(KCNK) Potassium Channels: Dynamic Roles in Neuronal Function EDMUND M

■ REVIEW Two-Pore-Domain (KCNK) Potassium Channels: Dynamic Roles in Neuronal Function EDMUND M. TALLEY, JAY E. SIROIS, QIUBO LEI, and DOUGLAS A. BAYLISS Department of Pharmacology University of Virginia Charlottesville

Leak K+ currents contribute to the resting membrane potential and are important for modulation of neuronal excitability. Within the past few years, an entire family of genes has been described whose members form leak K+ channels, insofar as they generate potassium-selective currents with little voltage- and time-dependence. They are often referred to as “two-pore-domain” channels because of their predicted topology, which includes two pore-forming regions in each subunit. These channels are modulated by a host of different endogenous and clinical compounds such as neurotransmitters and anesthetics, and by physicochemical factors such as temperature, pH, oxygen tension, and osmolarity. They also are subject to long-term regulation by changes in gene expression. In this review, the authors describe multiple roles that modulation of leak K+ channels play in CNS function and discuss evidence that members of the two-pore-domain family are molecular substrates for these processes. NEUROSCIENTIST 9(1):46–56, 2003. DOI: 10.1177/1073858402239590

KEY WORDS Potassium channel, KCNK, Two-pore-domain

A negative membrane potential is essential for electrical occurs via a host of different endogenous and clinical signaling in excitable tissues, and it has been recognized compounds such as neurotransmitters and anesthetics, for many years that this key cellular property is provid- and also by changes in such physicochemical parameters ed in large part by voltage- and time-independent potas- as temperature, oxygen tension, osmolarity, and pH. In sium (K+) currents (Hille 2001). However, until recently addition to these short-term regulatory phenomena, the molecular basis for these so-called leak K+ currents emerging evidence suggests that two-pore-domain K+ was a mystery. Over the past few years, an entire family channels also may be loci for longer-term modulation by of genes has been described whose members generate way of changes in gene expression. the hallmark properties of leak K+ currents: relative At present, these various forms of modulation are time- and voltage-independence, as well as potassium being clarified by ongoing investigations of cloned two- selectivity (Lesage and Lazdunski 2000; Goldstein and pore-domain K+ channels in heterologous expression others 2001; Patel and Honore 2001a). Genes from this systems, as has been extensively reviewed (Lesage and family differ from other K+ channel genes in their pre- Lazdunski 2000; Goldstein and others 2001; Patel and dicted subunit structure, with each subunit containing Honore 2001a). Here we describe the multiplicity of two pore-forming loops (P domains) instead of one (Fig. roles that modulation of leak K+ channels play in CNS 1). As a result, members of this family are variously function and the evidence that members of the two-pore- referred to as “two-P domain” or “two-pore-domain” K+ domain family are molecular substrates for these channels. processes. Because a polarized membrane potential is so fundamental, one might suspect that leak K+ channels perform Classification of Mammalian Two-Pore-Domain functions that are purely constitutive and unchanging. Channel Family Members Instead, it turns out that regulation of the resting con- + ductance is a primary mechanism for control of cellular One of the early reports of two-pore-domain K channel excitability. Thus, far from being mere “housekeeping genes came from analysis of the nematode genes” with static roles in cellular function, these chan- (Caenorhabditis elegans) genome (Wei and others nels are increasingly seen as important loci for dynamic 1996), which may contain as many as 50 of these genes, modulation of membrane properties. Such modulation with the potential for many more channel isoforms resulting from differential RNA splicing (Wang and others 1999; Salkoff and others 2001). Fortunately, the This work was supported by grants from the National Institutes of Health: F31MH12091 (EMT), F32HL10271 (JES), and NS33583 number of mammalian members of this family does not (DAB). appear to be quite so daunting. To date, 14 two-pore- domain channel genes from the human genome have Address correspondence to: Edmund M. Talley, Department of Pharmacology, University of Virginia Health System, P.O. Box 800735, been described. For 11 of these genes, heterologous Charlottesville, Virginia 22908-0735; e-mail: [email protected]. expression leads to functional channel activity at the

46 THE NEUROSCIENTIST Two-Pore-Domain Potassium Channels Copyright © 2003 Sage Publications ISSN 1073-8584 Fig. 1. Mammalian K+ channels are grouped into three structurally distinct types. Depicted are the basic topologies of subunits from the three main K+ channel families. The simplest structure belongs to members of the Kir family, which generate K+ currents that are inwardly rectifying (that is, they conduct current more effectively in the inward direction). Each subunit includes two transmembrane (TM) domains surrounding a so-called “P domain” or “pore-domain.” These subunits form channels as tetramers, with four P domains combining to create a single ion-conducting aperture. Subunits from the KV family, which typically generate voltage-gated currents, also form tetramers, with each subunit containing six transmembrane domains and one pore domain. Some calcium-activated K+ channels (KCa) have a similar structure but contain an extra transmembrane domain. As indicated by their name, two-pore-domain

(KCNK or K2P) channel subunits contain two P domains and four transmembrane domains; these subunits form channels by joining together as dimers. Most members of this third family generate currents with little voltage or time dependence, thus creating leak K+ channels. plasma membrane. In most cases, corresponding rodent important facet of the characterization of two-pore- orthologues also have been characterized. A similar domain channels is a description of their respective CNS number of genes, 11, has been predicted for the fruit fly expression patterns. Expression of these genes has been Drosophila melanogaster (Rubin and others 2000), one examined at the RNA level by array analysis, Northern of which has been studied extensively (Goldstein and blotting, RT-PCR, and in situ hybridization (see Table 1 others 2001). A yeast two-pore-domain channel also has for references). Four of the two-pore-domain K+ channel been characterized, although it has a subunit structure genes (TWIK-2, TASK-2, TALK-1, and TALK-2) are that so far is unique, because it has eight transmembrane absent or only found at low levels in the CNS (although domains instead of four (Ketchum and others 1995; TWIK-2 and TASK-2 are found in dorsal root ganglia). Lesage and others 1996a). Another three (KCNK7, THIK-2, and TASK-5) are of Currently there are two nomenclature systems in unknown function, as they fail to form plasma membrane prominent use for the description of these channels. One channels in heterologous expression systems. The remain- has been provided by the Human Genome Organization, ing seven genes are differentially distributed in the brain. which has assigned the prefix KCNK, followed by a dif- In rodents, they are primarily expressed in neurons, ferent number for each gene. Assignment of these num- although they also are found in some other cell types, bers has largely resulted from the order in which each such as the choroid plexus (TWIK-1), ependymal cells gene was discovered, and therefore the numbers do not lining the ventricles (TREK-2), and pia mater (TREK-1) reflect any classification by sequence or function. A sec- (Talley and others 2001). In addition to RNA, localization ond nomenclature utilizes acronyms that are derived of channel protein in rodents by immunohistochemistry from salient physical features of the cloned channels. has been reported for TREK-1, TRAAK, and TASK-1 Although this scheme is also not without problems (Kindler and others 2000; Maingret and others 2000; (Goldstein and others 2001), it can accommodate the Reyes and others 2000; Hervieu and others 2001). For categorization of these channels into subgroups based on the most part, the protein localization is consistent with sequence homology and functional similarity (Rajan, the patterns of RNA expression, although not unexpect- Wischmeyer, Karschin, and others 2000; Patel and oth- edly, there are some exceptions (Talley and others 2001). ers 2001). These proposed subfamilies and the acronyms To date no compounds have been described that selec- used to identify them are described in Figure 2. tively block or activate these channels, allowing definitive identification of their role in neuronal activity. As a CNS Expression of Two-Pore-Domain result, two-pore-domain channel correlates have been Channels identified in neurons by virtue of various constellations of properties, including their modulation (Kim and oth- A primary factor in establishing the role of a particular ers 1995; Millar and others 2000; Sirois and others 2000; gene in CNS function is a determination of the location Talley and others 2000; Bockenhauer and others 2001; and phenotype of cells expressing that gene. Hence, an Washburn and others 2002). Thus, identification of these

