In vivo activation of azipropofol prolongs anesthesia and reveals synaptic targets

Combining nano-differential scanning fluorimetry and microscale thermophoresis to investigate VDAC1 interaction with small molecules

The mitochondrial voltage-dependent anion channel 1 (VDAC1) plays a central role in metabolism and apoptosis, which makes it a promising therapeutic target. Nevertheless, molecular mechanisms governing VDAC1 functioning remain unclear. Small-molecule ligands specifically interacting with the channel provide an attractive way of exploring its structure-function relationships and can possibly be used as founding stones for future drug-candidates. While around 30 VDAC1 ligands have been identified over the years, various techniques have been used by research teams, making a fair and direct comparison between compounds impossible. To tackle this issue, we performed ligand-binding assays on a representative set of seventeen known VDAC1 ligands using nano-differential scanning fluorimetry and microscale thermophoresis. While all the compounds have been confirmed as VDAC1 ligands by at least one method, combining both technologies lead to the selection of four molecules (cannabidiol, curcumin, DIDS and VBIT4) as chemical starting points for future design of VDAC1 selective ligands.

In Vivo Photoadduction of Anesthetic Ligands in Mouse Brain Markedly Extends Sedation and Hypnosis

Photoaffinity ligands are best known as tools used to identify the specific binding sites of drugs to their molecular targets. However, photoaffinity ligands have the potential to further define critical neuroanatomic targets of drug action. In the brains of WT male mice, we demonstrate the feasibility of using photoaffinity ligands in vivo to prolong anesthesia via targeted yet spatially restricted photoadduction of azi-m-propofol (aziPm), a photoreactive analog of the general anesthetic propofol. Systemic administration of aziPm with bilateral near-ultraviolet photoadduction in the rostral pons, at the border of the parabrachial nucleus and locus coeruleus, produced a 20-fold increase in the duration of sedative and hypnotic effects compared with control mice without UV illumination. Photoadduction that missed the parabrachial-coerulean complex also failed to extend the sedative or hypnotic actions of aziPm and was indistinguishable from nonadducted controls. Paralleling the prolonged behavioral and EEG consequences of on target in vivo photoadduction, we conducted electrophysiologic recordings in rostral pontine brain slices. Using neurons within the locus coeruleus to further highlight the cellular consequences of irreversible aziPm binding, we demonstrate transient slowing of spontaneous action potentials with a brief bath application of aziPm that becomes irreversible on photoadduction. Together, these findings suggest that photochemistry-based strategies are a viable new approach for probing CNS physiology and pathophysiology.SIGNIFICANCE STATEMENT Photoaffinity ligands are drugs capable of light-induced irreversible binding, which have unexploited potential to identify the neuroanatomic sites of drug action. We systemically administer a centrally acting anesthetic photoaffinity ligand in mice, conduct localized photoillumination within the brain to covalently adduct the drug at its in vivo sites of action, and successfully enrich irreversible drug binding within a restricted 250 µm radius. When photoadduction encompassed the pontine parabrachial-coerulean complex, anesthetic sedation and hypnosis was prolonged 20-fold, thus illustrating the power of in vivo photochemistry to help unravel neuronal mechanisms of drug action.

A Novel Photoswitchable Azobenzene-Containing Local Anesthetic Ethercaine with Light-Controlled Biological Activity In Vivo

Pain is a common symptom that impairs the quality of life for people around the world. Local anesthetics widely used for pain relief have a number of side effects, which makes the development of both new drugs and new ways to control their activity particularly important. Photopharmacology makes it possible to reduce the side effects of an anesthetic and control its biological activity in the body. The purpose of this work was to create a new light-controlled local anesthetic and study its biological activity in animals. A compound with a simple scheme of synthesis was chosen to shift the UV-Vis absorption band towards the visible range of the spectrum and was synthesized for the first time. Some computer calculations were performed to make sure that the aforementioned changes would not lead to loss of biological activity. The micellar form of the new compound was prepared, and in vivo biological studies were carried out in rabbits. The existence of a local anesthetic effect, which disappeared almost completely on irradiation with light (λ = 395 nm), was shown using the surface anesthesia model. Moreover, the possibility of multiple reversible changes in the biological activity of ethercaine under the action of light was demonstrated. The latter compound manifests no local irritating effect, either. The data obtained indicate the prospects for the development of new compounds based on azobenzene for light-controlled local anesthesia.

