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Abstract
Genetic variation contributes to an individual’s sensitivity and response to a variety of drugs important to anesthetic practice. Early insights into the clinical impact of pharmacogenetics were provided by anesthesiology – investigations into prolonged apnea after succinylcholine administration, thiopental-induced porphyria and malignant hyperthermia contributed to the novel science of pharmacogenetics in the early 1960s. Genetic polymorphisms involved in pharmacokinetics (absorption, distribution, metabolism, and excretion of drugs) and pharmacodynamics (receptors, ion channels and enzymes) can affect an individual’s response to the drugs used in anesthetic practice. In addition, genetic variation in proteins directly unrelated to drug action or metabolism can influence responses to environmental changes that occur during anesthesia. This review will summarize the current knowledge of genetic variation in response to drugs relevant to anesthesia, and how this impacts upon clinical practice.
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Affiliation(s)
- Helen F Galley
- University of Aberdeen, Academic Unit of Anaesthesia & Intensive Care, School of Medicine,Institute of Medical Sciences, Aberdeen, AB25 2ZD, UK.
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52
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Belelli D, Herd MB, Mitchell EA, Peden DR, Vardy AW, Gentet L, Lambert JJ. Neuroactive steroids and inhibitory neurotransmission: mechanisms of action and physiological relevance. Neuroscience 2005; 138:821-9. [PMID: 16310966 DOI: 10.1016/j.neuroscience.2005.07.021] [Citation(s) in RCA: 111] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2005] [Revised: 06/28/2005] [Accepted: 07/12/2005] [Indexed: 01/27/2023]
Abstract
Dysfunction of GABA(A) receptor-mediated inhibition is implicated in a number of neurological and psychiatric conditions including epilepsy and affective disorders. Some of these conditions have been associated with abnormal levels of certain endogenously occurring neurosteroids, which potently and selectively enhance the function of the brain's major inhibitory receptor, the GABA(A) receptor. Consistent with their ability to enhance neuronal inhibition, such steroids exhibit in animals and humans anxiolytic, anticonvulsant and anesthetic actions. Neurosteroids, exemplified by the potent progesterone metabolite, 5alpha-pregnan-3alpha-ol-20-one can be synthesized de novo in the CNS both in neurones and glia in levels sufficient to modulate GABA(A) receptor function. Neurosteroid levels are not static, but are subject to dynamic fluctuations, for example during stress, or the later stages of pregnancy. These observations suggest that these endogenous modulators may refine the function of the brain's major inhibitory receptor and thus, play an important physiological and pathophysiological role. However, given the ubiquitous expression of GABA(A) receptors throughout the mammalian CNS, changes in neurosteroid levels should be widely experienced, causing a generalized enhancement of neuronal inhibition. Such a non-specific action would seem incompatible with a physiological role. However, neurosteroid action is both brain region and neurone selective. This specificity results from a variety of molecular mechanisms including receptor subunit composition, local steroid metabolism and phosphorylation. This paper will evaluate the relative contribution these mechanisms play in defining the interaction of neurosteroids with synaptic and extra-synaptic GABA(A) receptors.
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Affiliation(s)
- D Belelli
- Neurosciences Institute, Division of Pathology and Neuroscience, University of Dundee, Ninewells Hospital and Medical School, UK.
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53
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Grasshoff C, Rudolph U, Antkowiak B. Molecular and systemic mechanisms of general anaesthesia: the ‘multi-site and multiple mechanisms’ concept. Curr Opin Anaesthesiol 2005; 18:386-91. [PMID: 16534263 DOI: 10.1097/01.aco.0000174961.90135.dc] [Citation(s) in RCA: 84] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
PURPOSE OF REVIEW Amnesia, hypnosis and immobility are essential components of general anaesthesia. This review highlights recent advances in our understanding of how these components are achieved at a molecular level. RECENT FINDINGS Commonly used volatile anaesthetic agents such as isoflurane or sevoflurane cause immobility by modulating multiple molecular targets predominantly in the spinal cord, including gamma-aminobutyric acidA receptors, glycine receptors, glutamate receptors and TREK-1 potassium channels. In contrast, intravenously applied drugs such as propofol or etomidate depress spinal motor reflexes almost exclusively via enhancing gamma-aminobutyric acidA receptor function. Studies on knock-in animals showed that etomidate and propofol act via gamma-aminobutyric acidA receptors containing beta3 subunits, whereas gamma-aminobutyric acidA receptors including alpha2 and gamma subunits mediate the myorelaxant properties of diazepam. These findings suggest that a large fraction of gamma-aminobutyric acidA receptors in the spinal cord assemble from alpha2, beta3 and most probably gamma2 subunits. The hypnotic actions of etomidate are mediated by beta3-containing gamma-aminobutyric acidA receptors expressed in the brain. In contrast, gamma-aminobutyric acidA receptors harbouring beta2 subunits produce sedation, but not hypnosis. Furthermore, there is growing evidence that extrasynaptic gamma-aminobutyric acidA receptors in the hippocampus containing alpha5 subunits contribute to amnesia. SUMMARY Clinical anaesthesia is based on drug actions at multiple anatomical sites in the brain. The finding that amnesia, hypnosis and immobility involve distinct molecular targets opens new avenues for developing improved therapeutic strategies in anaesthesia.
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54
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Abstract
GABA(A) (gamma-aminobutyric acid type A) receptors mediate most of the 'fast' synaptic inhibition in the mammalian brain and are targeted by many clinically important drugs. Certain naturally occurring pregnane steroids can potently and specifically enhance GABA(A) receptor function in a nongenomic (direct) manner, and consequently have anxiolytic, analgesic, anticonvulsant, sedative, hypnotic and anaesthetic properties. These steroids not only act as remote endocrine messengers, but also can be synthesized in the brain, where they modify neuronal activity locally by modulating GABA(A) receptor function. Such 'neurosteroids' can influence mood and behaviour in various physiological and pathophysiological situations, and might contribute to the behavioural effects of psychoactive drugs.
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Affiliation(s)
- Delia Belelli
- Neurosciences Institute, Division of Pathology and Neuroscience, Ninewells Hospital and Medical School, Dundee University, Dundee DD19SY, UK
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55
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Asproni B, Talani G, Busonero F, Pau A, Sanna S, Cerri R, Mascia MP, Sanna E, Biggio G. Synthesis, structure-activity relationships at the GABA(A) receptor in rat brain, and differential electrophysiological profile at the recombinant human GABA(A) receptor of a series of substituted 1,2-diphenylimidazoles. J Med Chem 2005; 48:2638-45. [PMID: 15801854 DOI: 10.1021/jm049120y] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
A series of new 1,2-diphenylimidazole derivatives (1a-x) were synthesized and evaluated for their ability to potentiate gamma-aminobutyric acid (GABA)-evoked currents in Xenopus laevis oocytes expressing recombinant human GABA(A) receptors. Many of these compounds enhanced GABA action with potencies (EC(50) = 0.19-19 muM) and efficacies (maximal efficacies of up to 640%) similar to or greater than those of anesthetics such as etomidate, propofol, and alphaxalone. Structure-activity relationship analysis revealed that the presence of an ester moiety in the imidazole ring was required for full agonist properties, while modifications made in the phenyl rings affected potency and efficacy, with ethyl 2-(4-bromophenyl)-1-(2,4-dichlorophenyl)-1H-4-imidazolecarboxylate showing the highest potency. These compounds potentiated the [(3)H]GABA binding to rat brain membranes, suggesting a site of interaction different from that of GABA. As for etomidate, mutation of asparagine-265 in the beta2 subunit of the GABA(A) receptor into serine reduced the ability of derivative 1i to modulate the GABA function.
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Affiliation(s)
- Battistina Asproni
- Dipartimento Farmaco Chimico Tossicologico, Università di Sassari, Sassari, Italy.
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56
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Caraiscos VB, Newell JG, You-Ten KE, Elliott EM, Rosahl TW, Wafford KA, MacDonald JF, Orser BA. Selective enhancement of tonic GABAergic inhibition in murine hippocampal neurons by low concentrations of the volatile anesthetic isoflurane. J Neurosci 2005; 24:8454-8. [PMID: 15456818 PMCID: PMC6729903 DOI: 10.1523/jneurosci.2063-04.2004] [Citation(s) in RCA: 108] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
Volatile (inhaled) anesthetics cause amnesia at concentrations well below those that cause loss of consciousness and immobility; however, the underlying neuronal mechanisms are unknown. Although many anesthetics increase inhibitory GABAergic synaptic transmission, this effect occurs only at high concentrations (>100 microm). Molecular targets for low concentrations of inhaled anesthetics have not been identified. Here, we report that a tonic inhibitory conductance in hippocampal pyramidal neurons generated by alpha5 subunit-containing GABA(A) receptors is highly sensitive to low concentrations of the volatile anesthetic isoflurane (ISO) (25 and 83.3 microm). The alpha5 subunit is necessary for enhancement of the tonic current by these low concentrations of isoflurane because potentiation is absent in neurons from alpha5-/- mice. Furthermore, ISO (25 microm) potentiated recombinant human alpha5beta3gamma2L GABA(A) receptors, whereas this effect was not seen with alpha1beta3gamma2L GABA(A) receptors. These studies suggest that an increased tonic inhibition in the hippocampus may contribute to amnestic properties of volatile anesthetics.
