Ketamine and its preservative, benzethonium chloride, both inhibit human recombinant alpha7 and alpha4beta2 neuronal nicotinic acetylcholine receptors in Xenopus oocytes

Exploring Novel Antidepressants Targeting G Protein-Coupled Receptors and Key Membrane Receptors Based on Molecular Structures

Major Depressive Disorder (MDD) is a complex mental disorder that involves alterations in signal transmission across multiple scales and structural abnormalities. The development of effective antidepressants (ADs) has been hindered by the dominance of monoamine hypothesis, resulting in slow progress. Traditional ADs have undesirable traits like delayed onset of action, limited efficacy, and severe side effects. Recently, two categories of fast-acting antidepressant compounds have surfaced, dissociative anesthetics S-ketamine and its metabolites, as well as psychedelics such as lysergic acid diethylamide (LSD). This has led to structural research and drug development of the receptors that they target. This review provides breakthroughs and achievements in the structure of depression-related receptors and novel ADs based on these. Cryo-electron microscopy (cryo-EM) has enabled researchers to identify the structures of membrane receptors, including the N-methyl-D-aspartate receptor (NMDAR) and the 5-hydroxytryptamine 2A (5-HT2A) receptor. These high-resolution structures can be used for the development of novel ADs using virtual drug screening (VDS). Moreover, the unique antidepressant effects of 5-HT1A receptors in various brain regions, and the pivotal roles of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) and tyrosine kinase receptor 2 (TrkB) in regulating synaptic plasticity, emphasize their potential as therapeutic targets. Using structural information, a series of highly selective ADs were designed based on the different role of receptors in MDD. These molecules have the favorable characteristics of rapid onset and low adverse drug reactions. This review offers researchers guidance and a methodological framework for the structure-based design of ADs.

Solubility and Thermodynamics Profile of Benzethonium Chloride in Pure and Binary Solvents at Different Temperatures

Benzethonium chloride (BTC) has various applications in several industries. The solubility and solution thermodynamic properties of BTC were measured. The solubility of BTC in methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, water, dimethyl sulfoxide, acetic acid, and dimethyl formamide neat solvents and methanol + water and ethanol + water binary solvents at 298.15-318.15 K over an atmospheric pressure was measured. The solubility data of BTC is positively related to the temperature in all selected solvents. The solubility data was fitted by the Apelblat model, λh model, Yaws model, Van't Hoff equation, CNIBS/R-K model, and modified Jouyban-Acree equation. The RMSD and ARD were chosen to evaluate the fitting of each model. The dissolution thermodynamic parameters, enthalpy of the solution, entropy of the solution, and Gibbs energy of the solution were calculated. The solubility data and dissolution thermodynamic parameters of BTC will provide significant guidance for purification, crystallization, and separation in various areas.

Ketamine preservative benzethonium chloride potentiates hippocampal synaptic transmission and binds neurotransmitter receptors and transporters

Benzethonium chloride (BZT) is an excipient used in numerous products including (R,S)-ketamine (ketamine) drug formulations for human and veterinary use. Emerging evidence indicates BZT is pharmacologically active. BZT may therefore contribute to some of the clinical or preclinical effects observed with ketamine. In the present study, we evaluated: (i) the affinity of BZT for neurotransmitter receptors and transporters, (ii) the effects of BZT on hippocampal synaptic transmission in vitro, and (iii) plasma and brain concentrations of BZT following its intraperitoneal administration to male CD1 mice. Radioligand binding assays determined the affinity of BZT for neurotransmitter targets. Effects of BZT on field excitatory postsynaptic potentials (fEPSPs) were established via electrophysiological recordings from slices collected from male C57BL/6J mice. The binding assays revealed that BZT binds to numerous receptors (e.g., σ2 Ki = 7 nM) and transporters (e.g., dopamine transporter Ki = 545 nM). Bath application of BZT potentiated hippocampal fEPSPs in mouse hippocampal slices with an EC50 of 2.03 nM. Following intraperitoneal administration, BZT was detected in the plasma, but not in the brain of mice. These data highlight that studies measuring peripheral endpoints or directly exposing systems, in vitro, intracerebroventricularly, or intracortically, to BZT-containing formulations should account for the direct effects of BZT. Our findings also suggest that earlier data attributing pharmacological effects to ketamine may be confounded by BZT and that additional investigation into the functional impact of BZT is warranted. This article is part of the Special Issue on 'Ketamine and its Metabolites'.

Ketamine and its metabolites: Potential as novel treatments for depression

Depression is a well-known serious mental illness, and the onset of treatment using traditional antidepressants is frequently delayed by several weeks. Moreover, numerous patients with depression fail to respond to therapy. One major breakthrough in antidepressant therapy is that subanesthetic ketamine doses can rapidly alleviate depressive symptoms within hours of administering a single dose, even in treatment-resistant patients. However, specific mechanisms through which ketamine exerts its antidepressant effects remain elusive, leading to concerns regarding its rapid and long-lasting antidepressant effects. N-methyl-d-aspartate receptor (NMDAR) antagonists like ketamine are reportedly associated with serious side effects, such as dissociative symptoms, cognitive impairment, and abuse potential, limiting the large-scale clinical use of ketamine as an antidepressant. Herein, we reviewed the pharmacological properties of ketamine and the mechanisms of action underlying the rapid antidepressant efficacy, including the disinhibition hypothesis and synaptogenesis, along with common downstream effector pathways such as enhanced brain-derived neurotrophic factor and tropomyosin-related kinase B signaling, activation of the mechanistic target of rapamycin complex 1 and transforming growth factor β1. We focused on evidence supporting the relevance of these potential mechanisms of ketamine and its metabolites in mediating the clinical efficacy of the drug. Given its reported antidepressant efficacy in preclinical studies and limited undesirable adverse effects, (R)-ketamine may be a safer, more controllable, rapid antidepressant. Overall, understanding the potential mechanisms of action of ketamine and its metabolites in combination with pharmacology may help develop a new generation of rapid antidepressants that maximize antidepressant effects while avoiding unfavorable adverse effects. This article is part of the Special Issue on 'Ketamine and its Metabolites'.

Taurine, an essential β-amino acid insulates against ketamine-induced experimental psychosis by enhancement of cholinergic neurotransmission, inhibition of oxidative/nitrergic imbalances, and suppression of COX-2/iNOS immunoreactions in mice

Cholinergic, oxidative, nitrergic alterations, and neuroinflammation are some key neuropathological features common in schizophrenia disease. They involve complex biological processes that alter normal behavior. The present treatments used in the management of the disorder remain ineffective together with some serious side effects as one of their setbacks. Taurine is a naturally occurring essential β-amino acid reported to elicit antipsychotic property in first episode psychosis in clinical setting, thus require preclinical investigation. Hence, we set out to investigate the effects of taurine in the prevention and reversal of ketamine-induced psychotic-like behaviors and the associated putative neurobiological mechanisms underlying its effects. Adult male Swiss mice were sheared into three separate cohorts of experiments (n = 7): drug alone, preventive and reversal studies. Treatments consisted of saline (10 mL/kg/p.o./day), taurine (50 and 100 mg/kg/p.o./day) and risperidone (0.5 mg/kg/p.o./day) with concomitant ketamine (20 mg/kg/i.p./day) injections between days 8-14, or 14 days entirely. Behavioral hyperactivity, despair, cognitive impairment, and catalepsy were measured. Brain oxidative/nitrergic imbalance, immunoreactivity (COX-2 and iNOS), and cholinergic markers were determined in the striatum, prefrontal-cortex, and hippocampus. Taurine abates ketamine-mediated psychotic-like episodes without cataleptogenic potential. Taurine attenuated ketamine-induced decrease in glutathione, superoxide-dismutase and catalase levels in the striatum, prefrontal-cortex and hippocampus. Also, taurine prevented and reversed ketamine-mediated elevation of malondialdehyde, nitrite contents, acetylcholinesterase activity, and suppressed COX-2 and iNOS expressions in a brain-region dependent manner. Conclusively, taurine insulates against ketamine-mediated psychotic phenotype by normalizing brain central cholinergic neurotransmissions, oxidative, nitrergic and suppression of immunoreactive proteins in mice brains.

