Motoneurons express heteromeric TWIK-related acid-sensitive K+ (TASK) channels containing TASK-1 (KCNK3) and TASK-3 (KCNK9) subunits

A novel acid-sensitive K+ channel in rat dorsal root ganglia neurons

Recent studies have suggested that acid-sensitive background K+ channels such as TASK-1 and TASK-3, members of two-pore domain K+ (K2P) channel family, express and contribute to extracellular acidification-induced responses in dorsal root ganglia (DRG) neurons. However, it has remained to address whether other acid-sensitive background K+ channels are functionally expressed in DRG neurons. Here we characterized biophysical and pharmacological properties of a novel acid-sensitive background K+ channel in DRG neurons isolated from neonatal rats. We recorded an 80-pS K+ channel with a weak inward rectification current-voltage relationship in cell-attached patches in 150mM KCl bath solution. The 80-pS K+ channel was inhibited by extracellular low pH (pHo 6.3). Interestingly, the channel was similar to TASK-2 cloned from mouse and rat in biophysical and pharmacological properties. However, extracellular alkaline condition which activates TASK-2 channel, failed to activate the 80-pS K+ channel. Lidocaine and quinine more inhibited the channel activity of 80-pS K+ channel than that of TASK-2 channel. Our results suggest that the acid-sensitive 80-pS K+ channels may regulate resting membrane potential and may play a critical role in various processes such as cell metabolism, pH, and pain sensation in DRG neurons.

Membrane potential-regulated transcription of the resting K+ conductance TASK-3 via the calcineurin pathway

The 2P domain K(+) channel TASK-3 is highly expressed in cerebellar granule neurons where it has been proposed to underlie the K(+) leak conductance, IKso. In a previous work we showed that expression of TASK-3 increases in cerebellar granule neurons as they mature in culture. Here we show that within the cerebellum, levels of TASK-3 mRNA increase as granule neurons migrate to their adult positions and receive excitatory mossy fiber input. To understand the mechanism of this increase in TASK-3 expression we used an in vitro model culturing the neurons in either depolarizing conditions mimicking neuronal activity (25K, 25 mm KCl) or in conditions which approach deafferentation (5K, 5 mm KCl). An important increase in TASK-3 mRNA is uniquely observed in 25K and is specific since other background K(+) channel levels remain unchanged or decrease. The rise in TASK-3 mRNA leads to an increase in TASK-3 protein and the IKso conductance resulting in hyperpolarization. Blocking L-type calcium channels or their downstream effector calcineurin, abrogates TASK-3 expression and IKso, leading to hyperexcitability. This is the first study demonstrating that depolarization-induced Ca(2+) entry can directly regulate cellular excitability by dynamically regulating the transcription of a resting K(+) conductance. The appearance of this conductance may play an important role in the transition of depolarized immature neurons to their mature hyperpolarized state during neuronal development.

TASK-like conductances are present within hippocampal CA1 stratum oriens interneuron subpopulations

TASK-1 (KCNK3) and TASK-3 (KCNK9) are members of the two-pore domain potassium channel family and form either homomeric or heteromeric open-rectifier (leak) channels. Recent evidence suggests that these channels contribute to the resting potential and input resistance in several neuron types, including hippocampal CA1 pyramidal cells. However, the evidence for TWIK-related acid-sensitive potassium (TASK)-like conductances in inhibitory interneurons is less clear, and mRNA expression has suggested that TASK channels are expressed in only a subpopulation of interneurons. Here we use immunocytochemistry to demonstrate prominent TASK-3 protein expression in both parvalbumin-positive- and a subpopulation of glutamic acid decarboxylase (GAD)67-positive interneurons. In addition, a TASK-like current (modulated by both pH and bupivacaine) was detected in 30-50% of CA1 stratum oriens interneurons of various morphological classes. In most neurons, basic shifts in pH had a larger effect on the TASK-like current than acidic, suggesting that the current is mediated by TASK-1/TASK-3 heterodimers. These data suggest that TASK-like conductances are more prevalent in inhibitory interneurons than previously supposed.

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.

