Recent advances in the molecular understanding of voltage-gated Ca2+ channels

An ancestral MAGUK protein supports the modulation of mammalian voltage-gated Ca2+ channels through a conserved CaVβ-like interface

Eukaryote voltage-gated Ca²⁺ channels of the Ca_V2 channel family are hetero-oligomers formed by the pore-forming Ca_Vα1 protein assembled with auxiliary Ca_Vα2δ and Ca_Vβ subunits. Ca_Vβ subunits are formed by a Src homology 3 (SH3) domain and a guanylate kinase (GK) domain connected through a HOOK domain. The GK domain binds a conserved cytoplasmic region of the pore-forming Ca_Vα1 subunit referred as the "AID". Herein we explored the phylogenetic and functional relationship between Ca_V channel subunits in distant eukaryotic organisms by investigating the function of a MAGUK protein (XM_004990081) cloned from the choanoflagellate Salpingoeca rosetta (Sro). This MAGUK protein (Sroβ) features SH3 and GK structural domains with a 25% primary sequence identity to mammalian Ca_Vβ. Recombinant expression of its cDNA with mammalian high-voltage activated Ca²⁺ channel Ca_V2.3 in mammalian HEK cells produced robust voltage-gated inward Ca²⁺ currents with typical activation and inactivation properties. Like Ca_Vβ, Sroβ prevents fast degradation of total Ca_V2.3 proteins in cycloheximide assays. The three-dimensional homology model predicts an interaction between the GK domain of Sroβ and the AID motif of the pore-forming Ca_Vα1 protein. Substitution of AID residues Trp (W386A) and Tyr (Y383A) significantly impaired co-immunoprecipitation of Ca_V2.3 with Sroβ and functional upregulation of Ca_V2.3 currents. Likewise, a 6-residue deletion within the GK domain of Sroβ, similar to the locus found in mammalian Ca_Vβ, significantly reduced peak current density. Altogether our data demonstrate that an ancestor MAGUK protein reconstitutes the biophysical and molecular features responsible for channel upregulation by mammalian Ca_Vβ through a minimally conserved molecular interface.

Safety, Tolerability, and Efficacy of Pain Reduction by Gabapentin for Acute Headache and Meningismus After Aneurysmal Subarachnoid Hemorrhage: A Pilot Study

Introduction: Severe, often sudden-onset headache is the principal presenting symptoms of aneurysmal subarachnoid hemorrhage (aSAH). We hypothesized that gabapentin would be safe and tolerable for aSAH-induced headaches and would reduce concurrent opioid use.

Methods: We performed a single-center, double-blind, randomized, placebo-controlled trial (registered at ClinicalTrials.gov; NCT02330094) from November 24, 2014, to June 24, 2017, where aSAH patients received either dose-escalating gabapentin or oral placebo, both alongside a standard of care pain regimen. After 7 days, patients had the option to continue in an open-label period until 14 days after enrollment or until discharge from the intensive care unit. Our primary endpoint was the efficacy of gabapentin in reducing headache numeric pain scores and opioid usage in patients with aSAH compared to the placebo group. We identified 63 potential patients with aSAH for the study. After applying stringent exclusion criteria, 16 eligible patients were enrolled into one of two arms.

Results: The study ended prematurely after reaching a pre-specified funding period and an unexpected drop in aSAH cases. There was a trend toward lower headache numeric pain scores and opioid use in the gabapentin treated arm; however this was not significantly different. Gabapentin was well tolerated by participants and no adverse effects were reported.

Conclusions: While there was a trend toward lower pain scores and opioid requirement in the gabapentin group, the study was underpowered to detect a difference. Larger multicenter trials are required to evaluate the efficacy of gabapentin to reduce opioid requirements after aSAH.

Epilepsy and migraine-Are they comorbidity?