Binding Sites and the Mechanism of Action of Propofol and a Photoreactive Analogue in Prokaryotic Voltage-Gated Sodium Channels

Propofol, one of the most commonly used intravenous general anesthetics, modulates neuronal function by interacting with ion channels. The mechanisms that link propofol binding to the modulation of distinct ion channel states, however, are not understood. To tackle this problem, we investigated the prokaryotic ancestors of eukaryotic voltage-gated Na⁺ channels (Navs) using unbiased photoaffinity labeling (PAL) with a diazirine derivative of propofol (AziPm), electrophysiological methods, and mutagenesis. AziPm inhibits Nav function in a manner that is indistinguishable from that of the parent compound by promoting activation-coupled inactivation. In several replicates (8/9) involving NaChBac and NavMs, we found adducts at residues located at the C-terminal end of the S4 voltage sensor, the S4-S5 linker, and the N-terminal end of the S5 segment. However, the non-inactivating mutant NaChBac-T220A yielded adducts that were different from those found in the wild-type counterpart, which suggested state-dependent changes at the binding site. Then, using molecular dynamics simulations to further elucidate the structural basis of Nav modulation by propofol, we show that the S4 voltage sensors and the S4-S5 linkers shape two distinct propofol binding sites in a conformation-dependent manner. Supporting the PAL and MD simulation results, we also found that Ala mutations of a subset of adducted residues have distinct effects on gating modulation of NaChBac and NavMs by propofol. The results of this study provide direct insights into the structural basis of the mechanism through which propofol binding promotes activation-coupled inactivation to inhibit Nav channel function.

The Biology of General Anesthesia from Paramecium to Primate

General anesthesia serves a critically important function in the clinical care of human patients. However, the anesthetized state has foundational implications for biology because anesthetic drugs are effective in organisms ranging from paramecia, to plants, to primates. Although unconsciousness is typically considered the cardinal feature of general anesthesia, this endpoint is only strictly applicable to a select subset of organisms that are susceptible to being anesthetized. We review the behavioral endpoints of general anesthetics across species and propose the isolation of an organism from its environment - both in terms of the afferent arm of sensation and the efferent arm of action - as a generalizable definition. We also consider the various targets and putative mechanisms of general anesthetics across biology and identify key substrates that are conserved, including cytoskeletal elements, ion channels, mitochondria, and functionally coupled electrical or neural activity. We conclude with a unifying framework related to network function and suggest that general anesthetics - from single cells to complex brains - create inefficiency and enhance modularity, leading to the dissociation of functions both within an organism and between the organism and its surroundings. Collectively, we demonstrate that general anesthesia is not restricted to the domain of modern medicine but has broad biological relevance with wide-ranging implications for a diverse array of species.

Targeting the Multiple Physiologic Roles of VDAC With Steroids and Hydrophobic Drugs

There is accumulating evidence that endogenous steroids and non-polar drugs are involved in the regulation of mitochondrial physiology. Many of these hydrophobic compounds interact with the Voltage Dependent Anion Channel (VDAC). This major metabolite channel in the mitochondrial outer membrane (MOM) regulates the exchange of ions and water-soluble metabolites, such as ATP and ADP, across the MOM, thus governing mitochondrial respiration. Proteomics and biochemical approaches together with molecular dynamics simulations have identified an impressively large number of non-polar compounds, including endogenous, able to bind to VDAC. These findings have sparked speculation that both natural steroids and synthetic hydrophobic drugs regulate mitochondrial physiology by directly affecting VDAC ion channel properties and modulating its metabolite permeability. Here we evaluate recent studies investigating the effect of identified VDAC-binding natural steroids and non-polar drugs on VDAC channel functioning. We argue that while many compounds are found to bind to the VDAC protein, they do not necessarily affect its channel functions in vitro. However, they may modify other aspects of VDAC physiology such as interaction with its cytosolic partner proteins or complex formation with other mitochondrial membrane proteins, thus altering mitochondrial function.

