Functional characterization of an endogenous Xenopus oocyte adenosine receptor

The "www" of Xenopus laevis Oocytes: The Why, When, What of Xenopus laevis Oocytes in Membrane Transporters Research

After 50 years, the heterologous expression of proteins in Xenopus laevis oocytes is still essential in many research fields. New approaches and revised protocols, but also classical methods, such as the two-electrode voltage clamp, are applied in studying membrane transporters. New and old methods for investigating the activity and the expression of Solute Carriers (SLC) are reviewed, and the kinds of experiment that are still useful to perform with this kind of cell are reported. Xenopus laevis oocytes at the full-grown stage have a highly efficient biosynthetic apparatus that correctly targets functional proteins at the defined compartment. This small protein factory can produce, fold, and localize almost any kind of wild-type or recombinant protein; some tricks are required to obtain high expression and to verify the functionality. The methodologies examined here are mainly related to research in the field of membrane transporters. This work is certainly not exhaustive; it has been carried out to be helpful to researchers who want to quickly find suggestions and detailed indications when investigating the functionality and expression of the different members of the solute carrier families.

Hydrogen sulfide stimulates CFTR in Xenopus oocytes by activation of the cAMP/PKA signalling axis

Hydrogen sulfide (H₂S) has been recognized as a signalling molecule which affects the activity of ion channels and transporters in epithelial cells. The cystic fibrosis transmembrane conductance regulator (CFTR) is an epithelial anion channel and a key regulator of electrolyte and fluid homeostasis. In this study, we investigated the regulation of CFTR by H₂S. Human CFTR was heterologously expressed in Xenopus oocytes and its activity was electrophysiologically measured by microelectrode recordings. The H₂S-forming sulphur salt Na₂S as well as the slow-releasing H₂S-liberating compound GYY4137 increased transmembrane currents of CFTR-expressing oocytes. Na₂S had no effect on native, non-injected oocytes. The effect of Na₂S was blocked by the CFTR inhibitor CFTR_inh172, the adenylyl cyclase inhibitor MDL 12330A, and the protein kinase A antagonist cAMPS-Rp. Na₂S potentiated CFTR stimulation by forskolin, but not that by IBMX. Na₂S enhanced CFTR stimulation by membrane-permeable 8Br-cAMP under inhibition of adenylyl cyclase-mediated cAMP production by MDL 12330A. These data indicate that H₂S activates CFTR in Xenopus oocytes by inhibiting phosphodiesterase activity and subsequent stimulation of CFTR by cAMP-dependent protein kinase A. In epithelia, an increased CFTR activity may correspond to a pro-secretory response to H₂S which may be endogenously produced by the epithelium or H₂S-generating microflora.

Permissive role of reduced inwardly-rectifying potassium current density in the automaticity of the guinea pig pulmonary vein myocardium

The electrophysiological properties underlying the automaticity of the guinea pig pulmonary vein myocardium were studied. About 30% of the isolated pulmonary vein tissue preparations showed spontaneous electrical activity, as shown by glass microelectrode recordings from their myocardial layer. The remaining quiescent preparations had a resting membrane potential less negative than that in the left atria. Blockade of the acetylcholine activated potassium current (I_K-ACh) by tertiapin induced a depolarizing shift of the resting membrane potential and automatic electrical activity in the pulmonary vein, but not in the atria. The tertiapin-induced electrical activity, as well as the spontaneous activity, was inhibited by the application of carbachol or by chelation of intracellular Ca²⁺ by BAPTA. The isolated pulmonary vein cardiomyocytes had an I_K-ACh density similar to that of the atrial cardiomyocytes, but a lower density of the inwardly-rectifying potassium current (I_K1). Spontaneous Ca²⁺ transients were observed in about 30% of the isolated pulmonary vein cardiomyocytes, but not in atrial cardiomyocytes. The Ca²⁺ transients in the pulmonary vein cardiomyocytes were induced by tertiapin and inhibited by carbachol. These results indicate that the pulmonary vein cardiomyocytes have a reduced density of the inwardly-rectifying potassium current, which plays a permissive role in their intracellular Ca²⁺-dependent automaticity.

