General anaesthetic actions on ligand-gated ion channels

Menthol suppresses nicotinic acetylcholine receptor functioning in sensory neurons via allosteric modulation

In this study, we have investigated how the function of native and recombinant nicotinic acetylcholine receptors (nAChRs) is modulated by the monoterpenoid alcohol from peppermint (−) menthol. In trigeminal neurons (TG), we found that nicotine (75 μM)-activated whole-cell currents through nAChRs were reversibly reduced by menthol in a concentration-dependent manner with an IC₅₀ of 111 μM. To analyze the mechanism underlying menthol's action in more detail, we used single channel and whole-cell recordings from recombinant human α4β2 nAChR expressed in HEK tsA201 cells. Here, we found a shortening of channel open time and a prolongation of channel closed time, and an increase in single channel amplitude leading in summary to a reduction in single channel current. Furthermore, menthol did not affect nicotine's EC₅₀ value for currents through recombinant human α4β2 nAChRs but caused a significant reduction in nicotine's efficacy. Taken together, these findings indicate that menthol is a negative allosteric modulator of nAChRs.

The effect of pentobarbital sodium and propofol anesthesia on multifocal electroretinograms in rhesus macaques

We compared the suitability of pentobarbital sodium (PB) and propofol (PF) anesthetics for multifocal electroretinograms (mfERGs) in rhesus macaques. mfERGs were collected from 4 ocularly normal rhesus macaques. All animals were pre-anesthetized with intramuscular ketamine (10-15 mg/kg). Intravenous PB induction/maintenance levels were 15 mg/kg/2-10 mg/kg and for PF, 2-5 mg/kg/6-24 mg/kg/h. There were 3 testing sessions with PB anesthesia and 5-7 testing sessions with PF anesthesia. All PB sessions were carried out before PF. First-order (K1) and second-order (first slice) kernels (K2.1) response density amplitude (RDA), implicit time (IT), and root mean square signal-to-noise ratios (RMS SNR) of the low-frequency (LFC) and high-frequency (HFC) components were evaluated. The use of PF or PB anesthesia resulted in robust, replicable mfERGs in rhesus macaques; however, RMS SNR of K1 LFC in ring and quadrant analyses was significantly larger for PF than for PB. Additionally, K1 RDA under PF was significantly larger than under PB for N1, P1, and P2 components (ring and quadrant) and for N2 (quadrant). PF IT was significantly prolonged (<1 ms) relative to PB IT for N1, P1 (ring), and N1 (quadrant), while PB IT was significantly prolonged (0.8-4.2 ms) relative to PF IT for N2 and P2 (ring and quadrant). K1 HFC and K2.1 LFC did not differ significantly between PB and PF in the ring or quadrant analyses. The response differences found with PB and PF anesthesia likely arise from variable relative effects of the anesthetics on retinal γ-aminobutyric acid (GABA(A)) receptors, and in part, on glycine and on glutamate receptors. Given the advantages of a stable anesthetic plane with continuous intravenous infusion and a smoother, more rapid recovery, PF is an appealing alternative for mfERG testing in rhesus macaques.

p-(4-Azipentyl)propofol: a potent photoreactive general anesthetic derivative of propofol

We synthesized 2,6-diisopropyl-4-[3-(3-methyl-3H-diazirin-3-yl)propyl]phenol (p-(4-azipentyl)propofol), or p-4-AziC5-Pro, a novel photoactivable derivative of the general anesthetic propofol. p-4-AziC5-Pro has an anesthetic potency similar to that of propofol. Like propofol, the compound potentiates inhibitory GABA(A) receptor current responses and allosterically modulates binding to both agonist and benzodiazepine sites, assayed on heterologously expressed GABA(A) receptors. p-4-AziC5-Pro inhibits excitatory current responses of nACh receptors expressed in Xenopus oocytes and photoincorporates into native nACh receptor-enriched Torpedo membranes. Thus, p-4-AziC5-Pro is a functional general anesthetic that both modulates and photoincorporates into Cys-loop ligand-gated ion channels, making it an excellent candidate for use in identifying propofol binding sites.

Dynamic loss of surface-expressed AMPA receptors in mouse cortical and striatal neurons during anesthesia

Ionotropic glutamate receptors, especially the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subtype, undergo dynamic trafficking between the surface membrane and intracellular organelles. This trafficking activity determines the efficacy and strength of excitatory synapses and is subject to modulation by changing synaptic inputs. Given the possibility that glutamate receptors in the central nervous system might be a sensitive target of anesthetic agents, this study investigated the possible impact of anesthesia on trafficking and subcellular expression of AMPA receptors in adult mouse brain neurons in vivo. We found that anesthesia induced by a systemic injection of pentobarbital did not alter total protein levels of three AMPA receptor subunits (GluR1-3) in cortical neurons. However, an anesthetic dose of pentobarbital reduced GluR1 and GluR3 proteins in the surface pool and elevated these proteins in the intracellular pool of cortical neurons. The similar redistribution of GluR1/3 was observed in mouse striatal neurons. Pentobarbital did not significantly alter GluR2 expression in the two pools. Chloral hydrate at an anesthetic dose also reduced surface GluR1/3 expression and increased intracellular levels of these proteins. The effect of pentobarbital on subcellular distribution of AMPA receptors was reversible. Altered subcellular distribution of GluR1/3 returned to normal levels after the anesthesia subsided. These data indicate that anesthesia induced by pentobarbital and chloral hydrate can alter AMPA receptor trafficking in both cortical and striatal neurons. This alteration is characterized by the concurrent loss and addition of GluR1/3 subunits in the respective surface and intracellular pools.

Anesthetic effects on fictive locomotion in the rat isolated spinal cord

General anesthetic mechanisms are poorly understood. Anesthetic immobilizing effects occur in the spinal ventral horn. However, a detailed analysis of anesthetic effects on ventral motor networks is lacking. We delivered isoflurane, desflurane, or propofol during NMDA/5-HT-induced, or noxious tail stimulus-evoked, fictive locomotion in neonatal rat isolated spinal cords. Anesthetics changed the frequency, amplitude, and regularity of fictive locomotion with little effect on phase-lag. Isoflurane abolished pharmacologically-induced versus noxious stimulus-induced motor output at similar concentrations. Propofol abolished pharmacologically-induced fictive locomotion through a γ-aminobutyric acid type A-receptor mechanism. Anesthetic effects on pharmacologically-elicted fictive locomotion appear clinically-relevant, and support a ventral horn immobilizing effect on locomotor rhythm generation.

