden lisääntymistä, jonka on ajateltu johtavan muutoksiin synapsien ja hermosolujen toiminnassa. Näiden vaikutusten on esitetty aiheutuvan aivoperäisen hermokasvutekijän (BDNF) kohteena toimivan TrkB-reseptorin aktivaatiosta ja sitä seuraavista muutoksista glykogeenisyntaasikinaasi 3β:n toiminnassa ja rapamysiinin mekaanisen kohteen (mTOR) ohjaamassa proteiinisynteesissä. Ensimmäisessä tutkimuksessa keskityimme selvittämään isofluraanianestesian aiheuttamia molekulaarisia muutoksia hiiren hippokampuksessa, jonka tutkimisessa hyödynsimme fosfoproteomiikkaa. Havaitsimme 318 fosforylaatiomuutosta 237 eri proteiinissa. Vahvistaessamme havaittuja muutoksia valikoiduissa proteiineissa havaitsimme monien eri anesteettien, kuten uretaanin ja ketamiinin, aiheuttavan samankaltaisia muutoksia. Tutkimuksen toisessa osassa selvitimme N2O:n vaikutuksia ketamiinin säätelemiin solusignalointimekanismeihin hiiren etuaivokuorella. Ilokaasualtistuksen aikana havaittiin hermoston aktiivisuuteen liitettyjen merkkiaineiden lisääntymistä. Annostelun päätyttyä aivosähkökäyrässä havaittiin voimistunutta hidasaaltotoimintaa, jonka aikana kerätyissä aivonäytteissä havaittiin TrkB-välitteisen signaloinnin lisääntyneen. TrkB-signalointi lisääntyi myös näytteissä, jotka kerättiin flurotyylin aiheuttamien kouristuksien tai hidasaaltotoimintaa suoraan lisäävän medetomidiinin annostelun jälkeen. Medetomidiini ei kuitenkaan aiheuttanut masennuslääkkeille tyypillisiä käyttäytymisen muutoksia ns. opittu avuttomuus -mallissa. Lopuksi selvitimme ketamiinin annosriippuvaisia vaikutuksia TrkB-signalointiin hiiressä. Anesteettiset ketamiiniannokset lisäsivät aivosähkökäyrän hidasaaltotoimintaa, jonka aikana kerätyissä näytteissä havaittiin TrkB-signalointipolun aktivoituneen. Ketamiinin aiheuttamat muutokset näihin solusignalointimekanismeihin eivät näyttäisi olevan riippuvaisia sen metaboliatuotteesta hydroksinorketamiinista, jolla on esitetty olevan masennuslääkevaikutuksia koe-eläimiin. Tutkimustuloksemme viittaavat siihen, että hidasoskillaatioilla ja ketamiinin vaikutuksiin liitetyillä solusignalointimuutoksilla olisi yhteys. Lisäksi tulokset kannustavat selvittämään, ovatko aivokuoren ärtyvyys ja sitä seuraava hidasaaltotoiminnan lisääntyminen nopeavaikutteisten masennuslääkkeiden yhteinen ominaisuus

