Mechanisms of Anesthesia

A variety of quite different molecular structures have anesthetic activity. Other molecules, quite similar in structure to anesthetics, may have opposite effects and cause convulsions. Despite these paradoxes, imaginative attempts have been presented to unify a single mechanism for anesthesia and represent clues to the mechanism of consciousness.

Figure 7.5: Anesthesia results from prevention of protein switching between two or more different conformational states induced by binding of ligand, calcium ion, or voltage change. Anesthetic gas molecules bind by Van der Waals forces within hydrophobic pockets within which electron dipole oscillations may be a trigger, or switching mechanism for protein conformation.

Figure 7.5: Anesthesia results from prevention of protein switching between two or more different conformational states induced by binding of ligand, calcium ion, or voltage change. Anesthetic gas molecules bind by Van der Waals forces within hydrophobic pockets within which electron dipole oscillations may be a trigger, or switching mechanism for protein conformation.

Several neural functions and structures are inhibited by anesthetics, although some are inhibited at concentrations higher than that required for anesthesia. Propagation of action potentials resulting in nerve conduction as well as microtubule dependent axoplasmic transport are blocked by anesthetics such as halothane, which also causes depolymerization of MT. However, the measurable neural function which is sensitive to anesthetics at concentrations barely sufficient to erase consciousness is synaptic transmission. Neurotransmitter release and/or neurotransmitter receptor activation thus appear to be the specific actions whose inhibition results in anesthesia. Inhibition of collective communicative activities within the cytoskeleton and attached membrane proteins related to synaptic transmission are presently undetectable with current technologies and their inhibition may still account for anesthesia. Anatomical localization of sites of anesthetic action has focused on regions with many synapses. Accordingly, the reticular activating system which is involved with regulation of wakefulness and attention and is "polysynaptic" was originally viewed as a major site of anesthetic effect. However, many other brain regions are inhibited by anesthetics at concentrations relevant to anesthesia. Further, different anesthetics which have the same end result have dominant actions at differing areas of the brain, and have differing characteristics and side effects.

The mechanism of anesthesia, the "other side" of consciousness, is a beguiling enigma. Efforts to understand the actions of anesthetics began in mid-nineteenth century; Claude Bernard observed that the anesthetic chloroform inhibited "protoplasmic streaming" in slime mold. Around the turn of the twentieth century Meyer (1899) and Overton (1901) discovered that the anesthetic potency of a group of compounds directly correlated with their solubility in a lipid environment, specifically olive oil. Because membranes are largely lipid, the natural conclusion was that anesthetics exerted their effects through actions on lipids in membranes. Much later it was determined that critical, dynamic effects in membranes occurred via proteins, and that hydrophobic regions within proteins were "lipid-like" and hence able to bind anesthetic gas molecules by weak Van der Waals forces. Most contemporary theories agree that anesthesia results from alteration of dynamic, conformational functions of important brain neural proteins: membrane ion channels, synaptic receptors, cytoskeleton, neurotransmitter releasing mechanisms, and/or enzymes (Koplin and Eger 1979; Kaufman, 1977; Eyring, Woodbury and D'Arrigo, 1973). Opinions and theories disagree as to whether anesthetics alter these dynamic protein functions directly via intra-protein hydrophobic pockets, or indirectly via membrane lipids surrounding proteins, or at lipid protein interfaces (Franks and Lieb, 1982). There is further disagreement as to whether a single, unitary mechanism can explain actions of a wide range of anesthetic compounds acting on a presumably wide range of neural proteins, as well as explain reversal of anesthesia by increased pressure, and the paradoxical relationship between anesthetics and convulsants (Halsey, 1976).

Correlations of anesthetic potency with solubility in "lipid-like" solvents (olive oil, octanol, lecithin) are consistent with anesthetic effects occurring either in lipids or in "lipid-like" hydrophobic pockets within proteins (Eger, Lundgren, Miller and Stevens, 1969). Wulf and Featherstone (1957) were among the first to demonstrate binding of anesthetics within hydrophobic regions of whale myoglobin and other proteins. They suggested that anesthetic binding caused a protein conformational change with a resultant increase in exposure of charged groups at protein surfaces which sufficiently altered protein function to cause anesthesia. Altered charge groups can change protein binding of surface water and can explain the findings which led Pauling (1965) and Miller (1969) to propose that anesthetic induced water-protein complexes ("clathrates") were the cause of anesthesia. Other research including Brammall, Beard and Hulands (1974) demonstrated that anesthetics can cause conformational changes in functional proteins, but at concentrations significantly higher than that required to cause reversible clinical anesthesia.