Volume 9, Number 1, 2003 THE NEUROSCIENTIST 47 TWIK: This acronym for “tandem of P domains in a weak inwardly rectifying K+ channel” was used for the first mammalian two-pore-domain family member to be described, TWIK-1 (KCNK1) (Lesage and others 1996b). The name has retained its salience, in that both TWIK-1 and the related TWIK-2 (KCNK6) generate inwardly rectifying currents at the whole cell level (Lesage and others 1996b; Chavez and others 1999; Patel and others 2000), a characteristic shown by only one other two-pore-domain channel, THIK-1 (KCNK13) (Rajan, Wischmeyer, Karschin, and others 2000). All of the other channels are either outwardly rectifying or show virtually no voltage dependence. TWIK-2 is also unique among two-pore-domain channels in that it generates inactivating currents (Patel and others 2000).

THIK: “Tandem pore domain halothane-inhibited K+ channel” was chosen for THIK-1 (KCNK13) and the related but as yet nonfunctional THIK-2 (KCNK12) because of the inhibition of THIK-1 by supra-clinical concentrations of halothane (Rajan, Wischmeyer, Karschin, and others 2000), a volatile anesthetic compound that activates (rather than inhibits) a number of other two-pore-domain channels. However, it has since been discovered that THIK-1 is not the only two-pore-domain channel to be inhibited by halothane (Patel and Honore 2001b).

TREK: “TWIK-related K+ channel gene” was applied to the second mammalian two-pore-domain channel to be described, TREK-1 (KCNK2) (Fink and others 1996). This gene has two homologues, TREK-2 (KCNK10) and TRAAK (KCNK4). The lat- ter acronym (“TWIK-related arachidonic acid-stimulated K+ channel”) was chosen as a result of the ability of this channel to be stimulated by polyunsaturated fatty acids (Fink and others 1998), although it is now known that this property is shared by TREK-1, TREK-2, THIK-1, and TWIK-2 (Patel and others 2000, 2001; Rajan, Wischmeyer, Karschin, and others 2000).

TASK: “TWIK-related acid-sensitive K+ channel” was used to describe the third mammalian two-pore-domain K+ channel to be discovered, TASK-1 (KCNK3), as a result of its sensitivity to Fig. 2. Two-pore-domain K+ channels can be grouped into five physiologically relevant changes in extracellular pH (Duprat and subfamilies. The 14 known human two-pore-domain K+ channel others 1997). A number of channels subsequently discovered genes are presented in a phylogenetic tree based on analysis were given the name TASK, but it has since become clear that using CLUSTAL W (Thompson and others 1994), demonstrat- this set of genes should be divided into two categories. One ing their division into five subfamilies. These subgroups are rep- group is related to TASK-1 and includes TASK-3 (KCNK9) and resented in different colors, with genes that have not produced the as yet nonfunctional TASK-5 (KCNK15). functional channels shown in gray. The more distant relation- ships between the subgroups have been omitted. Two naming TALK: “TWIK-related alkaline pH activated K+ channel” has schemes for these channels are in current use; both are pre- been used to describe the second group of channels that are sented here. One is provided by the Human Genome sensitive to changes in extracellular pH (Girard and others Organization, in which the KCNK moniker precedes a numeri- 2001), which includes TASK-2 (KCNK5), TALK-1 (KCNK16), and cal distinction reflecting the order of discovery of each of the TALK-2/TASK-4 (KCNK17). The pK for proton inhibition of these genes. The other is a set of acronyms based on salient physi- channels is in the alkalized range; this pH sensitivity further dis- ological or pharmacological properties. The acronyms are as tinguishes channels in this group from TASK-1 and TASK-3, follows. which have a pK in the neutral-to-acidified range. channels and investigation of their function can go hand are under homeostatic control, in part through reflexes in hand, because as described below, their modulation is mediated by the central nervous system. Increasing evi- often physiologically and/or clinically relevant. dence indicates that two-pore-domain K+ channels participate as molecular substrates in regulating these Participation of Leak K+ Channels in homeostatic functions. Two of the physiological process- Homeostatic Regulation of es in which leak K+ channels have been strongly impli- Physicochemical Processes cated are the regulation of breathing and the mainte- nance of body temperature. Two-pore-domain channels are regulated by a diverse Control of respiration provides homeostatic regulation array of biologically important physical and chemical of blood/brain oxygen, carbon dioxide, and pH, and stimuli, including temperature, intracellular/extracellu- involves sensors in the CNS (primarily in the brainstem) lar pH, oxygen tension, and osmolarity and/or mem- and in the periphery (Feldman and Ellenberger 1988). brane stretch (Lesage and Lazdunski 2000; Goldstein The major peripheral site for this process is the carotid and others 2001; Patel and Honore 2001a). These factors body, which contains glomus cells that are sensitive to

