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Channel Inactivation (channel + inactivation)
Selected AbstractsThe Drosophila cacts2 mutation reduces presynaptic Ca2+ entry and defines an important element in Cav2.1 channel inactivationEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 12 2006G. T. Macleod Abstract Voltage-gated Ca2+ channels in nerve terminals open in response to action potentials and admit Ca2+, the trigger for neurotransmitter release. The cacophony gene encodes the primary presynaptic voltage-gated Ca2+ channel in Drosophila motor-nerve terminals. The cacts2 mutant allele of cacophony is associated with paralysis and reduced neurotransmission at non-permissive temperatures but the basis for the neurotransmission deficit has not been established. The cacts2 mutation occurs in the cytoplasmic carboxyl tail of the ,1 -subunit, not within the pore-forming trans-membrane domains, making it difficult to predict the mutation's impact. We applied a Ca2+ -imaging technique at motor-nerve terminals of mutant larvae to test the hypothesis that the neurotransmission deficit is a result of impaired Ca2+ entry. Presynaptic Ca2+ signals evoked by single and multiple action potentials showed a temperature-dependent reduction. The amplitude of the reduction was sufficient to account for the neurotransmission deficit, indicating that the site of the cacts2 mutation plays a role in Ca2+ channel activity. As the mutation occurs in a motif conserved in mammalian high-voltage-activated Ca2+ channels, we used a heterologous expression system to probe the effect of this mutation on channel function. The mutation was introduced into rat Cav2.1 channels expressed in human embryonic kidney cells. Patch-clamp analysis of mutant channels at the physiological temperature of 37 °C showed much faster inactivation rates than for wild-type channels, demonstrating that the integrity of this motif is critical for normal Cav2.1 channel inactivation. [source] Voltage-gated sodium channel isoform-specific effects of pompilidotoxinsFEBS JOURNAL, Issue 4 2010Emanuele Schiavon Pompilidotoxins (PMTXs, , and ,) are small peptides consisting of 13 amino acids purified from the venom of the solitary wasps Anoplius samariensis (,-PMTX) and Batozonellus maculifrons (,-PMTX). They are known to facilitate synaptic transmission in the lobster neuromuscular junction, and to slow sodium channel inactivation. By using ,-PMTX, ,-PMTX and four synthetic analogs with amino acid changes, we conducted a thorough study of the effects of PMTXs on sodium current inactivation in seven mammalian voltage-gated sodium channel (VGSC) isoforms and one insect VGSC (DmNav1). By evaluating three components of which the inactivating current is composed (fast, slow and steady-state components), we could distinguish three distinct groups of PMTX effects. The first group concerned the insect and Nav1.6 channels, which showed a large increase in the steady-state current component without any increase in the slow component. Moreover, the dose-dependent increase in this steady-state component was correlated with the dose-dependent decrease in the fast component. A second group of effects concerned the Nav1.1, Nav1.2, Nav1.3 and Nav1.7 isoforms, which responded with a large increase in the slow component, and showed only a small steady-state component. As with the first group of effects, the slow component was dose-dependent and correlated with the decrease in the fast component. Finally, a third group of effects concerned Nav1.4 and Nav1.5, which did not show any change in the slow or steady-state component. These data shed light on the complex and intriguing behavior of VGSCs in response to PMTXs, helping us to better understand the molecular determinants explaining isoform-specific effects. [source] The molecular interactions of pyrethroid insecticides with insect and mammalian sodium channels,PEST MANAGEMENT SCIENCE (FORMERLY: PESTICIDE SCIENCE), Issue 10 2001Horia Vais Abstract Recent progress in the cloning of , (para) and , (TipE) Na channel sub-units from Drosophila melanogaster (fruit fly) and Musca domestica (housefly) have facilitated functional expression studies of insect Na channels in Xenopus laevis oocytes, assayed by voltage clamp techniques. The effects of Type I and Type II pyrethroids on the biophysical properties of these channels are critically reviewed. Pyrethroid resistance mutations (termed kdr and super-kdr) that reduce the sensitivity of the insect Na channel to pyrethroids have been identified in a range of insect species. Some of these mutations (eg L1014F, M918T and T929I) have been incorporated into the para Na channel of Drosophila, either individually or in combination, to investigate their effects on the sensitivity of this channel to pyrethroids. The kdr mutation (L1014F) shifts the voltage dependence of both activation and steady-state inactivation