Site Of Anesthetic Action Research Articles

Nitrous oxide-induced slow and delta oscillations

ObjectivesSwitching from maintenance of general anesthesia with an ether anesthetic to maintenance with high-dose (concentration >50% and total gas flow rate >4liters per minute) nitrous oxide is a common practice used to facilitate emergence from general anesthesia. The transition from the ether anesthetic to nitrous oxide is associated with a switch in the putative mechanisms and sites of anesthetic action. We investigated whether there is an electroencephalogram (EEG) marker of this transition. MethodsWe retrospectively studied the ether anesthetic to nitrous oxide transition in 19 patients with EEG monitoring receiving general anesthesia using the ether anesthetic sevoflurane combined with oxygen and air. ResultsFollowing the transition to nitrous oxide, the alpha (8–12Hz) oscillations associated with sevoflurane dissipated within 3–12min (median 6min) and were replaced by highly coherent large-amplitude slow-delta (0.1–4Hz) oscillations that persisted for 2–12min (median 3min). ConclusionsAdministration of high-dose nitrous oxide is associated with transient, large amplitude slow-delta oscillations. SignificanceWe postulate that these slow-delta oscillations may result from nitrous oxide-induced blockade of major excitatory inputs (NMDA glutamate projections) from the brainstem (parabrachial nucleus and medial pontine reticular formation) to the thalamus and cortex. This EEG signature of high-dose nitrous oxide may offer new insights into brain states during general anesthesia.

Homology Modeling of a Human Glycine Alpha 1 Receptor Reveals a Plausible Anesthetic Binding Site.

The superfamily of ligand-gated ion channels (LGICs) has been implicated in anesthetic and alcohol responses. Mutations within glycine and GABA receptors have demonstrated that possible sites of anesthetic action exist within the transmembrane subunits of these receptors. The exact molecular arrangement of this transmembrane region remains at intermediate resolution with current experimental techniques. Homology modeling methods were therefore combined with experimental data to produce a more exact model of this region. A consensus from multiple bioinformatics techniques predicted the topology within the transmembrane domain of a glycine alpha one receptor (GlyRa1) to be alpha helical. This fold information was combined with sequence information using the SeqFold algorithm to search for modeling templates. Independently, the FoldMiner algorithm was used to search for templates that had structural folds similar to published coordinates of the homologous nAChR (1OED). Both SeqFold and Foldminer identified the same modeling template. The GlyRa1 sequence was aligned with this template using multiple scoring criteria. Refinement of the alignment closed gaps to produce agreement with labeling studies carried out on the homologous receptors of the superfamily. Structural assignment and refinement was achieved using Modeler. The final structure demonstrated a cavity within the core of a four-helix bundle. Residues known to be involved in modulating anesthetic potency converge on and line this cavity. This suggests that the binding sites for volatile anesthetics in the LGICs are the cavities formed within the core of transmembrane four-helix bundles.

Contradicting a Unitary Theory of General Anesthetic Action: a History of Three Compounds from 1901 to 2001

