Differential distribution of individual subunits of strongly inwardly rectifying potassium channels (Kir2 family) in rat brain

Ionic mechanisms involved in arginine vasopressin-mediated excitation of auditory cortical and thalamic neurons

The axons containing arginine vasopressin (AVP) from the hypothalamus innervate a variety of structures including the cerebral cortex, thalamus, hippocampus and amygdala. A plethora amount of evidence indicates that activation of the V1a subtype of the vasopressin receptors facilitates anxiety-like and fear responses. As an essential structure involved in fear and anxiety responses, the amygdala, especially the lateral nucleus of amygdala (LA), receives glutamatergic innervations from the auditory cortex and auditory thalamus where high density of V1a receptors have been detected. However, the roles and mechanisms of AVP in these two important areas have not been determined, which prevents the understanding of the mechanisms whereby V1a activation augments anxiety and fear responses. Here, we used coronal brain slices and studied the effects of AVP on neuronal activities of the auditory cortical and thalamic neurons. Our results indicate that activation of V1a receptors excited both auditory cortical and thalamic neurons. In the auditory cortical neurons, AVP increased neuronal excitability by depressing multiple subtypes of inwardly rectifying K+ (Kir) channels including the Kir2 subfamily, the ATP-sensitive K+ channels and the G protein-gated inwardly rectifying K+ (GIRK) channels, whereas activation of V1a receptors excited the auditory thalamic neurons by depressing the Kir2 subfamily of the Kir channels as well as activating the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and a persistent Na+ channel. Our results may help explain the roles of V1a receptors in facilitating fear and anxiety responses. Categories: Cell Physiology.

Pre- and postsynaptic modulation of hippocampal inhibitory synaptic transmission by pregnenolone sulphate

Neurosteroids are important endogenous modulators of GABA_A receptor-mediated neurotransmission within the CNS and play a vital role in maintaining normal healthy brain function. Research has mainly focussed on neurosteroids such as allopregnanolone and tetrahydro-deoxycorticosterone (THDOC) which are allosteric potentiators of GABA_A receptors, whilst the sulphated steroids, including pregnenolone sulphate (PS), which inhibit GABA_A receptor function, have been relatively neglected. Importantly, a full description of PS effects on inhibitory synaptic transmission, at concentrations that are expected to inhibit postsynaptic GABA_A receptors, is lacking. Here, we address this deficit by recording inhibitory postsynaptic currents (IPSCs) from rat hippocampal neurons both in culture and in acute brain slices and explore the impact of PS at micromolar concentrations. We reveal that PS inhibits postsynaptic GABA_A receptors, evident from reductions in IPSC amplitude and decay time. Concurrently, PS also causes an increase in synaptic GABA release which we discover is due to the activation of presynaptic TRPM3 receptors located close to presynaptic GABA release sites. Pharmacological blockade of TRPM3 receptors uncovers a PS-evoked reduction in IPSC frequency. This second presynaptic effect is caused by PS activation of inwardly-rectifying Kir2.3 channels on interneurons, which act to depress synaptic GABA release. Overall, we provide a comprehensive characterisation of pre- and postsynaptic modulation by PS of inhibitory synaptic transmission onto hippocampal neurons which elucidates the diverse mechanisms by which this understudied neurosteroid can modulate brain function.

Potassium Channels and Their Potential Roles in Substance Use Disorders

Substance use disorders (SUDs) are ubiquitous throughout the world. However, much remains to be done to develop pharmacotherapies that are very efficacious because the focus has been mostly on using dopaminergic agents or opioid agonists. Herein we discuss the potential of using potassium channel activators in SUD treatment because evidence has accumulated to support a role of these channels in the effects of rewarding drugs. Potassium channels regulate neuronal action potential via effects on threshold, burst firing, and firing frequency. They are located in brain regions identified as important for the behavioral responses to rewarding drugs. In addition, their expression profiles are influenced by administration of rewarding substances. Genetic studies have also implicated variants in genes that encode potassium channels. Importantly, administration of potassium agonists have been shown to reduce alcohol intake and to augment the behavioral effects of opioid drugs. Potassium channel expression is also increased in animals with reduced intake of methamphetamine. Together, these results support the idea of further investing in studies that focus on elucidating the role of potassium channels as targets for therapeutic interventions against SUDs.

A Single Prior Injection of Methamphetamine Enhances Methamphetamine Self-Administration (SA) and Blocks SA-Induced Changes in DNA Methylation and mRNA Expression of Potassium Channels in the Rat Nucleus Accumbens

The transition from occasional to escalated psychostimulant use is accelerated by prior drug exposure. These behavioral observations may be related to long-lasting transcriptional and/or epigenetic changes induced by the drug pre-exposure. Herein, we investigated if a single methamphetamine (METH) injection would enhance METH self-administration (SA) and impact any METH SA-induced epigenetic or transcriptional alterations. We thus injected a single METH dose (10 mg/kg) or saline to rats before training them to self-administer METH or saline. There were three experimental groups in SA experiments: (1) a single saline injection followed by saline SA (SS); (2) a single saline injection followed by METH SA (SM); and (3) a single METH injection followed by METH SA (MM). METH-pretreated rats escalated METH SA earlier and took more METH than saline-pretreated animals. Both groups showed similar incubation of cue-induced METH craving. Because compulsive METH takers and METH-abstinent rats show differences in potassium (K⁺) channel mRNA levels in their nucleus accumbens (NAc), we wondered if K⁺ channel expression might also help to distinguish between SM and MM groups. We found increases in mRNA and protein expression of shaker-related voltage-gated K⁺ channels (Kv1: Kcna1, Kcna3, and Kcna6) and calcium-activated K⁺ channels (Kcnn1) in the SM compared to MM rats. SM rats also showed decreased DNA methylation at the CpG-rich sites near the promoter region of Kcna1, Kcna3 and Kcnn1 genes in comparison to MM rats. Together, these results provide additional evidence for potentially using K⁺ channels as therapeutic targets against METH use disorder.

Memantine selectively prevented the induction of dynamic allodynia by blocking Kir2.1 channel and inhibiting the activation of microglia in spinal dorsal horn of mice in spared nerve injury model

Background

Memantine is one of the important clinical medications in treating moderate to severe Alzheimer disease. The effect of memantine on preventing or treating punctate allodynia has been thoroughly studied but not on the induction of dynamic allodynia. The aim of this study is to investigate whether memantine could prevent the induction of dynamic allodynia and its underlying spinal mechanisms.

Results

(1) In in vivo spared nerve injury pain model, pretreatment with memantine at a lower dose (10 nmol, intrathecal; memantine-10) selectively prevented the induction of dynamic allodynia but not the punctate allodynia. (2) Pretreatment with either MK801-10 (MK801-10 nmol, intrathecal) or higher dose of memantine (30 nmol, intrathecal; memantine-30) prevented the induction of both dynamic and punctate allodynia. (3) Memantine-10 showed significant effect on the inhibition of the spared nerve injury-induced overactivation of microglia in spinal dorsal horn. (4) In contrast, in complete freund′s adjuvant (CFA) model, memantine-10 neither affected the CFA injection-induced activation of microglia in spinal dorsal horn nor the induction of dynamic allodynia. (5) Immunohistological studies showed Kir2.1 channel distributed widely and co-localized with microglia in the spinal dorsal horn of mice. (6) Pretreatment with either minocycline, a microglia inhibitor, or ML133, a Kir2.1 inhibitor, both selectively prevented the overactivation of microglia in spinal dorsal horn and the induction of dynamic allodynia following spared nerve injury.

Conclusion

The selective inhibitory effect on the induction of dynamic allodynia in spared nerve injury model by low dose of the memantine (memantine-10) was tightly correlated with the blockade of microglia Kir2.1 channel to suppress the microglia activation.

