Changes in expression of some two-pore domain potassium channel genes (KCNK) in selected brain regions of developing mice

Ion channels of cold transduction and transmission

Thermosensation requires the activation of a unique collection of ion channels and receptors that work in concert to transmit thermal information. It is widely accepted that transient receptor potential melastatin 8 (TRPM8) activation is required for normal cold sensing; however, recent studies have illuminated major roles for other ion channels in this important somatic sensation. In addition to TRPM8, other TRP channels have been reported to contribute to cold transduction mechanisms in diverse sensory neuron populations, with both leak- and voltage-gated channels being identified for their role in the transmission of cold signals. Whether the same channels that contribute to physiological cold sensing also mediate noxious cold signaling remains unclear; however, recent work has found a conserved role for the kainite receptor, GluK2, in noxious cold sensing across species. Additionally, cold-sensing neurons likely engage in functional crosstalk with nociceptors to give rise to cold pain. This Review will provide an update on our understanding of the relationship between various ion channels in the transduction and transmission of cold and highlight areas where further investigation is required.

Structure of the human K2P13.1(THIK-1) channel reveals a novel hydrophilic pore restriction and lipid cofactor site

The halothane-inhibited K2P leak potassium channel K2P13.1 (THIK-1)1-3 is found in diverse cells1,4 including neurons1,5 and microglia6-8 where it affects surveillance6, synaptic pruning7, phagocytosis7, and inflammasome-mediated interleukin-1β release6,8,9. As with many K2Ps1,5,10-14 and other voltage-gated ion channel (VGIC) superfamily members3,15,16, polyunsaturated fatty acid (PUFA) lipids modulate K2P13.1 (THIK-1)1,5,14,17 via a poorly understood mechanism. Here, we present cryo-electronmicroscopy (cryo-EM) structures of human K2P13.1 (THIK-1) and mutants in lipid nanodiscs and detergent. These reveal that, unlike other K2Ps13,18-24, K2P13.1 (THIK-1) has a two-chamber aqueous inner cavity obstructed by a M4 transmembrane helix tyrosine (Tyr273, the flow restrictor). This hydrophilic barrier can be opened by an activatory mutation, S136P25, at natural break in the M2 transmembrane helix and by intrinsic channel dynamics. The structures also reveal a buried lipid in the P1/M4 intersubunit interface at a location, the PUFA site, that coincides with the TREK subfamily K2P modulator pocket for small molecule agonists18,26,27. This overlap, together with the effects of mutation on K2P13.1 (THIK-1) PUFA responses, indicates that the PUFA site lipids are K2P13.1 (THIK-1) cofactors. Comparison with the PUFA-responsive VGIC Kv7.1 (KCNQ1)28-31 reveals a shared role for the equivalent pore domain intersubunit interface in lipid modulation, providing a framework for dissecting the effects of PUFAs on the VGIC superfamily. Our findings reveal the unique architecture underlying K2P13.1 (THIK-1) function, highlight the importance of the P1/M4 interface in control of K2Ps by both natural and synthetic agents, and should aid development of THIK subfamily modulators for diseases such as neuroinflammation6,32 and autism6.

Electrophysiology and Morphology of Human Cortical Supragranular Pyramidal Cells in a Wide Age Range

The basic excitatory neurons of the cerebral cortex, the pyramidal cells, are the most important signal integrators for the local circuit. They have quite characteristic morphological and electrophysiological properties that are known to be largely constant with age in the young and adult cortex. However, the brain undergoes several dynamic changes throughout life, such as in the phases of early development and cognitive decline in the aging brain. We set out to search for intrinsic cellular changes in supragranular pyramidal cells across a broad age range: from birth to 85 years of age and we found differences in several biophysical properties between defined age groups. During the first year of life, subthreshold and suprathreshold electrophysiological properties changed in a way that shows that pyramidal cells become less excitable with maturation, but also become temporarily more precise. According to our findings, the morphological features of the three-dimensional reconstructions from different life stages showed consistent morphological properties and systematic dendritic spine analysis of an infantile and an old pyramidal cell showed clear significant differences in the distribution of spine shapes. Overall, the changes that occur during development and aging may have lasting effects on the properties of pyramidal cells in the cerebral cortex. Understanding these changes is important to unravel the complex mechanisms underlying brain development, cognition and age-related neurodegenerative diseases.

WNK3 kinase maintains neuronal excitability by reducing inwardly rectifying K+ conductance in layer V pyramidal neurons of mouse medial prefrontal cortex

The with-no-lysine (WNK) family of serine-threonine kinases and its downstream kinases of STE20/SPS1-related proline/alanine-rich kinase (SPAK) and oxidative stress-responsive kinase-1 (OSR1) may regulate intracellular Cl− homeostasis through phosphorylation of cation-Cl− co-transporters. WNK3 is expressed in fetal and postnatal brains, and its expression level increases during development. Its roles in neurons, however, remain uncertain. Using WNK3 knockout (KO) mice, we investigated the role of WNK3 in the regulation of the intracellular Cl− concentration ([Cl−]i) and the excitability of layer V pyramidal neurons in the medial prefrontal cortex (mPFC). Gramicidin-perforated patch-clamp recordings in neurons from acute slice preparation at the postnatal day 21 indicated a significantly depolarized reversal potential for GABAA receptor-mediated currents by 6 mV, corresponding to the higher [Cl−]i level by ~4 mM in KO mice than in wild-type littermates. However, phosphorylation levels of SPAK and OSR1 and those of neuronal Na+-K+-2Cl− co-transporter NKCC1 and K+-Cl− co-transporter KCC2 did not significantly differ between KO and wild-type mice. Meanwhile, the resting membrane potential of neurons was more hyperpolarized by 7 mV, and the minimum stimulus current necessary for firing induction was increased in KO mice. These were due to an increased inwardly rectifying K+ (IRK) conductance, mediated by classical inwardly rectifying (Kir) channels, in KO neurons. The introduction of an active form of WNK3 into the recording neurons reversed these changes. The potential role of KCC2 function in the observed changes of KO neurons was investigated by applying a selective KCC2 activator, CLP290. This reversed the enhanced IRK conductance in KO neurons, indicating that both WNK3 and KCC2 are intimately linked in the regulation of resting K+ conductance. Evaluation of synaptic properties revealed that the frequency of miniature excitatory postsynaptic currents (mEPSCs) was reduced, whereas that of inhibitory currents (mIPSCs) was slightly increased in KO neurons. Together, the impact of these developmental changes on the membrane and synaptic properties was manifested as behavioral deficits in pre-pulse inhibition, a measure of sensorimotor gating involving multiple brain regions including the mPFC, in KO mice. Thus, the basal function of WNK3 would be the maintenance and/or development of both intrinsic and synaptic excitabilities.

Gain-of-function mutations in KCNK3 cause a developmental disorder with sleep apnea

Sleep apnea is a common disorder that represents a global public health burden. KCNK3 encodes TASK-1, a K+ channel implicated in the control of breathing, but its link with sleep apnea remains poorly understood. Here we describe a new developmental disorder with associated sleep apnea (developmental delay with sleep apnea, or DDSA) caused by rare de novo gain-of-function mutations in KCNK3. The mutations cluster around the 'X-gate', a gating motif that controls channel opening, and produce overactive channels that no longer respond to inhibition by G-protein-coupled receptor pathways. However, despite their defective X-gating, these mutant channels can still be inhibited by a range of known TASK channel inhibitors. These results not only highlight an important new role for TASK-1 K+ channels and their link with sleep apnea but also identify possible therapeutic strategies.