Proarrhythmic effects induced by benzethonium chloride and domiphen bromide in vitro and in vivo

Benzethonium chloride (BZT) and domiphen bromide (DMP) are widely used as antimicrobials in drugs, vaccines and industry. However, no cardiac safety data has been developed on both compounds. Previously we reported BZT and DMP as high-affinity human ether-a-go-go related gene (HERG) channel inhibitors with unknown proarrhythmic risk. Here, we investigate the cardiotoxicity of BZT and DMP in vitro and in vivo, aiming to improve the safety-in-use of both antimicrobials. In the present study, human iPSC derived cardiomyocytes (hiPSC-CMs) were generated and rabbit models were used to examine the proarrhythmic potential of BZT and DMP. Our results found that BZT and DMP induced time- and dose-dependent decrease in the contractile parameters of hiPSC-CMs, prolonged FPDc (≥ 0.1 μM), caused tachycardia/fibrillation-like oscillation (0.3-1 μM), ultimately progressing to irreversible arrest of beating (≥ 1 μM). The IC50 values of BZT and DMP derived from normalized beat rate were 0.13 μM and 0.10 μM on hiPSC-CMs at 76 days. Moreover, in vivo rabbit ECG data demonstrated that 12.85 mg/kg BZT and 3.85 mg/kg DMP evoked QTc prolongation, noncomplex arrhythmias and ventricular tachycardias. Our findings support the cardiac safety of 0.01 μM BZT/DMP in vitro and the intravenous infusion of 3.85 mg/kg BZT and 1.28 mg/kg DMP in vivo, whereas higher concentrations of both compounds cause mild to moderate cardiotoxicity that should not be neglected during medical and industrial applications.

TEMPORARY REMOVAL: Acute cell stress screen with supervised machine learning predicts cytotoxicity of excipients

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Computational identification of preservatives with potential neuronal cytotoxicity

Preservatives play a vital role in cosmetics by preventing microbiological contamination for keeping products safe to use. However, a few commonly used preservatives have been suggested to be neurotoxic. Cytotoxicity to neuronal cells is commonly used as the first-tier assay for assessing chemical-induced neurotoxicity. Given the time and resources required for chemical screening, computational methods are attractive alternatives over experimental approaches in prioritizing chemicals prior to further experimental evaluations. In this study, we developed a Quantitative Structure-Activity Relationships (QSAR) model for the identification of potential neurotoxicants. A set of 681 chemicals was utilized to construct a robust prediction model using oversampling and Random Forest algorithms. Within a defined applicability domain, the independent test on 452 chemicals showed a high accuracy of 87.7%. The application of the model to 157 preservatives identified 15 chemicals potentially toxic to neuronal cells. Three of them were further validated by in vitro experiments. The results suggested that further experiments are desirable for assessing the neurotoxicity of the identified preservatives with potential neuronal cytotoxicity.

Synergistic Effect of Benzethonium Chloride Combined with Endoxan against Hepatocellular Carcinoma in Rats through Targeting Apoptosis Signaling Pathway

Combination therapy has been the trendy of care, particularly in cancer remedy, since it is a rational approach to increase response and tolerability and to diminish resistance. Hence, there is a growing interest in combining anticancer drugs to maximizing efficacy with minimum systemic toxicity through the delivery of lower drug doses. Therefore, in the present study, the value of combination between benzethonium chloride (benzo) and endoxan (endo) as anti-tumor drug sensitization of hepatocellular carcinoma HCC treatment were detected both in vitro and in vivo. Crystal violet test was performed to detect the proliferation of HepG2 cells treated with benzo or/and endo. In addition, the HCC rat model was established by diethylnitrosamine (DEN) administration. The antitumor effect was enhanced with the combined treatment of the two drugs, particularly in the group with benzo and endo. The results confirmed that the HCC condition was developed in response to lower expressions of caspase 3 and P53 which, in turn, was due to the overexpression of Bcl-2, and downregulation of cytochrome C. The treatment with benzo combined with endo caused significant activation of caspase-3 mediated apoptotic signals that could be responsible for its anti-HCC potential. Meantime, benzo combined with endo treatments could reduce the hepatocellular carcinogenesis by reducing the expression of MMP-9. Therefore, benzo and endo treatments may be a hopeful therapeutic drug for HCC. Also, more studies are recommended to feat the idea of this research for medical use.

Ketamine's dose related multiple mechanisms of actions: Dissociative anesthetic to rapid antidepressant

Ketamine induces safe and effective anesthesia and displays unusual cataleptic properties that gave rise to the term dissociative anesthesia. Since 1970, clinicians only utilized the drug as an anesthetic or analgesic for decades, but ketamine was found to have rapid acting antidepressant effects in 1990s. Accumulated evidence exhibits NMDAR antagonism may not be the only mechanism of ketamine. The contributions of AMPA receptor, mTor signal pathway, monoaminergic system, sigma-1 receptor, cholinergic, opioid and cannabinoid systems, as well as voltage-gated calcium channels and hyperpolarization cyclic nucleotide gated channels are discussed for the antidepressant effects. Also the effects of ketamine's enantiomers and metabolites are reviewed. Furthermore ketamine's anesthetic and analgesic mechanisms are briefly revisited. Overall, pharmacology of ketamine, its enantiomers and metabolites is very unique. Insight into multiple mechanisms of action will provide further development and desirable clinical effects of ketamine.

Ketamine: The final frontier or another depressing end?

Two decades ago, the observation of a rapid and sustained antidepressant response after ketamine administration provided an exciting new avenue in the search for more effective therapeutics for the treatment of clinical depression. Research elucidating the mechanism(s) underlying ketamine's antidepressant properties has led to the development of several hypotheses, including that of disinhibition of excitatory glutamate neurons via blockade of N-methyl-d-aspartate (NMDA) receptors. Although the prominent understanding has been that ketamine's mode of action is mediated solely via the NMDA receptor, this view has been challenged by reports implicating other glutamate receptors such as AMPA, and other neurotransmitter systems such as serotonin and opioids in the antidepressant response. The recent approval of esketamine (Spravato™) for the treatment of depression has sparked a resurgence of interest for a deeper understanding of the mechanism(s) underlying ketamine's actions and safe therapeutic use. This review aims to present our current knowledge on both NMDA and non-NMDA mechanisms implicated in ketamine's response, and addresses the controversy surrounding the antidepressant role and potency of its stereoisomers and metabolites. There is much that remains to be known about our understanding of ketamine's antidepressant properties; and although the arrival of esketamine has been received with great enthusiasm, it is now more important than ever that its mechanisms of action be fully delineated, and both the short- and long-term neurobiological/functional consequences of its treatment be thoroughly characterized.

