| JOURNAL HOME | CME HOME | THIS MONTH | PAST ISSUES | ETOC | COLLECTIONS |
| AUTHORS | REVIEWERS | EDITORIAL BOARD | FEEDBACK | RSS | HELP |
| ||||||||||||||
| ||||||||||||||
|
|
|
|
||||||||||||
|
||||||||||||||
University of Pennsylvania, Department of Anesthesia and the Johnson Research Foundation, Philadelphia, Pennsylvania
Address correspondence and reprint requests to Jonas S. Johansson, MD, PhD, 319C John Morgan Building, University of Pennsylvania, 3620 Hamilton Walk, Philadelphia, PA 19104. Address e-mail to JohanssJ{at}uphs.upenn.edu
The sites in the central nervous system where inhaled anesthetics exert their clinical effects remain to be defined. During the past two decades, most investigators in the field have shifted away from a nonspecific plasma membrane lipid site of action towards direct interactions with membrane proteins or even globular proteins. Currently, favored targets include ligand-gated ion channels, such as the 
-aminobutyric acid type A receptor, the glycine receptor, and the excitatory ionotropic glutamate receptor that is sensitive to N-methyl-D-aspartate (1). Enhancement of chloride ion conductance through the -aminobutyric acid type A or glycine receptor occurs in the presence of a number of inhaled anesthetics, and the resulting hyperpolarization of the plasma membrane is consistent with decreased neuronal activity. In addition, several inhaled anesthetics inhibit the excitatory N-methyl-D-aspartate receptor, which is also predicted to lead to decreased neuronal activity.
Another plausible group of targets for general anesthetics are the background potassium channels examined in the paper by Shin and Winegar (2) in this issue of Anesthesia & Analgesia. The activity of these background potassium channels was enhanced by halothane, isoflurane, and sevoflurane in whole-cell patch-clamp experiments on cultured cerebellar granule neurons from 7-day-old Sprague-Dawley rats. Such increased conductivity through these background potassium channels in the presence of inhaled anesthetics will hyperpolarize the plasma membrane and result in decreased neuronal activity. Nicoll and Madison (3) initially noted the ability of general anesthetics to hyperpolarize frog motoneurons and rat hippocampal CA1 pyramidal cells by increasing potassium conductance. These background or tandem pore (P) domain potassium channels are a recently described family of membrane proteins that mediate baseline or leak currents, set the resting membrane potential, and thereby influence the likelihood of neuronal action potential generation.
Potassium channels are the most diversified of the various ion channels, with more than 70 different types present in the human genome. Potassium channels have a characteristic conserved P domain in their primary sequences (4) that forms part of the ion channel itself and determines potassium selectivity over other ions and either two, four, or six transmembrane domains. Patel et al. (5) identified a family of mammalian potassium channels with a basic architecture consisting of two P domains in tandem and four transmembrane segments. The native functional form of these channels is thought to be a dimeric structure containing a total of four P domains. Voltage independence coupled with absent activation and inactivation kinetics are characteristics of conductances referred to as leak or background conductances. The five representatives of this family are designated TWIK-1, TASK, TREK-1, TRAAK, and TALK-1, where the first letter indicates tandem P domain. TWIK-1 is a weakly inward-rectifying K+ channel, and TASK (acid sensitive) is a background outward-rectifier (outward channel conductance increases with depolarization more than inward currents with hyperpolarization) potassium channel whose activity is inhibited by low pH. This potassium channel is very sensitive to changes in extracellular pH within the physiological range, exhibiting 10% of the maximal current at a pH value of 6.7 compared with 90% at a pH value of 7.7 (6). TREK-1 (TWIK-1 related K+ channel) and TRAAK (TWIK-1 related arachidonic acid-stimulated K+ channel) are both background outward-rectifier potassium channels that are activated by polyunsaturated fatty acids, including arachidonic acid. The current model explaining how polyunsaturated fatty acids alter channel activity involves changes in the curvature of the lipid bilayer rather than direct interactions with the channel protein (5). The alkaline-activated tandem P domain potassium channel TALK-1 is primarily expressed in the pancreas (7). This family of potassium channels is also described using an alternative nomenclature as KCNKx, where x is a number indicating the order in which each member was identified, and KCNK denotes K+ channel subfamily K. There are currently 17 members of this family, according to the Human Genome Organization Nomenclature Committee database (http://www.gene.ucl.ac.uk/nomenclature/).