Neurobiologiset ilmiöt nopean masennuslääkevasteen taustalla

English summaryPeer reviewe

Brief isoflurane anesthesia regulates striatal AKT-GSK3 beta signaling and ameliorates motor deficits in a rat model of early-stage Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative movement disorder primarily affecting the nigrostriatal dopaminergic system. The link between heightened activity of glycogen synthase kinase 3 beta (GSK313) and neurodegenerative processes has encouraged investigation into the potential disease-modifying effects of novel GSK3 beta inhibitors in experimental models of PD. Therefore, the intriguing ability of several anesthetics to readily inhibit GSK3 beta within the cortex and hippocampus led us to investigate the effects of brief isoflurane anesthesia on striatal GSK3 beta signaling in nave rats and in a rat model of early-stage PD. Deep but brief (20-min) isoflurane anesthesia exposure increased the phosphorylation of GSK3 beta at the inhibitory Ser9 residue, and induced phosphorylation of AKT(Thr308) (protein kinase B; negative regulator of GSK3 beta) in the striatum of naive rats and rats with unilateral striatal 6-hydroxydopamine (6-OHDA) lesion. The 6-OHDA protocol produced gradual functional deficiency within the nigrostriatal pathway, reflected as a preference for using the limb ipsilateral to the lesioned striatum at 2 weeks post 6-OHDA. Interestingly, such motor impairment was not observed in animals exposed to four consecutive isoflurane treatments (20-min anesthesia every 48 h; treatments started 7 days after 6-OHDA delivery). However, isoflurane had no effect on striatal or nigral tyrosine hydroxylase (a marker of dopaminergic neurons) protein levels. This brief report provides promising results regarding the therapeutic potential and neurobiological mechanisms of anesthetics in experimental models of PD and guides development of novel disease-modifying therapies.Peer reviewe

A wake-up call : Sleep physiology and related translational discrepancies in studies of rapid-acting antidepressants

Depression is frequently associated with sleep problems, and clinical improvement often coincides with the normalization of sleep architecture and realignment of circadian rhythm. The effectiveness of treatments targeting sleep in depressed patients, such as sleep deprivation, further demonstrates the confluence of sleep and mood. Moreover, recent studies showing that the rapid-acting antidepressant ketamine influences processes related to sleep-wake neurobiology have led to novel hypotheses explaining rapid and sustained antidepressant effects. Despite the available evidence, studies addressing ketamine's antidepressant effects have focused on pharmacology and often overlooked the role of physiology. To explore this discrepancy in research on rapid acting antidepressants, we examined articles published between 2009-2019. A keyword search algorithm indicated that vast majority of the articles completely ignored sleep. Out of the 100 most frequently cited pre clinical and clinical research papers, 89 % and 71 %, respectively, did not mention sleep at all. Furthermore, only a handful of these articles disclosed key experimental variables, such as the times of treatment administration or behavioral testing, let alone considered the potential association between these variables and experimental observations. Notably, in preclinical studies, treatments were preferentially administered during the inactive period, which is the polar opposite of clinical practice and research. We discuss the potential impact of this practice on the results in the field. Our hope is that this perspective will serve as a wake-up call to (re)-examine rapid-acting antidepressant effects with more appreciation for the role of sleep and chronobiology.Peer reviewe

Cortical Excitability and Activation of TrkB Signaling During Rebound Slow Oscillations Are Critical for Rapid Antidepressant Responses

Rapid antidepressant effects of ketamine become most evident when its psychotomimetic effects subside, but the neurobiological basis of this lag remains unclear. Laughing gas (N2O), another NMDA-R (N-methyl-d-aspartate receptor) blocker, has been reported to bring antidepressant effects rapidly upon drug discontinuation. We took advantage of the exceptional pharmacokinetic properties of N2O to investigate EEG (electroencephalogram) alterations and molecular determinants of antidepressant actions during and immediately after NMDA-R blockade. Effects of the drugs on brain activity were investigated in C57BL/6 mice using quantitative EEG recordings. Western blot and qPCR were used for molecular analyses. Learned helplessness (LH) was used to assess antidepressant-like behavior. Immediate-early genes (e.g., bdnf) and phosphorylation of mitogen-activated protein kinasemarkers of neuronal excitabilitywere upregulated during N2O exposure. Notably, phosphorylation of BDNF receptor TrkB and GSK3 (glycogen synthase kinase 3) became regulated only gradually upon N2O discontinuation, during a brain state dominated by slow EEG activity. Subanesthetic ketamine and flurothyl-induced convulsions (reminiscent of electroconvulsive therapy) also evoked slow oscillations when their acute pharmacological effects subsided. The correlation between ongoing slow EEG oscillations and TrkB-GSK3 signaling was further strengthened utilizing medetomidine, a hypnotic-sedative agent that facilitates slow oscillations directly through the activation of (2)-adrenergic autoreceptors. Medetomidine did not, however, facilitate markers of neuronal excitability or produce antidepressant-like behavioral changes in LH. Our results support a hypothesis that transient cortical excitability and the subsequent regulation of TrkB and GSK3 signaling during homeostatic emergence of slow oscillations are critical components for rapid antidepressant responses.Peer reviewe

