Altered regulation of potassium and calcium channels by GABA(B) and adenosine receptors in hippocampal neurons from mice lacking Galpha(o)

Neuronal G protein-gated K+ channels

G protein-gated inwardly rectifying K⁺ (GIRK/Kir3) channels exert a critical inhibitory influence on neurons. Neuronal GIRK channels mediate the G protein-dependent, direct/postsynaptic inhibitory effect of many neurotransmitters including γ-aminobutyric acid (GABA), serotonin, dopamine, adenosine, somatostatin, and enkephalin. In addition to their complex regulation by G proteins, neuronal GIRK channel activity is sensitive to PIP₂, phosphorylation, regulator of G protein signaling (RGS) proteins, intracellular Na⁺ and Ca²⁺, and cholesterol. The application of genetic and viral manipulations in rodent models, together with recent progress in the development of GIRK channel modulators, has increased our understanding of the physiological and behavioral impact of neuronal GIRK channels. Work in rodent models has also revealed that neuronal GIRK channel activity is modified, transiently or persistently, by various stimuli including exposure drugs of abuse, changes in neuronal activity patterns, and aversive experience. A growing body of preclinical and clinical evidence suggests that dysregulation of GIRK channel activity contributes to neurological diseases and disorders. The primary goals of this review are to highlight fundamental principles of neuronal GIRK channel biology, mechanisms of GIRK channel regulation and plasticity, the nascent landscape of GIRK channel pharmacology, and the potential relevance of GIRK channels to the pathophysiology and treatment of neurological diseases and disorders.

GABAB Receptors and Pain

A substantial fraction of the human population suffers from chronic pain states, which often cannot be sufficiently treated with existing drugs. This calls for alternative targets and strategies for the development of novel analgesics. There is substantial evidence that the G protein-coupled GABA_B receptor is involved in the processing of pain signals and thus has long been considered a valuable target for the generation of analgesics to treat chronic pain. In this review, the contribution of GABA_B receptors to the generation and modulation of pain signals, their involvement in chronic pain states as well as their target suitability for the development of novel analgesics is discussed.

Pediatric Encephalopathy: Clinical, Biochemical and Cellular Insights into the Role of Gln52 of GNAO1 and GNAI1 for the Dominant Disease

Heterotrimeric G proteins are immediate transducers of G protein-coupled receptors-the biggest receptor family in metazoans-and play innumerate functions in health and disease. A set of de novo point mutations in GNAO1 and GNAI1, the genes encoding the α-subunits (Gαo and Gαi1, respectively) of the heterotrimeric G proteins, have been described to cause pediatric encephalopathies represented by epileptic seizures, movement disorders, developmental delay, intellectual disability, and signs of neurodegeneration. Among such mutations, the Gln52Pro substitutions have been previously identified in GNAO1 and GNAI1. Here, we describe the case of an infant with another mutation in the same site, Gln52Arg. The patient manifested epileptic and movement disorders and a developmental delay, at the onset of 1.5 weeks after birth. We have analyzed biochemical and cellular properties of the three types of dominant pathogenic mutants in the Gln52 position described so far: Gαo[Gln52Pro], Gαi1[Gln52Pro], and the novel Gαo[Gln52Arg]. At the biochemical level, the three mutant proteins are deficient in binding and hydrolyzing GTP, which is the fundamental function of the healthy G proteins. At the cellular level, the mutants are defective in the interaction with partner proteins recognizing either the GDP-loaded or the GTP-loaded forms of Gαo. Further, of the two intracellular sites of Gαo localization, plasma membrane and Golgi, the former is strongly reduced for the mutant proteins. We conclude that the point mutations at Gln52 inactivate the Gαo and Gαi1 proteins leading to aberrant intracellular localization and partner protein interactions. These features likely lie at the core of the molecular etiology of pediatric encephalopathies associated with the codon 52 mutations in GNAO1/GNAI1.

Humanization of Drosophila Gαo to Model GNAO1 Paediatric Encephalopathies

Several hundred genes have been identified to contribute to epilepsy-the disease affecting 65 million people worldwide. One of these genes is GNAO1 encoding Gαo, the major neuronal α-subunit of heterotrimeric G proteins. An avalanche of dominant de novo mutations in GNAO1 have been recently described in paediatric epileptic patients, suffering, in addition to epilepsy, from motor dysfunction and developmental delay. Although occurring in amino acids conserved from humans to Drosophila, these mutations and their functional consequences have only been poorly analysed at the biochemical or neuronal levels. Adequate animal models to study the molecular aetiology of GNAO1 encephalopathies have also so far been lacking. As the first step towards modeling the disease in Drosophila, we here describe the humanization of the Gαo locus in the fruit fly. A two-step CRISPR/Cas9-mediated replacement was conducted, first substituting the coding exons 2-3 of Gαo with respective human GNAO1 sequences. At the next step, the remaining exons 4-7 were similarly replaced, keeping intact the gene Cyp49a1 embedded in between, as well as the non-coding exons, exon 1 and the surrounding regulatory sequences. The resulting flies, homozygous for the humanized GNAO1 loci, are viable and fertile without any visible phenotypes; their body weight, locomotion, and longevity are also normal. Human Gαo-specific antibodies confirm the endogenous-level expression of the humanized Gαo, which fully replaces the Drosophila functions. The genetic model we established will make it easy to incorporate encephalopathic GNAO1 mutations and will permit intensive investigations into the molecular aetiology of the human disease through the powerful toolkit of Drosophila genetics.

NPY2 Receptors Reduce Tonic Action Potential-Independent GABAB Currents in the Basolateral Amygdala

Although NPY has potent anxiolytic actions within the BLA, selective activation of BLA NPY Y₂ receptors (Y₂Rs) acutely increases anxiety by an unknown mechanism. Using ex vivo male rat brain slice electrophysiology, we show that the selective Y₂R agonist, [ahx^5-24]NPY, reduced the frequency of GABA_A-mediated mIPSCs in BLA principal neurons (PNs). [ahx^5-24]NPY also reduced tonic activation of GABA_B receptors (GABA_BR), which increased PN excitability through inhibition of a tonic, inwardly rectifying potassium current (K_IR ). Surprisingly, Y₂R-sensitive GABA_BR currents were action potential-independent, persisting after treatment with TTX. Additionally, the Ca²⁺-dependent, slow afterhyperpolarizing K⁺ current (I_sAHP ) was enhanced in approximately half of the Y₂R-sensitive PNs, possibly from enhanced Ca²⁺ influx, permitted by reduced GABA_BR tone. In male and female mice expressing tdTomato in Y₂R-mRNA cells (tdT-Y₂R mice), immunohistochemistry revealed that BLA somatostatin interneurons express Y₂Rs, as do a significant subset of BLA PNs. In tdT-Y₂R mice, [ahx^5-24]NPY increased excitability and suppressed the K_IR in nearly all BLA PNs independent of tdT-Y₂R fluorescence, consistent with presynaptic Y₂Rs on somatostatin interneurons mediating the above effects. However, only tdT-Y₂R-expressing PNs responded to [ahx^5-24]NPY with an enhancement of the I_sAHP Ultimately, increased PN excitability via acute Y₂R activation likely correlates with enhanced BLA output, consistent with reported Y₂R-mediated anxiogenesis. Furthermore, we demonstrate the following: (1) a novel mechanism whereby activity-independent GABA release can powerfully dampen BLA neuronal excitability via postsynaptic GABA_BRs; and (2) that this tonic inhibition can be interrupted by neuromodulation, here by NPY via Y₂Rs.SIGNIFICANCE STATEMENT Within the BLA, NPY is potently anxiolytic. However, selective activation of NPY₂ receptors (Y₂Rs) increases anxiety by an unknown mechanism. We show that activation of BLA Y₂Rs decreases tonic GABA release onto BLA principal neurons, probably from Y₂R-expressing somatostatin interneurons, some of which coexpress NPY. This increases principal neuron excitability by reducing GABA_B receptor (GABA_BR)-mediated activation of G-protein-coupled, inwardly rectifying K⁺ currents. Tonic, Y₂R-sensitive GABA_BR currents unexpectedly persisted in the absence of action potential firing, revealing, to our knowledge, the first report of substantial, activity-independent GABA_BR activation. Ultimately, we provide a plausible explanation for Y₂R-mediated anxiogenesis in vivo and describe a novel and modulatable means of damping neuronal excitability.

