Volatile anesthetics reduce agonist affinity at nicotinic acetylcholine receptors in the brain

In previous studies we and others have demonstrated that the activation of nicotinic acetylcholine receptors (nAChRs) is inhibited by subanesthetic concentrations of volatile anesthetics. The mechanism by which activation is inhibited is unknown. Studies of the evolutionarily related nAChRs from the electric fish Torpedo have suggested that volatile anesthetics alter the affinity of the agonist for the receptor. We studied the effect of two volatile anesthetics, isoflurane and sevoflurane, on equilibrium binding of the high-affinity nicotinic agonist epibatidine to nicotinic receptors from mouse brain. We studied binding to male and female brain separately, because sex differences in nicotine responses have been reported. Male and female brains have equal epibatidine binding without anesthetic. Isoflurane and sevoflurane reduce the binding of [(3)H]epibatidine to male and female nicotinic receptors, but only at concentrations at and above those required for anesthesia. The 50% inhibitory concentration for isoflurane inhibition of [(3)H]epibatidine binding to male brain was 0.58 +/- 0.07 mM and to female brain was 1.62 +/- 0.30 mM. The 50% inhibitory concentration for sevoflurane inhibition of [(3)H]epibatidine binding to male brain was 0.77 +/- 0.05 mM and to female brain was 0.77 +/- 0.04 mM. There was no statistically significant difference in the effect of either drug between sexes (P > 0.05). Although there is a slight decrease in agonist affinity at anesthetic concentrations, the marked reductions in nAChR function at subanesthetic concentrations cannot be attributed to changes in agonist affinity. IMPLICATIONS: Volatile anesthetics reduce the activation of nicotinic acetylcholine receptors by an unknown mechanism. We have demonstrated that although isoflurane and sevoflurane inhibit agonist affinity, the concentrations required are too large to be responsible for the dynamic changes observed.     

Mutation of alpha1G T-type calcium channels in mice does not change anesthetic requirements for loss of the righting reflex and minimum alveolar concentration but delays the onset of anesthetic induction

BACKGROUND: T-type calcium channels regulate neuronal membrane excitability and participate in a number of physiologic and pathologic processes in the central nervous system, including sleep and epileptic activity. Volatile anesthetics inhibit native and recombinant T-type calcium channels at concentrations comparable to those required to produce anesthesia. To determine whether T-type calcium channels are involved in the mechanisms of anesthetic action, the authors examined the effects of general anesthetics in mutant mice lacking alpha1G T-type calcium channels. METHODS: The hypnotic effects of volatile and intravenous anesthetics administered to mutant and C57BL/6 control mice were evaluated using the behavioral endpoint of loss of righting reflex. To investigate the immobilizing effects of volatile anesthetics in mice, the minimum alveolar concentration (MAC) values were determined using the tail-clamp method. RESULTS: The 50% effective concentration for loss of righting reflex and MAC values for volatile anesthetics were not altered after alpha1G channel knockout. However, mutant mice required significantly more time to develop anesthesia/hypnosis after exposure to isoflurane, halothane, and sevoflurane and after intraperitoneal administration of pentobarbital. CONCLUSIONS: The 50% effective concentration for loss of righting reflex and MAC values for the volatile anesthetics were not altered after alpha1G calcium channel knockout, indicating that normal functioning of alpha1G calcium channels is not required for the maintenance of anesthetic hypnosis and immobility. However, the timely induction of anesthesia/hypnosis by volatile anesthetic agents and some intravenous anesthetic agents may require the normal functioning of these channel subunits.     

Nonhalogenated anesthetic alkanes and perhalogenated nonimmobilizing alkanes inhibit alpha(4)beta(2) neuronal nicotinic acetylcholine receptors

