Localization of the tandem pore domain K+ channel TASK-1 in the rat central nervous system

Unlocking Therapeutic Synergy: Tailoring Drugs for Comorbidities such as Depression and Diabetes through Identical Molecular Targets in Different Cell Types

Research in the field of pharmacology aims to generate new treatments for pathologies. Nowadays, there are an increased number of chronic disorders that severely and durably handicap many patients. Among the most widespread pathologies, obesity, which is often associated with diabetes, is constantly increasing in incidence, and in parallel, neurodegenerative and mood disorders are increasingly affecting many people. For years, these pathologies have been so frequently observed in the population in a concomitant way that they are considered as comorbidities. In fact, common mechanisms are certainly at work in the etiology of these pathologies. The main purpose of this review is to show the value of anticipating the effect of baseline treatment of a condition on its comorbidity in order to obtain concomitant positive actions. One of the implications would be that by understanding and targeting shared molecular mechanisms underlying these conditions, it may be possible to tailor drugs that address both simultaneously. To this end, we firstly remind readers of the close link existing between depression and diabetes and secondly address the potential benefit of the pleiotropic actions of two major active molecules used to treat central and peripheral disorders, first a serotonin reuptake inhibitor (Prozac ®) and then GLP-1R agonists. In the second part, by discussing the therapeutic potential of new experimental antidepressant molecules, we will support the concept that a better understanding of the intracellular signaling pathways targeted by pharmacological agents could lead to future synergistic treatments targeting solely positive effects for comorbidities.

Deregulation of Astroglial TASK-1 K+ Channel Decreases the Responsiveness to Perampanel-Induced AMPA Receptor Inhibition in Chronic Epilepsy Rats

Tandem of P domains in a weak inwardly rectifying K+ channel (TWIK)-related acid sensitive K+-1 channel (TASK-1) is activated under extracellular alkaline conditions (pH 7.2–8.2), which are upregulated in astrocytes (particularly in the CA1 region) of the hippocampi of patients with temporal lobe epilepsy and chronic epilepsy rats. Perampanel (PER) is a non-competitive α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) antagonist used for the treatment of focal seizures and primary generalized tonic–clonic seizures. Since AMPAR activation leads to extracellular alkaline shifts, it is likely that the responsiveness to PER in the epileptic hippocampus may be relevant to astroglial TASK-1 regulation, which has been unreported. In the present study, we found that PER ameliorated astroglial TASK-1 upregulation in responders (whose seizure activities were responsive to PER), but not non-responders (whose seizure activities were not responsive to PER), in chronic epilepsy rats. ML365 (a selective TASK-1 inhibitor) diminished astroglial TASK-1 expression and seizure duration in non-responders to PER. ML365 co-treatment with PER decreased spontaneous seizure activities in non-responders to PER. These findings suggest that deregulation of astroglial TASK-1 upregulation may participate in the responsiveness to PER, and that this may be a potential target to improve the efficacies of PER.

Blockade of TASK-1 Channel Improves the Efficacy of Levetiracetam in Chronically Epileptic Rats

Tandem of P domains in a weak inwardly rectifying K⁺ channel (TWIK)-related acid sensitive K⁺-1 channel (TASK-1) is an outwardly rectifying K⁺ channel that acts in response to extracellular pH. TASK-1 is upregulated in the astrocytes (particularly in the CA1 region) of the hippocampi of patients with temporal lobe epilepsy and chronically epilepsy rats. Since levetiracetam (LEV) is an effective inhibitor for carbonic anhydrase, which has a pivotal role in buffering of extracellular pH, it is likely that the anti-epileptic action of LEV may be relevant to TASK-1 inhibition, which remains to be elusive. In the present study, we found that LEV diminished the upregulated TASK-1 expression in the CA1 astrocytes of responders (whose seizure activities were responsive to LEV), but not non-responders (whose seizure activities were not controlled by LEV) in chronically epileptic rats. ML365 (a selective TASK-1 inhibitor) only reduced seizure duration in LEV non-responders, concomitant with astroglial TASK-1 downregulation. Furthermore, ML365 co-treatment with LEV decreased the duration, frequency and severity of spontaneous seizures in non-responders to LEV. To the best of our knowledge, our findings suggest, for the first time, that the up-regulation of TASK-1 expression in CA1 astrocytes may be involved in refractory seizures in response to LEV. This may be a potential target to improve responsiveness to LEV.

On the electrical passivity of astrocyte potassium conductance

Predominant expression of leak-type K⁺ channels provides astrocytes a high membrane permeability to K⁺ ions and a hyperpolarized membrane potential that are crucial for astrocyte function in brain homeostasis. In functionally mature astrocytes, the expression of leak K⁺ channels creates a unique membrane K⁺ conductance that lacks voltage-dependent rectification. Accordingly, the conductance is named ohmic or passive K⁺ conductance. Several inwardly rectifying and two-pore domain K⁺ channels have been investigated for their contributions to passive conductance. Meanwhile, gap junctional coupling has been postulated to underlie the passive behavior of membrane conductance. It is now clear that the intrinsic properties of K⁺ channels and gap junctional coupling can each act alone or together to bring about a passive behavior of astrocyte conductance. Additionally, while the passive conductance can generally be viewed as a K⁺ conductance, the actual representation of this conductance is a combined expression of multiple known and unknown K⁺ channels, which has been further modified by the intricate morphology of individual astrocytes and syncytial gap junctional coupling. The expression of the inwardly rectifying K⁺ channels explains the inward-going component of passive conductance disobeying Goldman-Hodgkin-Katz constant field outward rectification. However, the K⁺ channels encoding the outward-going passive currents remain to be determined in the future. Here, we review our current understanding of ion channels and biophysical mechanisms engaged in the passive astrocyte K⁺ conductance, propose new studies to resolve this long-standing puzzle in astrocyte physiology, and discuss the functional implication(s) of passive behavior of K⁺ conductance on astrocyte physiology.

TASK-1 channel blockade by AVE1231 increases vasocontractile responses and BP in 1- to 2-week-old but not adult rats

Background and Purpose

The vasomotor role of K2P potassium channels during early postnatal development has never been investigated. We tested the hypothesis that TASK‐1 channel (K2P family member) contribution to arterial vascular tone and BP is higher in the early postnatal period than in adulthood.

Experimental Approach

We studied 10‐ to 15‐day‐old (“young”) and 2‐ to 3‐month‐old (“adult”) male rats performing digital PCR (dPCR) (using endothelium‐intact saphenous arteries), isometric myography, sharp microelectrode technique, quantitative PCR (qPCR) and Western blotting (using endothelium‐denuded saphenous arteries), and arterial pressure measurements under urethane anaesthesia.

Key Results

We found mRNA of Kcnk1–Kcnk7, Kcnk12, and Kcnk13 genes to be expressed in rat saphenous artery, and Kcnk3 (TASK‐1) and Kcnk6 (TWIK‐2) were most abundant in both age groups. The TASK‐1 channel blocker AVE1231 (1 μmol·L⁻¹) prominently depolarized arterial smooth muscle and increased basal tone level and contractile responses to methoxamine of arteries from young rats but had almost no effect in adult rats. The level of TASK‐1 mRNA and protein expression was higher in arteries from young compared with adult rats. Importantly, intravenous administration of AVE1231 (4 mg·kg⁻¹) had no effect on mean arterial pressure in adult rats but prominently raised it in young rats.

Conclusion and Implications

We showed that TASK‐1 channels are important for negative feedback regulation of vasocontraction in young but not adult rats. The influence of TASK‐1 channels most likely contributes to low BP level at perinatal age.