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Affiliation(s)
- Valerie B Caraiscos
- Institute of Medical Science, University of Toronto, Toronto, Ontario M5S 1A8, Canada
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57
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Bosman LWJ, Heinen K, Spijker S, Brussaard AB. Mice lacking the major adult GABAA receptor subtype have normal number of synapses, but retain juvenile IPSC kinetics until adulthood. J Neurophysiol 2005; 94:338-46. [PMID: 15758057 DOI: 10.1152/jn.00084.2005] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
There is a large variation in structurally and functionally different GABA(A) receptor subtypes. The expression pattern of GABA(A) receptor subunits is highly regulated, both temporarily and spatially. Especially during development, profound changes in subunit expression have been described. In most brain areas, the GABA(A) receptor alpha1 subunit replaces the alpha2 and/or alpha3 subunit as major alpha subunit. This is accompanied by a marked decrease in the open time of GABA(A) receptors and hence in the duration of postsynaptic responses. We describe here the development of GABAergic, synaptic transmission in mice lacking the alpha1 subunit. We show that alpha1 is to a large extent--but not entirely--responsible for the relatively short duration of postsynaptic responses in the developing and the mature brain. However, alpha1 already affects GABAergic transmission in the neonatal cerebral cortex when it is only sparsely expressed. It appears that the alpha1 -/- mice do not show a large reduction in GABAergic synapses but do retain long-lasting postsynaptic currents into adulthood. Hence, they form a good model to study the functional role of developmental GABA(A) receptor subunit switching.
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Affiliation(s)
- Laurens W J Bosman
- Department of Experimental Neurophysiology, Center for Neurogenomics and Cognitive Research, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
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58
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Abstract
Recently Urban (Br. J. Anaesth. 2002, 89, 167) and Trudell (Br. J. Anaesth. 2002, 89, 32) assessed the present state of the art in anesthesiological research. This article is an attempt to add to the discussion some ideas from the chemist's point of view. General anesthesia is a matter of molecular associations. Among the intermolecular interactions that can be involved, weak hydrogen bonding and van der Waals forces are believed to be most important. A pluralistic view is proposed, thereby different anesthetics can choose different interactions in conformity with their chemical structure. This can involve proteins, lipids, and sugars. Special attention is given to glycoproteins and glycosphingolipids. A review with 90 references.
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59
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60
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Abstract
The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.
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Affiliation(s)
- Joseph W Lynch
- School of Biomedical Sciences, Univ. of Queensland, Brisbane QLD 4072, Australia.
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61
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Xie Z, Currie KPM, Fox AP. Etomidate elevates intracellular calcium levels and promotes catecholamine secretion in bovine chromaffin cells. J Physiol 2004; 560:677-90. [PMID: 15331676 PMCID: PMC1665276 DOI: 10.1113/jphysiol.2004.070664] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
Etomidate, an intravenous imidazole general anaesthetic, is thought to produce anaesthesia by modulating or activating ionotropic Cl(-)-permeable GABA(A) receptors. Chromaffin cells are known to express functional GABA(A) receptors with properties similar to their neuronal counterparts. We have shown that activation of the GABA(A) receptors, with specific GABA(A) agonists, leads to cellular excitation. Our goal was to determine whether etomidate mimicked this response and to explore the functional consequences of this activation. Imaging experiments with the Ca(2+)-indicator dye fura-2 were used to assay [Ca(2+)](i). Bovine adrenal chromaffin cells were superfused with a variety of GABA(A)-selective drugs to determine their effects on [Ca(2+)](i). Amperometric measurements were used to assay catecholamine release in real-time. We show that bovine adrenal chromaffin cells were excited by etomidate at clinically relevant concentrations. Etomidate directly activated GABA(A) receptors found in chromaffin cells thereby elevating [Ca(2+)](i). The effects of etomidate were mimicked by the specific GABA(A) agonist muscimol and blocked by the specific antagonist bicuculline. Our data show that low concentrations of etomidate modulated GABA(A) receptor activation by muscimol. Blockade of voltage-dependent Ca(2+) channels prevented the elevation of [Ca(2+)](i) by GABA. Application of etomidate directly to the chromaffin cells elicited robust catecholamine secretion from these cells. The data indicate that clinically relevant concentrations of etomidate can directly activate GABA(A) receptors, which, due to the positive anion equilibrium potential, depolarizes chromaffin cells. This depolarization activates voltage-dependent Ca(2+) channels thereby stimulating catecholamine release. Our data suggest that circulating catecholamine levels may be elevated after etomidate application.
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Affiliation(s)
- Z Xie
- University of Chicago, Department of Anesthesia and Critical Care, 5841 S. Maryland, MC 4028, Chicago, IL 60637, USA.
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62
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Weir CJ, Ling ATY, Belelli D, Wildsmith JAW, Peters JA, Lambert JJ. The interaction of anaesthetic steroids with recombinant glycine and GABAA receptors. Br J Anaesth 2004; 92:704-11. [PMID: 15033889 DOI: 10.1093/bja/aeh125] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
BACKGROUND Anaesthetic steroids are established positive allosteric modulators of GABAA receptors, but little is known concerning steroid modulation of strychnine-sensitive glycine receptors, the principal mediators of fast, inhibitory neurotransmission in the brain stem and spinal cord. This study compared the modulatory actions of five anaesthetic pregnane steroids and two non-anaesthetic isomers at human recombinant alpha1 glycine and alpha1beta2gamma2L GABAA receptors. METHODS Recombinant alpha1 glycine or alpha1beta2gamma2L GABAA receptors were expressed in Xenopus laevis oocytes and agonist-evoked currents recorded under voltage-clamp. Steroid modulation of currents evoked by GABA, or glycine, was quantified by determining the potency (EC50) and maximal effect of the compounds. RESULTS The anaesthetics minaxolone (EC50=1.3 microM), Org20599 (EC50=1.1 microM) and alphaxalone (EC50=2.2 microM) enhanced currents mediated by GABAA receptors. The anaesthetics also enhanced currents mediated by glycine receptors, although with higher EC50 values (minaxolone 13.1 microM; Org20599=22.9 microM and alphaxalone=27.8 microM). The maximal enhancement (to 780-950% of control) produced by the three steroids acting at the GABAA receptor was similar, but currents evoked by glycine were potentiated with increasing effectiveness by alphaxalone (199%) <Org20599 (525%) <minaxolone (1197%). The anaesthetic isomers, 5alpha-pregnan-3alpha-ol-20-one and 5beta-pregnan-3alpha-ol-20-one (eltanolone) enhanced GABAA receptor-mediated currents with similar potency and efficacy, but only the former enhanced glycine, the latter causing inhibition. The non-anaesthetic steroids 5alpha-pregnan-3beta-ol-20-one and 5beta-pregnan-3beta-ol-20-one modulated neither GABAA, nor glycine, receptors. CONCLUSIONS The data demonstrate that structure-activity relationships for steroid modulation at glycine and GABAA receptors differ. Comparing the EC50 values reported here with free plasma concentrations during steroid-induced anaesthesia indicates that a selective modulation of GABAA receptor activity is likely to occur in vivo.
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Affiliation(s)
- C J Weir
- Department of Anaesthesia, Division of Pathology and Neuroscience, Ninewells Hospital and Medical School, The University of Dundee, Dundee DD1 9SY, UK.