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.

The Two-Pore Domain Potassium Channel TREK-1 Promotes Blood-Brain Barrier Breakdown and Exacerbates Neuronal Death After Focal Cerebral Ischemia in Mice

Earlier studies have shown the neuroprotective role of TWIK-related K⁺ channel 1 (TREK-1) in global cerebral and spinal cord ischemia, while its function in focal cerebral ischemia has long been debated. This study used TREK-1-deficient mice to directly investigate the role of TREK-1 after focal cerebral ischemia. First, immunofluorescence assays in the mouse cerebral cortex indicated that TREK-1 expression was mostly abundant in astrocytes, neurons, and oligodendrocyte precursor cells but was low in myelinating oligodendrocytes, microglia, or endothelial cells. TREK-1 deficiency did not affect brain weight and morphology or the number of neurons, astrocytes, or microglia but did increase glial fibrillary acidic protein (GFAP) expression in astrocytes of the cerebral cortex. The anatomy of the major cerebral vasculature, number and structure of brain micro blood vessels, and blood-brain barrier integrity were unaltered. Next, mice underwent 60 min of focal cerebral ischemia and 72 h of reperfusion induced by the intraluminal suture method. TREK-1-deficient mice showed less neuronal death, smaller infarction size, milder blood-brain barrier (BBB) breakdown, reduced immune cell invasion, and better neurological function. Finally, the specific pharmacological inhibition of TREK-1 also decreased infarction size and improved neurological function. These results demonstrated that TREK-1 might play a detrimental rather than beneficial role in focal cerebral ischemia, and inhibition of TREK-1 would be a strategy to treat ischemic stroke in the clinic.

TWIK-1 BAC-GFP Transgenic Mice, an Animal Model for TWIK-1 Expression

TWIK-1 is the first identified member of the two-pore domain potassium (K2P) channels that are involved in neuronal excitability and astrocytic passive conductance in the brain. Despite the physiological roles of TWIK-1, there is still a lack of information on the basic expression patterns of TWIK-1 proteins in the brain. Here, using a modified bacterial artificial chromosome (BAC), we generated a transgenic mouse (Tg mouse) line expressing green fluorescent protein (GFP) under the control of the TWIK-1 promoter (TWIK-1 BAC-GFP Tg mice). We confirmed that nearly all GFP-producing cells co-expressed endogenous TWIK-1 in the brain of TWIK-1 BAC-GFP Tg mice. GFP signals were highly expressed in various brain areas, including the dentate gyrus (DG), lateral entorhinal cortex (LEC), and cerebellum (Cb). In addition, we found that GFP signals were highly expressed in immature granule cells in the DG. Finally, our TWIK-1 BAC-GFP Tg mice mimic the upregulation of TWIK-1 mRNA expression in the hippocampus following the injection of kainic acid (KA). Our data clearly showed that TWIK-1 BAC-GFP Tg mice are a useful animal model for studying the mechanisms regulating TWIK-1 gene expression and the physiological roles of TWIK-1 channels in the brain.

Muscimol Directly Activates the TREK-2 Channel Expressed in GABAergic Neurons through Its N-Terminus

The two-pore domain K⁺ (K_2P) channel, which is involved in setting the resting membrane potential in neurons, is an essential target for receptor agonists. Activation of the γ-aminobutyric acid (GABA) receptors (GABA_AR and GABA_BR) reduces cellular excitability through Cl^- influx and K⁺ efflux in neurons. Relatively little is known about the link between GABA_AR and the K⁺ channel. The present study was performed to identify the effect of GABAR agonists on K_2P channel expression and activity in the neuroblastic B35 cells that maintain glutamic acid decarboxylase (GAD) activity and express GABA. TASK and TREK/TRAAK mRNA were expressed in B35 cells with a high level of TREK-2 and TRAAK. In addition, TREK/TRAAK proteins were detected in the GABAergic neurons obtained from GABA transgenic mice. Furthermore, TREK-2 mRNA and protein expression levels were markedly upregulated in B35 cells by GABA_AR and GABA_BR agonists. In particular, muscimol, a GABA_AR agonist, significantly increased TREK-2 expression and activity, but the effect was reduced in the presence of the GABA_AR antagonist bicuculine or TREK-2 inhibitor norfluoxetine. In the whole-cell and single-channel patch configurations, muscimol increased TREK-2 activity, but the muscimol effect disappeared in the N-terminal deletion mutant. These results indicate that muscimol directly induces TREK-2 activation through the N-terminus and suggest that muscimol can reduce cellular excitability by activating the TREK-2 channel and by inducing Cl^- influx in GABAergic neurons.

Negative Influence by the Force: Mechanically Induced Hyperpolarization via K2P Background Potassium Channels

The two-pore domain K_2P subunits form background (leak) potassium channels, which are characterized by constitutive, although not necessarily constant activity, at all membrane potential values. Among the fifteen pore-forming K_2P subunits encoded by the KCNK genes, the three members of the TREK subfamily, TREK-1, TREK-2, and TRAAK are mechanosensitive ion channels. Mechanically induced opening of these channels generally results in outward K⁺ current under physiological conditions, with consequent hyperpolarization and inhibition of membrane potential-dependent cellular functions. In the past decade, great advances have been made in the investigation of the molecular determinants of mechanosensation, and members of the TREK subfamily have emerged among the best-understood examples of mammalian ion channels directly influenced by the tension of the phospholipid bilayer. In parallel, the crucial contribution of mechano-gated TREK channels to the regulation of membrane potential in several cell types has been reported. In this review, we summarize the general principles underlying the mechanical activation of K_2P channels, and focus on the physiological roles of mechanically induced hyperpolarization.

Two-Pore Domain Potassium Channel in Neurological Disorders

K2P channel is the leaky potassium channel that is critical to keep up the negative resting membrane potential for legitimate electrical conductivity of the excitable tissues. Recently, many substances and medication elements are discovered that could either straightforwardly or in a roundabout way influence the 15 distinctive K⁺ ion channels including TWIK, TREK, TASK, TALK, THIK, and TRESK. Opening and shutting of these channels or any adjustment in their conduct is thought to alter the pathophysiological condition of CNS. There is no document available till now to explain in detail about the molecular mechanism of agents acting on K2P channel. Accordingly, in this review we cover the current research and mechanism of action of these channels, we have also tried to mention the detailed effect of drugs and how the channel behavior changes by focusing on recent advances regarding activation and modulation of ion channels.