Inhibitory Signaling to Ion Channels in Hippocampal Neurons Is Differentially Regulated by Alternative Macromolecular Complexes of RGS7

The neuromodulatory effects of GABA on pyramidal neurons are mediated by GABA_B receptors (GABA_BRs) that signal via a conserved G-protein-coupled pathway. Two prominent effectors regulated by GABA_BRs include G-protein inwardly rectifying K⁺ (GIRK) and P/Q/N type voltage-gated Ca²⁺ (Ca_V2) ion channels that control excitability and synaptic output of these neurons, respectively. Regulator of G-protein signaling 7 (RGS7) has been shown to control GABA_B effects, yet the specificity of its impacts on effector channels and underlying molecular mechanisms is poorly understood. In this study, we show that hippocampal RGS7 forms two distinct complexes with alternative subunit configuration bound to either membrane protein R7BP (RGS7 binding protein) or orphan receptor GPR158. Quantitative biochemical experiments show that both complexes account for targeting nearly the entire pool of RGS7 to the plasma membrane. We analyzed the effect of genetic elimination in mice of both sexes and overexpression of various components of RGS7 complex by patch-clamp electrophysiology in cultured neurons and brain slices. We report that RGS7 prominently regulates GABA_BR signaling to Ca_V2, in addition to its known involvement in modulating GIRK. Strikingly, only complexes containing R7BP, but not GPR158, accelerated the kinetics of both GIRK and Ca_V2 modulation by GABA_BRs. In contrast, GPR158 overexpression exerted the opposite effect and inhibited RGS7-assisted temporal modulation of GIRK and Ca_V2 by GABA. Collectively, our data reveal mechanisms by which distinctly composed macromolecular complexes modulate the activity of key ion channels that mediate the inhibitory effects of GABA on hippocampal CA1 pyramidal neurons.SIGNIFICANCE STATEMENT This study identifies the contributions of distinct macromolecular complexes containing a major G-protein regulator to controlling key ion channel function in hippocampal neurons with implications for understanding molecular mechanisms underlying synaptic plasticity, learning, and memory.

Testosterone rapidly increases Ca2+-activated K+ currents causing hyperpolarization in human coronary artery endothelial cells

Testosterone has endothelium-dependent vasodilatory effects on the coronary artery, with some reports suggesting endothelial ion channel involvement. This study employed the whole-cell patch clamp technique to investigate the effect of testosterone on ion channels in human coronary artery endothelial cells (HCAECs) and the mechanisms involved. We found that 0.03-3μM testosterone significantly induced a rapid, concentration-dependent increase in total HCAEC current (EC₅₀, 71.96±1.66nM; maximum increase, 59.13±8.37%; mean±SEM). The testosterone-enhanced currents consisted of small- and large-conductance Ca²⁺-activated K⁺ currents (SK_Ca and BK_Ca currents), but not Cl^- and nonselective cation currents. Either a non-permeant testosterone conjugate or the non-aromatizable androgen dihydrotestosterone (DHT) could increase HCAEC currents as well. The androgen receptor antagonist flutamide prevented this testosterone, testosterone conjugate, and DHT effect, while the estrogen receptor antagonist fulvestrant did not. Incubating HCAECs with pertussis toxin or protein kinase A inhibitor H-89 largely inhibited the testosterone effect, while pre-incubation with phospholipase C inhibitor U-73122, prostacyclin inhibitor indomethacin, nitric oxide synthase inhibitor L-NAME or cytochrome P450 inhibitor MS-PPOH, did not. Finally, testosterone application induced HCAEC hyperpolarization within minutes; this effect was prevented by SK_Ca and BK_Ca current inhibitors apamin and iberiotoxin. This is the first electrophysiological demonstration of androgen-induced K_Ca current increase, leading to hyperpolarization, in any endothelial cell, and the first report of SK_Ca as a testosterone target. Our data show that testosterone rapidly increased whole-cell HCAEC SK_Ca and BK_Ca currents via a surface androgen receptor, G_i/o protein, and protein kinase A. This mechanism may explain rapid testosterone-induced coronary vasodilation seen in vivo.

Prioritizing the development of mouse models for childhood brain disorders

Mutations in hundreds of genes contribute to cognitive and behavioral dysfunction associated with developmental brain disorders (DBDs). Due to the sheer number of risk factors available for study combined with the cost of developing new animal models, it remains an open question how genes should be prioritized for in-depth neurobiological investigations. Recent reviews have argued that priority should be given to frequently mutated genes commonly found in sporadic DBD patients. Intrigued by this idea, we explored to what extent "high priority" risk factors have been studied in animals in an effort to assess their potential for generating valuable preclinical models capable of advancing the neurobiological understanding of DBDs. We found that in-depth whole animal studies are lacking for many high priority genes, with relatively few neurobiological studies performed in construct valid animal models aimed at understanding the pathological substrates associated with disease phenotypes. However, some high priority risk factors have been extensively studied in animal models and they have generated novel insights into DBD patho-neurobiology while also advancing early pre-clinical therapeutic treatment strategies. We suggest that prioritizing model development toward genes frequently mutated in non-specific DBD populations will accelerate the understanding of DBD patho-neurobiology and drive novel therapeutic strategies. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

RGS Redundancy and Implications in GPCR-GIRK Signaling

Regulators of G protein signaling (RGS proteins) are key components of GPCR complexes, interacting directly with G protein α-subunits to enhance their intrinsic GTPase activity. The functional consequence is an accelerated termination of G protein effectors including certain ion channels. RGS proteins have a profound impact on the membrane-delimited gating behavior of G-protein-activated inwardly rectifying K(+) (GIRK) channels as demonstrated in reconstitution assays and recent RGS knockout mice studies. Akin to GPCRs and G protein αβγ subunits, multiple RGS isoforms are expressed within single GIRK-expressing neurons, suggesting functional redundancy and/or specificity in GPCR-GIRK channel signaling. The extent and impact of RGS redundancy in neuronal GPCR-GIRK channel signaling is currently not fully appreciated; however, recent studies from RGS knockout mice are providing important new clues on the impact of individual endogenous RGS proteins and the extent of RGS functional redundancy. Incorporating "tools" such as engineered RGS-resistant Gαi/o subunits provide an important assessment method for determining the impact of all endogenous RGS proteins on a given GPCR response and an accounting benchmark to assess the impact of individual RGS knockouts on overall RGS redundancy within a given neuron. Elucidating the degree of regulation attributable to specific RGS proteins in GIRK channel function will aid in the assessment of individual RGS proteins as viable therapeutic targets in epilepsy, ataxia's, memory disorders, and a growing list of neurological disorders.