Mechanosensitive activation of CFTR by increased cell volume and hydrostatic pressure but not shear stress

The cystic fibrosis transmembrane conductance regulator (CFTR) is a Cl(-) channel that is essential for electrolyte and fluid homeostasis. Preliminary evidence indicates that CFTR is a mechanosensitive channel. In lung epithelia, CFTR is exposed to different mechanical forces such as shear stress (Ss) and membrane distention. The present study questioned whether Ss and/or stretch influence CFTR activity (wild type, ∆F508, G551D). Human CFTR (hCFTR) was heterologously expressed in Xenopus oocytes and the response to the mechanical stimulus and forskolin/IBMX (FI) was measured by two-electrode voltage-clamp experiments. Ss had no influence on hCFTR activity. Injection of an intracellular analogous solution to increase cell volume alone did not affect hCFTR activity. However, hCFTR activity was augmented by injection after pre-stimulation with FI. The response to injection was similar in channels carrying the common mutations ∆F508 and G551D compared to wild type hCFTR. Stretch-induced CFTR activation was further assessed in Ussing chamber measurements using Xenopus lung preparations. Under control conditions increased hydrostatic pressure (HP) decreased the measured ion current including activation of a Cl(-) secretion that was unmasked by the CFTR inhibitor GlyH-101. These data demonstrate activation of CFTR in vitro and in a native pulmonary epithelium in response to mechanical stress. Mechanosensitive regulation of CFTR is highly relevant for pulmonary physiology that relies on ion transport processes facilitated by pulmonary epithelial cells.

Adenosine receptors mediated intracellular calcium in cumulus cells involved in the maintenance of first meiotic arrest

Keeping the intact germinal vesicle (GV) is essential for maintaining the capacity of mammals including human. It is maintained by very complex procedures along with folliculogenesis and is a critical step for getting competent oocyte. So far, a few mechanisms involved in folliculogenesis are known but GV arrest mechanisms are largely unrevealed. Cyclic AMP, a adenosine derived substance, have been used as inhibitor of germinal vesicle breakdown as a putative oocyte maturation inhibitor. In this study, we examined the potency of adenosine as GV maintainer and a possible signaling mediator for that. A1, A2b, and A3 were detected in cumulus cells of cumulus enclosed-oocyte (CEO). Intact of germinal vesicle was not kept like in follicle but the spontaneous maturation was inhibited by exogenous adenosine. It is inhibited with concentration dependent manners. Intracellular calcium level of cumulus was extensively increased after adenosine treatment. Based on these results it is suggested that one of the pathway for GV arrest by adenosine and its receptors is calcium mediated signaling pathway in CEO.

Purinergic signalling - a possible mechanism for KCNQ1 channel response to cell volume challenges

AIM

A number of K(+) channels are regulated by small, fast changes in cell volume. The mechanisms underlying cell volume sensitivity are not known, but one possible mechanism could be purinergic signalling. Volume activated ATP release could trigger signalling pathways that subsequently lead to ion channel stimulation and cell volume back-regulation. Our aim was to investigate whether volume sensitivity of the voltage-gated K(+) channel, KCNQ1, is dependent on ATP release and regulation by purinergic signalling.

METHODS

We used Xenopus oocytes heterologously expressing human KCNQ1, KCNE1, water channels (AQP1) and P2Y2 receptors. ATP release was monitored by a luciferin-luciferase assay and ion channel conductance was recorded by two-electrode voltage clamp.

RESULTS

The luminescence assay showed that oocytes released ATP in response to mechanical, hypoosmotic stimuli and hyperosmotic stimuli. Basal ATP release was approx. three times higher in the KCNQ1 + AQP1 and KCNQ1 injected oocytes compared to the non-injected ones. Exogenously added ATP (0.1 mm) did not have any substantial effect on volume-induced KCNQ1 currents. Nevertheless, apyrase decreased all currents by about 50%. Suramin inhibited about 23% of the KCNQ1 volume sensitivity. Expression of P2Y2 receptors stimulated endogenous Cl(-) channels, but it also led to 68% inhibition of the KCNQ1 currents. Adenosine (0.1 mm) also inhibited the KCNQ1 currents by about 56%.

CONCLUSION

Xenopus oocytes release ATP in response to mechanical stimuli and cell volume changes. Purinergic P2 and P1 receptors confer some of the KCNQ1 channel volume sensitivity, although endogenous adenosine receptors and expressed P2Y2 receptors do so in the negative direction.