48 THE NEUROSCIENTIST Two-Pore-Domain Potassium Channels Fig. 3. Differential distribution of mRNA coding for TASK-1 and TASK-3 in the rat CNS. Sagittal (top) and horizontal (bottom) sections from rat brain were hybridized with radioactive probes specific for TASK-1 and TASK-3 (Talley and others 2001). On the left are grayscale images of the resulting film autoradiograms. To generate the colorized panels shown on the right, the grayscale images were converted to a single color (TASK-1: green, TASK-3: red) and the respective images were merged. Only regions with moderate-to-high signal intensity were included. Areas of overlapping expression are shown in yellow. Particularly evident in this regard are granule cells in the cerebellum and olfactory bulb, and brainstem motoneurons. Not shown is the related gene TASK-5, which to date has been nonfunctional in heterologous expression systems. In situ hybridization for TASK-5 shows discrete expression in a number of areas associated with auditory processing (Karschin and others 2001). changes in oxygen tension. In rats, the response of these a particular site. A number of promising candidate cells to hypoxia includes a depolarization that is due, at regions have emerged, two of which are the locus coeru- least in part, to inhibition of a leak K+ channel with prop- lus and the caudal raphe nuclei (Ballantyne and Scheid erties of TASK-1 (Buckler 1997; Buckler and others 2000; Nattie 2001). Both of these groups of neurons 2000). Like TASK-1, the oxygen-sensitive channels in express high levels of mRNA for TASK-1 and TASK-3, carotid body glomus cells are voltage insensitive, and and accordingly, noradrenergic locus coeruleus and sero- they have single channel conductance and kinetic prop- tonergic raphe neurons are depolarized and increase erties that are very similar to those of the cloned chan- their firing in response to extracellular acidification nel. They also have pharmacological properties that are (Sirois and others 2000; Washburn and others 2002). For similar to TASK-1, such as inhibition by extracellular both cell types, there appear to be multiple conductances acidification and activation by halothane. Accordingly, involved in this effect, including a joint pH-sensitive, in situ hybridization indicates that TASK-1 mRNA is halothane-activated leak K+ conductance with the prop- expressed in carotid body glomus cells (Buckler and oth- erties of TASK-1 and TASK-3. Thus, one or both of ers 2000), and the cloned channel is partially inhibited these channels are likely to participate in the chemore- when exposed to hypoxia in HEK 293 cells (Lewis and ceptive response of these cells, and in turn, may con- others 2001). tribute to the regulation of breathing. In the CNS, pH is a major factor influencing chemore- In addition to expression in neurons that are putative ceptors that contribute to the regulation of breathing, and chemoreceptors, TASK-1 and TASK-3 are found at high correspondingly, brain pH is subject to tight control by levels in motoneurons, including those that innervate the the respiratory system. TASK-1 and the related TASK-3 respiratory musculature (Talley and others 2001). These have been implicated in this process, because these two include hypoglossal motoneurons, which have a respira- channels are exquisitely responsive to even minor tory-related output that is necessary for the control of changes in extracellular pH, and because they are upper airway patency during inspiration. Consistent with expressed in relevant neurons. The sensitivity of TASK-1 their expression of TASK channels, recordings from centers on physiological pH (pK ~ 7.4), whereas the pK these neurons reveal a K+ conductance with the pH-sen- of TASK-3 is more in the acidified range (pK ~ 6.7) sitivity of TASK-1, and these cells respond to acidifica- (Duprat and others 1997; Kim and others 2000; Lopes tion with a depolarization and enhanced excitability and others 2000; Rajan, Wischmeyer, Xin, and others (Talley and others 2000). Thus, it may be the case that 2000; Meadows and Randall 2001). Thus, they are well chemoreception occurs in part at the level of the output suited for a role in sensing changes in brain pH, as their motoneuron, as well as in the more traditionally defined inhibition by acidification would be expected to excite central respiratory chemoreceptors. neurons during periods of reduced respiratory output. A second homeostatic process in which leak K+ chan- The location of chemoreceptive neurons that are respon- nels have been implicated is the regulation of body tem- sible for transducing this signal to the brainstem respira- perature. Specifically, a voltage-insensitive K+ conduc- tory network has not been established with certainty, and tance has been proposed to support responses to cold it is becoming increasingly clear that they are distributed temperatures in thermoreceptive neurons of the dorsal diffusely in the brainstem, rather than being restricted to root and trigeminal ganglia (Reid and Flonta 2001a;

Volume 9, Number 1, 2003 THE NEUROSCIENTIST 49 d rodent orthologues have not d rodent a-Saenz and others (2001); 14. 01); 3. Salinas and others (1999); situ hybridization. Relative expres- ers (2001); 8. Fink and others (1998); s minimal or undetectable. Blank fields —— ———— —+++ —— ———— —+++ — — — — — — — + +++ ++++ — +++ +++++++ — — — + ++ — — +++ + + — + — ++ + ++ + ++ + +++ — + — +— — +++ — —+++ +++ +++++ + ++ +++ + +++++ —++ + ++++ +++ + +++ +++ ++ +++ + + +++ + + + — ++ +++ ++ + ++ + + ++ — — + ++ ++ + +++ +++ + — + +++ — ++ +++ +++ +++ ++ — + ++ — ++ ++ +++ ++ + ++ ++ + ++ ++ ++ +++ + ++ ++ ++ + ++ ++ ++ +++ + +++ + — + +++ + + +++ + + ++ ++ ++ ++ +++ +++ ++++++ +++ +++ +++ + ++ ++ + — + + — ++ + ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ + + + +++ + ++ +++ brain cerebellum thalamus hypothalamus nigra accumbens neocortex hippocampus amygdala cord ganglia whole substantia striatum/ spinal root dorsal RNA Expression of Human and Rodent Two-Pore-Domain Channels in Various Brain Regions in Various Channels Two-Pore-Domain of Human and Rodent RNA Expression Human (1,16)Rodent (2,16,17) — — — — Rodent (2,10) Human (1) Rodent (2,11) Human (1,12) Rodent (2,11,13) Human (14,15)Rodent (11) — Human (1) Rodent (2,8) Human (1,9) Rodent (2) Human (1,9) Human (1,4)Rodent (4,5)Human —Rodent (6) — Human (7) — Rodent (6) — Human (1) Rodent (2) Human (1) Rodent (3) — TWIK-1 indicate that no data are available. TALK-1 and TALK-2 have been excluded because they were not detected in the human brain, an not detected in the human brain, they were have been excluded because and TALK-2 available. TALK-1 indicate that no data are Data are compiled from studies in which RNA levels were assayed by Northern blotting, RT-PCR, expression array analysis, or in expression assayed by Northern were blotting, RT-PCR, studies in which RNA levels compiled from Data are wa ++, moderate; +, low), with dashes indicating that expression identified using pluses (+++, high expression; sion levels are and others (20 and others (2001); 2. Talley and others 2001). Notes: 1. Medhurst Girard been described (Decher and others 2001; oth and Karschin, and others (2000); 7. Girard and others (1999); 6. Rajan, Wischmeyer, 4. Patel and others (2000); 5. Pountney (2000); 13. Veg and others (2000); 11. Karschin and others (2001); 12. Chapman and others 9. Lesage and others (2000); 10. Bang (2000); 17. Reyes and others (1998). Ashmole and others (2001); 15. Kim and Gnatenco (2001); 16. Gray and others TWIK-2 THIK-1 Table 1. Table TASK-5 TASK-2 THIK-2 TRAAK TREK-1 TREK-2 TASK-1 TASK-3 KCNK7