The introduction of diethyl ether, chloroform, and nitrous oxide into clinical practice over 150 years ago revolutionized medicine and surgery. However, despite a long period of clinical use, it has been difficult to elucidate the molecular basis for general anesthesia. Even today this question continues to be a source of active inquiry and debate. Although investigators in the last half of the nineteenth century proposed various theories for the actions of anesthetics, the first ‘modern’ theory for general anesthetic action came from the independent works of Meyer and Overton. The observation by Meyer and Overton, supported by extensive experimental investigation, was that the anesthetic potency of certain compounds (‘narcotics’ in the older terminology) varied in direct proportion to the partition coefficient in a lipid solvent such as olive oil. While these results did not prove any particular molecular target, the results were often interpreted to indicate the lipid membrane as the site of anesthetic action (the Meyer-Overton ‘lipoid’ hypothesis), described as follows by Meyer: The narcotizing substance enters in a loose physico-chemical combination with the vitally important lipoids of the cell, perhaps with the lecithin, and in so doing changes their normal relationship to the other cell constituents, through which an inhibition of the entire cell chemism results.1 Although Meyer and Overton recognized limitations to their experimental observations, their work was taken as the basis for a unified ‘lipoid’ theory of anesthesia. The Meyer-Overton theory is not the only unitary theory of anesthesia action. At least seventeen distinct unified theories have been proposed between 1847 and 1997 (see Table 1). It is nearly impossible to find a review article or book chapter on the mechanism of anesthetic action that does not in some way mention the Meyer-Overton hypothesis or a related unified theory of anesthetic action. Table 1 Major Unitary Theories of General Anesthesia Formulated Between 1847 and 1997 While the legacy of Meyer and Overton has dominated investigations of general anesthetics, support for their hypothesis was hardly monolithic. In the last several decades in particular, unified theories of anesthesia have come under increasingly severe attacks and the general,2,3 although by no means unaminous,4–6 consensus is that anesthetics, as with most drugs in current medical use, act at protein receptor targets. A clear symbol of the ‘changing of the guard’ is illustrated by comparing two quotations from the 1990 and 1996 editions of the pharmacology textbook The Pharmacological Basis of Therapeutics. The 1990 edition reads: Most theories of anesthetic action are based on the physicochemical characteristics of the anesthetic drugs. These proposals relate closely to the correlation between the potency of an anesthetic agent and the solubility of the drug in oil…Interpretation of this fundamental result is thought to be crucial to the understanding of the action of anesthetics.7 In the 1996 edition of this text, the above passage was repeated but with the last sentence amended as follows: Interpretation of this fundamental results, once thought to be crucial to the understanding of the action of anesthetics, may be only an incidental finding.8 (italics added) The following sections will focus on three compounds whose actions are inconsistent with the Meyer-Overton hypothesis. Two of the compounds, α-chloralose and chloral hydrate, were anesthetics known to Meyer and Overton, and recognized at least by Overton as having properties which were difficult to explain by his hypothesis. The third compound, flurothyl (hexafluorodiethyl ether) is a volatile, fluorinated ether with convulsant properties which was discovered in 1957. While these three compounds posed a challenge for the Meyer-Overton hypothesis, their properties were mostly ignored by those favoring a unitary theory of anesthesia. The end of this manuscript will speculate on reasons for this.

The effect of rigidity, shape, unsaturation, and length on the anesthetic potency of hydrocarbons.

We previously hypothesized that anesthesia results from an action on two sites separated by 5 A. The hypothesis relied on the finding that fluorinated alkanes having active anesthetic sites at each end of the molecule produce anesthesia as long as the total number of carbon atoms in their structure does not exceed five (i.e., approximately 5 A), and on the sustaining of the 5-A separation by the rigidity produced by fluorination. In this study, we tested an alternative hypothesis: that the site of action cannot accommodate a rigid compound, particularly a rectilinear compound, having more than five carbon atoms, and that rigidity itself might limit the anesthetic potency of larger compounds. We tested the anesthetic potency of 11 hydrocarbons in which rigidity was increased by unsaturation. In 72 rats exposed to such compounds, we found that unsaturation, rigidity, or both produced by unsaturation either did not change (double bonds) or increased (triple bonds) potency for a given number of carbon atoms. For example, we found that the rectilinear, rigid 2,4-trans-trans-hexadiene was no less potent (minimum alveolar anesthetic concentration [MAC] 0.042 +/- 0.002 atm; mean +/- SD) than the flexible 1,5-hexadiene (0.047 +/- 0.005 atm) or n-hexane (0.0467 +/- 0.0055 atm) and that 3-hexyne was more potent (MAC 0.0146 +/- 0.0014 atm) than n-hexane (MAC 0.0467 +/- 0.0055 atm). We conclude that the site of anesthetic action can accommodate straight rigid structures of up to six carbons in length.

Clinical uses of alpha2 -adrenergic agonists.

Clinical uses of alpha2 -adrenergic agonists.

Tandem pore domain K channels: an important site of volatile anesthetic action?

