IRK(1-3) and GIRK(1-4) inwardly rectifying K+ channel mRNAs are differentially expressed in the adult rat brain

Directed midbrain and spinal cord neurogenesis from pluripotent stem cells to model development and disease in a dish

Induction of specific neuronal fates is restricted in time and space in the developing CNS through integration of extrinsic morphogen signals and intrinsic determinants. Morphogens impose regional characteristics on neural progenitors and establish distinct progenitor domains. Such domains are defined by unique expression patterns of fate determining transcription factors. These processes of neuronal fate specification can be recapitulated in vitro using pluripotent stem cells. In this review, we focus on the generation of dopamine neurons and motor neurons, which are induced at ventral positions of the neural tube through Sonic hedgehog (Shh) signaling, and defined at anteroposterior positions by fibroblast growth factor (Fgf) 8, Wnt1, and retinoic acid (RA). In vitro utilization of these morphogenic signals typically results in the generation of multiple neuronal cell types, which are defined at the intersection of these signals. If the purpose of in vitro neurogenesis is to generate one cell type only, further lineage restriction can be accomplished by forced expression of specific transcription factors in a permissive environment. Alternatively, cell-sorting strategies allow for selection of neuronal progenitors or mature neurons. However, modeling development, disease and prospective therapies in a dish could benefit from structured heterogeneity, where desired neurons are appropriately synaptically connected and thus better reflect the three-dimensional structure of that region. By modulating the extrinsic environment to direct sequential generation of neural progenitors within a domain, followed by self-organization and synaptic establishment, a reductionist model of that brain region could be created. Here we review recent advances in neuronal fate induction in vitro, with a focus on the interplay between cell intrinsic and extrinsic factors, and discuss the implications for studying development and disease in a dish.

Genetically induced dysfunctions of Kir2.1 channels: implications for short QT3 syndrome and autism-epilepsy phenotype

Short QT3 syndrome (SQT3S) is a cardiac disorder characterized by a high risk of mortality and associated with mutations in Kir2.1 (KCNJ2) channels. The molecular mechanisms leading to channel dysfunction, cardiac rhythm disturbances and neurodevelopmental disorders, potentially associated with SQT3S, remain incompletely understood. Here, we report on monozygotic twins displaying a short QT interval on electrocardiogram recordings and autism-epilepsy phenotype. Genetic screening identified a novel KCNJ2 variant in Kir2.1 that (i) enhanced the channel's surface expression and stability at the plasma membrane, (ii) reduced protein ubiquitylation and degradation, (iii) altered protein compartmentalization in lipid rafts by targeting more channels to cholesterol-poor domains and (iv) reduced interactions with caveolin 2. Importantly, our study reveals novel physiological mechanisms concerning wild-type Kir2.1 channel processing by the cell, such as binding to both caveolin 1 and 2, protein degradation through the ubiquitin-proteasome pathway; in addition, it uncovers a potential multifunctional site that controls Kir2.1 surface expression, protein half-life and partitioning to lipid rafts. The reported mechanisms emerge as crucial also for proper astrocyte function, suggesting the need for a neuropsychiatric evaluation in patients with SQT3S and offering new opportunities for disease management.

The interplay of seven subthreshold conductances controls the resting membrane potential and the oscillatory behavior of thalamocortical neurons

The signaling properties of thalamocortical (TC) neurons depend on the diversity of ion conductance mechanisms that underlie their rich membrane behavior at subthreshold potentials. Using patch-clamp recordings of TC neurons in brain slices from mice and a realistic conductance-based computational model, we characterized seven subthreshold ion currents of TC neurons and quantified their individual contributions to the total steady-state conductance at levels below tonic firing threshold. We then used the TC neuron model to show that the resting membrane potential results from the interplay of several inward and outward currents over a background provided by the potassium and sodium leak currents. The steady-state conductances of depolarizing Ih (hyperpolarization-activated cationic current), IT (low-threshold calcium current), and INaP (persistent sodium current) move the membrane potential away from the reversal potential of the leak conductances. This depolarization is counteracted in turn by the hyperpolarizing steady-state current of IA (fast transient A-type potassium current) and IKir (inwardly rectifying potassium current). Using the computational model, we have shown that single parameter variations compatible with physiological or pathological modulation promote burst firing periodicity. The balance between three amplifying variables (activation of IT, activation of INaP, and activation of IKir) and three recovering variables (inactivation of IT, activation of IA, and activation of Ih) determines the propensity, or lack thereof, of repetitive burst firing of TC neurons. We also have determined the specific roles that each of these variables have during the intrinsic oscillation.

Activation of a Gq-coupled membrane estrogen receptor rapidly attenuates α2-adrenoceptor-induced antinociception via an ERK I/II-dependent, non-genomic mechanism in the female rat

Though sex differences in pain and analgesia are known, underlying mechanisms remain elusive. This study addresses the selective contribution of membrane estrogen receptors (mERs) and mER-initiated non-genomic signaling mechanisms in our previously reported estrogen-induced attenuation of α2-adrenoceptor-mediated antinociception. By selectively targeting spinal mERs in ovariectomized female rats using β-estradiol 6-(O-carboxy-methyl)oxime bovine serum albumin (E2BSA) (membrane impermeant estradiol analog), and ERα selective agonist 4,4',4″-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT), ERβ selective agonist 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN), G-protein-coupled estrogen receptor 30 (GPR30) agonist G1 and Gq-coupled mER (Gq-mER) agonist STX, we provide strong evidence that Gq-mER activation may solely contribute to suppressing clonidine (an α2-adrenoceptor agonist)-induced antinociception, using the nociceptive tail-flick test. Increased tail-flick latencies (TFLs) by intrathecal (i.t.) clonidine were not significantly altered by i.t. PPT, DPN, or G1. In contrast, E2BSA or STX rapidly and dose-dependently attenuated clonidine-induced increase in TFL. ICI 182,780, the ER antagonist, blocked this effect. Consistent with findings with the lack of effect of ERα and ERβ agonists that modulate receptor-regulated transcription, inhibition of de novo protein synthesis using anisomycin also failed to alter the effect of E2BSA or STX, arguing against a contribution of genomic mechanisms. Immunoblotting of spinal tissue revealed that mER activation increased levels of phosphorylated extracellular signal-regulated kinase (ERK) but not of protein kinase A (PKA) or C (PKC). In vivo inhibition of ERK with U0126 blocked the effect of STX and restored clonidine antinociception. Although estrogen-induced delayed genomic mechanisms may still exist, data presented here indicate that Gq-mER may solely mediate estradiol-induced attenuation of clonidine antinociception via a rapid, reversible, and ERK-dependent, non-genomic mechanism, suggesting that Gq-mER blockade might provide improved analgesia in females.