50 THE NEUROSCIENTIST Two-Pore-Domain Potassium Channels Viana and others 2002). In both of these cell groups, a between mammalian TREK channels and the S channel, limited set of small- to medium-diameter neurons it seems likely that Aplysia californica expresses a gene respond to cooling with a depolarization and action with a high degree of sequence homology to TREK-1 potential discharge. The cooling-induced depolarization and TREK-2, although this remains to be determined. entails an increase in input resistance, and voltage clamp In mammals, inhibition of leak K+ channels is a data indicate that closure of a resting potassium conduc- prominent mechanism for neuromodulation in a variety tance is a primary mechanism for the effect, although of neurons, from regions as diverse as the cortex and other conductances are likely to be involved (Reid and thalamus (McCormick 1992), hippocampus (Brown and Flonta 2001b; McKemy and others 2002; Peier and oth- others 1993; Guerineau and others 1994), neurosecreto- ers 2002; Viana and others 2002). Of the cloned two- ry cells of the hypothalamus (Schrader and Tasker 1997), pore-domain K+ channels, TREK-1 has emerged as an submucosal neurons in the ileum (Shen and Surprenant excellent candidate for mediating this response, as this 1993), brainstem aminergic neurons (Pan and others channel is highly sensitive to temperature changes in the 1994), and cerebellar granule neurons (Millar and others appropriate range, and it is expressed in small to medi- 2000). This mechanism has been particularly well docu- um neurons of the dorsal root ganglia (Maingret and oth- mented in motoneurons, where it is induced by multiple ers 2000). TASK-1 is also a possible contributor to the transmitters, including serotonin, norepinephrine, Sub- response, as it is inhibited by cooling and is expressed in stance P (SP), thyrotropin-releasing hormone (TRH), dorsal root ganglia neurons, but its temperature sensitiv- and glutamate (Rekling and others 2000). As noted ity is not as great as that of TREK-1 (Maingret and oth- above, motoneurons express high levels of TASK-1 and ers 2000). TASK-3 mRNA, and hypoglossal motoneurons express a K+ conductance with the properties of TASK-1 (i.e., Leak K+ Currents Are a Basis for pH sensitivity, pK ~ 7.3, and a lack of voltage- and time- Modulation by Neurotransmitters dependence) (Talley and others 2000, 2001). Occlusion experiments, in which transmitters were applied under Neuromodulation entails slow effects of transmitters that different pH conditions, established that this pH-sensi- are distinct from fast actions of ligand-gated ion chan- tive conductance accounts for most, if not all, of the K+ nels, and it has been well established that up- and down- current that is inhibited by transmitters in these neurons modulation of K+ currents are among the primary ionic (Talley and others 2000). TASK-1 and/or TASK-3 also mechanisms for this process (Nicoll and others 1990). In may be substrates for the effect of acetylcholine on cere- particular, K+ channel inhibition in neurons often bellar granule neurons (Millar and others 2000). In other involves channels with linear or weakly rectifying volt- regions of the brain, the K+ channels that are subject to age dependence, and it seems likely that two-pore- transmitter modulation have yet to be identified. domain channels provide molecular substrates for many In most mammalian systems, little is known about the of these effects. signal transduction pathways mediating receptor modu- One of the early and classic descriptions of neuro- lation of leak K+ currents, but information from cloned modulation within a behavioral context involved the two-pore-domain channels indicates that various signal- inhibition of a background K+ conductance in sensory ing pathways contribute to their regulation. This is par- neurons of the sea slug Aplysia californica, which occurs ticularly evident for TREK-1 and TREK-2, both of during sensitization of the gill withdrawal reflex. This is which are subject to modulation by multiple mecha- a simple form of adaptation that was the starting point nisms. As noted above, they are inhibited by PKA phos- for a host of studies on more complex behavioral phorylation, and accordingly, both channels can be mod- changes (Byrne and Kandel 1996). In Aplysia, gill-with- ulated by receptors that activate adenylyl cyclase (Patel drawal is a defensive reflex that is potentiated following and others 1998; Lesage and others 2000). Moreover, a noxious stimulus to a different part of the body, such as both are inhibited by receptors that activate phospholi- the tail. The potentiation is mediated in part by seroton- pase C (PLC). For TREK-1, the relevant mechanism ergic inhibition of a resting K+ channel, which was des- entails the activation of protein kinase C (Qiubo Lei and ignated the S channel because of its modulation by sero- Douglas Bayliss, unpublished results), but TREK-2 inhi- tonin. The intracellular mechanism for S channel inhibi- bition involves a different, unknown mechanism (Lesage tion involves activation of protein kinase A (PKA) and and others 2000). In addition to phosphorylation by likely entails direct phosphorylation of the channel. PKA, TREK-1 is phosphorylated by cyclic GMP- Although the molecular identity of this Aplysia channel dependent protein kinase (PKG), a process that may con- is unknown, it has properties that are remarkably similar tribute to smooth muscle relaxation in response to nitric to the mammalian two-pore-domain K+ channels oxide (Koh and others 2001). The channel responses to TREK-1 and TREK-2. Like the S channel, TREK chan- PKA and PKG are remarkable insofar as they each nels produce outwardly rectifying currents that are require a distinct phosphorylation site on the cytoplas- inhibited by cAMP-dependent phosphorylation and are mic C-terminus of the channel, which are only separated activated by membrane stretch, arachidonic acid, and from one another by 17 amino acid residues. In spite of volatile anesthetics (Patel and others 1998, 1999; Bang the close proximity of the phosphorylated residues, the and others 2000; Lesage and others 2000; Bockenhauer two kinases produce opposite effects on the channel, that and others 2001). Given the extraordinary similarities is, inhibition by PKA and activation by PKG.

Volume 9, Number 1, 2003 THE NEUROSCIENTIST 51 Fig. 4. CNS expression of members of the TREK subfamily of two-pore-domain K+ channels. In situ hybridization reveals differential distribution of mRNA encoding TREK-1, TREK-2, and TRAAK. Top panels show autoradiograms of sagittal (left) and horizontal (right) sections hybridized with probes for these three genes (Talley and others 2001). The colorized (merged) images were generated as in Figure 3. Regions with expression of a single gene are represented by “primary” colors (blue: TREK-1, red: TREK-2, yellow: TRAAK); regions of overlapping expression are represented by purple (TREK-1 and TREK-2), green (TREK-1 and TRAAK), orange (TREK-2 and TRAAK), and white (regions with all three genes).