A novel GABAergic action mediated by functional coupling between GABAB-like receptor and two different high-conductance K+ channels in cricket Kenyon cells

The γ-aminobutyric acid type B (GABA(B)) receptor has been shown to attenuate high-voltage-activated Ca(2+) currents and enhance voltage-dependent or inwardly rectifying K(+) currents in a variety of neurons. In this study, we report a novel coupling of GABA(B)-like receptor with two different high-conductance K(+) channels, Na(+)-activated K(+) (K(Na)) channel and Ca(2+)-activated K(+) (K(Ca)) channel, in Kenyon cells isolated from the mushroom body of the cricket brain. Single-channel activities of K(Na) and K(Ca) channels in response to bath applications of GABA and the GABA(B)-specific agonist SKF97541 were recorded with the cell-attached patch configuration. The open probability (P(o)) of both K(Na) and K(Ca) channels was found to be increased by bath application of GABA, and this increase in Po was antagonized by coapplication of the GABAB antagonist CGP54626, suggesting that GABA(B)-like receptors mediate these actions. Similarly, GABA(B)-specific agonist SKF97541 increased the Po of both K(Na) and K(Ca) channels. Perforated-patch recordings using β-escin further revealed that SKF97541 increased the amplitude of the outward currents elicited by step depolarizations. Under current-clamp conditions, SKF97541 decreased the firing frequency of spontaneous action potential (AP) and changed the AP waveform. The amplitude and duration of AP were decreased, whereas the afterhyperpolarization of AP was increased. Resting membrane potential, however, was not significantly altered by SKF97541. Taken together, these results suggest that GABA(B)-like receptor is functionally coupled with both K(Na) and K(Ca) channels and this coupling mechanism may serve to prevent AP formation and limit excitatory synaptic input.

PTPBR7 binding proteins in myelinating neurons of the mouse brain

Mouse protein tyrosine phosphatase PTPBR7 is a receptor-like, transmembrane protein that is localized on the surface of neuronal cells. Its protein phosphatase activity is reduced upon multimerization, and PTPBR7-deficient mice display motor coordination defects. Extracellular molecules that may influence PTPBR7 activity, however, remain to be determined. We here show that the PTPBR7 extracellular domain binds to highly myelinated regions in mouse brain, in particular the white matter tracks in cerebellum. PTPBR7 deficiency does not alter this binding pattern, as witnessed by RAP in situ staining of Ptprr^-/- mouse brain sections. Additional in situ and in vitro experiments also suggest that sugar moieties of heparan sulphate and chondroitin sulphate glycosaminoglycans are not critical for PTPBR7 binding. Candidate binding proteins were affinity-purified exploiting the PTPBR7 extracellular domain and identified by mass spectrometric means. Results support the suggested link between PTPRR isoforms and cerebellar calcium ion homeostasis, and suggest an additional role in the process of cell-cell adhesion.

Involvement of protein kinase C and protein kinase A in the enhancement of L-type calcium current by GABAB receptor activation in neonatal hippocampus

In the early neonatal period activation of GABAB receptors attenuates calcium current through N-type calcium channels while enhancing current through L-type calcium channels in rat hippocampal neurons. The attenuation of N-type calcium current has been previously demonstrated to occur through direct interactions of the βγ subunits of Gi/o G-proteins, but the signal transduction pathway for the enhancement of L-type calcium channels in mammalian neurons remains unknown. In the present study, calcium currents were elicited in acute cultures from postnatal day 6-8 rat hippocampi in the presence of various modulators of protein kinase A (PKA) and protein kinase C (PKC) pathways. Overnight treatment with an inhibitor of Gi/o (pertussis toxin, 200 ng/ml) abolished the attenuation of calcium current by the GABAB agonist, baclofen (10 μM) with no effect on the enhancement of calcium current. These data indicate that while the attenuation of N-type calcium current is mediated by the Gi/o subtype of G-protein, the enhancement of L-type calcium current requires activation of a different G-protein. The enhancement of the sustained component of calcium current by baclofen was blocked by PKC inhibitors, GF-109203X (500 nM), chelerythrine chloride (5 μM), and PKC fragment 19-36 (2 μM) and mimicked by the PKC activator phorbol-12-myristate-13-acetate (1 μM). The enhancement of the sustained component of calcium current was blocked by PKA inhibitors H-89 (1 μM) and PKA fragment 6-22 (500 nM) but not Rp-cAMPS (30 μM) and it was not mimicked by the PKA activator, 8-Br-cAMP (500 μM-1 mM). The data suggest that activation of PKC alone is sufficient to enhance L-type calcium current but that PKA may also be involved in the GABAB receptor mediated effect.

Activity Modes in Thalamocortical Relay Neurons are Modulated by G(q)/G(11) Family G-proteins - Serotonergic and Glutamatergic Signaling

In thalamocortical relay (TC) neurons, G-protein-coupled receptors play an important part in the control of activity modes. A conditional Gα(q) knockout on the background of a constitutive Gα(11) knockout (Gα(q)/Gα(11) (-/-)) was used to determine the contribution of Gq/G11 family G-proteins to metabotropic serotonin (5-HT) and glutamate (Glu) function in the dorsal part of the lateral geniculate nucleus (dLGN). In control mice, current clamp recordings showed that α-m-5-HT induced a depolarization of V(rest) which was sufficient to suppress burst firing. This depolarization was concentration-dependent (100 μM: +6 ± 1 mV, n = 10; 200 μM: +10 ± 1 mV, n = 7) and had a conditioning effect on the activation of other Gα(q)-mediated pathways. The depolarization was significantly reduced in Gα(q)/Gα(11) (-/-) (100 μM: 3 ± 1 mV, n = 11; 200 μM: 5 ± 1 mV, n = 6) and was apparently insufficient to suppress burst firing. Activating Gα(q)-coupled muscarinic receptors affected the magnitude of α-m-5-HT-induced effects in a reciprocal manner. Furthermore, the depolarizing effect of mGluR1 agonists was significantly reduced in Gα(q)/Gα(11) (-/-) mice. Immunohistochemical stainings revealed binding of 5-HT(2C)R- and mGluR1α-, but not of 5-HT(2A)R-specific antibodies in the dLGN of Gα(q)/Gα(11) (-/-) mice. In conclusion, these findings demonstrate that transmitters of ascending brainstem fibers and corticofugal fibers both signal via a central element in the form of Gq/G11-mediated pathways to control activity modes in the TC system.