Activation of Kir2.3 Channels by Tenidap Suppresses Epileptiform Burst Discharges in Cultured Hippocampal Neurons

BACKGROUND & OBJECTIVE

Tenidap, a selective human inwardly rectifying potassium (Kir) 2.3 channel opener, has been reported to have antiepileptic effect in the pilocarpine temporal lobe epilepsy rat model in our previous study. However, the effect of tenidap on neurons and its relationship with the epileptiform bursting charges in neuron is still required to be explored.

METHODS

In this study, cyclothiazide (CTZ) induced cultured hippocampal neuron epileptic model was used to study the antiepileptic effect of tenidap and the relationship between Kir2.3 channel and the neuronal epileptiform burst.

RESULTS

Patch clamp recording showed that both acute (2h) and chronic (48h) CTZ pre-treatment all significantly induced robust epileptiform burst activities in cultured hippocampal neurons, and tenidap acutely application inhibited this highly synchronized abnormal activities. The effect of tenidap is likely due to increased activity of Kir2.3 channels, since tenidap significantly enhanced kir current recorded from those neurons. In addition, neurons overexpressing Kir2.3 channels, by transfection with Kir2.3 plasmid, showed a significant large increase of the Kir current, prevented CTZ treatment to induce epileptiform burst discharge.

CONCLUSION

Our current study demonstrated that over activation of Kir2.3 channel in hippocampal neurons could positively interference with epileptiform burst activities, and tenidap, as a selective Kir2.3 channel opener, could be a potential candidate for seizure therapy.

Thermal constraints on in vivo optogenetic manipulations

A key assumption of optogenetics is that light only affects opsin-expressing neurons. However, illumination invariably heats tissue, and many physiological processes are temperature-sensitive. Commonly used illumination protocols increased the temperature by 0.2-2 °C and suppressed spiking in multiple brain regions. In the striatum, light delivery activated an inwardly rectifying potassium conductance and biased rotational behavior. Thus, careful consideration of light-delivery parameters is required, as even modest intracranial heating can confound interpretation of optogenetic experiments.

Aquaporin 4 Suppresses Neural Hyperactivity and Synaptic Fatigue and Fine-Tunes Neurotransmission to Regulate Visual Function in the Mouse Retina

The bidirectional water channel aquaporin 4 (AQP4) is abundantly expressed in the neural tissue. The advantages and disadvantages of AQP4 neural tissue deficiency under pathological conditions, such as inflammation, and relationship with neural diseases, such as Alzheimer’s disease, have been previously reported. However, the physiological functions of AQP4 are not fully understood. Here, we evaluated the role of AQP4 in the mouse retina using Aqp4 knockout (KO) mice. Aqp4 was expressed in Müller glial cells surrounding the synaptic area between photoreceptors and bipolar cells. Both scotopic and photopic electroretinograms showed hyperactive visual responses in KO mice, gradually progressing with age. Moreover, the amplitude reduction after frequent stimuli and synaptic fatigue was more severe in KO mice. Glutamine synthetase, glutamate aspartate transporter, synaptophysin, and the inward potassium channel Kir2.1, but not Kir4.1, were downregulated in KO retinas. KIR2.1 colocalized with AQP4 in Müller glial cells at the synaptic area, and its expression was affected by Aqp4 levels in primary Müller glial cell cultures. Intraocular injection of potassium in wild-type mice led to visual function hyperactivity, as observed in Aqp4 KO mice. Mitochondria molecules, such as Pgc1α and CoxIV, were downregulated, while apoptotic markers were upregulated in KO retinas. AQP4 may fine-tune synaptic activity, most likely by regulating potassium metabolism, at least in part, via collaborating with KIR2.1, and possibly indirectly regulating glutamate kinetics, to inhibit neural hyperactivity and synaptic fatigue which finally affect mitochondria and cause neurodegeneration.

How Gastrin-Releasing Peptide Opens the Spinal Gate for Itch

Spinal transmission of pruritoceptive (itch) signals requires transneuronal signaling by gastrin-releasing peptide (GRP) produced by a subpopulation of dorsal horn excitatory interneurons. These neurons also express the glutamatergic marker vGluT2, raising the question of why glutamate alone is insufficient for spinal itch relay. Using optogenetics together with slice electrophysiology and mouse behavior, we demonstrate that baseline synaptic coupling between GRP and GRP receptor (GRPR) neurons is too weak for suprathreshold excitation. Only when we mimicked the endogenous firing of GRP neurons and stimulated them repetitively to fire bursts of action potentials did GRPR neurons depolarize progressively and become excitable by GRP neurons. GRPR but not glutamate receptor antagonism prevented this action. Provoking itch-like behavior by optogenetic activation of spinal GRP neurons required similar stimulation paradigms. These results establish a spinal gating mechanism for itch that requires sustained repetitive activity of presynaptic GRP neurons and postsynaptic GRP signaling to drive GRPR neuron output.

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Spinal itch relay requires effective communication from GRP to GRP receptor neurons

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Single action potentials in GRP neurons fail to release sufficient GRP

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Only burst firing releases enough GRP to prime GRP receptor neurons for activation

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GRP acts as a volume transmitter probably explaining why itch is hard to localize

Characterization of the relaxin family peptide receptor 3 system in the mouse bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) is a critical node involved in stress and reward-related behaviors. Relaxin family peptide receptor 3 (RXFP3) signaling in the BNST has been implicated in stress-induced alcohol seeking behavior. However, the neurochemical phenotype and connectivity of BNST RXFP3-expressing (RXFP3+) cells have yet to be elucidated. We interrogated the molecular signature and electrophysiological properties of BNST RXFP3+ neurons using a RXFP3-Cre reporter mouse line. BNST RXFP3+ cells are circumscribed to the dorsal BNST (dBNST) and are neurochemically heterogeneous, comprising a mix of inhibitory and excitatory neurons. Immunohistochemistry revealed that ~48% of BNST RXFP3+ neurons are GABAergic, and a quarter of these co-express the calcium-binding protein, calbindin. A subset of BNST RXFP3+ cells (~41%) co-express CaMKIIα, suggesting this subpopulation of BNST RXFP3+ neurons are excitatory. Corroborating this, RNAscope® revealed that ~35% of BNST RXFP3+ cells express vVGluT2 mRNA, indicating a subpopulation of RXFP3+ neurons are glutamatergic. RXFP3+ neurons show direct hyperpolarization to bath application of a selective RXFP3 agonist, RXFP3-A2, while around 50% of cells were depolarised by exogenous corticotrophin releasing factor. In behaviorally naive mice the majority of RXFP3+ neurons were Type II cells exhibiting I_h and T type calcium mediated currents. However, chronic swim stress caused persistent plasticity, decreasing the proportion of neurons that express these channels. These studies are the first to characterize the BNST RXFP3 system in mouse and lay the foundation for future functional studies appraising the role of the murine BNST RXFP3 system in more complex behaviors.

Kir2.1 Channel Regulation of Glycinergic Transmission Selectively Contributes to Dynamic Mechanical Allodynia in a Mouse Model of Spared Nerve Injury

Neuropathic pain is a chronic debilitating symptom characterized by spontaneous pain and mechanical allodynia. It occurs in distinct forms, including brush-evoked dynamic and filament-evoked punctate mechanical allodynia. Potassium channel 2.1 (Kir2.1), which exhibits strong inward rectification, is and regulates the activity of lamina I projection neurons. However, the relationship between Kir2.1 channels and mechanical allodynia is still unclear. In this study, we first found that pretreatment with ML133, a selective Kir2.1 inhibitor, by intrathecal administration, preferentially inhibited dynamic, but not punctate, allodynia in mice with spared nerve injury (SNI). Intrathecal injection of low doses of strychnine, a glycine receptor inhibitor, selectively induced dynamic, but not punctate allodynia, not only in naïve but also in ML133-pretreated mice. In contrast, bicuculline, a GABA_A receptor antagonist, induced only punctate, but not dynamic, allodynia. These results indicated the involvement of glycinergic transmission in the development of dynamic allodynia. We further found that SNI significantly suppressed the frequency, but not the amplitude, of the glycinergic spontaneous inhibitory postsynaptic currents (gly-sIPSCs) in neurons on the lamina II-III border of the spinal dorsal horn, and pretreatment with ML133 prevented the SNI-induced gly-sIPSC reduction. Furthermore, 5 days after SNI, ML133, either by intrathecal administration or acute bath perfusion, and strychnine sensitively reversed the SNI-induced dynamic, but not punctate, allodynia and the gly-sIPSC reduction in lamina IIi neurons, respectively. In conclusion, our results suggest that blockade of Kir2.1 channels in the spinal dorsal horn selectively inhibits dynamic, but not punctate, mechanical allodynia by enhancing glycinergic inhibitory transmission.

