Nonhalogenated Anesthetic Alkanes and Perhalogenated Nonimmobilizing Alkanes Inhibit α4β2 Neuronal Nicotinic Acetylcholine Receptors

Anesthetic synergy between two n-alkanes

OBJECTIVE

N-butane and n-pentane can both produce general anesthesia. Both compounds potentiate γ-aminobutyric acid type A (GABA_A) receptor function, but only butane inhibits N-methyl-d-aspartate (NMDA) receptors. It was hypothesized that butane and pentane would exhibit anesthetic synergy due to their different actions on ligand-gated ion channels.

STUDY DESIGN

Prospective experimental study.

ANIMALS

A total of four Xenopus laevis frogs and 43 Sprague-Dawley rats.

METHODS

Alkane concentrations for all studies were determined via gas chromatography. Using a Xenopus oocyte expression model, standard two-electrode voltage clamp techniques were used to measure NMDA and GABA_A receptor responses in vitro as a function of butane and pentane concentrations relevant to anesthesia. The minimum alveolar concentrations (MAC) of butane and pentane were measured separately in rats, and then pentane MAC was measured during coadministration of 0.25, 0.50 or 0.75 times MAC of butane. An isobole with 95% confidence intervals was constructed using regression analysis. A sum of butane and pentane that was statistically less than the lower-end confidence bound isobole indicated a synergistic interaction.

RESULTS

Both butane and pentane dose-dependently potentiated GABA_A receptor currents over the study concentration range. Butane dose-dependently inhibited NMDA receptor currents, but pentane did not modulate NMDA receptors. Butane and pentane MAC in rats was 39.4±0.7 and 13.7±0.4 %, respectively. A small but significant (p<0.03) synergistic anesthetic effect with pentane was observed during administration of either 0.50 or 0.75×MAC butane.

CONCLUSIONS

Butane and pentane show synergistic anesthetic effects in vivo consistent with their different in vitro receptor effects.

CLINICAL RELEVANCE

Findings support the relevance of NMDA receptors in mediating anesthetic actions for some, but not all, inhaled agents.

Inhibition of human alpha4beta2 neuronal nicotinic acetylcholine receptors by volatile aromatic anesthetics depends on drug hydrophobicity

BACKGROUND

Volatile aromatic compounds such as benzene are general anesthetics that cause amnesia, hypnosis, and immobility in response to noxious stimuli when inhaled. Although these compounds are not used clinically, they are frequently found in commercial items such as solvents and household cleaning products and are abused as inhalant drugs. Volatile aromatic anesthetics are useful pharmacological tools for probing the relationship between chemical structure and drug activity at putative general anesthetic targets. Neuronal nicotinic acetylcholine (nACh) receptors are ligand-gated ion channels widely expressed in the brain, which are thought to play important roles in learning and memory. In this study, we tested the hypothesis that aromatic anesthetics reversibly inhibit alpha(4)beta(2) neuronal nACh receptor function and sought to determine the structural correlates of receptor inhibition.

METHODS

Electrophysiological techniques were used to quantify the effects of 8 volatile aromatic anesthetics on currents elicited by 1 mM ACh and mediated by human alpha(4)beta(2) nACh receptors expressed in Xenopus oocytes.

RESULTS

All of the volatile aromatic anesthetics used in this study reversibly inhibited alpha(4)beta(2) nACh receptors with IC(50) values ranging from 0.00091 atm for 1,2-difluorobenzene to 0.045 atm for hexafluorobenzene. With the exception of hexafluorobenzene, all of the compounds had IC(50) values less than minimum alveolar concentration. Inhibitory potency correlated poorly with the cation-pi binding energies of the compounds (r(2) = 0.48, P = 0.059). However, there was a good correlation between inhibitory potency and the octanol/gas partition coefficient (r(2) = 0.87, P = 0.0008).

CONCLUSIONS

Volatile aromatic anesthetics potently and reversibly inhibit human alpha(4)beta(2) neuronal nACh receptors. This inhibition may play a role in producing amnesia. In contrast to N-methyl-d-aspartate receptors, the inhibitory potencies of aromatic anesthetics for alpha(4)beta(2) neuronal nACh receptors seem to be dependent on drug hydrophobicity rather than electrostatic properties. This implies that the volatile aromatic anesthetic binding site in the alpha(4)beta(2) neuronal nACh receptor is hydrophobic in character and differs from the nature of the binding site in N-methyl-D-aspartate receptors.

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.