[Identification of proteins in complexes with alpha-latrotoxin receptors]

A thorough analysis of proteins capable of interacting with presynaptic receptors of alpha-latrotoxin was carried out. The protein components of receptor complexes were isolated from rat brain membranes by affinity chromatography on immobilized alpha-latrotoxin and antibodies to the cytoplasmic moiety of the calcium-independent receptor of alpha-latrotoxin (CIRL) followed by analysis by mass spectrometry. Several proteins were identified, with structural proteins, intracellular signal proteins, and proteins involved in the endocytosis and transport of synaptic vesicles being among them.

Adenosine A1 receptors inhibit GABAergic transmission in rat tuberomammillary nucleus neurons

The adenosinergic modulation of GABAergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs) was investigated in mechanically dissociated rat tuberomammillary nucleus (TMN) neurons using a conventional whole-cell patch clamp technique. Adenosine (100 microM) reversibly decreased mIPSC frequency without affecting the current amplitude, indicating that adenosine acts presynaptically to decrease the probability of spontaneous GABA release. The adenosine action on GABAergic mIPSC frequency was completely blocked by 1 microM DPCPX, a selective A(1) receptor antagonist, and mimicked by 1 microM CPA, a selective A(1) receptor agonist. This suggests that presynaptic A(1) receptors were responsible for the adenosine-mediated inhibition of GABAergic mIPSC frequency. CPA still decreased GABAergic mIPSC frequency even either in the presence of 200 microM Cd(2+), a general voltage-dependent Ca(2+) channel blocker, or in the Ca(2+)-free external solution. However, the inhibitory effect of CPA on GABAergic mIPSC frequency was completely occluded by 1 mM Ba(2+), a G-protein coupled inwardly rectifying K(+) (GIRK) channel blocker. In addition, the CPA-induced decrease in mIPSC frequency was completely occluded by either 100 microM SQ22536, an adenylyl cyclase (AC) inhibitor, or 1 muM KT5720, a specific protein kinase A (PKA) inhibitor. The results suggest that the activation of presynaptic A(1) receptors decreases spontaneous GABAergic transmission onto TMN neurons via the modulation of GIRK channels as well as the AC/cAMP/PKA signal transduction pathway. This adenosine A(1) receptor-mediated modulation of GABAergic transmission onto TMN neurons may play an important role in the fine modulation of the excitability of TMN histaminergic neurons as well as the regulation of sleep-wakefulness.

Regulation of neurite outgrowth by G(i/o) signaling pathways

Neurogenesis is a long and winding journey. A neural progenitor cell migrates long distances, differentiates by forming a single axon and multiple dendrites, undergoes maturation, and ultimately survives. The initial formation of neurites during neuronal differentiation, commonly referred to as "neurite outgrowth," can be induced by a large repertoire of signals that stimulate an array of receptors and downstream signaling pathways. The G(i/o) family of heterotrimeric G-proteins are abundantly expressed in the brain and enriched at neuronal growth cones. Recent evidence has uncovered several G(i/o)-coupled receptors that induce neurite outgrowth and has begun to elucidate the underlying molecular mechanisms. Emerging data suggests that signals from several G(i/o)-coupled receptors converge at the transcription factor STAT3 to regulate neurite outgrowth and at Rac1 and Cdc42 to regulate cytoskeletal reorganization. Physiologically, signaling through G(i/o)-coupled cannabinoid receptors is critical for pro percentral nervous system development. As the mechanisms by which G(i/o)-coupled receptors regulate neurite outgrowth are clarified, it is becoming evident that modulating signals from G(i/o) and their receptors has great potential for the treatment of neurodegenerative diseases.

Muscarinic ACh receptor-mediated control of thalamic activity via G(q)/G (11)-family G-proteins

A genetic knock out was used to determine the specific contribution of G(q)/G(11)-family G-proteins to the function of thalamocortical relay (TC) neurons. Disruption of Galpha(q) function in a conditional forebrain-specific Galpha(q)/Galpha(11)-double-deficient mouse line (Galpha(q)/Galpha(11)(-/-) had no effects on the resting membrane potential (V (rest)) and the amplitude of the standing outward current (I (SO)). Stimulation of muscarinic acetylcholine (ACh) receptors (mAChR; muscarine, 50 microM) induced a decrease in I (SO) amplitude in wild-type mice (36 +/- 4%, n = 5), a constitutive Galpha(11)-deficient mouse line (Galpha(11)(-/-; 36 +/- 3%, n = 8), and Galpha(q)/Galpha(11)(-/-) (11 +/- 2%, n = 16). Current-clamp recordings revealed a muscarine-induced positive shift in V (rest) of 23 +/- 2 mV (n = 6), 18 +/- 5 mV (n = 5), and 2 +/- 1 mV (n = 9) in wild type, Galpha(11)(-/-), and Galpha(q)/Galpha(11)(-/-), respectively. This depolarization was associated with a change in TC neuron activity from burst to tonic firing in wild type and Galpha(11)(-/-), but not in Galpha(q)/Galpha(11)(-/-). The use of specific antibodies and of pharmacological agents with preferred affinity points to the contribution of m(1)AChR and m(3)AChR. In conclusion, we present two novel aspects of the physiology of the thalamocortical system by demonstrating that the depolarization of TC neurons, which is induced by the action of transmitters of ascending brainstem fibers, is governed roughly equally by both m(1)AChR and m(3)AChR and is transduced by Galpha(q) but not by Galpha(11).

Enhanced calcium cycling and contractile function in transgenic hearts expressing constitutively active G alpha o* protein

In contrast to the other heterotrimeric GTP-binding proteins (G proteins) Gs and Gi, the functional role of G o is still poorly defined. To investigate the role of G alpha o in the heart, we generated transgenic mice with cardiac-specific expression of a constitutively active form of G alpha o1* (G alpha o*), the predominant G alpha o isoform in the heart. G alpha o expression was increased 3- to 15-fold in mice from 5 independent lines, all of which had a normal life span and no gross cardiac morphological abnormalities. We demonstrate enhanced contractile function in G alpha o* transgenic mice in vivo, along with increased L-type Ca2+ channel current density, calcium transients, and cell shortening in ventricular G alpha o*-expressing myocytes compared with wild-type controls. These changes were evident at baseline and maintained after isoproterenol stimulation. Expression levels of all major Ca2+ handling proteins were largely unchanged, except for a modest reduction in Na+/Ca2+ exchanger in transgenic ventricles. In contrast, phosphorylation of the ryanodine receptor and phospholamban at known PKA sites was increased 1.6- and 1.9-fold, respectively, in G alpha o* ventricles. Density and affinity of beta-adrenoceptors, cAMP levels, and PKA activity were comparable in G alpha o* and wild-type myocytes, but protein phosphatase 1 activity was reduced upon G alpha o* expression, particularly in the vicinity of the ryanodine receptor. We conclude that G alpha o* exerts a positive effect on Ca2+ cycling and contractile function. Alterations in protein phosphatase 1 activity rather than PKA-mediated phosphorylation might be involved in hyperphosphorylation of key Ca2+ handling proteins in hearts with constitutive G alpha o activation.