Cloperastine rescues impairment of passive avoidance response in mice prenatally exposed to diethylstilbestrol

We previously reported that prenatal exposure to diethylstilbestrol (DES) impaired passive avoidance responses in mice. Apart from the above, we also found that cloperastine, a centrally acting antitussive, ameliorated depression-like and anxiety-like behaviors in rodents at antitussive-effective doses. In this study, we investigated whether or not cloperastine rescues impairment of passive avoidance responses in mice prenatally exposed to DES. Male DES-exposed mice were subcutaneously administered cloperastine at 10 or 30 mg/kg twice a day from 32 to 41 days after birth and subjected to behavioral testing 42 to 46 days after birth. Cloperastine at 10 and 30 mg/kg ameliorated DES-induced impairment of passive avoidance responses. In addition, cloperastine affected the levels of 5-HT1A receptors, GIRK and BDNF in the hippocampus of DES-exposed mice. However, the number of BrdU-positive cells in the hippocampus of DES-exposed mice was not changed by chronic administration of cloperastine. These findings suggest that the action of endocrine disruptors in the brain may not always be irreversible, and that the symptoms caused by endocrine disruptors might be curable with drugs such as cloperastine.

New insights into the therapeutic potential of Girk channels

G protein-dependent signaling pathways control the activity of excitable cells of the nervous system and heart, and are the targets of neurotransmitters, clinically relevant drugs, and drugs of abuse. G protein-gated inwardly rectifying potassium (K(+)) (Girk/Kir3) channels are a key effector in inhibitory signaling pathways. Girk-dependent signaling contributes to nociception and analgesia, reward-related behavior, mood, cognition, and heart-rate regulation, and has been linked to epilepsy, Down syndrome, addiction, and arrhythmias. We discuss recent advances in our understanding of Girk channel structure, organization in signaling complexes, and plasticity, as well as progress on the development of subunit-selective Girk modulators. These findings offer new hope for the selective manipulation of Girk channels to treat a variety of debilitating afflictions.

Electrophysiological investigations of synaptic connectivity between host and graft neurons

The functional synaptic integration of grafted stem cell-derived neurons is one of the key aspects of neural cell replacement therapies for neurological disorders such as Parkinson's disease. However, little is currently known about the synaptic connectivity between graft and host cells after transplantation, not only in the settings of clinical trials but also in experimental studies. This knowledge gap is primarily due to the lack of experimental electrophysiological approaches allowing interrogation of synaptic connectivity between prospectively identified host and graft neurons and hampers our understanding of the mechanisms underlying functional integration of stem cell-derived neurons in the host brain, as well as the optimization of protocols for deriving stem cells for neural cell replacement therapy. Recent optogenetic tools allow for direct investigation of connectivity between host and graft neural populations and have already been applied to show bidirectional integration of dopaminergic neurons in a host tissue. These new tools have potential to advance our understanding of functional integration in the near future. Here, we provide an overview of the current literature addressing functional integration of stem cell-derived neurons in the settings of Parkinson's disease models and discuss some experimental paradigms to approach this issue.

Repeated cocaine weakens GABA(B)-Girk signaling in layer 5/6 pyramidal neurons in the prelimbic cortex

Repeated cocaine exposure triggers adaptations in layer 5/6 glutamatergic neurons in the medial prefrontal cortex (mPFC) that promote behavioral sensitization and drug-seeking behavior. While suppression of metabotropic inhibitory signaling has been implicated in these behaviors, underlying mechanisms are unknown. Here, we show that Girk/K(IR)3 channels mediate most of the GABA(B) receptor (GABA(B)R)-dependent inhibition of layer 5/6 pyramidal neurons in the mPFC and that repeated cocaine suppresses this pathway. This adaptation was selective for GABA(B)R-dependent Girk signaling in layer 5/6 pyramidal neurons of the prelimbic cortex (PrLC) and involved a D₁/₅ dopamine receptor- and phosphorylation-dependent internalization of GABA(B)R and Girk channels. Persistent suppression of Girk signaling in layer 5/6 of the dorsal mPFC enhanced cocaine-induced locomotor activity and occluded behavioral sensitization. Thus, the cocaine-induced suppression of GABA(B)R-Girk signaling in layer 5/6 pyramidal neurons of the prelimbic cortex appears to represent an early adaptation critical for promoting addiction-related behavior.

Membrane channels as integrators of G-protein-mediated signaling

A variety of extracellular stimuli regulate cellular responses via membrane receptors. A well-known group of seven-transmembrane domain-containing proteins referred to as G protein-coupled receptors, directly couple with the intracellular GTP-binding proteins (G proteins) across cell membranes and trigger various cellular responses by regulating the activity of several enzymes as well as ion channels. Many specific populations of ion channels are directly controlled by G proteins; however, indirect modulation of some channels by G protein-dependent phosphorylation events and lipid metabolism is also observed. G protein-mediated diverse modifications affect the ion channel activities and spatio-temporally regulate membrane potentials as well as of intracellular Ca(2+) concentrations in both excitatory and non-excitatory cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

State-dependent and reflex drives to the upper airway: basic physiology with clinical implications

The root cause of the most common and serious of the sleep disorders is impairment of breathing, and a number of factors predispose a particular individual to hypoventilation during sleep. In turn, obstructive hypopneas and apneas are the most common of the sleep-related respiratory problems and are caused by dysfunction of the upper airway as a conduit for airflow. The overarching principle that underpins the full spectrum of clinical sleep-related breathing disorders is that the sleeping brain modifies respiratory muscle activity and control mechanisms and diminishes the ability to respond to respiratory distress. Depression of upper airway muscle activity and reflex responses, and suppression of arousal (i.e., "waking-up") responses to respiratory disturbance, can also occur with commonly used sedating agents (e.g., hypnotics and anesthetics). Growing evidence indicates that the sometimes critical problems of sleep and sedation-induced depression of breathing and arousal responses may be working through common brain pathways acting on common cellular mechanisms. To identify these state-dependent pathways and reflex mechanisms, as they affect the upper airway, is the focus of this paper. Major emphasis is on the synthesis of established and recent findings. In particular, we specifically focus on 1) the recently defined mechanism of genioglossus muscle inhibition in rapid-eye-movement sleep; 2) convergence of diverse neurotransmitters and signaling pathways onto one root mechanism that may explain pharyngeal motor suppression in sleep and drug-induced brain sedation; 3) the lateral reticular formation as a key hub of respiratory and reflex drives to the upper airway.

Discovery of 'molecular switches' within a GIRK activator scaffold that afford selective GIRK inhibitors

This letter describes a multi-dimensional SAR campaign based on a potent, efficacious and selective GIRK1/2 activator (~10-fold versus GIRK1/4 and inactive on nonGIRK 1-containing GIRKs, GIRK 2 or GIRK2/3). Further chemical optimization through an iterative parallel synthesis effort identified multiple 'molecular switches' that modulated the mode of pharmacology from activator to inhibitor, as well as engendering varying selectivity profiles for GIRK1/2 and GIRK1/4. Importantly, these compounds were all inactive on nonGIRK1 containing GIRK channels. However, SAR was challenging as subtle structural modifications had large effects on both mode of pharmacology and GIRK1/2 and GIRK1/4 channel selectivity.