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63
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Abstract
The GABAA receptor is a target of many general anesthetics. The low affinity of general anesthetics has complicated the search for the location of anesthetic binding sites. Attention has focused on two pairs of residues near the extracellular ends of the M2 and M3 membrane-spanning segments, alpha1Ser270/beta2Asn265 (15' M2) and alpha1Ala291/beta2Met286 (M3). In the 4-A resolution acetylcholine receptor structure, the aligned positions are separated by approximately 10 A. To determine whether these residues are part of a binding site for propofol, an intravenous anesthetic, we probed propofol's ability to protect cysteines substituted for these residues from modification by the sulfhydryl-specific reagent p-chloromercuribenzenesulfonate (pCMBS-). pCMBS- reacted with cysteines substituted at the four positions in the absence and presence of GABA. Because propofol binding induces conformational change in the GABAAreceptor, we needed to establish a reference state of the receptor to compare reaction rates in the absence and presence of propofol. We compared reaction rates in the presence of GABA with those in the presence of propofol +GABA. The GABA concentration was reduced to give a similar fraction of the maximal GABA current in both conditions. Propofol protected, in a concentration-dependent manner, the cysteine substituted for beta2Met286 from reaction with pCMBS-. Propofol did not protect the cysteine substituted for the aligned alpha1 subunit position or the 15' M2 segment Cys mutants in either subunit. We infer that propofol may bind near the extracellular end of the betasubunit M3 segment.
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Affiliation(s)
- Moez Bali
- Department of Physiology, Albert Einstein College of Medicine, Yeshiva University, Bronx, NY 10461, USA
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64
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Akabas MH. GABAA Receptor Structure–Function Studies: A Reexamination in Light of New Acetylcholine Receptor Structures. INTERNATIONAL REVIEW OF NEUROBIOLOGY 2004; 62:1-43. [PMID: 15530567 DOI: 10.1016/s0074-7742(04)62001-0] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Affiliation(s)
- Myles H Akabas
- Department of Physiology and Biophysics, Albert Einstein College of Medicine, Yeshiva University, Bronx, New York 10461, USA
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65
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Pau D, Belelli D, Callachan H, Peden DR, Dunlop JI, Peters JA, Guitart X, Gutierrez B, Lambert JJ. GABAA receptor modulation by the novel intravenous general anaesthetic E-6375. Neuropharmacology 2003; 45:1029-40. [PMID: 14614946 DOI: 10.1016/s0028-3908(03)00299-5] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
E-6375 (4-butoxy-2-[4-(2-cyanobenzoyl)-1-piperazinyl] pyrimidine hydrochloride) is a new intravenous general anaesthetic with an anaesthetic potency, in mice, comparable to propofol, or etomidate. Here, we examined the effect of E-6375 upon the GABAA receptor, a putative target of intravenous anaesthetic action. E-6375 reversibly enhanced GABA-evoked currents mediated by recombinant GABAA (alpha1beta2gamma2L) receptors expressed in Xenopus laevis oocytes, with little effect on NMDA- and kainate-evoked currents mediated by NR1a/NR2A and GluR1o/GluR2o glutamate receptors, respectively. E-6375 prolonged the decay of GABA-evoked miniature inhibitory postsynaptic currents recorded from rat Purkinje neurones demonstrating the anaesthetic also enhanced the activity of synaptic GABAA receptors. The GABA enhancing action of E-6375 on recombinant GABAA receptors was unaffected by the subtype of the alpha isoform (i.e. alphaxbeta2gamma2L; x=1-3) within the receptor, but was increased by the omission of the gamma2L subunit. Receptors incorporating beta2, or beta3, subunits were more sensitive to modulation by E-6375 than those containing the beta1 subunit. The selectivity of E-6375 was largely governed by the identity (serine or asparagine) of a single amino acid residue within the second transmembrane domain of the beta-subunit. The various in vivo actions of general anaesthetics may be mediated by GABAA receptor isoforms that have a differential distribution within the CNS. The identification of agents, such as E-6375, that discriminate between GABAA receptor subtypes may augur the development of general anaesthetics with an improved therapeutic profile.
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Affiliation(s)
- D Pau
- Neuroscience Institute, Department of Pharmacology and Neuroscience, Ninewells Hospital and Medical School, The University of Dundee, Dundee DD1 9SY, UK
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66
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Priestley CM, Williamson EM, Wafford KA, Sattelle DB. Thymol, a constituent of thyme essential oil, is a positive allosteric modulator of human GABA(A) receptors and a homo-oligomeric GABA receptor from Drosophila melanogaster. Br J Pharmacol 2003; 140:1363-72. [PMID: 14623762 PMCID: PMC1574153 DOI: 10.1038/sj.bjp.0705542] [Citation(s) in RCA: 233] [Impact Index Per Article: 11.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
The GABA-modulating and GABA-mimetic activities of the monoterpenoid thymol were explored on human GABAA and Drosophila melanogaster homomeric RDLac GABA receptors expressed in Xenopus laevis oocytes, voltage-clamped at -60 mV. The site of action of thymol was also investigated. Thymol, 1-100 microm, resulted in a dose-dependent potentiation of the EC20 GABA response in oocytes injected with either alpha1beta3gamma2s GABAA subunit cDNAs or the RDLac subunit RNA. At 100 microm thymol, current amplitudes in response to GABA were 416+/-72 and 715+/-85% of controls, respectively. On both receptors, thymol, 100 microm, elicited small currents in the absence of GABA. The EC50 for GABA at alpha1beta3gamma2s GABAA receptors was reduced by 50 microm thymol from 15+/-3 to 4+/-1 microm, and the Hill slope changed from 1.35+/-0.14 to 1.04+/-0.16; there was little effect on the maximum GABA response. Thymol (1-100 microm) potentiation of responses to EC20 GABA for alpha1beta1gamma2s, alpha6beta3gamma2s and alpha1beta3gamma2s human GABAA receptors was almost identical, arguing against actions at benzodiazepine or loreclezole sites. Neither flumazenil, 3-hydroxymethyl-beta-carboline (3-HMC), nor 5alpha-pregnane-3alpha, 20alpha-diol (5alpha-pregnanediol) affected thymol potentiation of the GABA response at alpha1beta3gamma2s receptors, providing evidence against actions at the benzodiazepine/beta-carboline or steroid sites. Thymol stimulated the agonist actions of pentobarbital and propofol on alpha1beta3gamma2s receptors, consistent with a mode of action distinct from that of either compound. These data suggest that thymol potentiates GABAA receptors through a previously unidentified binding site.
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Affiliation(s)
- Caroline M Priestley
- Department of Pharmaceutical and Biological Chemistry, The School of Pharmacy, 29-39 Brunswick Square, London WC1N 1AX.
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67
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Abstract
Certain metabolites of progesterone and deoxycorticosterone are established as potent and selective positive allosteric modulators of the gamma-aminobutyric acid type A (GABA(A)) receptor. Upon administration these steroids exhibit clear behavioural effects that include anxiolysis, sedation and analgesia, they are anticonvulsant and at high doses induce a state of general anaesthesia, a profile consistent with an action to enhance neuronal inhibition. Physiologically, peripherally synthesised pregnane steroids derived from endocrine glands such as the adrenals and ovaries function as hormones by crossing the blood brain barrier to influence neuronal signalling. However, the demonstration that certain neurons and glial cells within the central nervous system (CNS) can synthesize these steroids either de novo, or from peripherally derived progesterone, has led to the proposal that these steroids (neurosteroids) can additionally function in a paracrine manner, to locally influence GABAergic transmission. Steroid levels are known to change dynamically, for example in stress and during pregnancy. Given that GABA(A) receptors are ubiquitously expressed throughout the central nervous system, such changes in steroid levels would be predicted to cause a global enhancement of inhibitory neurotransmission throughout the brain, a scenario that would seem incompatible with a physiological role as a selective neuromodulator. Here, we will review emerging evidence that the GABA-modulatory actions of the pregnane steroids are highly selective, with their actions being brain region and indeed neuron dependent. Furthermore, the sensitivity of GABA(A) receptors is not static but can dynamically change. The molecular mechanisms underpinning this neuronal specificity will be discussed with particular emphasis being given to the role of GABA(A) receptor isoforms, protein phosphorylation and local steroid metabolism and synthesis.
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Affiliation(s)
- Jeremy J Lambert
- The Department of Pharmacology and Neuroscience, The Neuroscience Institute, Ninewells Hospital and Medical School, Dundee University, Dundee DD1 9SY, Scotland, UK.
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68
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Abstract
A number of important drugs act on GABA(A) receptors, pentameric GABA-gated chloride channels assembled from among 19 known subunits. In trying to discover the roles in the brain of the subunits and their combinations, with the goal of developing more selective drugs, one tool has been to reduce expression of the subunits and examine the functional consequences. After briefly examining the properties of GABA(A) receptors, this review surveys the means available for receptor subunit reduction, and some of the observations to which their application has led. The methods discussed include radiation-induced deletion, gene knockout, knock-in mutations, antisense, ribozymes, RNA interference, dominant negative constructs, and transcriptional regulation, e.g., via decoy oligonucleotides.