TREK-1 and TREK-2 are also activated by a number and others 1997; Leonoudakis and others 1998; Lopes of polyunsaturated fatty acids and lysophospholipids, as and others 2000; Czirjak and others 2001), or inhibition are TRAAK, THIK-1, and TWIK-2 (Rajan, Wischmeyer, TASK-like channels in neurons (Bayliss and others Karschin, and others 2000; Patel and Honore 2001a; 1994; Boyd and others 2000). This suggests that a novel Patel and others 2001). The most interesting of these in signaling pathway may be involved. Consistent with this regard to transmitter modulation is arachidonic acid, in possibility, neurotransmitter modulation of TASK-1 and that this lipid can be produced by receptor stimulation TASK-3 requires a channel region (at the start of the via the activation of phospholipase A2. Although arachi- cytoplasmic C-terminus) that does not contain any clas- donic acid is the precursor to a variety of metabolites sical motifs that would suggest a known signaling mech- with relevance for intracellular signaling, pharmacolog- anism (Talley and Bayliss 2002). ical evidence suggests that it acts directly in the opening of a leak K+ channel by somatostatin in CA1 hippocam- Potential Roles for Two-Pore-Domain K+ pal pyramidal neurons (Schweitzer and others 1998). Channels in Clinical Effects of Anesthetics The identity of this native channel has not been determined, but the arachidonic acid-activated two-pore- Despite more than a century of use, the mechanisms by domain K+ channels are likely candidates, because all which volatile anesthetics induce their clinically impor- except TWIK-2 are expressed in CA1 pyramidal neurons tant actions are only partly understood. The most widely (Rajan, Wischmeyer, Karschin, and others 2000; Talley recognized targets of these compounds are ligand-gated and others 2001). ion channels, and in particular, enhanced GABA recep- Unlike the modulation of TREK-1 and TREK-2, tor activity is believed to induce a general depression of where a number of classical signaling pathways have neuronal activity that is a major component of the anes- been shown, the mechanisms for receptor modulation of thetized state (Koblin 2000). However, other targets are TASK-1 and TASK-3 have been elusive. Evidence sug- likely to contribute, and two-pore-domain K+ channels gests that receptor inhibition may occur via activation of have emerged as possible substrates in this regard (Patel phospholipase C (Czirjak and others 2001), but common and Honore 2001b). Of the two-pore-domain channels downstream signaling events do not appear to be with prominent CNS expression, TREK-1, TREK-2, involved in modulation of the cloned channels (Duprat TASK-1, and TASK-3 are all activated by clinical con-

52 THE NEUROSCIENTIST Two-Pore-Domain Potassium Channels centrations of anesthetics such as halothane and isoflu- both cell types, there appear to be multiple ionic con- rane (Patel and others 1999; Lesage and others 2000; ductances contributing to the effect, but bath acidifica- Meadows and Randall 2001). TASK-1 and TASK-3 are tion revealed the existence of a joint pH-sensitive, particularly interesting in this context, as a result of their halothane-activated leak K+ conductance that is likely contribution to resting K+ currents in neurons that are carried by TASK-1 and/or TASK-3 channels. In these potentially involved in the clinical effects of these com- neurons, hyperpolarization would be expected to con- pounds (Sirois and others 2000; Washburn and others tribute to decreased arousal and thus may play a role in 2002). this aspect of the anesthetic response. Although anatomic substrates for the clinical actions Although this discussion has focused on the potential of volatile anesthetics are still a matter of investigation, contributions of TASK-1 and TASK-3 to general anes- it is believed that distinct components of these effects are thesia, it is likely that other channels such as TREK-1 mediated by different regions of the CNS and may and TREK-2 participate in the CNS actions of general involve different molecular targets (Koblin 2000). anesthetics. Possible roles for these channels in such Consistent with this view, the two fundamental clinical actions remain to be investigated. Also, a number of two- end-points of anesthesia, the loss of consciousness and pore-domain channels that are not prominently the absence of movement in response to a painful stimu- expressed in the CNS are affected by clinically relevant lus, occur at different doses of these compounds. doses of general anesthetics (Patel and Honore 2001b) Experimental evidence suggests that higher brain struc- and may contribute to some of their well-known periph- tures do not mediate neuromuscular effects, but soporif- eral effects. Finally, some of the two-pore-domain K+ ic actions likely occur supra-spinally (Collins and others channels are sensitive to the actions of local anesthetics 1995). The loss of movement is particularly interesting and may participate in clinical effects of these com- in this context, because it is accompanied by a decline in pounds (Kindler and others 1999; Patel and Honore motoneuronal excitability, both in experimental animals 2001b). and in humans (Rampil and King 1996; Zhou and others 1997, 1998). This decrease in excitability is believed to Regulation of Cellular Membrane Properties result from motoneuronal hyperpolarization, and experi- by Changes in Gene Expression ments in vitro suggest that the hyperpolarization occurs in part by activation of leak K+ channels (Sirois and oth- A final form of neuronal regulation in which leak K+ ers 1998, 2000). As noted above, motoneurons express channels have been implicated is the modulation of high levels of TASK-1 and TASK-3 mRNA, and accord- membrane properties through changes in gene expres- ingly, the anesthetic-activated K+ conductance in sion. This has been specifically addressed in a study of hypoglossal motoneurons has the voltage-dependent and animals that, as a result of a genetic deletion, do not kinetic properties of these channels and is completely express the α6 or δ GABA receptor subunits in their inhibited by acidification of the extracellular media cerebellar granule neurons (Brickley and others 2001).

(Sirois and others 2000). Most notably, the activation in In these neurons, there are two populations of GABAA motoneurons of this TASK-like conductance by general receptors, which differ in their subcellular localization, anesthetics (halothane and isofluorane) occurs at con- subunit composition, and presumed functions. One centrations that are precisely the same as those required group mediates phasic GABAergic synaptic currents, for the immobilization induced by these compounds. and the other group mediates a tonic, extrasynaptic Taken together, these data suggest that TASK-1 and/or “spillover” component that appears to contribute to the TASK-3 may participate in one of the fundamental clin- normal resting membrane potential in these cells. The ical effects of inhaled anesthetics. α6 and δ subunits are involved in mediation of the tonic Given the fact that anesthetic-sensitive two-pore- component, and therefore it was predicted that cells domain channels are widely distributed in the CNS, it is lacking these subunits would have a depolarized mem- plausible that they contribute to supra-spinal effects of brane potential relative to cells from wild-type mice. these compounds, in addition to their proposed neuro- However, this was not the case. As expected, the muscular actions. Once again, the anatomic substrates GABAergic contribution to the resting membrane poten- for these effects are not known with certainty, but one of tial was absent in the mutant mice. Nevertheless, these the traditional areas for consideration has been the neurons did not differ from their wild-type counterparts ascending arousal system (Koblin 2000), because these in membrane potential or in a host of other membrane groups of primarily aminergic neurons in the brainstem properties, suggesting that cells from the mutant animals play a role in consciousness and alertness. Two of the were subject to some sort of compensatory alteration. important sets of cells in this system are noradrenergic Examination of both groups of neurons in the presence neurons in the locus coeruleus and serotonergic neurons of GABAergic blockers showed that cerebellar granule in the dorsal raphe nucleus. Like motoneurons, cells in neurons from the mutant mice had larger non- each of these groups express high levels of TASK-1 and GABAergic outward currents than wild-type cells. TASK-3 mRNA (Karschin and others 2001; Talley and Underlying this difference was an enhanced pH-sensi- others 2001). Accordingly, both groups of neurons are tive leak K+ conductance, which likely resulted from hyperpolarized by clinically relevant doses of halothane increased expression of TASK-1. So it appears that these (Sirois and others 2000; Washburn and others 2002). In neurons may have a “set point” for certain membrane