The nonhalogenated anesthetic alkanes, cyclopropane and butane, do not enhance gamma-aminobutyric acid-elicited GABAergic currents, suggesting that these agents produce anesthesia via interactions with other molecular targets. Perhalogenated nonimmobilizing alkanes, such as 1,2-dichlorohexafluorocyclobutane and 2,3-dichlorooctafluorobutane, also fail to enhance GABAergic currents, but display specific behavioral effects that are distinct from those of structurally similar anesthetics. At concentrations predicted to be anesthetic, 1,2-dichlorohexafluorocyclobutane and 2,3-dichlorooctafluorobutane produce amnesia but fail to produce immobility. Neuronal nicotinic acetylcholine (nACh) receptors are sensitive to many anesthetics and are thought to have an important role in learning and memory. We postulated that neuronal nACh receptors might mediate the common amnestic action of nonhalogenated and perhalogenated alkanes. To test the hypothesis that neuronal nACh receptors have a role in mediating the behavioral effects of general anesthetics and nonimmobilizers, we quantified the inhibitory potencies of nonhalogenated anesthetic alkanes and perhalogenated nonimmobilizing alkanes on currents mediated by alpha(4)beta(2) neuronal nACh receptors. Our studies reveal that anesthetics and nonimmobilizers significantly inhibit alpha(4)beta(2) neuronal nACh receptors at concentrations that suppress learning and with potencies that correlate with their hydrophobicities. These results support the hypothesis that alpha(4)beta(2) neuronal nACh receptors mediate the amnestic actions of alkanes but not their immobilizing actions. IMPLICATIONS: The results of this study suggest that the immobilizing actions of general anesthetics do not result from the inhibition of alpha(4beta2) neuronal nicotinic acetylcholine receptors. However, the inhibition of neuronal nicotinic acetylcholine receptors may account for the amnestic activities of general anesthetics and nonimmobilizers.     

Anesthetic and Nonanesthetic Halogenated Volatile Compounds Have Dissimilar Activities on Nicotinic Acetylcholine Receptor Desensitization Kinetics

Background: The Meyer-Overton rule predicts that an anesthetic's potency will correlate with its oil solubility. A group of halogenated volatile compounds that disobey this rule has been characterized. These compounds do not induce anesthesia in rats at partial pressures exceeding those predicted by the Meyer-Overton rule to be anesthetic. The observation that potentiation of GABAA receptor responses by anesthetic and nonanesthetic halogenated volatile compounds correlates with their abilities to induce general anesthesia suggests that this receptor is involved in the mechanism of general anesthesia. However, the GABAA receptor is only one member of a superfamily of structurally similar ligand-gated ion channels. This study compares the actions of both anesthetic and nonanesthetic halogenated volatile compounds on another member of this superfamily of receptors, the nicotinic acetylcholine receptor (nAcChoR).

Methods: The actions of both anesthetic and nonanesthetic compounds on desensitization kinetics were characterized from the time-dependent binding of the fluorescent acetylcholine analogue, Dns-C6 -Cho, to the nAcChoR.

Results: At concentrations predicted by the Meyer-Overton rule to be equianesthetic, the anesthetics isoflurane and enflurane were significantly more effective than the nonanesthetics 1,2-dichlorohexafluorocyclobutane and 2,3-dichlorooctafluorobutane in enhancing the fraction of receptors preexisting in the slow desensitized state and increasing the apparent rates of agonist-induced fast and slow desensitization.     

The role of nicotinic inhibition in ketamine-induced behavior

Several anesthetic drugs are nicotinic antagonists at or below levels used for anesthesia, including ketamine and volatile anesthetics. In contrast, propofol does not inhibit nicotinic receptors. To determine the potential behavioral ramifications of nicotinic inhibition by ketamine, we determined the doses of ketamine required to induce immobility, impair the righting reflex, and cause analgesia in the absence and presence of several nicotinic ligands. Propofol was used as a control in similar experiments. When used as a sole anesthetic drug, 383 +/- 22 mg/kg ketamine intraperitoneally (IP) was required for immobility and 180 +/- 17 mg/kg IP impaired righting reflex. Propofol, 371 +/- 34 mg/kg IP, induced immobility whereas 199 mg/kg IP inhibited the righting reflex. Nicotinic antagonists had no effect on the dose of propofol or ketamine required for either end-point. When nociceptive responses were tested at subhypnotic doses, no pronociceptive or antinociceptive phase was identified for propofol, whereas analgesia was induced at ketamine doses larger than 60 mg/kg IP. The broad-spectrum nicotinic antagonist mecamylamine enhanced the analgesic action of ketamine. These findings are different than those seen with volatile anesthetics, where nicotinic inhibition is thought to be responsible for a pronociceptive action. Such a phase is possibly obscured by analgesia induced as a result of N-methyl-d-aspartic acid antagonism by ketamine. IMPLICATIONS: Ketamine and volatile anesthetics, but not propofol, inhibit neuronal nicotinic acetylcholine receptors in clinically relevant concentration ranges. Nicotinic inhibition by ketamine is not related to its immobilizing or sedating effects but may play a role in ketamine's analgesic action.     

