Anesthetic mechanisms in-vitro and in general anesthesia

Investigation of Anesthetic-Protein Interactions by a Thermodynamic Approach

Anesthetics can interact with a wide variety of proteins in the body, including ion channels and alter their activity, but little is known about the molecular mechanisms of the interactions responsible for the functional activity. Characterization of the nature of anesthetic-protein interactions therefore is important and requires the complete analysis of the binding energetics. Isothermal titration calorimetry (ITC) is the only technique that allows quantitative determination of all thermodynamic parameters, including the equilibrium binding constant (K_B), the standard Gibbs free energy change (ΔG), the enthalpy change (ΔH), the entropy change (ΔS), heat capacity change (ΔC_p), and stoichiometry (n) of the reaction. ITC does not require any labeling or modification of the interacting partners analyzed and can be performed in solution with small amounts of reagents. In this chapter we describe the general properties of the ITC method, highlighting some critical aspects of experimental planning and data analysis, with practical application to anesthetic-protein interactions.

QSAR Studies on the Anesthetic Action of Some Polyhalogenated Ethers

There has been an on-going debate about the mode of action of general anesthetics and until now, many sites have been postulated as target site for action of these compounds. Here, some quantum chemical-based quantitative structure-activity relationship (QSAR) models were developed for a set of polyhalogenated ethers in order to investigate the aspects of their anesthetic action, which is not completely defined yet, although some hypotheses have been suggested. A data set including 25 polyhalogenated methyl ethyl ethers were selected, and different descriptors were calculated for each molecule using density functional theory calculations, and subsequently some multilinear QSAR models were built by using different sets of the calculated molecular descriptors. The result showed that polar (polarizability) and non-polar (log P) parameters have mixed role on the anesthetic activity i.e. models with high statistical quality were obtained in combination with these two parameters. Also a good model between anesthetic action and electrostatic potentials was obtained, which may imply the important role of electronic interactions in the anesthetic activity of the compounds. Finally, a four-parametric QSAR model containing log P, molecular polarizability, most positive charge and an electrostatic potential parameters was obtained, which indicated that the anesthetic action of the polyhaloganted ethers may be proceeded through lipophilic, steric and columbic interactions.

Emergent properties of CNS neuronal networks as targets for pharmacology: application to anticonvulsant drug action

CNS drugs may act by modifying the emergent properties of complex CNS neuronal networks. Emergent properties are network characteristics that are not predictably based on properties of individual member neurons. Neuronal membership within networks is controlled by several mechanisms, including burst firing, gap junctions, endogenous and exogenous neuroactive substances, extracellular ions, temperature, interneuron activity, astrocytic integration and external stimuli. The effects of many CNS drugs in vivo may critically involve actions on specific brain loci, but this selectivity may be absent when the same neurons are isolated from the network in vitro where emergent properties are lost. Audiogenic seizures (AGS) qualify as an emergent CNS property, since in AGS the acoustic stimulus evokes a non-linear output (motor convulsion), but the identical stimulus evokes minimal behavioral changes normally. The hierarchical neuronal network, subserving AGS in rodents is initiated in inferior colliculus (IC) and progresses to deep layers of superior colliculus (DLSC), pontine reticular formation (PRF) and periaqueductal gray (PAG) in genetic and ethanol withdrawal-induced AGS. In blocking AGS, certain anticonvulsants reduce IC neuronal firing, while other agents act primarily on neurons in other AGS network sites. However, the NMDA receptor channel blocker, MK-801, does not depress neuronal firing in any network site despite potently blocking AGS. Recent findings indicate that MK-801 actually enhances firing in substantia nigra reticulata (SNR) neurons in vivo but not in vitro. Thus, the MK-801-induced firing increases in SNR neurons observed in vivo may involve an indirect effect via disinhibition, involving an action on the emergent properties of this seizure network.