Tipepidine activates VTA dopamine neuron via inhibiting dopamine D₂ receptor-mediated inward rectifying K⁺ current

We previously reported that the novel antidepressant-like effect of tipepidine may be produced at least partly through the activation of mesolimbic dopamine (DA) neurons via inhibiting G protein-coupled inwardly rectifying potassium (GIRK) channels. In this study, we investigated the action of tipepidine on DA D2 receptor-mediated GIRK currents (IDA(GIRK)) and membrane excitability in DA neurons using the voltage clamp and current clamp modes of the patch-clamp techniques, respectively. DA neurons were acutely dissociated from the ventral tegmental area (VTA) in rats and identified by the presence of the hyperpolarization-activated currents. Tipepidine reversibly inhibited IDA(GIRK) with IC50 7.0 μM and also abolished IDA(GIRK) irreversibly activated in the presence of intracellular GTPγS. Then tipepidine depolarized membrane potential and generated action potentials in the neurons current-clamped. Furthermore, the drug at 40 mg/kg, i.p. increased the number of cells immunopositive both for c-Fos and tyrosine hydroxylase (TH) in the VTA. These results suggest that tipepidine may activate DA neurons in VTA through the inhibition of GIRK channel-activated currents.

Association of Rgs7/Gβ5 complexes with Girk channels and GABAB receptors in hippocampal CA1 pyramidal neurons

In the hippocampus, signaling through G protein-coupled receptors is modulated by Regulators of G protein signaling (Rgs) proteins, which act to stimulate the rate of GTP hydrolysis, and consequently, G protein inactivation. The R7-Rgs subfamily selectively deactivates the G(i/o)-class of Gα subunits that mediate the action of several GPCRs. Here, we used co-immunoprecipitation, electrophysiology and immunoelectron microscopy techniques to investigate the formation of macromolecular complexes and spatial relationship of Rgs7/Gβ5 complexes and its prototypical signaling partners, the GABAB receptor and Girk channel. Co-expression of recombinant GABAB receptors and Girk channels in combination with co-immunoprecipitation experiments established that the Rgs7/Gβ5 forms complexes with GABAB receptors or Girk channels. Using electrophysiological experiments, we found that GABAB -Girk current deactivation kinetics was markedly faster in cells coexpressing Rgs7/Gβ5. At the electron microscopic level, immunolabeling for Rgs7 and Gβ5 proteins was found primarily in the dendritic layers of the hippocampus and showed similar distribution patterns. Immunoreactivity was mostly localized along the extrasynaptic plasma membrane of dendritic shafts and spines of pyramidal cells and, to a lesser extent, to that of presynaptic terminals. Quantitative analysis of immunogold particles for Rgs7 and Gβ5 revealed an enrichment of the two proteins around excitatory synapses on dendritic spines, virtually identical to that of Girk2 and GABAB1 . These data support the existence of macromolecular complexes composed of GABAB receptor-G protein-Rgs7-Girk channels in which Rgs7 and Gβ5 proteins may preferentialy modulate GABAB receptor signaling through the deactivation of Girk channels on dendritic spines. In contrast, Rgs7 and Girk2 were associated but mainly segregated from GABAB1 in dendritic shafts, where Rgs7/Gβ5 signaling complexes might modulate Girk-dependent signaling via a different metabotropic receptor(s).

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.

Discovery and SAR of a novel series of GIRK1/2 and GIRK1/4 activators

This Letter describes a novel series of GIRK activators identified through an HTS campaign. The HTS lead was a potent and efficacious dual GIRK1/2 and GIRK1/4 activator. Further chemical optimization through both iterative parallel synthesis and fragment library efforts identified dual GIRK1/2 and GIRK1/4 activators as well as the first examples of selective GIRK1/4 activators. Importantly, these compounds were inactive on GIRK2 and other non-GIRK1 containing GIRK channels, and SAR proved shallow.

The K(+) channel GIRK2 is both necessary and sufficient for peripheral opioid-mediated analgesia

The use of opioid agonists acting outside the central nervous system (CNS) is a promising therapeutic strategy for pain control that avoids deleterious central side effects such as apnea and addiction. In human clinical trials and rat models of inflammatory pain, peripherally restricted opioids have repeatedly shown powerful analgesic effects; in some mouse models however, their actions remain unclear. Here, we investigated opioid receptor coupling to K⁺ channels as a mechanism to explain such discrepancies. We found that GIRK channels, major effectors for opioid signalling in the CNS, are absent from mouse peripheral sensory neurons but present in human and rat. In vivo transgenic expression of GIRK channels in mouse nociceptors established peripheral opioid signalling and local analgesia. We further identified a regulatory element in the rat GIRK2 gene that accounts for differential expression in rodents. Thus, GIRK channels are indispensable for peripheral opioid analgesia, and their absence in mice has profound consequences for GPCR signalling in peripheral sensory neurons.

GIRK channels are indispensable for peripheral opioid analgesia. The absence of GIRK channels from mouse dorsal root ganglion neurons questions the predictive validity of mice as a model organism for investigating peripheral GPCRmediated analgesia.

GIRK2 expression in dopamine neurons of the substantia nigra and ventral tegmental area

G-protein-regulated inward-rectifier potassium channel 2 (GIRK2) is reported to be expressed only within certain dopamine neurons of the substantia nigra (SN), although very limited data are available in humans. We examined the localization of GIRK2 in the SN and adjacent ventral tegmental area (VTA) of humans and mice by using either neuromelanin pigment or immunolabeling with tyrosine hydroxylase (TH) or calbindin. GIRK2 immunoreactivity was found in nearly every human pigmented neuron or mouse TH-immunoreactive neuron in both the SN and VTA, although considerable variability in the intensity of GIRK2 staining was observed. The relative intensity of GIRK2 immunoreactivity in TH-immunoreactive neurons was determined; in both species nearly all SN TH-immunoreactive neurons had strong GIRK2 immunoreactivity compared with only 50-60% of VTA neurons. Most paranigral VTA neurons also contained calbindin immunoreactivity, and approximately 25% of these and nearby VTA neurons also had strong GIRK2 immunoreactivity. These data show that high amounts of GIRK2 protein are found in most SN neurons as well as in a proportion of nearby VTA neurons. The single previous human study may have been compromised by the fixation method used and the postmortem delay of their controls, whereas other studies suggesting that GIRK2 is located only in limited neuronal groups within the SN have erroneously included VTA regions as part of the SN. In particular, the dorsal layer of dopamine neurons directly underneath the red nucleus is considered a VTA region in humans but is commonly considered the dorsal tier of the SN in laboratory species.

Muscarinic depolarization of layer II neurons of the parasubiculum

The parasubiculum (PaS) is a component of the hippocampal formation that sends its major output to layer II of the entorhinal cortex. The PaS receives strong cholinergic innervation from the basal forebrain that is likely to modulate neuronal excitability and contribute to theta-frequency network activity. The present study used whole cell current- and voltage-clamp recordings to determine the effects of cholinergic receptor activation on layer II PaS neurons. Bath application of carbachol (CCh; 10–50 µM) resulted in a dose-dependent depolarization of morphologically-identified layer II stellate and pyramidal cells that was not prevented by blockade of excitatory and inhibitory synaptic inputs. Bath application of the M₁ receptor antagonist pirenzepine (1 µM), but not the M₂-preferring antagonist methoctramine (1 µM), blocked the depolarization, suggesting that it is dependent on M₁ receptors. Voltage-clamp experiments using ramped voltage commands showed that CCh resulted in the gradual development of an inward current that was partially blocked by concurrent application of the selective Kv7.2/3 channel antagonist XE-991, which inhibits the muscarine-dependent K⁺ current I_M. The remaining inward current also reversed near E_K and was inhibited by the K⁺ channel blocker Ba²⁺, suggesting that M₁ receptor activation attenuates both I_M as well as an additional K⁺ current. The additional K⁺ current showed rectification at depolarized voltages, similar to K⁺ conductances mediated by Kir 2.3 channels. The cholinergic depolarization of layer II PaS neurons therefore appears to occur through M₁-mediated effects on I_M as well as an additional K⁺ conductance.