Volume 9, Number 1, 2003 THE NEUROSCIENTIST 53 characteristics (e.g., membrane potential, input resist- Buckler KJ. 1997. A novel oxygen-sensitive potassium current in rat ance), and expression of a two-pore-domain K+ channel carotid body type I cells. J Physiol (Lond) 498:649–62. Buckler KJ, Williams BA, Honore E. 2000. An oxygen-, acid- and is one of the mechanisms for maintaining these proper- anaesthetic-sensitive TASK-like background potassium channel in ties. Experiments with these mutant mice provide evi- rat arterial chemoreceptor cells. J Physiol (Lond) 525:135–42. dence that two-pore-domain channels are substrates for Byrne JH, Kandel ER. 1996. Presynaptic facilitation revisited: state regulation of cellular excitability not only in the short and time dependence. J Neurosci 16:425–35. term but also over longer time scales. Chapman CG, Meadows HJ, Godden RJ, Campbell DA, Duckworth M, Kelsell RE, and others. 2000. Cloning, localisation and functional expression of a novel human, cerebellum specific, two pore Conclusions domain potassium channel. Brain Res Mol Brain Res 82:74–83. Chavez RA, Gray AT, Zhao BB, Kindler CH, Mazurek MJ, Mehta Y, In this review, we have described evidence for the and others. 1999. TWIK-2, a new weak inward rectifying member involvement of two-pore-domain K+ channels in a num- of the tandem pore domain potassium channel family. J Biol Chem ber of important CNS functions. We have focused on 19:7887–92. Collins JG, Kendig JJ, Mason P. 1995. Anesthetic actions within the processes in which there is specific evidence for partic- spinal cord: contributions to the state of general anesthesia. Trends + ipation of leak K channels and/or members of the two- Neurosci 18:549–53. pore-domain family as substrates. Although much of this Czirjak G, Petheo GL, Spat A, Enyedi P. 2001. Inhibition of TASK-1 evidence is compelling, it is entirely correlative, and potassium channel by phospholipase C. Am J Physiol Cell Physiol more definitive work needs to be done. Certainly the 281:C700–8. Decher N, Maier M, Dittrich W, Gassenhuber J, Bruggemann A, Busch production of genetically modified animals with dele- AE, and others. 2001. Characterization of TASK-4, a novel mem- tion or mutation of these genes will help, although as is ber of the pH-sensitive, two-pore domain potassium channel fami- evident from the preceding section, interpretation of ly. FEBS Lett 492:84–9. experiments using these animals will present a new set Duprat F, Lesage F, Fink M, Reyes R, Heurteaux C, Lazdunski M. 1997. TASK, a human background K+ channel to sense external pH of complications. In addition to genetic modifications, variations near physiological pH. EMBO J 16:5464–71. the development of more selective compounds for the Feldman JL, Ellenberger HH. 1988. Central coordination of respirato- activation and/or blockade of these channels will be use- ry and cardiovascular control in mammals. Annu Rev Physiol ful experimentally and may be of clinical benefit. 50:593–606. Fink M, Duprat F, Lesage F, Reyes R, Romey G, Heurteaux C, and others. 1996. Cloning, functional expression and brain localization of Note Added in Proof a novel unconventional outward rectifier K+ channel. EMBO J 15:6854–62. The International Union of Pharmacology (IUPHAR) Fink M, Lesage F, Duprat F, Heurteaux C, Reyes R, Fosset M, and oth- has adopted the nomenclature system put in place by the ers. 1998. A neuronal two P domain K+ channel stimulated by Human Genome Organization, replacing “KCNKx” arachidonic acid and polyunsaturated fatty acids. EMBO J with “K2Px.1.” They also suggest that a different 17:3297–308. Girard C, Duprat F, Terrenoire C, Tinel N, Fosset M, Romey G, and nomenclature (consolidating clearly related channels) is others. 2001. Genomic and functional characteristics of novel likely to be established in the future. See Catterall WA, human pancreatic 2P domain K+ channels. Biochem Biophys Res Chandy KG, Gutman GA, editors. 2002. The IUPHAR Commun 282:249–56. Compendium of Voltage-gated Ion Channels. Leeds: Goldstein SA, Bockenhauer D, O’Kelly I, Zilberberg N. 2001. IUPHAR Media. Potassium leak channels and the KCNK family of two-P-domain subunits. Nat Rev Neurosci 2:175–84. Gray AT, Zhao BB, Kindler CH, Winegar BD, Mazurek MJ, Xu J, and References others. 2000. Volatile anesthetics activate the human tandem pore domain baseline K+ channel KCNK5. Anesthesiology 92:1722–30. Ashmole I, Goodwin PA, Stanfield PR. 2001. TASK-5, a novel mem- Guerineau NC, Gahwiler BH, Gerber U. 1994. Reduction of resting K+ ber of the tandem pore K+ channel family. Pflugers Arch current by metabotropic glutamate and muscarinic receptors in rat 442:828–33. CA3 cells: mediation by G-proteins. J Physiol (Lond) 474:27–33. Ballantyne D, Scheid P. 2000. Mammalian brainstem chemosensitive Hervieu GJ, Cluderay JE, Gray CW, Green PJ, Ranson JL, Randall AD, neurones: linking them to respiration in vitro. J Physiol (Lond) and others. 2001. Distribution and expression of TREK-1, a two- 525:567–77. pore-domain potassium channel, in the adult rat CNS. Bang H, Kim Y, Kim D. 2000. TREK-2, a new member of the Neuroscience 103:899–919. mechanosensitive tandem-pore K+ channel family. J Biol Chem Hille B. 2001. Ion channels of excitable membranes. Sunderland 275:17412–19. (MA): Sinauer. Bayliss DA, Viana F, Berger AJ. 1994. Effects of thyrotropin-releasing Karschin C, Wischmeyer E, Preisig-Muller R, Rajan S, Derst C, hormone on rat motoneurons are mediated by G proteins. Brain Grzeschik KH, and others. 2001. Expression pattern in brain of Res 668:220–9. TASK-1, TASK-3, and a tandem pore domain K+ channel subunit, Bockenhauer D, Zilberberg N, Goldstein SA. 2001. KCNK2: TASK-5, associated with the central auditory nervous system. Mol reversible conversion of a hippocampal potassium leak into a volt- Cell Neurosci 18:632–48. age-dependent channel. Nat Neurosci 4:486–91. Ketchum KA, Joiner WJ, Sellers AJ, Kaczmarek LK, Goldstein SA. Boyd DF, Millar JA, Watkins CS, Mathie A. 2000. The role of Ca2+ 1995. A new family of outwardly rectifying potassium channel pro- + teins with two pore domains in tandem. Nature 376:690–5. stores in the muscarinic inhibition of the K current IK(SO) in neona- tal rat cerebellar granule cells. J Physiol (Lond) 529:321–31. Kim D, Gnatenco C. 2001. TASK-5, a new member of the tandem-pore Brickley SG, Revilla V, Cull-Candy SG, Wisden W, Farrant M. 2001. K+ channel family. Biochem Biophys Res Commun 284:923–30. Adaptive regulation of neuronal excitability by a voltage-inde- Kim D, Sladek CD, Aguado-Velasco C, Mathiasen JR. 1995. pendent potassium conductance. Nature 409:88–92. Arachidonic acid activation of a new family of K+ channels in cul- Brown LD, Kim KM, Nakajima Y, Nakajima S. 1993. The role of G tured rat neuronal cells. J Physiol (Lond) 484:643–60. protein in muscarinic depolarization near resting potential in cul- Kim Y, Bang H, Kim D. 2000. TASK-3, a new member of the tandem tured hippocampal neurons. Brain Res 612:200–9. pore K+ channel family. J Biol Chem 275:9340–7.