Neurointegrity and neurophysiology: astrocyte, glutamate, and carbon monoxide interactions

Astrocyte contributions to brain function and prevention of neuropathologies are as extensive as that of neurons. Astroglial regulation of glutamate, a primary neurotransmitter, is through uptake, release through vesicular and non-vesicular pathways, and catabolism to intermediates. Homeostasis by astrocytes is considered to be of primary importance in determining normal central nervous system health and central nervous system physiology - glutamate is central to dynamic physiologic changes and central nervous system stability. Gasotransmitters may affect diverse glutamate interactions positively or negatively. The effect of carbon monoxide, an intrinsic central nervous system gasotransmitter, in the complex astrocyte homeostasis of glutamate may offer insights to normal brain development, protection, and its use as a neuromodulator and neurotherapeutic. In this article, we will review the effects of carbon monoxide on astrocyte homeostasis of glutamate.

Ion channel dysfunction in cerebellar ataxia

Cerebellar ataxias constitute a heterogeneous group of disorders that result in impaired speech, uncoordinated limb movements, and impaired balance, often ultimately resulting in wheelchair confinement. Motor dysfunction in ataxia can be attributed to dysfunction and degeneration of neurons in the cerebellum and its associated pathways. Recent work has suggested the importance of cerebellar neuronal dysfunction resulting from mutations in specific ion-channels that regulate membrane excitability in the pathogenesis of cerebellar ataxia in humans. Importantly, even in ataxias not directly due to ion-channel mutations, transcriptional changes resulting in ion-channel dysfunction are tied to motor dysfunction and degeneration in models of disease. In this review, we describe the role that ion-channel dysfunction plays in a variety of cerebellar ataxias, and postulate that a potential therapeutic strategy that targets specific ion-channels exists for cerebellar ataxia.

An emerging role of astrocytes in vascular contributions to cognitive impairment and dementia

Vascular contributions to cognitive impairment and dementia (VCID) is understood to be the second most common cause of dementia after Alzheimer's disease, and is also a frequent comorbidity with Alzheimer's disease. While VCID is widely acknowledged as a key contributor to dementia, the mechanistic underpinnings of VCID remain poorly understood. In this review, we address the potential role of astrocytes in the pathophysiology of VCID. The vast majority of the blood vessels in the brain are surrounded by astrocytic end-feet. Given that astrocytes make up a significant proportion of the cells in the brain, and that astrocytes are usually passively connected to one another through gap junctions, we hypothesize that astrocytes are key mediators of cognitive impairment because of cerebrovascular disease. In this review, we discuss the existing body of literature regarding the role of astrocytes at the vasculature in the brain, and the known consequences of their dysfunction, as well as our hypotheses regarding the role astrocytes play in VCID. This article is part of the Special Issue "Vascular Dementia".

Diversity of astrocyte potassium channels: An update

Astrocyte K⁺ channels and the K⁺ currents they mediate dwarf all other transmembrane conductances in these cells. This defining feature of astrocytes and its functional implications have been investigated intensely over the past decades. Nonetheless, many aspects of astrocyte K⁺ handling and signaling remain incompletely understood. In this review, we provide an update on the diversity of K⁺ channels expressed by astrocytes and new functional implications. We focus on inwardly-rectifying K⁺ channels (particularly Kir4.1), two-pore K⁺ channels and voltage and Ca²⁺-dependent K⁺ channels. We further discuss new insights into the involvement of these K⁺ channels in K⁺ buffering, control of synaptic transmission, regulation of the vasculature and in diseases of the central nervous system.

Two-pore Domain Potassium Channels in Astrocytes

Two-pore domain potassium (K_2P) channels have a distinct structure and channel properties, and are involved in a background K⁺ current. The 15 members of the K_2P channels are identified and classified into six subfamilies on the basis of their sequence similarities. The activity of the channels is dynamically regulated by various physical, chemical, and biological effectors. The channels are expressed in a wide variety of tissues in mammals in an isoform specific manner, and play various roles in many physiological and pathophysiological conditions. To function as channels, the K_2P channels form dimers, and some isoforms form heterodimers that provide diversity in channel properties. In the brain, TWIK1, TREK1, TREK2, TRAAK, TASK1, and TASK3 are predominantly expressed in various regions, including the cerebral cortex, dentate gyrus, CA1-CA3, and granular layer of the cerebellum. TWIK1, TREK1, and TASK1 are highly expressed in astrocytes, where they play specific cellular roles. Astrocytes keep leak K⁺ conductance, called the passive conductance, which mainly involves TWIK1-TREK1 heterodimeric channel. TWIK1 and TREK1 also mediate glutamate release from astrocytes in an exocytosis-independent manner. The expression of TREK1 and TREK2 in astrocytes increases under ischemic conditions, that enhance neuroprotection from ischemia. Accumulated evidence has indicated that astrocytes, together with neurons, are involved in brain function, with the K_2P channels playing critical role in these astrocytes.

Neuronal Atrophy Early in Degenerative Ataxia Is a Compensatory Mechanism to Regulate Membrane Excitability

Neuronal atrophy in neurodegenerative diseases is commonly viewed as an early event in a continuum that ultimately results in neuronal loss. In a mouse model of the polyglutamine disorder spinocerebellar ataxia type 1 (SCA1), we tested the hypothesis that cerebellar Purkinje neuron atrophy serves an adaptive role rather than being simply a nonspecific response to injury. In acute cerebellar slices from SCA1 mice, we find that Purkinje neuron pacemaker firing is initially normal but, with the onset of motor dysfunction, becomes disrupted, accompanied by abnormal depolarization. Remarkably, subsequent Purkinje cell atrophy is associated with a restoration of pacemaker firing. The early inability of Purkinje neurons to support repetitive spiking is due to unopposed calcium currents resulting from a reduction in large-conductance calcium-activated potassium (BK) and subthreshold-activated potassium channels. The subsequent restoration of SCA1 Purkinje neuron firing correlates with the recovery of the density of these potassium channels that accompanies cell atrophy. Supporting a critical role for BK channels, viral-mediated increases in BK channel expression in SCA1 Purkinje neurons improves motor dysfunction and partially restores Purkinje neuron morphology. Cerebellar perfusion of flufenamic acid, an agent that restores the depolarized membrane potential of SCA1 Purkinje neurons by activating potassium channels, prevents Purkinje neuron dendritic atrophy. These results suggest that Purkinje neuron dendritic remodeling in ataxia is an adaptive response to increases in intrinsic membrane excitability. Similar adaptive remodeling could apply to other vulnerable neuronal populations in neurodegenerative disease.

SIGNIFICANCE STATEMENT

In neurodegenerative disease, neuronal atrophy has long been assumed to be an early nonspecific event preceding neuronal loss. However, in a mouse model of spinocerebellar ataxia type 1 (SCA1), we identify a previously unappreciated compensatory role for neuronal shrinkage. Purkinje neuron firing in these mice is initially normal, but is followed by abnormal membrane depolarization resulting from a reduction in potassium channels. Subsequently, these electrophysiological effects are counteracted by cell atrophy, which by restoring normal potassium channel membrane density, re-establishes pacemaker firing. Reversing the initial membrane depolarization improved motor function and Purkinje neuron morphology in the SCA1 mice. These results suggest that Purkinje neuron remodeling in ataxia is an active compensatory response that serves to normalize intrinsic membrane excitability.

The neuroglial potassium cycle during neurotransmission: role of Kir4.1 channels

Neuronal excitability relies on inward sodium and outward potassium fluxes during action potentials. To prevent neuronal hyperexcitability, potassium ions have to be taken up quickly. However, the dynamics of the activity-dependent potassium fluxes and the molecular pathways underlying extracellular potassium homeostasis remain elusive. To decipher the specific and acute contribution of astroglial Kir4.1 channels in controlling potassium homeostasis and the moment to moment neurotransmission, we built a tri-compartment model accounting for potassium dynamics between neurons, astrocytes and the extracellular space. We here demonstrate that astroglial Kir4.1 channels are sufficient to account for the slow membrane depolarization of hippocampal astrocytes and crucially contribute to extracellular potassium clearance during basal and high activity. By quantifying the dynamics of potassium levels in neuron-glia-extracellular space compartments, we show that astrocytes buffer within 6 to 9 seconds more than 80% of the potassium released by neurons in response to basal, repetitive and tetanic stimulations. Astroglial Kir4.1 channels directly lead to recovery of basal extracellular potassium levels and neuronal excitability, especially during repetitive stimulation, thereby preventing the generation of epileptiform activity. Remarkably, we also show that Kir4.1 channels strongly regulate neuronal excitability for slow 3 to 10 Hz rhythmic activity resulting from probabilistic firing activity induced by sub-firing stimulation coupled to Brownian noise. Altogether, these data suggest that astroglial Kir4.1 channels are crucially involved in extracellular potassium homeostasis regulating theta rhythmic activity.