Cell-type specific inactivation of hippocampal CA1 disrupts location-dependent object recognition in the mouse

The allatostatin receptor (AlstR)/ligand inactivation system enables potent regulation of neuronal circuit activity. To examine how different cell types participate in memory formation, we have used this system through Cre-directed, cell-type specific expression in mouse hippocampal CA1 in vivo and examined functional effects of inactivation of excitatory vs. inhibitory neurons on memory formation. We chose to use a hippocampus-dependent behavioral task involving location-dependent object recognition (LOR). The double transgenic mice, with the AlstRs selectively expressed in excitatory pyramidal neurons or inhibitory interneurons, were cannulated, targeting dorsal hippocampus to allow the infusion of the receptor ligand (the allatostatin [AL] peptide) in a time dependent manner. Compared to control animals, AL-infused animals showed no long-term memory for object location. While inactivation of excitatory or inhibitory neurons produced opposite effects on hippocampal circuit activity in vitro, the effects in vivo were similar. Both types of inactivation experiments resulted in mice exhibiting no long-term memory for object location. Together, these results demonstrate that the Cre-directed, AlstR-based system is a powerful tool for cell-type specific manipulations in a behaving animal and suggest that activity of either excitatory neurons or inhibitory interneurons is essential for proper long-term object location memory formation.

Structural elements in the Girk1 subunit that potentiate G protein-gated potassium channel activity

G protein-gated inwardly rectifying K(+) (Girk/K(IR)3) channels mediate the inhibitory effect of many neurotransmitters on excitable cells. Girk channels are tetramers consisting of various combinations of four mammalian Girk subunits (Girk1 to -4). Although Girk1 is unable to form functional homomeric channels, its presence in cardiac and neuronal channel complexes correlates with robust channel activity. This study sought to better understand the potentiating influence of Girk1, using the GABA(B) receptor and Girk1/Girk2 heteromer as a model system. Girk1 did not increase the protein levels or alter the trafficking of Girk2-containing channels to the cell surface in transfected cells or hippocampal neurons, indicating that its potentiating influence involves enhancement of channel activity. Structural elements in both the distal carboxyl-terminal domain and channel core were identified as key determinants of robust channel activity. In the distal carboxyl-terminal domain, residue Q404 was identified as a key determinant of receptor-induced channel activity. In the Girk1 core, three unique residues in the pore (P) loop (F137, A142, Y150) were identified as a collective potentiating influence on both receptor-dependent and receptor-independent channel activity, exerting their influence, at least in part, by enhancing mean open time and single-channel conductance. Interestingly, the potentiating influence of the Girk1 P-loop is tempered by residue F162 in the second membrane-spanning domain. Thus, discontinuous and sometime opposing elements in Girk1 underlie the Girk1-dependent potentiation of receptor-dependent and receptor-independent heteromeric channel activity.

Multiple pharmacological actions of centrally acting antitussives--Do they target G protein-coupled inwardly rectifying K⁺ (GIRK) channels?

Antitussive drugs have been used for decades and their modes of action are well elucidated. However, recent studies on the mechanism of their antitussive action seem to be opening a new way for discovery or development of novel drugs for intractable brain diseases including psychiatric disorders. Antitussives inhibit the currents caused by activation of G protein-coupled inwardly rectifying K⁺ (GIRK) channels in neurons. In our own studies carried out so far, we found that antitussives possessing an inhibitory action on GIRK channels, similar to the effects of an enriched environment, ameliorate symptoms of intractable brain diseases in various animal models. In this review, the multiple pharmacological actions of the antitussives are described, and their mechanisms are discussed addressing GIRK channels as a possible molecular target.

A tandem Di-hydrophobic motif mediates clathrin-dependent endocytosis via direct binding to the AP-2 ασ2 subunits

Select plasma membrane proteins can be marked as cargo for inclusion into clathrin-coated pits by common internalization signals (e.g. YXXΦ, dileucine motifs, NPXY) that serve as universal recognition sites for the AP-2 adaptor complex or other clathrin-associated sorting proteins. However, some surface proteins, such as the Kir2.3 potassium channel, lack canonical signals but are still targeted for clathrin-dependent endocytosis. Here, we explore the mechanism. We found an unusual endocytic signal in Kir2.3 that is based on two consecutive pairs of hydrophobic residues. Characterized by the sequence ΦΦXΦΦ (a tandem di-hydrophobic (TDH) motif, where Φ is a hydrophobic amino acid), the signal shows no resemblance to other endocytic motifs, yet it directly interacts with AP-2 to target the Kir2.3 potassium channel into the endocytic pathway. We found that the tandem di-hydrophobic motif directly binds to the ασ2 subunits of AP-2, interacting within a large hydrophobic cleft that encompasses part of the docking site for di-Leu signals, but includes additional structures. These observations expand the repertoire of clathrin-dependent internalization signals and the ways in which AP-2 can coordinate endocytosis of cargo proteins.

Towards therapeutic applications of arthropod venom k(+)-channel blockers in CNS neurologic diseases involving memory acquisition and storage

Potassium channels are the most heterogeneous and widely distributed group of ion channels and play important functions in all cells, in both normal and pathological mechanisms, including learning and memory processes. Being fundamental for many diverse physiological processes, K⁺-channels are recognized as potential therapeutic targets in the treatment of several Central Nervous System (CNS) diseases, such as multiple sclerosis, Parkinson's and Alzheimer's diseases, schizophrenia, HIV-1-associated dementia, and epilepsy. Blockers of these channels are therefore potential candidates for the symptomatic treatment of these neuropathies, through their neurological effects. Venomous animals have evolved a wide set of toxins for prey capture and defense. These compounds, mainly peptides, act on various pharmacological targets, making them an innumerable source of ligands for answering experimental paradigms, as well as for therapeutic application. This paper provides an overview of CNS K⁺-channels involved in memory acquisition and storage and aims at evaluating the use of highly selective K⁺-channel blockers derived from arthropod venoms as potential therapeutic agents for CNS diseases involving learning and memory mechanisms.

The mu opioid receptor modulates neurotransmission in the rat spinal ventral horn

BACKGROUND

Opioids inhibit excitatory neurotransmission and produce antinociception through μ opioid receptors (MORs). Although MORs are expressed in the spinal ventral horn, their functions and effects are largely unknown. Therefore, we examined the neuromodulatory effects of μ opioids in spinal lamina IX neurons at the cellular level.