Circadian photoperiod alters TREK-1 channel function and expression in dorsal raphe serotonergic neurons via melatonin receptor 1 signaling

Seasonal day length has been linked to the prevalence of mood disorders, and however, the mechanisms underlying this relationship remain unknown. Previous work in our laboratory has shown that developmental exposure to seasonal photoperiods has enduring effects on the activity of mouse dorsal raphe serotonergic neurons, their intrinsic electrical properties, as well as on depression and anxiety-related behaviors. Here we focus on the possible ionic mechanisms that underlie the observed programming of the electrophysiological properties of serotonin neurons, focusing on the twin-pore K + channels TREK-1 and TASK-1 that set resting membrane potential and regulate excitability. Pharmacological inhibition of TREK-1 significantly increased spike frequency in Short and Equinox photoperiods, but did not further elevate the firing rate in slices from Long photoperiod mice, suggesting that TREK-1 function is reduced in Long photoperiods. In contrast, inhibition of TASK-1 resulted in increases in firing rates across all photoperiods, suggesting that it contributes to setting excitability, but is not regulated by photoperiod. We then quantified Kcnk2 mRNA levels specifically in dorsal raphe 5-HT neurons using triple-label RNAscope. We found that Long photoperiod significantly reduced levels of Kcnk2 in serotonin neurons co-expressing Tph2, and Pet-1. Photoperiodic effects on the function and expression of TREK-1 were blocked in melatonin 1 receptor knockout (MT-1KO) mice, consistent with previous findings that MT-1 signaling is necessary for photoperiodic programming of dorsal raphe 5-HT neurons. Taken together these results indicate that photoperiodic regulation of TREK-1 expression and function plays a key role in photoperiodic programming the excitability of dorsal raphe 5-HT neurons.

TRESK and TREK-2 two-pore-domain potassium channel subunits form functional heterodimers in primary somatosensory neurons

Two-pore-domain potassium channels (K_2P) are the major determinants of the background potassium conductance. They play a crucial role in setting the resting membrane potential and regulating cellular excitability. These channels form homodimers; however, a few examples of heterodimerization have also been reported. The K_2P channel subunits TRESK and TREK-2 provide the predominant background potassium current in the primary sensory neurons of the dorsal root and trigeminal ganglia. A recent study has shown that a TRESK mutation causes migraine because it leads to the formation of a dominant negative truncated TRESK fragment. Surprisingly, this fragment can also interact with TREK-2. In this study, we determined the biophysical and pharmacological properties of the TRESK/TREK-2 heterodimer using a covalently linked TRESK/TREK-2 construct to ensure the assembly of the different subunits. The tandem channel has an intermediate single-channel conductance compared with the TRESK and TREK-2 homodimers. Similar conductance values were recorded when TRESK and TREK-2 were coexpressed, demonstrating that the two subunits can spontaneously form functional heterodimers. The TRESK component confers calcineurin-dependent regulation to the heterodimer and gives rise to a pharmacological profile similar to the TRESK homodimer, whereas the presence of the TREK-2 subunit renders the channel sensitive to the selective TREK-2 activator T2A3. In trigeminal primary sensory neurons, we detected single-channel activity with biophysical and pharmacological properties similar to the TRESK/TREK-2 tandem, indicating that WT TRESK and TREK-2 subunits coassemble to form functional heterodimeric channels also in native cells.

Metabotropic Modulation of Potassium Channels During Synaptic Plasticity

Besides their primary function mediating the repolarization phase of action potentials, potassium channels exquisitely and ubiquitously regulate the resting membrane potential of neurons and therefore have a key role establishing their intrinsic excitability. This group of proteins is composed of a very diverse collection of voltage-dependent and -independent ion channels, whose specific distribution is finely tuned at the level of the synapse. Both at the presynaptic and postsynaptic membranes, different types of potassium channels are subjected to modulation by second messenger signaling cascades triggered by metabotropic receptors, which in this way serve as a link between neurotransmitter actions and changes in the neuron membrane excitability. On the one hand, by regulating the resting membrane potential of the postsynaptic membrane, potassium channels appear to be critical towards setting the threshold for the induction of long-term potentiation and depression. On the other hand, these channels maintain the presynaptic membrane potential under control, therefore influencing the probability of neurotransmitter release underlying different forms of short-term plasticity. In the present review, we examine in detail the role of metabotropic receptors translating their activation by different neurotransmitters into a final effect modulating several types of potassium channels. Furthermore, we evaluate the consequences that this interplay has on the induction and maintenance of different forms of synaptic plasticity.

KCC2 Regulates Neuronal Excitability and Hippocampal Activity via Interaction with Task-3 Channels

KCC2 regulates neuronal transmembrane chloride gradients and thereby controls GABA signaling in the brain. KCC2 downregulation is observed in numerous neurological and psychiatric disorders. Paradoxical, excitatory GABA signaling is usually assumed to contribute to abnormal network activity underlying the pathology. We tested this hypothesis and explored the functional impact of chronic KCC2 downregulation in the rat dentate gyrus. Although the reversal potential of GABAA receptor currents is depolarized in KCC2 knockdown neurons, this shift is compensated by depolarization of the resting membrane potential. This reflects downregulation of leak potassium currents. We show KCC2 interacts with Task-3 (KCNK9) channels and is required for their membrane expression. Increased neuronal excitability upon KCC2 suppression altered dentate gyrus rhythmogenesis, which could be normalized by chemogenetic hyperpolarization. Our data reveal KCC2 downregulation engages complex synaptic and cellular alterations beyond GABA signaling that perturb network activity thus offering additional targets for therapeutic intervention.

Influence of Temperature on Motor Behaviors in Newborn Opossums (Monodelphis domestica): An In Vitro Study

External thermosensation is crucial to regulate animal behavior and homeostasis, but the development of the mammalian thermosensory system is not well known. We investigated whether temperature could play a role in the control of movements in a mammalian model born very immature, the opossum (Monodelphis domestica). Like other marsupials, at birth the opossum performs alternate and rhythmic movements with its forelimbs (FLs) to reach a teat where it attaches in order to continue its development. It was shown that FL movements can be induced by mechanical stimulation of the snout in in vitro preparations of newborns consisting of the neuraxis with skin and FLs intact. In the present study, we used puff ejections of cold, neutral (bath temperature) and hot liquid directed toward the snout to induce FL responses in such preparations. Either the responses were visually observed under a microscope or triceps muscle activity was recorded. Cold liquid systematically induced FL movements and triceps contractions, but neutral and hot temperatures were less potent to do so. Sections of the trigeminal nerves and removal of the facial skin diminished responses to cold and nearly abolished those to hot and neutral stimulations. Transient receptor potential melastatin 8 (TRPM8) being the major cold receptor cation channel in adult mammals, we employed immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) to test for its expression, but found that it is not expressed before 13 postnatal days. Overall our results indicate that cold thermosensation exerts a strong influence on motor behaviors in newborn opossums.

Early postnatal development of pyramidal neurons across layers of the mouse medial prefrontal cortex

Mammalian neocortex is a highly layered structure. Each layer is populated by distinct subtypes of principal cells that are born at different times during development. While the differences between principal cells across layers have been extensively studied, it is not known how the developmental profiles of neurons in different layers compare. Here, we provide a detailed morphological and functional characterisation of pyramidal neurons in mouse mPFC during the first postnatal month, corresponding to known critical periods for synapse and neuron formation in mouse sensory neocortex. Our data demonstrate similar maturation profiles of dendritic morphology and intrinsic properties of pyramidal neurons in both deep and superficial layers. In contrast, the balance of synaptic excitation and inhibition differs in a layer-specific pattern from one to four postnatal weeks of age. Our characterisation of the early development and maturation of pyramidal neurons in mouse mPFC not only demonstrates a comparable time course of postnatal maturation to that in other neocortical circuits, but also implies that consideration of layer- and time-specific changes in pyramidal neurons may be relevant for studies in mouse models of neuropsychiatric and neurodevelopmental disorders.