Electrophysiology and pharmacology of tandem domain potassium channel TREK-1 related BDNF synthesis in rat astrocytes

In the present study, the functional properties and pharmacology of two-pore domain potassium channel (K2P) TREK-1 in primary cultured rat brain astrocytes were investigated. Western blot, patch clamping techniques, and ELISA were used to detect the distribution and function of TREK-1 as well as the expression of brain-derived neurotrophic factor (BDNF) on the primary cultured astrocytes. It was shown that TREK-1 protein expressed in astrocytes was 2.4-fold higher than it was expressed in microglia. Single channel recording via patch clamping showed that the TREK-1 outward currents in astrocytes could be activated by arachidonic acid (AA) or chloroform with the conductance of 113 ± 14 and 120 ± 13 pS, respectively. The current was also sensitive to mechanical stretch and intracellular acidification. Negative pressure (-30 cm H2O) and acidification of intracellular solution (pH 6.8 or 6.3) both enhanced TREK-1 channel open probability significantly. Further pharmacological studies showed that TREK-1 antagonist penfluridol inhibited AA-induced currents, and both penfluridol and methionine (TREK-1 blockers) significantly increased BDNF level in astrocytes by 50 %. These results indicated that TREK-1 channel current was a major component of K2P currents in astrocytes. TREK-1 channels might play important roles in regulating the function of astrocytes and might be used as a drug target for neuroprotection.

From the background to the spotlight: TASK channels in pathological conditions

TWIK-related acid-sensitive potassium channels (TASK1-3) belong to the family of two-pore domain (K(2P) ) potassium channels. Emerging knowledge about an involvement of TASK channels in cancer development, inflammation, ischemia and epilepsy puts the spotlight on a leading role of TASK channels under these conditions. TASK3 has been especially linked to cancer development. The pro-oncogenic potential of TASK3 could be shown in cell lines and in various tumor entities. Pathophysiological hallmarks in solid tumors (e.g. low pH and oxygen deprivation) regulate TASK3 channels. These conditions can also be found in (autoimmune) inflammation. Inhibition of TASK1,2,3 leads to a reduction of T cell effector function. It could be demonstrated that TASK1(-/-) mice are protected from experimental autoimmune inflammation while the same animals display increased infarct volumes after cerebral ischemia. Furthermore, TASK channels have both an anti-epileptic as well as a pro-epileptic potential. The relative contribution of these opposing influences depends on their cell type-specific expression and the conditions of the cellular environment. This indicates that TASK channels are per se neither protective nor detrimental but their functional impact depends on the "pathophysiological" scenario. Based on these findings TASK channels have evolved from "mere background" channels to key modulators in pathophysiological conditions.

The two-pore domain K+ channel TASK-1 is closely associated with brain barriers and meninges

Impairment of the blood-brain barrier (BBB), the blood-cerebrospinal fluid (CSF) barrier and brain-CSF barrier has been implicated in neuropathology of several brain disorders, such as amyotrophic lateral sclerosis, cerebral edema, multiple sclerosis, neural inflammation, ischemia and stroke. Two-pore domain weakly inward rectifying K+ channel (TWIK)-related acid-sensitive potassium (TASK)-1 channels (K2p3.1; KCNK3) are among the targets that contribute to the development of these pathologies. For example TASK-1 activity is inhibited by acidification, ischemia, hypoxia and several signaling molecules released under pathologic conditions. We have used immuno-histochemistry to examine the distribution of the TASK-1 protein in structures associated with the BBB, blood-CSF barrier, brain-CSF barrier, and in the meninges of adult rat. Dense TASK-1 immuno-reactivity (TASK-1-IR) was observed in ependymal cells lining the fourth ventricle at the brain-CSF interface, in glial cells that ensheath the walls of blood vessels at the glio-vascular interface, and in the meninges. In these structures, TASK-1-IR often co-localized with glial fibrillary associated protein (GFAP) or vimentin. This study provides anatomical evidence for localization of TASK-1 K+ channels in cells that segregate distinct fluid compartments within and surrounding the brain. We suggest that TASK-1 channels, in coordination with other ion channels (e.g., aquaporins and chloride channels) and transporters (e.g., Na+-K+-ATPase and Na+-K+-2Cl⁻ and by virtue of its heterogeneous distribution, may differentially contribute to the varying levels of K+ vital for cellular function in these compartments. Our findings are likely to be relevant to recently reported roles of TASK-1 in cerebral ischemia, stroke and inflammatory brain disorders.

Functional identification of an outwardly rectifying pH- and anesthetic-sensitive leak K(+) conductance in hippocampal astrocytes

Astrocytes function as spatial K(+) buffers by expressing a rich repertoire of K(+) channels. Earlier studies suggest that acid-sensitive tandem-pore K(+) channels, mainly TWIK-related acid-sensitive K(+) (TASK) channels, mediate part of the passive astroglial membrane conductance. Here, using a combination of electrophysiology and pharmacology, we investigated the presence of TASK-like conductance in hippocampal astrocytes of rat brain slices. Extracellular pH shifts to below 7.4 (or above 7.4) induced a prominent inward (or outward) current in astrocytes in the presence of tetrodotoxin, a Na(+) channel blocker, and 4,4'-diisothiocyanatostilbene-2,2'-disulfonate, a co-transporter blocker. The pH-sensitive current was insensitive to quinine, a potent blocker of tandem-pore K(+) channels including TWIK-1 and TREK-1 channels. Voltage-clamp analysis revealed that the pH-sensitive current exhibited weak outward rectification with a reversal potential of -112 mV, close to the Nernst equilibrium potential for K(+) . Furthermore, the current-voltage relationship was well fitted with the Goldman-Hodgkin-Katz current equation for the classical open-rectifier 'leak' K(+) channel. The pH-sensitive K(+) current was potentiated by TASK channel modulators such as the volatile anesthetic isoflurane but depressed by the local anesthetic bupivacaine. However, unlike TASK channels, the pH-sensitive current was insensitive to Ba(2+) and quinine. Thus, the molecular identity of the pH-sensitive leak K(+) channel is unlikely to be attributable to TASK channels. Taken together, our results suggest a novel yet unknown leak K(+) channel underlying the pH- and anesthetic-sensitive background conductance in hippocampal astrocytes.

Heterogeneity of Kir4.1 channel expression in glia revealed by mouse transgenesis

The weakly inwardly rectifying K(+) channel Kir4.1 is found in many glial cells including astrocytes. However, questions remain regarding the relative contribution of Kir4.1 to the resting K(+) conductance of mature astrocytes in situ. We employed a bacterial artificial chromosome transgenic approach in mice to visualize Kir4.1 expression in vivo. These mice (Kir4.1-EGFP) express enhanced green fluorescent protein (EGFP) under the transcriptional control of the Kir4.1 promoter. The brains of adult Kir4.1-EGFP transgenic mice showed co-expression of EGFP and Kir4.1 in astrocytes. In addition, weaker expression of EGFP was detected in NG2+ glial cells when compared with EGFP expression in GFAP+ glial cells. Whole-cell voltage clamp recordings of EGFP+ glial cells in the CA1 area of the adult mouse hippocampus indicated astrocytes displaying properties consistent with both the "passive" and "complex" subpopulations. EGFP+ cells with bright fluorescence had the linear current-voltage (I-V) relationships and extensive gap junctional coupling characteristic of passive astrocytes. However, EGFP+ glia with weaker fluorescence displayed properties associated with complex astrocytes including nonlinear I-V relationships and lack of intercellular gap junctional coupling. Pharmacological blockade of inward currents implied that Kir4.1 channels constitute the dominant resting K(+) conductance in both glial cell types and are more highly expressed in passive astrocytes. These results suggest differential expression of Kir4.1 in glia and that this channel likely underlies the resting K(+) conductance in passive and complex astrocytes.