Tolerance to allopregnanolone with focus on the GABA-A receptor

Many studies have suggested a relationship between stress, sex steroids, and negative mental and mood changes in humans. The progesterone metabolite allopregnanolone is a potent endogenous ligand of the γ-amino butyric acid -A (GABA-A) receptor, and the most discussed neuroactive steroid. Variations in the levels of neuroactive steroids that influence the activity of the GABA-A receptor cause a vulnerability to mental and emotional pathology. There are physiological conditions in which allopregnanolone production increases acutely (e.g. stress) or chronically (e.g. menstrual cycle, pregnancy), thus exposing the GABA-A receptor to high and continuous allopregnanolone concentrations. In such conditions, tolerance to allopregnanolone may develop. We have shown that both acute and chronic tolerances can develop to the effects of allopregnanolone. Following the development of acute allopregnanolone tolerance, there is a decrease in the abundance of the GABA-A receptor α4 subunit and the expression of the α4 subunit mRNA in the ventral-posteriomedial nucleus of the thalamus. Little is known about the mechanism behind allopregnanolone tolerance and its effects on assembly of the GABA-A receptor composition. The exact mechanism of the allopregnanolone tolerance phenomena remains unclear. The purpose of this review is to summarize certain aspects of current knowledge concerning allopregnanolone tolerance and changes in the GABA-A receptors.

Alcohol-binding sites in distinct brain proteins: the quest for atomic level resolution

Defining the sites of action of ethanol on brain proteins is a major prerequisite to understanding the molecular pharmacology of this drug. The main barrier to reaching an atomic-level understanding of alcohol action is the low potency of alcohols, ethanol in particular, which is a reflection of transient, low-affinity interactions with their targets. These mechanisms are difficult or impossible to study with traditional techniques such as radioligand binding or spectroscopy. However, there has been considerable recent progress in combining X-ray crystallography, structural modeling, and site-directed mutagenesis to define the sites and mechanisms of action of ethanol and related alcohols on key brain proteins. We review such insights for several diverse classes of proteins including inwardly rectifying potassium, transient receptor potential, and neurotransmitter-gated ion channels, as well as protein kinase C epsilon. Some common themes are beginning to emerge from these proteins, including hydrogen bonding of the hydroxyl group and van der Waals interactions of the methylene groups of ethanol with specific amino acid residues. The resulting binding energy is proposed to facilitate or stabilize low-energy state transitions in the bound proteins, allowing ethanol to act as a "molecular lubricant" for protein function. We discuss evidence for characteristic, discrete alcohol-binding sites on protein targets, as well as evidence that binding to some proteins is better characterized by an interaction region that can accommodate multiple molecules of ethanol.

General anesthetic actions on GABA(A) receptors

General anesthetic drugs interact with many receptors in the nervous system, but only a handful of these interactions are critical for producing anesthesia. Over the last 20 years, neuropharmacologists have revealed that one of the most important target sites for general anesthetics is the GABA_A receptor. In this review we will discuss what is known about anesthetic – GABA_A receptor interactions.

Cyclohexanol analogues are positive modulators of GABA(A) receptor currents and act as general anaesthetics in vivo

GABA(A) receptors meet all the pharmacological criteria required to be considered important general anaesthetic targets. In the following study, the modulatory effects of various commercially available and novel cyclohexanols were investigated on recombinant human γ-aminobutyric acid (GABA(A), α(1)β(2)γ(2s)) receptors expressed in Xenopus oocytes, and compared to the modulatory effects on GABA currents observed with exposures to the intravenous anaesthetic agent, propofol. Submaximal EC(20) GABA currents were typically enhanced by co-applications of 3-300 μM cyclohexanols. For instance, at 30 μM 2,6-diisopropylcyclohexanol (a novel compound) GABA responses were increased ~3-fold (although similar enhancements were achieved at 3 μM propofol). As regards rank order for modulation by the cyclohexanol analogues at 30 μM, the % enhancements for 2,6-dimethylcyclohexanol~2,6-diethylcyclohexanol~2,6-diisopropylcyclohexanol~2,6-di-sec-butylcyclohexanol ≫2,6-di-tert-butylcyclohexanol~4-tert-butylcyclohexanol>cyclohexanol~cyclopentanol~2-methylcyclohexanol. We further tested the potencies of the cyclohexanol analogues as general anaesthetics using a tadpole in vivo assay. Both 2,6-diisopropylcyclohexanol and 2,6-dimethylcyclohexanol were effective as anaesthetics with EC(50)s of 14.0 μM and 13.1 μM respectively, while other cyclohexanols with bulkier side chains were less potent. In conclusion, our data indicate that cyclohexanols are both positive modulators of GABA(A) receptors currents and anaesthetics. The positioning and size of the alkyl groups at the 2 and 6 positions on the cyclohexanol ring were critical determinants of activity.

Clinical and molecular pharmacology of etomidate

This review focuses on the unique clinical and molecular pharmacologic features of etomidate. Among general anesthesia induction drugs, etomidate is the only imidazole, and it has the most favorable therapeutic index for single-bolus administration. It also produces a unique toxicity among anesthetic drugs: inhibition of adrenal steroid synthesis that far outlasts its hypnotic action and that may reduce survival of critically ill patients. The major molecular targets mediating anesthetic effects of etomidate in the central nervous system are specific γ-aminobutyric acid type A receptor subtypes. Amino acids forming etomidate binding sites have been identified in transmembrane domains of these proteins. Etomidate binding site structure models for the main enzyme mediating etomidate adrenotoxicity have also been developed. Based on this deepening understanding of molecular targets and actions, new etomidate derivatives are being investigated as potentially improved sedative-hypnotics or for use as highly selective inhibitors of adrenal steroid synthesis.

Anaesthesia for a patient with paraneoplastic limbic encephalitis with ovarian teratoma: relationship to anti-N-methyl-D-aspartate receptor antibodies

Paraneoplastic limbic encephalitis associated with ovarian teratoma has recently been related to the development of antibodies to specific heteromers of the N-methyl-d-aspartate receptor and exhibits various manifestations including psychiatric symptoms, hypoventilation, seizures and derangement of autonomic nervous system function. Although recovery can sometimes occur spontaneously, early tumour resection with immunotherapy facilitates earlier recovery. Herein, we describe anaesthetic management of a 20-year-old woman who developed general convulsions and decreased level of consciousness, whom we suspected of having paraneoplastic limbic encephalitis and was scheduled for left ovarian tumour resection. Anaesthetic management was successful with no complications but the case acts as focus of discussion for the potential interaction of N-methyl-D-aspartate receptors and anaesthetic sensitivity.