Despite over 150 years of clinical use, the mechanism and molecular elements by which volatile anesthetics produce unconsciousness are not established. Although enhanced activity of inhibitory neurotransmitter systems (GABAA) and depression of excitatory neurotransmitter systems (NMDA) probably contribute to the anesthetic state, the role of other ion channels families have also been studied. Potassium channels represent the largest group of mammalian ion channels and their activity to reduce neuronal excitability makes them viable candidates as sites of anesthetic action. Several studies from the 1970's and 80's identified volatile anesthetic enhancement of neuronal potassium currents. More recently, a new family of K channels with a unique structure (tandem pore domains) that may be responsible for baseline or background K currents have been isolated and some members of this family can be activated by volatile anesthetics. These emerging findings suggest a new molecular mechanism by which volatile anesthetics may mediate central nervous system depression.

Contributions of dipole moments, quadrupole moments, and molecular polarizabilities to the anesthetic potency of fluorobenzenes

Previous studies have emphasized the role of molecular polarizability and electric moments, especially dipole and quadrupole moments, in binding of drugs to sites of action. A recent publication of ED50s that prevent response to a noxious stimulus for eight fluorobenzenes has made it possible to compare anesthetic potency with ab initio Hartree–Fock calculations of molecular polarizability as well as dipole and quadrupole moments. Fluorobenzenes provide a stringent test of the role of electric moments in anesthetic potency because individual dipole moments range from 0 to 2.84 debye (D) while the quadrupole moment of benzene is large and negative (−30×10 −40 C m 2), that of hexafluorobenzene is large and positive (30×10 −40 C m 2), and that of 1,3,5-trifluorobenzene is nearly zero. We found that anesthetic potency of fluorobenzenes was not affected by the presence of either dipole or quadrupole moments. This result is surprising because fluoroalkanes and fluorocycloalkanes are most potent when half fluorinated and are usually not anesthetics when perfluorinated. The results suggest that electrostatic interactions are not important for binding of fluorobenzenes at sites of anesthetic action and that these sites are different from those that bind conventional anesthetics.

Inhalational anesthetic effects on rat cerebellar nitric oxide and cyclic guanosine monophosphate production.

Inhalational anesthetics interact with the nitric oxide-cyclic guanosine monophosphate (NO-cGMP) pathway in the central nervous system (CNS) and attenuate excitatory neurotransmitter-induced cGMP concentration. The site of anesthetic action on the NO-cGMP pathway in the CNS remains controversial. This study investigated the effect of inhalational anesthetics on N-methyl-D-aspartate (NMDA)-stimulated NO synthase activity and cyclic cGMP production in rat cerebellum slices. The interaction of inhalational anesthetics with NO synthase activation and cGMP concentration was determined in cerebellum slices of 10-day-old rats. Nitric oxide synthase activity in cerebellum slices was assessed by measuring the conversion of L-[3H]arginine to L-[3H]citrulline. The cGMP content of cerebellum slices was measured by radioimmunoassay. Isoflurane at 1.5% and 3% enhanced the NMDA-stimulated NO synthase activity by two times while halothane at 1.5% and 3% produced no significant effect. However, the NMDA-stimulated cGMP production was inhibited by both anesthetic agents. The anesthetic inhibition of cGMP accumulation was not significantly altered by a mixture of superoxide dismutase and catalase or by glycine, a coagonist of the NMDA receptor. The enhancement of NMDA-induced NO synthase activity by isoflurane and the inhibition of NMDA-stimulated cGMP production by halothane and isoflurane suggests that inhalational anesthetics interfere with the neuronal NO-cGMP pathway. This inhibitory effect of anesthetics on cGMP accumulation is not due to either their interaction with the glycine binding site of the NMDA receptor or to the action of superoxide anions.