Expression of Kir2.1 Inward Rectifying Potassium Channels in Optic Nerve Glia: Evidence for Heteromeric Association with Kir4.1 and Kir5.1

Inward rectifying potassium (Kir) channels comprise a large family with diverse biophysical properties. A predominant feature of central nervous system (CNS) glia is their expression of Kir4.1, which as homomers are weakly rectifying channels, but form strongly rectifying channels as heteromers with Kir2.1. However, the extent of Kir2.1 expression and their association with Kir4.1 in glia throughout the CNS is unclear. We have examined this in astrocytes and oligodendrocytes of the mouse optic nerve, a typical CNS white matter tract. Western blot and immunocytochemistry demonstrates that optic nerve astrocytes and oligodendrocytes express Kir2.1 and that it co-localises with Kir4.1. Co-immunoprecipitation analysis provided further evidence that Kir2.1 associate with Kir4.1 and, moreover, Kir2.1 expression was significantly reduced in optic nerves and brains from Kir4.1 knock-out mice. In addition, optic nerve glia express Kir5.1, which may associate with Kir2.1 to form silent channels. Immunocytochemical and co-immunoprecipitation analyses indicate that Kir2.1 associate with Kir5.1 in optic nerve glia, but not in the brain. The results provide evidence that astrocytes and oligodendrocytes may express heteromeric Kir2.1/Kir4.1 and Kir2.1/Kir5.1 channels, together with homomeric Kir2.1 and Kir4.1 channels. In astrocytes, expression of multiple Kir channels is the biophysical substrate for the uptake and redistribution of K+ released during neuronal electrical activity known as ‘potassium spatial buffering’. Our findings suggest a similar potential role for the diverse Kir channels expressed by oligodendrocytes, which by way of their myelin sheaths are intimately associated with the sites of action potential propagation and axonal K+ release.

Ion channel dysfunction in cerebellar ataxia

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

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.

Three pairs of weak interactions precisely regulate the G-loop gate of Kir2.1 channel

Kir2.1 (also known as IRK1) plays key roles in regulation of resting membrane potential and cell excitability. To achieve its physiological roles, Kir2.1 performs a series of conformational transition, named as gating. However, the structural basis of gating is still obscure. Here, we combined site-directed mutation, two-electrode voltage clamp with molecular dynamics simulations and determined that H221 regulates the gating process of Kir2.1 by involving a weak interaction network. Our data show that the H221R mutant accelerates the rundown kinetics and decelerates the reactivation kinetics of Kir2.1. Compared with the WT channel, the H221R mutation strengthens the interaction between the CD- and G-loops (E303-R221) which stabilizes the close state of the G-loop gate and weakens the interactions between C-linker and CD-loop (R221-R189) and the adjacent G-loops (E303-R312) which destabilizes the open state of G-loop gate. Our data indicate that the three pairs of interactions (E303-H221, H221-R189 and E303-R312) precisely regulate the G-loop gate by controlling the conformation of G-loop. Proteins 2016; 84:1929-1937. © 2016 Wiley Periodicals, Inc.

Membrane-associated guanylate kinase scaffolds organize a horizontal cell synaptic complex restricted to invaginating contacts with photoreceptors

Synaptic processes and plasticity of synapses are mediated by large suites of proteins. In most cases, many of these proteins are tethered together by synaptic scaffold proteins. Scaffold proteins have a large number and typically a variety of protein interaction domains that allow many different proteins to be assembled into functional complexes. Because each scaffold protein has a different set of protein interaction domains and a unique set of interacting partners, the presence of synaptic scaffolds can provide insight into the molecular mechanisms that regulate synaptic processes. In studies of rabbit retina, we found SAP102 and Chapsyn110 selectively localized in the tips of B-type horizontal cell processes, where they contact cone and rod photoreceptors. We further identified some known SAP102 binding partners, kainate receptor GluR6/7 and inward rectifier potassium channel Kir2.1, closely associated with SAP102 in photoreceptor invaginations. The kainate receptor occupies a position distinct from that of the majority of AMPA receptors that dominate the horizontal cell postsynaptic response. GluR6/7 and Kir2.1 presumably are involved in synaptic processes that govern cell-to-cell communication and could both contribute in different ways to synaptic currents that mediate feedback signaling. Notably, we failed to find evidence for the presence of Cx57 or Cx59 that might be involved in ephaptic feedback signaling in this complex. The presence of SAP102 and its binding partners in both cone and rod invaginating synapses suggests that whatever mechanism is supported by this protein complex is present in both types of photoreceptors. J. Comp. Neurol. 525:850-867, 2017. © 2016 Wiley Periodicals, Inc.

Clinical Concentrations of Local Anesthetics Bupivacaine and Lidocaine Differentially Inhibit Human Kir2.x Inward Rectifier K+ Channels

BACKGROUND

Inward rectifier K channels of the Kir2.x subfamily are widely expressed in neuronal tissues, controlling neuronal excitability. Previous studies reported that local anesthetics (LAs) do not affect Kir2 channels. However, the effects have not been studied at large concentrations used in regional anesthesia.

METHODS

This study used the patch-clamp technique to examine the effects of bupivacaine and lidocaine on Kir2.1, Kir2.2, and Kir2.3 channels expressed in human embryonic kidney 293 cells.

RESULTS

When applied extracellularly in whole-cell recordings, both LAs inhibited Kir2.x currents in a voltage-independent manner. Inhibition with bupivacaine was slow and irreversible, whereas that with lidocaine was fast and reversible. Kir2.3 displayed a greater sensitivity to bupivacaine than Kir2.1 and Kir2.2 (50% inhibitory concentrations at approximately 5 minutes, 0.6 vs 8-10 mM), whereas their sensitivities to lidocaine were similar (50% inhibitory concentrations, 1.5-2.7 mM). Increases in the charged/neutral ratio of the LAs at an acidic extracellular pH attenuated their inhibitory effects, and a permanently charged lidocaine derivative QX-314 exhibited no effects when applied extracellularly. Inside-out experiments demonstrated that inhibition of Kir2.1 with cytoplasmic lidocaine and QX-314 was rapid and reversible, whereas that induced by bupivacaine was slow and irreversible. Furthermore, dose-inhibition relations for the charged form of bupivacaine and lidocaine obtained at different cytoplasmic pHs could be approximated by a single relation for each LA.

CONCLUSIONS

The results indicate that both LAs at clinical concentrations equilibrated rapidly with the intracellular milieu, differentially inhibiting Kir2.x channel function from the cytoplasmic side.