Modeling sharp wave-ripple complexes through a CA3-CA1 network model with chemical synapses

The hippocampus, and particularly the CA3 and CA1 areas, exhibit a variety of oscillatory rhythms that span frequencies from the slow theta range (4-10 Hz) up to fast ripples (200 Hz). Various computational models of different complexities have been developed in an effort to simulate such population oscillations. Nevertheless the mechanism that underlies the so called Sharp Wave-Ripple complex (SPWR), observed in extracellular recordings in CA1, still remains elusive. We present here, the combination of two simple but realistic models of the rat CA3 and CA1 areas, connected together in a feedforward scheme mimicking Schaffer collaterals. Both network models are computationally simple one-dimensional arrays of excitatory and inhibitory populations interacting only via fast chemical synapses. Connectivity schemes and postsynaptic potentials are based on physiological data, yielding a realistic network topology. The CA3 model exhibits quasi-synchronous population bursts, which give rise to sharp wave-like deep depolarizations in the CA1 dendritic layer accompanied by transient field oscillations at ≈ 150-200 Hz in the somatic layer. The frequency and synchrony of these oscillations is based on interneuronal activity and fast-decaying recurrent inhibition in CA1. Pyramidal cell spikes are sparse and come from a subset of cells receiving stronger than average excitatory input from CA3. The model is shown to accurately reproduce a large number of basic characteristics of SPWRs and yields a new mechanism for the generation of ripples, offering an interpretation to a range of neurophysiological observations, such as the ripple disruption by halothane and the selective firing of pyramidal cells during ripples, which may have implications for memory consolidation during SPWRs.

Etomidate and propofol inhibit the neurotransmitter release machinery at different sites

The mechanism of general anaesthetic action is only partially understood. Facilitation of inhibitory GABAA receptors plays an important role in the action of most anaesthetics, but is thought to be especially relevant in the case of intravenous anaesthetics, like etomidate and propofol. Recent evidence suggests that anaesthetics also inhibit excitatory synaptic transmission via a presynaptic mechanism(s), but it has been difficult to determine whether these agents act on the neurotransmitter release machinery itself. In the present study we sought to determine whether the intravenous anaesthetics propofol and etomidate inhibit the release machinery. For these studies we used an experimental approach that directly regulated [Ca2+]i at neurotransmitter release sites, thereby bypassing anaesthetic effects on channels and receptors in order to allow anaesthetic effects on the neurotransmitter release machinery to be examined in isolation. The data show that clinically relevant concentrations of propofol and etomidate inhibited the neurotransmitter release machinery in neurosecretory cells and in cultured hippocampal neurons. md130A is a mutant form of syntaxin with a truncated C-terminus. Overexpressing md130A in PC12 cells completely eliminated the reduction in neurotransmitter release produced by propofol, without affecting release itself. In contrast, overexpressing md130A in PC12 cells had little or no effect on the response to etomidate. These results suggest that both propofol and etomidate inhibit neurotransmitter release by a direct interaction with SNAREs and/or SNARE-associated proteins but they do so at different sites.

Anaesthetic impairment of immune function is mediated via GABA(A) receptors

Background

GABA_A receptors are members of the Cys-loop family of neurotransmitter receptors, proteins which are responsible for fast synaptic transmission, and are the site of action of wide range of drugs [1]. Recent work has shown that Cys-loop receptors are present on immune cells, but their physiological roles and the effects of drugs that modify their function in the innate immune system are currently unclear [2]. We are interested in how and why anaesthetics increase infections in intensive care patients; a serious problem as more than 50% of patients with severe sepsis will die [3]–[6]. As many anaesthetics act via GABA_A receptors [7], the aim of this study was to determine if these receptors are present on immune cells, and could play a role in immunocompromising patients.

Principal Findings

We demonstrate, using RT-PCR, that monocytes express GABA_A receptors constructed of α1, α4, β2, γ1 and/or δ subunits. Whole cell patch clamp electrophysiological studies show that GABA can activate these receptors, resulting in the opening of a chloride-selective channel; activation is inhibited by the GABA_A receptor antagonists bicuculline and picrotoxin, but not enhanced by the positive modulator diazepam. The anaesthetic drugs propofol and thiopental, which can act via GABA_A receptors, impaired monocyte function in classic immunological chemotaxis and phagocytosis assays, an effect reversed by bicuculline and picrotoxin.

Significance

Our results show that functional GABA_A receptors are present on monocytes with properties similar to CNS GABA_A receptors. The functional data provide a possible explanation as to why chronic propofol and thiopental administration can increase the risk of infection in critically ill patients: their action on GABA_A receptors inhibits normal monocyte behaviour. The data also suggest a potential solution: monocyte GABA_A receptors are insensitive to diazepam, thus the use of benzodiazepines as an alternative anesthetising agent may be advantageous where infection is a life threatening problem.

Anaesthetic impairment of immune function is mediated via GABA(A) receptors

BACKGROUND

GABA(A) receptors are members of the Cys-loop family of neurotransmitter receptors, proteins which are responsible for fast synaptic transmission, and are the site of action of wide range of drugs. Recent work has shown that Cys-loop receptors are present on immune cells, but their physiological roles and the effects of drugs that modify their function in the innate immune system are currently unclear. We are interested in how and why anaesthetics increase infections in intensive care patients; a serious problem as more than 50% of patients with severe sepsis will die. As many anaesthetics act via GABA(A) receptors, the aim of this study was to determine if these receptors are present on immune cells, and could play a role in immunocompromising patients.

PRINCIPAL FINDINGS

We demonstrate, using RT-PCR, that monocytes express GABA(A) receptors constructed of α1, α4, β2, γ1 and/or δ subunits. Whole cell patch clamp electrophysiological studies show that GABA can activate these receptors, resulting in the opening of a chloride-selective channel; activation is inhibited by the GABA(A) receptor antagonists bicuculline and picrotoxin, but not enhanced by the positive modulator diazepam. The anaesthetic drugs propofol and thiopental, which can act via GABA(A) receptors, impaired monocyte function in classic immunological chemotaxis and phagocytosis assays, an effect reversed by bicuculline and picrotoxin.

SIGNIFICANCE

Our results show that functional GABA(A) receptors are present on monocytes with properties similar to CNS GABA(A) receptors. The functional data provide a possible explanation as to why chronic propofol and thiopental administration can increase the risk of infection in critically ill patients: their action on GABA(A) receptors inhibits normal monocyte behaviour. The data also suggest a potential solution: monocyte GABA(A) receptors are insensitive to diazepam, thus the use of benzodiazepines as an alternative anesthetising agent may be advantageous where infection is a life threatening problem.

Assessment of general anaesthetic cytotoxicity in murine cortical neurones in dissociated culture

General anaesthetics are proposed to cause unconsciousness by modulating neuronal excitability in the mammalian brain through mechanisms that include enhancement of inhibitory GABA(A) receptor currents and suppression of excitatory glutamate receptor responses. Both intravenous and volatile agents may produce neurotoxic effects during early postnatal rodent brain development through similar mechanisms. In the following study, we investigated anaesthetic cytotoxicity in primary cortical neurones and glia from postnatal day 2-8 mice. Cultures at 4-20 days in vitro were exposed to combinations of ketamine (100 μM to 3 mM), nitrous oxide (75%, v/v) and/or isoflurane (1.5-5%, v/v) for 6-12 h. Neuronal survival and cell death were measured via microtubule associated protein 2 immunoassay and lactate dehydrogenase release assays, respectively. Clinically relevant anaesthetic concentrations of ketamine, nitrous oxide and isoflurane had no significant neurotoxic effects individually or when given as anaesthetic cocktails, even with up to 12 h exposure. This lack of neurotoxicity was observed regardless of whether cultures were prepared from postnatal day 0-2 or day 8 mice, and was also unaffected by number of days in vitro (DIV 4-20). Significant neurotoxic effects were only observed at supraclinical concentrations (e.g. 1-3 mM ketamine). Our study suggests that neurotoxicity previously reported in vivo is not due to direct cytotoxicity of anaesthetic agents, but results from other impacts of the anaesthetised state during early brain development.