1 + (-1) = 0, or Not All Anesthetic Sites Are Created Equal

In Response: We thank Dr. Morgan for his interest and comments. As he points out, our use of polyhalogenated nonanesthetic alkanes [1] to test for a possible mechanism of anesthesia assumes a common (unitary) mechanism of inhaled anesthetic action. He believes, as does Dr. Eckenhoff [2], that there is more than one site of anesthetic action. As stated in our previous response [3], we agree that there are at least two sites for inhaled anesthetic action: the spinal cord likely modulates movement in response to noxious stimuli during general anesthesia, and the brain is the probable site of anesthetic action for amnesia and unconsciousness. However, different anatomic sites for different anesthetic end-points do not preclude a common mechanism of inhaled action when a single anesthetic end-point is evaluated (e.g., minimum alveolar anesthetic concentration). We have also noted [3] that the testing of a multiple-site theory of anesthesia is overwhelming since the combinations of possible sites of anesthetic action and anesthetic molecules approaches an astronomical value. Dr. Morgan, although believing in multiple sites of action, seems to accept the possibility of a limited number of sites (e.g., two or three). We suggest that even with a limited number of possible sites, the nonanesthetics [1] will be useful tools in defining the molecular and microscopic sites and mechanisms of anesthetic action. For example, if an investigator has a model in which only two sites are thought to be important in the production of the anesthetic end-point, then the nonanesthetics should not produce the same in vitro effects as anesthetics at both of these sites. The title of Dr. Morgan's letter presumably derives from his speculation that the absence of an anesthetic effect by a nonanesthetic might arise from an antagonism of excitatory and inhibitory effects of the compound and result in "behavioral effects [that] tend to cancel out, leaving the agent relatively impotent." However, we note that although several nonanesthetics had convulsant properties [1], some nonanesthetics (e.g., perfluoropropane) have little or no excitatory activity [4]. In addition, for the nonanesthetic convulsant compounds, the consistent finding of no appreciable change in the minimum alveolar anesthetic concentration of agents given concurrently (e.g., desflurane) would seem remarkably fortuitous. That is, it is difficult to imagine a precise balancing of excitatory and inhibitory effects by the diverse compounds that are nonanesthetics. (The nonanesthetics are not limited to alkanes--we have also found alcohols and ethers that are nonanesthetics [unpublished work]). Finally, some volatile compounds that cause convulsions (e.g., flurothyl) have anesthetic properties. Thus, it seems unlikely that the absence of anesthetic properties for these compounds is simply explained by an excitatory effect that "cancels out" an anesthetic effect. We conclude that 1 + 0 = 1. Donald D. Koblin, PhD, MD Edmond I Eger II, MD Department of Anesthesia University of California San Francisco, CA 94143-0464, and Anesthesiology Service (129) Veteran's Administration Hospital San Francisco, CA 94121 Michael J. Halsey, DPhil Nuffield Department of Anaesthetics University of Oxford Oxford, England

1 + (-1) = 0, or Not All Anesthetic Sites Are Created Equal

In Response: We thank Dr. Morgan for his interest and comments. As he points out, our use of polyhalogenated nonanesthetic alkanes [1] to test for a possible mechanism of anesthesia assumes a common (unitary) mechanism of inhaled anesthetic action. He believes, as does Dr. Eckenhoff [2], that there is more than one site of anesthetic action. As stated in our previous response [3], we agree that there are at least two sites for inhaled anesthetic action: the spinal cord likely modulates movement in response to noxious stimuli during general anesthesia, and the brain is the probable site of anesthetic action for amnesia and unconsciousness. However, different anatomic sites for different anesthetic end-points do not preclude a common mechanism of inhaled action when a single anesthetic end-point is evaluated (e.g., minimum alveolar anesthetic concentration). We have also noted [3] that the testing of a multiple-site theory of anesthesia is overwhelming since the combinations of possible sites of anesthetic action and anesthetic molecules approaches an astronomical value. Dr. Morgan, although believing in multiple sites of action, seems to accept the possibility of a limited number of sites (e.g., two or three). We suggest that even with a limited number of possible sites, the nonanesthetics [1] will be useful tools in defining the molecular and microscopic sites and mechanisms of anesthetic action. For example, if an investigator has a model in which only two sites are thought to be important in the production of the anesthetic end-point, then the nonanesthetics should not produce the same in vitro effects as anesthetics at both of these sites. The title of Dr. Morgan's letter presumably derives from his speculation that the absence of an anesthetic effect by a nonanesthetic might arise from an antagonism of excitatory and inhibitory effects of the compound and result in "behavioral effects [that] tend to cancel out, leaving the agent relatively impotent." However, we note that although several nonanesthetics had convulsant properties [1], some nonanesthetics (e.g., perfluoropropane) have little or no excitatory activity [4]. In addition, for the nonanesthetic convulsant compounds, the consistent finding of no appreciable change in the minimum alveolar anesthetic concentration of agents given concurrently (e.g., desflurane) would seem remarkably fortuitous. That is, it is difficult to imagine a precise balancing of excitatory and inhibitory effects by the diverse compounds that are nonanesthetics. (The nonanesthetics are not limited to alkanes--we have also found alcohols and ethers that are nonanesthetics [unpublished work]). Finally, some volatile compounds that cause convulsions (e.g., flurothyl) have anesthetic properties. Thus, it seems unlikely that the absence of anesthetic properties for these compounds is simply explained by an excitatory effect that "cancels out" an anesthetic effect. We conclude that 1 + 0 = 1. Donald D. Koblin, PhD, MD Edmond I Eger II, MD Department of Anesthesia University of California San Francisco, CA 94143-0464, and Anesthesiology Service (129) Veteran's Administration Hospital San Francisco, CA 94121 Michael J. Halsey, DPhil Nuffield Department of Anaesthetics University of Oxford Oxford, England