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.

Naringin directly activates inwardly rectifying potassium channels at an overlapping binding site to tertiapin-Q

BACKGROUND

G protein-coupled inwardly rectifying potassium (K(IR) 3) channels are important proteins that regulate numerous physiological processes including excitatory responses in the CNS and the control of heart rate. Flavonoids have been shown to have significant health benefits and are a diverse source of compounds for identifying agents with novel mechanisms of action.

EXPERIMENTAL APPROACH

The flavonoid glycoside, naringin, was evaluated on recombinant human K(IR) 3.1-3.4 and K(IR) 3.1-3.2 expressed in Xenopus oocytes using two-electrode voltage clamp methods. In addition, we evaluated the activity of naringin alone and in the presence of the K(IR) 3 channel blocker tertiapin-Q (0.5 nM, 1 nM and 3 nM) at recombinant K(IR) 3.1-3.4 channels. Site-directed mutagenesis was used to identify amino acids within the M1-M2 loop of the K(IR) 3.1(F137S) mutant channel important for naringin's activity.

KEY RESULTS

Naringin (100 µM) had minimal effect on uninjected oocytes but activated K(IR) 3.1-3.4 and K(IR) 3.1-3.2 channels. The activation by naringin of K(IR) 3.1-3.4 channels was inhibited by tertiapin-Q in a competitive manner. An alanine-scan performed on the K(IR) 3.1(F137S) mutant channel, replacing one by one aromatic amino acids within the M1-M2 loop, identified tyrosines 148 and 150 to be significantly contributing to the affinity of naringin as these mutations reduced the activity of naringin by 20- and 40-fold respectively.

CONCLUSIONS AND IMPLICATIONS

These results show that naringin is a direct activator of K(IR) 3 channels and that tertiapin-Q shares an overlapping binding site on the K(IR) 3.1-3.4. This is the first example of a ligand that activates K(IR) 3 channels by binding to the extracellular M1-M2 linker of the channel.

Aldehyde dehydrogenase activation prevents reperfusion arrhythmias by inhibiting local renin release from cardiac mast cells

BACKGROUND

Renin released by ischemia/reperfusion from cardiac mast cells activates a local renin-angiotensin system (RAS). This exacerbates norepinephrine release and reperfusion arrhythmias (ventricular tachycardia and fibrillation), making RAS a new therapeutic target in myocardial ischemia.

METHODS AND RESULTS

We investigated whether ischemic preconditioning (IPC) prevents cardiac RAS activation in guinea pig hearts ex vivo. When ischemia/reperfusion (20 minutes of ischemia/30 minutes of reperfusion) was preceded by IPC (two 5-minute ischemia/reperfusion cycles), renin and norepinephrine release and ventricular tachycardia and fibrillation duration were markedly decreased, a cardioprotective anti-RAS effect. Activation and blockade of adenosine A(2b)/A(3) receptors and activation and inhibition of protein kinase Cepsilon (PKCepsilon) mimicked and prevented, respectively, the anti-RAS effects of IPC. Moreover, activation of A(2b)/A(3) receptors or activation of PKCepsilon prevented degranulation and renin release elicited by peroxide in cultured mast cells (HMC-1). Activation and inhibition of mitochondrial aldehyde dehydrogenase type-2 (ALDH2) also mimicked and prevented, respectively, the cardioprotective anti-RAS effects of IPC. Furthermore, ALDH2 activation inhibited degranulation and renin release by reactive aldehydes in HMC-1. Notably, PKCepsilon and ALDH2 were both activated by A(2b)/A(3) receptor stimulation in HMC-1, and PKCepsilon inhibition prevented ALDH2 activation.

CONCLUSIONS

The results uncover a signaling cascade initiated by A(2b)/A(3) receptors, which triggers PKCepsilon-mediated ALDH2 activation in cardiac mast cells, contributing to IPC-induced cardioprotection by preventing mast cell renin release and the dysfunctional consequences of local RAS activation. Thus, unlike classic IPC in which cardiac myocytes are the main target, cardiac mast cells are the critical site at which the cardioprotective anti-RAS effects of IPC develop.

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.