Crucial role of mechanisms and modes of toxic action for understanding tissue residue toxicity and internal effect concentrations of organic chemicals

This article reviews the mechanistic basis of the tissue residue approach for toxicity assessment (TRA). The tissue residue approach implies that whole-body or organ concentrations (residues) are a better dose metric for describing toxicity to aquatic organisms than is the aqueous concentration typically used in the external medium. Although the benefit of internal concentrations as dose metrics in ecotoxicology has long been recognized, the application of the tissue residue approach remains limited. The main factor responsible for this is the difficulty of measuring internal concentrations. We propose that environmental toxicology can advance if mechanistic considerations are implemented and toxicokinetics and toxicodynamics are explicitly addressed. The variability in ecotoxicological outcomes and species sensitivity is due in part to differences in toxicokinetics, which consist of several processes, including absorption, distribution, metabolism, and excretion (ADME), that influence internal concentrations. Using internal concentrations or tissue residues as the dose metric substantially reduces the variability in toxicity metrics among species and individuals exposed under varying conditions. Total internal concentrations are useful as dose metrics only if they represent a surrogate of the biologically effective dose, the concentration or dose at the target site. If there is no direct proportionality, we advise the implementation of comprehensive toxicokinetic models that include deriving the target dose. Depending on the mechanism of toxicity, the concentration at the target site may or may not be a sufficient descriptor of toxicity. The steady-state concentration of a baseline toxicant associated with the biological membrane is a good descriptor of the toxicodynamics of baseline toxicity. When assessing specific-acting and reactive mechanisms, additional parameters (e.g., reaction rate with the target site and regeneration of the target site) are needed for characterization. For specifically acting compounds, intrinsic potency depends on 1) affinity for, and 2) type of interaction with, a receptor or a target enzyme. These 2 parameters determine the selectivity for the toxic mechanism and the sensitivity, respectively. Implementation of mechanistic information in toxicokinetic-toxicodynamic (TK-TD) models may help explain time-delayed effects, toxicity after pulsed or fluctuating exposure, carryover toxicity after sequential pulses, and mixture toxicity. We believe that this mechanistic understanding of tissue residue toxicity will lead to improved environmental risk assessment.

Identification and characterization of anesthetic targets by mouse molecular genetics approaches

PURPOSE

It is now generally accepted that proteins are the primary targets of general anesthetics. However, the demonstration that the activity of a protein is altered by general anesthetics at clinically relevant concentrations in vitro does not provide direct evidence that this target mediates pharmacological actions of general anesthetics. Here we report on advances that have been made in identifying the contribution of individual ligand-gated ion channels to defined anesthetic endpoints using molecular mouse genetics.

PRINCIPAL FINDINGS

Gamma-aminobutyric acid (GABA)(A) receptor subtypes defined by the presence of the α1, α4, α5, β2, and β3 subunits and two-pore domain potassium channels (TASK-1, TASK-3, and TREK) have been discovered to mediate, at least in part, the hypnotic, immobilizing or amnestic actions of intravenous and volatile general anesthetics. Moreover, using tissues from genetically modified mice, specific functions of GABA(A) receptor subtypes in cortical and spinal neuronal networks were identified.

CONCLUSION

Genetically modified mice have been very useful for research on mechanisms of anesthesia and have contributed to the functional identification of general anesthetic targets and of the role of these targets in neuronal networks.

Multiple binding sites for the general anesthetic isoflurane identified in the nicotinic acetylcholine receptor transmembrane domain

An extensive search for isoflurane binding sites in the nicotinic acetylcholine receptor (nAChR) and the proton gated ion channel from Gloebacter violaceus (GLIC) has been carried out based on molecular dynamics (MD) simulations in fully hydrated lipid membrane environments. Isoflurane introduced into the aqueous phase readily partitions into the lipid membrane and the membrane-bound protein. Specifically, isoflurane binds persistently to three classes of sites in the nAChR transmembrane domain: (i) An isoflurane dimer occludes the pore, contacting residues identified by previous mutagenesis studies; analogous behavior is observed in GLIC. (ii) Several nAChR subunit interfaces are also occupied, in a site suggested by photoaffinity labeling and thought to positively modulate the receptor; these sites are not occupied in GLIC. (iii) Isoflurane binds to the subunit centers of both nAChR alpha chains and one of the GLIC chains, in a site that has had little experimental targeting. Interpreted in the context of existing structural and physiological data, the present MD results support a multisite model for the mechanism of receptor-channel modulation by anesthetics.

5-HT(3) receptors: role in disease and target of drugs

Serotonin type 3 (5-HT(3)) receptors are pentameric ion channels belonging to the superfamily of Cys-loop receptors. Receptor activation either leads to fast excitatory responses or modulation of neurotransmitter release depending on their neuronal localisation. 5-HT(3) receptors are known to be expressed in the central nervous system in regions involved in the vomiting reflex, processing of pain, the reward system, cognition and anxiety control. In the periphery they are present on a variety of neurons and immune cells. 5-HT(3) receptors are known to be involved in emesis, pain disorders, drug addiction, psychiatric and GI disorders. Progress in molecular genetics gives direction to personalised medical strategies for treating complex diseases such as psychiatric and functional GI disorders and unravelling individual drug responses in pharmacogenetic approaches. Here we discuss the molecular basis of 5-HT(3) receptor diversity at the DNA and protein level, of which our knowledge has greatly extended in the last decade. We also evaluate their role in health and disease and describe specific case-control studies addressing the involvement of polymorphisms of 5-HT3 subunit genes in complex disorders and responses to drugs. Furthermore, we focus on the actual state of the pharmacological knowledge concerning not only classical 5-HT(3) antagonists--the setrons--but also compounds of various substance classes targeting 5-HT(3) receptors such as anaesthetics, opioids, cannabinoids, steroids, antidepressants and antipsychotics as well as natural compounds derived from plants. This shall point to alternative treatment options modulating the 5-HT(3) receptor system and open new possibilities for drug development in the future.