TWIK-1/TASK-3 heterodimeric channels contribute to the neurotensin-mediated excitation of hippocampal dentate gyrus granule cells

Two-pore domain K⁺ (K2P) channels have been shown to modulate neuronal excitability. The physiological role of TWIK-1, the first identified K2P channel, in neuronal cells is largely unknown, and we reported previously that TWIK-1 contributes to the intrinsic excitability of dentate gyrus granule cells (DGGCs) in mice. In the present study, we investigated the coexpression of TWIK-1 and TASK-3, another K2P member, in DGGCs. Immunohistochemical staining data showed that TASK-3 proteins were highly localized in the proximal dendrites and soma of DGGCs, and this localization is similar to the expression pattern of TWIK-1. TWIK-1 was shown to associate with TASK-3 in DGGCs of mouse hippocampus and when both genes were overexpressed in COS-7 cells. shRNA-mediated gene silencing demonstrated that TWIK-1/TASK-3 heterodimeric channels displayed outwardly rectifying currents and contributed to the intrinsic excitability of DGGCs. Neurotensin-neurotensin receptor 1 (NT-NTSR1) signaling triggered the depolarization of DGGCs by inhibiting TWIK-1/TASK-3 heterodimeric channels, causing facilitated excitation of DGGCs. Taken together, our study clearly showed that TWIK-1/TASK-3 heterodimeric channels contribute to the intrinsic excitability of DGGCs and that their activities are regulated by NT-NTSR1 signaling.

Leak potassium channels regulate sleep duration

A primary goal of sleep research is to understand the molecular basis of sleep. Although some sleep/wake-promoting circuits and secreted substances have been identified, the detailed molecular mechanisms underlying the regulation of sleep duration have been elusive. Here, to address these mechanisms, we developed a simple computational model of a cortical neuron with five channels and a pump, which recapitulates the cortical electrophysiological characteristics of slow-wave sleep (SWS) and wakefulness. Comprehensive bifurcation and detailed mathematical analyses predicted that leak K⁺ channels play a role in generating the electrophysiological characteristics of SWS, leading to a hypothesis that leak K⁺ channels play a role in the regulation of sleep duration. To test this hypothesis experimentally, we comprehensively generated and analyzed 14 KO mice, and found that impairment of the leak K⁺ channel (Kcnk9) decreased sleep duration. Based on these results, we hypothesize that leak K⁺ channels regulate sleep duration in mammals.

Regulatory Effect of General Anesthetics on Activity of Potassium Channels

General anesthesia is an unconscious state induced by anesthetics for surgery. The molecular targets and cellular mechanisms of general anesthetics in the mammalian nervous system have been investigated during past decades. In recent years, K⁺ channels have been identified as important targets of both volatile and intravenous anesthetics. This review covers achievements that have been made both on the regulatory effect of general anesthetics on the activity of K⁺ channels and their underlying mechanisms. Advances in research on the modulation of K⁺ channels by general anesthetics are summarized and categorized according to four large K⁺ channel families based on their amino-acid sequence homology. In addition, research achievements on the roles of K⁺ channels in general anesthesia in vivo, especially with regard to studies using mice with K⁺ channel knockout, are particularly emphasized.

Emerging Roles of TWIK-1 Heterodimerization in the Brain

Two-pore domain K⁺ (K2P) channels play essential roles in regulating resting membrane potential and cellular excitability. Although TWIK-1 (TWIK-tandem of pore domains in a weak inward rectifying K⁺ channel) was the first identified member of the K2P channel family, it is only in recent years that the physiological roles of TWIK-1 have been studied in depth. A series of reports suggest that TWIK-1 may underlie diverse functions, such as intrinsic excitability of neurons, astrocytic passive conductance, and astrocytic glutamate release, as a homodimer or heterodimer with other K2P isotypes. Here, we summarize expression patterns and newly identified functions of TWIK-1 in the brain.

T2N as a new tool for robust electrophysiological modeling demonstrated for mature and adult-born dentate granule cells

Compartmental models are the theoretical tool of choice for understanding single neuron computations. However, many models are incomplete, built ad hoc and require tuning for each novel condition rendering them of limited usability. Here, we present T2N, a powerful interface to control NEURON with Matlab and TREES toolbox, which supports generating models stable over a broad range of reconstructed and synthetic morphologies. We illustrate this for a novel, highly detailed active model of dentate granule cells (GCs) replicating a wide palette of experiments from various labs. By implementing known differences in ion channel composition and morphology, our model reproduces data from mouse or rat, mature or adult-born GCs as well as pharmacological interventions and epileptic conditions. This work sets a new benchmark for detailed compartmental modeling. T2N is suitable for creating robust models useful for large-scale networks that could lead to novel predictions. We discuss possible T2N application in degeneracy studies.

Prognostic significance of the TREK-1 K2P potassium channels in prostate cancer

Background

TREK-1 channels belong to the two-pore domain potassium channel superfamily and play an important role in central nervous system diseases. However, few studies have examined their role in carcinogenesis.

Methods

In this study, we assessed the expression of TREK-1 in 100 prostate cancer (PCa) tissues using immunohistochemistry and further analyzed its clinicopathological significance. Next, cell proliferation and cell cycle analysis were carried out on human PCa PC-3 cell lines where TREK-1 was stably knockdown.

Results

We found that compared with normal prostate tissues, PCa tissues showed overexpressed TREK-1 levels and TREK-1 levels were positively associated with Gleason score and T staging. High level of TREK-1 expression was related to shorter castration resistance free survival (CRFS). Furthermore, knockdown of TREK-1 significantly inhibited PCa cell proliferation in vitro and in vivo, and induced a G1/S cell cycle arrest.

Conclusion

Our results suggest that TREK-1 might be a biomarker in CRFS judgment of PCa, as well as a potential therapeutic target.

Functional study of TREK-1 potassium channels during rat heart development and cardiac ischemia using RNAi techniques

To explore the physiological and pathological significance of the 2-pore domain potassium channel TWIK-related K(+) (TREK)-1 in rat heart, its expression and role during heart development and cardiac ischemia were investigated. In the former study, the ventricles of Sprague Dawley rats were collected from embryo day 19 to postnatal 18 months and examined for mRNA and protein expression of TREK-1. It was found that both increased during development, reached a maximum at postnatal day 28, and remained higher at postnatal day 3 through to postnatal 18 months. In the latter study, protein expression of TREK-1 was examined after initiation of acute heart ischemia by ligation of the left anterior descending coronary artery. TREK-1 expression was found to be increased in the endocardium but unchanged in the epicardium. In primary cultured rat neonatal ventricular myocytes subjected to hypoxia (oxygen-glucose deprivation), TREK-1 expression was increased. In cultured neonatal cardiomyocytes, silencing of the TREK-1 gene by lentivirus delivery of the short-hairpin RNAs, L-sh-492 and L-sh-605, was found to promote their viability and number. In addition, both short-hairpin RNA provided protection against hypoxia-induced injury to cardiomyocytes in vitro. These results suggest that TREK-1 plays an important role in neonatal rat heart development and downregulation of TREK-1 may provide protection against ischemic injury. It seems that TREK-1 is a potential drug target for treatment of acute heart ischemia.

The role of K₂p channels in anaesthesia and sleep

Tandem two-pore potassium channels (K_2Ps) have widespread expression in the central nervous system and periphery where they contribute to background membrane conductance. Some general anaesthetics promote the opening of some of these channels, enhancing potassium currents and thus producing a reduction in neuronal excitability that contributes to the transition to unconsciousness. Similarly, these channels may be recruited during the normal sleep-wake cycle as downstream effectors of wake-promoting neurotransmitters such as noradrenaline, histamine and acetylcholine. These transmitters promote K_2P channel closure and thus an increase in neuronal excitability. Our understanding of the roles of these channels in sleep and anaesthesia has been largely informed by the study of mouse K_2P knockout lines and what is currently predicted by in vitro electrophysiology and channel structure and gating.

