RESUMEN
The master control of mammalian circadian rhythms is the suprachiasmatic nucleus (SCN), which is formed by the ventral and dorsal regions. In SCN neurons, GABA has an important function and even excitatory actions in adulthood. However, the physiological role of this neurotransmitter in the developing SCN is unknown. Here, we recorded GABAergic postsynaptic currents (in the perforated-patch configuration using gramicidin) to determine the chloride reversal potential (ECl) and also assessed the immunological expression of the Na-K-Cl cotransporter 1 (NKCC1) at early ages of the rat (postnatal days (P) 3 to 25), during the day and night, in the two SCN regions. We detected that ECl greatly varied with age and depending on the SCN region and time of day. Broadly speaking, ECl was more hyperpolarized with age, except for the oldest age studied (P20-25) in both day and night in the ventral SCN, where it was less negative. Likewise, ECl was more hyperpolarized in the dorsal SCN both during the day and at night; while ECl was more negative at night both in the ventral and the dorsal SCN. Moreover, the total NKCC1 fluorescent expression was higher during the day than at night. These results imply that NKCC1 regulates the circadian and developmental fluctuations in the [Cl-]i to fine-tune ECl, which is crucial for either excitatory or inhibitory GABAergic actions to occur in the SCN.
Asunto(s)
Cloruros , Ritmo Circadiano , Miembro 2 de la Familia de Transportadores de Soluto 12 , Núcleo Supraquiasmático , Animales , Núcleo Supraquiasmático/metabolismo , Ritmo Circadiano/fisiología , Ratas , Miembro 2 de la Familia de Transportadores de Soluto 12/metabolismo , Masculino , Cloruros/metabolismo , Ácido gamma-Aminobutírico/metabolismo , Ratas Wistar , Técnicas de Placa-Clamp , Envejecimiento/fisiologíaRESUMEN
Circadian rhythms synchronize to light through the retinohypothalamic tract (RHT), which is a bundle of axons coming from melanopsin retinal ganglion cells, whose synaptic terminals release glutamate to the ventral suprachiasmatic nucleus (SCN). Activation of AMPA-kainate and NMDA postsynaptic receptors elicits the increase in intracellular calcium required for triggering the signaling cascade that ends in phase shifts. During aging, there is a decline in the synchronization of circadian rhythms to light. With electrophysiological (whole-cell patch-clamp) and immunohistochemical assays, in this work, we studied pre- and postsynaptic properties between the RHT and ventral SCN neurons in young adult (P90-120) and old (P540-650) C57BL/6J mice. Incremental stimulation intensities (applied on the optic chiasm) induced much lesser AMPA-kainate postsynaptic responses in old animals, implying a lower recruitment of RHT fibers. Conversely, a higher proportion of old SCN neurons exhibited synaptic facilitation, and variance-mean analysis indicated an increase in the probability of release in RHT terminals. Moreover, both spontaneous and miniature postsynaptic events displayed larger amplitudes in neurons from aged mice, whereas analysis of the NMDA and AMPA-kainate components (evoked by RHT electrical stimulation) disclosed no difference between the two ages studied. Immunohistochemistry revealed a bigger size in the puncta of vGluT2, GluN2B, and GluN2A of elderly animals, and the number of immunopositive particles was increased, but that of PSD-95 was reduced. All these synaptic adaptations could be part of compensatory mechanisms in the glutamatergic signaling to ameliorate the loss of RHT terminals in old animals.
Asunto(s)
Envejecimiento , Ácido Glutámico , Ratones Endogámicos C57BL , Núcleo Supraquiasmático , Transmisión Sináptica , Animales , Ratones , Núcleo Supraquiasmático/fisiología , Núcleo Supraquiasmático/metabolismo , Transmisión Sináptica/fisiología , Envejecimiento/fisiología , Ácido Glutámico/metabolismo , Masculino , Potenciales Postsinápticos Excitadores/fisiología , Vías Visuales/fisiología , Proteína 2 de Transporte Vesicular de Glutamato/metabolismo , Técnicas de Placa-Clamp , Receptores de N-Metil-D-Aspartato/metabolismo , Homólogo 4 de la Proteína Discs Large/metabolismoRESUMEN
The suprachiasmatic nucleus (SCN) is the most important circadian clock in mammals. The SCN synchronizes to environmental light via the retinohypothalamic tract (RHT), which is an axon cluster derived from melanopsin-expressing intrinsic photosensitive retinal ganglion cells. Investigations on the development of the nonimage-forming pathway and the RHT are scarce. Previous studies imply that light stimulation during postnatal development is not needed to make the RHT functional at adult stages. Here, we examined the effects of light deprivation (i.e., constant darkness (DD) rearing) during postnatal development on the expression in the ventral SCN of two crucial proteins for the synchronization of circadian rhythms to light: the presynaptic vesicular glutamate transporter type 2 (vGluT2) and the GluN2B subunit of the postsynaptic NMDA receptor. We found that animals submitted to DD conditions exhibited a transitory reduction in the expression of vGluT2 (at P12-19) and of GluN2B (at P7-9) that was compensated at older stages. These findings support the hypothesis that visual stimulation during early ages is not decisive for normal development of the RHT-SCN pathway.