Biophysical and Pharmacological Characteristics of Native Two-Pore Domain TASK Channels in Rat Adrenal Glomerulosa Cells

Multiple genes of the TASK subfamily of two-pore domain K(+) channels are reported to be expressed in rat glomerulosa cells. To determine which TASK isoforms contribute to native leak channels controlling resting membrane potential, patch-clamp studies were performed to identify biophysical and pharmacological characteristics of macroscopic and unitary K(+) currents diagnostic of recombinant TASK channel isoforms. Results indicate K(+) conductance (gK(+)) is mediated almost exclusively by a weakly voltage-dependent (leak) K(+) channel closely resembling TASK-3. Leak channels exhibited a unitary conductance approximating that expected for TASK-3 under the recording conditions employed, brief mean open times and a voltage-dependent open probability. Extracellular H(+) induced voltage-independent inhibition of gK(+), exhibiting an IC(50) of 56 nM: (pH 7.25) and a Hill coefficient of 0.75. Protons inhibited leak channel open probability (P(o)) by promoting a long-lived closed state (tau > 500 ms). Extracellular Zn(2+) mimicked the effects of H(+); inhibition of gK(+) exhibited an IC(50) of 41 microM: with a Hill coefficient of 1.26, inhibiting channel gating by promoting a long-lived closed state. Ruthenium red (5 microM: ) inhibited gK(+) by 75.6% at 0 mV. Extracellular Mg(2+) induced voltage-dependent block of gK(+), inhibiting unitary current amplitude without affecting mean open time. Bupivacaine induced voltage-dependent block of gK(+), exhibiting IC(50) values of 116 microM: at -100 mV and 28 microM: at 40 mV with Hill coefficients of 1 at both potentials. Halothane induced a voltage-independent stimulation of gK(+) primarily by decreasing the leak channel closed-state dwell time.

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.

The contribution of TWIK-related acid-sensitive K+-containing channels to the function of dorsal lateral geniculate thalamocortical relay neurons

A genetic knockout was used to determine the specific contribution of TWIK-related acid-sensitive K+ (TASK)-1 channels to the function of dorsal lateral geniculate nucleus (DLG) thalamocortical relay (TC) neurons. Disruption of TASK-1 function produced an approximately 19% decrease in amplitude of the standing outward current (ISO) and a 3 +/- 1-mV depolarizing shift in resting membrane potential (Vrest) of DLG neurons. We estimated that current through TASK-1 homodimers or TASK-1/TASK-3 heterodimers contribute(s) approximately one third of the current sensitive to TASK channel modulators in DLG TC neurons. The effects of the TASK channel blocker bupivacaine (20 microM), of muscarine (50 microM), and of H+ on ISO were reduced to approximately 60%, 59%, and shifted to more acidic pH values, respectively. The blocking effect of anandamide on ISO [30 microM; 23 +/- 3% current decrease in wild type (WT)] was absent in TASK-1 knockout (TASK-1-/-) mice (9 +/- 6% current increase). Comparable results were obtained with the more stable anand-amide derivative methanandamide (20 microM; 20 +/- 2% decrease in WT; 4 +/- 6% increase in TASK-1-/-). Current-clamp recordings revealed a muscarine-induced shift in TC neuron activity from burst to tonic firing in both mouse genotypes. Electrocorticograms and sleep/wake times were unchanged in TASK-1-/- mice. In conclusion, our findings demonstrate a significant contribution of TASK-1 channels to ISO in DLG TC neurons, although the genetic knockout of TASK-1 did not produce severe deficits in the thalamocortical system.

Modifying the subunit composition of TASK channels alters the modulation of a leak conductance in cerebellar granule neurons

Two-pore domain potassium (K2P) channel expression is believed to underlie the developmental emergence of a potassium leak conductance [IK(SO)] in cerebellar granule neurons (CGNs), suggesting that K2P function is an important determinant of the input conductance and resting membrane potential. To investigate the role that different K2P channels may play in the regulation of CGN excitability, we generated a mouse lacking TASK-1, a K2P channel known to have high expression levels in CGNs. In situ hybridization and real-time PCR studies in wild-type and TASK-1 knock-outs (KOs) demonstrated that the expression of other K2P channels was unaltered in CGNs. TASK-1 knock-out mice were healthy and bred normally but exhibited compromised motor performance consistent with altered cerebellar function. Whole-cell recordings from adult cerebellar slice preparations revealed that the resting excitability of mature CGNs was no different in TASK-1 KO and littermate controls. However, the modulation of IK(SO) by extracellular Zn2+, ruthenium red, and H+ was altered. The IK(SO) recorded from TASK-1 knock-out CGNs was no longer sensitive to alkalization and was blocked by Zn2+ and ruthenium red. These results suggest that a TASK-1-containing channel population has been replaced by a homodimeric TASK-3 population in the TASK-1 knock-out. These data directly demonstrate that TASK-1 channels contribute to the properties of IK(SO) in adult CGNs. However, TASK channel subunit composition does not alter the resting excitability of CGNs but does influence sensitivity to endogenous modulators such as Zn2+ and H+.