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Affiliation(s)
- David R Burt
- Department of Pharmacology, University of Maryland School of Medicine, 655 W. Baltimore Street, Baltimore, MD 21201-1559, USA.
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69
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Chesnoy-Marchais D. Potentiation of glycine responses by dideoxyforskolin and tamoxifen in rat spinal neurons. Eur J Neurosci 2003; 17:681-91. [PMID: 12603258 DOI: 10.1046/j.1460-9568.2003.02481.x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Dideoxyforskolin, a forskolin analogue unable to stimulate adenylate cyclase, and tamoxifen, an antioestrogen widely used against breast cancer, are both known to block some Cl- channels. Their effects on Cl- responses to glycine or GABA have been tested here by using whole-cell recording from cultured spinal neurons. Dideoxyforskolin (4 or 16 microm) and tamoxifen (0.2-5 microm) both potentiate responses to low glycine concentrations. They also induce blocking effects, predominant at high glycine concentrations. At 5 microm, tamoxifen increased responses to 15 microm glycine by a factor >4.5, reaching 20 in some neurons. Potentiation by extracellular dideoxyforskolin or tamoxifen persisted after intracellular application of the modulator and was not due to Zn2+ contamination. Potentiation by tamoxifen also persisted in a Ca2+-free extracellular solution, after intracellular Ca2+ buffering and protein kinase C blockade. Thus, the critical sites of action are not intracellular. The EC50 for glycine was lowered 6.6-fold by 5 microm tamoxifen. The kinetics and voltage-dependence of the effects of tamoxifen on glycine responses support the idea that this hydrophobic drug may act from a site located within the membrane. Tamoxifen (5 micro m) also increased responses to 2 micro m GABA by a factor of 3.5, but barely affected peak responses to 20 microm GABA. The demonstration that tamoxifen affects some of the main inhibitory receptors should be useful for better evaluating its neurological effects. Furthermore, the results identify a new class of molecules that potentiate glycine receptor function.
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Affiliation(s)
- Dominique Chesnoy-Marchais
- Laboratoire de Neurobiologie Moléculaire et Cellulaire, CNRS UMR-8544, Ecole Normale Supérieure, 46 rue d'Ulm, 75005, Paris, France.
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Wan X, Puil E. Pentobarbital depressant effects are independent of GABA receptors in auditory thalamic neurons. J Neurophysiol 2002; 88:3067-77. [PMID: 12466430 DOI: 10.1152/jn.00365.2002] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
Pentobarbital, a general anesthetic, has received extensive study for its ability to potentiate inhibition at GABA(A) subtype of receptors for GABA. Using whole cell current-clamp techniques and bath applications, we determined the effects of pentobarbital and GABA receptor antagonists on the membrane properties and tonic or burst firing of medial geniculate neurons in thalamic slices. Pentobarbital (0.01-200 microM) induced depressant effects in 50 of 66 neurons (76%). Pentobarbital hyperpolarized neurons by a mean of 3 mV and decreased the number of action potentials in tonic firing, evoked by current pulse injection from near the resting potential. Pentobarbital also decreased burst firing or low threshold Ca(2+)-spikes, evoked by current pulse injection into neurons at potentials hyperpolarized from rest. The blockade of tonic and burst firing, as well as low threshold Ca(2+)-spikes, was surmountable by increasing the amplitude of input current. The GABA(A) receptor antagonists, bicuculline (100 microM) and picrotoxinin (50-100 microM), did not block the depressant effects of pentobarbital (10 microM). The GABA(B) receptor antagonist, saclofen (200 microM), and GABA(C) receptor antagonist, (1,2,3,6-tetrahydropyridine-4-yl)methylphosphinate (10-50 microM), did not significantly alter the depressant effects. Pentobarbital produced excitatory effects (0.1-50 microM) on 11 neurons (17%) but had no effects on 5 neurons (7%). The excitation consisted of approximately 3 mV depolarization, increased tonic and burst firing and the rate of rise and amplitude of low threshold Ca(2+) spikes. These effects were associated with a increase in input resistance. In contrast, the depressant effects of pentobarbital correlated to a decreased input resistance measured with hyperpolarizing current pulse injection (IC(50) = 7.8 microM). Pentobarbital reduced Na(+)-dependent rectification on depolarization and lowered the slope resistance over a wide voltage range. Tetrodotoxin eliminated both Na(+)-dependent rectification and the pentobarbital-induced decrease in membrane resistance at depolarized voltages in two-thirds of the neurons. The pentobarbital-induced decrease in membrane resistance at voltages hyperpolarized from rest was not evident during co-application with Cs(+), known to block the hyperpolarization-activated rectifiers. In summary, the pentobarbital acted at low concentrations to depress thalamocortical neurons. The depression resulted from decreased rectification on depolarization, which no longer boosted potentials over threshold, and an increased conductance that shunted spike generation. The depressant effects of pentobarbital did not involve known types of GABA receptor interactions.
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Affiliation(s)
- Xiang Wan
- Department of Pharmacology and Therapeutics and Department of Anesthesia, The University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada
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71
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72
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Lu H, Xu TL. The general anesthetic pentobarbital slows desensitization and deactivation of the glycine receptor in the rat spinal dorsal horn neurons. J Biol Chem 2002; 277:41369-78. [PMID: 12196533 DOI: 10.1074/jbc.m206768200] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Although many general anesthetics have been found to produce anesthetic and analgesic effects by augmenting GABA(A) receptor (GABA(A)R) function, the role of the glycine receptor (GlyR) in this process is not fully understood at the neuronal level in the spinal cord. We investigated the effects of a barbiturate general anesthetic, pentobarbital (PB), on the glycinergic miniature inhibitory postsynaptic currents (mIPSCs) and the responses to exogenously applied glycine, or taurine, a low affinity GlyR agonist, by using the whole-cell patch-clamp technique in the rat spinal dorsal horn neurons isolated using a novel mechanical method. Bath application of 30 microm PB significantly prolonged the decay time constant of the spontaneous glycinergic mIPSC without changing its amplitude and frequency. Co-application of 0.3 mm PB reduced the peak amplitude, affected the macroscopic desensitization and deactivation of the response to externally applied Gly in a concentration-dependent manner. In addition, the recovery of Gly response from desensitization was also prolonged by PB. However, PB did not change the desensitization and deactivation kinetics of the taurine-induced response. The GABA(A)R antagonist bicuculline (10 microm) did not affect the effect of PB on the Gly response. Thus, PB prolonged the spinal glycinergic mIPSCs by slowing desensitization and deactivation of GlyR. Two other structurally different intravenous anesthetics, i.e. propofol (10 microm) and etomidate (3 microm), prolonged the duration of the glycinergic mIPSC in the rat spinal dorsal horn neurons. In conclusion, on GlyR-Cl(-) channel complexes there may exist action site(s) of intravenous general anesthetics. GlyR and glycinergic neurotransmission may play an important role in the modulation of general anesthesia in the mammalian spinal cord.
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Affiliation(s)
- Hui Lu
- Laboratory of Receptor Pharmacology, Department of Neurobiology and Biophysics, University of Science and Technology of China, Hefei 230027, People's Republic of China
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73
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Nelson LE, Guo TZ, Lu J, Saper CB, Franks NP, Maze M. The sedative component of anesthesia is mediated by GABA(A) receptors in an endogenous sleep pathway. Nat Neurosci 2002; 5:979-84. [PMID: 12195434 DOI: 10.1038/nn913] [Citation(s) in RCA: 384] [Impact Index Per Article: 17.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2002] [Accepted: 08/02/2002] [Indexed: 01/22/2023]
Abstract
We investigated the role of regionally discrete GABA (gamma-aminobutyric acid) receptors in the sedative response to pharmacological agents that act on GABA(A) receptors (muscimol, propofol and pentobarbital; 'GABAergic agents') and to ketamine, a general anesthetic that does not affect GABA(A) receptors. Behavioral studies in rats showed that the sedative response to centrally administered GABAergic agents was attenuated by the GABA(A) receptor antagonist gabazine (systemically administered). The sedative response to ketamine, by contrast, was unaffected by gabazine. Using c-Fos as a marker of neuronal activation, we identified a possible role for the tuberomammillary nucleus (TMN): when gabazine was microinjected directly into the TMN, it attenuated the sedative response to GABAergic agents. Furthermore, the GABA(A) receptor agonist muscimol produced a dose-dependent sedation when it was administered into the TMN. We conclude that the TMN is a discrete neural locus that has a key role in the sedative response to GABAergic anesthetics.