TWIK-1 and TREK-1 are potassium channels contributing significantly to astrocyte passive conductance in rat hippocampal slices

Expression of a linear current-voltage (I-V) relationship (passive) K(+) membrane conductance is a hallmark of mature hippocampal astrocytes. However, the molecular identifications of the K(+) channels underlying this passive conductance remain unknown. We provide the following evidence supporting significant contribution of the two-pore domain K(+) channel (K(2P)) isoforms, TWIK-1 and TREK-1, to this conductance. First, both passive astrocytes and the cloned rat TWIK-1 and TREK-1 channels expressed in CHO cells conduct significant amounts of Cs(+) currents, but vary in their relative P(Cs)/P(K) permeability, 0.43, 0.10, and 0.05, respectively. Second, quinine, which potently inhibited TWIK-1 (IC(50) = 85 microm) and TREK-1 (IC(50) = 41 microm) currents, also inhibited astrocytic passive conductance by 58% at a concentration of 200 microm. Third, a moderate sensitivity of passive conductance to low extracellular pH (6.0) supports a combined expression of acid-insensitive TREK-1, and to a lesser extent, acid-sensitive TWIK-1. Fourth, the astrocyte passive conductance showed low sensitivity to extracellular Ba(2+), and extracellular Ba(2+) blocked TWIK-1 channels at an IC(50) of 960 microm and had no effect on TREK-1 channels. Finally, an immunocytochemical study showed colocalization of TWIK-1 and TREK-1 proteins with the astrocytic markers GLAST and GFAP in rat hippocampal stratum radiatum. In contrast, another K(2P) isoform TASK-1 was mainly colocalized with the neuronal marker NeuN in hippocampal pyramidal neurons and was expressed at a much lower level in astrocytes. These results support TWIK-1 and TREK-1 as being the major components of the long-sought K(+) channels underlying the passive conductance of mature hippocampal astrocytes.

Analysis of astroglial K+ channel expression in the developing hippocampus reveals a predominant role of the Kir4.1 subunit

Astrocytes in different brain regions display variable functional properties. In the hippocampus, astrocytes predominantly express time- and voltage-independent currents, but the underlying ion channels are not well defined. This ignorance is partly attributable to abundant intercellular coupling of these cells through gap junctions, impeding quantitative analyses of intrinsic membrane properties. Moreover, distinct types of cells with astroglial properties coexist in a given brain area, a finding that confused previous analyses. In the present study, we investigated expression of inwardly rectifying (Kir) and two-pore-domain (K2P) K+ channels in astrocytes, which are thought to be instrumental in the regulation of K+ homeostasis. Freshly isolated astrocytes were used to improve space-clamp conditions and allow for quantitative assessment of functional parameters. Patch-clamp recordings were combined with immunocytochemistry, Western blot analysis, and semiquantitative transcript analysis. Comparative measurements were performed in different CA1 subregions of astrocyte-targeted transgenic mice. While confirming weak Ba2+ sensitivity in situ, our data demonstrate that in freshly isolated astrocytes, the main proportion of membrane currents is sensitive to micromolar Ba2+ concentrations. Upregulation of Kir4.1 transcripts and protein during the first 10 postnatal days was accompanied by a fourfold increase in astrocyte inward current density. Hippocampal astrocytes from Kir4.1-/- mice lacked Ba2+-sensitive currents. In addition, we report functional expression of K2P channels of the TREK subfamily (TREK1, TREK2), which mediate astroglial outward currents. Together, our findings demonstrate that Kir4.1 constitutes the pivotal K+ channel subunit and that superposition of currents through Kir4.1 and TREK channels underlies the "passive" current pattern of hippocampal astrocytes.

Region-specific alterations in astroglial TWIK-related acid-sensitive K+-1 channel immunoreactivity in the rat hippocampal complex following pilocarpine-induced status epilepticus

In the present study, we performed an analysis of tandem of P domains in a weak inwardly rectifying K+ channel (TWIK)-related acid-sensitive K+ (TASK)-1 channel immunoreactivity in the rat hippocampal complex following pilocarpine-induced status epilepticus (SE). In control animals, TASK-1 immunoreactivity was strongly detected in astrocytes in the hippocampal complex. One day after SE, TASK-1 immunoreactivity in astrocytes was markedly reduced only in the molecular layer of the dentate gyrus. One week after SE, loss of astrocytes was observed in the molecular layer of the dentate gyrus. At this time point, TASK-1 immunoreactive cells were detected mainly in the subgranular region. These cells had bipolar, elongated cell bodies with fusiform-shaped nuclei and showed vimentin immunoreactivity. Four weeks after SE (when spontaneous seizure developed), typical reactive astrogliosis was observed in the dentate gyrus and the CA1 region. Almost no astrocytes in the molecular layer showed TASK-1 immunoreactivity, whereas astrocytes in the CA1 region showed strong TASK-1 immunoreactivity. These findings indicate that, after SE, TASK-1 immunoreactivity was differentially altered in astrocytes located in different regions of the hippocampal complex, and these changes were caused by astroglial degeneration/regeneration. Therefore, alteration in TASK-1 immunoreactivity may contribute to acquisition of the properties of the epileptic hippocampal complex.

Up-regulated astroglial TWIK-related acid-sensitive K+ channel-1 (TASK-1) in the hippocampus of seizure-sensitive gerbils: a target of anti-epileptic drugs

In order to identify the modulation of TASK (TWIK-related Acid-Sensitive K(+)) channel expressions in epilepsy, we conducted a comparative analysis of TASK channel immunoreactivities in the hippocampus of seizure-resistant (SR) and seizure-sensitive (SS) gerbils. There was no difference of the TASK-1 and TASK-2 channel expressions in the hippocampi of young SR and SS gerbils (1-2 months old). In adult SS gerbil hippocampus, TASK-1 immunoreactivity in astrocytes was higher than that in adult SR gerbil hippocampus. After seizures, TASK-1 immunoreactivity was significantly down-regulated in astrocytes of the SS gerbil hippocampus. In addition, various anti-epileptic drugs selectively affect TASK-1 immunoreactivity in astrocytes of the SS gerbil hippocampus. Gabapentin, lamotrigine, topiramate and valproic acid reduced the number of TASK-1(+) astrocytes in the hippocampus to 10-25% of that in saline-treated SS adult gerbils, whereas carbamazepine and vigabatrin decreased to approximately 50%. Therefore, the present study demonstrates that up-regulated TASK-1 immunoreactivity in astrocytes may be involved in the seizure activity of SS adult gerbils and suggests that the astroglial TASK-1 channel may be a target for epilepsy therapeutics.

Ion channel diversity, channel expression and function in the choroid plexuses

Knowledge of the diversity of ion channel form and function has increased enormously over the last 25 years. The initial impetus in channel discovery came with the introduction of the patch clamp method in 1981. Functional data from patch clamp experiments have subsequently been augmented by molecular studies which have determined channel structures. Thus the introduction of patch clamp methods to study ion channel expression in the choroid plexus represents an important step forward in our knowledge understanding of the process of CSF secretion.

Two K⁺ conductances have been identified in the choroid plexus: Kv1 channel subunits mediate outward currents at depolarising potentials; Kir 7.1 carries an inward-rectifying conductance at hyperpolarising potentials. Both K⁺ channels are localised at the apical membrane where they may contribute to maintenance of the membrane potential while allowing the recycling of K⁺ pumped in by Na⁺-K⁺ ATPase. Two anion conductances have been identified in choroid plexus. Both have significant HCO₃^- permeability, and may play a role in CSF secretion. One conductance exhibits inward-rectification and is regulated by cyclic AMP. The other is carried by an outward-rectifying channel, which is activated by increases in cell volume. The molecular identity of the anion channels is not known, nor is it clear whether they are expressed in the apical or basolateral membrane. Recent molecular evidence indicates that choroid plexus also expresses the non-selective cation channels such as transient receptor potential channels (TRPV4 and TRPM3) and purinoceptor type 2 (P2X) receptor operated channels. In conclusion, good progress has been made in identifying the channels expressed in the choroid plexus, but determining the precise roles of these channels in CSF secretion remains a challenge for the future.