A logical inference is that anesthetics, rather than causing protein conformational changes, prevented those that were necessary for normal function. Looking for model systems of functional dynamic activity in proteins, a number of researchers have focused on a seemingly obscure group of systems which, on the surface, are distantly removed from any semblance of cognitive function: anesthetic inhibition of luminescence from photoproteins in firefly and a variety of bacteria (Harvey, 1915; Halsey and Smith, 1970; White and Dundas, 1970). Firefly luciferase is a protein dimer of about 100 kilodaltons (not unlike tubulin). When combined with a 280 dalton hydrophobic molecule called luciferin, and in the presence of ATP and oxygen, an excited electron state is induced which results in photon emission. The light emission from the luciferase/luciferin complex is exquisitely sensitive to anesthetic gases in proportion to their anesthetic potency. Studying the light emission from firefly luciferase, Ueda (Ueda, 1965; Ueda and Karmaya, 1973) proposed that anesthetic binding at a hydrophobic site in the luciferase protein induced an inactive conformational state which prevented the ATP activated excited state which would normally result in luminescence. In more recent anesthetic studies on firefly luminescence, Franks and Lieb (1984) corroborated earlier work that anesthetic potency correlated with luciferase inhibition potency, much like the earlier correlations with solubility in "lipid-like" hydrophobic environments. They also reported that anesthetic binding did not cause a luciferase conformational change, and that the anesthetic effects occurred by competitive binding with luciferin for a hydrophobic site in luciferase. Franks and Lieb suggested that anesthesia occurs due to displacement of endogenous ligands from brain neural protein hydrophobic pockets which bear some resemblance to the luciferin site within firefly luciferase. Displacement of endogenous ligands would not explain, however, anesthetic inhibition of functional protein conformational changes which occur in response to stimuli other than hydrophobic ligands (i.e. voltage change, calcium, other ions).

Ultimately, anesthesia results from impairment of protein conformational dynamics. As described in Chapter 6, mechanisms which regulate protein states are not totally understood, but collective, cooperative mechanisms appear important. The conformational regulation of anesthetic sensitive neural proteins is schematically summarized in Figure 7.5. Under normal, non-anesthetic conditions, there is a class of neural proteins which undergoes functional conformational change in response to appropriate stimuli. These proteins may be membrane bound ion channels which become permeable to sodium, potassium, chloride, or calcium ions; membrane bound receptors for acetylcholine, gamma-amino butyric acid (GABA), glycine, aspartate, glutamate or other neurotransmitters which are coupled to ionic channels; cytoskeletal proteins which perform a wide range of dynamic intracellular functions; or various enzymes. Stimuli which induce functional conformational change in these proteins are thought to include binding of ligands (i.e. neurotransmitters), changes in voltage, or ionic flux (i.e. calcium ions). These functional conformational changes may either facilitate neuronal excitatory activity or have inhibitory effects. The common anesthetic sensitive link for both excitatory and inhibitory effects appears to be a protein conformational change.

The coupling of conformational states to appropriate stimuli is not clearly understood; possible explanations were discussed in Chapter 6 and appear to involve collective, nonlinear coupling of electronic mobility to mechanical movements. Conformationally coupled electron mobility within neural protein hydrophobic pockets may thus be essential to highest cognitive function and sensitive to the effects of anesthetic molecules. Evidence from gas chromatography (McNair and Bonelli, 1969) and corona discharge experiments (Hameroff and Watt, 1983) suggest that anesthetic gases can directly alter electron energy, capturing, retarding, or enhancing their mobility by Van der Waals-London forces (instantaneous dipole coupling). Thus regulation of protein conformational state, as proposed by Fröhlich's theory of coherence, electret behavior or Davydov's soliton model, would be directly inhibited by anesthetic occupancy of protein hydrophobic pockets because the environmental medium for electron mobility would be significantly altered. Hydrophobic pockets vary in size and shape among different proteins which would account for the variability among different anesthetic gas molecules. The weak, reversible Van der Waals interactions by which anesthetics bind within hydrophobic regions explain the reversibility of anesthesia. The flip side of anesthesia, consciousness, may be inferred to occur due to dynamic protein conformational effects among a distributed class of brain neural proteins. Because consciousness is the brain function most susceptible to anesthetics (which can spare breathing control and many reflexes at appropriate concentrations) it appears related to collective effects of protein conformartional dynamics. Inhibition of a subset of the collective dynamics would therefore inhibit consciousness.

Conversely, excitation of a specific subset involved in collective protein dynamics could lead to alteration or enhancement of consciousness. Psychoactive drugs such as amphetamines and LSD can exert profound effects on the brain/mind at extremely low concentrations. They are thought to bind and to exert their effects at membrane protein receptors such as the serotonin receptor in the case of LSD. Kang and Green (1970) correlated the potency of psychoactive compounds with their electronic bond structure and molecular orbitals. They found that the drug molecules' electronic orbital energy available to the receptors (and connected membrane and cytoskeleton) was an index of drug potency. Thus collective effects related to electron mobility within key proteins in a cooperative assembly may be important to consciousness.

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