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Affiliation(s)
- L E Nelson
- Department of Anaesthetics & Intensive Care, Chelsea & Westminster Hospital, Imperial College School of Medicine, London SW10 9NH, UK
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74
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Dong XP, Xu TL. The actions of propofol on gamma-aminobutyric acid-A and glycine receptors in acutely dissociated spinal dorsal horn neurons of the rat. Anesth Analg 2002; 95:907-14, table of contents. [PMID: 12351266 DOI: 10.1097/00000539-200210000-00021] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
UNLABELLED The spinal cord plays an important role in modulating anesthetic-induced suppression of nociceptive transmission. To gain some insight into the anesthetic mechanisms of propofol at the spinal level, we investigated the direct action of propofol and its modulation on the gamma-aminobutyric acid-A receptor (GABA(A)R) and the glycine receptor (GlyR) in acutely dissociated rat spinal dorsal horn neurons by using whole-cell patch-clamp electrophysiology. Propofol induced Cl(-) currents (I(Cl)), which were sensitive to bicuculline and, to a lesser extent, to strychnine. The activation, desensitization, and deactivation of propofol-induced I(Cl) were slower than those of GABA- and glycine-induced I(Cl). In addition, this study revealed similar modulatory actions of propofol on GABA(A)R and GlyR. Propofol potentiated both GABA- and glycine-induced I(Cl) at small con-centrations and inhibited both GABA- and glycine-induced I(Cl) at large concentrations. The potentiation of propofol on I(Cl) was caused by slowing current desensitization and deactivation, whereas the inhibition actions might be involved in the cross-desensitization between GABA- and propofol-induced I(Cl) and the cross-inhibition between the GABA(A)R and GlyR. The results suggest that propofol facilitation of GABA(A)R and GlyR at the spinal level could contribute significantly to general anesthetic-induced analgesia and anesthesia. IMPLICATIONS The actions of propofol on the gamma-aminobutyric acid-A receptor (GABA(A)R) and the glycine receptor (GlyR) were investigated in acutely dissociated rat spinal dorsal horn neurons by using whole-cell patch-clamp electrophysiology. Propofol was found to potentiate the functions of GABA(A)R and GlyR at the spinal level, which might contribute to propofol-induced analgesia and anesthesia.
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Affiliation(s)
- Xian-Ping Dong
- Laboratory of Receptor Pharmacology, Department of Neurobiology and Biophysics, University of Science and Technology of China, Hefei, People's Republic of China
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75
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The Actions of Propofol on γ-Aminobutyric Acid-A and Glycine Receptors in Acutely Dissociated Spinal Dorsal Horn Neurons of the Rat. Anesth Analg 2002. [DOI: 10.1213/00000539-200210000-00021] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
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76
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Beckstead MJ, Phelan R, Trudell JR, Bianchini MJ, Mihic SJ. Anesthetic and ethanol effects on spontaneously opening glycine receptor channels. J Neurochem 2002; 82:1343-51. [PMID: 12354281 DOI: 10.1046/j.1471-4159.2002.01086.x] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Strychnine-sensitive glycine receptors mediate inhibitory neurotransmission occurring in the brain stem and spinal cord. Alcohols, volatile anesthetics and inhaled drugs of abuse are positive allosteric modulators of glycine receptor function, normally enhancing function only in the presence of glycine. A complication in studying allosteric actions on ligand-gated ion channels is in the dissection of their effects on neurotransmitter binding from their effects on channel opening. Mutation of an aspartate residue at position 97 to arginine in the glycine receptor alpha1 subunit simulated the effects of glycine binding, producing receptors that exhibited tonic channel opening in the absence of neurotransmitter; i.e. these receptors demonstrated a dissociation of channel opening from neurotransmitter binding. In these receptors, ethanol, enflurane, chloroform, halothane, 1,1,1-trichloroethane and toluene elicited inward currents in the absence of glycine. We previously identified mutations on ligand-gated ion channels that eliminate ethanol, anesthetic and inhalant actions (such as S267I on alpha1 glycine receptors). The double mutant (D97R and S267I) receptors were both constitutively active and resistant to the enhancing effects of ethanol and enflurane. These data demonstrate that ethanol and volatile anesthetics can affect glycine receptor channel opening independently of their effects on enhancing neurotransmitter binding.
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Affiliation(s)
- Michael J Beckstead
- Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Winston-Salem, North Carolina, USA
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77
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Affiliation(s)
- B Antkowiak
- Department of Anaesthesiology, University of Tübingen, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
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78
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Abstract
We report the occurrence of delayed postoperative coma in five of nine consecutive children following surgery for insertion of baclofen infusion pumps. We suspect that improper technique resulted in an inadvertent administration of an intrathecal baclofen bolus. We suggest ways to minimize this risk.
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Affiliation(s)
- Keith J Anderson
- Department of Pediatric Anaesthesia, Neurosurgery and Oncology Montreal Childrens' Hospital, McGill University Healthcare Center, Montreal, Quebec, Canada.
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79
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Engblom AC, Carlson BX, Olsen RW, Schousboe A, Kristiansen U. Point mutation in the first transmembrane region of the beta 2 subunit of the gamma--aminobutyric acid type A receptor alters desensitization kinetics of gamma--aminobutyric acid- and anesthetic-induced channel gating. J Biol Chem 2002; 277:17438-47. [PMID: 11877425 DOI: 10.1074/jbc.m111215200] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
A conserved glycine residue in the first transmembrane (TM1) domain of the beta2 subunit has been identified to be involved with desensitization induced by gamma-aminobutyric acid (GABA) and anesthetics. Recombinant GABA(A) receptors expressed in Sf9 cells were recorded using semi-fast agonist application. Upon direct activation by GABA or anesthetics, the main effect of the TM1 point mutation on the beta2 subunit (G219F) was to slow the time constant (tau) of desensitization. At GABA concentrations eliciting maximum currents, the corresponding median tau values were 0.87 s (25-75% interval (0.76; 1.04 s)), 0.93 s (0.76; 1.23 s), and 1.36 s (1.17; 1.57 s) for alpha1beta2gamma2, alpha1(G223F)beta2gamma2, and alpha1beta2(G219F)gamma2, respectively. The tau value for the beta2-mutant receptor was significantly longer than alpha1beta2gamma2 (p < 0.01) and alpha1(G223F)beta2gamma2 (p < 0.05). For pentobarbital-induced currents (500 microm), the corresponding median tau values were 1.36 s (0.81; 1.41 s), 1.47 s (1.31; 2.38 s), and 2.82 s (2.21; 5.56 s) for alpha1beta2gamma2, alpha1(G223F)beta2gamma2, and alpha1beta2(G219F)gamma2, respectively. The tau value for the beta2-mutant receptor was significantly longer than that for alpha1beta2gamma2 (p < 0.01). The present findings suggest that this TM1 glycine residue is critical for the rate at which desensitization occurs and that both GABA and intravenous anesthetics implement an analogous pathway for generating desensitization.
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Affiliation(s)
- A Christine Engblom
- Department of Pharmacology, The Royal Danish School of Pharmacy, Copenhagen 2100, Denmark
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80
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Coyne L, Lees G, Nicholson RA, Zheng J, Neufield KD. The sleep hormone oleamide modulates inhibitory ionotropic receptors in mammalian CNS in vitro. Br J Pharmacol 2002; 135:1977-87. [PMID: 11959801 PMCID: PMC1573315 DOI: 10.1038/sj.bjp.0704651] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2001] [Revised: 01/29/2002] [Accepted: 02/04/2002] [Indexed: 11/09/2022] Open
Abstract
1. We examine the sensitivity of GABA(A) and glycine receptors (same ionotropic superfamily) to oleamide. We address subunit-dependence/modulatory mechanisms and analogies with depressant drugs. 2. Oleamide modulated human GABA(A) currents (alpha(1)beta(2)gamma(2L)) in oocytes (EC(50), 28.94+/-s.e.mean of 1.4 microM; Maximum 216%+/-35 of control, n=4). Modulation of human alpha1 glycine homo-oligomers (significant), was less marked, with a lower EC(50) (P<0.05) than GABA receptors (EC(50), 22.12+/-1.4 microM; Maximum 171%+/-30, n=11). 3. Only the hypnogenic cis geometric isomer enhanced glycine currents (without altering slope or maximal current, it reduced the glycine EC(50) from 322 to 239 microM: P<0.001). Modulation was not voltage-dependent or associated with a shift in E(r). 4. beta 1 containing GABA(A) receptors (insensitive to many depressant drugs) were positively modulated by oleamide. Oleamide efficacy was circa 2x greater at alpha(1)beta(1)gamma(2L) than alpha(1)beta(2)gamma(2L) (P=0.007). Splice variation in gamma subunits did not alter oleamide sensitivity. 5. cis-9,10-Octadecenoamide had no effect on the equilibrium binding of [(3)H]-muscimol or [(3)H]-EBOB to mouse brain membranes. It does not directly mimic GABA, or operate as a neurosteroid-, benzodiazepine- or barbiturate-like modulator of GABA(A)-receptors. 6. The transport of [(3)H]-GABA into mouse brain synaptoneurosomes was unaffected by high micromolar concentrations of cis-9,10-octadecenoamide. Oleamide does not enhance GABA-ergic currents or prolong IPSCs by inhibiting GABA transport. 7. Oleamide is a non-selective modulator of inhibitory ionotropic receptors. The sleep lipid exerts its effects indirectly, or at a novel recognition site on the GABA(A) complex.