Epilepsy and migraine often co-occur. From the clinical symptoms, they often have some signs of symptoms before onset; from the pathogenesis of epilepsy and migraine, both of them have a high degree of neuronal excitement and ion channel abnormalities; in terms of treatment, many antiepileptic drugs are work in migraine. All of this indicates that they interact with each other. But it is undeniable that there are interactions and relationships between them, and there are also some differences such as the different clinical episodes, the different ways of neuronal haperexcitability and the different drug treatment programs. And are they comorbidity? If we can better understand the correlation between seizures and migraines, then this will help develop better guidelines for clinical diagnosis and treatment.

The cellular building blocks of breathing

Respiratory brainstem neurons fulfill critical roles in controlling breathing: they generate the activity patterns for breathing and contribute to various sensory responses including changes in O2 and CO2. These complex sensorimotor tasks depend on the dynamic interplay between numerous cellular building blocks that consist of voltage-, calcium-, and ATP-dependent ionic conductances, various ionotropic and metabotropic synaptic mechanisms, as well as neuromodulators acting on G-protein coupled receptors and second messenger systems. As described in this review, the sensorimotor responses of the respiratory network emerge through the state-dependent integration of all these building blocks. There is no known respiratory function that involves only a small number of intrinsic, synaptic, or modulatory properties. Because of the complex integration of numerous intrinsic, synaptic, and modulatory mechanisms, the respiratory network is capable of continuously adapting to changes in the external and internal environment, which makes breathing one of the most integrated behaviors. Not surprisingly, inspiration is critical not only in the control of ventilation, but also in the context of "inspiring behaviors" such as arousal of the mind and even creativity. Far-reaching implications apply also to the underlying network mechanisms, as lessons learned from the respiratory network apply to network functions in general.

Cooperative activation of the T-type CaV3.2 channel: interaction between Domains II and III

T-type CaV3 channels are important mediators of Ca(2+) entry near the resting membrane potential. Little is known about the molecular mechanisms responsible for channel activation. Homology models based upon the high-resolution structure of bacterial NaV channels predict interaction between the S4-S5 helix of Domain II (IIS4-S5) and the distal S6 pore region of Domain II (IIS6) and Domain III (IIIS6). Functional intra- and inter-domain interactions were investigated with a double mutant cycle analysis. Activation gating and channel kinetics were measured for 47 single mutants and 20 pairs of mutants. Significant coupling energies (ΔΔG(interact) ≥ 1.5 kcal mol(-1)) were measured for 4 specific pairs of mutants introduced between IIS4-S5 and IIS6 and between IIS4-S5 and IIIS6. In agreement with the computer based models, Thr-911 in IIS4-S5 was functionally coupled with Ile-1013 in IIS6 during channel activation. The interaction energy was, however, found to be stronger between Val-907 in IIS4-S5 and Ile-1013 in IIS6. In addition Val-907 was significantly coupled with Asn-1548 in IIIS6 but not with Asn-1853 in IVS6. Altogether, our results demonstrate that the S4-S5 and S6 helices from adjacent domains are energetically coupled during the activation of a low voltage-gated T-type CaV3 channel.

A quartet of leucine residues in the guanylate kinase domain of CaVβ determines the plasma membrane density of the CaV2.3 channel

Ca(V)β subunits are formed by a Src homology 3 domain and a guanylate kinase-like (GK) domain connected through a variable HOOK domain. Complete deletion of the Src homology 3 domain (75 residues) as well as deletion of the HOOK domain (47 residues) did not alter plasma membrane density of Ca(V)2.3 nor its typical activation gating. In contrast, six-residue deletions in the GK domain disrupted cell surface trafficking and functional expression of Ca(V)2.3. Mutations of residues known to carry nanomolar affinity binding in the GK domain of Ca(V)β (P175A, P179A, M195A, M196A, K198A, S295A, R302G, R307A, E339G, N340G, and A345G) did not significantly alter cell surface targeting or gating modulation of Ca(V)2.3. Nonetheless, mutations of a quartet of leucine residues (either single or multiple mutants) in the α3, α6, β10, and α9 regions of the GK domain were found to significantly impair cell surface density of Ca(V)2.3 channels. Furthermore, the normalized protein density of Ca(V)2.3 was nearly abolished with the quadruple Ca(V)β3 Leu mutant L200G/L303G/L337G/L342G. Altogether, our observations suggest that the four leucine residues in Ca(V)β3 form a hydrophobic pocket surrounding key residues in the α-interacting domain of Ca(V)2.3. This interaction appears to play an essential role in conferring Ca(V)β-induced modulation of the protein density of Ca(V)α1 subunits in Ca(V)2 channels.