METHODS

The effects of the selective μ agonist [D-Ala(2),-N-Me-Phe(4), Gly(5)-ol]enkephalin (DAMGO) on synaptic transmission were examined in spinal lamina IX neurons of neonatal rats using the whole-cell patch-clamp technique.

RESULTS

DAMGO produced outward currents in 56% of the lamina IX neurons recorded, with a 50% effective concentration of 0.1 μM. Analysis of the current-voltage relationship revealed a reversal potential of approximately -86 mV. These currents were not blocked by tetrodotoxin but were inhibited by Ba(2+) or a selective μ antagonist. Moreover, the currents were suppressed by the addition of Cs(+) and tetraethylammonium or guanosine 5'-[β-thio]diphosphate trilithium salt to the pipette solution. In addition, DAMGO decreased the frequency of spontaneous excitatory and inhibitory postsynaptic currents, and these effects were unaltered by treatment with tetrodotoxin.

CONCLUSION

Our results suggest that DAMGO hyperpolarizes spinal lamina IX neurons by G protein-mediated activation of K(+) channels after activation of MORs. Furthermore, activation of MORs on presynaptic terminals reduces both excitatory and inhibitory transmitter release. Although traditionally opioids are not thought to affect motor function, the present study documents neuromodulatory effects of μ opioids in spinal lamina IX neurons, suggesting that MORs can influence motor activity.

G-protein-gated inwardly rectifying K+ channel 4 (GIRK4) immunoreactivity in chemically defined neurons of the hypothalamic arcuate nucleus that control body weight

G-protein-gated inwardly rectifying K(+) channels (GIRKs; also called Kir3) are a family of K(+) channels, which are activated (opened) via a signal transduction cascade starting with ligand-stimulated G-protein-coupled receptors (GPCRs). Four GIRK genes have been identified (GIRK1-4). GIRK4 (Kir3.4) has a role in regulating energy homeostasis, since mice with a targeted mutation in the GIRK4 gene exhibit a predisposition to late-onset obesity. GIRK4 mRNA is expressed in hypothalamic regions that harbor neurons involved in the regulation of food intake and body weight. Using goat and rabbit antisera to the GIRK4 protein, the cellular localization and transmitter content of GIRK4-immunoreactive neurons was determined in the hypothalamic arcuate nucleus, a region that contains neurons which are accessible to circulating hormones and is intimately associated with the control of body weight. GIRK4-immunoreactive large cell bodies were demonstrated in the ventrolateral part of the arcuate nucleus, with smaller neuronal cell bodies in the ventromedial part of the nucleus. Double-labeling showed presence of GIRK4 immunoreactivity in large neurons of the ventrolateral arcuate nucleus containing the peptides α-melanocyte-stimulating hormone (α-MSH), a marker for pro-opiomelanocortin (POMC) neurons, and cocaine- and amphetamine-regulated transcript (CART). GIRK4 immunoreactivity was also seen in neurons of the ventromedial part of the arcuate nucleus containing agouti-regulated peptide (AgRP) and neuropeptide Y (NPY). The results suggest that the GIRK4 channel protein plays a role in regulating membrane excitability in chemically defined neurons of the arcuate nucleus that control body weight.

RGS proteins maintain robustness of GPCR-GIRK coupling by selective stimulation of the G protein subunit Gαo

Termination of heterotrimeric guanine nucleotide-binding protein (G protein) signaling downstream of activated G protein-coupled receptors (GPCRs) is accelerated by regulator of G protein signaling (RGS) proteins, which act as guanosine triphosphatase (GTPase)-activating proteins (GAPs). Using a Xenopus oocyte expression system, we found that although RGS proteins had a negative effect of accelerating the kinetics of GPCR-coupled potassium ion (K+) channel (GIRK) deactivation, they also had positive effects of increasing the amplitudes and activation kinetics of neurotransmitter-evoked GIRK currents. The RGS box domain alone was sufficient to stimulate neurotransmitter-dependent activation of GIRK currents. Moreover, RGS4 mutants with compromised GAP activity augmented GPCR-GIRK coupling (as assessed by measurement of the GIRK current elicited by neurotransmitter). By accelerating G protein activation kinetics, RGS4 specifically stimulated Gα₀, which stimulated GPCR-GIRK coupling despite its GAP activity. Opposing actions of RGS proteins thus both stimulate and inhibit G proteins to modulate the amplitude and kinetics of neurotransmitter-induced GIRK currents, thereby distinguishing the responses to activation of different G protein isoforms.

Receptor signaling and the cell biology of synaptic transmission

This volume describes a series of psychiatric and neuropsychiatric disorders, connects some aspects of somatic and psychiatric medicine, and describes various current and emerging therapies. The purpose of this chapter is to set the stage for the volume by developing the theoretical basis of synaptic transmission and introducing the various neurotransmitters and their receptors involved in the process. The intent is to provide not only a historical context through which to understand neurotransmitters, but a current contextual basis for understanding neuronal signal transduction and applying this knowledge to facilitate treatment of maladies of the brain and mind.

Inhibition of G protein-activated inwardly rectifying K+ channels by different classes of antidepressants

Various antidepressants are commonly used for the treatment of depression and several other neuropsychiatric disorders. In addition to their primary effects on serotonergic or noradrenergic neurotransmitter systems, antidepressants have been shown to interact with several receptors and ion channels. However, the molecular mechanisms that underlie the effects of antidepressants have not yet been sufficiently clarified. G protein-activated inwardly rectifying K⁺ (GIRK, Kir3) channels play an important role in regulating neuronal excitability and heart rate, and GIRK channel modulation has been suggested to have therapeutic potential for several neuropsychiatric disorders and cardiac arrhythmias. In the present study, we investigated the effects of various classes of antidepressants on GIRK channels using the Xenopus oocyte expression assay. In oocytes injected with mRNA for GIRK1/GIRK2 or GIRK1/GIRK4 subunits, extracellular application of sertraline, duloxetine, and amoxapine effectively reduced GIRK currents, whereas nefazodone, venlafaxine, mianserin, and mirtazapine weakly inhibited GIRK currents even at toxic levels. The inhibitory effects were concentration-dependent, with various degrees of potency and effectiveness. Furthermore, the effects of sertraline were voltage-independent and time-independent during each voltage pulse, whereas the effects of duloxetine were voltage-dependent with weaker inhibition with negative membrane potentials and time-dependent with a gradual decrease in each voltage pulse. However, Kir2.1 channels were insensitive to all of the drugs. Moreover, the GIRK currents induced by ethanol were inhibited by sertraline but not by intracellularly applied sertraline. The present results suggest that GIRK channel inhibition may reveal a novel characteristic of the commonly used antidepressants, particularly sertraline, and contributes to some of the therapeutic effects and adverse effects.