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Affiliation(s)
- Leanne Coyne
- Institute of Pharmacy, Chemistry & Biomedical Sciences, School of Sciences, University of Sunderland, Wharncliffe Street, Sunderland SR1 3SD
| | - George Lees
- Institute of Pharmacy, Chemistry & Biomedical Sciences, School of Sciences, University of Sunderland, Wharncliffe Street, Sunderland SR1 3SD
| | - Russell A Nicholson
- Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada
| | - Jian Zheng
- Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada
| | - Katherine D Neufield
- Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada
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81
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Downie DL, Vicente-Agullo F, Campos-Caro A, Bushell TJ, Lieb WR, Franks NP. Determinants of the anesthetic sensitivity of neuronal nicotinic acetylcholine receptors. J Biol Chem 2002; 277:10367-73. [PMID: 11741933 DOI: 10.1074/jbc.m107847200] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Some neurotransmitter-gated ion channels are very much more sensitive to general anesthetics than others, even when they are genetically and structurally related. The most striking example of this is the extreme sensitivity of heteromeric neuronal nicotinic acetylcholine receptors to inhalational general anesthetics compared with the marked insensitivity of the closely related homomeric neuronal nicotinic receptors. Here we investigate the role of the alpha subunit in determining the anesthetic sensitivity of these receptors by using alpha(3)/alpha(7) chimeric subunits that are able to form functional homomeric receptors. By comparing the sensitivities of a number of chimeras to the inhalational agent halothane we show that the short (13 amino acids) putative extracellular loop connecting the second and third transmembrane segments is a critical determinant of anesthetic sensitivity. In addition, using site-directed mutagenesis, we show that two particular amino acids in this loop play a dominant role. When mutations are made in this loop, there is a good correlation between increasing anesthetic sensitivity and decreasing acetylcholine sensitivity. We conclude that this extracellular loop probably does not participate directly in anesthetic binding, but rather determines receptor sensitivity indirectly by playing a critical role in transducing anesthetic binding into an effect on channel gating.
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Affiliation(s)
- David Lindsay Downie
- Biophysics Group, The Blackett Laboratory, Imperial College of Science, Technology & Medicine, London SW7 2BW, United Kingdom
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82
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Lambert JJ, Harney SC, Belelli D, Peters JA. Neurosteroid modulation of recombinant and synaptic GABAA receptors. INTERNATIONAL REVIEW OF NEUROBIOLOGY 2002; 46:177-205. [PMID: 11599300 DOI: 10.1016/s0074-7742(01)46063-6] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/21/2023]
Abstract
Certain pregnane steroids are now established as potent, positive allosteric modulators of the gamma-aminobutyric acid type A (GABAA) receptor. These compounds are known to be synthesized in the periphery by endocrine glands, such as the ovaries and the adrenal glands, and can rapidly cross the blood-brain barrier. Therefore, such steroids could act as endogeneous modulators of the major inhibitory receptor in the mammalian central nervous system. However, the demonstration that certain neurons and glia can synthesize the pregnane steroids (i.e., neurosteroids) additionally suggests that they may serve a paracrine role by influencing GABAA-receptor function through their local release in the brain itself. Here, we demonstrate that these neurosteroids are highly selective and extremely potent modulators of the GABAA receptor. The subunit composition of the GABAA receptor may influence the actions of the neurosteroids, particularly when considering concentrations of these agents thought to occur physiologically, which may underlie their reported differential effects at certain inhibitory synapses. However, recent work suggests that the phosphorylation status of either the synaptic GABAA receptor or its associated proteins may also influence neurosteroid sensitivity; these findings are discussed. Upon administration, the neurosteroids exhibit clear behavioral effects, including sedation, anticonvulsant actions, and behaviors predictive of anxiolysis; when given at high doses, they induce general anesthesia. Numerous synthetic steroids have been synthesized in an attempt to therapeutically exploit these properties, and these data are reviewed in this chapter. However, targeting the brain enzymes that synthesize and metabolize the neurosteroids may offer a new approach to exploit this novel endocrine-paracrine neurotransmitter interaction.
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Affiliation(s)
- J J Lambert
- Department of Pharmacology and Neuroscience, Ninewells Hospital and Medical School, Dundee University, Dundee, DD1 9SY Scotland
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83
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Christopoulos A. Allosteric binding sites on cell-surface receptors: novel targets for drug discovery. Nat Rev Drug Discov 2002; 1:198-210. [PMID: 12120504 DOI: 10.1038/nrd746] [Citation(s) in RCA: 493] [Impact Index Per Article: 22.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Cell-surface receptors are the targets for more than 60% of current drugs. Traditionally, optimizing the interaction of lead molecules with the binding site for the endogenous agonist (orthosteric site) has been viewed as the best means of achieving selectivity of action. However, recent developments have highlighted the fact that drugs can interact with binding sites on the receptor molecule that are distinct from the orthosteric site, known as allosteric sites. Allosteric modulators could offer several advantages over orthosteric ligands, including greater selectivity and saturability of their effect.
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Affiliation(s)
- Arthur Christopoulos
- Department of Pharmacology, University of Melbourne, Grattan Street, Parkville, Victoria 3010, Australia.
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84
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Krasowski MD, Nishikawa K, Nikolaeva N, Lin A, Harrison N. Methionine 286 in transmembrane domain 3 of the GABAA receptor beta subunit controls a binding cavity for propofol and other alkylphenol general anesthetics. Neuropharmacology 2001; 41:952-64. [PMID: 11747900 PMCID: PMC2855216 DOI: 10.1016/s0028-3908(01)00141-1] [Citation(s) in RCA: 100] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
gamma-Aminobutyric acid type A (GABA(A)) receptors are an important target for general anesthetics in the central nervous system. Site-directed mutagenesis techniques have identified amino acid residues that are important for the positive modulation of GABA(A) receptors by general anesthetics. In the present study, we investigate the role of an amino acid residue in transmembrane (TM) domain 3 of the GABA(A) receptor beta(2) subunit for modulation by the general anesthetic 2,6-diisopropylphenol (propofol). Mutation of methionine 286 to tryptophan (M286W) in the beta(2) subunit abolished potentiation of GABA responses by propofol but did not affect direct receptor activation by propofol in the absence of GABA. In contrast, substitution of methionine 286 by alanine, cysteine, glutamate, lysine, phenylalanine, serine, or tyrosine was permissive for potentiation of GABA responses and direct activation by propofol. Using propofol analogs of varying molecular size, we show that the beta(2)(M286W) mutation resulted in a decrease in the 'cut-off' volume for propofol analog molecules to enhance GABA responses at GABA(A) alpha(1)beta(2)gamma(2s) receptors. This suggests that mutation of M286 in the GABA(A) beta(2) subunit alters the dimensions of a 'binding pocket' for propofol and related alkylphenol general anesthetics.