Nitric oxide signalling augments neuronal voltage-gated L-type (Ca(v)1) and P/q-type (Ca(v)2.1) channels in the mouse medial nucleus of the trapezoid body

Nitric Oxide (NO) is a diffusible second messenger that modulates ion channels, intrinsic excitability and mediates synaptic plasticity. In light of its activity-dependent generation in the principal neurons of the medial nucleus of the trapezoid body (MNTB), we have investigated its potential modulatory effects on native voltage-gated calcium channels (Ca_V) within this nucleus. Whole-cell patch recordings were made from brain slices from P13–15 CBA mice. Slices were incubated with the inhibitor of neuronal nitric oxide synthase (nNOS) 7-nitroindazole (10 µM) and pharmacological blockers used to isolate Ca²⁺ current subtypes. Unpaired observations in the presence and absence of the NO-donors sodium nitroprusside (SNP, 100 µM) or Diethyl-ammonium-nonoate (DEA, 100 µM) were made to elucidate NO-dependent modulation of the expressed Ca_V subtypes. A differential effect of NO on the calcium channel subtypes was observed: Ca_V1 and Ca_V2.1 (L+R- and P/Q+R-type) conductances were potentiated, whereas N+R-type (Ca_V2.2) and R-type (Ca_V2.3) current amplitudes were unaffected. L+R-type currents increased from 0.36±0.04 nA to 0.64±0.11 nA and P/Q+R-type from 0.55±0.09 nA to 0.94±0.05 nA, thereby changing the balance and relative contribution of each subtype to the whole cell calcium current. In addition, N+R-type half-activation voltage was left shifted following NO exposure. NO-dependent modulation of P/Q+R and N+R-type, but not L+R-type, channels was removed by inhibition of soluble guanylyl cyclase (sGC) activity. This data demonstrates a differential effect of NO signalling on voltage-gated calcium entry, by distinct NO-dependent pathways.

Double mutant cycle analysis identified a critical leucine residue in the IIS4S5 linker for the activation of the Ca(V)2.3 calcium channel

Mutations in distal S6 were shown to significantly alter the stability of the open state of Ca(V)2.3 (Raybaud, A., Baspinar, E. E., Dionne, F., Dodier, Y., Sauvé, R., and Parent, L. (2007) J. Biol. Chem. 282, 27944-27952). By analogy with K(V) channels, we tested the hypothesis that channel activation involves electromechanical coupling between S6 and the S4S5 linker in Ca(V)2.3. Among the 11 positions tested in the S4S5 linker of domain II, mutations of the leucine residue at position 596 were found to destabilize significantly the closed state with a -50 mV shift in the activation potential and a -20 mV shift in its charge-voltage relationship as compared with Ca(V)2.3 wt. A double mutant cycle analysis was performed by introducing pairs of glycine residues between S4S5 and S6 of Domain II. Strong coupling energies (ΔΔG(interact) > 2 kcal mol(-1)) were measured for the activation gating of 12 of 39 pairs of mutants. Leu-596 (IIS4S5) was strongly coupled with distal residues in IIS6 from Leu-699 to Asp-704. In particular, the double mutant L596G/I701G showed strong cooperativity with a ΔΔG(interact) ≈6 kcal mol(-1) suggesting that both positions contribute to the activation gating of the channel. Altogether, our results highlight the role of a leucine residue in S4S5 and provide the first series of evidence that the IIS4S5 and IIS6 regions are energetically coupled during the activation of a voltage-gated Ca(V) channel.

