Anesthetic Mechanisms of Action

Over the past decade, considerable evidence has been gathered to indicate that anesthetics may produce their effects via discrete molecular mechanisms.9 This contrasts with the considerable historical data on nonspecific effects of anesthetics, especially general anesthetics, on cell membrane lipids and proteins.

6.15.3.1 Protein Interactions

Considerable efforts have been focused on studying the interaction of general anesthetics with artificial proteins.16'17 These entities are distinct from receptors and ion channels. Millimolar binding constants have been determined for the interactions of isoflurane and halothane with bovine serum albumin using radioligand binding, 19F nuclear magnetic resonance, and tryptophan fluoresence quenching.15 Such binding involves hydrophobic, electrostatic, and van der Waals interaction as well as hydrogen bonding. General anesthetics are thought to bind to cavities within the transmembrane four-helix bundles of receptors, perhaps acting as allosteric modulators. Binding of halothane and isoflurane to ferritin, a 24mer of four-helix bundles, identified a motif at which micromolar binding was observed, higher than that previously reported for general anesthetic-protein interactions, that may reflect a common anesthetic binding pocket within the interhelical dimerization interface. This high-affinity anesthetic-apoferritin complex suggests a greater selectivity than had previously been thought and that direct protein actions, provided these interactions result in alterations in membrane function, may explain the physiological effects of anesthetics at concentrations lower than surgical levels of anesthetic, including loss of awareness.17 The o-3 fatty acids, e.g., eicosapentanoic acid, represent another class of druglike agent that may produce their effects by changing membrane fluidity.18,19

6.15.3.2 Lipid Interactions

With the focus of early studies of the effects of anesthetics at the membrane level, considerable attention has been paid to the lipophilicity and hydrophobicity of anesthetics.3,16 Coupled with their structural diversity and the high concentrations required to elicit anesthesia, Bernard's unitary theory of anesthesia, stating that all anesthetics produce their effects by a simple perturbation of the cell membrane, had a certain logic. The lack of pharmacological antagonists of general anesthetics and the debate on the modest stereoselectivity of anesthesics3'16 has made the unitary theory ''both elegant and attractive.''3 The increased anesthetic efficacy observed with increases in methylene chain length, until the anesthetic cut-off (C = 12 and 13), is also consistent with this role of hydrophobicity and is further supported by the reversal in anesthesia when the hydrostatic pressure is increased, a phenomenon known as pressure reversal.

The search for hydrophobic pockets with which anesthetics interact has been limited by the modest changes in membrane lipid-ordering produced by clinical doses of anesthetics. Nonetheless, at high concentrations, general anesthetics can shift the conformational equilibrium of the neuronal nicotinic receptor (nAChR) from a resting to desensitized state,20 while halothane can photolabel nAChR subunits. 3-Trifluoromethyl-3-(3-iodophenyl)diazirine (TID: Figure 3), which has undergone preliminary characterization as a novel general anesthetic in tadpole with an EC50 value of approximately 600 nmol L _ 1, 21 can label the lipid-protein interface of nAChR subunits. However TID is better known as a hydrophobic photoreactive probe than a general anesthetic.22,23 Point mutations of the nAChR have resulted in the identification of a discrete binding site for isoflurane in the pore-forming M2 domain.24 Using large-scale molecular dynamic simulations in a gramicidin A (gA) channel model, halothane interactions were localized to the channel-lipid-water interface, reinforcing a global, as opposed to local, change in channel dynamics.25 Studies with hexafluoroethane (HFE: Figure 1), an analog of halothane that is devoid of anesthetic activity in the gA channel model, showed that this inactive control, in contrast to halothane, had no significant effect on gA channel dynamics, further supporting a role for changes in global membrane dynamics in the action of general anesthetics.26 One caveat to these studies, however, is that the gA channel is a highly reductionistic model, not a physiological drug target.

6.15.3.3 Ion Channels

Several ion channels have been proposed as potential molecular targets for anesthetic actions.5-10,14,27-30 These include voltage-gated calcium,27 voltage-gated and two-pore-domain potassium,6 sodium, NMDA and hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels,5^14 nAChRs,3,10 and GABAa and glycine receptors.5,10,28