by ,5,mV towards more positive potentials and facilitates Na channel inactivation. Incorporation of the super-kdr mutation (M918T) into the Drosophila Na channel also increases channel inactivation and causes a >100-fold reduction in deltamethrin sensitivity. These effects are shared by T929I, an alternative mutation that confers super-kdr -like resistance. Parallel studies have been undertaken using the rat IIA Na channel to investigate the molecular basis for the low sensitivity of mammalian brain Na channels to pyrethroids. Rat IIA channels containing the mutation L1014F exhibit a shift in their mid-point potential for Na activation, but their overall sensitivity to permethrin remains similar to that of the wild-type rat channel (ie both are 1000-fold less sensitive than the wild-type insect channel). Mammalian neuronal Na channels have an isoleucine rather than a methionine at the position (874) corresponding to the super-kdr (M918) residue of the insect channel. Replacement of the isoleucine of the wild-type rat IIA Na channel with a methionine (I874M) increases deltamethrin sensitivity 100-fold. In this way, studies of wild-type and mutant Na channels of insects and mammals are providing a molecular understanding of kdr and super-kdr resistance in insects, and of the low pyrethroid sensitivity of most mammalian Na channels. They are also giving valuable insights into the binding sites for pyrethroids on these channels. © 2001 Society of Chemical Industry [source] Glu 87 of Channelrhodopsin-1 Causes pH-dependent Color Tuning and Fast Photocurrent Inactivation,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2009Satoshi P. Tsunoda Channelrhodopsins (ChR1 and ChR2) are directly light-gated ion channels acting as sensory photoreceptors in the green alga Chlamydomonas reinhardtii. These channels open rapidly after light absorption and both become permeable for cations such as H+, Li+, Na+, K+ and Ca2+. Km for Ca2+ is 16.6 mm in ChR1 and 18.3 mm in ChR2 whereas the Km values for Na+ are higher than 100 mm for both ChRs. Action spectra of ChR1 peak between 470 and 500 nm depending on the pH conditions, whereas ChR2 peaks at 470 nm regardless of the pH value. Now we created two chimeric ChRs possessing helix 1,5 of ChR1 and 6, 7 of ChR2 (ChR1/25/2), or 1, 2 from ChR1 and 3,7 from ChR2 (ChR1/22/5). Both ChR-chimera still showed pH-dependent action spectra shifts. Finally, a mutant ChR1E87Q was generated that inactivated only slowly in the light and showed no spectral shift upon pH change. The results indicate that protonation/deprotonation of E87 in helix 1 alters the chromophore polarity, which shifts the absorption and modifies channel inactivation accordingly. We propose a trimodal counter ion complex for ChR1 but only a bimodal complex for ChR2. [source] Conduction velocity is regulated by sodium channel inactivation in unmyelinated axons innervating the rat cranial meningesTHE JOURNAL OF PHYSIOLOGY, Issue 4 2008Roberto De Col Axonal conduction velocity varies according to the level of preceding impulse activity. In unmyelinated axons this typically results in a slowing of conduction velocity and a parallel increase in threshold. It is currently held that Na+,K+ -ATPase-dependent axonal hyperpolarization is responsible for this slowing but this has long been equivocal. We therefore examined conduction velocity changes during repetitive activation of single unmyelinated axons innervating the rat cranial meninges. In direct contradiction to the currently accepted postulate, Na+,K+ -ATPase blockade actually enhanced activity-induced conduction velocity slowing, while the degree of velocity slowing was curtailed in the presence of lidocaine (10,300 ,m) and carbamazepine (30,500 ,m) but not tetrodotoxin (TTX, 10,80 nm). This suggests that a change in the number of available sodium channels is the most prominent factor responsible for activity-induced changes in conduction velocity in unmyelinated axons. At moderate stimulus frequencies, axonal conduction velocity is determined by an interaction between residual sodium channel inactivation following each impulse and the retrieval of channels from inactivation by a concomitant Na+,K+ -ATPase-mediated hyperpolarization. Since the process is primarily dependent upon sodium channel availability, tracking conduction velocity provides a means of accessing relative changes in the excitability of nociceptive neurons. [source] Molecular determinants of inactivation in voltage-gated Ca2+ channelsTHE JOURNAL OF PHYSIOLOGY, Issue 2 2000Steffen Hering Evolution has created a large family of different classes of voltage-gated Ca2+ channels and a variety of additional splice variants with different inactivation properties. Inactivation controls the amount of Ca2+ entry during an action potential and is, therefore, believed to play an important role in tissue-specific Ca2+ signalling. Furthermore, mutations