Blockage of A2A and A3 adenosine receptors decreases the desensitization of human GABA(A) receptors microtransplanted to Xenopus oocytes

We previously found that the endogenous anticonvulsant adenosine, acting through A(2A) and A(3) adenosine receptors (ARs), alters the stability of currents (I(GABA)) generated by GABA(A) receptors expressed in the epileptic human mesial temporal lobe (MTLE). Here we examined whether ARs alter the stability (desensitization) of I(GABA) expressed in focal cortical dysplasia (FCD) and in periglioma epileptic tissues. The experiments were performed with tissues from 23 patients, using voltage-clamp recordings in Xenopus oocytes microinjected with membranes isolated from human MTLE and FCD tissues or using patch-clamp recordings of pyramidal neurons in epileptic tissue slices. On repetitive activation, the epileptic GABA(A) receptors revealed instability, manifested by a large I(GABA) rundown, which in most of the oocytes (approximately 70%) was obviously impaired by the new A(2A) antagonists ANR82, ANR94, and ANR152. In most MTLE tissue-microtransplanted oocytes, a new A(3) receptor antagonist (ANR235) significantly improved I(GABA) stability. Moreover, patch-clamped pyramidal neurons from human neocortical slices of periglioma epileptic tissues exhibited altered I(GABA) rundown on ANR94 treatment. Our findings indicate that antagonizing A(2A) and A(3) receptors increases the I(GABA) stability in different epileptic tissues and suggest that adenosine derivatives may offer therapeutic opportunities in various forms of human epilepsy.

Pregnenolone sulfate potentiates the inwardly rectifying K channel Kir2.3

Background

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

Methodology/Principal Findings

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

Conclusions/Significance

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

Interaction of neurochondrin with the melanin-concentrating hormone receptor 1 interferes with G protein-coupled signal transduction but not agonist-mediated internalization

Screening of a human brain cDNA library using the C-terminal tail of the melanin-concentrating hormone receptor 1 (MCHR1) as bait in a yeast two-hybrid assay resulted in the identification of the neurite-outgrowth related factor, neurochondrin. This interaction was verified in overlay, pulldown, and co-immunoprecipitation assays. Deletion mapping confined the binding to the C terminus of neurochondrin and to the proximal C terminus of MCHR1, a region known to be involved in G protein binding and signal transduction. This region of the MCHR1 is also able to interact with the actin- and intermediate filament-binding protein, periplakin. Interactions of MCHR1 with neurochondrin and periplakin were competitive, indicating that these two proteins bind to overlapping regions of MCHR1. Although neurochondrin did not interfere with melanin-concentrating hormone-mediated internalization of the receptor, it did inhibit G protein-coupled signal transduction via both Galpha(i/o) and Galpha(q/11) family G proteins as measured by each of melanin-concentrating hormone-induced G protein-activated inwardly rectifying K(+) channel activity of voltage-clamped amphibian oocytes, by calcium mobilization in transfected mammalian cells, and by reduction in the capacity of melanin-concentrating hormone to promote binding of [(35)S]guanosine 5'-3-O-(thio)triphosphate to both Galpha(o1) and Galpha(11). Immunohistochemistry revealed co-expression of neurochondrin and MCHR1 within the rodent brain, suggesting that neurochondrin may be involved in the regulation of MCHR1 signaling and play a role in modulating melanin-concentrating hormone-mediated functions in vivo.

Inhibition of G protein-activated inwardly rectifying K+ channels by ifenprodil

G protein-activated inwardly rectifying K+ channels (GIRK, also known as Kir3) are regulated by various G-protein-coupled receptors. Activation of GIRK channels plays an important role in reducing neuronal excitability in most brain regions and the heart rate. Ifenprodil, which is a clinically used cerebral vasodilator, interacts with several receptors, such as alpha1 adrenergic, N-methyl-D-aspartate, serotonin and sigma receptors. However, the molecular mechanisms underlying the various clinically related effects of ifenprodil remain to be clarified. Here, we examined the effects of ifenprodil on GIRK channels by using Xenopus oocyte expression assays. In oocytes injected with mRNAs for GIRK1/GIRK2, GIRK2 or GIRK1/GIRK4 subunits, ifenprodil reversibly reduced inward currents through the basal GIRK activity. The inhibition was concentration-dependent, but voltage- and time-independent, suggesting that ifenprodil may not act as an open channel blocker of the channels. In contrast, Kir1.1 and Kir2.1 channels in other Kir channel subfamilies were insensitive to ifenprodil. Furthermore, GIRK current responses activated by the cloned kappa-opioid receptor were similarly inhibited by ifenprodil. The inhibitory effects of ifenprodil were not observed when ifenprodil was applied intracellularly, and were not affected by extracellular pH, which changed the proportion of the uncharged to protonated ifenprodil, suggesting its action from the extracellular side. The GIRK currents induced by ethanol were also attenuated in the presence of ifenprodil. Our results suggest that direct inhibition of GIRK channels by ifenprodil, at submicromolar concentrations or more, may contribute to some of its therapeutic effects and adverse side effects.