Unifying electrostatic mechanism for receptor-ligand activity

A prior article in skeletal form proposed an electrostatic mechanism for receptor-ligand activity. The present review provides an elaboration, including supporting evidence. The fundamental aspect entails the presence of molecular electrostatic potential associated with ions and dipoles in the ligand. The ligand can be regarded as an electrical link that joins prevalent electrostatic fields present in the surrounding protein matrix. The exact role of these fields is speculative. One possibility is to function as conduits for electrons and radicals in cell signaling. There is increasing support for important participation of these species in signal transduction. There might also be a favorable influence on energetics involving the electron transfer process. A summary of receptor biology is also provided, including receptors for acetylcholine (nicotinic and muscarinic), GABA, adrenergic, and glutamate.

Correlating the clinical actions and molecular mechanisms of general anesthetics

PURPOSE OF REVIEW

To summarize recent in-vitro and in-vivo research on molecular mechanisms of general anesthetics' actions.

RECENT FINDINGS

Classes of general anesthetics with distinct clinical profiles appear to induce amnesia, hypnosis, and immobility via different molecular targets. Propofol, etomidate, and barbiturates produce profound amnesia and hypnosis, but weak immobility, by enhancing the activity of specific gamma-aminobutyric acid typeA receptors. In contrast, nitrous oxide, xenon, and ketamine produce analgesia, but weak hypnosis and amnesia, by inhibiting glutamate and nicotinic receptors and activating potassium 'leak' channels such as TREK-1. Volatile halogenated anesthetics show little selectivity for molecular targets. They act on all the channels mentioned above, and other targets such as glycine receptors and mediators of neurotransmitter release.

SUMMARY

Several clinically distinct 'anesthetic states' are induced by different classes of drugs acting on neuronal circuits via different molecular targets. Understanding the mechanisms underlying the therapeutic and toxic actions of general anesthetics helps us reframe the 'art' of anesthesia into more of a 'science'. These studies also enhance efforts to develop new drugs with improved clinical utility.

Mechanisms of general anesthesia: from molecules to mind

Despite the widespread presence of clinical anesthesiology in medical practice, the mechanism by which diverse inhalational agents result in the state of general anesthesia remains unknown. Over recent decades, our understanding of general anesthetic mechanisms has evolved dramatically from early unitary hypotheses, largely due to the development and influence of a myriad of scientific disciplines ranging from molecular biology to cognitive neuroscience. These discoveries have led to a renaissance of investigation into the mechanisms of general anesthetics and have generated both novel answers and questions. In this chapter, we review the major hypotheses of general anesthetic mechanisms of action and present an expanded overview of current investigation into those mechanisms. We also present a framework to aid in thinking about the actions of these agents, highlighting the relationship between putative targets at the molecular level and the more integrated functional changes in behavior and consciousness.

The N-Methyl-d-aspartate Receptor Inhibitory Potencies of Aromatic Inhaled Drugs of Abuse: Evidence for Modulation by Cation-π Interactions

Benzene and several close structural analogs are inhaled drugs of abuse with general anesthetic activity. By virtue of their pi electron clouds, they may engage in attractive electrostatic interactions with cationic atomic charges on protein targets. In this study, we tested the hypothesis that inhaled drugs of abuse inhibit human N-methyl-D-aspartate (NMDA) receptors with potencies that correlate with their abilities to engage in cation-pi interactions. Electrophysiological techniques were used to define the NR1/NR2B NMDA receptor inhibitory concentrations of volatile benzene analogs, and computer modeling was used to quantify their abilities to engage in cation-pi interactions and their molecular volumes. In addition, each compound's octanol/gas partition coefficient (a measure of hydrophobicity) was quantified. All 18 compounds inhibited human NR1/NR2B NMDA receptors reversibly and in a concentration-dependent manner. NMDA receptor inhibitory potency correlated strongly with the ability to engage in cation-pi interactions, weakly with hydrophobicity, and was independent of molecular volume. This is consistent with the hypothesis that cation-pi interactions enhance the binding of inhaled drugs of abuse to the NMDA receptor and suggests that the receptor binding site(s) for these drugs possesses significant cationic character.