Ketamine inhibits synaptic transmission and nicotinic acetylcholine receptor-mediated responses in rat intracardiac ganglia in situ

The intravenous anaesthetic ketamine, has been demonstrated to inhibit nicotinic acetylcholine receptor (nAChR)-mediated currents in dissociated rat intracardiac ganglion (ICG) neurons (Weber et al., 2005). This effect would be predicted to depress synaptic transmission in the ICG and would account for the inhibitory action of ketamine on vagal transmission to the heart (Inoue and König, 1988). This investigation was designed to examine the activity of ketamine on (i) postsynaptic responses to vagal nerve stimulation, (ii) the membrane potential, and (iii) membrane current responses evoked by exogenous application of ACh and nicotine in ICG neurons in situ. Intracellular recordings were made using sharp intracellular microelectrodes in a whole mount ICG preparation. Preganglionic nerve stimulation and recordings in current- and voltage-clamp modes were used to assess the action of ketamine on ganglionic transmission and nAChR-mediated responses. Ketamine attenuated the postsynaptic responses evoked by nerve stimulation. This reduction was significant at clinically relevant concentrations at high frequencies. The excitatory membrane potential and current responses to focal application of ACh and nicotine were inhibited in a concentration-dependent manner by ketamine. In contrast, ketamine had no effect on either the directly-evoked action potential or excitatory responses evoked by focal application of γ-aminobutyric acid (GABA). Taken together, ketamine inhibits synaptic transmission and nicotine- and ACh-evoked currents in adult rat ICG. Ketamine inhibition of synaptic transmission and nAChR-mediated responses in the ICG contributes significantly to its attenuation of the bradycardia observed in response to vagal stimulation in the mammalian heart.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

Biosensors Show the Pharmacokinetics of S-Ketamine in the Endoplasmic Reticulum

The target for the “rapid” (<24 h) antidepressant effects of S-ketamine is unknown, vitiating programs to rationally develop more effective rapid antidepressants. To describe a drug’s target, one must first understand the compartments entered by the drug, at all levels—the organ, the cell, and the organelle. We have, therefore, developed molecular tools to measure the subcellular, organellar pharmacokinetics of S-ketamine. The tools are genetically encoded intensity-based S-ketamine-sensing fluorescent reporters, iSKetSnFR1 and iSKetSnFR2. In solution, these biosensors respond to S-ketamine with a sensitivity, S-slope = delta(F/F₀)/(delta[S-ketamine]) of 0.23 and 1.9/μM, respectively. The iSKetSnFR2 construct allows measurements at <0.3 μM S-ketamine. The iSKetSnFR1 and iSKetSnFR2 biosensors display >100-fold selectivity over other ligands tested, including R-ketamine. We targeted each of the sensors to either the plasma membrane (PM) or the endoplasmic reticulum (ER). Measurements on these biosensors expressed in Neuro2a cells and in human dopaminergic neurons differentiated from induced pluripotent stem cells (iPSCs) show that S-ketamine enters the ER within a few seconds after appearing in the external solution near the PM, then leaves as rapidly after S-ketamine is removed from the extracellular solution. In cells, S-slopes for the ER and PM-targeted sensors differ by <2-fold, indicating that the ER [S-ketamine] is less than 2-fold different from the extracellular [S-ketamine]. Organelles represent potential compartments for the engagement of S-ketamine with its antidepressant target, and potential S-ketamine targets include organellar ion channels, receptors, and transporters.

Anesthetics: from modes of action to unconsciousness and neurotoxicity

Modern anesthetic compounds and advanced monitoring tools have revolutionized the field of medicine, allowing for complex surgical procedures to occur safely and effectively. Faster induction times and quicker recovery periods of current anesthetic agents have also helped reduce health care costs significantly. Moreover, extensive research has allowed for a better understanding of anesthetic modes of action, thus facilitating the development of more effective and safer compounds. Notwithstanding the realization that anesthetics are a prerequisite to all surgical procedures, evidence is emerging to support the notion that exposure of the developing brain to certain anesthetics may impact future brain development and function. Whereas the data in support of this postulate from human studies is equivocal, the vast majority of animal research strongly suggests that anesthetics are indeed cytotoxic at multiple brain structure and function levels. In this review, we first highlight various modes of anesthetic action and then debate the evidence of harm from both basic science and clinical studies perspectives. We present evidence from animal and human studies vis-à-vis the possible detrimental effects of anesthetic agents on both the young developing and the elderly aging brain while discussing potential ways to mitigate these effects. We hope that this review will, on the one hand, invoke debate vis-à-vis the evidence of anesthetic harm in young children and the elderly, and on the other hand, incentivize the search for better and less toxic anesthetic compounds.

Quantitative Systems Pharmacological Analysis of Drugs of Abuse Reveals the Pleiotropy of Their Targets and the Effector Role of mTORC1

Existing treatments against drug addiction are often ineffective due to the complexity of the networks of protein-drug and protein-protein interactions (PPIs) that mediate the development of drug addiction and related neurobiological disorders. There is an urgent need for understanding the molecular mechanisms that underlie drug addiction toward designing novel preventive or therapeutic strategies. The rapidly accumulating data on addictive drugs and their targets as well as advances in machine learning methods and computing technology now present an opportunity to systematically mine existing data and draw inferences on potential new strategies. To this aim, we carried out a comprehensive analysis of cellular pathways implicated in a diverse set of 50 drugs of abuse using quantitative systems pharmacology methods. The analysis of the drug/ligand-target interactions compiled in DrugBank and STITCH databases revealed 142 known and 48 newly predicted targets, which have been further analyzed to identify the KEGG pathways enriched at different stages of drug addiction cycle, as well as those implicated in cell signaling and regulation events associated with drug abuse. Apart from synaptic neurotransmission pathways detected as upstream signaling modules that “sense” the early effects of drugs of abuse, pathways involved in neuroplasticity are distinguished as determinants of neuronal morphological changes. Notably, many signaling pathways converge on important targets such as mTORC1. The latter emerges as a universal effector of the persistent restructuring of neurons in response to continued use of drugs of abuse.

Flumazenil but not bicuculline counteract the impairing effects of anesthetic ketamine on recognition memory in rats. Evidence for a functional interaction between the GABAA-benzodiazepine receptor and ketamine?

Experimental evidence indicates that anesthetic doses of the non-competitive NMDA receptor antagonist ketamine impair memory abilities in rodents. The mechanism by which anesthetic ketamine produces its adverse behavioural effects is not yet clarified. In this context, it has been proposed that the effects of anesthetic ketamine on memory might be attributed to its agonistic properties on the GABA type A receptor. The present study was designed to address this issue. Thus, we investigated the ability of the benzodiazepine receptor antagonist flumazenil (1, 3, 6 mg/kg, i.p.) and the GABA_A receptor antagonist bicuculline (0.5, 1.5, 3 mg/kg, i.p.) to counteract recognition memory deficits produced by anesthetic ketamine (100 mg/kg, i.p.) in rats. For this purpose, the novel object recognition task, a behavioural paradigm assessing recognition memory abilities in rodents was used. Compounds were coadministered 24 h before testing or retention. Pre (24 h before testing) or post-training (24 h before retention) administration of flumazenil (6 mg/kg, i.p.) counteracted anesthetic ketamine-induced performance deficits in the novel object recognition memory task. Conversely, bicuculline failed to attenuate the recognition memory deficits caused by anesthetic ketamine. Our findings propose a functional interaction between anesthetic ketamine and the GABA_A receptor allosteric modulator flumazenil on recognition memory.