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Affiliation(s)
- M. D. Krasowski
- Department of Anesthesia and Critical Care, University of Chicago, Chicago, IL 60637, USA
- Committee on Neurobiology, University of Chicago, Chicago, IL 60637, USA
| | - K. Nishikawa
- Department of Anesthesiology, A-1050, Weill Medical College of Cornell University, 525 East 68 Street, New York, NY 10021, USA
| | - N. Nikolaeva
- Department of Anesthesia and Critical Care, University of Chicago, Chicago, IL 60637, USA
| | - A. Lin
- Department of Anesthesia and Critical Care, University of Chicago, Chicago, IL 60637, USA
| | - N.L. Harrison
- Department of Anesthesiology, A-1050, Weill Medical College of Cornell University, 525 East 68 Street, New York, NY 10021, USA
- Corresponding author: Tel. +1-212-746-5325; fax: +1-212-746-4879. (N.L. Harrison)
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85
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Lambert JJ, Belelli D, Harney SC, Peters JA, Frenguelli BG. Modulation of native and recombinant GABA(A) receptors by endogenous and synthetic neuroactive steroids. BRAIN RESEARCH. BRAIN RESEARCH REVIEWS 2001; 37:68-80. [PMID: 11744075 DOI: 10.1016/s0165-0173(01)00124-2] [Citation(s) in RCA: 132] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Upon administration, certain pregnane steroids produce clear behavioural effects including, anxiolysis, sedation, analgesia, anaesthesia and are anti-convulsant. This behavioural profile is characteristic of compounds that act to enhance the actions of GABA acting at the GABA(A) receptor. In agreement, numerous studies have now demonstrated these steroids to be potent, positive allosteric modulators of the GABA(A) receptor. The pregnane steroids are synthesized in the periphery by endocrine glands such as the adrenals and the ovaries, but are also made by neurons and glial cells in the central nervous system itself. Hence, these compounds could play both an endocrine and a paracrine role to influence neuronal excitability by promoting inhibition. Here we review evidence that the pregnane steroids are highly selective and extremely potent GABA(A) receptor modulators and that their effects at 'physiological' concentrations (low nanomolar) may be influenced by the subunit composition of the GABA(A) receptor. This feature may underlie recent findings demonstrating the effects of the neurosteroids on inhibitory synaptic transmission to be brain region dependent, although recent reports suggest that phosphorylation mechanisms may additionally influence neurosteroid sensitivity of the GABA(A) receptor. Numerous synthetic steroids have been synthesized in an attempt to therapeutically exploit the behavioural effects of the pregnane steroids and progress with this approach will be discussed. However, the demonstration that the steroids may be made within the central nervous system offers the alternative strategy of targeting the enzymes that synthesize/metabolise the neurosteroids to exploit this novel endocrine/paracrine interaction.
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Affiliation(s)
- J J Lambert
- Department of Pharmacology and Neuroscience, Neurosciences Institute, University of Dundee, Ninewells Hospital and Medical School, Ninewells Hospital, DD1 9SY, Dundee, UK.
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86
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Anderson A, Belelli D, Bennett DJ, Buchanan KI, Casula A, Cooke A, Feilden H, Gemmell DK, Hamilton NM, Hutchinson EJ, Lambert JJ, Maidment MS, McGuire R, McPhail P, Miller S, Muntoni A, Peters JA, Sansbury FH, Stevenson D, Sundaram H. Alpha-amino acid phenolic ester derivatives: novel water-soluble general anesthetic agents which allosterically modulate GABA(A) receptors. J Med Chem 2001; 44:3582-91. [PMID: 11606122 DOI: 10.1021/jm010903i] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
In the search for a novel water-soluble general anesthetic agent the activity of an alpha-amino acid phenolic ester lead, identified from patent literature, was markedly improved. In addition to improving in vivo activity in mice, good in vitro activity at GABA(A) receptors was also conferred. Within the series of compounds good enantioselectivity for both in vitro and in vivo activity was found, supporting a protein-mediated mechanism of action for anesthesia involving allosteric modulation of GABA(A) receptors. alpha-Amino acid phenolic ester 19, as the hydrobromide salt Org 25435, was selected for clinical evaluation since it retained the best overall anesthetic profile coupled with improved stability and water solubility. In the clinic it proved to be an effective intravenous anesthetic in man with rapid onset of and recovery from anesthesia at doses of 3 and 4 mg/kg.
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Affiliation(s)
- A Anderson
- Organon Research, Newhouse, Lanarkshire ML1 5SH, Scotland, U.K
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Craig AD, Krout K, Andrew D. Quantitative response characteristics of thermoreceptive and nociceptive lamina I spinothalamic neurons in the cat. J Neurophysiol 2001; 86:1459-80. [PMID: 11535691 DOI: 10.1152/jn.2001.86.3.1459] [Citation(s) in RCA: 136] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The physiological characteristics of antidromically identified lamina I spinothalamic (STT) neurons in the lumbosacral spinal cord were examined using quantitative thermal and mechanical stimuli in barbiturate-anesthetized cats. Cells belonging to the three main recognized classes were included based on categorization with natural cutaneous stimulation of the hindpaw: nociceptive-specific (NS), polymodal nociceptive (HPC), or thermoreceptive-specific (COOL) cells. The mean central conduction latencies of these classes differed significantly; NS = 130.8 +/- 55.5 (SD) ms (n = 100), HPC = 72.1 +/- 28.0 ms (n = 128), and COOL = 58.6 +/- 25.3 ms (n = 136), which correspond to conduction velocities of 2.5, 4.6, and 5.6 m/s. Based on recordings made prior to any noxious stimulation, the mean spontaneous discharge rates of these classes also differed: NS = 0.5 +/- 0.7 imp/s (n = 47), HPC = 0.9 +/- 0.7 imp/s (n = 59), and COOL = 3.3 +/- 2.6 imp/s (n = 107). Standard, quantitative, thermal stimulus sequences applied with a Peltier thermode were used to characterize the stimulus-response functions of 76 COOL cells, 47 HPC cells, and 37 NS cells. The COOL cells showed a very linear output from 34 degrees C down to approximately 15 degrees C and a maintained plateau thereafter. The HPC cells showed a fairly linear but accelerating response to cold below a median threshold of approximately 24 degrees C and down to 9 degrees C (measured at the skin-thermode interface with a thermode temperature of 2 degrees C). The HPC cells and the NS cells both showed rapidly increasing, sigmoidal response functions to noxious heat with a fairly linear response between 45 and 53 degrees C, but they had significantly different thresholds; half of the HPC cells were activated at ~45.5 degrees C and half of the NS cells at approximately 43 degrees C. The 20 HPC lamina I STT cells and 10 NS cells tested with quantitative pinch stimuli showed fairly linear responses above a threshold of approximately 130 g/mm(2) for HPC cells and a threshold of approximately 100 g/mm(2) for NS cells. All of these response functions compare well (across species) with the available data on the characteristics of thermoreceptive and nociceptive primary afferent fibers and the appropriate psychophysics in humans. Together these results support the concept that these classes of lamina I STT cells provide discrete sensory channels for the sensations of temperature and pain.
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Affiliation(s)
- A D Craig
- Atkinson Pain Research Laboratory, Division of Neurosurgery, Barrow Neurological Institute, 350 W. Thomas Rd., Phoenix, AZ 85013, USA.
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88
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Yamakura T, Bertaccini E, Trudell JR, Harris RA. Anesthetics and ion channels: molecular models and sites of action. Annu Rev Pharmacol Toxicol 2001; 41:23-51. [PMID: 11264449 DOI: 10.1146/annurev.pharmtox.41.1.23] [Citation(s) in RCA: 198] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The mechanisms of general anesthesia in the central nervous system are finally yielding to molecular examination. As a result of research during the past several decades, a group of ligand-gated ion channels have emerged as plausible targets for general anesthetics. Molecular biology techniques have greatly accelerated attempts to classify ligand-gated ion channel sensitivity to general anesthetics, and have identified the sites of receptor subunits critical for anesthetic modulation using chimeric and mutated receptors. The experimental data have facilitated the construction of tenable molecular models for anesthetic binding sites, which in turn allows structural predictions to be tested. In vivo significance of a putative anesthetic target can now be examined by targeted gene manipulations in mice. In this review, we summarize from a molecular perspective recent advances in our understanding of mechanisms of action of general anesthetics on ligand-gated ion channels.
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Affiliation(s)
- T Yamakura
- Waggoner Center for Alcohol and Addiction Research, University of Texas, Austin, Texas 78712, USA.
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89
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Findlay GS, Ueno S, Harrison NL, Harris RA. Allosteric modulation in spontaneously active mutant γ-aminobutyric acidA receptors. Neurosci Lett 2001; 305:77-80. [PMID: 11356312 DOI: 10.1016/s0304-3940(01)01646-9] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Tryptophan substitutions were made in the second transmembrane domain of the gamma-aminobutyric acid(A) (GABAA) receptor alpha and beta subunits and the resulting mutant receptors, containing alpha2(S270W) and/or beta1 (S265W), were expressed in Xenopus oocytes. Mutation of either or both subunits resulted in receptors that exhibited enhanced sensitivity to agonist and were spontaneously active in the absence of GABA. The spontaneous activity was blocked by picrotoxin or bicuculline. The enhancement of GABA-induced currents by pentobarbital, by the neurosteroid 5alpha-pregnan-3alpha-ol-20-one, and by the benzodiazepine flunitrazepam was dramatically reduced in the mutant receptors. These results are consistent with the idea that a mutation that promotes gating behavior in a ligand-gated ion channel will also show reduced effects of all positive allosteric modulators in a generalized manner, even when these modulators act at distinct sites on the receptor.