Asunto(s)
Receptores de N-Metil-D-Aspartato , Núcleo Supraquiasmático , Proteína 2 de Transporte Vesicular de Glutamato , Animales , Ratas , Ritmo Circadiano/fisiología , Mamíferos/metabolismo , Receptores de N-Metil-D-Aspartato/genética , Receptores de N-Metil-D-Aspartato/metabolismo , Células Ganglionares de la Retina/metabolismo , Núcleo Supraquiasmático/metabolismo , Proteína 2 de Transporte Vesicular de Glutamato/metabolismoRESUMEN
Inhibitory regulation of the heart is determined by both cholinergic M2 receptors (M2R) and adenosine A1 receptors (A1R) that activate the same signaling pathway, the ACh-gated inward rectifier K+ (KACh) channels via Gi/o proteins. Previously, we have shown that the agonist-specific voltage sensitivity of M2R underlies several voltage-dependent features of IKACh, including the 'relaxation' property, which is characterized by a gradual increase or decrease of the current when cardiomyocytes are stepped to hyperpolarized or depolarized voltages, respectively. However, it is unknown whether membrane potential also affects A1R and how this could impact IKACh. Upon recording whole-cell currents of guinea-pig cardiomyocytes, we found that stimulation of the A1R-Gi/o-IKACh pathway with adenosine only caused a very slight voltage dependence in concentration-response relationships (~1.2-fold EC50 increase with depolarization) that was not manifested in the relative affinity, as estimated by the current deactivation kinetics (τ = 4074 ± 214 ms at -100 mV and τ = 4331 ± 341 ms at +30 mV; P = 0.31). Moreover, IKACh did not exhibit relaxation. Contrarily, activation of the M2R-Gi/o-IKACh pathway with acetylcholine induced the typical relaxation of the current, which correlated with the clear voltage-dependent effect observed in the concentration-response curves (~2.8-fold EC50 increase with depolarization) and in the IKACh deactivation kinetics (τ = 1762 ± 119 ms at -100 mV and τ = 1503 ± 160 ms at +30 mV; P = 0.01). Our findings further substantiate the hypothesis of the agonist-specific voltage dependence of GPCRs and that the IKACh relaxation is consequence of this property.
Asunto(s)
Acetilcolina/farmacología , Agonistas del Receptor de Adenosina A1/farmacología , Adenosina/farmacología , Activación del Canal Iónico/efectos de los fármacos , Miocitos Cardíacos/metabolismo , Canales de Potasio/metabolismo , Receptor de Adenosina A1/metabolismo , Animales , Femenino , Cobayas , Masculino , Receptor Muscarínico M2/agonistas , Receptor Muscarínico M2/metabolismoRESUMEN
The hypothalamic suprachiasmatic nucleus (SCN) is the leading circadian pacemaker in mammals, which synchronizes with environmental light through the retinohypothalamic tract (RHT). Although the SCN regulates circadian rhythms before birth, postnatal synaptic changes are needed for the RHT-SCN pathway to achieve total functional development. However, it is unknown whether visual experience affects developmental maturation. Here, we studied the effects of constant darkness (DD) rearing on the physiology (at pre- and postsynaptic levels) of glutamatergic neurotransmission between RHT and SCN during postnatal development in rats. Upon recording spontaneous and evoked excitatory postsynaptic currents (EPSCs) by electrical stimulation of RHT fibers, we found that DD animals at early postnatal ages (P3-19) exhibited different frequencies of spontaneous EPSCs and lower synaptic performance (short-term depression, release sites, and recruitment of RHT fibers) when compared with their normal light/dark (LD) counterparts. At the oldest age evaluated (P30-35), there was a synaptic response strengthening (probability of release, vesicular re-filling rate, and reduced synaptic depression) in DD rats, which functionally equaled (or surmounted) that of LD animals. Control experiments evaluating EPSCs in ventral SCN neurons of LD rats during day and night revealed no significant differences in spontaneous or evoked EPSCs by high-frequency trains in the RHT at any postnatal age. Our results suggest that DD conditions induce a compensatory mechanism in the glutamatergic signaling of the circadian system to increase the chances of synchronization to light at adult ages, and that the synaptic properties of RHT terminals during postnatal development are not critically influenced by environmental light.