The first member of this potassium channel family, TOK1 (tandem P domain outward rectifying K+ channel), was described in the genome (chromosome X) of the yeast Saccharomyces cerevisiae (8). Expression of the channel in Xenopus laevis oocytes and examination using two-electrode voltage clamp revealed potassium selectivity, outward rectification, and noninactivation. Subsequent work has shown that these tandem P domain potassium channels are widely distributed in the mammalian central nervous system (6,9–13). Tandem P domain potassium channels are found in abundance in the olfactory system, cerebral cortex, hippocampal formation, basal ganglia, and cerebellum (both Purkinje and granule cell layers). In the spinal cord, tandem P domain potassium channels are located primarily in the gray matter in the laminae of both dorsal and ventral horns, which include sensory and motor neuron cell bodies.
Initial studies with anesthetics were performed using invertebrate neurons. Halothane was shown to activate potassium currents in the right parietal ganglion neurons of the great pond snail Lymnaea stagnalis with a 50% effective concentration (EC50) of 0.2 mM (14). Similarly, halothane activated baseline serotonin-sensitive potassium channels from ganglion neurons of the marine mollusk Aplysia californica with an EC50 of 0.13 mM (15,16). Of interest is that the halothane-induced activation of these baseline potassium channels, with the resulting hyperpolarization of the plasma membrane, is associated with a decreased frequency of action potential generation by the neurons, demonstrating direct coupling between changes in channel activity and whole-cell function.
Gray et al. (17) examined the ability of volatile anesthetics to activate the background potassium channel TOK1 from S. cerevisiae expressed in Xenopus laevis oocytes. Halothane, isoflurane, and desflurane all increased the potassium currents through the TOK1 channel with EC50 values of approximately 0.8 mM. Patel et al. (18) demonstrated activation of murine background tandem P domain potassium channels expressed in COS cells (a transformed African green monkey kidney cell line) by general anesthetics. Halothane at 0.3 mM increased current flow through the TREK-1 channel by approximately 60%. One study showed that the human central nervous system tandem P domain baseline potassium channels KCNK5 (also known as TASK-2), expressed in Xenopus laevis oocytes, are activated by desflurane, enflurane, and halothane (19). Future studies will fill in the current gaps in our understanding of which of the 17 members of the KCNK family identified are activated by general anesthetics and which are resistant.
An appreciation of how general anesthetics modulate the activity of these background potassium channels at the molecular level will be enhanced by future studies using site-directed mutagenesis, high-resolution structural approaches, and molecular dynamics simulations. Recent success with solving the 3.2 Å x-ray crystal structure of the potassium channel from Streptomyces lividans (20) and large-scale molecular dynamics simulations of general anesthetic interactions with the membrane-associated gramicidin A channel (21) provide the foundation and support the feasibility of such studies. Because these tandem P domain potassium channels are present both pre- and postsynaptically, their activation by general anesthetics represents an attractive way to reversibly inhibit central nervous system function.
Acknowledgments
Supported, in part, by NIH grants GM55876 and GM65218.
References
This article has been cited by other articles:
|
|
 |
|
  I. Y. Churbanova, A. Tronin, J. Strzalka, T. Gog, I. Kuzmenko, J. S. Johansson, and J. K. Blasie Monolayers of a Model Anesthetic-Binding Membrane Protein: Formation, Characterization, and Halothane-Binding Affinity Biophys. J., May 1, 2006; 90(9): 3255 - 3266. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Ye, J. Strzalka, I. Y. Churbanova, S. Zheng, J. S. Johansson, and J. K. Blasie A Model Membrane Protein for Binding Volatile Anesthetics Biophys. J., December 1, 2004; 87(6): 4065 - 4074. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