Developmental regulation of G protein-gated inwardly-rectifying K+ (GIRK/Kir3) channel subunits in the brain

G protein-gated inwardly-rectifying K(+) (GIRK/family 3 of inwardly-rectifying K(+) ) channels are coupled to neurotransmitter action and can play important roles in modulating neuronal excitability. We investigated the temporal and spatial expression of GIRK1, GIRK2 and GIRK3 subunits in the developing and adult brain of mice and rats using biochemical, immunohistochemical and immunoelectron microscopic techniques. At all ages analysed, the overall distribution patterns of GIRK1-3 were very similar, with high expression levels in the neocortex, cerebellum, hippocampus and thalamus. Focusing on the hippocampus, histoblotting and immunohistochemistry showed that GIRK1-3 protein levels increased with age, and this was accompanied by a shift in the subcellular localization of the subunits. Early in development (postnatal day 5), GIRK subunits were predominantly localized to the endoplasmic reticulum in the pyramidal cells, but by postnatal day 60 they were mostly found along the plasma membrane. During development, GIRK1 and GIRK2 were found primarily at postsynaptic sites, whereas GIRK3 was predominantly detected at presynaptic sites. In addition, GIRK1 and GIRK2 expression on the spine plasma membrane showed identical proximal-to-distal gradients that differed from GIRK3 distribution. Furthermore, although GIRK1 was never found within the postsynaptic density (PSD), the level of GIRK2 in the PSD progressively increased and GIRK3 did not change in the PSD during development. Together, these findings shed new light on the developmental regulation and subcellular diversity of neuronal GIRK channels, and support the contention that distinct subpopulations of GIRK channels exert separable influences on neuronal excitability. The ability to selectively target specific subpopulations of GIRK channels may prove effective in the treatment of disorders of excitability.

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.

Estrogen regulation of the dopamine-activated GIRK channel in pituitary lactotrophs: implications for regulation of prolactin release during the estrous cycle

Prolactin (PRL), synthesized and secreted from lactotrophs of the anterior pituitary gland, is tonically inhibited by hypothalamic dopamine (DA) throughout the female reproductive (estrous) cycle. Our laboratory has shown that DA hyperpolarizes these cells by activating G protein-coupled inwardly rectifying K(+) (GIRK) channels; however, this response is only observed on proestrus. While the cellular mechanisms that allow for functional expression of this unique DA-signaling pathway are unclear, we hypothesized that activation of the DA-GIRK effector pathway is due to the rise in circulating estrogen (E₂) during the preceding day of diestrus. Thus, we examined the effects of E₂ on primary lactotrophs isolated from female rats. Treatment with a physiological concentration of E₂ (40-80 pg/ml, in vivo or in vitro) induced a proestrous phenotype in diestrous lactotrophs. These cells exhibited a DA-induced membrane hyperpolarization, as well as a secretory rebound of PRL following DA withdrawal (characteristic of proestrous cells). Internal dialysis of GTPγS demonstrated that E₂ exposure enabled functional expression of GIRK channels, and this regulation by E₂ did not involve the D₂R. The effect of E₂ was blocked by the receptor antagonist, ICI 182,780, and by the protein synthesis inhibitor, cycloheximide. Single-cell analysis revealed increased mRNA expression of GIRK channel subunits in E₂-treated lactotrophs. While E₂ is known to have multiple actions on the lactotroph, the present findings illuminate a novel action of E₂ in lactotrophs-regulation of the expression of a DA effector, the GIRK channel.

GABAB receptor-mediated tonic inhibition of noradrenergic A7 neurons in the rat

Noradrenergic (NAergic) A7 neurons that project axonal terminals to the dorsal horn of the spinal cord to modulate nociceptive signaling are suggested to receive tonic inhibition from local GABAergic interneurons, which are under the regulation of descending analgesic pathways. In support of this argument, we presently report GABA(B) receptor (GABA(B)R)-mediated tonic inhibition of NAergic A7 neurons. Bath application of baclofen induced an outward current (I(Bac)) in NAergic A7 neurons that was blocked by CGP 54626, a GABA(B)R blocker. The I(Bac) was reversed at about -99 mV, displayed inward rectification, and was blocked by Ba(2+) or Tertipian-Q, showing it was mediated by G protein-activated inward-rectifying K(+) (GIRK) channels. Single-cell RT-PCR results suggested that GIRK1/3 heterotetramers might dominate functional GIRK channels in NAergic A7 neurons. Under conditions in which GABA(A) and glycine receptors were blocked, bath application of GABA inhibited the spontaneous firing of NAergic A7 neurons in a dose-dependent manner. Interestingly, CGP 54626 application not only blocked the effect of GABA but also increased the firing rate to 126.9% of the control level, showing that GABA(B)Rs were constitutively active at an ambient GABA concentration of 2.8 μM and inhibited NAergic A7 neurons. GABA(B)Rs were also found at presynaptic excitatory and inhibitory axonal terminals in the A7 area. Pharmacological activation of these GABA(B)Rs inhibited the release of neurotransmitters. No physiological role was found for GABA(B)Rs on excitatory terminals, whereas those on the inhibitory terminals were found to exert autoregulatory control of GABA release.

Infraslow (<0.1 Hz) oscillations in thalamic relay nuclei basic mechanisms and significance to health and disease states

In the absence of external stimuli, the mammalian brain continues to display a rich variety of spontaneous activity. Such activity is often highly stereotypical, is invariably rhythmic, and can occur with periodicities ranging from a few milliseconds to several minutes. Recently, there has been a particular resurgence of interest in fluctuations in brain activity occurring at < 0.1 Hz, commonly referred to as very slow or infraslow oscillations (ISOs). Whilst this is primarily due to the emergence of functional magnetic resonance imaging (fMRI) as a technique which has revolutionized the study of human brain dynamics, it is also a consequence of the application of full band electroencephalography (fbEEG). Despite these technical advances, the precise mechanisms which lead to ISOs in the brain remain unclear. In a host of animal studies, one brain region that consistently shows oscillations at < 0.1 Hz is the thalamus. Importantly, similar oscillations can also be observed in slices of isolated thalamic relay nuclei maintained in vitro. Here, we discuss the nature and mechanisms of these oscillations, paying particular attention to a potential role for astrocytes in their genesis. We also highlight the relationship between this activity and ongoing local network oscillations in the alpha (α; ~8-13 Hz) band, drawing clear parallels with observations made in vivo. Last, we consider the relevance of these thalamic ISOs to the pathological activity that occurs in certain types of epilepsy.

Epistatic interaction of CREB1 and KCNJ6 on rumination and negative emotionality

G protein-activated K+ channel 2 (GIRK2) and cAMP-response element binding protein (CREB1) are involved in synaptic plasticity and their genes have been implicated depression and memory processing. Excessive rumination is a core cognitive feature of depression which is also present in remission. High scores on the Ruminative Response Scale (RRS) questionnaire are predictive of relapse and recurrence. Since rumination involves memory, we tested the hypothesis that variation in the genes encoding GIRK2 (KCNJ6) and CREB1 mechanisms would influence RRS scores. GIRK2 and CREB1 polymorphisms were studied in two independent samples (n=651 and n=1174) from the general population. Strongly significant interaction between the TT genotype of rs2070995 (located in KCNJ6) and the GG genotype of rs2253206 (located in CREB1) on RRS were found in both samples. These results were validated in an independent third sample (n=565; individuals with personality disorders) showing significant main effect of the variants mentioned as well as significant interaction on a categorical diagnosis of Cluster C personality disorder (obsessional-compulsive, avoidant and dependent) in which rumination is a prominent feature. Our results suggest that genetic epistasis in post-receptor signaling pathways in memory systems may have relevance for depression and its treatment.