Asunto(s)
Neuronas del Núcleo Supraquiasmático , Núcleo Supraquiasmático , Animales , Ritmo Circadiano , Potenciales Postsinápticos Excitadores , Luz , Ratas , Transmisión SinápticaRESUMEN
Inwardly rectifying potassium (Kir) channels are broadly expressed in both excitable and nonexcitable tissues, where they contribute to a wide variety of cellular functions. Numerous studies have established that rectification of Kir channels is not an inherent property of the channel protein itself, but rather reflects strong voltage dependence of channel block by intracellular cations, such as polyamines and Mg2+. Here, we identify a previously unknown mechanism of inward rectification in Kir4.1/Kir5.1 channels in the absence of these endogenous blockers. This novel intrinsic rectification originates from the voltage-dependent behavior of Kir4.1/Kir5.1, which is generated by the flux of potassium ions through the channel pore; the inward K+-flux induces the opening of the gate, whereas the outward flux is unable to maintain the gate open. This gating mechanism powered by the K+-flux is convergent with the gating of PIP2 because, at a saturating concentration, PIP2 greatly reduces the inward rectification. Our findings provide evidence of the coexistence of two rectification mechanisms in Kir4.1/Kir5.1 channels: the classical inward rectification induced by blocking cations and an intrinsic voltage-dependent mechanism generated by the K+-flux gating.
Asunto(s)
Canales de Potasio de Rectificación Interna , Iones , Potasio , Bloqueadores de los Canales de PotasioRESUMEN
It has been reported that muscarinic type-2 receptors (M2R) are voltage sensitive in an agonist-specific manner. In this work, we studied the effects of membrane potential on the interaction of M2R with the superagonist iperoxo (IXO), both functionally (using the activation of the ACh-gated K+ current (IKACh) in cardiomyocytes) and by molecular dynamics (MD) simulations. We found that IXO activated IKACh with remarkable high potency and clear voltage dependence, displaying a larger effect at the hyperpolarized potential. This result is consistent with a greater affinity, as validated by a slower (τ = 14.8 ± 2.3 s) deactivation kinetics of the IXO-evoked IKACh than that at the positive voltage (τ = 6.7 ± 1.2 s). The voltage-dependent M2R-IXO interaction induced IKACh to exhibit voltage-dependent features of this current, such as the 'relaxation gating' and the modulation of rectification. MD simulations revealed that membrane potential evoked specific conformational changes both at the external access and orthosteric site of M2R that underlie the agonist affinity change provoked by voltage on M2R. Moreover, our experimental data suggest that the 'tyrosine lid' (Y104, Y403, and Y426) is not the previously proposed voltage sensor of M2R. These findings provide an insight into the structural and functional framework of the biased signaling induced by voltage on GPCRs.
Asunto(s)
Activación del Canal Iónico/efectos de los fármacos , Isoxazoles/farmacología , Simulación de Dinámica Molecular , Compuestos de Amonio Cuaternario/farmacología , Receptor Muscarínico M2/fisiología , Acetilcolina/farmacología , Animales , Gatos , Células Cultivadas , Estimulación Eléctrica , Femenino , Activación del Canal Iónico/fisiología , Masculino , Potenciales de la Membrana/efectos de los fármacos , Modelos Moleculares , Agonistas Muscarínicos/farmacología , Miocitos Cardíacos/efectos de los fármacos , Miocitos Cardíacos/fisiología , Oocitos/efectos de los fármacos , Oocitos/fisiología , Técnicas de Placa-Clamp , Conformación Proteica , Receptor Muscarínico M2/química , Receptor Muscarínico M2/metabolismo , Xenopus laevisRESUMEN
The suprachiasmatic nucleus (SCN) is the main brain clock in mammals. The SCN synchronizes to the light-dark cycle through the retinohypothalamic tract (RHT). RHT axons release glutamate to activate AMPA-kainate and N-methyl-D-aspartate (NMDA) postsynaptic receptors in ventral SCN neurons. Stimulation of SCN NMDA receptors is necessary for the activation of the signaling cascades that govern the advances and delays of phase. To our knowledge, no research has been performed to analyze the functional synaptic modifications occurring during postnatal development that prepare the circadian system for a proper synchronization to light at adult ages. Here, we studied the pre- and postsynaptic developmental changes between the unmyelinated RHT-SCN connections. Spontaneous NMDA excitatory postsynaptic currents (EPSCs) were greater in amplitude and frequency at postnatal day 34 (P34) than at P8. Similarly, both quantal EPSCs (miniature NMDA and evoked quantal AMPA-kainate) showed a development-dependent increase at analyzed stages, P3-5, P7-9, and P13-18. Moreover, the electrically evoked NMDA and AMPA-kainate components were augmented with age, although the increment was larger for the latter, and the membrane resting potential was more depolarized at early postnatal ages. Finally, the short-term synaptic plasticity was significantly modified during postnatal development as was the estimated number of quanta released and the initial release probability. All of these synaptic modifications in the unmyelinated RHT-SCN synapses suggest that synchronization to light at adult ages requires developmental changes similar to those that occur in myelinated fast communication systems.