TWIK-1 contributes to the intrinsic excitability of dentate granule cells in mouse hippocampus

BACKGROUND

Two-pore domain K(+) (K2P) channels have been shown to modulate neuronal excitability. However, physiological function of TWIK-1, the first identified member of the mammalian K2P channel family, in neuronal cells is largely unknown.

RESULTS

We found that TWIK-1 proteins were expressed and localized mainly in the soma and proximal dendrites of dentate gyrus granule cells (DGGCs) rather than in distal dendrites or mossy fibers. Gene silencing demonstrates that the outwardly rectifying K(+) current density was reduced in TWIK-1-deficient granule cells. TWIK-1 deficiency caused a depolarizing shift in the resting membrane potential (RMP) of DGGCs and enhanced their firing rate in response to depolarizing current injections. Through perforant path stimulation, TWIK-1-deficient granule cells showed altered signal input-output properties with larger EPSP amplitude values and increased spiking compared to control DGGCs. In addition, supra-maximal perforant path stimulation evoked a graded burst discharge in 44% of TWIK-1-deficient cells, which implies impairment of EPSP-spike coupling.

CONCLUSIONS

These results showed that TWIK-1 is functionally expressed in DGGCs and contributes to the intrinsic excitability of these cells. The TWIK-1 channel is involved in establishing the RMP of DGGCs; it attenuates sub-threshold depolarization of the cells during neuronal activity, and contributes to EPSP-spike coupling in perforant path-to-granule cell synaptic transmission.

The two-pore domain potassium channel KCNK5 deteriorates outcome in ischemic neurodegeneration

Potassium channels can fulfill both beneficial and detrimental roles in neuronal damage during ischemic stroke. Earlier studies have characterized a neuroprotective role of the two-pore domain potassium channels KCNK2 (TREK1) and KCNK3 (TASK1). Protective neuronal hyperpolarization and prevention of intracellular Ca(2+) overload and glutamate excitotoxicity were suggested to be the underlying mechanisms. We here identify an unexpected role for the related KCNK5 channel in a mouse model of transient middle cerebral artery occlusion (tMCAO). KCNK5 is strongly upregulated on neurons upon cerebral ischemia, where it is most likely involved in the induction of neuronal apoptosis. Hypoxic conditions elevated neuronal expression levels of KCNK5 in acute brain slices and primary isolated neuronal cell cultures. In agreement, KCNK5 knockout mice had significantly reduced infarct volumes and improved neurologic function 24 h after 60 min of tMCAO and this protective effect was preserved at later stages of infarct development. KCNK5 deficiency resulted in a significantly reduced number of apoptotic neurons, a downregulation of pro-apoptotic and upregulation of anti-apoptotic factors. Results of adoptive transfer experiments of wild-type and Kcnk5 (-/-) immune cells into Rag1 (-/-) mice prior to tMCAO exclude a major role of KCNK5 in poststroke inflammatory reactions. In summary, KCNK5 expression is induced on neurons under ischemic conditions where it most likely exerts pro-apoptotic effects. Hence, pharmacological blockade of KCNK5 might have therapeutic potential in preventing ischemic neurodegeneration.

Regulation of breathing and autonomic outflows by chemoreceptors

Lung ventilation fluctuates widely with behavior but arterial PCO2 remains stable. Under normal conditions, the chemoreflexes contribute to PaCO2 stability by producing small corrective cardiorespiratory adjustments mediated by lower brainstem circuits. Carotid body (CB) information reaches the respiratory pattern generator (RPG) via nucleus solitarius (NTS) glutamatergic neurons which also target rostral ventrolateral medulla (RVLM) presympathetic neurons thereby raising sympathetic nerve activity (SNA). Chemoreceptors also regulate presympathetic neurons and cardiovagal preganglionic neurons indirectly via inputs from the RPG. Secondary effects of chemoreceptors on the autonomic outflows result from changes in lung stretch afferent and baroreceptor activity. Central respiratory chemosensitivity is caused by direct effects of acid on neurons and indirect effects of CO2 via astrocytes. Central respiratory chemoreceptors are not definitively identified but the retrotrapezoid nucleus (RTN) is a particularly strong candidate. The absence of RTN likely causes severe central apneas in congenital central hypoventilation syndrome. Like other stressors, intense chemosensory stimuli produce arousal and activate circuits that are wake- or attention-promoting. Such pathways (e.g., locus coeruleus, raphe, and orexin system) modulate the chemoreflexes in a state-dependent manner and their activation by strong chemosensory stimuli intensifies these reflexes. In essential hypertension, obstructive sleep apnea and congestive heart failure, chronically elevated CB afferent activity contributes to raising SNA but breathing is unchanged or becomes periodic (severe CHF). Extreme CNS hypoxia produces a stereotyped cardiorespiratory response (gasping, increased SNA). The effects of these various pathologies on brainstem cardiorespiratory networks are discussed, special consideration being given to the interactions between central and peripheral chemoreflexes.

Tandem pore domain halothane-inhibited K+ channel subunits THIK1 and THIK2 assemble and form active channels

Despite a high level of sequence homology, tandem pore domain halothane-inhibited K(+) channel 1 (THIK1) produces background K(+) currents, whereas THIK2 is silent. This lack of activity is due to a unique combination of intracellular retention and weak basal activity in the plasma membrane. Here, we designed THIK subunits containing dominant negative mutations (THIK1(DN) and THIK2(DN)). THIK2(DN) mutant inhibits THIK1 currents, whereas THIK1(DN) inhibits an activated form of THIK2 (THIK2-A155P-I158D). In situ proximity ligation assays and Förster/fluorescence resonance energy transfer (FRET) experiments support a physical association between THIK1 and THIK2. Next, we expressed covalent tandems of THIK proteins to obtain expression of pure heterodimers. Td-THIK1-THIK2 (where Td indicates tandem) produces K(+) currents of amplitude similar to Td-THIK1-THIK1 but with a noticeable difference in the current kinetics. Unlike Td-THIK2-THIK2 that is mainly detected in the endoplasmic reticulum, Td-THIK1-THIK2 distributes at the plasma membrane, indicating that THIK1 can mask the endoplasmic reticulum retention/retrieval motif of THIK2. Kinetics and unitary conductance of Td-THIK1-THIK2 are intermediate between THIK1 and THIK2. Altogether, these results show that THIK1 and THIK2 form active heteromeric channels, further expanding the known repertoire of K(+) channels.