[What do we know about anesthetic mechanisms?: hypnosis, unresponsiveness to surgical incision and amnesia]

Despite the increase of molecular knowledge in anesthesia research over the past decades there is still ongoing discussion about the mechanisms of anesthesia. This article focuses on presenting anesthetic sensitive ligand and voltage gated ion channels. The impact on anesthetic modulated ion channels is summarized for clinically commonly used anesthetics isoflurane, propofol and ketamine. Furthermore, the anesthetic features hypnosis, unresponsiveness to surgical incision and amnesia and their putative relevant anatomical sites in the central nervous system are briefly introduced.

The activation mechanism of alpha1beta2gamma2S and alpha3beta3gamma2S GABAA receptors

The α₁β₂γ₂ and α₃β₃γ₂ are two isoforms of γ-aminobutyric acid type A (GABA_A) receptor that are widely distributed in the brain. Both are found at synapses, for example in the thalamus, where they mediate distinctly different inhibitory postsynaptic current profiles, particularly with respect to decay time. The two isoforms were expressed in HEK293 cells, and single-channel activity was recorded from outside-out patches. The kinetic characteristics of both isoforms were investigated by analyzing single-channel currents over a wide range of GABA concentrations. α₁β₂γ₂ channels exhibited briefer active periods than α₃β₃γ₂ channels over the entire range of agonist concentrations and had lower intraburst open probabilities at subsaturating concentrations. Activation mechanisms were constructed by fitting postulated schemes to data recorded at saturating and subsaturating GABA concentrations simultaneously. Reaction mechanisms were ranked according to log-likelihood values and how accurately they simulated ensemble currents. The highest ranked mechanism for both channels consisted of two sequential binding steps, followed by three conducting and three nonconducting configurations. The equilibrium dissociation constant for GABA at α₃β₃γ₂ channels was ∼2.6 µM compared with ∼19 µM for α₁β₂γ₂ channels, suggesting that GABA binds to the α₃β₃γ₂ channels with higher affinity. A notable feature of the mechanism was that two consecutive doubly liganded shut states preceded all three open configurations. The lifetime of the third shut state was briefer for the α₃β₃γ₂ channels. The longer active periods, higher affinity, and preference for conducting states are consistent with the slower decay of inhibitory currents at synapses that contain α₃β₃γ₂ channels. The reaction mechanism we describe here may also be appropriate for the analysis of other types of GABA_A receptors and provides a framework for rational investigation of the kinetic effects of a variety of therapeutic agents that activate or modulate GABA_A receptors and hence influence synaptic and extrasynaptic inhibition in the central nervous system.

Genetic reduction of GABA(A) receptor gamma2 subunit expression potentiates the immobilizing action of isoflurane

Potentiation of inhibitory gamma-aminobutyric acid subtype A (GABA(A)) receptor function is involved in the mechanisms of anesthetic action. The present study examined the immobilizing action of the volatile anesthetic isoflurane in mice with double knockout (DKO) of phospholipase C-related inactive protein (PRIP)-1 and -2. Both of these proteins play important roles in the expression of GABA(A) receptors containing the gamma2 subunit on the neuronal cell surface. Immunohistochemistry for GABA(A) receptor subunits demonstrated reduced expression of gamma2 subunits in the spinal cord of the DKO mice. Immunohistochemistry also revealed up-regulation of the alpha1 and beta3 subunits even though there were no apparent differences in the immunoreactivities for the beta2 subunits between wild-type and DKO mice. The tail-clamp method was used to evaluate the anesthetic/immobilizing effect of isoflurane and the minimum alveolar concentration (MAC) was significantly lower in DKO mice compared with wild-type controls (1.07+/-0.01% versus 1.36+/-0.04% atm), indicating an increased sensitivity to isoflurane in DKO mice. These immunohistochemical and pharmacological findings suggest that reduced expression of the GABA(A) receptor gamma2 subunit affects the composition and function of spinal GABA(A) receptors and potentiates the immobilizing action of isoflurane.

Anaesthetic mechanisms: update on the challenge of unravelling the mystery of anaesthesia

General anaesthesia is administered each day to thousands of patients worldwide. Although more than 160 years have passed since the first successful public demonstration of anaesthesia, a detailed understanding of the anaesthetic mechanism of action of these drugs is still lacking. An important early observation was the Meyer-Overton correlation, which associated the potency of an anaesthetic with its lipid solubility. This work focuses attention on the lipid membrane as a likely location for anaesthetic action. With the advent of cellular electrophysiology and molecular biology techniques, tools to dissect the components of the lipid membrane have led, in recent years, to the widespread acceptance of proteins, namely receptors and ion channels, as more likely targets for the anaesthetic effect. Yet these accumulated data have not produced a comprehensive explanation for how these drugs produce central nervous system depression. In this review, we follow the story of anaesthesia mechanisms research from its historical roots to the intensely neurophysiological research regarding it today. We will also describe recent findings that identify specific neuroanatomical locations mediating the actions of some anaesthetic agents.

Anesthetic sensitivity of the Gloeobacter violaceus proton-gated ion channel

A prokaryotic member of the gamma-aminobutyric acid type A receptor superfamily (GLIC) was recently cloned from the cyanobacterium Gloeobacter violaceus, its function characterized, and its 3-dimensional x-ray diffraction crystal structure determined. We report its modulation by 9 anesthetics using 2-electrode voltage clamping in Xenopus laevis oocytes. Desflurane, halothane, isoflurane, sevoflurane, and propofol inhibited currents through GLIC at and below concentrations used clinically. Hill numbers averaged 0.3, indicating negative cooperativity or multiple sites or mechanisms of action. A 2-site model fit the data for desflurane and halothane better than a 1-site model. Xenon and etomidate modulated GLIC at or above clinical concentrations, with no cooperativity. Ethanol and nitrous oxide did not modulate GLIC at surgical anesthetic concentrations. These investigations lay the groundwork for further structural and functional studies of anesthetic actions on GLIC.

Loop 2 structure in glycine and GABA(A) receptors plays a key role in determining ethanol sensitivity

The present study tests the hypothesis that the structure of extracellular domain Loop 2 can markedly affect ethanol sensitivity in glycine receptors (GlyRs) and gamma-aminobutyric acid type A receptors (GABA(A)Rs). To test this, we mutated Loop 2 in the alpha1 subunit of GlyRs and in the gamma subunit of alpha1beta2gamma2GABA(A)Rs and measured the sensitivity of wild type and mutant receptors expressed in Xenopus oocytes to agonist, ethanol, and other agents using two-electrode voltage clamp. Replacing Loop 2 of alpha1GlyR subunits with Loop 2 from the deltaGABA(A)R (deltaL2), but not the gammaGABA(A)R subunit, reduced ethanol threshold and increased the degree of ethanol potentiation without altering general receptor function. Similarly, replacing Loop 2 of the gamma subunit of GABA(A)Rs with deltaL2 shifted the ethanol threshold from 50 mm in WT to 1 mm in the GABA(A) gamma-deltaL2 mutant. These findings indicate that the structure of Loop 2 can profoundly affect ethanol sensitivity in GlyRs and GABA(A)Rs. The deltaL2 mutations did not affect GlyR or GABA(A)R sensitivity, respectively, to Zn(2+) or diazepam, which suggests that these deltaL2-induced changes in ethanol sensitivity do not extend to all allosteric modulators and may be specific for ethanol or ethanol-like agents. To explore molecular mechanisms underlying these results, we threaded the WT and deltaL2 GlyR sequences onto the x-ray structure of the bacterial Gloeobacter violaceus pentameric ligand-gated ion channel homologue (GLIC). In addition to being the first GlyR model threaded on GLIC, the juxtaposition of the two structures led to a possible mechanistic explanation for the effects of ethanol on GlyR-based on changes in Loop 2 structure.