Characterization of the interactions between volatile anesthetics and neuromuscular blockers at the muscle nicotinic acetylcholine receptor

Volatile anesthetics enhance the neuromuscular blockade produced by nondepolarizing muscle relaxants (NDMRs). The neuromuscular junction is a postulated site of this interaction. We tested the hypothesis that volatile anesthetic enhancement of muscle relaxation is the result of combined drug effects on the nicotinic acetylcholine receptor. The adult mouse muscle nicotinic acetylcholine receptor (alpha(2), beta, delta, epsilon) was heterologously expressed in Xenopus laevis oocytes. Concentration-effect curves for the inhibition of acetylcholine-induced currents were established for vecuronium, d-tubocurarine, isoflurane, and sevoflurane. Subsequently, inhibitory effects of NDMRs were studied in the presence of the volatile anesthetics at a concentration equivalent to half the concentration producing a 50% inhibition alone. All individually tested compounds produced rapid and readily reversible concentration-dependent inhibition. The calculated 50% inhibitory concentration values were 9.9 nM (95% confidence interval [CI], 8.4-11.4 nM), 43.4 nM (95% CI, 33.6-53.3 nM), 897 microM (95% CI, 699-1150 microM), and 818 microM (95% CI, 685-1001 microM) for vecuronium, d-tubocurarine, isoflurane, and sevoflurane, respectively. Coapplication of either isoflurane or sevoflurane significantly enhanced the inhibitory effects of vecuronium and d-tubocurarine, especially so at small concentrations of NDMRs. Volatile anesthetics increase the potency of NDMRs, possibly by enhancing antagonist affinity at the receptor site. This effect may contribute to the clinically observable enhancement of neuromuscular blockade by volatile anesthetics. IMPLICATIONS: Isoflurane and sevoflurane enhance the receptor blocking effects of nondepolarizing muscle relaxants on nicotinic acetylcholine receptors.     

Heteromeric nicotinic inhibition by isoflurane does not mediate MAC or loss of righting reflex

BACKGROUND: Neuronal nicotinic acetylcholine receptors (nAChRs) have been implicated in the mechanism of action of isoflurane as they are inhibited at subanesthetic concentrations. Despite clear evidence for nicotinic inhibition at relevant isoflurane concentrations, it is unclear what behavioral result ensues, if any. METHODS: The authors have modeled two behaviors common to all general anesthetics, immobility and hypnosis, as minimum alveolar concentration that prevents movement in response to a supramaximal stimulus (MAC) and loss of righting reflex (LORR). They have tested the ability of nicotinic pharmacologic modulators and congenital absence of most heteromeric nAChRs to affect concentration of isoflurane required for these behaviors. RESULTS: Neither mecamylamine, 5 mg/kg, nor chlorisondamine, 10 mg/kg, affected isoflurane MAC. Nicotine caused a small decrease in MAC. None of the above agents had any effect on the concentration of isoflurane required for LORR. Mice genetically engineered to lack the beta 2 nicotinic gene product were not different in MAC or LORR from controls. CONCLUSIONS: Nicotinic antagonists do not cause MAC or LORR. Inhibition of nicotinic acetylcholine receptors by isoflurane is not likely related to its ability to provide immobility and hypnosis in a surgical setting. This is perhaps not surprising as the inhibition of nAChRs in vitro is complete at an isoflurane concentration equal to one half of MAC. Nicotinic inhibition may, however, be involved in anesthetic behaviors such as amnesia and analgesia, which occur at lower anesthetic concentrations.     

Nonanesthetic Volatile Drugs Obey the Meyer-Overton Correlation in Two Molecular Protein Site Models

Background: Nonanesthetic volatile compounds fail to inhibit movement in response to noxious stimulation at concentrations predicted to induce anesthesia from their oil-water partitioning. Thus they represent tools to determine whether molecular models behave like the targets that mediate in vivo anesthetic actions. The effects of volatile anesthetics and non-anesthetics were examined in two experimental models in which anesthetics interact directly with proteins: the pore of the nicotinic acetylcholine receptor and human serum albumin.