Neuronal and glial expression of inward rectifier potassium channel subunits Kir2.x in rat dorsal root ganglion and spinal cord

Inward rectifier K(+) channels of the Kir2.x subfamily play important roles in controlling the neuronal excitability. Although their cellular localization in the brain has been extensively studied, only a few studies have examined their expression in the spinal cord and peripheral nervous system. In this study, immunohistochemical analyses of Kir2.1, Kir2.2, and Kir2.3 expression were performed in rat dorsal root ganglion (DRG) and spinal cord using bright-field and confocal microscopy. In DRG, most ganglionic neurons expressed Kir2.1, Kir2.2 and Kir2.3, whereas satellite glial cells chiefly expressed Kir2.3. In the spinal cord, Kir2.1, Kir2.2 and Kir2.3 were all expressed highly in the gray matter of dorsal and ventral horns and moderately in the white matter also. Within the gray matter, the expression was especially high in the substantia gelatinosa (lamina II). Confocal images obtained using markers for neuronal cells, NeuN, and astrocytes, Sox9, showed expression of all three Kir2 subunits in both neuronal somata and astrocytes in lamina I-III of the dorsal horn and the lateral spinal nucleus of the dorsolateral funiculus. Immunoreactive signals other than those in neuronal and glial somata were abundant in lamina I and II, which probably located mainly in nerve fibers or nerve terminals. Colocalization of Kir2.1 and 2.3 and that of Kir2.2 and 2.3 were present in neuronal and glial somata. In the ventral horn, motor neurons and interneurons were also immunoreactive with the three Kir2 subunits. Our study suggests that Kir2 channels composed of Kir2.1-2.3 subunits are expressed in neuronal and glial cells in the DRG and spinal cord, contributing to sensory transduction and motor control.

Remote and reversible inhibition of neurons and circuits by small molecule induced potassium channel stabilization

Manipulating the function of neurons and circuits that translate electrical and chemical signals into behavior represents a major challenges in neuroscience. In addition to optogenetic methods using light-activatable channels, pharmacogenetic methods with ligand induced modulation of cell signaling and excitability have been developed. However, they are largely based on ectopic expression of exogenous or chimera proteins. Now, we describe the remote and reversible expression of a Kir2.1 type potassium channel using the chemogenetic technique of small molecule induced protein stabilization. Based on shield1-mediated shedding of a destabilizing domain fused to a protein of interest and inhibition of protein degradation, this principle has been adopted for biomedicine, but not in neuroscience so far. Here, we apply this chemogenetic approach in brain research for the first time in order to control a potassium channel in a remote and reversible manner. We could show that shield1-mediated ectopic Kir2.1 stabilization induces neuronal silencing in vitro and in vivo in the mouse brain. We also validated this novel pharmacogenetic method in different neurobehavioral paradigms.The DD-Kir2.1 may complement the existing portfolio of pharmaco- and optogenetic techniques for specific neuron manipulation, but it may also provide an example for future applications of this principle in neuroscience research.

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

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

SIGNIFICANCE STATEMENT

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

A digital atlas of ion channel expression patterns in the two-week-old rat brain

The approximately 350 ion channels encoded by the mammalian genome are a main pillar of the nervous system. We have determined the expression pattern of 320 channels in the two-week-old (P14) rat brain by means of non-radioactive robotic in situ hybridization. Optimized methods were developed and implemented to generate stringently coronal brain sections. The use of standardized methods permits a direct comparison of expression patterns across the entire ion channel expression pattern data set and facilitates recognizing ion channel co-expression. All expression data are made publically available at the Genepaint.org database. Inwardly rectifying potassium channels (Kir, encoded by the Kcnj genes) regulate a broad spectrum of physiological processes. Kcnj channel expression patterns generated in the present study were fitted with a deformable subdivision mesh atlas produced for the P14 rat brain. This co-registration, when combined with numerical quantification of expression strengths, allowed for semi-quantitative automated annotation of expression patterns as well as comparisons among and between Kcnj subfamilies. The expression patterns of Kcnj channel were also cross validated against previously published expression patterns of Kcnj channel genes.

Synergistic action of dendritic mitochondria and creatine kinase maintains ATP homeostasis and actin dynamics in growing neuronal dendrites

The distribution of mitochondria within mature, differentiated neurons is clearly adapted to their regional physiological needs and can be perturbed under various pathological conditions, but the function of mitochondria in developing neurons has been less well studied. We have studied mitochondrial distribution within developing mouse cerebellar Purkinje cells and have found that active delivery of mitochondria into their dendrites is a prerequisite for proper dendritic outgrowth. Even when mitochondria in the Purkinje cell bodies are functioning normally, interrupting the transport of mitochondria into their dendrites severely disturbs dendritic growth. Additionally, we find that the growth of atrophic dendrites lacking mitochondria can be rescued by activating ATP-phosphocreatine exchange mediated by creatine kinase (CK). Conversely, inhibiting cytosolic CKs decreases dendritic ATP levels and also disrupts dendrite development. Mechanistically, this energy depletion appears to perturb normal actin dynamics and enhance the aggregation of cofilin within growing dendrites, reminiscent of what occurs in neurons overexpressing the dephosphorylated form of cofilin. These results suggest that local ATP synthesis by dendritic mitochondria and ATP-phosphocreatine exchange act synergistically to sustain the cytoskeletal dynamics necessary for dendritic development.

Gating of the kir2.1 channel at the bundle crossing region by intracellular spermine and other cations

In the Kir2.1 channel, the flow-dependent blocking effect of intracellular spermine (SPM) strongly indicates coupled movement of ions in a segment of the pore. We have shown that the bundle crossing region of M2 constitutes this critical segment of the pore. Moreover, this segment may undergo opening/closing conformational changes mimicking channel gating. In this study, we further investigate these "gating" conformational changes and relevant controlling mechanisms at this critical segment. We demonstrate that A184R mutation in the inner end of the bundle crossing region not only abolishes the inward rectifying features of SPM block but also tends to close the channel pore, which can then only be opened by intracellular (e.g., Na(+) , or equally effectively, K(+) ) but not extracellular cations. We also found that the exit (back to the intracellular milieu) of the blocking in the deep site is facilitated rather than deterred by the presence of the other SPM in the superficial site. We conclude that intracellular SPM may bind to a deep site in the pore and serve as a flow-dependent blocker. The SPM in the superficial site, on the other hand, serves both as a docking form ready for permeation to the deep site, and as a gating particle capable of opening the bundle crossing region. This inner end of the bundle crossing region of the Kir2.1 channel pore thus constitutes a pivotal segment, which, in collaboration with intracellular SPM and K(+) ions, closely couple channel gating to (inward rectifying) ion permeation.

Flow- and voltage-dependent blocking effect of ethosuximide on the inward rectifier K⁺ (Kir2.1) channel

Absence seizures are manifestations of abnormal thalamocortical oscillations characterized by spike-and-wave complexes in EEG. Ethosuximide (ETX) is one of the principal medications against absence seizures. We investigate the effect of ETX on the Kir2.1 channel, a prototypical inward rectifier K(+) channel possibly playing an important role in the setting of neuronal membrane potential. We demonstrate that the outward currents of Kir2.1 channels are significantly inhibited by intracellular ETX. We further show that the movement of neutral molecule ETX in the Kir2.1 channel is accompanied by ∼1.2 K(+), giving rise to the vivid voltage dependence of ETX unbinding rate. Moreover, the apparent affinity (K d ) of ETX in the channels are decreased by single-point mutations involving M183, E224, and S165, and especially by double mutations involving T141/S165, which always also disrupt the flux-coupling feature of ETX block. Molecular dynamics simulation demonstrates narrowing of the pore at ∼D172 by binding of ETX to S165 or T141. ETX block of the Kir2.1 channels may cause a modest but critical depolarization of the relevant neurons, decreasing available T-type Ca(2+) channels and consequently lessening pathological thalamocortical burst discharges.