Dorsal raphe serotonin neurons in mice: immature hyperexcitability transitions to adult state during first three postnatal weeks suggesting sensitive period for environmental perturbation

Trauma during early life is a major risk factor for the development of anxiety disorders and suggests that the developing brain may be particularly sensitive to perturbation. Increased vulnerability most likely involves altering neural circuits involved in emotional regulation. The role of serotonin in emotional regulation is well established, but little is known about the postnatal development of the raphe where serotonin is made. Using whole-cell patch-clamp recording and immunohistochemistry, we tested whether serotonin circuitry in the dorsal and median raphe was functionally mature during the first 3 postnatal weeks in mice. Serotonin neurons at postnatal day 4 (P4) were hyperexcitable. The increased excitability was due to depolarized resting membrane potential, increased resistance, increased firing rate, lack of 5-HT1A autoreceptor response, and lack of GABA synaptic activity. Over the next 2 weeks, membrane resistance decreased and resting membrane potential hyperpolarized due in part to potassium current activation. The 5-HT1A autoreceptor-mediated inhibition did not develop until P21. The frequency of spontaneous inhibitory and excitatory events increased as neurons extended and refined their dendritic arbor. Serotonin colocalized with vGlut3 at P4 as in adulthood, suggesting enhanced release of glutamate alongside enhanced serotonin release. Because serotonin affects circuit development in other brain regions, altering the developmental trajectory of serotonin neuron excitability and release could have many downstream consequences. We conclude that serotonin neuron structure and function change substantially during the first 3 weeks of life during which external stressors could potentially alter circuit formation.

Modulatory mechanisms and multiple functions of somatodendritic A-type K (+) channel auxiliary subunits

Auxiliary subunits are non-conducting, modulatory components of the multi-protein ion channel complexes that underlie normal neuronal signaling. They interact with the pore-forming α-subunits to modulate surface distribution, ion conductance, and channel gating properties. For the somatodendritic subthreshold A-type potassium (ISA) channel based on Kv4 α-subunits, two types of auxiliary subunits have been extensively studied: Kv channel-interacting proteins (KChIPs) and dipeptidyl peptidase-like proteins (DPLPs). KChIPs are cytoplasmic calcium-binding proteins that interact with intracellular portions of the Kv4 subunits, whereas DPLPs are type II transmembrane proteins that associate with the Kv4 channel core. Both KChIPs and DPLPs genes contain multiple start sites that are used by various neuronal populations to drive the differential expression of functionally distinct N-terminal variants. In turn, these N-terminal variants generate tremendous functional diversity across the nervous system. Here, we focus our review on (1) the molecular mechanism underlying the unique properties of different N-terminal variants, (2) the shaping of native ISA properties by the concerted actions of KChIPs and DPLP variants, and (3) the surprising ways that KChIPs and DPLPs coordinate the activity of multiple channels to fine-tune neuronal excitability. Unlocking the unique contributions of different auxiliary subunit N-terminal variants may provide an important opportunity to develop novel targeted therapeutics to treat numerous neurological disorders.

Properties of BK-type Ca(+) (+)-dependent K(+) channel currents in medial prefrontal cortex pyramidal neurons in rats of different ages

The medial prefrontal cortex (PFC) is involved in cognitive functions, which undergo profound changes during adolescence. This alteration of the PFC function derives from neuron activity, which, in turn, may depend on age-dependent properties and the expression of neuronal ion channels. BK-type channels are involved in controlling both the Ca(+) (+) ion concentration in the cell interior and cell excitability. The purpose of this study was to test the properties of BK currents in the medial PFC pyramidal neurons of young (18- to 22-day-old), adolescent (38- to 42-day-old), and adult (60- to 65-day-old) rats. Whole-cell currents evoked by depolarizing voltage steps were recorded from dispersed medial PFC pyramidal neurons. A selective BK channel blocker - paxilline (10 μM) - irreversibly decreased the non-inactivating K(+) current in neurons that were isolated from the young and adult rats. This current was not significantly affected by paxilline in the neurons obtained from adolescent rats. The properties of single-channel K(+) currents were recorded from the soma of dispersed medial PFC pyramidal neurons in the cell-attached configuration. Of the K(+) channel currents that were recorded, ~90% were BK and leak channel currents. The BK-type channel currents were dependent on the Ca(+) (+) concentration and the voltage and were inhibited by paxilline. The biophysical properties of the BK channel currents did not differ among the pyramidal neurons isolated from young, adolescent, and adult rats. Among all of the recorded K(+) channel currents, 38.9, 12.7, and 21.1% were BK-type channel currents in the neurons isolated from the young, adolescent, and adult rats, respectively. Furthermore, application of paxilline effectively prolonged the half-width of the action potential in pyramidal neurons in slices isolated from young and adult rats but not in neurons isolated from adolescent rats. We conclude that the availability of BK channel currents decreases in medial PFC pyramidal neurons of adolescent rats compared with those in the neurons of young and adult rats while their properties did not change across ages.

The inhibitor of volume-regulated anion channels DCPIB activates TREK potassium channels in cultured astrocytes

BACKGROUND AND PURPOSE

The ethacrynic acid derivative, 4-(2-butyl-6,7-dichlor-2-cyclopentylindan-1-on-5-yl) oxobutyric acid (DCPIB) is considered to be a specific and potent inhibitor of volume-regulated anion channels (VRACs). In the CNS, DCPIB was shown to be neuroprotective through mechanisms principally associated to its action on VRACs. We hypothesized that DCPIB could also regulate the activity of other astroglial channels involved in cell volume homeostasis.

EXPERIMENTAL APPROACH

Experiments were performed in rat cortical astrocytes in primary culture and in hippocampal astrocytes in situ. The effect of DCPIB was evaluated by patch-clamp electrophysiology and immunocytochemical techniques. Results were verified by comparative analysis with recombinant channels expressed in COS-7 cells.

KEY RESULTS

In cultured astrocytes, DCPIB promoted the activation of a K(+) conductance mediated by two-pore-domain K(+) (K(2P) ) channels. The DCPIB effect occluded that of arachidonic acid, which activates K(2P) channels K(2P) 2.1 (TREK-1) and K(2P) 10.1 (TREK-2) in cultured astrocytes. Immunocytochemical analysis suggests that cultured astrocytes express K(2P) 2.1 and K(2P) 10.1 proteins. Moreover, DCPIB opened recombinant K(2P) 2.1 and K(2P) 10.1 expressed in heterologous system. In brain slices, DCPIB did not augment the large background K(+) conductance in hippocampal astrocytes, but caused an increment in basal K(+) current of neurons.

CONCLUSION AND IMPLICATIONS

Our results indicate that the neuroprotective effect of DCPIB could be due, at least in part, to activation of TREK channels. DCPIB could be used as template to build new pharmacological tools able to increase background K(+) conductance in astroglia and neuronal cells.

Severe hyperaldosteronism in neonatal Task3 potassium channel knockout mice is associated with activation of the intraadrenal renin-angiotensin system

Task3 K(+) channels are highly expressed in the adrenal cortex and contribute to the angiotensin II and K(+) sensitivity of aldosterone-producing glomerulosa cells. Adult Task3(-/-) mice display a partially autonomous aldosterone secretion, subclinical hyperaldosteronism, and salt-sensitive hypertension. Here, we investigated the age dependence of the adrenal phenotype of Task3(-/-) mice. Compared with adults, newborn Task3(-/-) mice displayed a severe adrenal phenotype with strongly increased plasma levels of aldosterone, corticosterone, and progesterone. This adrenocortical dysfunction was accompanied by a modified gene expression profile. The most strongly up-regulated gene was the protease renin. Real-time PCR corroborated the strong increase in adrenal renin expression, and immunofluorescence revealed renin-expressing cells in the zona fasciculata. Together with additional factors, activation of the local adrenal renin system is probably causative for the severely disturbed steroid hormone secretion of neonatal Task3(-/-) mice. The changes in gene expression patterns of neonatal Task3(-/-) mice could also be relevant for other forms of hyperaldosteronism.