Consensus Guidelines on the Use of Intravenous Ketamine Infusions for Chronic Pain From the American Society of Regional Anesthesia and Pain Medicine, the American Academy of Pain Medicine, and the American Society of Anesthesiologists

BACKGROUND

Over the past 2 decades, the use of intravenous ketamine infusions as a treatment for chronic pain has increased dramatically, with wide variation in patient selection, dosing, and monitoring. This has led to a chorus of calls from various sources for the development of consensus guidelines.

METHODS

In November 2016, the charge for developing consensus guidelines was approved by the boards of directors of the American Society of Regional Anesthesia and Pain Medicine and, shortly thereafter, the American Academy of Pain Medicine. In late 2017, the completed document was sent to the American Society of Anesthesiologists' Committees on Pain Medicine and Standards and Practice Parameters, after which additional modifications were made. Panel members were selected by the committee chair and both boards of directors based on their expertise in evaluating clinical trials, past research experience, and clinical experience in developing protocols and treating patients with ketamine. Questions were developed and refined by the committee, and the groups responsible for addressing each question consisted of modules composed of 3 to 5 panel members in addition to the committee chair. Once a preliminary consensus was achieved, sections were sent to the entire panel, and further revisions were made. In addition to consensus guidelines, a comprehensive narrative review was performed, which formed part of the basis for guidelines.

RESULTS

Guidelines were prepared for the following areas: indications; contraindications; whether there was evidence for a dose-response relationship, or a minimum or therapeutic dose range; whether oral ketamine or another N-methyl-D-aspartate receptor antagonist was a reasonable treatment option as a follow-up to infusions; preinfusion testing requirements; settings and personnel necessary to administer and monitor treatment; the use of preemptive and rescue medications to address adverse effects; and what constitutes a positive treatment response. The group was able to reach consensus on all questions.

CONCLUSIONS

Evidence supports the use of ketamine for chronic pain, but the level of evidence varies by condition and dose range. Most studies evaluating the efficacy of ketamine were small and uncontrolled and were either unblinded or ineffectively blinded. Adverse effects were few and the rate of serious adverse effects was similar to placebo in most studies, with higher dosages and more frequent infusions associated with greater risks. Larger studies, evaluating a wider variety of conditions, are needed to better quantify efficacy, improve patient selection, refine the therapeutic dose range, determine the effectiveness of nonintravenous ketamine alternatives, and develop a greater understanding of the long-term risks of repeated treatments.

Ketamine and Ketamine Metabolite Pharmacology: Insights into Therapeutic Mechanisms

Ketamine, a racemic mixture consisting of (S)- and (R)-ketamine, has been in clinical use since 1970. Although best characterized for its dissociative anesthetic properties, ketamine also exerts analgesic, anti-inflammatory, and antidepressant actions. We provide a comprehensive review of these therapeutic uses, emphasizing drug dose, route of administration, and the time course of these effects. Dissociative, psychotomimetic, cognitive, and peripheral side effects associated with short-term or prolonged exposure, as well as recreational ketamine use, are also discussed. We further describe ketamine's pharmacokinetics, including its rapid and extensive metabolism to norketamine, dehydronorketamine, hydroxyketamine, and hydroxynorketamine (HNK) metabolites. Whereas the anesthetic and analgesic properties of ketamine are generally attributed to direct ketamine-induced inhibition of N-methyl-D-aspartate receptors, other putative lower-affinity pharmacological targets of ketamine include, but are not limited to, γ-amynobutyric acid (GABA), dopamine, serotonin, sigma, opioid, and cholinergic receptors, as well as voltage-gated sodium and hyperpolarization-activated cyclic nucleotide-gated channels. We examine the evidence supporting the relevance of these targets of ketamine and its metabolites to the clinical effects of the drug. Ketamine metabolites may have broader clinical relevance than was previously considered, given that HNK metabolites have antidepressant efficacy in preclinical studies. Overall, pharmacological target deconvolution of ketamine and its metabolites will provide insight critical to the development of new pharmacotherapies that possess the desirable clinical effects of ketamine, but limit undesirable side effects.

Solution NMR Studies of Anesthetic Interactions with Ion Channels

NMR spectroscopy is one of the major tools to provide atomic resolution protein structural information. It has been used to elucidate the molecular details of interactions between anesthetics and ion channels, to identify anesthetic binding sites, and to characterize channel dynamics and changes introduced by anesthetics. In this chapter, we present solution NMR methods essential for investigating interactions between ion channels and general anesthetics, including both volatile and intravenous anesthetics. Case studies are provided with a focus on pentameric ligand-gated ion channels and the voltage-gated sodium channel NaChBac.

Identification of FDA-Approved Small Molecules Capable of Disrupting the Calmodulin-Adenylyl Cyclase 8 Interaction through Direct Binding to Calmodulin

Adenylyl cyclases (AC) catalyze the formation of cyclic AMP (cAMP) from ATP and are involved in a number of disease states, making them attractive potential drug targets. AC8, in particular, has been implicated in several neurological disorders. While development of small molecule AC inhibitors has generated some chemical leads, the lack of inhibitor specificity among AC family members has limited the identification of successful drug candidates. Therefore, finding alternative novel methods to suppress AC activity are needed. Because only AC1 and AC8 are robustly stimulated by calmodulin (CaM), we set out to explore the mechanism of disrupting the AC/CaM interaction as a way to selectively inhibit AC8. Through the development and implementation of a novel biochemical high-throughput-screening paradigm, we identified six small molecules from an FDA-approved compound library that are capable of disrupting the AC8/CaM interaction. These compounds were also shown to be able disrupt formation of this complex in cells, ultimately leading to decreased AC8 activity. Interestingly, further mechanistic analysis determined that these compounds functioned by binding to CaM and blocking its interaction with AC8. While these particular compounds could inhibit CaM interaction with both AC1 and AC8, they provide significant proof of concept for inhibition of ACs through disruption of CaM binding. These compounds, as dual AC1/AC8 inhibitors, provide important tools for probing pathological conditions where AC1/AC8 activity are enhanced, such as chronic pain and ethanol consumption. Furthermore, unlike tools such as genetic deletion, these compounds can be used in a dose-dependent fashion to determine the role of AC/CaM interactions in these pathologies.

Effect fingerprinting of new psychoactive substances (NPS): What can we learn from in vitro data?

The use of new psychoactive substances (NPS) is increasing and currently >600 NPS have been reported. However, limited information on neuropharmacological and toxicological effects of NPS is available, hampering risk characterization. We reviewed the literature on the in vitro neuronal modes of action to obtain effect fingerprints of different classes of illicit drugs and NPS. The most frequently reported NPS were selected for review: cathinones (MDPV, α-PVP, mephedrone, 4-MEC, pentedrone, methylone), cannabinoids (JWH-018), (hallucinogenic) phenethylamines (4-fluoroamphetamine, benzofurans (5-APB, 6-APB), 2C-B, NBOMes (25B-NBOMe, 25C-NBOMe, 25I-NBOMe)), arylcyclohexylamines (methoxetamine) and piperazine derivatives (mCPP, TFMPP, BZP). Our effect fingerprints highlight the main modes of action for the different NPS studied, including inhibition and/or reversal of monoamine reuptake transporters (cathinones and non-hallucinogenic phenethylamines), activation of 5-HT₂receptors (hallucinogenic phenethylamines and piperazines), activation of cannabinoid receptors (cannabinoids) and inhibition of NDMA receptors (arylcyclohexylamines). Importantly, we identified additional targets by relating reported effect concentrations to the estimated human brain concentrations during recreational use. These additional targets include dopamine receptors, α- and β-adrenergic receptors, GABA_Areceptors and acetylcholine receptors, which may all contribute to the observed clinical symptoms following exposure. Additional data is needed as the number of NPS continues to increase. Also, the effect fingerprints we have obtained are still incomplete and suffer from a large variation in the reported effects and effect sizes. Dedicated in vitro screening batteries will aid in complementing specific effect fingerprints of NPS. These fingerprints can be implemented in the risk assessments of NPS that are necessary for eventual control measures to reduce Public Health risks.

