The in vivo contributions of TASK-1-containing channels to the actions of inhalation anesthetics, the alpha(2) adrenergic sedative dexmedetomidine, and cannabinoid agonists

TASK channels determine pH sensitivity in select respiratory neurons but do not contribute to central respiratory chemosensitivity

Central respiratory chemoreception is the mechanism by which the CNS maintains physiologically appropriate pH and PCO2 via control of breathing. A prominent hypothesis holds that neural substrates for this process are distributed widely in the respiratory network, especially because many neurons that make up this network are chemosensitive in vitro. We and others have proposed that TASK channels (TASK-1, K(2P)3.1 and/or TASK-3, K(2P)9.1) may serve as molecular sensors for central chemoreception because they are highly expressed in multiple neuronal populations in the respiratory pathway and contribute to their pH sensitivity in vitro. To test this hypothesis, we examined the chemosensitivity of two prime candidate chemoreceptor neurons in vitro and tested ventilatory responses to CO2 using TASK channel knock-out mice. The pH sensitivity of serotonergic raphe neurons was abolished in TASK channel knock-outs. In contrast, pH sensitivity of neurons in the mouse retrotrapezoid nucleus (RTN) was fully maintained in a TASK null background, and pharmacological evidence indicated that a K+ channel with properties distinct from TASK channels contributes to the pH sensitivity of rat RTN neurons. Furthermore, the ventilatory response to CO2 was completely retained in single or double TASK knock-out mice. These data rule out a strict requirement for TASK channels or raphe neurons in central respiratory chemosensation. Furthermore, they indicate that a non-TASK K+ current contributes to chemosensitivity of RTN neurons, which are profoundly pH-sensitive and capable of driving respiratory output in response to local pH changes in vivo.

Invalidation of TASK1 potassium channels disrupts adrenal gland zonation and mineralocorticoid homeostasis

TASK1 (KCNK3) and TASK3 (KCNK9) are two-pore domain potassium channels highly expressed in adrenal glands. TASK1/TASK3 heterodimers are believed to contribute to the background conductance whose inhibition by angiotensin II stimulates aldosterone secretion. We used task1-/- mice to analyze the role of this channel in adrenal gland function. Task1-/- exhibited severe hyperaldosteronism independent of salt intake, hypokalemia, and arterial 'low-renin' hypertension. The hyperaldosteronism was fully remediable by glucocorticoids. The aldosterone phenotype was caused by an adrenocortical zonation defect. Aldosterone synthase was absent in the outer cortex normally corresponding to the zona glomerulosa, but abundant in the reticulo-fasciculata zone. The impaired mineralocorticoid homeostasis and zonation were independent of the sex in young mice, but were restricted to females in adults. Patch-clamp experiments on adrenal cells suggest that task3 and other K+ channels compensate for the task1 absence. Adrenal zonation appears as a dynamic process that even can take place in adulthood. The striking changes in the adrenocortical architecture in task1-/- mice are the first demonstration of the causative role of a potassium channel in development/differentiation.

Identification of a region in the TASK3 two pore domain potassium channel that is critical for its blockade by methanandamide

BACKGROUND AND PURPOSE

The TASK subfamily of two pore domain potassium channels (K2P) encodes for leak K currents, contributing to the resting membrane potential of many neurons and regulating their excitability. TASK1 and TASK3 channels are regulated by a number of pharmacological and physiological mediators including cannabinoids such as methanandamide. In this study, we investigate how methanandamide blocks these channels.

EXPERIMENTAL APPROACH

Currents through wild type and mutated TASK1 and TASK3 channels expressed in modified HEK-293 cells were measured using whole-cell electrophysiological recordings in the presence and absence of methanandamide.

KEY RESULTS

Methanandamide (3 microM) produced substantial block of hTASK1, hTASK3 and mTASK3 channels but was most potent at blocking hTASK3 channels. Block of these channels was irreversible unless cells were washed with buffer containing bovine serum albumin. Mutation of the distal six amino acids of TASK1 did not alter methanandamide inhibition, whilst C terminal truncation of TASK3 channels caused a small but significant reduction of inhibition. However, deletion of six amino acids (VLRFLT) at the interface between the final transmembrane domain and cytoplasmic C terminus of TASK3 channels gave functional currents that were no longer inhibited by methanandamide or by activation of GPCRs.