Molecular determinants of the CaVbeta-induced plasma membrane targeting of the CaV1.2 channel

Ca(V)beta subunits modulate cell surface expression and voltage-dependent gating of high voltage-activated (HVA) Ca(V)1 and Ca(V)2 alpha1 subunits. High affinity Ca(V)beta binding onto the so-called alpha interaction domain of the I-II linker of the Ca(V)alpha1 subunit is required for Ca(V)beta modulation of HVA channel gating. It has been suggested, however, that Ca(V)beta-mediated plasma membrane targeting could be uncoupled from Ca(V)beta-mediated modulation of channel gating. In addition to Ca(V)beta, Ca(V)alpha2delta and calmodulin have been proposed to play important roles in HVA channel targeting. Indeed we show that co-expression of Ca(V)alpha2delta caused a 5-fold stimulation of the whole cell currents measured with Ca(V)1.2 and Ca(V)beta3. To gauge the synergetic role of auxiliary subunits in the steady-state plasma membrane expression of Ca(V)1.2, extracellularly tagged Ca(V)1.2 proteins were quantified using fluorescence-activated cell sorting analysis. Co-expression of Ca(V)1.2 with either Ca(V)alpha2delta, calmodulin wild type, or apocalmodulin (alone or in combination) failed to promote the detection of fluorescently labeled Ca(V)1.2 subunits. In contrast, co-expression with Ca(V)beta3 stimulated plasma membrane expression of Ca(V)1.2 by a 10-fold factor. Mutations within the alpha interaction domain of Ca(V)1.2 or within the nucleotide kinase domain of Ca(V)beta3 disrupted the Ca(V)beta3-induced plasma membrane targeting of Ca(V)1.2. Altogether, these data support a model where high affinity binding of Ca(V)beta to the I-II linker of Ca(V)alpha1 largely accounts for Ca(V)beta-induced plasma membrane targeting of Ca(V)1.2.

The Role of Distal S6 Hydrophobic Residues in the Voltage-dependent Gating of CaV2.3 Channels

The hydrophobic locus VAVIM is conserved in the S6 transmembrane segment of domain IV (IVS6) in Ca(V)1 and Ca(V)2 families. Herein we show that glycine substitution of the VAVIM motif in Ca(V)2.3 produced whole cell currents with inactivation kinetics that were either slower (A1719G approximately V1720G), similar (V1718G), or faster (I1721G approximately M1722G) than the wild-type channel. The fast kinetics of I1721G were observed with a approximately +10 mV shift in its voltage dependence of activation (E(0.5,act)). In contrast, the slow kinetics of A1719G and V1720G were accompanied by a significant shift of approximately -20 mV in their E(0.5,act) indicating that the relative stability of the channel closed state was decreased in these mutants. Glycine scan performed with Val (349) in IS6, Ile(701) in IIS6, and Leu(1420) in IIIS6 at positions predicted to face Val(1720) in IVS6 also produced slow inactivating currents with hyperpolarizing shifts in the activation and inactivation potentials, again pointing out a decrease in the stability of the channel closed state. Mutations to other hydrophobic residues at these positions nearly restored the channel gating. Altogether these data indicate that residues at positions equivalent to 1720 exert a critical control upon the relative stability of the channel closed and open states and more specifically, that hydrophobic residues at these positions promote the channel closed state. We discuss a three-dimensional homology model of Ca(V)2.3 based upon Kv1.2 where hydrophobic residues at positions facing Val(1720) in IS6, IIS6, and IIIS6 play a critical role in stabilizing the closed state in Ca(V)2.3.