Ketamine Inhibition of the Pentameric Ligand-Gated Ion Channel GLIC

Ketamine inhibits pentameric ligand-gated ion channels (pLGICs), including the bacterial pLGIC from Gloeobacter violaceus (GLIC). The crystal structure of GLIC shows R-ketamine bound to an extracellular intersubunit cavity. Here, we performed molecular dynamics simulations of GLIC in the absence and presence of R- or S-ketamine. No stable binding of S-ketamine in the original cavity was observed in the simulations, largely due to its unfavorable access to residue D154, which provides important electrostatic interactions to stabilize R-ketamine binding. Contrary to the symmetric binding shown in the crystal structure, R-ketamine moved away from some of the binding sites and was bound to GLIC asymmetrically at the end of simulations. The asymmetric binding is consistent with the experimentally measured negative cooperativity of ketamine binding to GLIC. In the presence of R-ketamine, all subunits showed changes in structure and dynamics, irrespective of binding stability; the extracellular intersubunit cavity expanded and intersubunit electrostatic interactions involved in channel activation were altered. R-ketamine binding promoted a conformational shift toward closed GLIC. Conformational changes near the ketamine-binding site were propagated to the interface between the extracellular and transmembrane domains, and further to the pore-lining TM2 through two pathways: pre-TM1 and the β1-β2 loop. Both signaling pathways have been predicted previously using the perturbation-based Markovian transmission model. The study provides a structural and dynamics basis for the inhibitory modulation of ketamine on pLGICs.

Classics in Chemical Neuroscience: Ketamine

Ketamine, a molecule of many faces, has contributed immeasurably to numerous realms of clinical practice and scientific inquiry. From anesthesia and analgesia to depression and schizophrenia, it continues to shed light on the molecular underpinnings of pain, consciousness, and the pathophysiology of neuropsychiatric disorders. In particular, research on ketamine's mechanism of action is providing new hope in the search for therapies for treatment-resistant depression and affords insights into disorders of glutamatergic dysfunction. In this Review, we will cover aspects of ketamine's synthesis, manufacturing, metabolism, pharmacology, approved and off-label indications, and adverse effects. We will also discuss the captivating history of this molecule, its influence on neuropsychiatry, and its potential to advance the fields of chemical neuroscience and neuropharmacology.

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.

Neuroprotective effects of the ethanol stem bark extracts of Terminalia ivorensis in ketamine-induced schizophrenia-like behaviors and oxidative damage in mice

CONTEXT

Schizophrenia is a heterogenous neurological disorder, which has been hypothetically linked to oxidative imbalance and associated behavioral perturbations. Preliminary evidence from animal models predictive of human psychosis suggests that Terminalia ivorensis A. Chev. (Combretaceae) has antipsychotic-like activity in mice.

OBJECTIVE

This study investigates the neuroprotective property of the ethanol stem bark extracts of T. ivorensis (EETI) in reversal treatment of ketamine-induced schizophrenia-like behaviors and oxidative alteration in adult male Swiss albino mice.

MATERIALS AND METHODS

Animals were divided into six treatment groups (n = 5). Animals received distilled water or ketamine (20 mg/kg) once daily intraperitoneally (i.p.) for 14 days, and from the 8th to the 14th day, they were treated with EETI (125, 250 or 500 mg/kg), risperidone (RIS) or vehicle orally once daily. Behaviors related to positive (locomotor activity) and cognitive (Y maze) symptoms of schizophrenia were assessed. Glutathione (GSH) levels, superoxide dismutase (SOD) and catalase (CAT) activities, including malondialdehyde (MDA) concentration were measured in mice whole brains.

RESULT

The LD₅₀ of EETI was 2236.06 mg/kg, p.o. body weight. EETI (125, 250 or 500 mg/kg, p.o.) demonstrated significant (p < 0.05) inhibition of ketamine-induced hyperlocomotion and cognitive dysfunction. The extract decreased MDA concentration (39.0, 62.6 and 67.5%) in a dose-dependent manner. Moreover, EETI significantly (p < 0.05) reversed the depletion of GSH, and increased activities of SOD and CAT in brain tissues.

DISCUSSION AND CONCLUSION

These findings suggest that EETI probably exert its antipsychotic-like activity, via a neuroprotective compensatory mechanism of action, and as such, could be relevant in the management of schizophrenia.

Potential involvement of serotonergic signaling in ketamine's antidepressant actions: A critical review

A single i.v. infusion of ketamine, classified as an N-methyl-d-aspartate (NMDA) receptor antagonist, may alleviate depressive symptoms within hours of administration in treatment resistant depressed patients, and the antidepressant effect may last for several weeks. These unique therapeutic properties have prompted researchers to explore the mechanisms mediating the antidepressant effects of ketamine, but despite many efforts, no consensus on its antidepressant mechanism of action has been reached. Recent preclinical reports have associated the neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) with the antidepressant-like action of ketamine. Here, we review the current evidence for a serotonergic role in ketamine's antidepressant effects. The pharmacological profile of ketamine may include equipotent activity on several non-NMDA targets, and the current hypotheses for the mechanisms responsible for ketamine's antidepressant activity do not appear to preclude the possibility that non-glutamate neurotransmitters are involved in the antidepressant effects. At multiple levels, the serotonergic and glutamatergic systems interact, and such crosstalk could support the notion that changes in serotonergic neurotransmission may impact ketamine's antidepressant potential. In line with these prospects, ketamine may increase 5-HT levels in the prefrontal cortex of rats, plausibly via hippocampal NMDA receptor inhibition and activation of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors. In addition, a number of preclinical studies suggest that the antidepressant-like effects of ketamine may depend on endogenous activation of 5-HT receptors. Recent imaging and behavioral data predominantly support a role for 5-HT1A or 5-HT1B receptors, but the full range of 5-HT receptors has currently not been systematically investigated in this context. Furthermore, the nature of any 5-HT dependent mechanism in ketamine's antidepressant effect is currently not understood, and therefore, more studies are warranted to confirm this hypothesis and explore the specific pathways that might implicate 5-HT.

Enantioselective inhibition of d-serine transport by (S)-ketamine

BACKGROUND AND PURPOSE

Patients with major depressive disorder receiving racemic ketamine, (R,S)-ketamine, experience transient increases in Clinician-Administered Dissociative States Scale scores and a coincident drop in plasma d-serine levels. The results suggest that (R,S)-ketamine produces an immediate, concentration-dependent pharmacological effect on d-serine plasma concentrations. One potential source of this effect is (R,S)-ketamine-induced inhibition of the transporter ASCT2, which regulates intracellular d-serine concentrations. In this study, we tested this hypothesis by examining the effect of (S)- and (R)-ketamine on ASCT2-mediated transport of d-serine in PC-12 and 1321N1 cells and primary neuronal cells in culture.