in a neuronal Ca2+ channel (Cav2.1) that are associated with the aetiology of neurological disorders such as familial hemiplegic migraine and ataxia cause significant changes in the process of channel inactivation. Ca2+ channels of a given subtype may inactivate by three different conformational changes: a fast and a slow voltage-dependent inactivation process and in some channel types by an additional Ca2+ -dependent inactivation mechanism. Inactivation kinetics of Ca2+ channels are determined by the intrinsic properties of their pore-forming ,1 -subunits and by interactions with other channel subunits. This review focuses on structural determinants of Ca2+ channel inactivation in different parts of Ca2+ channel ,1 -subunits, including pore-forming transmembrane segments and loops, intracellular domain linkers and the carboxyl terminus. Inactivation is also affected by the interaction of the ,1 -subunits with auxiliary ,-subunits and intracellular regulator proteins. The evidence shows that pore-forming S6 segments and conformational changes in extra- (pore loop) and intracellular linkers connected to pore-forming segments may play a principal role in the modulation of Ca2+ channel inactivation. Structural concepts of Ca2+ channel inactivation are discussed. [source] Evolving mechanisms of action of alverine citrate on phasic smooth musclesBRITISH JOURNAL OF PHARMACOLOGY, Issue 8 2007M Hayase Background and purpose: We have investigated the mechanisms underlying the paradoxical ability of the antispasmodic, alverine, to enhance spontaneous activity in smooth muscles while suppressing evoked activity. Experimental approach: The effects of alverine on spontaneous and induced contractile activity were examined in preliminary experiments with various smooth muscles. More detailed effects were also investigated by recording membrane potential, intracellular Ca2+ concentration ([Ca2+]i) and tension from single-bundle detrusor smooth muscle (DSM) of the guinea-pig urinary bladder. Key results: Alverine (10 ,M) increased the frequency and amplitude of spontaneous action potentials, transient increases in [Ca2+]i and associated contractions. Alverine also decreased action potential rate of decay, suggesting inhibition of L -type Ca channel inactivation. Charybdotoxin (50 nM) but neither cyclopiazonic acid (10 ,M) nor Bay K 8644 (10 ,M) attenuated alverine-induced enhancement of spontaneous contractions. Alverine suppressed contractions produced by high K (40 mM) or ACh (10 ,M), without affecting electrical responses and with little suppression of increases in [Ca2+]i. This feature was very similar to that of the effects of the Rho kinase inhibitor Y-27632 (10 ,M). Conclusions and implications: Alverine may increase Ca influx during action potentials due to inhibition of the inactivation of L -type Ca channels, but may also suppress evoked activity by inhibiting the sensitivity of contractile proteins to Ca2+. The proportional contribution of Ca-dependent and Ca-independent contractions in DSM may differ between spontaneous and evoked activity, necessitating further investigations into the interactions between these pathways for assessing the therapeutic potential of alverine to treat DSM dysfunction. British Journal of Pharmacology (2007) 152, 1228,1238; doi:10.1038/sj.bjp.0707496; published online 15 October 2007 [source] The P/Q-type voltage-dependent calcium channel: a therapeutic target in spinocerebellar ataxia type 6ACTA NEUROLOGICA SCANDINAVICA, Issue 5 2007J. Gazulla Background,,, Voltage-dependent calcium channels (VDCCs) are heteromultimeric complexes that mediate calcium influx into cells; the ,1A subunit is the pore-forming subunit specific to the neuronal P/Q-type VDCCs. Spinocerebellar ataxia type 6 (SCA 6) is caused by an abnormal expansion of a CAG repeat in CACNA1A, which encodes the ,1A subunit. Heterologous expression of mutated ,1A subunits resulted in increased channel inactivation in electrophysiological tests. Gabapentin and pregabalin interact with the ,2, subunit of the VDCCs and improved ataxia in cases of cortical cerebellar atrophy (CCA) and ataxia-telangiectasia. Materials and methods,,, A bibliographical review was performed in order to find out if gabapentin and pregabalin coud prove useful in the treatment of SCA 6. Results,,, Gabapentin and pregabalin slowed the rate of inactivation in recombinant P/Q-type VDCCs. SCA 6 shares neuropathological findings with CCA. Conclusions,,, On the basis of the neuropathological identity of SCA 6 with CCA, and of the effect of gabapentin and pregabalin on recombinant VDCCs the authors put forward the hypothesis that these drugs might prove beneficial in SCA 6, as the ataxia would be expected to improve. The authors hope that researchers working with this illness will be encouraged to undertake the appropriate clinical and experimental work. [source] | ||||||||||||||||