Somatostatin decreases voltage-gated Ca2+ currents in GH3 cells through activation of somatostatin receptor 2

The secretion of growth hormone (GH) is inhibited by hypothalamic somatostatin (SRIF) in somatotropes through five subtypes of the somatostatin receptor (SSTR1-SSTR5). We aimed to characterize the subtype(s) of SSTRs involved in the Ca2+ current reduction in GH3 somatotrope cells using specific SSTR subtype agonists. We used nystatin-perforated patch clamp to record voltage-gated Ca2+ currents, using a holding potential of -80 mV in the presence of K+ and Na+ channel blockers. We first established the presence of T-, L-, N-, and P/Q-type Ca2+ currents in GH3 cells using a variety of channel blockers (Ni+, nifedipine, omega-conotoxin GVIA, and omega-agatoxin IVA). SRIF (200 nM) reduced L- and N-type but not T- or P/Q-type currents in GH3 cells. A range of concentrations of each specific SSTR agonist was tested on Ca2+ currents to find the maximal effective concentration. Activation of SSTR2 with 10(-7) and 10(-8) M L-797,976 decreased the voltage-gated Ca2+ current and abolished any further decrease by SRIF. SSTR1, SSTR3, SSTR4, and SSTR5 agonists at 10(-7) M did not modify the voltage-gated Ca2+ current and did not affect the Ca2+ current response to SRIF. These results indicate that SSTR2 is involved mainly in regulating voltage-gated Ca2+ currents by SRIF, which contributes to the decrease in intracellular Ca2+ concentration and GH secretion by SRIF.

Voltage-gated currents of tilapia prolactin cells

The first recordings of neuron-like electrical activity from endocrine cells were made from fish pituitary cells. However, patch-clamping studies have predominantly utilized mammalian preparations. This study used whole-cell patch-clamping to characterize voltage-gated ionic currents of anterior pituitary cells of Oreochromis mossambicus in primary culture. Due to their importance for control of hormone secretion we emphasize analysis of calcium currents (I(Ca)), including using peptide toxins diagnostic for mammalian neuronal Ca(2+) channel types. These appear not to have been previously tested on fish endocrine cells. In balanced salines, inward currents consisted of a rapid TTX-sensitive sodium current and a smaller, slower I(Ca); there followed outward potassium currents dominated by delayed, sustained TEA-sensitive K(+) current. About half of cells tested from a holding potential (V(h)) of -90 mV showed early transient K(+) current; most cells showed a small Ca(2+)-mediated outward current. I-V plots of isolated I(Ca) with 15 mM [Ca(2+)](o) showed peak currents (up to 20 pA/pF from V(h) -90 mV) at approximately +10 mV, with approximately 60% I(Ca) for V(h) -50 mV and approximately 30% remaining at V(h) -30 mV. Plots of normalized conductance vs. voltage at several V(h)s were nearly superimposable. Well-sustained I(Ca) with predominantly Ca(2+)-dependent inactivation and inhibition of approximately 30% of total I(Ca) by nifedipine or nimodipine suggests participation of L-type channels. Each of the peptide toxins (omega-conotoxin GVIA, omega-agatoxin IVA, SNX482) alone blocked 36-54% of I(Ca). Inhibition by any of these toxins was additive to inhibition by nifedipine. Combinations of the toxins failed to produce additive effects. I(Ca) of up to 30% of total remained with any combination of inhibitors, but 0.1mM cadmium blocked all I(Ca) rapidly and reversibly. We did not find differences among cells of differing size and hormone content. Thus, I(Ca) is carried by high voltage-activated Ca(2+) channels of at least three types, but the molecular types may differ from those characterized from mammalian neurons.

The Role of the GX9GX3G Motif in the Gating of High Voltage-activated Ca2+ Channels