Involvement of inward rectifier and M-type currents in carbachol-induced epileptiform synchronization

Exposure to cholinergic agonists is a widely used paradigm to induce epileptogenesis in vivo and synchronous activity in brain slices maintained in vitro. However, the mechanisms underlying these effects remain unclear. Here, we used field potential recordings from the lateral entorhinal cortex in horizontal rat brain slices to explore whether two different K(+) currents regulated by muscarinic receptor activation, the inward rectifier (K(IR)) and the M-type (K(M)) currents, have a role in carbachol (CCh)-induced field activity, a prototypical model of cholinergic-dependent epileptiform synchronization. To establish whether K(IR) or K(M) blockade could replicate CCh effects, we exposed slices to blockers of these currents in the absence of CCh. K(IR) channel blockade with micromolar Ba(2+) concentrations induced interictal-like events with duration and frequency that were lower than those observed with CCh; by contrast, the K(M) blocker linopirdine was ineffective. Pre-treatment with Ba(2+) or linopirdine increased the duration of epileptiform discharges induced by subsequent application of CCh. Baclofen, a GABA(B) receptor agonist that activates K(IR), abolished CCh-induced field oscillations, an effect that was abrogated by the GABA(B) receptor antagonist CGP 55845, and prevented by Ba(2+). Finally, when applied after CCh, the K(M) activators flupirtine and retigabine shifted leftward the cumulative distribution of CCh-induced event duration; this effect was opposite to what seen during linopirdine application under similar experimental conditions. Overall, our findings suggest that K(IR) rather than K(M) plays a major regulatory role in controlling CCh-induced epileptiform synchronization.

Evidence for oligomerization between GABAB receptors and GIRK channels containing the GIRK1 and GIRK3 subunits

The stimulation of inhibitory neurotransmitter receptors, such as γ-aminobutyric acid type B (GABA(B) ) receptors, activates G protein-gated inwardly-rectifying K(+) (GIRK) channels, which influence membrane excitability. There is now evidence suggesting that G protein-coupled receptors and G protein-gated inwardly-rectifying K(+) [GIRK/family 3 of inwardly-rectifying K(+) (Kir3)] channels do not diffuse freely within the plasma membrane, but instead there are direct protein-protein interactions between them. Here, we used bioluminescence resonance energy transfer, co-immunoprecipitation, confocal and electron microscopy techniques to investigate the oligomerization of GABA(B) receptors with GIRK channels containing the GIRK3 subunit, whose contribution to functional channels is still unresolved. Co-expression of GABA(B) receptors and GIRK channels in human embryonic kidney-293 cells in combination with co-immunoprecipitation experiments established that the metabotropic receptor forms stable complexes with GIRK channels. Using bioluminescence resonance energy transfer, we have shown that, in living cells under physiological conditions, GABA(B) receptors interact directly with GIRK1/GIRK3 heterotetramers. In addition, we have provided evidence that the receptor-effector complexes are also found in vivo and identified that the cerebellar granule cells are one neuron population where the interaction probably takes place. Altogether, our data show that signalling complexes containing GABA(B) receptors and GIRK channels are formed shortly after biosynthesis, probably in the endoplasmic reticulum and/or endoplasmic reticulum/Golgi apparatus complex, suggesting that this might be a general feature of receptor-effector ion channel signal transduction and supporting a channel-forming role for the GIRK3 subunit.

Using 'omics' to define pathogenesis and biomarkers of Parkinson's disease

Although great effort has been put forth to uncover the complex molecular mechanisms exploited in the pathogenesis of Parkinson's disease, a satisfactory explanation remains to be discovered. The emergence of several -omics techniques, transcriptomics, proteomics and metabolomics, have been integral in confirming previously identified pathways that are associated with dopaminergic neurodegeneration and subsequently Parkinson's disease, including mitochondrial and proteasomal function and synaptic neurotransmission. Additionally, these unbiased techniques, particularly in the brain regions uniquely associated with the disease, have greatly enhanced our ability to identify novel pathways, such as axon-guidance, that are potentially involved in Parkinson's pathogenesis. A comprehensive appraisal of the results obtained by different -omics has also reconfirmed the increase in oxidative stress as a common pathway likely to be critical in Parkinson's development/progression. It is hoped that further integration of these techniques will yield a more comprehensive understanding of Parkinson's disease etiology and the biological pathways that mediate neurodegeneration.

The potent inhibitory effect of tipepidine on marble-burying behavior in mice

Our previous study revealed that centrally acting non-narcotic antitussives inhibited G-protein-coupled inwardly rectifying K(+) (GIRK) channel currents in brain neurons, and that the tipepidine antitussives had a novel antidepressive-like effect on rats. Furthermore, the antitussives revealed multiplexed ameliorating actions on intractable brain disease models. This study evaluated the therapeutic potential of tipepidine in obsessive-compulsive disorder (OCD) subjects using marble-burying behavior (MBB) tests in mice. In fact, OCD is classified as an anxiety disorder characterized by obsession or compulsion. Although selective 5-HT reuptake inhibitors (SSRIs) are considered first choice agents for the pharmacological treatment of OCD, 50% of patients with OCD failed to respond to SSRIs. The burying of harmless objects such as marbles by mice might reflect the formation of compulsive behavior. The results show that tipepidine reduced MBB in a dose-dependent manner. The effect of tipepidine was significant even at a dosage as small as 5 mg/kg. The tipepidine at 10 mg/kg s.c. nearly abolished MBB without reducing the locomotor activity in mice. It is particularly interesting that the dopamine D₂ antagonist or 5-HT(1A) antagonist partly inhibited the effect of tipepidine on MBB. The results suggest that tipepidine has more of a potent inhibitory effect on MBB, compared with known drugs used for the treatment of OCD, and that the tipepidine action mechanism might differ from that of known drugs.

Substance P Modulation of Hypoglossal Motoneuron Excitability During Development: Changing Balance Between Conductances

Although Substance P (SP) acts primarily through neurokinin 1 (NK1) receptors to increase the excitability of virtually all motoneurons (MNs) tested, the ontogeny of this transmitter system is not known for any MN pool. Hypoglossal (XII) MNs innervate tongue protruder muscles and participate in several behaviors that must be functional from birth including swallowing, suckling and breathing. We used immunohistochemistry, Western immunoblotting, and whole cell recording of XII MNs in brain stem slices from rats ranging in age from postnatal day zero (P0) to P23 to explore developmental changes in: NK1 receptor expression; currents evoked by SPNK1 (an NK1-selective SP receptor agonist) and; the efficacy of transduction pathways transforming ligand binding into channel modulation. Despite developmental reductions in XII MN NK1 receptor expression, SPNK1 current density remained constant at 6.1 ± 1.0 (SE) pA/pF. SPNK1 activated at least two conductances. Activation of a pH-insensitive Na+ conductance dominated in neonates (P0–P5), but its contribution fell from ∼80 to ∼55% in juveniles (P14–P23). SPNK1 also inhibited a pH-sensitive, two-pore domain K+ (TASK)-like K+ current. Its contribution increased developmentally. First, the density of this pH-sensitive K+ current doubled between P0 and P23. Second, SPNK1 did not affect this current in neonates, but reduced it by 20% at P7–P10 and 80% in juveniles. In addition, potentiation of repetitive firing was greatest in juveniles. These data establish that despite apparent reductions in NK1 receptor density, SP remains an important modulator of XII MN excitability throughout postnatal development due, in part, to increased expression of a pH-sensitive, TASK-like conductance.