CONCLUSIONS AND IMPLICATIONS

Methanandamide potently blocked TASK3 and TASK1 channels and both methanandamide and G protein-mediated inhibition converged on the same intracellular gating pathway. Physiologically, methanandamide block of TASK1 and TASK3 channels may underpin a number of CNS effects of cannabinoids that are not mediated through activation of CB1 or CB2 receptors.

Biophysical, pharmacological, and functional characteristics of cloned and native mammalian two-pore domain K+ channels

The mammalian family of two-pore domain K+ (K2P) channel proteins are encoded by 15 KCNK genes and subdivided into six subfamilies on the basis of sequence similarities: TWIK, TREK, TASK, TALK, THIK, and TRESK. K2P channels are expressed in cells throughout the body and have been implicated in diverse cellular functions including maintenance of the resting potential and regulation of excitability, sensory transduction, ion transport, and cell volume regulation, as well as metabolic regulation and apoptosis. In recent years K2P channel isoforms have been identified as important targets of several widely employed drugs, including: general anesthetics, local anesthetics, neuroprotectants, and anti-depressants. An important goal of future studies will be to identify the basis of drug actions and channel isoform selectivity. This goal will be facilitated by characterization of native K2P channel isoforms, their pharmacological properties and tissue-specific expression patterns. To this end the present review examines the biophysical, pharmacological, and functional characteristics of cloned mammalian K2P channels and compares this information with the limited data available for native K2P channels in order to determine criteria which may be useful in identifying ionic currents mediated by native channel isoforms and investigating their pharmacological and functional characteristics.

TASK-3 two-pore domain potassium channels enable sustained high-frequency firing in cerebellar granule neurons

The ability of neurons, such as cerebellar granule neurons (CGNs), to fire action potentials (APs) at high frequencies during sustained depolarization is usually explained in relation to the functional properties of voltage-gated ion channels. Two-pore domain potassium (K(2P)) channels are considered to simply hyperpolarize the resting membrane potential (RMP) by increasing the potassium permeability of the membrane. However, we find that CGNs lacking the TASK-3 type K(2P) channel exhibit marked accommodation of action potential firing. The accommodation phenotype was not associated with any change in the functional properties of the underlying voltage-gated sodium channels, nor could it be explained by the more depolarized RMP that resulted from TASK-3 channel deletion. A functional rescue, involving the introduction of a nonlinear leak conductance with a dynamic current clamp, was able to restore wild-type firing properties to adult TASK-3 knock-out CGNs. Thus, in addition to the accepted role of TASK-3 channels in limiting neuronal excitability, by increasing the resting potassium conductance TASK-3 channels also increase excitability by supporting high-frequency firing once AP threshold is reached.

TASK-3 knockout mice exhibit exaggerated nocturnal activity, impairments in cognitive functions, and reduced sensitivity to inhalation anesthetics

The TASK-3 channel is an acid-sensitive two-pore-domain K+ channel, widely expressed in the brain and probably involved in regulating numerous neuronal populations. Here, we characterized the behavioral and pharmacological phenotypes of TASK-3 knockout (KO) mice. Circadian locomotor activity measurements revealed that the nocturnal activity of the TASK-3 KO mice was increased by 38% (P < 0.01) compared with wild-type littermate controls, light phase activity being similar. Although TASK-3 channels are abundant in cerebellar granule cells, the KO mice performed as well as the wild-type mice in walking on a rotating rod or along a 1.2-cm-diameter beam. However, they fell more frequently from a narrower 0.8-cm beam. The KO mice showed impaired working memory in the spontaneous alternation task, with the alternation percentage being 62 +/- 3% for the wild-type mice and 48 +/- 4% (P < 0.05) for the KO mice. Likewise, during training for the Morris water-maze spatial memory task, the KO mice were slower to find the hidden platform, and in the probe trial, the female KO mice visited fewer times the platform quadrant than the male KO and wild-type mice. In pharmacological tests, the TASK-3 KO mice showed reduced sensitivity to the inhalation anesthetic halothane and the cannabinoid receptor agonist WIN55212-2 mesylate [(R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate] but unaltered responses to the alpha2 adrenoceptor agonist dexmedetomidine, the i.v. anesthetic propofol, the opioid receptor agonist morphine, and the local anesthetic lidocaine. Overall, our results suggest important contributions of TASK-3 channels in the neuronal circuits regulating circadian rhythms, cognitive functions, and mediating specific pharmacological effects.