Zinc and mercuric ions distinguish TRESK from the other two-pore-domain K+ channels

TWIK-related spinal cord K+ channel (TRESK) is the most recently cloned two-pore-domain potassium (2PK+) channel, regulated by the calcium/calmodulin-dependent protein phosphatase calcineurin. Functional identification of endogenous TRESK and its distinction from the other 2PK+ channels, producing similar background K+ current, are impeded by the lack of specific inhibitors. Therefore, we searched for antagonists selective against TRESK among the mouse 2PK+ channels by screening more than 200 substances. Mibefradil, zinc, and mercuric ions inhibited TRESK expressed in Xenopus laevis oocytes with IC50 values lower than 10 microM. The specificity of the identified agents was determined by measuring their effects on mouse TALK-1, TASK-1, TASK-2, TASK-3, THIK-1, TRAAK, TREK-1, and TREK-2. Mibefradil failed to discriminate well among the functional 2PK+ channels; however, Zn2+ and Hg2+ exerted a significantly stronger inhibitory effect on TRESK than on the other channels. Sensitivity to zinc but insensitivity to ruthenium red were distinctive features of TRESK. Whereas both Zn2+ and Hg2+ were selective blockers of TRESK among the mouse 2PK+ channels, human TRESK was resistant to Zn2+; it was blocked only by Hg2+. His132 of mouse TRESK was partly responsible for this difference. Mouse TRESK expressed in COS-7 cells was also inhibited by Zn2+ and Hg2+, and TRESK single-channel current was diminished in outside-out patches, indicating that the action of the ions was membrane-delimited, most probably targeting the channel itself. Thus, both Zn2+ and Hg2+ are expected to inhibit endogenous TRESK in isolated mouse cells, and these ions can be applied to identify the calcineurin-activated 2PK+ channel in its natural environment.

The Ventilatory Stimulant Doxapram Inhibits TASK Tandem Pore (K2P) Potassium Channel Function but Does Not Affect Minimum Alveolar Anesthetic Concentration

TWIK-related acid-sensitive K(+)-1 (TASK-1 [KCNK3]) and TASK-3 (KCNK9) are tandem pore (K(2P)) potassium (K) channel subunits expressed in carotid bodies and the brainstem. Acidic pH values and hypoxia inhibit TASK-1 and TASK-3 channel function, and halothane enhances this function. These channels have putative roles in ventilatory regulation and volatile anesthetic mechanisms. Doxapram stimulates ventilation through an effect on carotid bodies, and we hypothesized that stimulation might result from inhibition of TASK-1 or TASK-3 K channel function. To address this, we expressed TASK-1, TASK-3, TASK-1/TASK-3 heterodimeric, and TASK-1/TASK-3 chimeric K channels in Xenopus oocytes and studied the effects of doxapram on their function. Doxapram inhibited TASK-1 (half-maximal effective concentration [EC50], 410 nM), TASK-3 (EC50, 37 microM), and TASK-1/TASK-3 heterodimeric channel function (EC50, 9 microM). Chimera studies suggested that the carboxy terminus of TASK-1 is important for doxapram inhibition. Other K2P channels required significantly larger concentrations for inhibition. To test the role of TASK-1 and TASK-3 in halothane-induced immobility, the minimum alveolar anesthetic concentration for halothane was determined and found unchanged in rats receiving doxapram by IV infusion. Our data indicate that TASK-1 and TASK-3 do not play a role in mediating the immobility produced by halothane, although they are plausible molecular targets for the ventilatory effects of doxapram.