EXPERIMENTAL APPROACH

Intracellular and extracellular d-serine levels were determined using capillary electrophoresis-laser-induced fluorescence and liquid chromatography-mass spectrometry respectively. Expression of ASCT2, Asc-1 and serine racemase was determined utilizing Western blotting.

KEY RESULTS

(S)-Ketamine produced a concentration-dependent increase in intracellular d-serine and reduced extracellular d-serine accumulation. In contrast, (R)-ketamine decreased both intracellular and extracellular d-serine levels. The ASCT2 inhibitor, benzyl-d-serine (BDS), and ASCT2 gene knockdown mimicked the action of (S)-ketamine on d-serine in PC-12 cells, while the Asc-1 agonist d-isoleucine reduced intracellular d-serine and increased extracellular d-serine accumulation. This response to d-isoleucine was not affected by BDS or (S)-ketamine. Primary cultures of rat neuronal cells expressed ASCT2 and were responsive to (S)-ketamine and BDS. (S)- and (R)-ketamine increased the expression of monomeric serine racemase in all the cells studied, with (S)-ketamine having the greatest effect.

CONCLUSIONS AND IMPLICATIONS

(S)-Ketamine decreased cellular export of d-serine via selective inhibition of ASCT2, and this could represent a possible source of dissociative effects observed with (R,S)-ketamine.

Molecular blueprint of allosteric binding sites in a homologue of the agonist-binding domain of the α7 nicotinic acetylcholine receptor

The α7 nicotinic acetylcholine receptor (nAChR) belongs to the family of pentameric ligand-gated ion channels and is involved in fast synaptic signaling. In this study, we take advantage of a recently identified chimera of the extracellular domain of the native α7 nicotinic acetylcholine receptor and acetylcholine binding protein, termed α7-AChBP. This chimeric receptor was used to conduct an innovative fragment-library screening in combination with X-ray crystallography to identify allosteric binding sites. One allosteric site is surface-exposed and is located near the N-terminal α-helix of the extracellular domain. Ligand binding at this site causes a conformational change of the α-helix as the fragment wedges between the α-helix and a loop homologous to the main immunogenic region of the muscle α1 subunit. A second site is located in the vestibule of the receptor, in a preexisting intrasubunit pocket opposite the agonist binding site and corresponds to a previously identified site involved in positive allosteric modulation of the bacterial homolog ELIC. A third site is located at a pocket right below the agonist binding site. Using electrophysiological recordings on the human α7 nAChR we demonstrate that the identified fragments, which bind at these sites, can modulate receptor activation. This work presents a structural framework for different allosteric binding sites in the α7 nAChR and paves the way for future development of novel allosteric modulators with therapeutic potential.

Pathophysiology of depression and innovative treatments: remodeling glutamatergic synaptic connections

Despite the complexity and heterogeneity of mood disorders, basic and clinical research studies have begun to elucidate the pathophysiology of depression and to identify rapid, efficacious antidepressant agents. Stress and depression are associated with neuronal atrophy, characterized by loss of synaptic connections in key cortical and limbic brain regions implicated in depression. This is thought to occur in part via decreased expression and function of growth factors, such as brain-derived neurotrophic factor (BDNF), in the prefrontal cortex (PFC) and hippocampus. These structural alterations are difficult to reverse with typical antidepressants. However, recent studies demonstrate that ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist that produces rapid antidepressant actions in treatment-resistant depressed patients, rapidly increases spine synapses in the PFC and reverses the deficits caused by chronic stress. This is thought to occur by disinhibition of glutamate transmission, resulting in a rapid but transient burst of glutamate, followed by an increase in BDNF release and activation of downstream signaling pathways that stimulate synapse formation. Recent work demonstrates that the rapid-acting antidepressant effects of scopolamine, a muscarinic receptor antagonist, are also associated with increased glutamate transmission and synapse formation. These findings have resulted in testing and identification of additional targets and agents that influence glutamate transmission and have rapid antidepressant actions in rodent models and in clinical trials. Together these studies have created tremendous excitement and hope for a new generation of rapid, efficacious antidepressants.

Effects of antihistamines on the function of human α7-nicotinic acetylcholine receptors

Effects of the histamine H₁ receptor (H1R) antagonists (antihistamines), promethazine (PMZ), orphenadrine (ORP), chlorpheniramine (CLP), pyrilamine (PYR), diphenhydramine (DPH), citerizine (CTZ), and triprolidine (TRP) on the functional properties of the cloned α7 subunit of the human nicotinic acetylcholine receptor expressed in Xenopus oocytes were investigated. Antihistamines inhibited the α7-nicotinic acetylcholine receptor in the order PYR>CLP>TRP>PMZ>ORP≥DPH≥CTZ. Among the antihistamines, PYR showed the highest reversible inhibition of acetylcholine (100 µM)-induced responses with IC₅₀ of 6.2 µM. PYR-induced inhibition was independent of the membrane potential and could not be reversed by increasing the concentration of acetylcholine. Specific binding of [¹²⁵I] α-bungarotoxin, a selective antagonist for α7-nicotinic acetylcholine receptor, was not changed in the presence of PYR suggesting a non-competitive inhibition of nicotinic receptors. In line with functional experiments, docking studies indicated that PYR can potentially bind allosterically with the α7 transmembrane domain. Our results indicate that the H₂-H₄ receptor antagonists tested in this study (10 µM) showed negligible inhibition of α7-nicotinic acetylcholine receptors. On the other hand, H₁ receptor antagonists inhibited the function of human α7-nicotinic acetylcholine receptor, with varying potencies. These results emphasize the importance of α7-nicotinic acetylcholine receptor for future pharmacological/toxicological profiling.

(R,S)-Ketamine metabolites (R,S)-norketamine and (2S,6S)-hydroxynorketamine increase the mammalian target of rapamycin function

BACKGROUND

Subanesthetic doses of (R,S)-ketamine are used in the treatment of neuropathic pain and depression. In the rat, the antidepressant effects of (R,S)-ketamine are associated with increased activity and function of mammalian target of rapamycin (mTOR); however, (R,S)-ketamine is extensively metabolized and the contribution of its metabolites to increased mTOR signaling is unknown.

METHODS

Rats (n = 3 per time point) were given (R,S)-ketamine, (R,S)-norketamine, and (2S,6S)-hydroxynorketamine and their effect on the mTOR pathway determined after 20, 30, and 60 min. PC-12 pheochromocytoma cells (n = 3 per experiment) were treated with escalating concentrations of each compound and the impact on the mTOR pathway was determined.

RESULTS

The phosphorylation of mTOR and its downstream targets was significantly increased in rat prefrontal cortex tissue by more than ~2.5-, ~25-, and ~2-fold, respectively, in response to a 60-min postadministration of (R,S)-ketamine, (R,S)-norketamine, and (2S,6S)-hydroxynorketamine (P < 0.05, ANOVA analysis). In PC-12 pheochromocytoma cells, the test compounds activated the mTOR pathway in a concentration-dependent manner, which resulted in a significantly higher expression of serine racemase with ~2-fold increases at 0.05 nM (2S,6S)-hydroxynorketamine, 10 nM (R,S)-norketamine, and 1,000 nM (R,S)-ketamine. The potency of the effect reflected antagonistic activity of the test compounds at the α7-nicotinic acetylcholine receptor.