G protein-activated inwardly rectifying K+ channel inhibition and rescue of weaver mouse motor functions by antidepressants

Antidepressants, including tricyclic antidepressants (TCAs) and selective serotonin reuptake inhibitors (SSRIs), have been widely used for the treatment of not only depression but also other psychiatric disorders, although the molecular mechanisms of the drug effects have not yet been sufficiently revealed. Here, we investigated the in vivo effects of these antidepressants on G protein-activated inwardly rectifying K+ (GIRK) channels, which are important for regulating the excitability of various cells, by using weaver (wv) mice, which have mutant GIRK channels and show abnormal neuronal cell death and motor disturbances. First, we found that a widely used SSRI fluoxetine (also known as Prozac) effectively inhibited wv GIRK2 channels like wild-type GIRK channels, expressed in Xenopus oocytes. Next, we found that weaver motor disturbances were remarkably alleviated by chronic treatment with fluoxetine or desipramine. Furthermore, the chronic fluoxetine treatment substantially suppressed the abnormal neuronal cell death in the weaver mouse cerebellum and pontine nuclei. These results suggest that continuous inhibition of wv GIRK2 channels by a group of antidepressants caused substantial suppression of the neuronal cell death and resulted in improvement of motor abilities in weaver mice. These results provide evidence for in vivo GIRK channel inhibition by a group of antidepressants.

Inhibition of G Protein-Activated Inwardly Rectifying K+ Channels by the Antidepressant Paroxetine

Paroxetine is commonly used as a selective serotonin reuptake inhibitor for the treatment of depression and other psychiatric disorders. However, the molecular mechanisms of the paroxetine effects have not yet been sufficiently clarified. Using Xenopus oocyte expression assays, we investigated the effects of paroxetine on G protein-activated inwardly rectifying K+ (GIRK) channels, which play an important role in reducing neuronal excitability in most brain regions and the heart rate. In oocytes injected with mRNAs for GIRK1/GIRK2, GIRK2, or GIRK1/GIRK4 subunits, paroxetine reversibly reduced inward currents through the expressed GIRK channels. The inhibition was concentration-dependent, but voltage-independent and time-independent during each voltage pulse. However, two structurally different antidepressants: milnacipran and trazodone, caused only a small inhibition of basal GIRK currents. Additionally, Kir1.1 and Kir2.1 channels were insensitive to all of the antidepressants. Furthermore, the GIRK currents induced by activation of A1 adenosine receptors or by ethanol were inhibited by extracellularly applied paroxetine in a concentration-dependent manner, but not affected by intracellularly applied paroxetine. Our results suggest that inhibition of GIRK channels by paroxetine may contribute partly to some of its therapeutic effects and adverse side effects.