Methods: Wild-type mouse muscle nicotinic receptors and receptors containing pore mutations ([Greek small leter alpha] S252I + [Greek small letter beta] T263I) were studied electrophysiologically in membrane patches from Xenopus oocytes. Patch currents evoked by brief pulses of acetylcholine were measured in the presence of enflurane and two nonanesthetics, 1,2-dichlorohexafluorocyclobutane and 2,3-dichlorooctafluorobutane. Nonanesthetic interactions with human serum album were assessed by quenching of intrinsic protein fluorescence.

Results: Both anesthetic and nonanesthetic volatile compounds inhibited wild-type and [Greek small letter alpha] S252I + [Greek small letter beta] T263I mutant nicotinic channels but displayed different selectivity for open versus resting receptor states. Median inhibitory concentrations (IC50 s) in wild-type nicotinic receptors were 870 +/- 20 [micro sign]M for enflurane, 37 +/- 3 [micro sign]M for 1,2-dichlorohexafluorocylcobutane, and 11.3 +/- 5.6 [micro sign]M for 2,3-dichlorooctafluorobutane. For all three drugs, ratios of wild-type IC50 s to mutant IC50mut ranged from 7-10, and ratios of wild-type IC50 s to predicted anesthetic median effective concentrations (EC50 s) ranged from 1.8-2.3. 1,2-Dichlorohexafluorocyclobutane quenched human serum albumin with an apparent dissociation constant (Kd) of 160 +/- 11 [micro sign]M. The ratios of dissociation constants to predicted EC50 s for the nonanesthetics were within a factor of two of the dissociation constant:EC (50) ratios calculated for halothane and chloroform from previous published results.     

Thermogenesis inhibition in brown adipocytes is a specific property of volatile anesthetics

BACKGROUND: This investigation examined the possibility that the inhibitory effect of halothane on nonshivering thermogenesis (heat production) in brown adipocytes is not a universal effect of all anesthetic agents but related to the type of anesthetic. METHODS: Brown adipocytes from hamster were isolated with a collagenase digestion method and incubated with anesthetic agents. The rate of oxygen consumption was measured with an oxygen electrode. The effect of clinically relevant (and higher) doses of anesthetics of different classes on basal and norepinephrine-induced thermogenesis (oxygen consumption) was tested. RESULTS: Two distinct groups of anesthetics could be distinguished: thermogenesis inhibitors and noninhibitors. Thermogenesis inhibitors include volatile anesthetics such as halothane (IC(50), 1.1 mm), ether (IC(50), 20 mm), and chloroform (IC(50), 2.2 mm) (nominal concentrations), but also tribromoethanol (IC(50), 0.6 mm), all inducing inhibition of norepinephrine-induced thermogenesis without affecting the EC for norepinephrine. Thermogenesis noninhibitors include the nonvolatile anesthetics pentobarbital, propofol, ketamine, and urethane, the inhalation anesthetic nitrous oxide, and, notably, also the volatile nonanesthetics (nonimmobilizers) 1,2-dichlorohexafluorocyclobutane and 2,3-dichlorooctafluorobutane; none of these compounds had any effect on norepinephrine-induced thermogenesis at any concentration tested. CONCLUSIONS: There are two distinct classes of anesthetics with regard to effects on thermogenesis, thermogenesis inhibitors and thermogenesis noninhibitors. The results are important for the interpretation of studies in thermal biology in general; specifically, they indicate that conclusions concerning regulation of nonshivering thermogenesis during anesthesia depend on the type of anesthetic used. Of clinical importance is that the volatile anesthetics are inhibitory for nonshivering thermogenesis and thus for an alternative heat production when myorelaxants prevent shivering. As the distinction between thermogenesis inhibitors and thermogenesis noninhibitors corresponds to the distinction between volatile and nonvolatile anesthetics, it may be related to the mode of action of the volatile anesthetics.     

Laryngeal Mask Airway insertion with total intravenous anesthesia for transsternal thymectomy in patients with myasthenia gravis: report of 5 cases

Myasthenia gravis is a chronic autoimmune disease characterized by a reduction of postsynaptic nicotinic acetylcholine receptors at the neuromuscular junction. Most myasthenia gravis patients require thymectomy. Intravenous (IV) anesthetics may be superior to inhalation agents in these patients. The Laryngeal Mask Airway (LMA), when compared with the endotracheal tube, causes less airway resistance, which in turn may lead to a decreased bronchoconstrictive reflex, less atelectasis, and fewer pulmonary infections. We report 5 patients with myasthenia gravis, who underwent transsternal thymectomy with total IV anesthesia and LMA.     

