Thalamic Microinfusion of Antibody to a Voltage-gated Potassium Channel Restores Consciousness during Anesthesia

Abstract

POTASSIUM (K) channels play a major role in regulating tissue excitability. There are at least four different types of K channels that serve slightly different functions in the nervous system, including voltage-gated (Kv), calcium-activated (KCa), inward rectifying (Kir), and 2-pore (K2P) domain background leak channels.1Many authors have suggested that K channels are involved in producing the effects of anesthesia.1–6Much recent focus has been placed on distinguishing K2P channels as possible targets of anesthetic action,6,7yet interactions with various other K channels like the Kv channels may also be important.2,3,8,9Kv channels are further divided into 12 families (Kv1–Kv12) on the basis of sequence homology and similarity to Drosophila melanogaster (i.e. , fruit-fly) genes.10In Drosophila , the Kv channel genes produce multiple versions of a particular channel and are more commonly known by historical nomenclature such as: Shaker (Kv1.1–Kv1.8), Shab (Kv2.1–Kv2.2), Shaw (Kv3.1–Kv3.4), Shal (Kv4.1–Kv4.3), and ether-a-go-go (Kv10.1–Kv10.2). The eight members of the Shaker -related K channel family (Kv1.1–Kv1.8) are involved with generating voltage-dependent outward currents that regulate action potential threshold, as well as waveform and pacemaker activity in excitable tissue. Kv channels are generally composed of tetramers of alpha subunits. When they are expressed as homomeric channels, most have “delayed rectifier” properties, and the others will exhibit fairly rapid inactivation.11The different Kv1.x family members (where x is any one of the possible eight different subunits) can coassemble into channels with mixed heteromeric alpha subunit compositions. Furthermore, Kv-beta subunits also exist and add on another layer of complex functional diversity in vivo .11The subunit composition of the Kv1 channels not only determines their gating and kinetic properties, it also dramatically affects their expression and localization.11The idea that Kv channels might play a role in anesthesia emerged from the discovery that Drosophila Shaker mutants shake their legs vigorously during ether anesthesia.12,13The Shaker mutant lacks a normal functioning Kv1.x channel, suggesting that the suppression of neural activity under ether anesthesia depends to some extent on a properly functioning Kv1.x channel. Indeed, the amount of isoflurane needed to anesthetize a Shaker mutant with a completely nonfunctioning Kv1.x channel is more than twice the dose needed to anesthetize wild-type flies.14Importantly, the changes in isoflurane doses needed to anesthetize various other Shaker mutants parallels the expected reductions in ionic currents mediated through the respective malfunctioning K channels.14In other words, the more defective the K channel (and the less current that passes through it), the greater the dose of isoflurane needed to anesthetize a particular Shaker mutant. This seems to suggest that anesthesia might work in part by hijacking the functioning of the Kv1.x channel.Recently, the unconsciousness of sleep was also linked to a voltage-dependent K channel.15Mutagenesis analysis was used to screen more than 9,000 Drosophila lines to identify those with a limited ability to sleep. Genetic analysis of these flies revealed a point mutation in a conserved domain of the Shaker gene that involves the voltage sensing portion of Kv1.2 channel.15,16Thus, for Drosophila to sleep, it appears to need current to properly flow through its Kv1.2 channels. Taken together with the earlier anesthesia work, this suggests Kv1.2 channels might be involved with mediating the effects of volatile anesthetics on consciousness.Concurrent with the developments in sleep neurophysiology, a possible role for the central medial thalamus (CMT) in contributing to the unconsciousness of anesthesia was recently identified.17Anesthetic-induced unconsciousness can be reversed with a localized and site-specific microinfusion of nicotine into the CMT of rats.17As the loss of consciousness associated with sleep and anesthesia may share overlapping neurobiological mechanisms,18–21and nicotine is known to also block various K channels,22,23including some Kv channels,24the hypothesis is raised that anesthetic effects on consciousness might involve interactions with Kv1.2 channels located, in part, in the CMT. Herein, we open the investigation into this area of research by microinfusing a Kv1.2 channel blocking antibody directly into the CMT of rats placed in an anesthetic chamber exposed to a dose of inhalational agent that is just sufficient to render them unconscious. As in the case of Drosophila Shaker mutants, which have a chronic malfunction of Kv1.x channels, the acute conduction blockade of the Kv1.2 channels in the CMT should act to rapidly increase the anesthetic dose required to keep the animals unconscious. As the dose of anesthesia will be held constant after the localized antibody microinfusion, a positive result (indicating the possible contr