The putative hinge point revealed by the crystal structure of the MthK potassium channel is a glycine residue that is conserved in many ion channels. In high voltage-activated (HVA) Ca(V) channels, the mid-S6 glycine residue is only present in IS6 and IIS6, corresponding to G422 and G770 in Ca(V)1.2. Two additional glycine residues are found in the distal portion of IS6 (Gly(432) and Gly(436) in Ca(V)1.2) to form a triglycine motif unique to HVA Ca(V) channels. Lethal arrhythmias are associated with mutations of glycine residues in the human L-type Ca(2+) channel. Hence, we undertook a mutational analysis to investigate the role of S6 glycine residues in channel gating. In Ca(V)1.2, alpha-helix-breaking proline mutants (G422P and G432P) as well as the double G422A/G432A channel did not produce functional channels. The macroscopic inactivation kinetics were significantly decreased with Ca(V)1.2 wild type > G770A > G422A congruent with G436A >> G432A (from the fastest to the slowest). Mutations at position Gly(432) produced mostly nonfunctional mutants. Macroscopic inactivation kinetics were markedly reduced by mutations of Gly(436) to Ala, Pro, Tyr, Glu, Arg, His, Lys, or Asp residues with stronger effects obtained with charged and polar residues. Mutations within the distal GX(3)G residues blunted Ca(2+)-dependent inactivation kinetics and prevented the increased voltage-dependent inactivation kinetics brought by positively charged residues in the I-II linker. In Ca(V)2.3, mutation of the distal glycine Gly(352) impacted significantly on the inactivation gating. Altogether, these data highlight the role of the GX(3)G motif in the voltage-dependent activation and inactivation gating of HVA Ca(V) channels with the distal glycine residue being mostly involved in the inactivation gating.

Voltage-gated calcium channel subunits from platyhelminths: potential role in praziquantel action

Voltage-gated calcium (Ca2+) channels provide the pathway for Ca2+ influxes that underlie Ca2+ -dependent responses in muscles, nerves and other excitable cells. They are also targets of a wide variety of drugs and toxins. Ca2+ channels are multisubunit protein complexes consisting of a pore-forming alpha(1) subunit and other modulatory subunits, including the beta subunit. Here, we review the structure and function of schistosome Ca2+ channel subunits, with particular emphasis on variant Ca2+ channel beta subunits (Ca(v)betavar) found in these parasites. In particular, we examine the role these beta subunits may play in the action of praziquantel, the current drug of choice against schistosomiasis. We also present evidence that Ca(v)betavar homologs are found in other praziquantel-sensitive platyhelminths such as the pork tapeworm, Taenia solium, and that these variant beta subunits may thus represent a platyhelminth-specific gene family.

Synthesis and SAR studies of a novel series of T-type calcium channel blockers

For the novel, potent, and selective T-type Ca2+ channel blockers, a series of sulfonamido-containing 3,4-dihydroquinazoline derivatives were prepared and evaluated for their blocking actions on T- and N-type Ca2+ channels. Among them, 9c (KYS05064, IC50 = 0.96 +/- 0.22 microM) was found to be as potent as Mibefradil and also showed the highest selectivity for T-type Ca2+ channel with no effect on N-type Ca2+ channel.

Pattern-Specific Synaptic Mechanisms in a Multifunctional Network. I. Effects of Alterations in Synapse Strength

Many neuronal networks are multifunctional, producing different patterns of activity in different circumstances, but the mechanisms responsible for this reconfiguration are in many cases unresolved. The mammalian respiratory network is an example of such a system. Normal respiratory activity (eupnea) is periodically interrupted by distinct large-amplitude inspirations known as sighs. Both rhythms originate from a single multifunctional neural network, and both are preserved in the in vitro transverse medullary slice of mice. Here we show that the generation of fictive sighs were more sensitive than eupnea to reductions of excitatory synapse strength caused by either the P/Q-type (α1A-containing) calcium channel antagonist ω-agatoxin TK or the non- N-methyl-d-aspartate (NMDA) glutamate receptor antagonist 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX). In contrast, the NMDA receptor antagonist MK-801, while also inhibiting eupnea, increased the occurrence of sighs. This suggests that among the glutamatergic synapses subserving eupneic rhythmogenesis, there is a specific subset—highly sensitive to agatoxin and insensitive to NMDA receptor blockade—that is essential for sighs. Blockade of N-type calcium channels with ω-conotoxin GVIA also had pattern-specific effects: eupneic activity was not affected, but sigh frequency was increased and postsigh apnea decreased. We hypothesize that N-type (α1B) calcium channels selectively coupled to calcium-activated potassium channels contribute to the generation of the postsigh apnea.