Inhaled anesthetics and immobility: mechanisms, mysteries, and minimum alveolar anesthetic concentration

Studies using molecular modeling, genetic engineering, neurophysiology/pharmacology, and whole animals have advanced our understanding of where and how inhaled anesthetics act to produce immobility (minimum alveolar anesthetic concentration; MAC) by actions on the spinal cord. Numerous ligand- and voltage-gated channels might plausibly mediate MAC, and specific amino acid sites in certain receptors present likely candidates for mediation. However, in vivo studies to date suggest that several channels or receptors may not be mediators (e.g., gamma-aminobutyric acid A, acetylcholine, potassium, 5-hydroxytryptamine-3, opioids, and alpha(2)-adrenergic), whereas other receptors/channels (e.g., glycine, N-methyl-D-aspartate, and sodium) remain credible candidates.

Isoflurane antagonizes the capacity of flurothyl or 1,2-dichlorohexafluorocyclobutane to impair fear conditioning to context and tone

In animals, the conventional inhaled anesthetic, isoflurane, impairs learning fear to context and fear to tone, doing so at concentrations that produce amnesia in humans. Nonimmobilizers are inhaled compounds that do not produce immobility in response to noxious stimulation, nor do they decrease the requirement for conventional inhaled anesthetics. Like isoflurane, the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N) impairs learning at concentrations less than those predicted from its lipophilicity to produce anesthesia. The capacity of the nonimmobilizer di-(2,2,2,-trifluoroethyl) ether (flurothyl) to affect learning and memory has not been studied. Both nonimmobilizers can cause convulsions. We hypothesized that if isoflurane, 2N, and flurothyl act by the same mechanism to impair learning and memory, their effects should be additive. We found that isoflurane, 2N, and flurothyl (each, alone) impaired learning fear to context and fear to tone in rats, with the nonimmobilizers doing so at concentrations less than those that cause convulsions. (Fear was defined by freezing [volitional immobility] in the presence of the conditioned stimulus [context or tone].) However, the combination of isoflurane and 2N or flurothyl produced an antagonistic rather than an additive effect on learning, a finding in conflict with our hypothesis. And flurothyl was no less potent than 2N (at least no less potent relative to the concentration of each that produced convulsions) in its capacity to impair learning. We conclude that conventional inhaled anesthetics and nonimmobilizers impair learning and memory by different mechanisms. The basis for this impairment remains unknown.

IMPLICATIONS

Conventional inhaled anesthetics and nonimmobilizers are antagonistic in their effects on learning and memory, and this finding suggests that they impair learning and memory, at least in part, by different mechanisms.

Acetylcholine receptors and thresholds for convulsions from flurothyl and 1,2-dichlorohexafluorocyclobutane

There are acetylcholine receptors throughout the central nervous system, and they may mediate some forms and aspects of convulsive activity. Most high-affinity binding sites on nicotinic acetylcholine receptors for nicotine, cytisine, and epibatidine in the brain contain the beta2 subunit of the receptor. Transitional inhaled compounds (compounds less potent than predicted from their lipophilicity and the Meyer-Overton hypothesis) and nonimmobilizers (compounds that do not produce immobility despite a lipophilicity that suggests anesthetic qualities as predicted from the Meyer-Overton hypothesis) can produce convulsions. The nonimmobilizer flurothyl [di-(2,2,2,-trifluoroethyl)ether] blocks the action of gamma-aminobutyric acid on gamma-aminobutyric acid(A) receptors, whereas the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N, also called F6) does not. 2N can block the action of acetylcholine on nicotinic acetylcholine receptors. We examined the relative capacities of these compounds to cause convulsions in mice having and lacking the beta2 subunit of the acetylcholine receptor. The partial pressure causing convulsions in half the mice (the 50% effective concentration [EC(50)]) was the same as in control mice. For the knockout mice, the EC(50) for flurothyl was 0.00170 +/- 0.00030 atm (mean +/- SD), and for 2N, it was 0.0345 +/- 0.0041 atm. For the control mice, the respective values were 0.00172 +/- 0.00057 atm and 0.0341 +/- 0.0048 atm. The ratio of the 2N to flurothyl EC(50) values was 20.8 +/- 3.5 for the knockout mice and 21.7 +/- 7.0 for the control mice. These results do not support the notion that acetylcholine receptors are important mediators of the capacity of 2N or flurothyl to cause convulsions. However, we also found that both nonimmobilizers inhibit rat alpha4beta2 neuronal nicotinic acetylcholine receptors at EC(50) partial pressures (0.00091 atm and 0.062 atm for flurothyl and 2N, respectively) that approximate those that produce convulsions (0.0015 atm and 0.04 atm).

IMPLICATIONS

The results from the present study provide conflicting data concerning the notion that acetylcholine receptors mediate the capacity of nonimmobilizers to produce convulsions.