Anesthetic Potency Is Not Altered after Hypothermic Spinal Cord Transection in Rats

In essence, the clinical goal of general anesthesia is to produce a state of unresponsiveness and amnesia. These endpoints are commonly achieved with drugs like isoflurane, but the sites and mechanisms by which these specific endpoints are achieved remain unknown. Blocking the somatic motor response to painful stimuli is widely used as an indicator of anesthetic adequacy, and the concentration of anesthetic agent (minimum alveolar concentration [MAC]) required to achieve this unresponsiveness is the benchmark of anesthetic potency. Recent work has demonstrated that precollicular decerebration does not alter MAC in rats, suggesting that the forebrain is not a major site of action of isoflurane in blocking motor responses. The brain stem contains systems that modulate pain processing in the spinal cord. The current study was undertaken to assess the relative roles of the brain stem and spinal cord as sites of anesthetic action in blocking somatic responsiveness. In seven rats, anesthesia was induced and maintained with isoflurane in oxygen. MAC was determined by observing the response to tail clamp and fore- and hind limb toe pinch at three times: after intubation, after cervical laminectomy, and after staged hypothermic spinal cord transection. MAC determined by tail clamp did not change during the protocol (1.28 +/- 0.08% [mean +/- standard deviation] baseline vs. 1.25 +/- 0.18% postlaminectomy vs. 1.03 +/- 0.40% posttransection). In one animal, the MAC value decreased from a prelesion value of 1.2% to 0.25%, accounting for most of the variance in the postlesion mean; the MAC value as determined by withdrawal to rear paw pinch was unchanged from its prelesion value in this animal. The MAC values as determined by toe pinch in all animals remained unchanged after spinal transection of the lesion both rostrally and caudally. Somatic motor responsiveness and its sensitivity to isoflurane appeared to be unaltered despite acute loss of descending cortical and bulbar controls. This observation suggests that the site of anesthetic inhibition of motor response may be in the spinal cord.

Spinal Cord as a Site of Anesthetic Action

Spinal Cord as a Site of Anesthetic Action

Exaggerated anesthetic requirements in the preferentially anesthetized brain.

The brain is assumed to be the site of anesthetic action, but anesthetics have effects elsewhere, such as the spinal cord. A preferentially anesthetized goat brain model was used to determine the importance of anesthetic action in the brain. Six goats were anesthetized with isoflurane; after tracheal intubation and insertion of a femoral arterial catheter, bilateral neck dissections were performed to isolate the external carotid arteries and external jugular veins. The occipital arteries were ligated to prevent vertebral blood from entering the carotid system. (Goats do not have direct, significant vertebral artery contributions to the brain, and they lack internal jugular veins.) Control isoflurane minimum alveolar concentration (MAC) was determined using a dew-claw clamp as the painful stimulus. Following this, cranial venous blood was drained into a bubble oxygenator in which an isoflurane vaporizer was placed in line with the gas flow. Oxygenator arterial isoflurane concentration was estimated from the isoflurane partial pressure in the oxygenator exhaust. Isoflurane administration via the lungs was discontinued and the isoflurane partial pressure in the blood delivered via the carotid artery was increased by an amount required to bracket the partial pressures permitting and preventing movement in response to dew-claw stimulation. The native circulation was reestablished and MAC determined again. Cerebral isoflurane requirements were 1.2 +/- 0.3% (mean +/- SD) before bypass, increased to 2.9 +/- 0.7% during bypass when the brain was preferentially anesthetized, and decreased to 1.3 +/- 0.1% after bypass. The results support the importance of subcortical structures, such as the spinal cord, in the generation of purposeful movement in response to a painful stimulus under general anesthesia.