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Affiliation(s)
- G S Findlay
- Waggoner Center for Alcohol and Addiction Research, The University of Texas at Austin (A4800), 2500 Speedway MBB1.124, TX, Austin 78712-1095, USA
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90
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Menon DK. Mapping the anatomy of unconsciousness--imaging anaesthetic action in the brain. Br J Anaesth 2001; 86:607-10. [PMID: 11575332 DOI: 10.1093/bja/86.5.607] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
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91
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Findlay GS, Ueno S, Harrison NL, Harris RA. Allosteric modulation in spontaneously active mutant gamma-aminobutyric acid(A) receptors [corrected]. Neurosci Lett 2000; 293:155-8. [PMID: 11036184 DOI: 10.1016/s0304-3940(00)01503-2] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Tryptophan substitutions were made in the second transmembrane domain of the gamma-aminobutyric acid(A) (GABA(A)) receptor alpha and beta subunits and the resulting mutant receptors, containing alpha(2)(S270W) and/or beta(1)(S265W), were expressed in Xenopus oocytes. Mutation of either or both subunits resulted in receptors that exhibited enhanced sensitivity to agonist and were spontaneously active in the absence of GABA. The spontaneous activity was blocked by picrotoxin or bicuculline. The enhancement of GABA-induced currents by pentobarbital, by the neurosteroid 5alpha-pregnan-3alpha-ol-20-one, and by the benzodiazepine flunitrazepam was dramatically reduced in the mutant receptors. These results are consistent with the idea that a mutation that promotes gating behavior in a ligand-gated ion channel will also show reduced effects of all positive allosteric modulators in a generalized manner, even when these modulators act at distinct sites on the receptor.
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Affiliation(s)
- G S Findlay
- Waggoner Center for Alcohol and Addiction Research, The University of Texas at Austin (A4800), 2500 Speedway MBB1.124, Austin, TX 78712-1095, USA
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92
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Pratt MB, Husain SS, Miller KW, Cohen JB. Identification of sites of incorporation in the nicotinic acetylcholine receptor of a photoactivatible general anesthetic. J Biol Chem 2000; 275:29441-51. [PMID: 10859324 DOI: 10.1074/jbc.m004710200] [Citation(s) in RCA: 67] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
Most general anesthetics including long chain aliphatic alcohols act as noncompetitive antagonists of the nicotinic acetylcholine receptor (nAChR). To locate the sites of interaction of a long chain alcohol with the Torpedo nAChR, we have used the photoactivatible alcohol 3-[(3)H]azioctanol, which inhibits the nAChR and photoincorporates into nAChR subunits. At 1 and 275 microm, 3-[(3)H]azioctanol photoincorporated into nAChR subunits with increased incorporation in the alpha-subunit in the desensitized state. The incorporation into the alpha-subunit was mapped to two large proteolytic fragments. One fragment of approximately 20 kDa (alpha V8-20), containing the M1, M2, and M3 transmembrane segments, showed enhanced incorporation in the presence of agonist whereas the other of approximately 10 kDa (alpha V8-10), containing the M4 transmembrane segment, did not show agonist-induced incorporation of label. Within alpha V8-20, the primary site of incorporation was alpha Glu-262 at the C-terminal end of alpha M2, labeled preferentially in the desensitized state. The incorporation at alpha Glu-262 approached saturation between 1 microm, with approximately 6% labeled, and 275 microm, with approximately 30% labeled. Low level incorporation was seen in residues at the agonist binding site and the protein-lipid interface at approximately 1% of the levels in alpha Glu-262. Therefore, the primary binding site of 3-azioctanol is within the ion channel with additional lower affinity interactions within the agonist binding site and at the protein-lipid interface.
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Affiliation(s)
- M B Pratt
- Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115, USA
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93
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Bovill JG. Mechanisms of anaesthesia: time to say farewell to the Meyer-Overton rule. Curr Opin Anaesthesiol 2000; 13:433-6. [PMID: 17016337 DOI: 10.1097/00001503-200008000-00006] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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94
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Irifune M, Sato T, Kamata Y, Nishikawa T, Dohi T, Kawahara M. Evidence for GABAA Receptor Agonistic Properties of Ketamine: Convulsive and Anesthetic Behavioral Models in Mice. Anesth Analg 2000. [DOI: 10.1213/00000539-200007000-00043] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
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95
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Irifune M, Sato T, Kamata Y, Nishikawa T, Dohi T, Kawahara M. Evidence for GABA(A) receptor agonistic properties of ketamine: convulsive and anesthetic behavioral models in mice. Anesth Analg 2000; 91:230-6. [PMID: 10866918 DOI: 10.1097/00000539-200007000-00043] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
UNLABELLED We examined the potentiation by ketamine of the gamma-aminobutyric acid(A) (GABA(A)) receptor function using convulsive and anesthetic behavioral models in adult male ddY mice. General anesthetic potencies were evaluated by a rating scale, which provided the data for anesthetic scores, loss of righting reflex, duration, and recovery time. All drugs were administered intraperitoneally. Small subanesthetic doses of ketamine did inhibit tonic seizures induced by a large dose of the GABA(A) receptor antagonist bicuculline (8 mg/kg). The 50% effective dose value was 15 (95% confidence limits 10-22) mg/kg. Even large anesthetic doses (100-150 mg/kg) did not suppress clonic seizures in 50% of the animals. The GABA(A) receptor agonist, muscimol (0.32-1.12 mg/kg), potentiated ketamine-induced anesthesia in a dose-dependent fashion (P < 0.05). Similarly, the benzodiazepine receptor agonist, diazepam (1-3 mg/kg), augmented ketamine anesthesia in a dose-dependent manner (P < 0.05). Bicuculline (2-5 mg/kg) dose-dependently antagonized ketamine-induced anesthesia (P < 0.05). Neither the benzodiazepine receptor antagonist, flumazenil (2-20 mg/kg), nor the GABA synthesis inhibitor, L-allylglycine (200 mg/kg), affected the anesthetic action of ketamine. These results suggest that ketamine has GABA(A) receptor agonistic properties and that ketamine-induced anesthesia is mediated, at least in part, by GABA(A) receptors. IMPLICATIONS We examined the potentiation by ketamine of the gamma-aminobutyric acid(A) receptor function using convulsive and anesthetic behavioral models in mice. Subanesthetic doses of ketamine-inhibited tonic convulsions induced by the gamma-aminobutyric acid(A) receptor antagonist bicuculline. The gamma-aminobutyric acid(A) receptor agonist, muscimol, potentiated ketamine-induced anesthesia. Bicuculline antagonized ketamine anesthesia, but the benzodiazepine receptor antagonist, flumazenil, and the gamma-aminobutyric acid synthesis inhibitor, L-allyglycine, did not. The effects of ketamine on the gamma-aminobutyric acid(A) receptors appear to correlate with its anesthetic actions.
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Affiliation(s)
- M Irifune
- Department of Pharmacology, Kagoshima University Dental School, Japan.
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96
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Akk G, Steinbach JH. Activation and block of recombinant GABA(A) receptors by pentobarbitone: a single-channel study. Br J Pharmacol 2000; 130:249-58. [PMID: 10807661 PMCID: PMC1572086 DOI: 10.1038/sj.bjp.0703335] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
Recombinant GABA(A) receptors (alpha1beta2gamma2L) were transiently expressed in HEK 293 cells. We have investigated activation and block of these receptors by pentobarbitone (PB) using cell-attached single-channel patch clamp. Clusters of single-channel activity elicited by 500 microM PB were analysed to estimate rate constants for agonist binding and channel gating. The minimal model able to describe the kinetic data involved two sequential binding steps, followed by channel opening. The estimated channel opening rate constant is approximately 1500 s(-1), and the estimated equilibrium dissociation constants for the binding steps involved in activation are approximately 2 mM. Our results show a dose-dependent block of receptors at millimolar concentrations of PB that results in reduced open interval durations. The reduction in mean open time is linearly proportional to PB concentration, indicating that block can be produced by binding of a single PB molecule. Addition of millimolar concentrations of PB in the presence of GABA also produces a reduction of open channel lifetime in addition to a progressive increase in the closed interval durations within a cluster. The data suggest that the receptor contains two or more blocking sites while occupancy of only one of the sites is sufficient for channel block. Neither the blocking rate constant nor return rate from the blocked state(s) is affected by pH (ionization status of the PB molecule) demonstrating that both neutral and anionic forms of PB cause channel block.
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Affiliation(s)
- G Akk
- Department of Anesthesiology, Washington University School of Medicine, St Louis, Missouri, MO 63110, USA.
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