CONCLUSIONS

The data demonstrate that (R,S)-norketamine and (2S,6S)-hydroxynorketamine have potent pharmacological activity both in vitro and in vivo and contribute to the molecular effects produced by subanesthetic doses of (R,S)-ketamine. The results suggest that the determination of the mechanisms underlying the antidepressant and analgesic effects of (R,S)-ketamine requires a full study of the parent compound and its metabolites.

Cellular registration without behavioral recall of olfactory sensory input under general anesthesia

BACKGROUND

Previous studies suggest that sensory information is "received" but not "perceived" under general anesthesia. Whether and to what extent the brain continues to process sensory inputs in a drug-induced unconscious state remain unclear.

METHODS

One hundred seven rats were randomly assigned to 12 different anesthesia and odor exposure paradigms. The immunoreactivities of the immediate early gene products c-Fos and Egr1 as neural activity markers were combined with behavioral tests to assess the integrity and relationship of cellular and behavioral responsiveness to olfactory stimuli under a surgical plane of ketamine-xylazine general anesthesia.

RESULTS

The olfactory sensory processing centers could distinguish the presence or absence of experimental odorants even when animals were fully anesthetized. In the anesthetized state, the c-Fos immunoreactivity in the higher olfactory cortices revealed a difference between novel and familiar odorants similar to that seen in the awake state, suggesting that the anesthetized brain functions beyond simply receiving external stimulation. Reexposing animals to odorants previously experienced only under anesthesia resulted in c-Fos immunoreactivity, which was similar to that elicited by familiar odorants, indicating that previous registration had occurred in the anesthetized brain. Despite the "cellular memory," however, odor discrimination and forced-choice odor-recognition tests showed absence of behavioral recall of the registered sensations, except for a longer latency in odor recognition tests.

CONCLUSIONS

Histologically distinguishable registration of sensory processing continues to occur at the cellular level under ketamine-xylazine general anesthesia despite the absence of behavioral recognition, consistent with the notion that general anesthesia causes disintegration of information processing without completely blocking cellular communications.

The suppression of endogenous adrenalin in the prolongation of ketamine anesthesia

This study investigated whether or not the anesthetic effect of ketamine in rats is dependent on adrenal gland hormones. The study was performed on two main rat groups, intact and adrenalectomized. Rat were divided into subgroups and given appropriate doses of ketamine, metyrapone or metyrosine. Durations of anesthesia in the groups were then recorded. Endogenous catecholamine levels were measured in samples taken from peripheral blood. This experimental results showed that ketamine did not induce anesthesia in intact rats at doses of 15 or 30mg/kg, and that at 60mg/kg anesthesia was established for only 11min. However, ketamine induced significant anesthesia even at a dose of 30mg/kg in animals in which production of endogenous catecholamine (adrenalin, noradrenalin dopamine) was inhibited with metyrosine at a level of 45-47%. Ketamine at 60mg/kg in animals in which endogenous catecholamine was inhibited at a level of 45-47% established anesthesia for 47.6min. However, ketamine at 30 and 60mg/kg induced longer anesthesia in adrenalectomized rats with higher noradrenalin and dopamine levels but suppressed adrenalin production. Adrenalin plays an important role in the control of duration of ketamine anesthesia, while noradrenalin, dopamine and corticosterone have no such function. If endogenous adrenalin is suppressed, ketamine can even provide sufficient anesthesia at a 2-fold lower dose. This makes it possible for ketamine to be used in lengthy surgical procedures.

NMR structures of the human α7 nAChR transmembrane domain and associated anesthetic binding sites

The α7 nicotinic acetylcholine receptor (nAChR), assembled as homomeric pentameric ligand-gated ion channels, is one of the most abundant nAChR subtypes in the brain. Despite its importance in memory, learning and cognition, no structure has been determined for the α7 nAChR TM domain, a target for allosteric modulators. Using solution state NMR, we determined the structure of the human α7 nAChR TM domain (PDB ID: 2MAW) and demonstrated that the α7 TM domain formed functional channels in Xenopus oocytes. We identified the associated binding sites for the anesthetics halothane and ketamine; the former cannot sensitively inhibit α7 function, but the latter can. The α7 TM domain folds into the expected four-helical bundle motif, but the intra-subunit cavity at the extracellular end of the α7 TM domain is smaller than the equivalent cavity in the α4β2 nAChRs (PDB IDs: 2LLY; 2LM2). Neither drug binds to the extracellular end of the α7 TM domain, but two halothane molecules or one ketamine molecule binds to the intracellular end of the α7 TM domain. Halothane and ketamine binding sites are partially overlapped. Ketamine, but not halothane, perturbed the α7 channel-gate residue L9'. Furthermore, halothane did not induce profound dynamics changes in the α7 channel as observed in α4β2. The study offers a novel high-resolution structure for the human α7 nAChR TM domain that is invaluable for developing α7-specific therapeutics. It also provides evidence to support the hypothesis: only when anesthetic binding perturbs the channel pore or alters the channel motion, can binding generate functional consequences.

Antidepressant effects of ketamine: mechanisms underlying fast-acting novel antidepressants

Newer antidepressants are needed for the many individuals with major depressive disorder (MDD) that do not respond adequately to treatment and because of a delay of weeks before the emergence of therapeutic effects. Recent evidence from clinical trials shows that the NMDA antagonist ketamine is a revolutionary novel antidepressant because it acts rapidly and is effective for treatment-resistant patients. A single infusion of ketamine alleviates depressive symptoms in treatment-resistant depressed patients within hours and these effects may be sustained for up to 2 weeks. Although the discovery of ketamine's effects has reshaped drug discovery for antidepressants, the psychotomimetic properties of this compound limit the use of this therapy to the most severely ill patients. In order to develop additional antidepressants like ketamine, adequate preclinical behavioral screening paradigms for fast-acting antidepressants need to be established and used to identify the underlying neural mechanisms. This review examines the preclinical literature attempting to model the antidepressant-like effects of ketamine. Acute administration of ketamine has produced effects in behavioral screens for antidepressants like the forced swim test, novelty suppression of feeding and in rodent models for depression. Protracted behavioral effects of ketamine have been reported to appear after a single treatment that last for days. This temporal pattern is similar to its clinical effects and may serve as a new animal paradigm for rapid antidepressant effects in humans. In addition, protracted changes in molecules mediating synaptic plasticity have been implicated in mediating the antidepressant-like behavioral effects of ketamine. Current preclinical studies are examining compounds with more specific pharmacological effects at glutamate receptors and synapses in order to develop additional rapidly acting antidepressants without the hallucinogenic side effects or abuse potential of ketamine.