Azi-medetomidine: Synthesis and Characterization of a Novel α2 Adrenergic Photoaffinity Ligand

Agonists at the α₂ adrenergic receptor produce sedation, increase focus, provide analgesia, and induce centrally mediated hypotension and bradycardia, yet neither their dynamic interactions with adrenergic receptors nor their modulation of neuronal circuit activity is completely understood. Photoaffinity ligands of α₂ adrenergic agonists have the potential both to capture discrete moments of ligand-receptor interactions and to prolong naturalistic drug effects in discrete regions of tissue in vivo. We present here the synthesis and characterization of a novel α₂ adrenergic agonist photolabel based on the imidazole medetomidine called azi-medetomidine. Azi-medetomidine shares protein association characteristics with its parent compound in experimental model systems and by molecular dynamics simulation of interactions with the α_2A adrenergic receptor. Azi-medetomidine acts as an agonist at α_2A adrenergic receptors, and produces hypnosis in Xenopus laevis tadpoles. Azi-medetomidine competes with the α₂ agonist clonidine at α_2A adrenergic receptors, which is potentiated by photolabeling, and azi-medetomidine labels moieties on the α_2A adrenergic receptor as determined by mass spectrometry in a manner consistent with a simulated model. This novel α₂ adrenergic agonist photolabel can serve as a powerful tool for in vitro and in vivo investigations of adrenergic signaling.

Optoanesthesia: Use of Anesthetic Photolabels In Vivo

Investigation of how anesthetics produce hypnosis requires knowledge of their effects at the molecular, neuronal, circuit, and whole-brain network level. Anesthetic photolabels have long been used to explore how anesthetics bind and affect known protein targets, but they could potentially assist in investigation of anesthetic effects at higher organizational levels of the central nervous system. Here, we advocate the use and provide detailed methods for the application of anesthetic photolabels in slice electrophysiology and in intact animals as a means of investigating anesthetic effects on distinct circuits and brain centers.

Zebrafish: A Pharmacogenetic Model for Anesthesia

General anesthetics are small molecules that interact with and effect the function of many different proteins to promote loss of consciousness, amnesia, and sometimes, analgesia. Owing to the complexity of this state transition and the transient nature of these drug/protein interactions, anesthetics can be difficult to study. The zebrafish is an emerging model for the discovery of both new genes required for the response to and side effects of anesthesia. Here we discuss the tools available to manipulate the zebrafish genome, including both genetic screens and genome engineering approaches. Additionally, there are various robust behavior assays available to study anesthetic and other drug responses. These assays are available for single-gene study or high throughput for genetic or drug discovery. Finally, we present a case study of using propofol as an anesthetic in the zebrafish. These techniques and protocols make the zebrafish a powerful model to study anesthetic mechanisms and drug discovery.

Identification of General Anesthetic Target Protein-Binding Sites by Photoaffinity Labeling and Mass Spectrometry

General anesthetics are unique in that they represent a diverse range of chemical structures. Therefore, it is not surprising that the desired and undesired molecular targets, and binding sites therein, are as equally diverse and unique. Photoaffinity labeling has proven to be a valuable strategy for the identification of anesthetic molecular targets, as well as binding sites within those targets. In combination with the advances in mass spectrometry-based proteomics, along with the ability to comprehensively map posttranslational modifications, the method is likely to undergo continued improvement. Here, we provide the fundamentals for the design and development of an anesthetic photolabel. We also outline a protocol for the identification of photolabeled residues by mass spectrometry. The major steps include the photolabeling experiment, sample preparation, high-resolution mass spectrometry, and data analysis. The protocol can be used as a foundation for further optimization for the specific protein of interest and conditions of an experiment. The use of photoaffinity labeling adds an advantageous alternative and/or complementary approach to increase understanding of anesthetic molecular mechanisms.

Recent progress on the molecular pharmacology of propofol

The precise mechanism by which propofol enhances GABAergic transmission remains unclear, but much progress has been made regarding the underlying structural and dynamic mechanisms. Furthermore, it is now clear that propofol has additional molecular targets, many of which are functionally influenced at concentrations achieved clinically. Focusing primarily on molecular targets, this brief review attempts to summarize some of this recent progress while pointing out knowledge gaps and controversies. It is not intended to be comprehensive but rather to stimulate further thought, discussion, and study on the mechanisms by which propofol produces its pleiotropic effects.