Functional consequences of the interactions among the neural cell adhesion molecule NCAM, the receptor tyrosine kinase TrkB, and the inwardly rectifying K+ channel KIR3.3

Cell adhesion molecules and neurotrophin receptors are crucial for the development and the function of the nervous system. Among downstream effectors of neurotrophin receptors and recognition molecules are ion channels. Here, we provide evidence that G protein-coupled inwardly rectifying K(+) channel Kir3.3 directly binds to the neural cell adhesion molecule (NCAM) and neurotrophin receptor TrkB. We identified the binding sites for NCAM and TrkB at the C-terminal intracellular domain of Kir3.3. The interaction between NCAM, TrkB, and Kir3.3 was supported by immunocytochemical co-localization of Kir3.3, NCAM, and/or TrkB at the surface of hippocampal neurons. Co-expression of TrkB and Kir3.1/3.3 in Xenopus oocytes increased the K(+) currents evoked by Kir3.1/3.3 channels. This current enhancement was reduced by the concomitant co-expression with NCAM. Both surface fluorescence measurements of microinjected oocytes and cell surface biotinylation of transfected CHO cells indicated that the cell membrane localization of Kir3.3 is regulated by TrkB and NCAM. Furthermore, the level of Kir3.3, but not of Kir3.2, at the plasma membranes was reduced in TrkB-deficient mice, supporting the notion that TrkB regulates the cell surface expression of Kir3.3. The premature expression of developmentally late appearing Kir3.1/3.3 in hippocampal neurons led to a reduction of NCAM-induced neurite outgrowth. Our observations indicate a decisive role for the neuronal K(+) channel in regulating NCAM-dependent neurite outgrowth and attribute a physiologically meaningful role to the functional interplay of Kir3.3, NCAM, and TrkB in ontogeny.

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.

Altered neurotransmission in the mesolimbic reward system of Girk mice

Mice lacking the Girk2 subunit of G protein-gated inwardly rectifying K+ (Girk) channels exhibit dopamine-dependent hyperactivity and elevated responses to drugs that stimulate dopamine neurotransmission. The dopamine-dependent phenotypes seen in Girk2(-/-) mice could reflect increased intrinsic excitability of or diminished inhibitory feedback to midbrain dopamine neurons, or secondary adaptations triggered by Girk2 ablation. We addressed these possibilities by evaluating Girk(-/-) mice in behavioral, electrophysiological, and cell biological assays centered on the mesolimbic dopamine system. Despite differences in the contribution of Girk1 and Girk2 subunits to Girk signaling in midbrain dopamine neurons, Girk1(-/-) and Girk2(-/-) mice exhibited comparable baseline hyperactivities and enhanced responses to cocaine. Girk ablation also correlated with altered afferent input to dopamine neurons in the ventral tegmental area. Dopamine neurons from Girk1(-/-) and Girk2(-/-) mice exhibited elevated glutamatergic neurotransmission, paralleled by increased synaptic levels of alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate glutamate receptors. In addition, synapse density, alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor levels, and glutamatergic neurotransmission were elevated in medium spiny neurons of the nucleus accumbens from Girk1(-/-) and Girk2(-/-) mice. We conclude that dopamine-dependent phenotypes in Girk2(-/-) mice are not solely attributable to a loss of Girk signaling in dopamine neurons, and likely involve secondary adaptations facilitating glutamatergic signaling in the mesolimbic reward system.

Inhibition of G-protein-activated inwardly rectifying K+ channels by the selective norepinephrine reuptake inhibitors atomoxetine and reboxetine

Atomoxetine and reboxetine are commonly used as selective norepinephrine reuptake inhibitors (NRIs) for the treatment of attention-deficit/hyperactivity disorder and depression, respectively. Furthermore, recent studies have suggested that NRIs may be useful for the treatment of several other psychiatric disorders. However, the molecular mechanisms underlying the various effects of NRIs have not yet been sufficiently clarified. G-protein-activated inwardly rectifying K(+) (GIRK or Kir3) channels have an important function in regulating neuronal excitability and heart rate, and GIRK channel modulation has been suggested to be a potential treatment for several neuropsychiatric disorders and cardiac arrhythmias. In this study, we investigated the effects of atomoxetine and reboxetine on GIRK channels using the Xenopus oocyte expression assay. In oocytes injected with mRNA for GIRK1/GIRK2, GIRK2, or GIRK1/GIRK4 subunits, extracellular application of atomoxetine or reboxetine reversibly reduced GIRK currents. The inhibitory effects were concentration-dependent, but voltage-independent, and time-independent during each voltage pulse. However, Kir1.1 and Kir2.1 channels were insensitive to atomoxetine and reboxetine. Atomoxetine and reboxetine also inhibited GIRK currents induced by activation of cloned A(1) adenosine receptors or by intracellularly applied GTPgammaS, a nonhydrolyzable GTP analogue. Furthermore, the GIRK currents induced by ethanol were concentration-dependently inhibited by extracellularly applied atomoxetine but not by intracellularly applied atomoxetine. The present results suggest that atomoxetine and reboxetine inhibit brain- and cardiac-type GIRK channels, revealing a novel characteristic of clinically used NRIs. GIRK channel inhibition may contribute to some of the therapeutic effects of NRIs and adverse side effects related to nervous system and heart function.

Ca(V)3.1 is a tremor rhythm pacemaker in the inferior olive

The rhythmic motor pathway activation by pacemaker neurons or circuits in the brain has been proposed as the mechanism for the timing of motor coordination, and the abnormal potentiation of this mechanism may lead to a pathological tremor. Here, we show that the potentiation of Ca(V)3.1 T-type Ca(2+) channels in the inferior olive contributes to the onset of the tremor in a pharmacological model of essential tremor. After administration of harmaline, 4- to 10-Hz synchronous neuronal activities arose from the IO and then propagated to cerebellar motor circuits in wild-type mice, but those rhythmic activities were absent in mice lacking Ca(V)3.1 gene. Intracellular recordings in brain-stem slices revealed that the Ca(V)3.1-deficient inferior olive neurons lacked the subthreshold oscillation of membrane potentials and failed to trigger 4- to 10-Hz rhythmic burst discharges in the presence of harmaline. In addition, the selective knockdown of Ca(V)3.1 gene in the inferior olive by shRNA efficiently suppressed the harmaline-induced tremor in wild-type mice. A mathematical model constructed based on data obtained from patch-clamping experiments indicated that harmaline could efficiently potentiate Ca(V)3.1 channels by changing voltage-dependent responsiveness in the hyperpolarizing direction. Thus, Ca(V)3.1 is a molecular pacemaker substrate for intrinsic neuronal oscillations of inferior olive neurons, and the potentiation of this mechanism can be considered as a pathological cause of essential tremor.