Inward rectifier potassium (Kir) current in dopaminergic periglomerular neurons of the mouse olfactory bulb

Dopaminergic (DA) periglomerular (PG) neurons are critically placed at the entry of the bulbar circuitry, directly in contact with both the terminals of olfactory sensory neurons and the apical dendrites of projection neurons; they are autorhythmic and are the target of numerous terminals releasing a variety of neurotransmitters. Despite the centrality of their position, suggesting a critical role in the sensory processing, their properties -and consequently their function- remain elusive. The current mediated by inward rectifier potassium (Kir) channels in DA-PG cells was recorded by adopting the perforated-patch configuration in thin slices; IKir could be distinguished from the hyperpolarization-activated current (I h ) by showing full activation in <10 ms, no inactivation, suppression by Ba(2+) in a typical voltage-dependent manner (IC50 208 μM) and reversal potential nearly coincident with EK. Ba(2+) (2 mM) induces a large depolarization of DA-PG cells, paralleled by an increase of the input resistance, leading to a block of the spontaneous activity, but the Kir current is not an essential component of the pacemaker machinery. The Kir current is negatively modulated by intracellular cAMP, as shown by a decrease of its amplitude induced by forskolin or 8Br-cAMP. We have also tested the neuromodulatory effects of the activation of several metabotropic receptors known to be present on these cells, showing that the current can be modulated by a multiplicity of pathways, whose activation in some case increases the amplitude of the current, as can be observed with agonists of D2, muscarinic, and GABAA receptors, whereas in other cases has the opposite effect, as it can be observed with agonists of α1 noradrenergic, 5-HT and histamine receptors. These characteristics of the Kir currents provide the basis for an unexpected plasticity of DA-PG cell function, making them potentially capable to reconfigure the bulbar network to allow a better flexibility.

Molecular cloning of ion channels in Felis catus that are related to periodic paralyses in man: a contribution to the understanding of the genetic susceptibility to feline neck ventroflexion and paralysis

Neck ventroflexion in cats has different causes; however, the most common is the hypokalemia associated with flaccid paralysis secondary to chronic renal failure. In humans, the most common causes of acute flaccid paralysis are hypokalemia precipitated by thyrotoxicosis and familial forms linked to mutations in sodium, potassium, and calcium channel genes. Here, we describe the sequencing and analysis of skeletal muscle ion channels in Felis catus that could be related to periodic paralyses in humans, contributing to the understanding of the genetic susceptibility to feline neck ventroflexion and paralysis. We studied genomic DNA from eleven cats, including five animals that were hyperthyroid with hypokalemia, although only one presented with muscle weakness, and six healthy control domestic cats. We identified the ion channel ortholog genes KCNJ2, KCNJ12, KCNJ14, CACNA1S and SCN4A in the Felis catus genome, together with several polymorphic variants. Upon comparative alignment with other genomes, we found that Felis catus provides evidence for a high genomic conservation of ion channel sequences. Although we hypothesized that neck ventroflexion in cats could be associated with a thyrotoxic or familial periodic paralysis channel mutation, we did not identify any previously detected human channel mutation in the hyperthyroid cat presenting hypokalemia. However, based on the small number of affected cats in this study, we cannot yet rule out this molecular mechanism. Notwithstanding, hyperthyroidism should still be considered as a differential diagnosis in hypokalemic feline paralysis.

Intrinsic properties and neuropharmacology of midline paraventricular thalamic nucleus neurons

Neurons in the midline and intralaminar thalamic nuclei are components of an interconnected brainstem, limbic and prefrontal cortex neural network that is engaged during arousal, vigilance, motivated and addictive behaviors, and stress. To better understand the cellular mechanisms underlying these functions, here we review some of the recently characterized electrophysiological and neuropharmacological properties of neurons in the paraventricular thalamic nucleus (PVT), derived from whole cell patch clamp recordings in acute rat brain slice preparations. PVT neurons display firing patterns and ionic conductances (I_T and I_H) that exhibit significant diurnal change. Their resting membrane potential (RMP) is maintained by various ionic conductances that include inward rectifier (Kir), hyperpolarization-activated nonselective cation (HCN) and TWIK-related acid sensitive (TASK) K⁺ channels. Firing patterns are regulated by high voltage-activated (HVA) and low voltage-activated (LVA) Ca²⁺ conductances. Moreover, transient receptor potential (TRP)-like nonselective cation channels together with Ca²⁺- and Na⁺-activated K⁺ conductances (K_Ca; K_Na) contribute to unique slow afterhyperpolarizing potentials (sAHPs) that are generally not detectable in lateral thalamic or reticular thalamic nucleus neurons. The excitability of PVT neurons is also modulated by activation of neurotransmitter receptors associated with afferent pathways to PVT and other thalamic midline nuclei. We report on receptor-mediated actions of GABA, glutamate, monoamines and several neuropeptides: arginine vasopressin, gastrin-releasing peptide, thyrotropin releasing hormone and the orexins (hypocretins). This review represents an initial survey of intrinsic and transmitter-sensitive ionic conductances that are deemed to be unique to this population of midline thalamic neurons, information that is fundamental to an appreciation of the role these thalamic neurons may play in normal central nervous system (CNS) physiology and in CNS disorders that involve the dorsomedial thalamus.

K+ channel modulation causes genioglossus inhibition in REM sleep and is a strategy for reactivation

Rapid eye movement (REM) sleep is accompanied by periods of upper airway motor suppression that cause hypoventilation and obstructive apneas in susceptible individuals. A common idea has been that upper airway motor suppression in REM sleep is caused by the neurotransmitters glycine and γ-amino butyric acid (GABA) acting at pharyngeal motor pools to inhibit motoneuron activity. Data refute this as a workable explanation because blockade of this putative glycine/GABAergic mechanism releases pharyngeal motor activity in all states, and least of all in REM sleep. Here we summarize a novel motor-inhibitory mechanism that suppresses hypoglossal motor activity largely in REM sleep, this being a muscarinic receptor mechanism linked to G-protein-coupled inwardly rectifying potassium (GIRK) channels. We then outline how this discovery informs efforts to pursue therapeutic targets to reactivate hypoglossal motor activity throughout sleep via potassium channel modulation. One such target is the inwardly rectifying potassium channel Kir2.4 whose expression in the brain is almost exclusive to cranial motor nuclei.

GIRK-like and TRPC-like conductances mediate thyrotropin-releasing hormone-induced increases in excitability in thalamic paraventricular nucleus neurons

The thalamic paraventricular nucleus (PVT), reported to participate in arousal and motivated behaviors, contains abundant receptors for thyrotropin-releasing hormone (TRH), a neuropeptide also known to modulate arousal and mood. To test the hypothesis that TRH could influence the excitability of PVT neurons, whole cell patch-clamp recordings obtained in rat brain slice preparations were evaluated during bath applied TRH. In the majority of neurons tested, TRH induced reversible TTX-resistant membrane depolarization. Under voltage-clamp, TRH induced a concentration-dependent G protein- mediated inward current. The mean net TRH-induced current exhibited a decrease in membrane conductance. Further analyses identified two concurrent conductances contributing to the TRH-induced response. One conductance featured a Na(+)-independent and K(+)-dependent net current that displayed rectification and was suppressed by micromolar concentrations of Ba(2+) and two GIRK antagonists, tertiapin Q and SCH 23390. The second conductance featured a Na(+)-dependent net inward current with an I-V relationship that exhibited double rectification with a negative slope conductance below -40 mV. This conductance was suppressed by nonselective TRPC channel blockers 2-APB, flufenamic acid and ML204, enhanced by La(3+) in a subpopulation of cells, and unchanged by the TRPV1 antagonist capsazepine or a Na(+)/Ca(2+) exchanger blocker KB-R7943. TRH also enhanced hyperpolarization-activated low threshold spikes, a feature that was sensitive to pretreatment with either 2-APB or ML204. Collectively, the data imply that TRH enhances excitability in PVT neurons via concurrently decreasing a G-protein-gated inwardly rectifying K(+) conductance and activating a cationic conductance with characteristics reminiscent of TRPC-like channels, possibly involving TRPC4/C5 subunits.