Shedding Light on Anesthetic Mechanisms: Application of Photoaffinity Ligands

Anesthetic photoaffinity ligands have had an increasing presence within anesthesiology research. These ligands mimic parent general anesthetics and allow investigators to study anesthetic interactions with receptors and enzymes; identify novel targets; and determine distribution within biological systems. To date, nearly all general anesthetics used in medicine have a corresponding photoaffinity ligand represented in the literature. In this review, we examine all aspects of the current methodologies, including ligand design, characterization, and deployment. Finally we offer points of consideration and highlight the future outlook as more photoaffinity ligands emerge within the field.

Photoaffinity Ligand for the Inhalational Anesthetic Sevoflurane Allows Mechanistic Insight into Potassium Channel Modulation

Sevoflurane is a commonly used inhaled general anesthetic. Despite this, its mechanism of action remains largely elusive. Compared to other anesthetics, sevoflurane exhibits distinct functional activity. In particular, sevoflurane is a positive modulator of voltage-gated Shaker-related potassium channels (K_v1.x), which are key regulators of action potentials. Here, we report the synthesis and validation of azisevoflurane, a photoaffinity ligand for the direct identification of sevoflurane binding sites in the K_v1.2 channel. Azisevoflurane retains major sevoflurane protein binding interactions and pharmacological properties within in vivo models. Photoactivation of azisevoflurane induces adduction to amino acid residues that accurately reported sevoflurane protein binding sites in model proteins. Pharmacologically relevant concentrations of azisevoflurane analogously potentiated wild-type K_v1.2 and the established mutant K_v1.2 G329T. In wild-type K_v1.2 channels, azisevoflurane photolabeled Leu317 within the internal S4-S5 linker, a vital helix that couples the voltage sensor to the pore region. A residue lining the same binding cavity was photolabeled by azisevoflurane and protected by sevoflurane in the K_v1.2 G329T. Mutagenesis of Leu317 in WT K_v1.2 abolished sevoflurane voltage-dependent positive modulation. Azisevoflurane additionally photolabeled a second distinct site at Thr384 near the external selectivity filter in the K_v1.2 G329T mutant. The identified sevoflurane binding sites are located in critical regions involved in gating of K_v channels and related ion channels. Azisevoflurane has thus emerged as a new tool to discover inhaled anesthetic targets and binding sites and investigate contributions of these targets to general anesthesia.

A Novel Bifunctional Alkylphenol Anesthetic Allows Characterization of γ-Aminobutyric Acid, Type A (GABAA), Receptor Subunit Binding Selectivity in Synaptosomes

Propofol, an intravenous anesthetic, is a positive modulator of the GABAA receptor, but the mechanistic details, including the relevant binding sites and alternative targets, remain disputed. Here we undertook an in-depth study of alkylphenol-based anesthetic binding to synaptic membranes. We designed, synthesized, and characterized a chemically active alkylphenol anesthetic (2-((prop-2-yn-1-yloxy)methyl)-5-(3-(trifluoromethyl)-3H-diazirin-3-yl)phenol, AziPm-click (1)), for affinity-based protein profiling (ABPP) of propofol-binding proteins in their native state within mouse synaptosomes. The ABPP strategy captured ∼4% of the synaptosomal proteome, including the unbiased capture of five α or β GABAA receptor subunits. Lack of γ2 subunit capture was not due to low abundance. Consistent with this, independent molecular dynamics simulations with alchemical free energy perturbation calculations predicted selective propofol binding to interfacial sites, with higher affinities for α/β than γ-containing interfaces. The simulations indicated hydrogen bonding is a key component leading to propofol-selective binding within GABAA receptor subunit interfaces, with stable hydrogen bonds observed between propofol and α/β cavity residues but not γ cavity residues. We confirmed this by introducing a hydrogen bond-null propofol analogue as a protecting ligand for targeted-ABPP and observed a lack of GABAA receptor subunit protection. This investigation demonstrates striking interfacial GABAA receptor subunit selectivity in the native milieu, suggesting that asymmetric occupancy of heteropentameric ion channels by alkylphenol-based anesthetics is sufficient to induce modulation of activity.