Influence of positive allosteric modulators on GABA(B) receptor coupling in rat brain: a scintillation proximity assay characterisation of G protein subtypes

Little is known concerning coupling of cerebral GABA(B) receptors to G protein subtypes, and the influence of positive allosteric modulators (PAMs) has not been evaluated. These questions were addressed by an antibody-capture/scintillation proximity assay strategy. GABA concentration-dependently enhanced the magnitude of [(35)S]GTPgammaS binding to Galphao and, less markedly, Galphai(1/3) in cortex, whereas Gq and Gs/olf were unaffected. (R)-baclofen and SKF97581 likewise activated Galphao and Galphai(1/3), expressing their actions more potently than GABA. Similar findings were acquired in hippocampus and cerebellum, and the GABA(B) antagonist, CGP55845A, abolished agonist-induced activation of Galphao and Galphai(1/3) in all structures. The PAMs, GS39783, CGP7930 and CGP13501, inactive alone, enhanced efficacy and potency of agonist-induced [(35)S]GTPgammaS binding to Galphao in all regions, actions abolished by CGP55845A. In contrast, they did not modify efficacies at Galphai(1/3). Similarly, in human embryonic kidney cells expressing GABA(B(1a+2)) or GABA(B(1b+2)) receptors, allosteric modulators did not detectably enhance efficacy of GABA at Galphai(1/3), though they increased its potency. To summarise, GABA(B) receptors coupled both to Galphao and to Galphai, but not Gq and Gs/olf, in rat brain. PAMs more markedly enhanced efficacy of coupling to Go versus Gi(1/3). It will be of interest to confirm these observations employing complementary techniques and to evaluate their potential therapeutic significance.

GABA(B) receptors: structure and function

In the basal ganglia the effects of gamma-aminobutyrate (GABA) are mediated by both ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. Although the existence and widespread distribution in the CNS of the GABA(B) receptor had been established in the 1980s the field of GABA(B) research was revolutionized with the discovery that two related G-protein-coupled receptors (GPCRs) needed to dimerize to form the functional GABA(B) receptor at the cell surface. This finding lead to a number of studies of oligomerization in GPCRs and detailed pharmacological studies of the cloned receptors and their splice variants. Particular interest has focused on the proteins interacting with the receptor which may be important in mediating the longer term signalling effects of the receptor and modifying its cellular localization or physiology. The cloning of the GABA(B) receptors also lead to the identification of the first compounds interacting in an allosteric fashion with the receptor some of which may have therapeutic value. Most recently "knockouts" of both the GABA(B) subunits have been produced where in general as expected there is a loss of the majority of the inhibitory effects of the GABA(B) receptor.

Regional differences in the coupling of 5-hydroxytryptamine-1A receptors to G proteins in the rat brain

Numerous data showed that 5-hydroxytryptamine-1A (5-HT1A) receptors couple to Galpha(o)/alpha(i) proteins for signal transduction. However, the alpha subunit isoforms really involved in 5-HT1A receptor coupling in brain remain to be identified. Moreover, regional differences in the functional characteristics of brain 5-HT1A receptors have been evidenced repeatedly. Because such differences could be due to variations in G proteins interacting with the same receptor, relevant approaches were used for identifying alpha subunits physically coupled to 5-HT1A receptors in different regions of the rat brain. Using immunoaffinity chromatography coupled to Western blot detection, 5-HT1A receptors were found to interact equally with Galpha(o) and Galpha(i3) in the cerebral cortex, mainly with Galpha(o) and weakly with Galpha(i3) in the hippocampus and exclusively with Galpha(i3) in the anterior raphe area. In the hypothalamus, 5-HT(1A) receptors seemed to be coupled to the latter two G proteins plus Galpha(i1) and Galpha(z). Complementary experiments based on an antibody capture technique coupled to both classic radioactivity and scintillation proximity assay detections showed that hippocampal 5-HT1A receptor stimulation induced 5'-O-(3-[35S]thio)triphosphate binding to immunoprecipitates with Galpha(i3) and Galpha(o) antisera. In the anterior raphe, such 5-HT1A receptor-mediated effect was obtained with Galpha(i3) antiserum only. These results demonstrated the existence of regional differences in the coupling of 5-HT1A receptors to G proteins in the rat brain. In the anterior raphe, 5-HT1A receptors seem to interact specifically with Galpha(i3), whereas in the hippocampus, they are mainly coupled to Galpha(o) proteins. Such a disparity in G-protein coupling might explain regional differences in adaptive regulations of brain 5-HT1A receptors.

Altered expression of G(q/11alpha) protein shapes mGlu1 and mGlu5 receptor-mediated single cell inositol 1,4,5-trisphosphate and Ca(2+) signaling

The metabotropic glutamate (mGlu) receptors mGlu1 and mGlu5 mediate distinct inositol 1,4,5-trisphosphate (IP(3)) and Ca(2+) signaling patterns, governed in part by differential mechanisms of feedback regulation after activation. Single cell imaging has shown that mGlu1 receptors initiate sustained elevations in IP(3) and Ca(2+), which are sensitive to agonist concentration. In contrast, mGlu5 receptors are subject to cyclical PKC-dependent uncoupling and consequently mediate coincident IP(3) and Ca(2+) oscillations that are largely independent of agonist concentration. In this study, we investigated the contribution of G(q/11)alpha protein expression levels in shaping mGlu1/5 receptor-mediated IP(3) and Ca(2+) signals, using RNA interference (RNAi). RNAi-mediated knockdown of G(q/11)alpha almost abolished the single-cell increase in IP(3) caused by mGlu1 and mGlu5 receptor activation. For the mGlu1 receptor, this unmasked baseline Ca(2+) oscillations that persisted even at maximal agonist concentrations. mGlu5 receptor-activated Ca(2+) oscillations were still observed but were only initiated at high agonist concentrations. Recombinant overexpression of G(q)alpha enhanced IP(3) signals after mGlu1 and mGlu5 receptor activation. It is noteworthy that although mGlu5 receptor-mediated IP(3) and Ca(2+) oscillations in control cells were largely insensitive to agonist concentration, increasing G(q)alpha expression converted these oscillatory signatures to sustained plateau responses in a high proportion of cells. In addition to modulating temporal Ca(2+) signals, up- or down-regulation of G(q/11)alpha expression alters the threshold for the concentration of glutamate at which a measurable Ca(2+) signal could be detected. These experiments indicate that altering G(q/11)alpha expression levels differentially affects spatiotemporal aspects of IP(3) and Ca(2+) signaling mediated by the mGlu1 and mGlu5 receptors.