54 THE NEUROSCIENTIST Two-Pore-Domain Potassium Channels Kindler CH, Pietruck C, Yost CS, Sampson ER, Gray AT. 2000. Patel AJ, Honore E, Maingret F, Lesage F, Fink M, Duprat F, and oth- Localization of the tandem pore domain K+ channel TASK-1 in ers. 1998. A mammalian two pore domain mechano-gated S-like the rat central nervous system. Brain Res Mol Brain Res K+ channel. EMBO J 17:4283–90. 80:99–108. Patel AJ, Lazdunski M, Honore E. 2001. Lipid and mechano-gated 2P Kindler CH, Yost CS, Gray AT. 1999. Local anesthetic inhibition of domain K+ channels. Curr Opin Cell Biol 13:422–8. baseline potassium channels with two pore domains in tandem. Patel AJ, Maingret F, Magnone V, Fosset M, Lazdunski M, Honore E. Anesthesiology 90:1092–102. 2000. TWIK-2, an inactivating 2P domain K+ channel. J Biol Chem Koblin DD. 2000. Mechanisms of action. In: Miller RD, editor. 275:28722–30. Anesthesia. Philadelphia: Churchill Livingston. p 48–73. Peier AM, Moqrich A, Hergarden AC, Reeve AJ, Andersson DA, Story Koh SD, Monaghan K, Sergeant GP, Ro S, Walker RL, Sanders KM, GM, and others. 2002. A TRP channel that senses cold stimuli and and others. 2001. TREK-1 regulation by nitric oxide and cGMP- menthol. Cell 108:705–15. dependent protein kinase. An essential role in smooth muscle Pountney DJ, Gulkarov I, Vega-Saenz dM, Holmes D, Saganich M, inhibitory neurotransmission. J Biol Chem 276:44338–46. Rudy B, and others. 1999. Identification and cloning of TWIK- Leonoudakis D, Gray AT, Winegar BD, Kindler CH, Harada M, Taylor originated similarity sequence (TOSS): a novel human 2-pore K+ DM, and others. 1998. An open rectifier potassium channel with channel principal subunit. FEBS Lett 450:191–6. two pore domains in tandem cloned from rat cerebellum. J Rajan S, Wischmeyer E, Karschin C, Preisig-Muller R, Grzeschik KH, Neurosci 18:868–77. Daut J, and others. 2000. THIK-1 and THIK-2, a novel subfamily Lesage F, Guillemare E, Fink M, Duprat F, Lazdunski M, Romey G, of tandem pore domain K+ channels. J Biol Chem 275:16650–7. and others. 1996a. A pH-sensitive yeast outward rectifier K+ chan- Rajan S, Wischmeyer E, Xin LG, Preisig-Muller R, Daut J, Karschin nel with two pore domains and novel gating properties. J Biol A, and others. 2000. TASK-3, a novel tandem pore domain acid- Chem 271:4183–7. sensitive K+ channel. An extracellular histiding as pH sensor. J Biol Lesage F, Guillemare E, Fink M, Duprat F, Lazdunski M, Romey G, Chem 275:16650–7. and others. 1996b. TWIK-1, a ubiquitous human weakly inward Rampil IJ, King BS. 1996. Volatile anesthetics depress spinal motor rectifying K+ channel with a novel structure. EMBO J 15:1004–11. neurons. Anesthesiology 85:129–34. Lesage F, Lazdunski M. 2000. Molecular and functional properties of Reid G, Flonta M. 2001a. Cold transduction by inhibition of a back- two-pore-domain potassium channels. Am J Physiol ground potassium conductance in rat primary sensory neurones. 279:F793–801. Neurosci Lett 297:171–4. Lesage F, Terrenoire C, Romey G, Lazdunski M. 2000. Human Reid G, Flonta ML. 2001b. Physiology. Cold current in thermorecep- TREK2, a 2P domain mechano-sensitive K+ channel with multiple tive neurons. Nature 413:480. regulations by polyunsaturated fatty acids, lysophospholipids, and Rekling JC, Funk GD, Bayliss DA, Dong XW, Feldman JL. 2000. Synaptic Gs, Gi, and Gq protein-coupled receptors. J Biol Chem control of motoneuronal excitability. Physiol Rev 80:399–410. 275:28398–405. Reyes R, Duprat F, Lesage F, Fink M, Salinas M, Farman N, and oth- Lewis A, Hartness ME, Chapman CG, Fearon IM, Meadows HJ, Peers ers. 1998. Cloning and expression of a novel pH-sensitive two pore + + C, and others. 2001. Recombinant hTASK1 is an O2-sensitive K domain K channel from human kidney. J Biol Chem 273:30863–9. channel. Biochem Biophys Res Commun 285:1290–4. Reyes R, Lauritzen I, Lesage F, Ettaiche M, Fosset M, Lazdunski M. Lopes CM, Gallagher PG, Buck ME, Butler MH, Goldstein SA. 2000. 2000. Immunolocalization of the arachidonic acid and Proton block and voltage gating are potassium-dependent in the mechanosensitive baseline TRAAK potassium channel in the nerv- cardiac leak channel Kcnk3. J Biol Chem 275:16969–78. ous system. Neuroscience 95:893–901. Maingret F, Lauritzen I, Patel AJ, Heurteaux C, Reyes R, Lesage F, and Rubin GM, Yandell MD, Wortman JR, Gabor Miklos GL, Nelson CR, others. 2000. TREK-1 is a heat-activated background K+ channel. Hariharan IK, and others. 2000. Comparative genomics of the EMBO J 19:2483–91. eukaryotes. Science 287:2204–15. McCormick DA. 1992. Neurotransmitter actions in the thalamus and Salinas M, Reyes R, Lesage F, Fosset M, Heurteaux C, Romey G, and cerebral cortex and their role in neuromodulation of thalamocorti- others. 1999. Cloning of a new mouse two-P domain channel sub- cal activity. Prog Neurobiol 39:337–88. unit and a human homologue with a unique pore structure. J Biol McKemy DD, Neuhausser WM, Julius D. 2002. Identification of a cold Chem 274:11751–60. receptor reveals a general role for TRP channels in thermosensa- Salkoff L, Butler A, Fawcett G, Kunkel M, McArdle C, Mino G, and tion. Nature 416:52–8. others. 2001. Evolution tunes the excitability of individual neu- Meadows HJ, Randall AD. 2001. Functional characterisation of human rons. Neuroscience 103:853–9. TASK-3, an acid-sensitive two-pore domain potassium channel. Schrader LA, Tasker JG. 1997. Modulation of multiple potassium cur- Neuropharmacology 40:551–9. rents by metabotropic glutamate receptors in neurons of the hypo- Medhurst AD, Rennie G, Chapman CG, Meadows H, Duckworth MD, thalamic supraoptic nucleus. J Neurophysiol 78:3428–37. Kelsell RE, and others. 2001. Distribution analysis of human two Schweitzer P, Madamba SG, Siggins GR. 1998. Somatostatin increas- pore domain potassium channels in tissues of the central nervous es a voltage-insensitive K+ conductance in rat CA1 hippocampal system and periphery. Brain Res Mol Brain Res 86:101–14. neurons. J Neurophysiol 79:1230–8. Millar JA, Barratt L, Southan AP, Page KM, Fyffe RE, Robertson B, Shen KZ, Surprenant A. 1993. Common ionic mechanisms of excita- and others. 2000. A functional role for the two-pore domain potas- tion by substance P and other transmitters in guinea-pig submu- sium channel TASK-1 in cerebellar granule neurons. Proc Natl cosal neurones. J Physiol (Lond) 462:483–501. Acad Sci U S A 97:3614–18. Sirois JE, Lei Q, Talley EM, Lynch C, Bayliss DA. 2000. The TASK-1 Nattie EE. 2001. Central chemosensitivity, sleep, and wakefulness. two-pore domain K+ channel is a molecular substrate for neuronal Respir Physiol 129:257–68. effects of inhalation anesthetics. J Neurosci 20:6347–54. Nicoll RA, Malenka RC, Kauer JA. 1990. Functional comparison of Sirois JE, Pancrazio JJ, Lynch III C, Bayliss DA. 1998. Multiple ionic neurotransmitter receptor subtypes in mammalian central nervous mechanisms mediate inhibition of rat motoneurones by inhalation system. Physiol Rev 70:513–65. anaesthetics. J Physiol (Lond) 512:851–62. α Pan ZZ, Grudt TJ, Williams JT. 1994. 1-adrenoceptors in rat dorsal Talley EM, Bayliss DA. 2002. Modulation of TASK-1 (Kcnk3) and raphe neurons: regulation of two potassium conductances. J TASK-3 (Kcnk9) potassium channels: volatile anesthetics and neu- Physiol (Lond) 478(Pt 3):437–47. rotransmitters share a molecular site of action. J Biol Chem Patel AJ, Honore E. 2001b. Anesthetic-sensitive 2P domain K+ chan- 277:17733–42. nels. Anesthesiology 95:1013–21. Talley EM, Lei Q, Sirois JE, Bayliss DA. 2000. TASK-1, a two-pore Patel AJ, Honore E. 2001a. Properties and modulation of mammalian domain K+ channel, is modulated by multiple neurotransmitters in 2P domain K+ channels. Trends Neurosci 24:339–46. motoneurons. Neuron 25:399–410. Patel AJ, Honore E, Lesage F, Fink M, Romey G, Lazdunski M. 1999. Talley EM, Solorzano G, Lei Q, Kim D, Bayliss DA. 2001. CNS dis- Inhalational anesthetics activate two-pore-domain background K+ tribution of members of the two-pore-domain (KCNK) potassium channels. Nat Neurosci 2:422–6. channel family. J Neurosci 21:7491–505.