Asunto(s)
Ritmo Circadiano , Potenciales Postsinápticos Excitadores , Núcleo Supraquiasmático/fisiología , Animales , Femenino , Ácido Glutámico/metabolismo , Masculino , Potenciales de la Membrana , Fotoperiodo , Ratas , Ratas Sprague-Dawley , Receptores de N-Metil-D-Aspartato/fisiología , Transmisión SinápticaRESUMEN
The suprachiasmatic nucleus (SCN) is the main brain clock that regulates circadian rhythms in mammals. The SCN synchronizes to the LD cycle through the retinohypothalamic tract (RHT), which projects to ventral SCN neurons via glutamatergic synapses. Released glutamate activates N-methyl-D-aspartate (NMDA) receptors, which play a critical role in the activation of signaling cascades to enable phase shifts. Previous evidence indicates that presynaptic changes during postnatal development consist of an increase in RHT fibers impinging on SCN neurons between postnatal day (P) 1 to 4 and P15. The aim of this study was to evaluate postsynaptic developmental changes in the NR2 subunits that determine the pharmacological and biophysical properties of the neuronal NMDA receptors in the ventral SCN. To identify the expression of NR2 subtypes, we utilized RT-PCR, immunohistochemical fluorescence, and electrophysiological recordings of synaptic activity. We identified development-dependent changes in NR2A, C, and D subtypes in mRNA and protein expression, whereas NR2B protein was equally present at all analyzed postnatal ages. The NR2A antagonist PEAQX (100 nM) reduced the frequency of NMDA excitatory postsynaptic currents (EPSCs) at P8 significantly more than at P34, but the antagonists for NR2B (3 µM Ro 25-6981) and NR2C/D (150 nM PPDA) did not influence NMDA EPSCs differently at the 2 analyzed postnatal ages. Our results point to P8 as the earliest analyzed postnatal age that shows mRNA and protein expression similar to those found at the juvenile stage P34. Taken together, our findings indicate that postsynaptic development-dependent modifications in the NR2 subtypes of the NMDA receptor could be important for the synchronization of ventral SCN neurons to the LD cycle at adult stages.
Asunto(s)
Envejecimiento , Ritmo Circadiano , Receptores de N-Metil-D-Aspartato/fisiología , Neuronas del Núcleo Supraquiasmático/fisiología , Animales , Encéfalo/fisiología , Ratas , Ratas Wistar , Receptores de N-Metil-D-Aspartato/antagonistas & inhibidores , Sinapsis/fisiologíaRESUMEN
The cardiac muscarinic receptor (M2R) regulates heart rate, in part, by modulating the acetylcholine (ACh) activated K+ current IK,ACh through dissociation of G-proteins, that in turn activate KACh channels. Recently, M2Rs were noted to exhibit intrinsic voltage sensitivity, i.e. their affinity for ligands varies in a voltage dependent manner. The voltage sensitivity of M2R implies that the affinity for ACh (and thus the ACh effect) varies throughout the time course of a cardiac electrical cycle. The aim of this study was to investigate the contribution of M2R voltage sensitivity to the rate and shape of the human sinus node action potentials in physiological and pathophysiological conditions. We developed a Markovian model of the IK,ACh modulation by voltage and integrated it into a computational model of human sinus node. We performed simulations with the integrated model varying ACh concentration and voltage sensitivity. Low ACh exerted a larger effect on IK,ACh at hyperpolarized versus depolarized membrane voltages. This led to a slowing of the pacemaker rate due to an attenuated slope of phase 4 depolarization with only marginal effect on action potential duration and amplitude. We also simulated the theoretical effects of genetic variants that alter the voltage sensitivity of M2R. Modest negative shifts in voltage sensitivity, predicted to increase the affinity of the receptor for ACh, slowed the rate of phase 4 depolarization and slowed heart rate, while modest positive shifts increased heart rate. These simulations support our hypothesis that altered M2R voltage sensitivity contributes to disease and provide a novel mechanistic foundation to study clinical disorders such as atrial fibrillation and inappropriate sinus tachycardia.