A new TASK for Dipeptidyl Peptidase-like Protein 6

Dipeptidyl Peptidase-like Protein 6 (DPP6) is widely expressed in the brain where it co-assembles with Kv4 channels and KChIP auxiliary subunits to regulate the amplitude and functional properties of the somatodendritic A-current, I_SA. Here we show that in cerebellar granule (CG) cells DPP6 also regulates resting membrane potential and input resistance by increasing the amplitude of the I_K(SO) resting membrane current. Pharmacological analysis shows that DPP6 acts through the control of a channel with properties matching the K_2P channel TASK-3. Heterologous expression and co-immunoprecipitation shows that DPP6 co-expression with TASK-3 results in the formation of a protein complex that enhances resting membrane potassium conductance. The co-regulation of resting and voltage-gated channels by DPP6 produces coordinate shifts in resting membrane potential and A-current gating that optimize the sensitivity of I_SA inactivation gating to subthreshold fluctuations in resting membrane potential.

The influence of cold temperature on cellular excitability of hippocampal networks

The hippocampus plays an important role in short term memory, learning and spatial navigation. A characteristic feature of the hippocampal region is its expression of different electrical population rhythms and activities during different brain states. Physiological fluctuations in brain temperature affect the activity patterns in hippocampus, but the underlying cellular mechanisms are poorly understood. In this work, we investigated the thermal modulation of hippocampal activity at the cellular network level. Primary cell cultures of mouse E17 hippocampus displayed robust network activation upon light cooling of the extracellular solution from baseline physiological temperatures. The activity generated was dependent on action potential firing and excitatory glutamatergic synaptic transmission. Involvement of thermosensitive channels from the transient receptor potential (TRP) family in network activation by temperature changes was ruled out, whereas pharmacological and immunochemical experiments strongly pointed towards the involvement of temperature-sensitive two-pore-domain potassium channels (K_2P), TREK/TRAAK family. In hippocampal slices we could show an increase in evoked and spontaneous synaptic activity produced by mild cooling in the physiological range that was prevented by chloroform, a K_2P channel opener. We propose that cold-induced closure of background TREK/TRAAK family channels increases the excitability of some hippocampal neurons, acting as a temperature-sensitive gate of network activation. Our findings in the hippocampus open the possibility that small temperature variations in the brain in vivo, associated with metabolism or blood flow oscillations, act as a switch mechanism of neuronal activity and determination of firing patterns through regulation of thermosensitive background potassium channel activity.

Dysfunction of KCNK potassium channels impairs neuronal migration in the developing mouse cerebral cortex

Development of the cerebral cortex depends partly on neural activity, but the identity of the ion channels that might contribute to the activity-dependent cortical development is unknown. KCNK channels are critical determinants of neuronal excitability in the mature cerebral cortex, and a member of the KCNK family, KCNK9, is responsible for a maternally transmitted mental retardation syndrome. Here, we have investigated the roles of KCNK family potassium channels in cortical development. Knockdown of KCNK2, 9, or 10 by RNAi using in utero electroporation impaired the migration of late-born cortical excitatory neurons destined to become Layer II/III neurons. The migration defect caused by KCNK9 knockdown was rescued by coexpression of RNAi-resistant functional KCNK9 mutant. Furthermore, expression of dominant-negative mutant KCNK9, responsible for the disease, and electrophysiological experiments demonstrated that ion channel function was involved in the migration defect. Calcium imaging revealed that KCNK9 knockdown or expression of dominant-negative mutant KCNK9 increased the fraction of neurons showing calcium transients and the frequency of spontaneous calcium transients. Mislocated neurons seen after KCNK9 knockdown stayed in the deep cortical layers, showing delayed morphological maturation. Taken together, our results suggest that dysfunction of KCNK9 causes a migration defect in the cortex via an activity-dependent mechanism.

Hyperpolarization of resting membrane potential causes retraction of spontaneous Ca(i)²⁺ transients during mouse embryonic circuit development

Abstract  Spontaneous activity supports developmental processes in many brain regions during embryogenesis, and the spatial extent and frequency of the spontaneous activity are tightly regulated by stage. In the developing mouse hindbrain, spontaneous activity propagates widely and the waves can cover the entire hindbrain at E11.5. The activity then retracts to waves that are spatially restricted to the rostral midline at E13.5, before disappearing altogether by E15.5. However, the mechanism of retraction is unknown. We studied passive membrane properties of cells that are spatiotemporally relevant to the pattern of retraction in mouse embryonic hindbrain using whole-cell patch clamp and imaging techniques. We find that membrane excitability progressively decreases due to hyperpolarization of resting membrane potential and increased resting conductance density between E11.5 and E15.5, in a spatiotemporal pattern correlated with the retraction sequence. Retraction can be acutely reversed by membrane depolarization at E15.5, and the induced events propagate similarly to spontaneous activity at earlier stages, though without involving gap junctional coupling. Manipulation of [K(+)](o) or [Cl(-)](o) reveals that membrane potential follows E(K) more closely than E(Cl), suggesting a dominant role for K(+) conductance in the membrane hyperpolarization. Reducing membrane excitability by hyperpolarization of the resting membrane potential and increasing resting conductance are effective mechanisms to desynchronize spontaneous activity in a spatiotemporal manner, while allowing information processing to occur at the synaptic and cellular level.

Age-related changes in two-pore domain acid-sensitive K⁺ channel expression in rat dorsal root ganglion neurons

1. Two-pore domain K⁺ (K(2P) ) channel expression influences brain development. The K(2P) channels, including two-pore domain acid-sensitive K⁺ (TASK) channels, contribute to the setting of the resting membrane potential of neurons. In addition to neurons in the brain, dorsal root ganglion (DRG) neurons also express K(2P) channels. The aim of the present study was to identify postnatal changes in the expression of TASK channels in DRG neurons. 2. Expression of TASK channels (TASK-1, TASK-2 and TASK-3) was compared between neonatal (postnatal Day (P) 1 or P2) and adult (P120) rat DRG using semiquantitative polymerase chain reaction, western blot analysis, immunostaining and the patch-clamp technique. 3. In adult (P120) rat DRG, expression of TASK-2 mRNA and protein was downregulated, whereas TASK-3 mRNA and protein expression was upregulated. There were no consistent changes in TASK-1 mRNA and protein expression. Single-channel recordings showed very low TASK-2- and TASK-3-like channel expression in P1-P2 DRG neurons (∼10% in TASK-2 and ∼3% in TASK-3). In P120 DRG, there was a reduction in the detection of TASK-2-like channels, whereas the detection of TASK-3-like channels increased. 4. These results show that TASK-2 and TASK-3 mRNA and protein expression undergoes age-related changes in DRG neurons, indicating that TASK-2 and TASK-3 channels are likely to contribute to the setting of the resting membrane potential of DRG neurons in neonates and adults, separately or together, during DRG development.