GABAB-receptor-mediated suppression of sympathetic outflow from the spinal cord of neonatal rats

Using a splanchnic nerve-spinal cord preparation in vitro that could spontaneously generate sympathetic nerve discharge (SND), we investigated the roles of intraspinal GABAB receptors in the regulation of SND. Despite an age-dependent difference in sensitivity, bath applications of baclofen (Bac; GABAB-receptor agonist) consistently reduced SND in a concentration-dependent manner. The drug specificity of Bac in activation of GABAB receptors was verified by application of its antagonist saclofen (Sac) or CGP-46381 (CGP). Sac or CGP alone did not change SND. However, in the presence of Sac or CGP, the effects of Bac on SND inhibition were reversibly attenuated. The splanchnic sympathetic preganglionic neuron (SPN) was recorded by blind whole cell, patch-clamp techniques. We examined Bac effects on electrical membrane properties of SPNs. Applications of Bac reduced excitatory synaptic events, induced membrane hyperpolarizations, and inhibited SPN firing. In the presence of 12 mM Mg2+ or 0.5 μM TTX to block Ca2+- or action potential-dependent synaptic transmissions, applications of Bac induced an outward baseline current that reversed at −29 ± 6 mV. Because the K+ equilibrium potential in our experimental conditions was −100 mV, the Bac-induced currents could not simply be attributed to an alteration of K+ conductance. On the other hand, applications of Bac to Cs+-loaded SPNs reduced Cd2+-sensitive and high-voltage-activated inward currents, indicating an inhibition of voltage-gated Ca2+ currents. Our results suggest that the activation of intraspinal GABAB receptors suppresses SND via a mixture of ion events that may link to a change in Ca2+ conductance.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

Pertussis-toxin-sensitive Galpha subunits selectively bind to C-terminal domain of neuronal GIRK channels: evidence for a heterotrimeric G-protein-channel complex

Neuronal G-protein-gated inwardly rectifying potassium (Kir3; GIRK) channels are activated by G-protein-coupled receptors that selectively interact with PTX-sensitive (Galphai/o) G proteins. Although the Gbetagamma dimer is known to activate GIRK channels, the role of the Galphai/o subunit remains unclear. Here, we established that Galphao subunits co-immunoprecipitate with neuronal GIRK channels. In vitro binding studies led to the identification of six amino acids in the GIRK2 C-terminal domain essential for Galphao binding. Further studies suggested that the Galphai/obetagamma heterotrimer binds to the GIRK2 C-terminal domain via Galpha and not Gbetagamma. Galphai/o binding-impaired GIRK2 channels exhibited reduced receptor-activated currents, but retained normal ethanol- and Gbetagamma-activated currents. Finally, PTX-insensitive Galphaq or Galphas subunits did not bind to the GIRK2 C-terminus. Together, these results suggest that the interaction of PTX-sensitive Galphai/o subunit with the GIRK2 C-terminal domain regulates G-protein receptor coupling, and may be important for establishing specific Galphai/o signaling pathways.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

A retinal-specific regulator of G-protein signaling interacts with Galpha(o) and accelerates an expressed metabotropic glutamate receptor 6 cascade

G(o) is the most abundant G-protein in the brain, but its regulators are essentially unknown. In retina, Galpha(o1) is obligatory in mediating the metabotropic glutamate receptor 6 (mGluR6)-initiated ON response. To identify the interactors of G(o), we conducted a yeast two-hybrid screen with constituitively active Galpha(o) as a bait. The screen frequently identified a regulator of G-protein signaling (RGS), Ret-RGS1, the interaction of which we confirmed by coimmunoprecipitation with Galpha(o) in transfected cells and in retina. Ret-RGS1 localized to the dendritic tips of ON bipolar neurons, along with mGluR6 and Galpha(o1). When Ret-RGS1 was coexpressed in Xenopus oocytes with mGluR6, Galpha(o1), and a GIRK (G-protein-gated inwardly rectifying K+) channel, it accelerated the deactivation of the channel response to glutamate in a concentration-dependent manner. Because light onset suppresses glutamate release from photoreceptors onto the ON bipolar dendrites, Ret-RGS1 should accelerate the rising phase of the light response of the ON bipolar cell. This would tend to match its kinetics to that of the OFF bipolar that arises directly from ligand-gated channels.

Mouse models to study G-protein-mediated signaling

The G-protein-mediated signaling system has evolved as one of the most widely used transmembrane signaling mechanisms in eukaryotic organisms. Mammalian cells express many G-protein-coupled receptors as well as several types of heterotrimeric G-proteins and effectors. This review focuses on recent data from studies in mutant mice, which have elucidated some of the roles of G-protein-mediated signaling in physiology and pathophysiology.

Gbetagamma-activated inwardly rectifying K(+) (GIRK) channel activation kinetics via Galphai and Galphao-coupled receptors are determined by Galpha-specific interdomain interactions that affect GDP release rates

Gbetagamma-activated inwardly rectifying K(+) (GIRK) channels have distinct gating properties when activated by receptors coupled specifically to Galpha(o) versus Galpha(i) subunit isoforms, with Galpha(o)-coupled currents having approximately 3-fold faster agonist-evoked activation kinetics. To identify the molecular determinants in Galpha subunits mediating these kinetic differences, chimeras were constructed using pertussis toxin (PTX)-insensitive Galpha(oA) and Galpha(i2) mutant subunits (Galpha(oA(C351G)) and Galpha(i2(C352G))) and examined in PTX-treated Xenopus oocytes expressing muscarinic m2 receptors and Kir3.1/3.2a channels. These experiments revealed that the alpha-helical N-terminal region (amino acids 1-161) and the switch regions of Galpha(i2) (amino acids 162-262) both partially contribute to slowing the GIRK activation time course when compared with the Galpha(oA(C351G))-coupled response. When present together, they fully reproduce Galpha(i2(C352G))-coupled GIRK kinetics. The Galpha(i2) C-terminal region (amino acids 263-355) had no significant effect on GIRK kinetics. Complementary responses were observed with chimeras substituting the Galpha(o) switch regions into the Galpha(i2(C352G)) subunit, which partially accelerated the GIRK activation rate. The Galpha(oA)/Galpha(i2) chimera results led us to examine an interaction between the alpha-helical domain and the Ras-like domain previously implicated in mediating a 4-fold slower in vitro basal GDP release rate in Galpha(i1) compared with Galpha(o). Mutations disrupting the interdomain contact in Galpha(i2(C352G)) at either the alphaD-alphaE loop (R145A) or the switch III loop (L233Q/A236H/E240T/M241T), significantly accelerated the GIRK activation kinetics consistent with the Galpha(i2) interdomain interface regulating receptor-catalyzed GDP release rates in vivo. We propose that differences in Galpha(i) versus Galpha(o)-coupled GIRK activation kinetics are due to intrinsic differences in receptor-catalyzed GDP release that rate-limit Gbetagamma production and is attributed to heterogeneity in Galpha(i) and Galpha(o) interdomain contacts.

G-proteins as transducers in transmembrane signalling

The G-protein-mediated signalling system has evolved as one of the most widely used transmembrane signalling mechanisms in mammalian organisms. All mammalian cells express G-protein-coupled receptors as well as several types of heterotrimeric G-proteins and effectors. G-protein-mediated signalling is involved in many physiological and pathological processes. This review summarizes some general aspects of G-protein-mediated signalling and focusses on recent data especially from studies in mutant mice which have elucidated some of the cellular and biological functions of heterotrimeric G-prtoteins.