Osteoblast Ca(2+) permeability and voltage-sensitive Ca(2+) channel expression is temporally regulated by 1,25-dihydroxyvitamin D(3)

The cardiac subtype of the L-type voltage-sensitive Ca(2+) channel (VSCC) Cav1.2 (alpha(1C)) is the primary voltage-sensitive channel responsible for Ca(2+) influx into actively proliferating osteoblasts. This channel also serves as the major transducer of Ca(2+) signals in growth-phase osteoblasts in response to hormone treatment. In this study, we have demonstrated that 24-h treatment of MC3T3-E1 preosteoblasts with 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], a coupling factor for bone resorption, coordinately downregulates Cav1.2 (alpha(1C)) and uniquely upregulates T-type channel Cav3.2 (alpha(1H)). No other voltage-sensitive channel alpha-subunit of the 10 that were surveyed was upregulated by 1,25(OH)(2)D(3). The shift from predominantly L-type to T-type channel expression has been demonstrated to occur at both mRNA and protein levels detected using quantitative PCR and immunohistochemistry with antibodies specific for each channel type. Functional and pharmacological studies using specific inhibitors have revealed that treatment with 1,25(OH)(2)D(3) also alters the Ca(2+) permeability properties of the osteoblast membrane from a state of primarily L-current sensitivity to T-current sensitivity. We conclude that the L-type channel is likely to support proliferation of osteoblast cells, whereas T-type channels are more likely to be involved in supporting differentiated functions after 1,25(OH)(2)D(3)-mediated reversal of remodeling has occurred. This latter observation is consistent with the unique expression of the T-type VSCC Cav3.2 (alpha(1H)) in terminally differentiated osteocytes as we recently reported.

Presynaptic ‘Cav2.3-containing’ E-type Ca2+channels share dual roles during neurotransmitter release

Ca2+ influx into excitable cells is a prerequisite for neurotransmitter release and regulated exocytosis. Within the group of ten cloned voltage-gated Ca2+ channels, the Ca(v)2.3-containing E-type Ca2+ channels are involved in various physiological processes, such as neurotransmitter release and exocytosis together with other voltage-gated Ca2+ channels of the Ca(v)1, Ca(v)2 and Ca(v)3 subfamily. However, E-type Ca2+ channels also exhibit several subunit-specific features, most of which still remain poorly understood. Ca(v)2.3-containing R-type channels (here called 'E-type channels') are also located in presynaptic terminals and interact with some synaptic vesicle proteins, the so-called SNARE proteins, although lacking the classical synprint interaction site. E-type channels trigger exocytosis and are also involved in long-term potentiation. Recently, it was shown that the interaction of Ca(v)2.3 with the EF-hand motif containing protein EFHC1 is involved in the aetiology and pathogenesis of juvenile myoclonic epilepsy.

Neuroprotection in the rat lateral fluid percussion model of traumatic brain injury by SNX-185, an N-type voltage-gated calcium channel blocker

Overload of intracellular calcium ([Ca(2+)](i)) following traumatic brain injury (TBI) has been implicated in the pathogenesis of neuronal injury and death. Voltage-gated calcium channels (VGCCs) provide one of the major sources of Ca(2+) entry into cells. Therefore, the potential neuroprotective activity of SNX-185, a specific N-type VGCC blocker, was tested in rats using the lateral fluid percussion (LFP) model of TBI. SNX-185 (50, 100, or 200 pmol) or vehicle was injected 5 min after injury into the CA2-3 subregion of the hippocampus ipsilateral to TBI. Acute neuronal degeneration was visualized in brain sections 24 h postinjury using the histofluorescent marker Fluoro-Jade (FJ), and the number of surviving neurons in the CA2-3 subregion of the hippocampus 42 days after injury was determined stereologically. Behavioral outcome after TBI and drug treatment was assessed in the beam walk test and Morris water maze. Direct injection of SNX-185 into the CA2-3 region of the hippocampus reduced neuronal injury 24 h after TBI and increased neuronal survival at 42 days at each of the three drug concentrations. Behavioral outcome in both the beam walk and Morris water maze were also improved by SNX-185, with 100 and 200 pmol, but not 50 pmol SNX-185 providing neuroprotection. These data support previous studies demonstrating substantial neuroprotection after TBI by treatment with N-type VGCC blockers.