Isoflurane produces antidepressant effects and induces TrkB signaling in rodents

A brief burst-suppressing isoflurane anesthesia has been shown to rapidly alleviate symptoms of depression in a subset of patients, but the neurobiological basis of these observations remains obscure. We show that a single isoflurane anesthesia produces antidepressant-like behavioural effects in the learned helplessness paradigm and regulates molecular events implicated in the mechanism of action of rapid-acting antidepressant ketamine: activation of brain-derived neurotrophic factor (BDNF) receptor TrkB, facilitation of mammalian target of rapamycin (mTOR) signaling pathway and inhibition of glycogen synthase kinase 3 beta (GSK3 beta). Moreover, isoflurane affected neuronal plasticity by facilitating long-term potentiation in the hippocampus. We also found that isoflurane increased activity of the parvalbumin interneurons, and facilitated GABAergic transmission in wild type mice but not in transgenic mice with reduced TrkB expression in parvalbumin interneurons. Our findings strengthen the role of TrkB signaling in the antidepressant responses and encourage further evaluation of isoflurane as a rapid-acting antidepressant devoid of the psychotomimetic effects and abuse potential of ketamine.Peer reviewe

Ketamine-induced regulation of TrkB-GSK3β signaling is accompanied by slow EEG oscillations and sedation but is independent of hydroxynorketamine metabolites

Subanesthetic rather than anesthetic doses are thought to bring the rapid antidepressant effects of the NMDAR (N-methyl-D-aspartate receptor) antagonist ketamine. Among molecular mechanisms, activation of BDNF receptor TrkB along with the inhibition of GSK3 beta (glycogen synthase kinase 3 beta) are considered as critical molecular level determinants for ketamine's antidepressant effects. Hydroxynorketamines (2R,6R)-HNK and (2S,6S) HNK), non-anesthetic metabolites of ketamine, have been proposed to govern the therapeutic effects of ketamine through a mechanism not involving NMDARs. However, we have shown that nitrous oxide, another NMDAR blocking anesthetic and a putative rapid-acting antidepressant, evokes TrkB-GSK3 beta signaling alterations during rebound slow EEG (electroencephalogram) oscillations. We investigated here the acute effects of ketamine, 6,6-d(2)-ketamine (a ketamine analogue resistant to metabolism) and cis-HNK that contains (2R,6R) and (2S,6S) enantiomers in 1:1 ratio, on TrkB-GSK3 beta signaling and concomitant electroencephalographic (EEG) alterations in the adult mouse cortex. Ketamine dose-dependently increased slow oscillations and phosphorylations of TrkB(Y816) and GSK3 beta(59) in crude brain homogenates (i.e. sedative/anesthetic doses ( > 50 mg/kg, i.p.) produced more prominent effects than a subanesthetic dose (10 mg/kg, i.p.)). Similar, albeit less obvious, effects were seen in crude synaptosomes. A sedative dose of 6,6-d(2)-ketamine (100 mg/kg, i.p.) recapitulated the effects of ketamine on TrkB and GSK3 beta phosphorylation while cis-HNK at a dose of 20 mg/kg produced negligible acute effects on TrkB-GSK3 beta signaling or slow oscillations. These findings suggest that the acute effects of ketamine on TrkB-GSK3 beta signaling are by no means restricted to subanesthetic (i.e. antidepressant) doses and that cis-HNK is not responsible for these effects.Peer reviewe