Noncanonical signaling by ionotropic kainate receptors

The potent neurotoxin kainate activates ion channel-forming receptors. However, it can also activate a G protein-coupled signaling pathway to inhibit transmitter release in central neurons. It remains unclear whether the same receptor complex is involved in both signaling activities. Here we show that in a population of dorsal root ganglion cells, exposure to kainate elicits a G protein-dependent increase in intracellular Ca2+. Furthermore, in these cells a brief exposure to kainate inhibited the K+-induced Ca2+ increase, a process that was sensitive to the G protein inhibitor Pertussis toxin and inhibitors of protein kinase C. This metabotropic action did not require ion channel activity and was not observed in neurons prepared from mice deficient for the ion channel-forming subunit GluR5. These results indicate that GluR5, an ion channel-forming subunit, signals through a second messenger cascade, inhibiting voltage-dependent Ca2+ channels. Thus, such a system represents a noncanonical signaling route of ion channel-forming receptors.

The dynamics of formation and action of the ternary complex revealed in living cells using a G-protein-gated K+ channel as a biosensor

Traditionally the consequences of activation of G-protein-coupled receptors (GPCRs) by an agonist are studied using biochemical assays. In this study we use live cells and take advantage of a G-protein-gated inwardly rectifying potassium channel (Kir3.1+3.2A) that is activated by the direct binding of Gbetagamma subunit to the channel complex to report, in real-time, using the patch clamp technique the activity of the "ternary complex" of agonist/receptor/G-protein. This analysis is further facilitated by the use of pertussis toxin-resistant fluorescent and non-fluorescent Galpha(i/o) subunits and a series of HEK293 cell lines stably expressing both channel and receptors (including the adenosine A(1) receptor, the adrenergic alpha(2A) receptor, the dopamine D(2S) receptor, the M4 muscarinic receptor, and the dimeric GABA-B(1b/2) receptor). We systematically analyzed the contribution of the various inputs to the observed kinetic response of channel activation. Our studies indicate that the combination of agonist, GPCR, and G-protein isoform uniquely specify the behavior of these channels and thus support the importance of the whole ternary complex at a kinetic level.

Adenosine A1 receptor-mediated presynaptic inhibition of GABAergic transmission in immature rat hippocampal CA1 neurons

In the mechanically dissociated rat hippocampal CA1 neurons with native presynaptic nerve endings, namely "synaptic bouton" preparation, the purinergic modulation of spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) was investigated using whole-cell recording mode under the voltage-clamp conditions. In immature neurons, adenosine (10 microM) reversibly decreased GABAergic mIPSC frequency without affecting the mean current amplitude. The inhibitory effect of adenosine transmission was completely blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 nM), a selective Alpha(1) receptor antagonist, and was mimicked by N(6)-cyclopentyladenosine (CPA, 1 microM), a selective Alpha(1) receptor agonist. However, CPA had no effect on GABAergic mIPSC frequency in postnatal 30 day neurons. N-ethylmaleimide (10 microM), a guanosine 5'-triphosphate binding protein uncoupler, and Ca(2+)-free external solution removed the CPA-induced inhibition of mIPSC frequency. K(+) channel blockers, 4-aminopyridine (100 microM) and Ba(2+) (1 mM), had no effect on the inhibitory effect of CPA on GABAergic mIPSC frequency. Stimulation of adenylyl cyclase with forskolin (10 microM) prevented the CPA action on GABAergic mIPSC frequency. Rp-cAMPS (100 microM), a selective PKA inhibitor, also blocked the CPA action. It was concluded that the activation of presynaptic Alpha(1) receptors modulates the probability of spontaneous GABA release via cAMP- and protein kinase A dependent pathway. This Alpha(1) receptor-mediated modulation of GABAergic transmission may play an important role in the regulation of excitability of immature hippocampal CA1 neurons.

Molecular biology and ontogeny of gamma-aminobutyric acid (GABA) receptors in the mammalian central nervous system

gamma-Aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the mammalian central nervous system. After release from nerve terminals, GABA binds to at least two classes of postsynaptic receptors (ie, GABAA and GABAB), which are nearly ubiquitous in the brain. GABAA receptors are postsynaptic heteropentameric complexes that display unique physiologic and pharmacologic properties based on subunit composition. Activation of GABAA receptors in mature neurons results in membrane hyperpolarization, which is mediated principally by inward chloride flux, whereas in early stages of brain development, GABAA receptor activation causes depolarization of the postsynaptic membrane. GABA, receptors reside both presynaptically and postsynaptically, exist as heterodimers and are coupled to voltage-dependent ion channels through interactions with heterotrimeric G proteins. This review summarizes the molecular biology and ontogeny of GABAA and GABAB receptors, highlighting some of their putative roles during normal brain development as well as in disease states such as epilepsy.

Gating properties of GIRK channels activated by Galpha(o)- and Galpha(i)-coupled muscarinic m2 receptors in Xenopus oocytes: the role of receptor precoupling in RGS modulation

'Regulators of G protein Signalling' (RGSs) accelerate the activation and deactivation kinetics of G protein-gated inwardly rectifying K(+) (GIRK) channels. In an apparent paradox, RGSs do not reduce steady-state GIRK current amplitudes as expected from the accelerated rate of deactivation when reconstituted in Xenopus oocytes. We present evidence here that this kinetic anomaly is dependent on the degree of G protein-coupled receptor (GPCR) precoupling, which varies with different Galpha(i/o)-RGS complexes. The gating properties of GIRK channels (Kir3.1/Kir3.2a) activated by muscarinic m2 receptors at varying levels of G protein expression were examined with or without the co-expression of either RGS4 or RGS7 in Xenopus oocytes. Different levels of specific m2 receptor-Galpha coupling were established by uncoupling endogenous pertussis toxin (PTX)-sensitive Galpha(i/o) subunits with PTX, while expressing varying amounts of a single PTX-insensitive subunit (Galpha(i1(C351G)), Galpha(i2(C352G)), Galpha(i3(C351G)), Galpha(oA(C351G)), or Galpha(oB(C351G))). Co-expression of each of the PTX-insensitive Galpha(i/o) subunits rescued acetylcholine (ACh)-elicited GIRK currents (I(K,ACh)) in a concentration-dependent manner, with Galpha(o) isoforms being more effective than Galpha(i) isoforms. Receptor-independent 'basal' GIRK currents (I(K,basal)) were reduced with increasing expression of PTX-insensitive Galpha subunits and were accompanied by a parallel rise in I(K,ACh). These effects together are indicative of increased Gbetagamma scavenging by the expressed Galpha subunit and the subsequent formation of functionally coupled m2 receptor-G protein heterotrimers (Galpha((GDP))betagamma). Co-expression of RGS4 accelerated all the PTX-insensitive Galpha(i/o)-coupled GIRK currents to a similar extent, yet reduced I(K,ACh) amplitudes 60-90 % under conditions of low Galpha(i/o) coupling. Kinetic analysis indicated the RGS4-dependent reduction in steady-state GIRK current was fully explained by the accelerated deactivation rate. Thus kinetic inconsistencies associated with RGS4-accelerated GIRK currents occur at a critical threshold of G protein coupling. In contrast to RGS4, RGS7 selectively accelerated Galpha(o)-coupled GIRK currents. Co-expression of Gbeta5, in addition to enhancing the kinetic effects of RGS7, caused a significant reduction (70-85 %) in steady-state GIRK currents indicating RGS7-Gbeta5 complexes disrupt Galpha(o) coupling. Altogether these results provide further evidence for a GPCR-Galphabetagamma-GIRK signalling complex that is revealed by the modulatory affects of RGS proteins on GIRK channel gating. Our functional experiments demonstrate that the formation of this signalling complex is markedly dependent on the concentration and composition of G protein-RGS complexes.