Asunto(s)
Modelos Cardiovasculares , Receptor Muscarínico M2/fisiología , Nodo Sinoatrial/fisiología , Acetilcolina/metabolismo , Biología Computacional , Humanos , Cadenas de MarkovRESUMEN
Transient receptor potential vanilloid 1 (TRPV1) channels have been implicated in depression and anxiety. The aim of this study was to evaluate the antidepressant-like properties of the TRPV1 agonist capsaicin using the forced swimming test (FST) in rats. Capsaicin (0.001-0.25â¯mg/kg, i.p.) produced a reduction of immobility in the FST. A maximally effective dose of the tricyclic antidepressant amitriptyline (12â¯mg/kg) reduced immobility as well. Notably, doses of capsaicin (1â¯pg/kg, 1â¯ng/kg, and 0.001â¯mg/kg) that were ineffective when applied alone produced a significant decrease in immobility when combined with a subthreshold dose of amitriptyline (5â¯mg/kg). Rats treated with capsaicin (0.01â¯mg/kg)â¯+â¯amitriptyline (5â¯mg/kg) displayed less immobility than those treated with a maximally effective dose of amitriptyline. The non-pungent TRPV1 channel agonist palvanil (0.05-0.1â¯mg/kg, i.p.) also decreased immobility in the FST. Capsaicin (0.05â¯mg/kg) did not affect general locomotion in the open field test, nor performance in the elevated plus maze, or skeletal muscle contraction strength measured in vitro after the FST (at 0.25â¯mg/kg). Altogether, our results imply that low doses of capsaicin produce antidepressant-like effects, and enhance the effect of a subthreshold dose of amitriptyline in the FST.
Asunto(s)
Amitriptilina/farmacología , Antidepresivos/farmacología , Capsaicina/farmacología , Trastorno Depresivo/tratamiento farmacológico , Animales , Ansiedad , Capsaicina/análogos & derivados , Relación Dosis-Respuesta a Droga , Masculino , Contracción Muscular/efectos de los fármacos , Distribución Aleatoria , Ratas Wistar , Natación , Canales Catiónicos TRPV/agonistasRESUMEN
The acetylcholine (ACh)-gated inwardly rectifying K+ current (IKACh) plays a vital role in cardiac excitability by regulating heart rate variability and vulnerability to atrial arrhythmias. These crucial physiological contributions are determined principally by the inwardly rectifying nature of IKACh. Here, we investigated the relative contribution of two distinct mechanisms of IKACh inward rectification measured in atrial myocytes: a rapid component due to KACh channel block by intracellular Mg2+ and polyamines; and a time- and concentration-dependent mechanism. The time- and ACh concentration-dependent inward rectification component was eliminated when IKACh was activated by GTPγS, a compound that bypasses the muscarinic-2 receptor (M2R) and directly stimulates trimeric G proteins to open KACh channels. Moreover, the time-dependent component of IKACh inward rectification was also eliminated at ACh concentrations that saturate the receptor. These observations indicate that the time- and concentration-dependent rectification mechanism is an intrinsic property of the receptor, M2R; consistent with our previous work demonstrating that voltage-dependent conformational changes in the M2R alter the receptor affinity for ACh. Our analysis of the initial and time-dependent components of IKACh indicate that rapid Mg2+-polyamine block accounts for 60-70% of inward rectification, with M2R voltage sensitivity contributing 30-40% at sub-saturating ACh concentrations. Thus, while both inward rectification mechanisms are extrinsic to the KACh channel, to our knowledge, this is the first description of extrinsic inward rectification of ionic current attributable to an intrinsic voltage-sensitive property of a G protein-coupled receptor.
Asunto(s)
Potenciales de Acción , Canales de Potasio Rectificados Internamente Asociados a la Proteína G/metabolismo , Miocitos Cardíacos/metabolismo , Receptor Muscarínico M2/metabolismo , Acetilcolina/metabolismo , Animales , Gatos , Células Cultivadas , Femenino , Atrios Cardíacos/citología , Magnesio/metabolismo , Masculino , Miocitos Cardíacos/fisiología , Poliaminas/metabolismoRESUMEN
Amitriptyline (AMIT) is a compound widely prescribed for psychiatric and non-psychiatric conditions including depression, migraine, chronic pain, and anorexia. However, AMIT has been associated with risks of cardiac arrhythmia and sudden death since it can induce prolongation of the QT interval on the surface electrocardiogram and torsade de pointes ventricular arrhythmia. These complications have been attributed to the inhibition of the rapid delayed rectifier potassium current (IKr). The slow delayed rectifier potassium current (IKs) is the main repolarizing cardiac current when IKr is compromised and it has an important role in cardiac repolarization at fast heart rates induced by an elevated sympathetic tone. Therefore, we sought to characterize the effects of AMIT on Kv7.1/KCNE1 and homomeric Kv7.1 channels expressed in HEK-293H cells. Homomeric Kv7.1 and Kv7.1/KCNE1 channels were inhibited by AMIT in a concentration-dependent manner with IC50 values of 8.8⯱â¯2.1⯵M and 2.5⯱â¯0.8⯵M, respectively. This effect was voltage-independent for both homomeric Kv7.1 and Kv7.1/KCNE1 channels. Moreover, mutation of residues located on the P-loop and S6 domain along with molecular docking, suggest that T312, I337 and F340 are the most important molecular determinants for AMIT-Kv7.1 channel interaction. Our experimental findings and modeling suggest that AMIT preferentially blocks the open state of Kv7.1/KCNE1 channels by interacting with specific residues that were previously reported to be important for binding of other compounds, such as chromanol 293B and the benzodiazepine L7.