Isoflurane inhibits the neurotransmitter release machinery

Despite their importance, the mechanism of action of general anesthetics is still poorly understood. Facilitation of inhibitory GABA(A) receptors plays an important role in anesthesia, but other targets have also been linked to anesthetic actions. Anesthetics are known to suppress excitatory synaptic transmission, but it has been difficult to determine whether they act on the neurotransmitter release machinery itself. By directly elevating [Ca(2+)](i) at neurotransmitter release sites without altering plasma membrane channels or receptors, we show that the commonly used inhalational general anesthetic, isoflurane, inhibits neurotransmitter release at clinically relevant concentrations, in a dose-dependent fashion in PC12 cells and hippocampal neurons. We hypothesized that a SNARE and/or SNARE-associated protein represents an important target(s) for isoflurane. Overexpression of a syntaxin 1A mutant, previously shown in Caenorhabditis elegans to block the behavioral effects of isoflurane, completely eliminated the reduction in neurotransmitter release produced by isoflurane, without affecting release itself, thereby establishing the possibility that syntaxin 1A is an intermediary in isoflurane's ability to inhibit neurotransmitter release.

Network actions of pentobarbital in the rat mesopontine tegmentum on sensory inflow through the spinothalamic tract

The recent discovery of a barbiturate-sensitive "general anesthesia switch" mechanism localized in the rat brain stem mesopontine tegmental anesthesia area (MPTA) has challenged the current view of the nonspecific actions of general anesthetic agents in the CNS. In this study we provide electrophysiological evidence that the antinociception, which accompanies the behavioral state resembling general anesthesia following pentobarbital (PB) microinjections into the MPTA of awake rats, could be accompanied by the attenuation of sensory transmission through the spinothalamic tract (STT). Following bilateral microinjections of PB into the MPTA spontaneous firing rate (SFR), antidromic firing index (FI), and sciatic (Sc) as well as sural (Su) nerve-evoked responses (ER) of identified lumbar STT neurons in the isoflurane-anesthetized rat were quantified using extracellular recording techniques. Microinjections of PB into the MPTA significantly suppressed the SFR (47%), magnitudes of Sc- (26%) and Su-ER (36%), and FI (41%) of STT neurons. Microinjections of PB-free vehicle control did not alter any of the above-cited electrophysiological parameters. The results from this study suggest that antinociception, which occurs during the anesthesia-like state following PB microinjections into the MPTA, may be due, in part, to (in)direct inhibition of STT neurons via switching mechanism(s) located in the MPTA. This study provides a provenance for investigating electrophysiologically the actions on STT neurons of other current agents used clinically to maintain the state of general anesthesia.

What makes a molecule an anaesthetic? Studies on the mechanisms of anaesthesia using a physicochemical approach

Recent studies of mechanisms of anaesthesia have been mainly 'target orientated', investigating the activity of both volatile and i.v. agents at putative sites of action. An alternative approach is one that is 'ligand orientated', focusing on the properties of molecules that define their immobilizing ability and secondly define their potency. The use of conventional descriptors (such as non-polar solubility or the octanol-water partition coefficient [Log P]) are limited in their utility as predictors of potency as they represent three-dimensional molecular properties as a one-dimensional parameter. Using different computer-based molecular modelling methods (molecular similarity studies and comparative molecular field analysis [CoMFA]), we have identified the molecular bases of the activity of structurally diverse anaesthetics, such that they can be described as a single model based on the spatial distribution of molecular bulk and electrostatic potential. The same approach can also be used to model other properties of anaesthetic agents, such as cardiovascular depression. The present data suggest that, for the i.v. agents, it may be difficult to separate immobilizing (anaesthetic) activity and cardiovascular depression within a single molecule.

Awareness during anesthesia: how sure can we be that the patient is sleeping indeed?

Awareness during surgery is a very serious problem for the anesthetist and the patient as well. Such incidents are the cause for 2% of the legal claims against anesthetists while patients with intraoperative awareness experience describe it as the worst thing they have ever suffered from. Pain, anxiety and inability to react due to muscle paralysis often lead to the situation called posttraumatic stress disorder which demands psychiatric support. The fact that there are patients who report intraoperative experience, even several days after surgery, raises questions about the way the anesthetic drugs interfere with the mechanisms of memory and consciousness while, in bibliography, there are studies proving that even deeply anesthetized patients can be influenced by auditory stimuli without being able to recall them. Intraoperative monitoring of the anesthesia depth is important for the prevention of this problem. From all the available devices only the Bispectral Index Monitoring (BIS) has been proven to be effective for this purpose but the high cost per person and the low specificity in preventing awareness episodes do not allow its everyday use. The surgeon and especially the anesthesiologist must be aware of the risk factors, the prevention measures and the actions that must be taken after an awareness incident in order to minimize the unfortunate complications for both the patient and the doctors.

Loss of surface N-methyl-D-aspartate receptor proteins in mouse cortical neurones during anaesthesia induced by chloral hydrate in vivo

BACKGROUND

Anaesthetics may target ionotropic glutamate receptors in brain cells to produce their biological actions. Membrane-bound ionotropic glutamate receptors undergo dynamic trafficking between the surface membrane and intracellular organelles. Their subcellular distribution is subject to modulation by changing synaptic inputs and determines the efficacy and strength of excitatory synapses. It has not been explored whether anaesthesia has any impact on surface glutamate receptor expression. In this study, the effect of general anaesthesia on expression of N-methyl-D-aspartate (NMDA) receptors in the surface and intracellular pools of cortical neurones was investigated in vivo.

METHODS

General anaesthesia was induced by intraperitoneal injection of chloral hydrate in adult male mice. Surface protein cross-linking assays were performed to detect changes in distribution of NMDA receptor subunits (NR1, NR2A, and NR2B) in the surface and intracellular compartments of cerebral cortical neurones.