Two-pore-domain potassium channels contribute to neuronal potassium release and glial potassium buffering in the rat hippocampus

Two-pore-domain potassium (K2P) channels have been suggested to be involved in neuronal K+ release and glial K+ uptake. We studied effects of the K2P channel blockers quinine (200 or 500 microM), quinidine (500 microM), and bupivacaine (200 microM) on stimulus-induced and iontophoretically induced transient increases of the extracellular potassium concentration ([K+]o) in area CA1 of rat hippocampal slices, always in presence of AMPA/kainate and NMDA receptor antagonists. Increases in [K+]o evoked by repetitive alvear stimulation (20 Hz) were blocked by quinine and quinidine but amplitudes of population spikes were only modestly reduced. Bupivacaine suppressed both rises in [K+]o and population spikes. In contrast, iontophoretically induced rises in [K+]o were moderately augmented by quinine and quinidine while bupivacaine had no effect. Barium at concentrations of 2 mM which should block both potassium inward rectifier (Kir) and some K2P channels doubled iontophoretically induced rises in [K+]o also in presence of quinine, quinidine, and bupivacaine. The data suggest that quinine/quinidine-sensitive K2P channels mediate K+ release from neurons and possibly contribute to glial K+ buffering.

Deafness associated changes in two-pore domain potassium channels in the rat inferior colliculus

Two-pore potassium channels can influence neuronal excitability by regulating background leakage of potassium ions and resting membrane potential. The present study used quantitative real time PCR and in situ hybridization to determine if the decreased activity from deafness would induce changes in two-pore potassium channel subunit expression in the rat inferior colliculus (IC). Ten subunits were assessed with quantitative real-time PCR at 3 days, 3 weeks and 3 months following bilateral cochlear ablation. TASK-1, TASK-5 and THIK-2 showed significant decreases in expression at all three times assessed. TASK-5, relatively specific to auditory neurons, had the greatest decrease. TWIK-1 was significantly decreased at 3 weeks and 3 months following deafness and TREK-2 was only significantly decreased at 3 days. TASK-3, TWIK-2, THIK-1, TRAAK and TREK-1 did not show any significant changes in gene expression. In situ hybridization was used to examine TASK-1, TASK-5, TWIK-1 and THIK-2 in the central nucleus, dorsal cortex and lateral (external) cortex of the IC in normal hearing animals and at 3 weeks following deafening. All four subunits showed expression in neurons throughout IC subdivisions in normal hearing rats, with TASK-5 having the greatest overall number of labeled neurons. There was no co-localization of subunit expression with glial fibrillary acidic protein immunostaining, indicating no expression in glia. Three weeks following deafening there was a significant decrease in the number of neurons expressing TASK-1 and THIK-2 in the IC, while TASK-5 had significant decreases in the central nucleus and dorsal cortex and TWIK-1 in the lateral and dorsal cortices.

Short-Term Effects of Adolescent Methylphenidate Exposure on Brain Striatal Gene Expression and Sexual/Endocrine Parameters in Male Rats

Exposure to methylphenidate (MPH) during adolescence is the elective therapy for attention deficit/hyperactivity disorder (ADHD) children, but raises major concerns for public health, due to possibly persistent neurobehavioral changes. Rats (30- to 44-days old) were administered MPH (2 mg/kg, i.p once daily) or saline (SAL). At the end of the treatment we collected plasma, testicular, liver, and brain (striatum) samples. The testes and liver were used to evaluate conventional reproductive and metabolic endpoints. Testes of MPH-exposed rats weighed more and contained an increased quantity of sperm, whereas testicular levels of testosterone (TST) were markedly decreased. The MPH treatment exerted an inductive effect on enzymatic activity of TST hydroxylases, resulting in increased hepatic TST catabolism. These findings suggest that subchronic MPH exposure in adolescent rats could have a trophic action on testis growth and a negative impact on TST metabolism. We have analyzed striatal gene expression profiles as a consequence of MPH exposure during adolescence, using microarray technology. More than 700 genes were upregulated in the striatum of MPH-treated rats (foldchange >1.5). A first group of genes were apparently involved in migration of immature neural/glial cells and/or growth of novel axons. These genes include matrix proteases (ADAM-1, MMP14), their inhibitors (TIMP-2, TIMP-3), the hyaluronan-mediated motility receptor (RHAMM), and growth factors (transforming growth factor-beta3 [TGF-beta3] and fibroblast growth factor 14 [FGF14]). A second group of genes were suggestive of active axonal myelination. These genes mediate survival of immature cells after contact with newly produced axonal matrix (laminin B1, collagens, integrin alpha 6) and stabilization of myelinating glia-axon contacts (RAB13, contactins 3 and 4). A third group indicated the appearance and/or upregulation of mature processes. The latter included genes for: K+ channels (TASK-1, TASK-5), intercellular junctions (connexin30), neurotransmitter receptors (adrenergic alpha 1B, kainate 2, serotonin 7, GABA-A), as well as major proteins responsible for their transport and/or anchoring (Homer 1, MAGUK MPP3, Shank2). All these genes were possibly involved in synaptic plasticity, namely the formation, maturation, and stabilization of new neural connections within the striatum. MPH treatment seems to potentiate synaptic plasticity, which is an age-dependent developmental phenomenon that adolescent rats are very likely to show, compared to adults. Our observations suggest that adolescent MPH exposure causes only transient changes in reproductive and hormonal parameters, and a more enduring enhancement of neurobehavioral plasticity.

Tandem-pore K+ channels mediate inhibition of orexin neurons by glucose

Glucose-inhibited neurons orchestrate behavior and metabolism according to body energy levels, but how glucose inhibits these cells is unknown. We studied glucose inhibition of orexin/hypocretin neurons, which promote wakefulness (their loss causes narcolepsy) and also regulate metabolism and reward. Here we demonstrate that their inhibition by glucose is mediated by ion channels not previously implicated in central or peripheral glucose sensing: tandem-pore K(+) (K(2P)) channels. Importantly, we show that this electrical mechanism is sufficiently sensitive to encode variations in glucose levels reflecting those occurring physiologically between normal meals. Moreover, we provide evidence that glucose acts at an extracellular site on orexin neurons, and this information is transmitted to the channels by an intracellular intermediary that is not ATP, Ca(2+), or glucose itself. These results reveal an unexpected energy-sensing pathway in neurons that regulate states of consciousness and energy balance.

Expression of a two-pore domain K+ channel (TASK-1) in developing avian and mouse ventricular conduction systems

In this study, we report the identification and amino acid sequence of a novel two-pore domain potassium channel (TASK-1) in chicken. This protein, cTASK-1, is highly similar to mouse and human TASK-1 particularly within the pore regions. We describe the expression profile of both chicken and mouse TASK-1 in the embryonic heart as the ventricular conduction system develops. The developmental distribution of TASK-1 is similar in chicken and mouse. Initially, TASK-1 is expressed throughout the myocardium of the early heart tube. However, as cardiogenesis proceeds, ventricular expression becomes restricted to the trabeculated myocardium and eventually the bundle of His, bundle branches, and Purkinje fibers of the mature conduction system. This finding suggests that components of the ventricular conduction system differentiate from TASK-1-positive myocytes of the early heart tube that retain TASK-1 expression as they mature. Our results are consistent with a common mechanism for ventricular conduction system development in avians and mammals, despite differences in the anatomy of the mature conduction systems of these organisms.