Halogenation and Anesthetic Potency

Previous studies have shown that the anesthetic potency of organic compounds increases as a given halogen is replaced with successively larger halogens. These studies often are limited in the accuracy of determination of potency, rarely correlate potency with physical properties, and usually fail to include ether compounds. Because establishing relationships between structure and activity may shed light on anesthetic action, we studied the new anesthetic, I-537 (CHF2-O-CHBr-CF3), relative to two other ether anesthetics, I-653 (CHF2-O-CHF-CF3) and isoflurane (CHF2-O-CHCl-CF3) for both of which MAC and oil/gas partition coefficients are accurately known. The oil/gas partition coefficient of I-537 at 37 degrees C was found to be 245 +/- 6 (mean +/- SD) and the MAC in Sprague-Dawley rats 0.52 +/- 0.07%. Increasing atomic weight of the 1-ethyl halogen (i.e., F in I-653, Cl in isoflurane, and Br in I-537) progressively decreases MAC (increases potency) and increases lipid solubility. Although potency and solubility change by more than 10-fold, the product of MAC and the oil/gas partition coefficient remains essentially constant (120 +/- 11). However, this product is significantly less than that for other inhaled anesthetics, a finding which either challenges the unitary theory of narcosis or suggests that the lipid solvent classically used to model the site of anesthetic action (olive oil) is inappropriate.

Subcellular distribution of an inhalational anesthetic in situ.

To better understand the mechanisms and sites of anesthetic action, we determined the subcellular partitioning of halothane in a tissue model. A method was found to fix the in vivo distribution of halothane in rat atrial tissue for subsequent electron microscopy and x-ray microanalysis. Atrial strips were exposed to various concentrations of halothane, rapidly frozen, cryo-sectioned, and cryo-transferred into an electron microscope. Irradiation of the hydrated cryosections with the electron beam caused halothane radiolysis, which allowed retention of the halogen-containing fragments after dehydration of the sections. The bromine from halothane was detected and quantified with x-ray microanalysis in various microregions of atrial myocytes. Halothane (bromine) partitioned largely to mitochondria, with progressively lower concentrations in sarcolemma, nuclear membrane, cytoplasm, sarcomere, and nucleus. Partitioning could not be explained solely by distribution of cellular lipid, suggesting significant and differential physicochemical solubility in protein. However, we found no saturable compartment in atrial myocytes within the clinical concentration range, which implies little specific protein binding.

Halothane inhibits 5-hydroxytryptamine uptake by synaptosomes from rat brain

The activity of 5-hydroxytryptamine (serotonin; 5-HT) in the central nervous system modulates sleep, the perception of pain and other functions of the body which might possibly relate to mechanisms of general anesthetic action. While administration of anesthetics has inconsistent effects on the content of 5-HT in brain, in vivo, accumulated data suggest that anesthetic drugs alter 5-HT homeostasis in the central nervous system. In an effort to identify one possible site of anesthetic action, the effect of halothane on the uptake of 5-HT was studied in synaptosomes isolated from the brain of rat. Established techniques were used to prepare the synaptosomal fractions and measure high affinity transport of radiolabelled 5-HT. Halothane inhibited synaptosomal accumulation of 5HT in a concentration-dependent manner, but had little effect on the passive or spontaneous release of the accumulated 5-HT. Rates of uptake of 5HT were inhibited by 43% in the presence of 1 mM halothane and by 75% of control in the presence of 5 mM halothane; the apparent I 50 for halothane was 1.0 ± 0.1 mM and Lineweaver-Burk analysis indicated the inhibition to be competitive at concentrations around the I 50.