Role for the propofol hydroxyl in anesthetic protein target molecular recognition

Propofol is a widely used intravenous general anesthetic. We synthesized 2-fluoro-1,3-diisopropylbenzene, a compound that we call "fropofol", to directly assess the significance of the propofol 1-hydroxyl for pharmacologically relevant molecular recognition in vitro and for anesthetic efficacy in vivo. Compared to propofol, fropofol had a similar molecular volume and only a small increase in hydrophobicity. Isothermal titration calorimetry and competition assays revealed that fropofol had higher affinity for a protein site governed largely by van der Waals interactions. Within another protein model containing hydrogen bond interactions, propofol demonstrated higher affinity. In vivo, fropofol demonstrated no anesthetic efficacy, but at high concentrations produced excitatory activity in tadpoles and mice; fropofol also antagonized propofol-induced hypnosis. In a propofol protein target that contributes to hypnosis, α1β2γ2L GABAA receptors, fropofol demonstrated no significant effect alone or on propofol positive allosteric modulation of the ion channel, suggesting an additional requirement for the 1-hydroxyl within synaptic GABAA receptor site(s). However, fropofol caused similar adverse cardiovascular effects as propofol by a dose-dependent depression of myocardial contractility. Our results directly implicate the propofol 1-hydroxyl as contributing to molecular recognition within protein targets leading to hypnosis, but not necessarily within protein targets leading to side effects of the drug.

Macroscopic and macromolecular specificity of alkylphenol anesthetics for neuronal substrates

We used a photoactive general anesthetic called meta-azi-propofol (AziPm) to test the selectivity and specificity of alkylphenol anesthetic binding in mammalian brain. Photolabeling of rat brain sections with [³H]AziPm revealed widespread but heterogeneous ligand distribution, with [³H]AziPm preferentially binding to synapse-dense areas compared to areas composed largely of cell bodies or myelin. With [³H]AziPm and propofol, we determined that alkylphenol general anesthetics bind selectively and specifically to multiple synaptic protein targets. In contrast, the alkylphenol anesthetics do not bind to specific sites on abundant phospholipids or cholesterol, although [³H]AziPm shows selectivity for photolabeling phosphatidylethanolamines. Together, our experiments suggest that alkylphenol anesthetic substrates are widespread in number and distribution, similar to those of volatile general anesthetics, and that multi-target mechanisms likely underlie their pharmacology.

Propofol inhibits SIRT2 deacetylase through a conformation-specific, allosteric site

meta-Azi-propofol (AziPm) is a photoactive analog of the general anesthetic propofol. We photolabeled a myelin-enriched fraction from rat brain with [(3)H]AziPm and identified the sirtuin deacetylase SIRT2 as a target of the anesthetic. AziPm photolabeled three SIRT2 residues (Tyr(139), Phe(190), and Met(206)) that are located in a single allosteric protein site, and propofol inhibited [(3)H]AziPm photolabeling of this site in myelin SIRT2. Structural modeling and in vitro experiments with recombinant human SIRT2 determined that propofol and [(3)H]AziPm only bind specifically and competitively to the enzyme when co-equilibrated with other substrates, which suggests that the anesthetic site is either created or stabilized in enzymatic conformations that are induced by substrate binding. In contrast to SIRT2, specific binding of [(3)H]AziPm or propofol to recombinant human SIRT1 was not observed. Residues that line the propofol binding site on SIRT2 contact the sirtuin co-substrate NAD(+) during enzymatic catalysis, and assays that measured SIRT2 deacetylation of acetylated α-tubulin revealed that propofol inhibits enzymatic function. We conclude that propofol inhibits the mammalian deacetylase SIRT2 through a conformation-specific, allosteric protein site that is unique from the previously described binding sites of other inhibitors. This suggests that propofol might influence cellular events that are regulated by protein acetylation state.

Sites and functional consequence of VDAC-alkylphenol anesthetic interactions

General anesthetics have previously been shown to bind mitochondrial VDAC. Here, using a photoactive analog of the anesthetic propofol, we determined that alkylphenol anesthetics bind to Gly56 and Val184 on rat VDAC1. By reconstituting rat VDAC into planar bilayers, we determined that propofol potentiates VDAC gating with asymmetry at the voltage polarities; in contrast, propofol does not affect the conductance of open VDAC. Additional experiments showed that propofol also does not affect gramicidin A properties that are sensitive to lipid bilayer mechanics. Together, this suggests propofol affects VDAC function through direct protein binding, likely at the lipid-exposed channel surface, and that gating can be modulated by ligand binding to the distal ends of VDAC β-strands where Gly56 and Val184 are located.