General Anesthetics Modify the Kinetics of Nicotinic Acetylcholine Receptor Desensitization at Clinically Relevant Concentrations

Background: General anesthetics are thought to induce anesthesia through their actions on ligand-gated ion channels. One such channel, the nicotinic acetylcholine receptor (nAcChoR), can be found in different subtypes in the central nervous system and at the periphery in the neuromuscular junction. The latter subtype of the nAcChoR is a useful model for examining interactions between general anesthetics and ligand-gated ion channels, because it can be isolated and purified in sufficient quantities to allow for biophysical and biochemical studies. This study examines the actions of general anesthetics on agonist-induced conversion of the nAcChoR to inactive desensitized conformational states.

Methods: Nicotinic acetylcholine receptor membranes were purified from the electric organ of Torpedo nobiliana. Agonist-induced desensitization was characterized from the time-dependent increase in fluorescence intensity that results from the binding of the fluorescent acetylcholine analog, Dns-C6 -Cho, to the nAcChoR.

Results: Mixing Dns-C6 -Cho with nAcChoR-rich membranes results in an increase in fluorescence that is characterized by four rate processes. Concentrations of isoflurane and butanol, which range from subclinical to toxic increase the rates of the third and fourth components of fluorescence, corresponding to fast and slow desensitization, respectively. At concentrations that are twice their EC sub 50 s for anesthesia, isoflurane, butanol, chloroform, methanol, and cyclopentane-methanol increase the apparent rates of fast and slow desensitization by an average of 92 plus/minus 22% and 108 plus/minus 22%, respectively.     

Anesthesia and myotonic dystrophy type 2: a case series

Background

Myotonic dystrophy type 2 (DM2) is a genetically distinct disorder that shares some phenotypical features of myotonic dystrophy type 1 (DM1). However, anesthetic management of patients with DM2 has not been described. The purpose of this study is to report the anesthetic management of a series of patients with DM2 and to describe their response to anesthesia.

Methods

We performed a computerized search of the Mayo Clinic medical records database looking for patients with DM2 who underwent general anesthesia. The medical records were reviewed for anesthetic technique, medications used, and postoperative complications.

Results

We identified 19 patients with DM2 who underwent 39 general anesthetics, 17 monitored anesthetic care cases, and two regional anesthetics. The patients exhibited normal responses to succinylcholine, nondepolarizing neuromuscular blockers, neostigmine, induction agents, and volatile anesthetics. Serious postoperative complications related to DM2 did not occur.

Conclusion

In our series, patients with DM2 tolerated commonly used anesthetics without obvious complications, and they exhibited normal responses to muscle relaxants. These observations suggest that these medications may be used safely in patients with DM2.     

Electrophysiological approach to mechanisms for actions of general anesthetics

Although general anesthetics were first used more than 160 years ago, their mechanisms have remained mysterious. During the past decade, significant progress in our understanding of general anesthetic action at the cellular and network system levels has been made. Our recent work demonstrates (a) that intravenous anesthetics, but not volatile agents, enhance the discharge of GABA from presynaptic terminals, (b) that intravenous anesthetics produce frequency-dependent modification (FDM) of anesthesia, and (c) that FDM is responsible for the unsuccessful immobilization or hypnosis during intravenous anesthesia. In addition, we review the development of hypothesis for anesthetic action, non-specific versus specific action, cutoff phenomenon in n-alcohols, and anesthesiological approach to consciousness.     

Considerations for general anesthesia combined with epidural anesthesia in a patient with stiff-person syndrome

We report the successful management of anesthesia in a patient with stiff-person syndrome (SPS) undergoing a thymectomy using a volatile anesthetic combined with epidural anesthesia. The anesthetic concern in patients with SPS is the possibility of postoperative hypotonia due to the presence of excessive γ-aminobutyric acid (GABA) resulting from an interaction between the anesthetic agents and preoperatively taken therapeutic drugs. Epidural anesthesia has the advantages of decreasing the required amount of anesthetics with GABAergic action, and relieving the postoperative pain that causes the symptoms of SPS. Epidural anesthesia could be a useful technique in SPS patients.     