Asunto(s)
Amitriptilina/farmacología , Canal de Potasio KCNQ1/antagonistas & inhibidores , Canales de Potasio con Entrada de Voltaje/antagonistas & inhibidores , Potenciales de Acción , Amitriptilina/química , Antidepresivos Tricíclicos/química , Antidepresivos Tricíclicos/farmacología , Regulación de la Expresión Génica/efectos de los fármacos , Células HEK293 , Humanos , Canal de Potasio KCNQ1/metabolismo , Modelos Moleculares , Simulación del Acoplamiento Molecular , Estructura Molecular , Canales de Potasio con Entrada de Voltaje/metabolismo , Conformación ProteicaRESUMEN
Inwardly rectifying potassium (Kir) channels are expressed in many cell types and contribute to a wide range of physiological processes. Particularly, Kir4.1 channels are involved in the astroglial spatial potassium buffering. In this work, we examined the effects of the cationic amphiphilic drug quinacrine on Kir4.1 channels heterologously expressed in HEK293 cells, employing the patch clamp technique. Quinacrine inhibited the currents of Kir4.1 channels in a concentration and voltage dependent manner. In inside-out patches, quinacrine inhibited Kir4.1 channels with an IC50 value of 1.8±0.3µM and with extremely slow blocking and unblocking kinetics. Molecular modeling combined with mutagenesis studies suggested that quinacrine blocks Kir4.1 by plugging the central cavity of the channels, stabilized by the residues E158 and T128. Overall, this study shows that quinacrine blocks Kir4.1 channels, which would be expected to impact the potassium transport in several tissues.
Asunto(s)
Canales de Potasio de Rectificación Interna/efectos de los fármacos , Canales de Potasio de Rectificación Interna/metabolismo , Quinacrina/farmacología , Animales , Astrocitos/metabolismo , Células HEK293 , Humanos , Activación del Canal Iónico/fisiología , Técnicas de Placa-Clamp/métodos , Potasio/metabolismo , Canales de Potasio/metabolismo , Canales de Potasio de Rectificación Interna/antagonistas & inhibidores , Quinacrina/metabolismo , RatasRESUMEN
Kir4.1 channels have been implicated in various physiological processes, mainly in the K+ homeostasis of the central nervous system and in the control of glial function and neuronal excitability. Even though, pharmacological research of these channels is very limited. Chloroquine (CQ) is an amino quinolone derivative known to inhibit Kir2.1 and Kir6.2 channels with different action mechanism and binding site. Here, we employed patch-clamp methods, mutagenesis analysis, and molecular modeling to characterize the molecular pharmacology of Kir4.1 inhibition by CQ. We found that this drug inhibits Kir4.1 channels heterologously expressed in HEK-293 cells. CQ produced a fast-onset voltage-dependent pore-blocking effect on these channels. In inside-out patches, CQ showed notable higher potency (IC50 ≈0.5µM at +50mV) and faster onset of block when compared to whole-cell configuration (IC50 ≈7µM at +60mV). Also, CQ showed a voltage-dependent unblock with repolarization. These results suggest that the drug directly blocks Kir4.1 channels by a pore-plugging mechanism. Moreover, we found that two residues (Thr128 and Glu158), facing the central cavity and located within the transmembrane pore, are particularly important structural determinants of CQ block. This evidence was similar to what was previously reported with Kir6.2, but distinct from the interaction site (cytoplasmic pore) CQ-Kir2.1. Thus, our findings highlight the diversity of interaction sites and mechanisms that underlie amino quinolone inhibition of Kir channels.