Inward-rectifying potassium (Kir) channels regulate pacemaker activity in spinal nociceptive circuits during early life

Pacemaker neurons in neonatal spinal nociceptive circuits generate intrinsic burst firing and are distinguished by a lower "leak" membrane conductance compared with adjacent nonbursting neurons. However, little is known about which subtypes of leak channels regulate the level of pacemaker activity within the developing rat superficial dorsal horn (SDH). Here we demonstrate that a hallmark feature of lamina I pacemaker neurons is a reduced conductance through inward-rectifying potassium (K(ir)) channels at physiological membrane potentials. Differences in the strength of inward rectification between pacemakers and nonpacemakers indicate the presence of functionally distinct K(ir) currents in these two populations at room temperature. However, K(ir) currents in both groups showed high sensitivity to block by extracellular Ba²⁺ (IC₅₀ ~ 10 μm), which suggests the presence of "classical" K(ir) (K(ir)2.x) channels in the neonatal SDH. The reduced K(ir) conductance within pacemakers is unlikely to be explained by an absence of particular K(ir)2.x isoforms, as immunohistochemical analysis revealed the expression of K(ir)2.1, K(ir)2.2, and K(ir)2.3 within spontaneously bursting neurons. Importantly, Ba²⁺ application unmasked rhythmic burst firing in ∼42% of nonbursting lamina I neurons, suggesting that pacemaker activity is a latent property of a sizeable population of SDH cells during early life. In addition, the prevalence of spontaneous burst firing within lamina I was enhanced in the presence of high internal concentrations of free Mg²⁺, consistent with its documented ability to block K(ir) channels from the intracellular side. Collectively, the results indicate that K(ir) channels are key modulators of pacemaker activity in newborn central pain networks.

Kir2.4 surface expression and basal current are affected by heterotrimeric G-proteins

Kir2.4, a strongly rectifying potassium channel that is localized to neurons and is especially abundant in retina, was fished with yeast two-hybrid screen using a constitutively active Gαo1. Here, we wished to determine whether and how Gαo affects this channel. Using transfected HEK 293 cells and retinal tissue, we showed that Kir2.4 interacts with Gαo, and this interaction is stronger with the GDP-bound form of Gαo. Using two-electrode voltage clamp, we recorded from oocytes that were injected with Kir2.4 mRNA and a combination of G-protein subunit mRNAs. We found that the wild type and the inactive mutant of Gαo reduce the Kir2.4 basal current, whereas the active mutant has little effect. Other pertussis-sensitive Gα subunits also reduce this current, whereas Gαs increases it. Gβγ increases the current, whereas m-phosducin, which binds Gβγ without affecting the state of Gα, reduces it. We then tested the effect of G-protein subunits on the surface expression of the channel fused to cerulean by imaging the plasma membranes of the oocytes. We found that the surface expression is affected, with effects paralleling those seen with the basal current. This suggests that the observed effects on the current are mainly indirect and are due to surface expression. Similar results were obtained in transfected HEK cells. Moreover, we show that in retinal ON bipolar cells lacking Gβ3, localization of Kir2.4 in the dendritic tips is reduced. We conclude that Gβγ targets Kir2.4 to the plasma membrane, and Gαo slows this down by binding Gβγ.

Immunocytochemical localization of TASK-3 channels in rat motor neurons

Motor neurons are large cholinergic neurons located in the brain stem and spinal cord. In recent years, a functional role for TASK channels in cellular excitability and vulnerability to anesthetics of motor neurons has been described. Using a polyclonal monospecific antibody against the tandem pore domain K(+) channel (K2P channel) TWIK-related acid-sensitive K(+) channel (TASK-3), we analyzed the expression of the TASK-3 protein in motor systems of the rat CNS. Immunocytochemical staining showed strong TASK-3 expression in motor neurons of the facial, trigeminal, ambiguus, and hypoglossal nuclei. Oculomotor nuclei (including trochlear and abducens nucleus) were also strongly positive for TASK-3. The parasympathetic Edinger-Westphal nucleus and dorsal vagal nucleus showed significant, but weaker expression compared with somato- and branchiomotoric neurons. In addition, motor neurons in the anterior horn of the spinal cord were also strongly labeled for TASK-3 immunoreactivity. Based on morphological criteria, TASK-3 was found in the somatodendritic compartment of motor neurons. Cellular staining using methyl green and immunofluorescence double-labeling with anti-vesicular acetylcholine transporter (anti-vAChT) indicated ubiquitous TASK-3 expression in motor neurons, whereas in other brain regions TASK-3 showed a widespread but not ubiquitous expression. In situ hybridization using a TASK-3 specific riboprobe verified the expression of TASK-3 in motor neurons at the mRNA level.

Verification of expressions of Kir2 as potential peripheral biomarkers in lymphocytes from patients with Parkinson's disease

Prior studies have reported gene expression alterations in peripheral blood lymphocytes (PBLs) obtained from patients with Parkinson's disease (PD) as compared to healthy controls. These alterations can not only be regarded as potential biomarkers, but also enhance understanding of the pathogenic mechanism of PD. In the present study, the gene expression levels of dopamine receptor (D2, D3), inward rectified potassium channels subunits Kir2 (Kir2.1, Kir2.2, Kir2.3, Kir2.4) and ATP-sensitive potassium channel subunit Kir6.2 in PBLs were analyzed using quantitative real-time PCR among 20 PD patients with medication, 10 PD patients without medication and 16 healthy controls, respectively. The results showed that there was a significantly decrease of the D2, D3 mRNA expression in PBLs of PD patients compared with that in healthy controls. The four inward rectified potassium channels Kir2.1, Kir2.2, Kir2.3, and Kir2.4 mRNA expression in PBLs from PD patients were also significantly down-regulated than that from age-matched healthy controls. However, there was no apparent difference in expression of another potassium channel Kir6.2 mRNA between PD patients and healthy controls. We proposed that the Kir2 potassium channels mRNA on blood lymphocytes may be regarded as a potential biomarker for PD screening.

Moderate traumatic brain injury causes acute dendritic and synaptic degeneration in the hippocampal dentate gyrus

Hippocampal injury-associated learning and memory deficits are frequent hallmarks of brain trauma and are the most enduring and devastating consequences following traumatic brain injury (TBI). Several reports, including our recent paper, showed that TBI brought on by a moderate level of controlled cortical impact (CCI) induces immature newborn neuron death in the hippocampal dentate gyrus. In contrast, the majority of mature neurons are spared. Less research has been focused on these spared neurons, which may also be injured or compromised by TBI. Here we examined the dendrite morphologies, dendritic spines, and synaptic structures using a genetic approach in combination with immunohistochemistry and Golgi staining. We found that although most of the mature granular neurons were spared following TBI at a moderate level of impact, they exhibited dramatic dendritic beading and fragmentation, decreased number of dendritic branches, and a lower density of dendritic spines, particularly the mushroom-shaped mature spines. Further studies showed that the density of synapses in the molecular layer of the hippocampal dentate gyrus was significantly reduced. The electrophysiological activity of neurons was impaired as well. These results indicate that TBI not only induces cell death in immature granular neurons, it also causes significant dendritic and synaptic degeneration in pathohistology. TBI also impairs the function of the spared mature granular neurons in the hippocampal dentate gyrus. These observations point to a potential anatomic substrate to explain, in part, the development of posttraumatic memory deficits. They also indicate that dendritic damage in the hippocampal dentate gyrus may serve as a therapeutic target following TBI.