Computational investigation of cholesterol binding sites on mitochondrial VDAC

The mitochondrial voltage-dependent anion channel (VDAC) allows passage of ions and metabolites across the mitochondrial outer membrane. Cholesterol binds mammalian VDAC, and we investigated the effects of binding to human VDAC1 with atomistic molecular dynamics simulations that totaled 1.4 μs. We docked cholesterol to specific sites on VDAC that were previously identified with NMR, and we tested the reliability of multiple docking results in each site with simulations. The most favorable binding modes were used to build a VDAC model with cholesterol occupying five unique sites, and during multiple 100 ns simulations, cholesterol stably and reproducibly remained bound to the protein. For comparison, VDAC was simulated in systems with identical components but with cholesterol initially unbound. The dynamics of loops that connect adjacent β-strands were most affected by bound cholesterol, with the averaged root-mean-square fluctuation (RMSF) of multiple residues altered by 20–30%. Cholesterol binding also stabilized charged residues inside the channel and localized the surrounding electrostatic potentials. Despite this, ion diffusion through the channel was not significantly affected by bound cholesterol, as evidenced by multi-ion potential of mean force measurements. Although we observed modest effects of cholesterol on the open channel, our model will be particularly useful in experiments that investigate how cholesterol affects VDAC function under applied electrochemical forces and also how other ligands and proteins interact with the channel.

Explaining general anesthesia: a two-step hypothesis linking sleep circuits and the synaptic release machinery

Several general anesthetics produce their sedative effect by activating endogenous sleep pathways. We propose that general anesthesia is a two-step process targeting sleep circuits at low doses, and synaptic release mechanisms across the entire brain at the higher doses required for surgery. Our hypothesis synthesizes data from a variety of model systems, some which require sleep (e.g. rodents and adult flies) and others that probably do not sleep (e.g. adult nematodes and cultured cell lines). Non-sleeping systems can be made insensitive (or hypersensitive) to some anesthetics by modifying a single pre-synaptic protein, syntaxin1A. This suggests that the synaptic release machinery, centered on the highly conserved SNARE complex, is an important target of general anesthetics in all animals. A careful consideration of SNARE architecture uncovers a potential mechanism for general anesthesia, which may be the primary target in animals that do not sleep, but a secondary target in animals that sleep.

Mechanisms revealed through general anesthetic photolabeling

General anesthetic photolabels are used to reveal molecular targets and molecular binding sites of anesthetic ligands. After identification, the relevance of anesthetic substrates or binding sites can be tested in biological systems. Halothane and photoactive analogs of isoflurane, propofol, etomidate, neurosteroids, anthracene, and long chain alcohols have been used in anesthetic photolabeling experiments. Interrogated protein targets include the nicotinic acetylcholine receptor, GABA_A receptor, tubulin, leukocyte function-associated antigen-1, and protein kinase C. In this review, we summarize insights revealed by photolabeling these targets, as well as general features of anesthetics, such as their propensity to partition to mitochondria and bind voltage-dependent anion channels. The theory of anesthetic photolabel design and the experimental application of photoactive ligands are also discussed.

Direct modulation of microtubule stability contributes to anthracene general anesthesia

Recently, we identified 1-aminoanthracene as a fluorescent general anesthetic. To investigate the mechanism of action, a photoactive analogue, 1-azidoanthracene, was synthesized. Administration of 1-azidoanthracene to albino stage 40-47 tadpoles was found to immobilize animals upon near-UV irradiation of the forebrain region. The immobilization was often reversible, but it was characterized by a longer duration consistent with covalent attachment of the ligand to functionally important targets. IEF/SDS-PAGE examination of irradiated tadpole brain homogenate revealed labeled protein, identified by mass spectrometry as β-tubulin. In vitro assays with aminoanthracene-cross-linked tubulin indicated inhibition of microtubule polymerization, similar to colchicine. Tandem mass spectrometry confirmed anthracene binding near the colchicine site. Stage 40-47 tadpoles were also incubated 1 h with microtubule stabilizing agents, epothilone D or discodermolide, followed by dosing with 1-aminoanthracene. The effective concentration of 1-aminoanthracene required to immobilize the tadpoles was significantly increased in the presence of either microtubule stabilizing agent. Epothilone D similarly mitigated the effects of a clinical neurosteroid general anesthetic, allopregnanolone, believed to occupy the colchicine site in tubulin. We conclude that neuronal microtubules are "on-pathway" targets for anthracene general anesthetics and may also represent functional targets for some neurosteroid general anesthetics.