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

Inhalation anesthetics activate and cannabinoid agonists inhibit TWIK-related acid-sensitive K(+) channels (TASK)-1 two-pore domain leak K(+) channels in vitro. Many neuromodulators, such as noradrenaline, might also manifest some of their actions by modifying TASK channel activity. Here, we have characterized the basal behavioral phenotype of TASK-1 knockout mice and tested their sensitivity to the inhalation anesthetics halothane and isoflurane, the alpha(2) adrenoreceptor agonist dexmedetomidine, and the cannabinoid agonist WIN55212-2 mesylate [R-(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3,-de]-1,4-benzoxazinyl]-(1-naphtalenyl)methanone mesylate)]. TASK-1 knockout mice had a largely normal behavioral phenotype. Male, but not female, knockout mice displayed an enhanced acoustic startle response. The knockout mice showed increased sensitivity to thermal nociception in a hot-plate test but not in a tail-flick test. The analgesic, sedative, and hypothermic effects of WIN55212-2 (2-6 mg/kg s.c.) were reduced in TASK-1 knockout mice. These results implicate TASK-1-containing channels in supraspinal pain pathways, in particular those modulated by endogenous cannabinoids. TASK-1 knockout mice were less sensitive to the anesthetic effects of halothane and isoflurane than wild-type littermates, requiring higher anesthetic concentrations to induce immobility as reflected by loss of the tail-withdrawal reflex. Our results support the idea that the activation of multiple background K(+) channels is crucial for the high potency of inhalation anesthetics. Furthermore, TASK-1 knockout mice were less sensitive to the sedative effects of dexmedetomidine (0.03 mg/kg s.c.), suggesting a role for the TASK-1 channels in the modulation of function of the adrenergic locus coeruleus nuclei and/or other neuronal systems.

A TASK-like pH- and amine-sensitive ‘leak’ K+ conductance regulates neonatal rat facial motoneuron excitability in vitro

A 'leak' potassium (K+) conductance (gK(Leak)) modulated by amine neurotransmitters is a major determinant of neonatal rat facial motoneuron excitability. Although the molecular identity of gK(Leak) is unknown, TASK-1 and TASK-3 channel mRNA is found in facial motoneurons. External pH, across the physiological range (pH 6-8), and noradrenaline (NA) modulated a conductance that displayed a relatively linear current/voltage relationship and reversed at the K+ equilibrium potential, consistent with inhibition of gK(Leak). The pH-sensitive current (I(pH)), was maximal around pH 8, fully inhibited near pH 6 and was described by a modified Hill equation with a pK of 7.1. The NA-induced current (I(NA)) was occluded at pH 6 and enhanced at pH 7.7. The TASK-1 selective inhibitor anandamide (10 microM), its stable analogue methanandamide (10 microM), the TASK-3 selective inhibitor ruthenium red (10 microM) and Zn2+ (100-300 microM) all failed to alter facial motoneuron membrane current or block I(NA) or I(pH). Isoflurane, a volatile anaesthetic that enhances heteromeric TASK-1/TASK-3 currents, increased gK(Leak). Ba2+, Cs+ and Rb+ blocked I(NA) and I(pH) voltage-dependently with maximal block at hyperpolarized potentials. 4-Aminopyridine (4-AP, 4 mM) voltage-independently blocked I(NA) and I(pH). In summary, gK(Leak) displays some of the properties of a TASK-like conductance. The linearity of gK(Leak) and an independence of activation on external [K+] suggests against pH-sensitive inwardly rectifying K+ channels. Our results argue against principal contributions to gK(Leak) by homomeric TASK-1 or TASK-3 channels, while the potentiation by isoflurane supports a predominant role for heterodimeric TASK-1/TASK-3 channels.

Tandem-pore domain potassium channels are functionally expressed in retinal (Müller) glial cells

Tandem-pore domain (2P-domain) K+-channels regulate neuronal excitability, but their function in glia, particularly, in retinal glial cells, is unclear. We have previously demonstrated the immunocytochemical localization of the 2P-domain K+ channels TASK-1 and TASK-2 in retinal Müller glial cells of amphibians. The purpose of the present study was to determine whether these channels were functional, by employing whole-cell recording from frog and mammalian (guinea pig, rat and mouse) Müller cells and confocal microscopy to monitor swelling in rat Müller cells. TASK-like immunolabel was localized in these cells. The currents mediated by 2P-domain channels were studied in isolation after blocking Kir, K(A), K(D), and BK channels. The remaining cell conductance was mostly outward and was depressed by acid pH, bupivacaine, methanandamide, quinine, and clofilium, and activated by alkaline pH in a manner consistent with that described for TASK channels. Arachidonic acid (an activator of TREK channels) had no effect on this conductance. Blockade of the conductance with bupivacaine depolarized the Müller cell membrane potential by about 50%. In slices of the rat retina, adenosine inhibited osmotic glial cell swelling via activation of A1 receptors and subsequent opening of 2P-domain K+ channels. The swelling was strongly increased by clofilium and quinine (inhibitors of 2P-domain K+ channels). These data suggest that 2P-domain K+ channels are involved in homeostasis of glial cell volume, in activity-dependent spatial K+ buffering and may play a role in maintenance of a hyperpolarized membrane potential especially in conditions where Kir channels are blocked or downregulated.

Postnatal changes in TASK-1 and TREK-1 expression in rat brain stem and cerebellum

Developmental changes in expression of two-pore domain K+ channels, TASK-1 and TREK-1, were investigated in the juvenile (postnatal day 13; P13) and adult (P105) rat brain stem and cerebellum using immunohistochemistry. In the juvenile, extensive TASK-1-like immunoreactivity (TASK-1-LIR) was seen among glial cells in the white matter (e.g., radial glia), which showed marked reduction in the adult. In contrast, TASK-1-LIR in neurons including cerebellar Purkinje and granule cells, hypoglossal and facial motoneurons, and ventrolateral medulla neurons was increased in the adult. TASK-1-LIR in neuroglia surrounding peripheral axons of cranial nerves was persistent. TREK-1-LIR was similar between ages, although TREK-1-LIR was neuronal and present only in juvenile cerebellar external germinal layer. Present results suggest roles for TASK-1 and K+ homeostasis in neuro-glial interaction, neurogenesis, differentiation, migration, axon guidance, synaptogenesis and myelination.

Immunolocalization of TASK-3 (KCNK9) to a subset of cortical neurons in the rat CNS

Tandem pore domain (2P) K channels constitute the most diverse family of K channels and are responsible for background (leak or baseline) K currents. Of the 15 human 2P K channels, TASK-1, TASK-2, and TASK-3 are uniquely sensitive to physiologic pH changes as well as being inhibited by local anesthetics and activated by volatile anesthetics. In this study polyclonal antibodies selective for TASK-3 have been used to localize its expression in the rat central nervous system (CNS). TASK-3 immunostaining was found in rat cortex, hypothalamus, and hippocampus. Double immunofluorescent studies identified a discrete population of TASK-3 expressing neurons scattered throughout cortex. Using immunogold electron microscopy TASK-3 was identified at the cell surface associated with synapses and within the intracellular synthetic compartments. These results provide a more finely detailed picture of TASK-3 expression and indicate a role for TASK-3 in modulating cerebral synaptic transmission and responses to CNS active drugs.

Developmental expression of two-pore domain K+ channels, TASK-1 and TREK-1, in the rat cochlea

Developmental expression of two-pore domain potassium (2P K) channels, TASK-1 and TREK-1, was investigated in the rat cochlea at onset of hearing and after maturity using RT-PCR and immunocytochemistry. TASK-1 and TREK-1 mRNAs were detected by RT-PCR at postnatal day (P) 9-12. TASK-1 like immunoreactivity (LIR) in the P13 cochlea was observed in Deiters', pillar, Claudius' and outer sulcus cells, spiral limbus fibrocytes, and neuroglia. At P13, TREK-1-LIR was more wide-spread, and included sensory and supporting cells of the organ of Corti, spiral ganglion, stria vascularis, Reissner's membrane, inner and outer sulcus cells, connective and support tissues surrounding modiolus. By P105 the pattern of TASK-1- and TREK-1-LIR became limited to a subset of the above structures, suggesting developmental regulation. During postnatal development, TASK-1 may be important in the onset (around P11) and maturation (by P22) of endocochlear potential and hearing. The distribution of TASK-1 and TREK-1 suggest a role in K cycling and homeostasis. As TASK-1 and TREK-1 are inhibited by local anesthetics at doses used to treat tinnitus, 2P K channels may also be important in cochlear dysfunction.