A transcriptomic analysis of type I-III neurons in the bed nucleus of the stria terminalis

The activity of neurons in the anterolateral cell group of the bed nucleus of the stria terminalis (BNST(ALG)) plays a critical role in anxiety- and stress-related behaviors. Histochemical studies have suggested that multiple distinct neuronal phenotypes exist in the BNST(ALG). Consistent with this observation, the physiological properties of BNST(ALG) neurons are also heterogeneous, and three distinct cell types can be defined (Types I-III) based primarily on their expression of four key membrane currents, namely I(h), I(A), I(T), and I(K(IR)). Significantly, all four channels are multimeric proteins and can comprise of more than one pore-forming α subunit. Hence, differential expression of α subunits may further diversify the neuronal population. However, nothing is known about the relative expression of these ion channel α subunits in BNST(ALG) neurons. We have addressed this lacuna by combining whole-cell patch-clamp recording together with single-cell reverse transcriptase polymerase chain reaction (scRT-PCR) to assess the mRNA transcript expression for each of the subunits for the four key ion channels in Type I-III neurons of the BNST(ALG.) Here, cytosolic mRNA from single neurons was probed for the expression of transcripts for each of the α subunits of I(h) (HCN1-HCN4), I(T) (Ca(v)3.1-Ca(v)3.3), I(A) (K(v)1.4, K(v)3.4, K(v)4.1-K(v) 4.3) and I(K(IR)) (Kir2.1-Kir2.4). An unbiased hierarchical cluster analysis followed by discriminant function analysis revealed that a positive correlation exists between the physiological and genetic phenotype of BNST(ALG) neurons. Thus, the analysis segregated BNST(ALG) neurons into 3 distinct groups, based on their α subunit mRNA expression profile, which positively correlated with our existing electrophysiological classification (Types I-III). Furthermore, analysis of mRNA transcript expression in Type I-Type III neurons suggested that, whereas Type I and III neurons appear to represent genetically homologous cell populations, Type II neurons may be further subdivided into three genetically distinct subgroups. These data not only validate our original classification scheme, but further refine the classification at the molecular level, and thus identifies novel targets for potential disruption and/or pharmacotherapeutic intervention in stress-related anxiety-like behaviors.

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.

Immunocytochemical localization of TASK-3 (K(2P)9.1) channels in monoaminergic and cholinergic neurons

Monoaminergic and cholinergic systems are important regulators of cortical and subcortical systems, and a variety of vegetative functions are controlled by the respective neurotransmitters. Neuronal excitability and transmitter release of these neurons are strongly regulated by their potassium conductances carried by Kir and K(2P) channels. Here we describe the generation and characterization of a polyclonal monospecific antibody against rat TASK-3, a major brain K(2P) channel. After removal of cross-reactivities and affinity purification the antibody was characterized by ELISA, immunocytochemistry of TASK-3 transfected cells, and Western blots indicating that the antibody only detects TASK-3 protein, but not its paralogs TASK-1 and TASK-5. Western blot analysis of brain membrane fractions showed a single band around 45 kD, close to the predicted molecular weight of the TASK-3 protein. In addition, specific immunolabeling using the anti-TASK-3 antibody in Western blot analysis and immunocytochemistry was blocked in a concentration dependent manner by its cognate antigen only. Immunocytochemical analysis of rat brain revealed strong expression of TASK-3 channels in serotoninergic neurons of the dorsal and median raphe, noradrenergic neurons of the locus coeruleus, histaminergic neurons of the tuberomammillary nucleus and in the cholinergic neurons of the basal nucleus of Meynert. Immunofluorescence double-labeling experiments with appropriate marker enzymes confirmed the expression of TASK-3 in cholinergic, serotoninergic, and noradrenergic neurons. In the dopaminergic system strong TASK-3 expression was found in the ventral tegmental area, whereas TASK-3 immunoreactivity in the substantia nigra compacta was only weak. All immunocytochemical results were supported by in situ hybridization using TASK-3 specific riboprobes.

A KCNJ6 (Kir3.2, GIRK2) gene polymorphism modulates opioid effects on analgesia and addiction but not on pupil size

AIM

KCNJ6 coding for potassium inwardly rectifying channels (Kir3.2, GIRK2) is important for opioid receptor transmission. The KCNJ6 rs2070995 AA genotype has been associated with increased opioid analgesic requirements in Japanese. We analyzed its consequences for other opioid effects.

METHODS

Genotyping was done in 85 methadone-substituted former heroin addicts, 352 opioid-treated chronic pain patients, and in 51 healthy volunteers where miotic effects of levomethadone had been measured. Expression of Kir3.2 in the Edinger-Westphal nucleus of rat brains was analyzed by means of immunohistochemistry.

RESULTS

Average daily methadone substitution doses during the first therapy year were larger in the AA genotype (n=4, 119.7+/-49.6 mg/day) than in other rs2070995 genotypes (77.5+/-26.2 mg/day, P=0.003) whereas AA carriers lacked opioid withdrawal symptoms. A similar tendency toward less opioid effectiveness was observed toward higher opioid dosing demands for analgesia in the AA genotype (n=17, opioid dose 2.03+/-0.45 log mg oral morphine equivalents per day, controls: 1.81+/-0.52 log mg oral morphine equivalents/day, P=0.093). In contrast, no pharmacogenetic effects were observed on miotic opioid effects. This could be traced back to the absence of Kir3.2 from the Edinger-Westphal nucleus in rat brains, a key cerebral structure governing pupil constriction.

CONCLUSION

The association of the KCNJ6 rs2070995 AA genotype with increased opioid requirements extends from analgesia to opiate substitution therapy. Opioid induced miosis is exempted for molecular histological reasons.

The mammalian K(IR)2.x inward rectifier ion channel family: expression pattern and pathophysiology

Inward rectifier currents based on K(IR)2.x subunits are regarded as essential components for establishing a stable and negative resting membrane potential in many excitable cell types. Pharmacological inhibition, null mutation in mice and dominant positive and negative mutations in patients reveal some of the important functions of these channels in their native tissues. Here we review the complex mammalian expression pattern of K(IR)2.x subunits and relate these to the outcomes of functional inhibition of the resultant channels. Correlations between expression and function in muscle and bone tissue are observed, while we recognize a discrepancy between neuronal expression and function.

Probing potassium channel function in vivo by intracellular delivery of antibodies in a rat model of retinal neurodegeneration

Inward rectifying potassium (Kir) channels participate in regulating potassium concentration (K(+)) in the central nervous system (CNS), including in the retina. We explored the contribution of Kir channels to retinal function by delivering Kir antibodies (Kir-Abs) into the rat eye in vivo to interrupt channel activity. Kir-Abs were coupled to a peptide carrier to reach intracellular epitopes. Functional effects were evaluated by recording the scotopic threshold response (STR) and photopic negative response (PhNR) of the electroretinogram (ERG) noninvasively with an electrode on the cornea to determine activity of the rod and cone pathways, respectively. Intravitreal delivery of Kir2.1-Ab coupled to the peptide carrier diminished these ERG responses equivalent to dimming the stimulus 10- to 100-fold. Immunohistochemistry (IHC) showed Kir2.1 immunostaining of retinal bipolar cells (BCs) matching the labeling pattern obtained with conventional IHC of applying Kir2.1-Ab to fixed retinal sections postmortem. Whole-cell voltage-clamp BC recordings in rat acute retinal slices showed suppression of barium-sensitive Kir2.1 currents upon inclusion of Kir2.1-Ab in the patch pipette. The in vivo functional and structural results implicate a contribution of Kir2.1 channel activity in these electronegative ERG potentials. Studies with Kir4.1-Ab administered in vivo also suppressed the ERG components and showed immunostaining of Müller cells. The strategy of administering Kir antibodies in vivo, coupled to a peptide carrier to facilitate intracellular delivery, identifies roles for Kir2.1 and Kir4.1 in ERG components arising in the proximal retina and suggests this approach could be of further value in research.