Reduced inhibition of cortical glutamate and GABA release by halothane in mice lacking the K+ channel, TREK-1

BACKGROUND AND PURPOSE

Deletion of TREK-1, a two-pore domain K(+) channel (K(2P)) activated by volatile anaesthetics, reduces volatile anaesthetic potency in mice, consistent with a role for TREK-1 as an anaesthetic target. We used TREK-1 knockout mice to examine the presynaptic function of TREK-1 in transmitter release and its role in the selective inhibition of glutamate vs GABA release by volatile anaesthetics.

EXPERIMENTAL APPROACH

The effects of halothane on 4-aminopyridine-evoked and basal [(3)H]glutamate and [(14)C]GABA release from cerebrocortical nerve terminals isolated from TREK-1 knockout (KO) and littermate wild-type (WT) mice were compared. TREK-1 was quantified by immunoblotting of nerve terminal preparations.

KEY RESULTS

Deletion of TREK-1 significantly reduced the potency of halothane inhibition of 4-aminopyridine-evoked release of both glutamate and GABA without affecting control evoked release or the selective inhibition of glutamate vs GABA release. TREK-1 deletion also reduced halothane inhibition of basal glutamate release, but did not affect basal GABA release.

CONCLUSIONS AND IMPLICATIONS

The reduced sensitivity of glutamate and GABA release to inhibition by halothane in TREK-1 KO nerve terminals correlates with the reduced anaesthetic potency of halothane in TREK-1 KO mice observed in vivo. A presynaptic role for TREK-1 was supported by the enrichment of TREK-1 in isolated nerve terminals determined by immunoblotting. This study represents the first evidence for a link between an anaesthetic-sensitive 2-pore domain K(+) channel and presynaptic function, and provides further support for presynaptic mechanisms in determining volatile anaesthetic action.

mGlu4 potentiation of K(2P)2.1 is dependant on C-terminal dephosphorylation

Two-pore domain potassium (K(2P)) channels are proposed to underlie the background or leak current found in many excitable cells. Extensive studies have been performed investigating the inhibition of K(2P)2.1 by Galpha(q)- and Galpha(s)-coupled G-protein-coupled receptors (GPCRs), whereas in the present study we investigate the mechanisms underlying Galpha(i)/Galpha(o)-coupled GPCR increases in K(2P)2.1 activity. Activation of mGlu4 increases K(2P)2.1 activity, with pharmacological inhibition of protein kinases and phosphatases revealing the involvement of PKA whereas PKC, PKG or protein phosphatases play no role. Mutational analysis of potential C-terminal phosphorylation sites indicates S333 to control approximately 70%, with S300 controlling approximately 30% of the increase in K(2P)2.1 activity following mGlu4 activation. These data reveal that activation of mGlu4 leads to an increase in K(2P)2.1 activity through a reduction in C-terminal phosphorylation, which represents a novel mechanism by which group III mGlu receptors may regulate cell excitability and synaptic activity.

Differential effects of volatile and intravenous anesthetics on the activity of human TASK-1

Volatile anesthetics have been shown to activate various two-pore (2P) domain K(+) (K(2P)) channels such as TASK-1 and TREK-1 (TWIK-related acid-sensitive K(+) channel), and mice deficient in these channels are resistant to halothane-induced anesthesia. Here, we investigated whether K(2P) channels were also potentially important targets of intravenous anesthetics. Whole cell patch-clamp techniques were used to determine the effects of the commonly used intravenous anesthetics etomidate and propofol on the acid-sensitive K(+) current in rat ventricular myocytes (which strongly express TASK-1) and selected human K(2P) channels expressed in Xenopus laevis oocytes. In myocytes, etomidate decreased both inward rectifier K(+) (K(ir)) current (I(K1)) and acid-sensitive outward K(+) current at positive potentials, suggesting that this drug may inhibit TASK channels. Indeed, in addition to inhibiting guinea pig Kir2.1 expressed in oocytes, etomidate inhibited human TASK-1 (and TASK-3) in a concentration-dependent fashion. Propofol had no effect on human TASK-1 (or TASK-3) expressed in oocytes. Moreover, we showed that, similar to the known effect of halothane, sevoflurane and the purified R-(-)- and S-(+)-enantiomers of isoflurane, without stereoselectivity, activated human TASK-1. We conclude that intravenous and volatile anesthetics have dissimilar effects on K(2P) channels. Human TASK-1 (and TASK-3) are insensitive to propofol but are inhibited by supraclinical concentrations of etomidate. In contrast, stimulatory effects of sevoflurane and enantiomeric isoflurane on human TASK-1 can be observed at clinically relevant concentrations.