Effects of general anesthetics on neuronal nicotinic acetylcholine receptors and their roles in the mechanism of anesthesia]

Neuronal nicotinic acetylcholine receptors (nAchRs) are widely expressed in the central and autonomic nervous systems and have subunit compositions with biophysical and pharmacological properties distinct from those of the receptors at the neuromuscular junction. They are thought to modulate synaptic transmission in the central nervous system (CNS) mainly by regulating the release of neurotransmitters. Although roles of neuronal nAchRs in the CNS are poorly understood, these receptors are involved in cognitive performance, nociception and psychoneurological disorders such as Alzheimer's and Parkinson disease. It is known that both central and peripheral neuronal nAchRs are sensitive to various types of anesthetics. Among those, barbiturates, ketamine, volatile and gaseous anesthetics depress neuronal nAchRs at or below clinical concentrations. Inhibition of neuronal nAchRs by barbiturates is unlikely to contribute to the anesthetic action of barbiturates, since this effect does not correlate with the anesthetic potencies of barbiturate stereoisomers. Relevance of inhibition of these receptors is controversial for anesthetic effects of other anesthetics, because conflicting results have been obtained from comparison of this effect with anesthetic actions of stereoisomers or structurally related compounds. However, it is possible that inhibition of central nAchRs contributes to secondary effects attributed to anesthesia such as impairment in memory and cognitive performance.     

Naloxone does not antagonize the analgesic effects of inhalation anesthetics

A previous demonstration that the ratio of analgesic to anesthetic endpoints is not constant across inhalation anesthetic agents implies that more than one mechanism of action may be operant in general anesthesia. We hypothesized that the endogenous opiate systems might account for this observed disparity in ratios. The tail flick ED50 (TFED50) in response to a heat stimulus, as an index of analgesia, and MAC as an index of anesthesia, were determined in rats treated with either saline or naloxone, 20 mg/kg, and exposed to halothane, enflurane, or isoflurane. Our findings confirmed those of Deady et al., showing a lack of uniformity of ratios of TFED50/MAC, with values of 0.90 +/- 0.03 for halothane, 0.80 +/- 0.04 for enflurane, and 0.70 +/- 0.04 for isoflurane. Naloxone had no effect on TFED50, MAC, or their ratio. If the endogenous opiate system were involved in the analgesic effect of general anesthetics, naloxone would have affected the ratios. We conclude that opiate systems are not involved in the analgesic action of general anesthetics.     

Differential membrane effects of general and local anesthetics.

The authors studied the effects of varying Na+ and Ca++ concentrations and of replacing H2O with D2O in Ringer's solution upon the actions of general and local anesthetics on isolated frog sciatic nerves. This experimental model was used to study whether general anesthetics affect excitable membranes in a manner similar to that of typical membrane stabilizers (local anesthetics). Procaine (2.5-7.5 mM), halothane (9, 18, and 36 mM), enflurane (8 mM), and ketamine (0.15 and 0.73 mM) raised threshold and lowered spike amplitude, and their effects were facilitated by reducing Na+ concentration in the Ringer's solution. The local anesthetic effects of procaine (2.5-7.5 mM) and ketamine (0.73 mM) were antagonized by Ca++, while the axonal depressant effect of halothane was facilitated by increasing Ca++ concentration in the Ringer's solution, indicating a different mode of action. General anesthetics also differed from local anesthetics in their interaction with water: replacement by D2O of H2O in the Ringer's solution selectively increased the axonal depressant effects of halothane and enflurane but not those of ketamine or procaine. Since D2O differs from H2O in its greater ice-likeness, these results are consistent with the view that general anesthetics stabilize excitable membranes via stabilization of the water-biopolymer lattice, as predicted by the hydrate-microcrystal theory of anesthesia. In contrast, local anesthetics may stabilize excitable tissues by binding to the same fixed negative charges of the membrane to which Ca++ is normally bound. (Key words: Theories of anesthesia, hydrate-microcrystal; Nerve, mode of action of anesthetics; Anesthetics, volatile, halothane; Anesthetics, volatile, enflurane; Anesthetics, local, procaine; Anesthetics, intravenous, ketamine.)     