Leak K⁺ channel mRNAs in dorsal root ganglia: relation to inflammation and spontaneous pain behaviour

Two pore domain potassium (K2P) channels (KCNKx.x) cause K + leak currents and are major contributors to resting membrane potential. Their roles in dorsal root ganglion (DRG) neurons normally, and in pathological pain models, are poorly understood. Therefore, we examined mRNA levels for 10 K2P channels in L4 and L5 rat DRGs normally, and 1 day and 4 days after unilateral cutaneous inflammation, induced by intradermal complete Freund's adjuvant (CFA) injections. Spontaneous foot lifting (SFL) duration (spontaneous pain behaviour) was measured in 1 day and 4 day rats < 1 h before DRG harvest. mRNA levels for KCNK channels and Kv1.4 relative to GAPDH (n = 4–6 rats/group) were determined with real-time RT-PCR. This study is the first to demonstrate expression of THIK1, THIK2 and TWIK2 mRNA in DRGs. Abundance in normal DRGs was, in descending order:

Kv1.4 > TRESK(KCNK18) > TRAAK(KCNK4) > TREK2(KCNK10) = TWIK2(KCNK6) > TREK1 (KCNK2) = THIK2(KCNK12) > TASK1(KCNK3) > TASK2(KCNK5) > THIK1(KCNK13) = TASK3(KCNK9).

During inflammation, the main differences from normal in DRG mRNA levels were bilateral, suggesting systemic regulation, although some channels showed evidence of ipsilateral modulation. By 1 day, bilateral K2P mRNA levels had decreased (THIK1) or increased (TASK1, THIK2) but by 4 days they were consistently decreased (TASK2, TASK3) or tended to decrease (excluding TRAAK). The decreased TASK2 mRNA was mirrored by decreased protein (TASK2-immunoreactivity) at 4 days. Ipsilateral mRNA levels at 4 days compared with 1 day were lower (TRESK, TASK1, TASK3, TASK2 and THIK2) or higher (THIK1). Ipsilateral SFL duration during inflammation was positively correlated with ipsilateral TASK1 and TASK3 mRNAs, and contralateral TASK1, TRESK and TASK2 mRNAs. Thus changes in K2P mRNA levels occurred during inflammation and for 4 K2P channels were associated with spontaneous pain behaviour (SFL). K2P channels and their altered expression are therefore associated with inflammation-induced pain.

Identification of the muscarinic pathway underlying cessation of sleep-related burst activity in rat thalamocortical relay neurons

Modulation of the standing outward current (I (SO)) by muscarinic acetylcholine (ACh) receptor (MAChR) stimulation is fundamental for the state-dependent change in activity mode of thalamocortical relay (TC) neurons. Here, we probe the contribution of MAChR subtypes, G proteins, phospholipase C (PLC), and two pore domain K(+) (K(2P)) channels to this signaling cascade. By the use of spadin and A293 as specific blockers, we identify TWIK-related K(+) (TREK)-1 channel as new targets and confirm TWIK-related acid-sensitve K(+) (TASK)-1 channels as known effectors of muscarinic signaling in TC neurons. These findings were confirmed using a high affinity blocker of TASK-3 and TREK-1, namely, tetrahexylammonium chloride. It was found that the effect of muscarinic stimulation was inhibited by M(1)AChR-(pirenzepine, MT-7) and M(3)AChR-specific (4-DAMP) antagonists, phosphoinositide-specific PLCβ (PI-PLC) inhibitors (U73122, ET-18-OCH(3)), but not the phosphatidylcholine-specific PLC (PC-PLC) blocker D609. By comparison, depleting guanosine-5'-triphosphate (GTP) in the intracellular milieu nearly completely abolished the effect of MAChR stimulation. The block of TASK and TREK channels was accompanied by a reduction of the muscarinic effect on I (SO). Current-clamp recordings revealed a membrane depolarization following MAChR stimulation, which was sufficient to switch TC neurons from burst to tonic firing under control conditions but not during block of M(1)AChR/M(3)AChR and in the absence of intracellular GTP. These findings point to a critical role of G proteins and PLC as well as TASK and TREK channels in the muscarinic modulation of thalamic activity modes.

Molecular regulations governing TREK and TRAAK channel functions

K+ channels with two-pore domain (K2p) form a large family of hyperpolarizing channels. They produce background currents that oppose membrane depolarization and cell excitability. They are involved in cellular mechanisms of apoptosis, vasodilatation, anesthesia, pain, neuroprotection and depression. This review focuses on TREK-1, TREK-2 and TRAAK channels subfamily and on the mechanisms that contribute to their molecular heterogeneity and functional regulations. Their molecular diversity is determined not only by the number of genes but also by alternative splicing and alternative initiation of translation. These channels are sensitive to a wide array of biophysical parameters that affect their activity such as unsaturated fatty acids, intra- and extracellular pH, membrane stretch, temperature, and intracellular signaling pathways. They interact with partner proteins that influence their activity and their plasma membrane expression. Molecular heterogeneity, regulatory mechanisms and protein partners are all expected to contribute to cell specific functions of TREK currents in many tissues.

Fluoxetine attenuates the inhibitory effect of glucocorticoid hormones on neurogenesis in vitro via a two-pore domain potassium channel, TREK-1

RATIONALE

Sustained stress and elevated glucocorticoid reduces neurogenesis, whereas chronic treatment with antidepressants increases neurogenesis and blocks the effects of stress. Recently, TREK-1, a two-pore domain (K(2)p) potassium channel, has been shown to be involved in the mechanisms of major depression.

OBJECTIVES

This study aimed to investigate whether TREK-1 is involved in the alteration of neurogenesis according to glucocorticoids and antidepressants.

RESULTS

The present study addressed the expression of TREK-1 in neural stem cells (NSCs) and found TREK-1 was only associated with NSC proliferation. Bupivacaine and curcumin, two strong TREK-1 channel inhibitors, significantly increased embryonic NSC viability and proliferation while transfection of hTREK-1 decreased cell proliferation in embryonic NSCs. Dexamethasone, a glucocorticoid hormone receptor agonist, upregulated both protein and mRNA levels of TREK-1 leading to decreased NSC proliferation which could be reversed by bupivacaine. Fluoxetine, a serotonin reuptake inhibitor antidepressant that has been previously found to inhibit TREK-1 channels, robustly, could attenuate the upregulation of TREK-1 expression and the inhibition of NSC proliferation induced by dexamethasone.

CONCLUSIONS

Taken together, these data suggest that TREK-1 is associated with NSC proliferation and probably is a modulator of the effect that fluoxetine attenuates the inhibitory neurogenesis induced by glucocorticoid hormones.

Potassium channel expression in adult murine neural progenitor cells

Neural progenitor cells (NPCs) are a source of new neurons and glia in the adult brain. Most NPCs reside in the forebrain subventricular zone (SVZ) and in the subgranular zone of the dentate gyrus, where they contribute to plasticity in the adult brain. To use their potential for repair, it is essential to identify the molecules that regulate their growth, migration and differentiation. Potassium (K+) channels are promising molecule candidates for NPC regulation as they are important components of signal transduction and their diversity is ideal to cover the complex functions required for cell proliferation and differentiation. There is increasing evidence that K+ channels influence cell growth and neurogenesis, however, very little is known regarding K+ channel distribution in NPCs. We therefore explored the expression of a variety of voltage-gated (Kv), inwardly rectifying (Kir) and two-pore (K2P) K+ channels in the SVZ of adult mice and in neurosphere cultures of NPCs during growth and differentiation. Immunocytochemical analysis revealed a differential expression pattern of K+ channels in nestin+ SVZ precursor cells, early SVZ doublecortin+ neurons and (sub)ependymal cells. These findings were confirmed in neurosphere cultures at the protein and mRNA levels. The expression of some K+ channel proteins, such as Kir4.1, Kir6.1, TREK1 or TASK1, suggests a role of K+ channels in the complex regulation of NPC proliferation, maturation and differentiation.