Determinants of the Anesthetic Sensitivity of Two-pore Domain Acid-sensitive Potassium Channels

Certain two-pore domain K(+) channels are plausible targets for volatile general anesthetics, yet little is known at the molecular level about how these simple agents cause channel activation. The first anesthetic-activated K(+) current I(K(An)) that was characterized was discovered in the mollusk Lymnaea stagnalis and is remarkable for both its sensitivity to general anesthetics and its stereoselective responses to anesthetic enantiomers (Franks, N. P., and Lieb, W. R. (1988) Nature 333, 662-664 and Franks, N. P., and Lieb, W. R. (1991) Science 254, 427-430). Here we report the molecular cloning of a two-pore domain K(+) channel LyTASK from L. stagnalis and show that, when expressed in HEK-293 cells, it displays the same biophysical characteristics as the anesthetic-activated K(+) current I(K(An)). Sequence analysis and functional properties show it to be a member of the TASK family of channels with approximately 47% identity at the amino acid level when compared with human TASK-1 and TASK-3. By using chimeric channel constructs and site-directed mutagenesis we have identified the specific amino acid 159 to be a critical determinant of anesthetic sensitivity, which, when mutated to alanine, essentially eliminates anesthetic activation in the human channels and greatly reduces activation in LyTASK. The L159A mutation in LyTASK disrupts the stereoselective response to isoflurane while having no effect on the pH sensitivity of the channel, suggesting this critical amino acid may form part of an anesthetic binding site.

Does ethanol act preferentially via selected brain GABAA receptor subtypes? the current evidence is ambiguous

In rodent models, gamma-aminobutyric acid A (GABAA) receptors with the alpha6 and delta subunits, expressed in the cerebellar and cochlear nucleus granule cells, have been linked to ethanol sensitivity and voluntary ethanol drinking. Here, we review the findings. When considering both in vivo contributions and data on cloned receptors, the evidence for direct participation of the alpha6-containing receptors to increased ethanol sensitivity is poor. The alpha6 subunit-knockout mouse lines do not have any changed sensitivity to ethanol, although these mice do display increased benzodiazepine sensitivity. However, in general the compensations occurring in knockout mice (regardless of which particular gene is knocked out) tend to fog interpretations of drug actions at the systems level. For example, the alpha6 knockout mice have increased TASK-1 channel expression in their cerebellar granule cells, which could influence sensitivity to ethanol in the opposite direction to that obtained with the alpha6 knockouts. Indeed, TASK-1 knockout mice are more impaired than wild types in motor skills when given ethanol; this might explain why GABAA receptor alpha6 knockout mice have unchanged ethanol sensitivities. As an alternative to studying knockout mice, we examined the claimed delta subunit-dependent/gamma2 subunit-independent ethanol/[3H]Ro 15-4513 binding sites on GABAA receptors. We looked at [3H]Ro 15-4513 binding in HEK 293 cell membrane homogenates containing rat recombinant alpha6/4beta3delta receptors and in mouse brain sections. Specific high-affinity [3H]Ro 15-4513 binding could not be detected under any conditions to the recombinant receptors or to the cerebellar sections of gamma2(F77I) knockin mice, nor was this binding to brain sections of wild-type C57BL/6 inhibited by 1-100 mM ethanol. Since ethanol may act on many receptor and channel protein targets in neuronal membranes, we consider the alpha6 (and alpha4) subunit-containing GABAA receptors unlikely to be directly responsible for any major part of ethanol's actions. Therefore, we finish the review by discussing more generally alcohol and GABAA receptors and by suggesting potential future directions for this research.