Functional evidence of a role for two-pore domain potassium channels in rat mesenteric and pulmonary arteries

1. Experiments were performed to elucidate the mechanism by which alterations of extracellular pH (pH(o)) change membrane potential (E(M)) in rat mesenteric and pulmonary arteries. 2. Changing pH(o) from 7.4 to 6.4 or 8.4 produced a depolarisation or hyperpolarisation, respectively, in mesenteric and pulmonary arteries. Anandamide (10 microm) or bupivacaine (100 microm) reversed the hyperpolarisation associated with alkaline pH(o), shifting the E(M) of both vessels to levels comparable to that at pH 6.4. In pulmonary arteries, clofilium (100 microm) caused a significant reversal of hyperpolarisation seen at pH 8.4 but was without effect at pH 7.4. 3. K(+) channel blockade by 4-aminopyridine (4-AP) (5 mm), tetraethylammonium (TEA) (10 mm), Ba(2+) (30 microm) and glibenclamide (10 microm) depolarised the pulmonary artery. However, shifts in E(M) with changes in pH(o) remained and were sensitive to anandamide (10 microm), bupivacaine (100 microm) or Zn(2+) (200 microm). 4. Anandamide (0.3-60 microm) or bupivacaine (0.3-300 microm) caused a concentration-dependent increase in basal tone in pulmonary arteries. 5. RT-PCR demonstrated the expression of TASK-1, TASK-2, THIK-1, TRAAK, TREK-1, TWIK-1 and TWIK-2 in mesenteric arteries and TASK-1, TASK-2, THIK-1, TREK-2 and TWIK-2 in pulmonary arteries. TASK-1, TASK-2, TREK-1 and TWIK-2 protein was demonstrated in both arteries by immunostaining. 6. These experiments provide evidence for the presence of two-pore domain K(+) channels in rat mesenteric and pulmonary arteries. Collectively, they strongly suggest that modulation of TASK-1 channels is most likely to have mediated the pH-induced changes in membrane potential observed in these vessels, and that blockade of these channels by anandamide or bupivacaine generates a small increase in pulmonary artery tone.

Tandem-pore K+channels display an uneven distribution in amphibian retina

Previous studies in retinal glial (Müller) cells have suggested that the dominant membrane currents are mediated by K(+) inward-rectifier (Kir) channels. After blockade of inwardly (Kir) and outwardly (KD and BK) conducting channels, a large K(+) conductance remains, but its nature has not been determined. Tandem-pore K(+) channels are likely candidates for this potassium conductance and the purpose of the present study was to determine, using immunocytochemistry, whether Müller cells express TASK-1, TASK-2, TREK-1 and/or TREK-2 potassium channel subunits. The results reveal that retinal glial cells express TASK-1 and TASK-2 subunits, but not TREK-1 or TREK-2 subunits. Furthermore, the distribution of TASK subunits differs from that of Kir channels and may contribute to the potassium conductance of Müller cells.

What are the roles of the many different types of potassium channel expressed in cerebellar granule cells?

Potassium (K) channels have a key role in the regulation of neuronal excitability. Over a hundred different subunits encoding distinct K channel subtypes have been identified so far. A major challenge is to relate these many different channel subunits to the functional K currents observed in native neurons. In this review, we have concentrated on cerebellar granule neurons (CGNs). We have considered each of the three principal super families of K channels in turn, namely, the six transmembrane domain, voltage-gated super family, the two transmembrane domain, inward-rectifier super family and the four transmembrane domain, leak channel super family. For each super family, we have identified the subunits that are expressed in CGNs and related the properties of these expressed channel subunits to the functional currents seen in electrophysiological recordings from these neurons. In some cases, there are strong molecular candidates for proteins underlying observed currents. In other cases the correlation is less clear. We show that at least 26 potassium channel alpha subunits are moderately or strongly expressed in CGNs. Nevertheless, a good empirical model of CGN function has been obtained with just six distinct K conductances. The transient KA current in CGNs, seems due to expression of Kv4.2 channels or Kv4.2/4.3 heteromers, while the KCa current is due to expression of large-conductance slo channels. The G-protein activated KIR current is probably due to heteromeric expression of KIR3.1 and KIR3.2. Perhaps KIR2.2 subunits underlie the KIR current when it is constitutively active. The leak conductance can be attributed to TASK-1 and or TASK-3 channels. With less certainty, the IK-slow current may be due to expression of one or more members of the KCNQ or EAG family. Lastly, the delayed-rectifier Kv current has as many as six different potential contributors from the extensive Kv family of alpha subunits. Since many of these subunits are highly regulated by neurotransmitters, physiological regulators and, often, auxiliary subunits, the resulting electrical properties of CGNs may be highly dynamic and subject to constant fine-tuning.

Modulation of noninactivating K+ channels in rat cerebellar granule neurons by halothane, isoflurane, and sevoflurane

Neuronal baseline K(+) channels were activated by several volatile anesthetics. Whole-cell recordings from cultured cerebellar granule neurons of 7-day-old male Sprague-Dawley rats showed outward-rectifying K(+) currents with a conductance of approximately 1.1 +/- 0.3 nS (n = 20) at positive potentials. The channel activity was noninactivating, exhibited no voltage gating, and was insensitive to conventional K(+) channel blockers. Clinically relevant concentrations of halothane (112, 224, 336, and 448 micro M) dissolved in Ringer's solution increased outward currents by 29%, 50%, 63%, and 94%, respectively (n = 5; P < 0.05; analysis of variance [ANOVA]). Similar increases in currents were produced by isoflurane (274, 411, 548, and 822 micro M), which increased outward currents by 22%, 47%, 52%, and 60%, respectively (n = 5; P < 0.05; ANOVA). Sevoflurane 518 micro M increased outward currents by 225% (n = 10; P < 0.05; ANOVA). In all experiments, channel activity quickly returned to baseline levels during wash. The outward-rectifying whole-cell current-voltage curves were consistent with the properties of anesthetic-sensitive KCNK channels. These results support the idea that noninactivating baseline K(+) channels are important target sites of volatile general anesthetics.

IMPLICATIONS

The volatile anesthetics halothane, isoflurane, and sevoflurane, reversibly enhanced a noninactivating outwardly rectifying K(+) current in rat cerebellar granule neurons. These findings support a model of anesthesia that includes a site of action at baseline K(+) channels.

Two-pore-Domain (KCNK) potassium channels: dynamic roles in neuronal function

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.

Pharmacological characterization of a non-inactivating outward current observed in mouse cerebellar Purkinje neurones

Whole-cell patch clamp recordings were used to investigate the properties of a non-inactivating outward current observed in mouse cerebellar Purkinje neurones at a holding potential of -20 mV. Increasing the external potassium (K(+)) concentration from 3 mM to 20 mM produced a rightward shift in the observed reversal potential of approximately 30 mV or approximately 40 mV for a K(+)-or a caesium (Cs(+))-based intracellular solution respectively, indicating the outward current was a K(+) current. The outward current was partially inhibited by the K(+) channel blocker, tetraethylammonium (TEA; IC(50)=0.15 mM). Subsequently, the background or TEA-insensitive current was measured in the presence of 1 mM TEA. The background current was reversibly inhibited by barium (Ba(2+); 300 microM, 50%) and potentiated by the application of arachidonic acid (AA; 1 mM, 62%). The volatile anaesthetic, halothane (1 mM), and the neuroprotectant, riluzole (500 microM), both reversibly inhibited the background current by 54% and 36% respectively. The background current was insensitive to changes in both intracellular and extracellular acidification. The GABA(B) and mu-opioid receptor agonists, baclofen and [D-Ala(2), N-MePhe(4)-Gly-ol(5)] enkephalin (DAMGO) both reversibly potentiated the outward current by 42% and 26% respectively. In contrast, the metabotropic glutamate receptor and acetylcholine receptor agonists, (S)-3,5-dihydroxyphenylglycine (DHPG) and muscarine both reversibly inhibited the outward current by 48% and 42% respectively. These data suggest that cerebellar Purkinje neurones possess a background current which shares several properties with recently cloned two-pore K(+) channels, particularly THIK-1.