Asunto(s)
Cloroquina/farmacología , Bloqueadores de los Canales de Potasio/farmacología , Canales de Potasio de Rectificación Interna/antagonistas & inhibidores , Canales de Potasio de Rectificación Interna/química , Sitios de Unión , Cloroquina/metabolismo , Citoplasma/efectos de los fármacos , Citoplasma/metabolismo , Células HEK293 , Humanos , Activación del Canal Iónico/efectos de los fármacos , Cinética , Simulación del Acoplamiento Molecular , Porosidad , Bloqueadores de los Canales de Potasio/metabolismo , Canales de Potasio de Rectificación Interna/metabolismo , Conformación ProteicaRESUMEN
Potassium (K(+)) channels are crucial for determining the shape, duration, and frequency of action-potential firing in excitable cells. Broadly speaking, K(+) channels can be classified based on whether their macroscopic current outwardly or inwardly rectifies, whereby rectification refers to a change in conductance with voltage. Outwardly rectifying K(+) channels conduct greater current at depolarized membrane potentials, whereas inward rectifier channels conduct greater current at hyperpolarized membrane potentials. Under most circumstances, outward currents through inwardly rectifying K(+) channels are reduced at more depolarized potentials. However, the acetylcholine-gated K(+) channel (KACh) conducts current that inwardly rectifies when activated by some ligands (such as acetylcholine), and yet conducts current that outwardly rectifies when activated by other ligands (for example, pilocarpine and choline). The perplexing and paradoxical behavior of KACh channels is due to the intrinsic voltage sensitivity of the receptor that activates KACh channels, the M2 muscarinic receptor (M2R). Emerging evidence reveals that the affinity of M2R for distinct ligands varies in a voltage-dependent and ligand-specific manner. These intrinsic receptor properties determine whether current conducted by KACh channels inwardly or outwardly rectifies. This review summarizes the most recent concepts regarding the intrinsic voltage sensitivity of muscarinic receptors and the consequences of this intriguing behavior on cardiac physiology and pharmacology of KACh channels.
Asunto(s)
Canales de Potasio de Rectificación Interna/metabolismo , Canales de Potasio/metabolismo , Receptores Muscarínicos/metabolismo , Animales , Corazón/fisiología , Humanos , Activación del Canal Iónico , Sistema Nervioso Parasimpático/fisiologíaRESUMEN
Recently, it has been shown that G protein-coupled receptors (GPCRs) display intrinsic voltage sensitivity. We reported that the voltage sensitivity of M2 muscarinic receptor (M2R) is also ligand specific. Here, we provide additional evidence to understand the mechanism underlying the ligand-specific voltage sensitivity of the M2R. Using ACh, pilocarpine (Pilo), and bethanechol (Beth), we evaluated the agonist-specific effects of voltage by measuring the ACh-activated K(+) current (I KACh) in feline and rabbit atrial myocytes and in HEK-293 cells expressing M2R-Kir3.1/Kir3.4. The activation of I KACh by the muscarinic agonist Beth was voltage insensitive, suggesting that the voltage-induced conformational changes in M2R do not modify its affinity for this agonist. Moreover, deactivation of the Beth-evoked I KACh was voltage insensitive. By contrast, deactivation of the ACh-induced I KACh was significantly slower at -100 mV than at +50 mV, while an opposite effect was observed when I KACh was activated by Pilo. These findings are consistent with the voltage affinity pattern observed for these three agonists. Our findings suggest that independent of how voltage disturbs the receptor binding site, the voltage dependence of the signaling pathway is ultimately determined by the agonist. These observations emphasize the pharmacological potential to regulate the M2R-parasympathetic associated cardiac function and also other cellular signaling pathways by exploiting the voltage-dependent properties of GPCRs.