Modulation of hippocampal glutamatergic transmission by ATP is dependent on adenosine a(1) receptors

Excitatory glutamatergic synapses in the hippocampal CA1 region of rats are potently inhibited by purines, including adenosine, ATP, and ATP analogs. Adenosine A(1) receptors are known to mediate at least part of the response to adenine nucleotides, either because adenine nucleotides activate A(1) receptors directly, or activate them secondarily upon the nucleotides' conversion to adenosine. In the present studies, the inhibitory effects of adenosine, ATP, the purportedly stable ATP analog adenosine-5'-O-(3-thio)triphosphate (ATPgammaS), and cyclic AMP were examined in mice with a null mutation in the adenosine A(1) receptor gene. ATPgammaS displaced the binding of A(1)-selective ligands to intact brain sections and brain homogenates from adenosine A(1) receptor wild-type animals. In homogenates, but not in intact brain sections, this displacement was abolished by adenosine deaminase. In hippocampal slices from wild-type mice, purines abolished synaptic responses, but slices from mice lacking functional A(1) receptors showed no synaptic modulation by adenosine, ATP, cAMP, or ATPgammaS. In slices from heterozygous mice the dose-response curve for both adenosine and ATP was shifted to the right. In all cases, inhibition of synaptic responses by purines could be blocked by prior treatment with the competitive adenosine A(1) receptor antagonist 8-cyclopentyltheophylline. Taken together, these results show that even supposedly stable adenine nucleotides are rapidly converted to adenosine at sites close to the A(1) receptor, and that inhibition of synaptic transmission by purine nucleotides is mediated exclusively by A(1) receptors.

Nonselective cation conductance activated by muscarinic and purinergic receptors in rat spiral ganglion neurons

The present study characterizes the ionic conductances activated by acetylcholine (ACh) and ATP, two candidate neuromodulators, in isolated spiral ganglion neurons (SGNs). Brief application (1 s) of ACh evoked in a dose-dependent manner (EC(50) = 4.1 microM) a reversible inward current with a long latency (average 1.3 s), at holding potential (V(h)) = -50 mV. This current was reversibly blocked by atropine and mimicked by muscarine. Application of ATP also evoked a reversible inward current at V(h) = -50 mV, but the current showed two components. A fast component with a short latency was largely reduced when N-methyl-D-glucamine (NMDG) replaced extracellular sodium, implying a P2X-like ionotropic conductance. The second component had a longer latency (average 1.1 s) and was presumably activated by metabotropic P2Y-like receptors. The second component of ATP-evoked current shared similar characteristics with the responses evoked by ACh: the current reversed near 0 mV, displayed inward rectification, could be carried by NMDG, and was insensitive to extracellular and intracellular calcium. This ACh-/ATP-evoked conductance was reversibly inhibited by preapplication of ionomycin. These results suggest that muscarinic receptors and purinergic metabotropic receptors activate a similar large nonselective cation conductance via a common intracellular pathway in SGNs, a candidate mechanism to regulate neuronal excitability of SGNs.

Functional specificity of G alpha q and G alpha 11 in the cholinergic and glutamatergic modulation of potassium currents and excitability in hippocampal neurons

In hippocampal and other cortical neurons, action potentials are followed by a slow afterhyperpolarization (sAHP) generated by the activation of small-conductance Ca(2+)-activated K(+) channels and controlling spike frequency adaptation. The corresponding current, the apamin-insensitive sI(AHP), is a well known target of modulation by different neurotransmitters, including acetylcholine (via M(3) receptors) and glutamate (via metabotropic glutamate receptor 5, mGluR(5)), in CA1 pyramidal neurons. The actions of muscarinic and mGluR agonists on sI(AHP) involve the activation of pertussis toxin-insensitive G-proteins. However, the pharmacological tools available so far did not permit the identification of the specific G-protein subtypes transducing the effects of M(3) and mGluR(5) on sI(AHP). In the present study, we used mice deficient in the Galpha(q) and Galpha(11) genes to investigate the specific role of these G-protein alpha subunits in the cholinergic and glutamatergic modulation of sI(AHP) in CA1 neurons. In mice lacking Galpha(q), the effects of muscarinic and glutamatergic agonists on sI(AHP) were nearly abolished, whereas beta-adrenergic agonists acting via Galpha(s) were still fully effective. Modulation of sI(AHP) by any of these agonists was instead unchanged in mice lacking Galpha(11). The additional depolarizing effects of muscarinic and glutamatergic agonists on CA1 neurons were preserved in mice lacking Galpha(q) or Galpha(11). Thus, Galpha(q), but not Galpha(11), mediates specifically the action of cholinergic and glutamatergic agonists on sI(AHP), without affecting the modulation of other currents. These results provide to our knowledge one of the first examples of the functional specificity of Galpha(q) and Galpha(11) in central neurons.

The chemokine interleukin-8 acutely reduces Ca(2+) currents in identified cholinergic septal neurons expressing CXCR1 and CXCR2 receptor mRNAs

The chemokine IL-8 is known to be synthesized by glial cells in the brain. It has traditionally been shown to have an important role in neuroinflammation but recent evidence indicates that it may also be involved in rapid signaling in neurons. We investigated how IL-8 participates in rapid neuronal signaling by using a combination of whole-cell recording and single-cell RT-PCR on dissociated rat septal neurons. We show that IL-8 can acutely reduce Ca(2+) currents in septal neurons, an effect that was concentration-dependent, involved the closure of L- and N-type Ca(2+) channels, and the activation of G(ialpha1) and/or G(ialpha2) subtype(s) of G-proteins. Analysis of the mRNAs from the recorded neurons revealed that the latter were all cholinergic in nature. Moreover, we found that all cholinergic neurons that responded to IL-8, expressed mRNAs for either one or both IL-8 receptors CXCR1 and CXCR2. This is the first report of a chemokine that modulates ion channels in neurons via G-proteins, and the first demonstration that mRNAs for CXCR1 are expressed in the brain. Our results suggest that IL-8 release by glial cells in vivo may activate CXCR1 and CXCR2 receptors on cholinergic septal neurons and acutely modulate their excitability by closing calcium channels.

The light response of ON bipolar neurons requires G[alpha]o

ON bipolar neurons in retina detect the glutamate released by rods and cones via metabotropic glutamate receptor 6 (mGluR6), whose cascade is unknown. The trimeric G-protein G(o) might mediate this cascade because it colocalizes with mGluR6. To test this, we studied the retina in mice negative for the alpha subunit of G(o) (Galpha(o)-/-). Retinal layering, key cell types, synaptic structure, and mGluR6 expression were all normal, as was the a-wave of the electroretinogram, which represents the rod and cone photocurrents. However, the b-wave of the electroretinogram, both rod- and cone-driven components, was entirely missing. Because the b-wave represents the massed response of ON bipolar cells, its loss in the Galpha(o) null mouse establishes that the light response of the ON bipolar cell requires G(o). This represents the first function to be defined in vivo for the alpha subunit of the most abundant G-protein of the brain.