The pressure reversal of a variety of anesthetic agents in mice

The aim of this work was to study in mammals the ability of high pressures to reverse the anesthesia produced by a wide range of general anesthetics. Dose-response curves were obtained using mice at pressures ranging from 1 to 125 atm for five agents, namely alpha-chloralose, ethylcarbamate, phenobarbital and, for comparison, nitrogen and argon. The increase of ED50 was found to be a linear function of pressure in each case, but the proportionate increases in ED50 with pressure were greater for the three non-inhalation agents than for the two gases. Thus, the ratio of ED50 at 100 atm to that at 1 atm was 1.74 for alpha-chloralose, 1.68 for ethylcarbamate, and 1.54 for phenobarbital. On the other hand, the corresponding ratios for argon and nitrogen were only 1.36 and 1.34. The potencies of three short-acting agents (trichloroethanol, ketamine, and alphadione) were shown to increase with decreasing pressure, although ED50 values could not be obtained. It is concluded that pressure reverses the actions of a wide variety of anesthetics in mice. The results of this study are not inconsistent with either the fluidized lipid membrane or the critical volume hypotheses of anesthetic action.     

Mutation of α1G T-type Calcium Channels in Mice Does Not Change Anesthetic Requirements for Loss of the Righting Reflex and Minimum Alveolar Concentration but Delays the Onset of Anesthetic Induction

Background: T-type calcium channels regulate neuronal membrane excitability and participate in a number of physiologic and pathologic processes in the central nervous system, including sleep and epileptic activity. Volatile anesthetics inhibit native and recombinant T-type calcium channels at concentrations comparable to those required to produce anesthesia. To determine whether T-type calcium channels are involved in the mechanisms of anesthetic action, the authors examined the effects of general anesthetics in mutant mice lacking [alpha]1G T-type calcium channels.

Methods: The hypnotic effects of volatile and intravenous anesthetics administered to mutant and C57BL/6 control mice were evaluated using the behavioral endpoint of loss of righting reflex. To investigate the immobilizing effects of volatile anesthetics in mice, the minimum alveolar concentration (MAC) values were determined using the tail-clamp method.

Results: The 50% effective concentration for loss of righting reflex and MAC values for volatile anesthetics were not altered after [alpha]1G channel knockout. However, mutant mice required significantly more time to develop anesthesia/hypnosis after exposure to isoflurane, halothane, and sevoflurane and after intraperitoneal administration of pentobarbital.     

Inhibitory effects of isoflurane and nonimmobilizing halogenated compounds on neuronal nicotinic acetylcholine receptors

BACKGROUND: Neuronal nicotinic acetylcholine receptors (nAchRs) are inhibited by low concentrations of volatile anesthetics. However, it is not clear whether this phenomenon contributes to the anesthetic effects of volatile anesthetics. Effects of a volatile anesthetic (isoflurane) and structurally related nonimmobilizers (F6: 1,2-dichlorohexafluorocyclobutane, F8: 2,3-dichlorooctafluorobutane) on the current mediated through neuronal nAchRs were studied. METHOD: This study investigated neuronal nAchRs in PC12 cells and acutely dissociated rat medial habenula (MHb) neurons. Whole cell currents elicited by 30 microm nicotine were recorded in the absence and presence of the halogenated agents. The minimum alveolar concentrations (MACs) for F6 and F8 were predicted from Meyer-Overton correlation. RESULTS: All halogenated compounds inhibited the nicotine-induced current in a concentration-dependent manner in PC12 cells. In MHb neurons, while isoflurane and F6 significantly inhibited the nicotine-induced peak current, F8 failed to inhibit it. The peak currents in the presence of isoflurane at 1.7 MAC, of F6 at 2.4 MAC, and of F8 at 2.2 MAC were 12, 31, and 97% of control, respectively. CONCLUSIONS: Isoflurane, F6, and F8 inhibited ganglion-type nAchRs in PC12 cells independent from their abilities to produce the anesthetic state. In MHb neurons, isoflurane and F6, which lack the immobilizing effect but has the amnesic effect, inhibited nAchRs. Native brain nicotinic receptors in MHb neurons were almost insensitive to F8, which lacks both the immobilizing and the amnesic effect. These results are consistent with the hypothesis that inhibition of nAchRs in MHb neurons is not important for the anesthetic effect but may contribute to the amnesic effect of these agents.