Comparison of behavioral effects of the NMDA receptor channel blockers memantine and ketamine in rats

Memantine and ketamine block N-methyl-D-aspartate (NMDA) receptors with similar affinity and kinetics, yet their behavioral consequences differ: e.g., memantine is used to alleviate symptoms of Alzheimer's disease, whereas ketamine reproduces symptoms of schizophrenia. The two drugs exhibit different pharmacokinetics, which may play a principal role in their differential behavioral effects. To gain insight into the drugs' behavioral consequences, we treated adult male rats acutely with varying doses (0-40 mg/kg i.p.) of memantine or ketamine and assessed exploratory behavior and spatial working memory. To examine the importance of pharmacokinetics, we assessed behavior either 15 or 45 min after drug administration. Both drugs decreased ambulation, fine movements, and rearing at the beginning of the exploratory activity test; however, at the end of the test, high doses of only memantine increased ambulation and fine movements. High doses of both drugs disrupted spontaneous alternation, a measure of working memory, but high doses of only memantine elicited perseverative behavior. Surprisingly, ketamine's effects were influenced by the delay between drug administration and testing no more frequently than were memantine's. Our findings show that, regardless of test delay, memantine and ketamine evoke similar behavioral effects at lower doses, consistent with NMDA receptors being both drugs' principal site of action, but can have divergent effects at higher doses. Our results suggest that the divergence of memantine's and ketamine's behavioral consequences is likely to result from differences in mechanisms of NMDA receptor antagonism or actions at other targets.

Mechanism of HERG potassium channel inhibition by tetra-n-octylammonium bromide and benzethonium chloride

Tetra-n-octylammonium bromide and benzethonium chloride are synthetic quaternary ammonium salts that are widely used in hospitals and industries for the disinfection and surface treatment and as the preservative agent. Recently, the activities of HERG channel inhibition by these compounds have been found to have potential risks to induce the long QT syndrome and cardiac arrhythmia, although the mechanism of action is still elusive. This study was conducted to investigate the mechanism of HERG channel inhibition by these compounds by using whole-cell patch clamp experiments in a CHO cell line stably expressing HERG channels. Tetra-n-octylammonium bromide and benzethonium chloride exhibited concentration-dependent inhibitions of HERG channel currents with IC(50) values of 4nM and 17nM, respectively, which were also voltage-dependent and use-dependent. Both compounds shifted the channel activation I-V curves in a hyperpolarized direction for 10-15mV and accelerated channel activation and inactivation processes by 2-fold. In addition, tetra-n-octylammonium bromide shifted the inactivation I-V curve in a hyperpolarized direction for 24.4mV and slowed the rate of channel deactivation by 2-fold, whereas benzethonium chloride did not. The results indicate that tetra-n-octylammonium bromide and benzethonium chloride are open-channel blockers that inhibit HERG channels in the voltage-dependent, use-dependent and state-dependent manners.

Alpha7 neuronal nicotinic receptors as a drug target in schizophrenia

INTRODUCTION

Schizophrenia is a profoundly debilitating disease that represents not only an individual, but a societal problem. Once characterized solely by the hyperactivity of the dopaminergic system, therapies directed to dampen dopaminergic neurotransmission were developed. However, these drugs do not address the significant impairments in cognition and the negative symptoms of the disease, and it is now apparent that disequilibrium of many neurotransmitter systems is involved. Despite enormous efforts, minimal progress has been made toward the development of safer, more effective therapies to date.

AREAS COVERED

The high preponderance of smoking in schizophrenics suggests that nicotine may provide symptomatic improvement, which has led to investigation for selective molecules targeted to individual nicotinic receptor (nAChR) subtypes. Of special interest is activation of the homomeric α7nAChR, which is widely distributed in the brain and has been implicated in the pathophysiology of schizophrenia through numerous approaches.

EXPERT OPINION

Preclinical and clinical data suggest that neuronal α7nAChRs play an important role in cognitive functions. Moreover, some, but not all, early clinical trials conducted with α7nAChR agonists show cognitive benefits in schizophrenics. These encouraging results suggest that development of compounds targeting α7nAChRs will represent a valuable tool to mitigate symptoms associated with schizophrenia, and open new strategies for better pharmacological treatment of these patients.

Sub-anesthetic concentrations of (R,S)-ketamine metabolites inhibit acetylcholine-evoked currents in α7 nicotinic acetylcholine receptors

The effect of the (R,S)-ketamine metabolites (R,S)-norketamine, (R,S)-dehydronorketamine, (2S,6S)-hydroxynorketamine and (2R,6R)-hydroxynorketamine on the activity of α7 and α3β4 neuronal nicotinic acetylcholine receptors was investigated using patch-clamp techniques. The data indicated that (R,S)-dehydronorketamine inhibited acetylcholine-evoked currents in α7-nicotinic acetylcholine receptor, IC(50) = 55 ± 6 nM, and that (2S,6S)-hydroxynorketamine, (2R,6R)-hydroxynorketamine and (R,S)-norketamine also inhibited α7-nicotinic acetylcholine receptor function at concentrations ≤ 1 μM, while (R,S)-ketamine was inactive at these concentrations. The inhibitory effect of (R,S)-dehydronorketamine was voltage-independent and the compound did not competitively displace selective α7-nicotinic acetylcholine receptor ligands [(125)I]-α-bungarotoxin and [(3)H]-epibatidine indicating that (R,S)-dehydronorketamine is a negative allosteric modulator of the α7-nicotinic acetylcholine receptor. (R,S)-Ketamine and (R,S)-norketamine inhibited (S)-nicotine-induced whole-cell currents in cells expressing α3β4-nicotinic acetylcholine receptor, IC(50) 3.1 and 9.1 μM, respectively, while (R,S)-dehydronorketamine, (2S,6S)-hydroxynorketamine and (2R,6R)-hydroxynorketamine were weak inhibitors, IC(50) >100 μM. The binding affinities of (R,S)-dehydronorketamine, (2S,6S)-hydroxynorketamine and (2R,6R)-hydroxynorketamine at the NMDA receptor were also determined using rat brain membranes and the selective NMDA receptor antagonist [(3)H]-MK-801. The calculated K(i) values were 38.95 μM for (S)-dehydronorketamine, 21.19 μM for (2S,6S)-hydroxynorketamine and>100 μM for (2R,6R)-hydroxynorketamine. The results suggest that the inhibitory activity of ketamine metabolites at the α7-nicotinic acetylcholine receptor may contribute to the clinical effect of the drug.

NMR resolved multiple anesthetic binding sites in the TM domains of the α4β2 nAChR

The α4β2 nicotinic acetylcholine receptor (nAChR) has significant roles in nervous system function and disease. It is also a molecular target of general anesthetics. Anesthetics inhibit the α4β2 nAChR at clinically relevant concentrations, but their binding sites in α4β2 remain unclear. The recently determined NMR structures of the α4β2 nAChR transmembrane (TM) domains provide valuable frameworks for identifying the binding sites. In this study, we performed solution NMR experiments on the α4β2 TM domains in the absence and presence of halothane and ketamine. Both anesthetics were found in an intra-subunit cavity near the extracellular end of the β2 transmembrane helices, homologous to a common anesthetic binding site observed in X-ray structures of anesthetic-bound GLIC (Nury et al., [32]). Halothane, but not ketamine, was also found in cavities adjacent to the common anesthetic site at the interface of α4 and β2. In addition, both anesthetics bound to cavities near the ion selectivity filter at the intracellular end of the TM domains. Anesthetic binding induced profound changes in protein conformational exchanges. A number of residues, close to or remote from the binding sites, showed resonance signal splitting from single to double peaks, signifying that anesthetics decreased conformation exchange rates. It was also evident that anesthetics shifted population of two conformations. Altogether, the study comprehensively resolved anesthetic binding sites in the α4β2 nAChR. Furthermore, the study provided compelling experimental evidence of anesthetic-induced changes in protein dynamics, especially near regions of the hydrophobic gate and ion selectivity filter that directly regulate channel functions.