Effects of interferon-α on cloned opioid receptors expressed in Xenopus oocytes

Interferon-alpha (IFNalpha) affects the opioid system. However, the direct action of IFNalpha on cloned opioid receptors remains unknown. Taking advantage of the functional coupling of cloned opioid receptors to G protein-activated inwardly rectifying K+ (GIRK) channels in a Xenopus oocyte expression system, we investigated the effects of recombinant IFNalpha on cloned mu-, delta- and kappa-opioid receptors. In oocytes co-injected with mRNAs for either the delta- or kappa-opioid receptor and for GIRK channel subunits, IFNalpha at high concentrations induced small GIRK currents that were abolished by naloxone, an opioid-receptor antagonist, compared with the control responses to each selective opioid agonist. Additionally, IFNalpha induced no significant current response in oocytes injected with mRNA(s) for either opioid receptor alone or GIRK channels. In oocytes expressing the mu-opioid receptor and GIRK channels, IFNalpha had little or no effect. Moreover, in oocytes expressing each opioid receptor and GIRK channels, GIRK current responses to each selective opioid agonist were not affected by the presence of IFNalpha, indicating no significant antagonism of IFNalpha toward the opioid receptors. Furthermore, IFNalpha had little or no effect on the mu/delta-, delta/kappa- or mu/kappa-opioid receptors expressed together with GIRK channels in oocytes. Our results suggest that IFNalpha weakly activates the delta and kappa-opioid receptors. The direct activation of the delta- and kappa-opioid receptors by IFNalpha may partly contribute to some of the IFNalpha effects under its high-dose medication.

Inhibition of G protein-activated inwardly rectifying K+ channels by various antidepressant drugs

G protein-activated inwardly rectifying K+ channels (GIRK, also known as Kir3) are activated by various G protein-coupled receptors. GIRK channels play an important role in the inhibitory regulation of neuronal excitability in most brain regions and the heart rate. Modulation of GIRK channel activity may affect many brain functions. Here, we report the inhibitory effects of various antidepressants: imipramine, desipramine, amitriptyline, nortriptyline, clomipramine, maprotiline, and citalopram, on GIRK channels. In Xenopus oocytes injected with mRNAs for GIRK1/GIRK2, GIRK2 or GIRK1/GIRK4 subunits, the various antidepressants tested, except fluvoxamine, zimelidine, and bupropion, reversibly reduced inward currents through the basal GIRK activity at micromolar concentrations. The inhibitions were concentration-dependent with various degrees of potency and effectiveness, but voltage- and time-independent. In contrast, Kir1.1 and Kir2.1 channels in other Kir channel subfamilies were insensitive to all of the drugs. Furthermore, GIRK current responses activated by the cloned A1 adenosine receptor were similarly inhibited by the tricyclic antidepressant desipramine. The inhibitory effects of desipramine were not observed when desipramine was applied intracellularly, and were not affected by extracellular pH, which changed the proportion of the uncharged to protonated desipramine, suggesting its action from the extracellular side. The GIRK currents induced by ethanol were also attenuated in the presence of desipramine. Our results suggest that inhibition of GIRK channels by the tricyclic antidepressants and maprotiline may contribute to some of the therapeutic effects and adverse side effects, especially seizures and atrial arrhythmias in overdose, observed in clinical practice.

Inhibition of G protein-activated inwardly rectifying K+ channels by fluoxetine (Prozac)

1. The effects of fluoxetine, a commonly used antidepressant drug, on G protein-activated inwardly rectifying K(+) channels (GIRK, Kir3) were investigated using Xenopus oocyte expression assays. 2. In oocytes injected with mRNAs for GIRK1/GIRK2, GIRK2 or GIRK1/GIRK4 subunits, fluoxetine reversibly reduced inward currents through the basal GIRK activity. The inhibition by fluoxetine showed a concentration-dependence, a weak voltage-dependence and a slight time-dependence with a predominant effect on the instantaneous current elicited by voltage pulses and followed by slight further inhibition. Furthermore, in oocytes expressing GIRK1/2 channels and the cloned Xenopus A(1) adenosine receptor, GIRK current responses activated by the receptor were inhibited by fluoxetine. In contrast, ROMK1 and IRK1 channels in other Kir channel subfamilies were insensitive to fluoxetine. 3. The inhibitory effect on GIRK channels was not obtained by intracellularly applied fluoxetine, and not affected by extracellular pH, which changed the proportion of the uncharged to protonated fluoxetine, suggesting that fluoxetine inhibits GIRK channels from the extracellular side. 4. The GIRK currents induced by ethanol were also attenuated in the presence of fluoxetine. 5. We demonstrate that fluoxetine, at low micromolar concentrations, inhibits GIRK channels that play an important role in the inhibitory regulation of neuronal excitability in most brain regions and the heart rate through activation of various G-protein-coupled receptors. The present results suggest that inhibition of GIRK channels by fluoxetine may contribute to some of its therapeutic effects and adverse side effects, particularly seizures in overdose, observed in clinical practice.