Asunto(s)
Acetilcolina/farmacología , Activación del Canal Iónico/efectos de los fármacos , Agonistas Muscarínicos/farmacología , Canales de Potasio/metabolismo , Potasio/metabolismo , Receptor Muscarínico M2/metabolismo , Animales , Sitios de Unión/efectos de los fármacos , Gatos , Canales de Potasio Rectificados Internamente Asociados a la Proteína G/metabolismo , Células HEK293 , Humanos , Masculino , Potenciales de la Membrana/efectos de los fármacos , Miocitos Cardíacos/efectos de los fármacos , Miocitos Cardíacos/metabolismo , Conejos , Receptores Acoplados a Proteínas G/metabolismo , Transducción de Señal/efectos de los fármacosRESUMEN
INTRODUCTION: Voltage- and state-dependent blocks are important mechanisms by which drugs affect voltage-gated ionic channels. However, spontaneous (i.e. drug-free) time-dependent changes in the activation and inactivation of hERG and Na(+) channels have been reported when using conventional whole-cell patch-clamp in HEK-293 cells. METHODS: hERG channels were heterologously expressed in HEK-293 cells and in Xenopus laevis oocytes. hERG current (IhERG) was recorded using both conventional and perforated whole-cell patch-clamp (HEK-293 cells), and two microelectrode voltage-clamp (Xenopus oocytes) in drug-free solution, and in the presence of the drug trazodone. RESULTS: In conventional whole-cell setup, we observed a spontaneous time-dependent hyperpolarizing shift in the activation curve of IhERG. Conversely, in perforated patch whole-cell (HEK-293 cells) or in two microelectrode voltage-clamp (Xenopus oocytes) activation curves of IhERG were very stable for periods ~50min. Voltage-dependent inactivation of IhERG was not significantly altered in the three voltage clamp configurations tested. When comparing voltage- and state-dependent effects of the antidepressant drug trazodone on IhERG, similar changes between the three voltage clamp configurations were observed as under drug-free conditions. DISCUSSION: The comparative analysis performed in this work showed that only under conventional whole-cell voltage-clamp conditions, a leftward shift in the activation curve of IhERG occurred, both in the presence and absence of drugs. These spontaneous time-dependent changes in the voltage activation gate of IhERG are a potential confounder in pharmacological studies on hERG channels expressed in HEK-293 cells.
Asunto(s)
Canales de Potasio Éter-A-Go-Go/efectos de los fármacos , Técnicas de Placa-Clamp/métodos , Trazodona/farmacología , Animales , Antidepresivos de Segunda Generación , Canal de Potasio ERG1 , Canales de Potasio Éter-A-Go-Go/metabolismo , Células HEK293 , Humanos , Oocitos , Factores de Tiempo , Xenopus laevisRESUMEN
The human intestinal pathogen Giardia lamblia is a flagellated unicellular protozoan with pronounced medical and biological relevance. However, the basic physiology of Giardia trophozoites has been sparsely studied, especially the electrical and ionic properties of their cellular membrane which are virtually unknown. In this study, we were able to record and characterize the macroscopic ionic currents of Giardia trophozoite membrane by electrophysiological methods of the patch clamp technique. Giardia trophozoites showed a high current density (â¼600 pA/pF at -140 mV) that was activated upon hyperpolarization. This current was carried by a chloride-selective channel (I Cl-G) and it was the most important determinant of the membrane potential in Giardia trophozoites. Moreover, this conductance was able to carry other halide anions and the sequence of permeability was Br(-) > Cl(-) ≈ I(-) â« F(-). Besides the voltage-dependent inward-rectifying nature of I Cl-G, its activation and deactivation kinetics were comparable to those observed in ClC-2 channels. Extracellular pH modified the voltage-dependent properties of I Cl-G, shifting the activation curve from a V 1/2 = -79 ± 1 mV (pH 7.4) to -93 ± 2 mV (pH 8.4) and -112 ± 2 mV (pH 5.4). Furthermore, the maximal amplitude of I Cl-G measured at -100 mV showed dependence to external pH in a bell-shaped fashion reported only for ClC-2 channels. Therefore, our results suggest that I Cl-G possesses several functional properties similar to the mammalian ClC-2 channels.
Asunto(s)
Potenciales de Acción , Canales de Cloruro/metabolismo , Giardia lamblia/metabolismo , Proteínas Protozoarias/metabolismo , Trofozoítos/metabolismo , Canales de Cloruro CLC-2 , Cloruros/metabolismo , Giardia lamblia/crecimiento & desarrollo , Giardia lamblia/fisiología , Potenciales de la Membrana , Trofozoítos/fisiologíaRESUMEN
Choline (Ch) is a precursor and metabolite of the neurotransmitter acetylcholine (ACh). In canine and guinea pig atrial myocytes, Ch was shown to activate an outward K(+) current in a delayed rectifier fashion. This current has been suggested to modulate cardiac electrical activity and to play a role in atrial fibrillation pathophysiology. However, the exact nature and identity of this current has not been convincingly established. We recently described the unique ligand- and voltage-dependent properties of muscarinic activation of ACh-activated K(+) current (IKACh) and showed that, in contrast to ACh, pilocarpine induces a current with delayed rectifier-like properties with membrane depolarization. Here, we tested the hypothesis that Ch activates IKACh in feline atrial myocytes in a voltage-dependent manner similar to pilocarpine. Single-channel recordings, biophysical profiles, specific pharmacological inhibition and computational data indicate that the current activated by Ch is IKACh. Moreover, we show that membrane depolarization increases the potency and efficacy of IKACh activation by Ch and thus gives the appearance of a delayed rectifier activating K(+) current at depolarized potentials. Our findings support the emerging concept that IKACh modulation is both voltage- and ligand-specific and reinforce the importance of these properties in understanding cardiac physiology.