Inhibition of a background potassium channel by Gq protein alpha-subunits

Two-pore-domain K(+) channels provide neuronal background currents that establish resting membrane potential and input resistance; their modulation provides a prevalent mechanism for regulating cellular excitability. The so-called TASK channel subunits (TASK-1 and TASK-3) are widely expressed, and they are robustly inhibited by receptors that signal through Galphaq family proteins. Here, we manipulated G protein expression and membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) levels in intact and cell-free systems to provide electrophysiological and biochemical evidence that inhibition of TASK channels by Galphaq-linked receptors proceeds unabated in the absence of phospholipase C (PLC) activity, and instead involves association of activated Galphaq subunits with the channels. Receptor-mediated inhibition of TASK channels was faster and less sensitive to a PLCbeta1-ct minigene construct than inhibition of PIP(2)-sensitive Kir3.4(S143T) homomeric channels that is known to be dependent on PLC. TASK channels were strongly inhibited by constitutively active Galphaq, even by a mutated version that is deficient in PLC activation. Receptor-mediated TASK channel inhibition required exogenous Galphaq expression in fibroblasts derived from Galphaq/11 knockout mice, but proceeded unabated in a cell line in which PIP(2) levels were reduced by regulated overexpression of a lipid phosphatase. Direct application of activated Galphaq, but not other G protein subunits, inhibited TASK channels in excised patches, and constitutively active Galphaq subunits were selectively coimmunoprecipitated with TASK channels. These data indicate that receptor-mediated TASK channel inhibition is independent of PIP(2) depletion, and they suggest a mechanism whereby channel modulation by Galphaq occurs through direct interaction with the ion channel or a closely associated intermediary.

Computational Estimation of the Distribution of L-type Ca2+Channels in Motoneurons Based on Variable Threshold of Activation of Persistent Inward Currents

In the presence of neuromodulators such as serotonin and noradrenaline, motoneurons exhibit persistent inward currents (PICs) that serve to amplify synaptic inputs. A major component of these PICs is mediated by L-type Ca(2+) channels. Estimates based on electrophysiological studies indicate that these channels are located on the dendrites, but immunohistochemical studies of their precise distribution have yielded different results. Our goal was to determine the distribution of these channels using computational methods. A theoretical analysis of the activation of PICs by a somatic current injection in the absence or presence of synaptic activity suggests that L-type Ca(2+) channels may be segregated to discrete hot spots 25-200 microm long and centered 100-400 microm from the soma in the dendritic tree. Compartmental models based on detailed anatomical measurements of the structure of feline neck motoneurons with L-type Ca(2+) channels incorporated in these regions produced plateau potentials resulting from PIC activation. Furthermore, we replicated the experimental observation that the somatic threshold at which PICs were activated was depolarized by tonic activation of inhibitory synapses and hyperpolarized by tonic activation of excitatory synapses. Models with L-type Ca(2+) channels distributed uniformly were unable to replicate the change in somatic threshold of PIC activation. Therefore we conclude that the set of L-type Ca(2+) channels mediating plateau potentials is restricted to discrete regions in the dendritic tree. Furthermore, this distribution leads to the compartmentalization of the dendritic tree of motoneurons into subunits whose sequential activation lead to the graded amplification of synaptic inputs.

Differential expression of TASK channels between horizontal interneurons and pyramidal cells of rat hippocampus

Among the electrophysiological properties differentiating stratum oriens horizontal interneurons from pyramidal neurons of the CA1 hippocampal subfield are the more depolarized resting potential and the higher input resistance; additionally, these interneurons are also less sensitive to ischemic damage than pyramidal cells. A differential expression of pH-sensitive leakage potassium channels (TASK) could contribute to all of these differences. To test this hypothesis, we studied the expression and properties of TASK channels in the two cell types. Electrophysiological recordings from acute slices showed that barium- and bupivacaine-sensitive TASK currents were detectable in pyramidal cells but not in interneurons and that extracellular acidification caused a much stronger depolarization in pyramidal cells than in interneurons. This pyramidal cell depolarization was paralleled by an increase of the input resistance, suggesting the blockade of a background conductance. Single-cell reverse transcription-PCR experiments showed that the expression profile of TASK channels differ between the two cell types and suggested that these channels mediate an important share of the leakage current of pyramidal cells. We suggest that the different expression of TASK channels in these cell types contribute to their electrophysiological differences and may result in cell-specific sensitivity to extracellular acidification in conditions such as epilepsy and ischemia.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

Species-Specific Differences in Response to Anesthetics and Other Modulators by the K2P Channel TRESK

TRESK (TWIK-related spinal cord K+ channel) is the most recently characterized member of the tandem-pore domain potassium channel (K2P) family. Human TRESK is potently activated by halothane, isoflurane, sevoflurane, and desflurane, making it the most sensitive volatile anesthetic-activated K2P channel yet described. Herein, we compare the anesthetic sensitivity and pharmacologic modulation of rodent versions of TRESK to their human orthologue. Currents passed by mouse and rat TRESK were enhanced by isoflurane at clinical concentrations but with significantly lower efficacy than human TRESK. Unlike human TRESK, the rodent TRESKs are strongly inhibited by acidic extracellular pH in the physiologic range. Zinc inhibited currents passed by both rodent TRESK in the low micromolar range but was without effect on human TRESK. Enantiomers of isoflurane that have stereoselective anesthetic potency in vivo produced stereospecific enhancement of the rodent TRESKs in vitro. Amide local anesthetics inhibited the rodent TRESKs at almost 10-fold smaller concentrations than that which inhibit human TRESK. These results identified interspecies differences and similarities in the pharmacology of TRESK. Further characterization of TRESK expression patterns is needed to understand their role in anesthetic mechanisms.