RESULTS

Chloral hydrate did not alter the total amounts of NR1, NR2A, and NR2B proteins in cortical neurones. However, the drug reduced NR1 proteins in the surface pool of these neurones, and induced a proportional increase in NR1 in the intracellular pool. Similar redistribution of NR2B subunits was observed between the two distinct pools. The changes in NR1 and NR2B were rapid and remained throughout the duration of anaesthesia. NR2A proteins were not altered in the surface or intracellular pool in response to chloral hydrate.

CONCLUSIONS

These data demonstrate that subcellular expression of NR1 and NR2B in cortical neurones is sensitive to anaesthesia. Chloral hydrate reduces surface-expressed NMDA receptors (specifically NR2B-containing NMDA receptors) in these neurones in vivo.

Modeling the GABAergic action of etomidate on the thalamocortical system

BACKGROUND

We have used a computational model of the thalamocortical system to investigate the effects of a GABAergic anesthetic (etomidate) on cerebral cortical and thalamic neuronal function. We examined the effects of phasic and tonic inhibition, as well as the relative importance of anesthetic action in the thalamus and cortex.

METHODS

The amount of phasic GABAergic inhibition was adjusted in the model to simulate etomidate concentrations of between 0.25 and 2 microM, with the concentration range producing unconsciousness assumed to be between 0.25 and 0.5 microM. In addition, we modeled tonic inhibition separately, and then phasic and tonic inhibition together. We also introduced phasic and tonic inhibition into the cerebral cortex and thalamus separately to determine the relative importance of each of these structures to anesthetic-induced depression of the thalamocortical system.

RESULTS

Phasic inhibition decreased cortical neuronal firing by 11%-18% in the 0.25-0.5 microM range and by 38% at 2 microM. Tonic inhibition produced similar depression (11%-21%) in the 0.25-0.5 microM range but 65% depression at 2 microM; phasic and tonic inhibition combined produced the most inhibition (76% depression at 2 microM). When the thalamus and cortex were separately subjected to phasic and tonic inhibition, cortical firing rates decreased less compared to when both structures were targeted. In the 0.25-0.5 microM range, cortical firing rate was minimally affected when etomidate action was simulated in the thalamus only.

CONCLUSIONS

This computational model of the thalamocortical system indicated that tonic GABAergic inhibition seems to be more important than phasic GABAergic inhibition (especially at larger etomidate concentrations), although both combined had the most effect on cerebral cortical firing rates. Furthermore, etomidate action in the thalamus, by itself, does not likely explain etomidate-induced unconsciousness.

GABA signaling in the nucleus tractus solitarius sets the level of activity in dorsal motor nucleus of the vagus cholinergic neurons in the vagovagal circuit

It has been proposed that there is an "apparent monosynaptic" connection between gastric vagal afferent nerve terminals and inhibitory projection neurons in the nucleus tractus solitarius (NTS) and that two efferent parallel pathways from the dorsal motor nucleus of the vagus (DMV) influence peripheral organs associated with these reflexes (6). The purpose of our study was to verify the validity of these views as they relate to basal control of gastric motility. To test the validity of a direct connection of vagal afferent terminals (known to release l-glutamate) directly impacting second-order projection neurons, we evaluated the effect of GABA(A) receptor blockade in the area of the medial subnucleus of the tractus solitarius (mNTS) on gastric motility. Microinjection of bicuculline methiodide into the mNTS produced robust decreases in gastric motility (-1.6 +/- 0.2 mmHg, P < 0.05, n = 23), which were prevented by cervical vagotomy and by pretreatment with kynurenic acid microinjected into the mNTS. Kynurenic acid per se had no effect on gastric motility. However, after GABA(A) receptor blockade in the mNTS, kynurenic acid produced a robust increase in gastric motility. To test for the contribution of two parallel efferent DMV pathways, we assessed the effect of either intravenous atropine methylbromide or N(G)-nitro-l-arginine methyl ester on baseline motility and on decreases in gastric motility induced by GABA(A) receptor blockade in the mNTS. Only atropine methylbromide altered baseline motility and prevented the effects of GABA(A) receptor blockade on gastric motility. Our data demonstrate the presence of intra-NTS GABAergic signaling between the vagal afferent nerve terminals and inhibitory projection neurons in the NTS and that the cholinergic-cholinergic excitatory pathway comprises the functionally relevant efferent arm of the vagovagal circuit.

A transmembrane residue influences the interaction of propofol with the strychnine-sensitive glycine alpha1 and alpha1beta receptor

BACKGROUND

Propofol, well known for its anesthetic effects, acts as a positive allosteric modulator of the alpha-aminobutyric acid type A (GABA(A)) receptor but also enhances the function of the glycine receptor. The GABA modulatory effects of propofol are influenced by an amino acid residue located within the second transmembrane domain (TM2) of the GABA(A) receptor beta subunit. In glycine alpha(1) subunits, the homologous residue (serine 267) affects the glycine modulatory actions of alcohols and alkane anesthetics. In the present study we investigated the role of this residue on the interaction of propofol with the glycine alpha(1) and alpha(1)beta receptor.

METHODS

The influence of propofol on wild type and mutant (alpha(1)S267M, alpha(1)S267I, alpha(1)S267Mbeta, alpha(1)S267Ibeta) glycine receptors expressed in human embryonic kidney 293 cells was investigated by using the whole-cell clamp technique.

RESULTS

Mutation of the alpha(1) subunit TM2 serine residue to either isoleucine or methionine decreased the sensitivity of the receptor to glycine, and abolished the direct activation of the glycine receptor by propofol. Additionally, the methionine and particularly the isoleucine mutation decreased the glycine-enhancing actions of propofol.

CONCLUSIONS

The nature of the TM2 residue (267) of the glycine alpha(1) subunit influences the glycine modulatory effect of propofol and direct activation of the receptor by this anesthetic. A comparison of the impact of such complementary mutations on the interaction of propofol with glycine and GABA(A) receptors should permit a better understanding of the molecular determinants of action of propofol on these structurally related receptors and may aid in the development of selective glycine receptor modulators.

Spatial memory is intact in aged rats after propofol anesthesia

BACKGROUND

We have previously demonstrated that aged rats have persistent impairment of spatial memory after sedation with nitrous oxide or general anesthesia with isoflurane-nitrous oxide. Propofol has different receptor mechanisms of action and a favorable short-term recovery profile, and it has been proposed that propofol is devoid of enduring effects on cognitive performance. No studies have investigated this question in aged subjects, however, so we designed an experiment to examine the long-term effects of propofol anesthesia on spatial working memory.

METHODS

Eighteen-mo-old rats were randomized to 2 h of 100% oxygen-propofol anesthesia (n=11) or to a control group that breathed 100% oxygen (n=10). Propofol was administered by continuous infusion via a tail vein catheter. Rats breathed spontaneously and rectal temperature was maintained. Mean arterial blood pressure was measured noninvasively and a venous blood gas was obtained just before discontinuation of propofol. After a 2-day recovery, spatial working memory was assessed for 14 days using a 12-arm radial maze. The number of total errors, number of correct choices to first error, and time to complete the maze was recorded and analyzed using a repeated measure analysis of variance (ANOVA), with P<0.05 being considered statistically significant.