Origin of active states in local neocortical networks during slow sleep oscillation

Slow-wave sleep is characterized by spontaneous alternations of activity and silence in corticothalamic networks, but the causes of transition from silence to activity remain unknown. We investigated local mechanisms underlying initiation of activity, using simultaneous multisite field potential, multiunit recordings, and intracellular recordings from 2 to 4 nearby neurons in naturally sleeping or anesthetized cats. We demonstrate that activity may start in any neuron or recording location, with tens of milliseconds delay in other cells and sites. Typically, however, activity originated at deep locations, then involved some superficial cells, but appeared later in the middle of the cortex. Neuronal firing was also found to begin, after the onset of active states, at depths that correspond to cortical layer V. These results support the hypothesis that switch from silence to activity is mediated by spontaneous synaptic events, whereby any neuron may become active first. Due to probabilistic nature of activity onset, the large pyramidal cells from deep cortical layers, which are equipped with the most numerous synaptic inputs and large projection fields, are best suited for switching the whole network into active state.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Pregnenolone sulfate potentiates the inwardly rectifying K channel Kir2.3

Background

Neurosteroids have various physiological and neuropsychopharmacological effects. In addition to the genomic effects of steroids, some neurosteroids modulate several neurotransmitter receptors and channels, such as N-methyl-D-aspartate receptors, γ-aminobutyric acid type A (GABA_A) receptors, and σ₁ receptors, and voltage-gated Ca²⁺ and K⁺ channels. However, the molecular mechanisms underlying the various effects of neurosteroids have not yet been sufficiently clarified. In the nervous system, inwardly rectifying K⁺ (Kir) channels also play important roles in the control of resting membrane potential, cellular excitability and K⁺ homeostasis. Among constitutively active Kir2 channels in a major Kir subfamily, Kir2.3 channels are expressed predominantly in the forebrain, a brain area related to cognition, memory, emotion, and neuropsychiatric disorders.

Methodology/Principal Findings

The present study examined the effects of various neurosteroids on Kir2.3 channels using the Xenopus oocyte expression assay. In oocytes injected with Kir2.3 mRNA, only pregnenolone sulfate (PREGS), among nine neurosteroids tested, reversibly potentiated Kir2.3 currents. The potentiation effect was concentration-dependent in the micromolar range, and the current-voltage relationship showed inward rectification. However, the potentiation effect of PREGS was not observed when PREGS was applied intracellularly and was not affected by extracellular pH conditions. Furthermore, although Kir1.1, Kir2.1, Kir2.2, and Kir3 channels were insensitive to PREGS, in oocytes injected with Kir2.1/Kir2.3 or Kir2.2/Kir2.3 mRNA, but not Kir2.1/Kir2.2 mRNA, PREGS potentiated Kir currents. These potentiation properties in the concentration-response relationships were less potent than for Kir2.3 channels, suggesting action of PREGS on Kir2.3-containing Kir2 heteromeric channels.

Conclusions/Significance

The present results suggest that PREGS acts as a positive modulator of Kir2.3 channels. Kir2.3 channel potentiation may provide novel insights into the various effects of PREGS.

Upregulation of inward rectifier K+ (Kir2) channels in dentate gyrus granule cells in temporal lobe epilepsy

In humans, temporal lobe epilepsy (TLE) is often associated with Ammon's horn sclerosis (AHS) characterized by hippocampal cell death, gliosis and granule cell dispersion (GCD) in the dentate gyrus. Granule cells surviving TLE have been proposed to be hyperexcitable and to play an important role in seizure generation. However, it is unclear whether this applies to conditions of AHS. We studied granule cells using the intrahippocampal kainate injection mouse model of TLE, brain slice patch-clamp recordings, morphological reconstructions and immunocytochemistry. With progressing AHS and GCD, 'epileptic' granule cells of the injected hippocampus displayed a decreased input resistance, a decreased membrane time constant and an increased rheobase. The resting leak conductance was doubled in epileptic granule cells and roughly 70-80% of this difference were sensitive to K(+) replacement. Of the increased K(+) leak, about 50% were sensitive to 1 mm Ba(2+). Approximately 20-30% of the pathological leak was mediated by a bicuculline-sensitive GABA(A) conductance. Epileptic granule cells had strongly enlarged inwardly rectifying currents with a low micromolar Ba(2+) IC(50), reminiscent of classic inward rectifier K(+) channels (Irk/Kir2). Indeed, protein expression of Kir2 subunits (Kir2.1, Kir2.2, Kir2.3, Kir2.4) was upregulated in epileptic granule cells. Immunolabelling for two-pore weak inward rectifier K(+) channels (Twik1/K2P1.1, Twik2/K2P6.1) was also increased. We conclude that the excitability of granule cells in the sclerotic focus of TLE is reduced due to an increased resting conductance mainly due to upregulated K(+) channel expression. These results point to a local adaptive mechanism that could counterbalance hyperexcitability in epilepsy.

Cross-talk between membrane-initiated and nuclear-initiated oestrogen signalling in the hypothalamus

It is increasingly evident that 17beta-oestradiol (E(2)), via a distinct membrane oestrogen receptor (Gq-mER), can rapidly activate kinase pathways to have multiple downstream actions in central nervous system (CNS) neurones. We have found that E(2) can rapidly reduce the potency of the GABA(B) receptor agonist baclofen and mu-opioid receptor agonist DAMGO to activate G-protein-coupled, inwardly rectifying K(+) (GIRK) channels in hypothalamic neurones, thereby increasing the excitability (firing activity) of pro-opiomelanocortin (POMC) and dopamine neurones. These effects are mimicked by the membrane impermeant E(2)-BSA and a new ligand (STX) that is selective for the Gq-mER that does not bind to ERalpha or ERbeta. Both E(2) and STX are fully efficacious in attenuating the GABA(B) response in ERalpha, ERbeta and GPR 30 knockout mice in an ICI 182 780 reversible manner. These findings are further proof that E(2) signals through a unique plasma membrane ER. We have characterised the coupling of this Gq-mER to a Gq-mediated activation of phospholipase C leading to the up-regulation of protein kinase Cdelta and protein kinase A activity in these neurones, which ultimately alters gene transcription. Finally, as proof of principle, we have found that STX, similar to E(2), reduces food intake and body weight gain in ovariectomised females. STX, presumably via the Gq-mER, also regulates gene expression of a number of relevant targets including cation channels and signalling molecules that are critical for regulating (as a prime example) POMC neuronal excitability. Therefore, E(2) can activate multiple receptor-mediated pathways to modulate excitability and gene transcription in CNS neurones that are critical for controlling homeostasis and motivated behaviors.

Differential excitability and modulation of striatal medium spiny neuron dendrites

The loss of striatal dopamine (DA) in Parkinson's disease (PD) models triggers a cell-type-specific reduction in the density of dendritic spines in D(2) receptor-expressing striatopallidal medium spiny neurons (D(2) MSNs). How the intrinsic properties of MSN dendrites, where the vast majority of DA receptors are found, contribute to this adaptation is not clear. To address this question, two-photon laser scanning microscopy (2PLSM) was performed in patch-clamped mouse MSNs identified in striatal slices by expression of green fluorescent protein (eGFP) controlled by DA receptor promoters. These studies revealed that single backpropagating action potentials (bAPs) produced more reliable elevations in cytosolic Ca(2+) concentration at distal dendritic locations in D(2) MSNs than at similar locations in D(1) receptor-expressing striatonigral MSNs (D(1) MSNs). In both cell types, the dendritic Ca(2+) entry elicited by bAPs was enhanced by pharmacological blockade of Kv4, but not Kv1 K(+) channels. Local application of DA depressed dendritic bAP-evoked Ca(2+) transients, whereas application of ACh increased these Ca(2+) transients in D(2) MSNs, but not in D(1) MSNs. After DA depletion, bAP-evoked Ca(2+) transients were enhanced in distal dendrites and spines in D(2) MSNs. Together, these results suggest that normally D(2) MSN dendrites are more excitable than those of D(1) MSNs and that DA depletion exaggerates this asymmetry, potentially contributing to adaptations in PD models.