IMPLICATIONS

Mouse and rat TRESK (TWIK-related spinal cord K+ channel) have different pharmacologic responses compared with human TRESK. In particular, we found stereospecific differences in response to isoflurane by the rodent TRESKs but not by human TRESK. TRESK may be a target site for the mechanism of action of volatile anesthetics.

TASK-like K+ channels mediate effects of 5-HT and extracellular pH in rat dorsal vagal neurones in vitro

Dorsal vagal neurones (DVN) receive serotonergic projections from the medullary raphé nuclei, suggesting that 5-HT modulates vagal activity. A previous study has shown that 5-HT excites DVN in part by inhibition of a K+ current via postsynaptic 5-HT2A receptors. As mRNA for the two-pore-domain K+ channels TASK-1 (KCNK3) and TASK-3 (KCNK9) has been found in DVN, we investigated the possibility that 5-HT exerts its effects via inhibition of these K+ channels using whole-cell patch-clamp techniques. In current clamp, 5-HT (20 microM) elicited a depolarization by 5.1+/-1.5 mV and an increase in firing rate. In voltage clamp, 5-HT reduced the standing outward current (ISO) at -20 mV by 106+/-17 pA, inhibiting a conductance (reversal, -95+/-4 mV) which displayed Goldman-Hodgkin-Katz outward rectification, supportive of a TASK-like K+ current. Since TASK channels are modulated by extracellular pH (pHo), we next investigated the pH sensitivity of ISO in Hepes-buffered ACSF. At pHo 7.3, DVN exhibited an ISO of 147+/-15 pA at -20 mV. Acidification to pHo 6.3 reduced ISO to 85+/-13 pA, whereas raising pHo to 8.5 increased ISO to 216+/-26 pA. At pHo 7.3, ISO was inhibited by BaCl2 (IC50 465 microM), but unaffected by ZnCl2 (100 microM). 5-HT (10 microM) reduced ISO by 114+/-17 pA at pHo 7.3, but at pHo 6.3 the 5-HT-induced inhibition of ISO was significantly smaller. The present data suggest that the excitatory effects of 5-HT on DVN are mediated in part by inhibition of a TASK-like, pH-sensitive K+ conductance. The pharmacological profile of this conductance excludes TASK-3 homomers, but rather implicates TASK-1-containing channels.

TRPV1b, a functional human vanilloid receptor splice variant

Transient receptor potential (TRP) genes encode a family of related ion-channel subunits. This family consists of cation-selective, calcium-permeable channels that include a group of vanilloid receptor channels (TRPV) implicated in pain and inflammation. These channels are activated by diverse stimuli, including capsaicin, lipids, membrane deformation, heat, and protons. Six members of the TRPV family have been identified that differ predominantly in their activation properties. However, in neurons, TRPV channels do not account for the observed diversity of responses to activators. By probing human and rat brain cDNA libraries to identify TRPV subunits, we identified a novel human TRPV1 RNA splice variant, TRPV1b, which forms functional ion channels that are activated by temperature (threshold, approximately 47 degrees C), but not by capsaicin or protons. Channels with similar activation properties were found in trigeminal ganglion neurons, suggesting that TRPV1b receptors are expressed in these cells and contribute to thermal nociception.

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.

A phospholipid sensor controls mechanogating of the K+ channel TREK-1

TREK-1 (KCNK2 or K(2P)2.1) is a mechanosensitive K(2P) channel that is opened by membrane stretch as well as cell swelling. Here, we demonstrate that membrane phospholipids, including PIP(2), control channel gating and transform TREK-1 into a leak K(+) conductance. A carboxy-terminal positively charged cluster is the phospholipid-sensing domain that interacts with the plasma membrane. This region also encompasses the proton sensor E306 that is required for activation of TREK-1 by cytosolic acidosis. Protonation of E306 drastically tightens channel-phospholipid interaction and leads to TREK-1 opening at atmospheric pressure. The TREK-1-phospholipid interaction is critical for channel mechano-, pH(i)- and voltage-dependent gating.