RESULTS

The average propofol infusion rate was 0.6+/-0.1 mg.kg (-1).min(-1), a rate corresponding to a 50% effective concentration dose in adult rats. Mean arterial blood pressure during anesthesia was 100+/-2 mm Hg and venous blood gases remained in the normal range. There was no difference between the control and previously anesthetized rats on any measure of radial arm maze performance, indicating propofol anesthesia produces no lasting impairment in spatial working memory in aged rats.

CONCLUSIONS

In aged rats, propofol anesthesia is devoid of the persistent memory effects observed with other general anesthetics in this model. Thus, while it appears that the state of general anesthesia is neither necessary nor sufficient for development of postanesthetic memory impairment, the choice of anesthetics may play a role in late cognitive outcome in the aged.

Population based models of cortical drug response: insights from anaesthesia

A great explanatory gap lies between the molecular pharmacology of psychoactive agents and the neurophysiological changes they induce, as recorded by neuroimaging modalities. Causally relating the cellular actions of psychoactive compounds to their influence on population activity is experimentally challenging. Recent developments in the dynamical modelling of neural tissue have attempted to span this explanatory gap between microscopic targets and their macroscopic neurophysiological effects via a range of biologically plausible dynamical models of cortical tissue. Such theoretical models allow exploration of neural dynamics, in particular their modification by drug action. The ability to theoretically bridge scales is due to a biologically plausible averaging of cortical tissue properties. In the resulting macroscopic neural field, individual neurons need not be explicitly represented (as in neural networks). The following paper aims to provide a non-technical introduction to the mean field population modelling of drug action and its recent successes in modelling anaesthesia.

A hypothesis on the origin and evolution of the response to inhaled anesthetics

In this article, I present an evolutionary explanation for why organisms respond to inhaled anesthetics. It is conjectured that organisms today respond to inhaled anesthetics owing to the sensitivity of ion channels to inhaled anesthetics, which in turn has arisen by common descent from ancestral, anesthetic-sensitive ion channels in one-celled organisms (i.e., that the response to anesthetics did not arise as an adaptation of the nervous system, but rather of ion channels that preceded the origin of multicellularity). This sensitivity may have been refined by continuing selection at synapses in multicellular organisms. In particular, it is hypothesized that 1) the beneficial trait that was selected for in one-celled organisms was the coordinated response of ion channels to compounds that were present in the environment, which influenced the conformational equilibrium of ion channels; 2) this coordinated response prevented the deleterious consequences of entry of positive charges into the cell, thereby increasing the fitness of the organism; and 3) these compounds (which may have included organic anions, cations, and zwitterions as well as uncharged compounds) mimicked inhaled anesthetics in that they were interfacially active, and modulated ion channel function by altering bilayer properties coupled to channel function. The proposed hypothesis is consistent with known properties of inhaled anesthetics. In addition, it leads to testable experimental predictions of nonvolatile compounds having anesthetic-like modulatory effects on ion channels and in animals, including endogenous compounds that may modulate ion channel function in health and disease. The latter included metabolites that are increased in some types of end-stage organ failure, and genetic metabolic diseases. Several of these predictions have been tested and proved to be correct.

Is a new paradigm needed to explain how inhaled anesthetics produce immobility?

A paradox arises from present information concerning the mechanism(s) by which inhaled anesthetics produce immobility in the face of noxious stimulation. Several findings, such as additivity, suggest a common site at which inhaled anesthetics act to produce immobility. However, two decades of focused investigation have not identified a ligand- or voltage-gated channel that alone is sufficient to mediate immobility. Indeed, most putative targets provide minimal or no mediation. For example, opioid, 5-HT3, gamma-aminobutyric acid type A and glutamate receptors, and potassium and calcium channels appear to be irrelevant or play only minor roles. Furthermore, no combination of actions on ligand- or voltage-gated channels seems sufficient. A few plausible targets (e.g., sodium channels) merit further study, but there remains the possibility that immobilization results from a nonspecific mechanism.

General anesthetics have additive actions on three ligand gated ion channels

BACKGROUND

The purpose of this study was to determine whether pairs of compounds, including general anesthetics, could simultaneously modulate receptor function in a synergistic manner, thus demonstrating the existence of multiple intraprotein anesthetic binding sites.

METHODS

Using standard electrophysiologic methods, we measured the effects of at least one combination of benzene, isoflurane (ISO), halothane (HAL), chloroform, flunitrazepam, zinc, and pentobarbital on at least one of the following ligand gated ion channels: N-methyl-D-aspartate receptors, glycine receptors and gamma-aminobutyric acid type A receptors.

RESULTS

All drug-drug-receptor combinations were found to exhibit additive, not synergistic modulation. ISO with benzene additively depressed N-methyl-D-aspartate receptors function. ISO with HAL additively enhanced glycine receptors function, as did ISO with zinc. ISO with HAL additively enhanced gamma-aminobutyric acid type A receptors function as did all of the following: HAL with chloroform, pentobarbital with ISO, and flunitrazepam with ISO.

CONCLUSION

The simultaneous allosteric modulation of ligand gated ion channels by general anesthetics is entirely additive. Where pairs of general anesthetic drugs interact synergistically to produce general anesthesia, they must do so on systems more complex than a single receptor.

Coincident activity of converging pathways enables simultaneous long-term potentiation and long-term depression in hippocampal CA1 network in vivo

Memory is believed to depend on activity-dependent changes in the strength of synapses, e.g. long-term potentiation (LTP) and long-term depression (LTD), which can be determined by the sequence of coincident pre- and postsynaptic activity, respectively. It remains unclear, however, whether and how coincident activity of converging efferent pathways can enable LTP and LTD in the pathways simultaneously. Here, we report that, in pentobarbital-anesthetized rats, stimulation (600 pulses, 5 Hz) to Schaffer preceding to commissural pathway within a 40-ms timing window induced similar magnitudes of LTP in both pathways onto synapses of CA1 neurons, with varied LTP magnitudes after reversal of the stimulation sequence. In contrast, in urethane-anesthetized or freely-moving rats, the stimulation to Schaffer preceding to commissural pathway induced Schaffer LTP and commissural LTD simultaneously within a 40-ms timing window, without affecting synaptic efficacy in the reversed stimulation sequence. Coincident activity of Schaffer pathways confirmed the above findings under pentobarbital and urethane anesthesia. Thus, coincident activity of converging afferent pathways tends to switch the pathways to be LTP only or LTP/LTD depending on the activity states of the hippocampus. This network rule strengthens the view that activity-dependent synaptic plasticity may well contribute to memory process of the hippocampal network with flexibility or stability from one state to another.