Effects of non-electrolyte molecules with anesthetic activity on the physical properties of DMPC multilammelar liposomes

The effects of 13 non-electrolytes with moderate anesthetic potency on the order of DMPC liposomes were examined. Changes in order were monitored by steady-state fluorescence polarization techniques using 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). At 30°C, all of the compounds tested decreased the DPH steady-state anisotropy ( r s), with potencies highly correlated to their oil/water partition coefficients. However, only the most hydrophobic anesthetics decreased TMA-DPH r s. Some of the most hydrophilic compounds, including ethanol and urethane, actually increased TMA-DPH r s, suggestive of an increase in membrane order. The concept of selectivity was borrowed from partitioning theory and used to explain some effects on anesthetic potency of decreasing temperatures to 18°C. In the gel as opposed to the liquid crystalline phase, selectivity for decreasing membrane order (as monitored by DPH) markedly increased, suggesting that anesthetic partitioning and/or the site of anesthetic action was occurring in a more hydrophobic domain. The solute-independent difference (or capacity) between two membranes for perturbation was defined as membrane sensitivity. Sensitivity appeared to also decrease with decreasing temperature, despite the decrease in membrane partitioning. This effect is thought to result from the selective delivery of the anesthetic solute to the membrane interior and away from more hydrophilic domains where anesthetics may order membrane structure.

Reversal by pressure of seed germination promoted by anesthetics

Germination of Panicum capillare L. caryopses induced by solutions of ethanol and ethyl ether was prevented by application of pressure >1 MPa during the period of exposure to the anesthetic. This effect of pressure indicates that germination is correlated with expansion at a site of anesthetic action in a cell membrane. The effects of several other anesthetics were measured on germination of P. capillare seeds. Ethanol, chloroform, and ethyl ether had the highest activity. Methanol and isopropanol were inactive. The effective compounds are thought to distribute preferentially to lipid-solution interfaces in cell membranes of the seeds.

Biophysical Mechanisms of Anesthetic Action

The large number and diversity of anesthetic agents were evident to investigators 80 years ago, and suggested a physicochemical theory of anesthesia. Meyer and Overton were the first to offer a quantitative relationship between a physicochemical property and potency of anesthetic agents. They also focused attention on the lipid phase as the site of anesthetic action. Ferguson realized that the concentration of an agent at its site of action bears a generally unknown relation to the concentration in the external phase. However, at equilibrium the activity of an agent is the same in every phase, motivating Ferguson to suggest that activities rather than concentrations be used as indices of dosage. The critical-volume theory resulted from modification of the Meyer-Overton theory to include the molal volume of the anesthetic. The allowance for molal volume resulted initially from an attempt further to regularize the experimental data. The concept of a critical-volume fraction of anesthetic being necessary for narcosis was discussed in most detail by Mullins. Subsequently, the concept of the effect of the anesthetic has changed from filling of free space to expansion and fluidization of the membrane. The ability of pressure to cause excitant phenomena and antagonize anesthetics is predictable from the critical-volume theory and is therefore highly significant evidence. K. W. Miller and associates are perhaps most prominent in the recent quantification and formalization of the critical-volume theory and HPNS. The existence of a separate convulsant site(s) is suggested by the demonstration of significantly different compressibilities associated with anesthesia and convulsions. Work corroborating a separate convulsant site involved measurement of the partial molal volumes of a series of related convulsant and anesthetic ethers and calculation of each compound's solubility parameter. Multiple convulsant sites may exist, and these two methods may not have accessed the same site. Understanding the anesthetic-convulsant duality will have important practical application to deepwater diving, and may well offer important insight into the neurophysiologic and electrophysiologic effects of anesthetics. The application of ESR and NMR allows investigation at the molecular level of effects of anesthetics on biological and model membranes. Magnetic resonance techniques have generally supported the concept of membrane fluidization by anesthetics. Some investigators have recently attempted to displace the focus of attention from the lipid phase. However, the evidence is clearly against the aqueous-phase theory of Pauling and S.L. Miller. The microtubule theory of Allison and Nunn has not accumulated supporting evidence comparable to the lipid theories. Contradictory evidence makes any evaluation of this theory speculative. Additionally, the interspecies and intracellular variability of microtubules raises questions of the relevance of many studies...