6.15.3.3.1 y-Aminobutyric acidA (GABAa) receptors

GABAA receptors are a site of action of the major inhibitory transmitter in the CNS, GABA. The benzodiazepines (BZs) are a class of drugs with anxiolytic, hypnotic, muscle relaxant, and alcohol-potentiating actions that also interact with the GABAA receptor via an allosteric site, the BZ receptor. Similarly, the barbiturates can also interact with a discrete allosteric site on the GABAA receptor with pentobarbital (Figure 1), facilitating GABA currents and inhibitory neurotransmission in the brain and spinal cord. Inhalation anesthetics like halothane, enflurane, and isoflurane also interact with GABAA receptors.27 Halothane, for instance, has an EC50 value of 230 mmolL _ 1, close to its EC50 value for general anesthesia.31 The S-isomer of isoflurane (Figure 1) is twice as potent as R-isoflurane in prolonging GABA-evoked inhibitory postsynaptic potentials (i.p.s.ps) in cultured rat hippocampal neurons.32 Similarly, the steroid anesthetic, alphaxalone (Figure 2), but not its 3-b-hydroxy-isomer, betaxalone (Figure 2), which is devoid of anesthetic activity, can enhance GABA currents.33 Point mutation/chimeric studies of the GABAA receptor have identified two amino acid residues in TM2 and TM3 that are critical for enflurane and isoflurane, but not propofol, interactions with the GABAA receptor.29 The sedative effects of propofol and pentobarbital are mediated via GABAA receptors in the tuberomammillary nucleus, as shown by their blockade with the GABAA receptor antagonist, gabazine. Those of ketamine (Figure 2), which produces its anesthetic actions via the NMDA receptor, were unaffected by gabazine.5 Propofol delays desensitization of GABAA receptors and it, and a series of analogs, shows a distinct structure-activity relationship for their anesthetic potency that does not correlate with lipid solubility.34

6.15.3.3.2 Voltage-gated potassium channels

Voltage-gated potassium channels (VKCs or Kv channels), a family that comprises 12 distinct members,35 have been implicated as the target of anesthetics, with considerable interest in the Shaw-related family, Kv4, as a potential target for anesthetics.36 Isoflurane can also inhibit 'leak' or two-pore-domain potassium channels,37 including the weakly inward-rectifying K+ channel (TWIK), the acid-sensitive K+ channel (TASK), the TWIK-related K+ channel (TREK), and the TWIK-related arachidonic acid-stimulated K+ channel (TRAAK).

6.15.3.3.3 Voltage-sensitive calcium channels

Voltage-sensitive calcium channels (Cav) are comprised of three major families, Cav1-3, of which there are a total of 10 members.38 While these have been implicated in anesthetic interactions, this only occurs at high concentrations in the millimolar range27 and may not be of physiological significance.

6.15.3.3.4 Voltage-gated sodium channels

Voltage-gated sodium channels (Nav) are a family of nine distinct receptors, Nav 1.1-1.9, with distinct pharmacology.13 While sodium currents in giant axons of the squid and crayfish are insensitive to inhalation anesthetics, those in small unmyelinated axons in the hippocampus are depressed. Nav1.2 is inhibited via a voltage-independent block of peak current and a hyperpolarizing shift in voltage dependence of the steady state.39 Volatile anesthetics also interact with Nav1.4, Nav1.5, and Nav1.6 channels,9 leading to the potential to modulate neurotransmitter release.

6.15.3.3.5 N-Methyl-D-aspartate (NMDA) and glycine receptors

The best-known anesthetic interacting with NMDA receptors is ketamine (Figure 2), which, as has already been noted, is an atypical dissociative anesthetic with hallucinogenic side effects. The NMDA receptor may also be the site at which ethanol, xenon, nitrous oxide, and cyclopropane produce their anesthetic effects.9 Studies on the glycine receptor, a component allosteric modulatory site on the NMDA receptor, have shown that propofol can enhance activation of the glycine receptor and, by extrapolation, NMDA receptor function.

6.15.3.3.6 Neuronal nicotinic receptors nAChRs were the first ion channels to be available in sufficient quantities to study interactions with pharmacological agents. As such, there is a considerable body of data to support an interaction with nAChRs in the context of the unitary hypothesis of anesthetic action. The isomers of isoflurane do however show a twofold stereoselectivity in its ability to inhibit nAChR-mediated currents,40 while the N-terminal domain of the a7 subunit is involved in the effects of inhalation anesthetics.41

6.15.3.3.7 Hyperpolarization-activated, cyclic nucleotide gated (HCN) or pacemaker channels

HCN or pacemaker channels can be inhibited by enflurane and halothane that decrease Ih conductance (which is involved in resting membrane potential) in brainstem motor neurons42and thalamic neurons.43 Propofol slows HCN1 channel activation with an EC50 value of 6 mmol L _ and can also interact with HCN2 and HCN4 channels.44 While HCN neurons may represent ''an under-appreciated anesthetic target site,''9 propofol interactions with HCN channels may contribute to bradyarrhythmias.44

Do Not Panic

Do Not Panic

This guide Don't Panic has tips and additional information on what you should do when you are experiencing an anxiety or panic attack. With so much going on in the world today with taking care of your family, working full time, dealing with office politics and other things, you could experience a serious meltdown. All of these things could at one point cause you to stress out and snap.

Get My Free Ebook


Post a comment