Effects of volatile and intravenous anesthetics on the uptake of GABA, glutamate and dopamine by their transporters heterologously expressed in COS cells and in rat brain synaptosomes

Although the neurotransmitter uptake system is considered a possible target for the presynaptic action of anesthetic agents, observations are inconsistent concerning effects on the transporter and their clinical relevance. The present study examined the effects of volatile and intravenous anesthetics on the uptake of GABA, glutamate and dopamine in COS cells heterologously expressing the transporters for these neurotransmitters and in the rat brain synaptosomes. Halothane and isoflurane, but not thiamylal or thiopental, significantly inhibited uptake by COS cell systems of GABA, dopamine and glutamic acid in a concentration-dependent manner within clinically relevant ranges for anesthesia induced by these agents. Similarly, in synaptosomes halothane and isoflurane but not thiopental significantly suppressed the uptake of GABA and glutamic acid, respectively. These results do not support the hypothesis that volatile and intravenous anesthetics exert their action via specific inhibition of GABA uptake to enhance inhibitory GABAergic neuronal activity. Rather, they suggest that presynaptic uptake systems for various neurotransmitters including GABA may be the molecular targets for volatile anesthetic agents.

Ethanol and halothane differently modulate HLA class I and class II oligomerization. A new look at the mode of action of anesthetic agents through fluorescence spectroscopy

The field of research considering the working mechanism of anesthetic agents is a complex one and the site or sites of action of general anesthetics are yet to be elucidated. Through the years, on the molecular level, the discussion has shifted from the lipid theories to the more specific interaction with the proteins responsible for the signal transduction. While this approach led to several models, they offer, at best, partial explanations for the observed phenomena. Anesthetic agents interact with many systems, of which the neuronal is best studied, leaving interaction with the immune defense system relatively unexplored. In this study we focus on the interaction of ethanol and halothane with the co-localization on the membrane of HLA I and II molecules. We show that ethanol tends to randomize the distribution of HLA I and II molecules, while halothane increases the clustering of HLA I proteins. The notion that anesthetics modulate cell function by disrupting clustering and thereby promoting a random distribution is a novel approach that may explain the general involvement of many systems during exposition to anesthetic drugs. In this study we show the disturbance of co-localization of molecules that may form a functional network. The relevance of this finding depends on the importance of these networks for extracellular and intracellular processes.

Effects of halothane on the membrane potential in skeletal muscle of the frog

Halothane has many effects on the resting membrane potential (V(m)) of excitable cells and exerts numerous effects on skeletal muscle one of which is the enhancement of Ca(2+) release by the sarcoplasmic reticulum (SR) resulting in a sustained contracture. The aim of this study was to analyse the effects of clinical doses of halothane on V(m), recorded using intracellular microelectrodes on cleaned and non stimulated sartorius muscle which was freshly isolated from the leg of the frog Rana esculenta. We assessed the mechanism of effects of superfused halothane on V(m) by the administration of selective antagonists of membrane bound Na(+), K(+) and Cl(-) channels and by inhibition of SR Ca(2+) release. Halothane (3%) induced an early and transient depolarization (4.5 mV within 7 min) and a delayed and sustained hyperpolarization (about 11 mV within 15 min) of V(m). The halothane-induced transient depolarization was sensitive to ryanodine (10 microM) and to 4-acetamido-4'-isothiocyanatostilbene 2,2' disulphonic acid (SITS, 1 mM). The hyperpolarization of V(m) induced by halothane (0.1 - 3%) was dose-dependent and reversible. It was insensitive to SITS (1 mM), tetrodotoxin (0.6 microM), and tetraethylammonium (10 mM) but was blocked and/or prevented by ryanodine (10 microM), charybdotoxin (CTX, 1 microM), and glibenclamide (10 nM). Our observations revealed that the effects of halothane on V(m) may be related to the increase in intracellular Ca(2+) concentration produced by the ryanodine-sensitive Ca(2+) release from the SR induced by the anaesthetic. The depolarization may be attributed to the activation of Ca(2+)-dependent Cl(-) (blocked by SITS) channels and the hyperpolarization to the activation of large conductance Ca(2+)-dependent K(+) channels, blocked by CTX, and to the opening of ATP-sensitive K(+) channels, inhibited by glibenclamide.