Volume 9, Number 1, 2003 THE NEUROSCIENTIST 55 Thompson JD, Higgins DG, Gibson TJ. 1994. CLUSTAL W: improv- Washburn CP, Sirois JE, Talley EM, Guyenet PG, Bayliss DA. 2002. ing the sensitivity of progressive multiple sequence alignment Serotonergic raphe neurons express TASK channel transcripts and through sequence weighting, position-specific gap penalties and a TASK-like pH- and halothane-sensitive K+ conductance. J weight matrix choice. Nucleic Acids Res 22:4673–80. Neurosci 22:1256–65. Vega-Saenz de Miena E, Lau DH, Zhadina M, Pountney D, Coetzee Wei A, Jegla T, Salkoff L. 1996. Eight potassium channel families WA, Rudy B. 2001. KT3.2 and KT3.3, two novel human two-pore revealed by the C. elegans genome project. Neuropharmacology K+ channels closely related to TASK-1. J Neurophysiol 86:130–42. 35:805–29. Viana F, De la Peña E, Belmonte C. 2002. Specificity of cold thermo- Zhou HH, Jin TT, Qin B, Turndorf H. 1998. Suppression of spinal cord transduction is determined by differential ionic channel expression. motoneuron excitability correlates with surgical immobility during Nat Neurosci 5:254–60. isoflurane anesthesia. Anesthesiology 88:955–61. Wang ZW, Kunkel MT, Wei A, Butler A, Salkoff L. 1999. Genomic Zhou HH, Mehta M, Leis AA. 1997. Spinal cord motoneuron organization of nematode 4TM K+ channels. Ann N Y Acad Sci excitability during isoflurane and nitrous oxide anesthesia. 868:286–303. Anesthesiology 86:302–7.

56 THE NEUROSCIENTIST Two-Pore-Domain Potassium Channels