Localization of the tandem pore domain K+ channel KCNK5 (TASK-2) in the rat central nervous system

Tandem pore domain K+ channels (2P K+ channels) are responsible for background K+ currents. 2P K+ channels are the most numerous encoded K+ channels in the Caenorhabditis elegans and Drosophila melanogaster genomes and to date 14 human 2P K+ channels have been identified. The 2P K+ channel TASK-2 (also named KCNK5) is sensitive to changes in extracellular pH, inhibited by local anesthetics and activated by volatile anesthetics. While TASK-1 has been shown to be involved in controlling neuronal cell excitability, much less is known about the cellular expression and function of TASK-2, originally cloned from human kidney. Previous studies demonstrated TASK-2 mRNA expression in high abundance in human kidney, liver, and pancreas, but only low expression in mouse brain or even absent expression in human brain was reported. In this study we have used immunohistochemical methods to localize TASK-2 at the cellular level in the rat central nervous system. TASK-2 immunoreactivity is prominently found in the rat hippocampal formation with the strongest staining observed in the pyramidal cell layer and in the dentate gyrus, and the Purkinje and granule cells of cerebellum. Additional immunofluorescence studies in cultured cerebellar granule cells demonstrate TASK-2 localization to the neuronal soma and to the proximal regions of neurites of cerebellar granule cells. The superficial layers of spinal cord and small-diameter neurons of dorsal root ganglia also showed strong TASK-2 immunoreactivity. These results suggest a possible involvement of TASK-2 in central mechanisms for controlling cell excitability and in peripheral signal transduction.

Effects of barium, furosemide, ouabaine and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) on ionophoretically-induced changes in extracellular potassium concentration in hippocampal slices from rats and from patients with epilepsy

Glial cells limit local K(+)-accumulation by K(+)-uptake through different mechanisms, sensitive to Ba(2+), ouabaine, furosemide, or DIDS. Since the relative contribution of these mechanisms has not yet been determined, we studied the effects of bath-applied barium (2 mM), ouabaine (9 microM), furosemide (2 mM), and DIDS (1 mM) on ionophoretically-induced rises in [K(+)](o) in the pyramidal layer of area CA1 from normal rat slices, in the presence of glutamate receptor (Glu-R) antagonists. We also investigated the effect of barium on ionophoretically-induced tetrapropylammonium (TPA(+))-signals in order to test for barium-induced changes of the extracellular space. Finally, we repeated the barium experiment on slices from human non-sclerotic and sclerotic hippocampal specimens to assess a reduced glial capability for barium-sensitive K(+)-uptake in sclerotic tissue from epilepsy patients. In normal rat slices barium augmented ionophoretically-induced rises in [K(+)](o) by approximately 120%, also in the presence of tetrodotoxin (TTX) (by approximately 150%), but did not significantly affect the TPA(+)-signal. Ouabaine also augmented the K(+)-signal, but only by 27%. Furosemide and DIDS had negligible effects. In slices from sclerotic human hippocampus an augmentation of the K(+)-signal by barium was absent. Thus barium augments ionophoretically-induced K(+)-signals to a similar extent as previously shown for stimulus-induced signals. We suggest that glial barium-sensitive K(+)-buffer mechanisms reduce fast local rises of [K(+)](o) by at least 50%. This capability of glial cells is extremely reduced in area CA1 of slices from human sclerotic hippocampal specimens.

Expression pattern in brain of TASK-1, TASK-3, and a tandem pore domain K(+) channel subunit, TASK-5, associated with the central auditory nervous system

TWIK-related acid-sensitive K(+) (TASK) channels contribute to setting the resting potential of mammalian neurons and have recently been defined as molecular targets for extracellular protons and volatile anesthetics. We have isolated a novel member of this subfamily, hTASK-5, from a human genomic library and mapped it to chromosomal region 20q12-20q13. hTASK-5 did not functionally express in Xenopus oocytes, whereas chimeric TASK-5/TASK-3 constructs containing the region between M1 and M3 of TASK-3 produced K(+) selective currents. To better correlate TASK subunits with native K(+) currents in neurons the precise cellular distribution of all TASK family members was elucidated in rat brain. A comprehensive in situ hybridization analysis revealed that both TASK-1 and TASK-3 transcripts are most strongly expressed in many neurons likely to be cholinergic, serotonergic, or noradrenergic. In contrast, TASK-5 expression is found in olfactory bulb mitral cells and Purkinje cells, but predominantly associated with the central auditory pathway. Thus, TASK-5 K(+) channels, possibly in conjunction with auxiliary proteins, may play a role in the transmission of temporal information in the auditory system.

Cns distribution of members of the two-pore-domain (KCNK) potassium channel family

Two-pore-domain potassium (K(+)) channels are substrates for resting K(+) currents in neurons. They are major targets for endogenous modulators, as well as for clinically important compounds such as volatile anesthetics. In the current study, we report on the CNS distribution in the rat and mouse of mRNA encoding seven two-pore-domain K(+) channel family members: TASK-1 (KCNK3), TASK-2 (KCNK5), TASK-3 (KCNK9), TREK-1 (KCNK2), TREK-2 (KCNK10), TRAAK (KCNK4), and TWIK-1 (KCNK1). All of these genes were expressed in dorsal root ganglia, and for all of the genes except TASK-2, there was a differential distribution in the CNS. For TASK-1, highest mRNA accumulation was seen in the cerebellum and somatic motoneurons. TASK-3 was much more widely distributed, with robust expression in all brain regions, with particularly high expression in somatic motoneurons, cerebellar granule neurons, the locus ceruleus, and raphe nuclei and in various nuclei of the hypothalamus. TREK-1 was highest in the striatum and in parts of the cortex (layer IV) and hippocampus (CA2 pyramidal neurons). mRNA for TRAAK also was highest in the cortex, whereas expression of TREK-2 was primarily restricted to the cerebellar granule cell layer. There was widespread distribution of TWIK-1, with highest levels in the cerebellar granule cell layer, thalamic reticular nucleus, and piriform cortex. The differential expression of each of these genes likely contributes to characteristic excitability properties in distinct populations of neurons, as well as to diversity in their susceptibility to modulation.

Properties and modulation of mammalian 2P domain K+ channels

Mammalian 2P domain K(+) channels are responsible for background or 'leak' K(+) currents. These channels are regulated by various physical and chemical stimuli, including membrane stretch, temperature, acidosis, lipids and inhalational anaesthetics. Furthermore, channel activity is tightly controlled by membrane receptor stimulation and second messenger phosphorylation pathways. Several members of this novel family of K(+) channels are highly expressed in the central and peripheral nervous systems in which they are proposed to play an important physiological role. The pharmacological modulation of this novel class of ion channels could be of interest for both general anaesthesia and ischaemic neuroprotection.

The endocannabinoid anandamide is a direct and selective blocker of the background K(+) channel TASK-1

TASK-1 encodes an acid- and anaesthetic-sensitive background K(+) current, which sets the resting membrane potential of both cerebellar granule neurons and somatic motoneurons. We demonstrate that TASK-1, unlike the other two pore (2P) domain K(+) channels, is directly blocked by submicromolar concentrations of the endocannabinoid anandamide, independently of the CB1 and CB2 receptors. In cerebellar granule neurons, anandamide also blocks the TASK-1 standing-outward K(+) current, IKso, and induces depolarization. Anandamide-induced neurobehavioural effects are only partly reversed by antagonists of the cannabinoid receptors, suggesting the involvement of alternative pathways. TASK-1 constitutes a novel sensitive molecular target for this endocannabinoid.