Synthesis and biological activity of 3,4-dihydroquinazolines for selective T-type Ca2+ channel blockers

We have synthesized 3,4-dihydroquinazoline derivatives for the potent and selective T-type Ca(2+) channel blockers and evaluated for their inhibitory activities against two subtypes T-type Ca(2+) channels and N-type Ca(2+) channels. Among them, 5b (KYS05044, IC(50)=0.56+/-0.10 microM) was identified as potent T-type Ca(2+) channel blocker with in vitro selectivity profile at meaningful level (T/N-type, SI=>100).

Gabapentin fails to alter P/Q-type Ca2+ channel-mediated synaptic transmission in the hippocampus in vitro

Gabapentin (Neurontin) has been successfully used in the treatment of both epilepsy and neuropathic pain. Despite its widespread clinical use, its mechanism of action is very poorly understood. Indeed, the only protein it is known to interact with is the alpha2delta subunit of the voltage-gated Ca(2+) channel complex. In a recent article, gabapentin was reported to inhibit synaptic transmission in the spinal cord through an inhibitory effect on presynaptic P/Q-type Ca(2+) channels in both glutamatergic primary afferents and glycinergic interneuones. To examine if such inhibition of P/Q-channel-mediated synaptic transmission by gabapentin generalised to other synaptic pathways, we tested the actions of gabapentin of P/Q-type Ca(2+) channel-mediated synaptic responses in the CA1 subfield of the hippocampus. We found that gabapentin was completely inactive on such synaptic responses even at 10 times the maximally effective concentration used in the spinal cord. A small ( approximately 10%) but consistent depression of control synaptic responses was elicited by 10 microM gabapentin. No greater response was observed at a 10 times higher concentration. From these data we conclude that gabapentin is not a generic inhibitor of presynaptic P/Q-type channels and its actions at the spinal level must represent a feature of the P/Q-type channel not present in the hippocampus. Given the known interactions of this compound, the best candidate for this is the presence, subtype, or state of the alpha2delta subunit.

Are Ca2+ channels targets of praziquantel action?

Praziquantel is the current drug of choice for the control of schistosomiasis. It is highly effective against all species of schistosomes and shows minimal adverse effects. Though introduced for the treatment of schistosomiasis more than 20 years ago, the mode of action of praziquantel remains to be elucidated. This review will focus on advances in defining the molecular target of praziquantel action, with particular emphasis on recent work indicating an important role for voltage-gated calcium channels.

Cerebellar ataxia, seizures, premature death, and cardiac abnormalities in mice with targeted disruption of the Cacna2d2 gene

CACNA2D2 is a putative tumor suppressor gene located in the human chromosome 3p21.3 region that shows frequent allelic imbalances in lung, breast, and other cancers. The alpha2delta-2 protein encoded by the gene is a regulatory subunit of voltage-dependent calcium channels and is expressed in brain, heart, and other tissues. Here we report that mice homozygous for targeted disruption of the Cacna2d2 gene exhibit growth retardation, reduced life span, ataxic gait with apoptosis of cerebellar granule cells followed by Purkinje cell depletion, enhanced susceptibility to seizures, and cardiac abnormalities. The Cacna2d2(tm1NCIF) null phenotype has much in common with that of Cacna1a mutants, such as cerebellar neuro-degeneration associated with ataxia, seizures, and premature death. A tendency to bradycardia and limited response of null mutants to isoflurane implicate alpha2delta-2 in sympathetic regulation of cardiac function. In summary, our findings provide genetic evidence that the alpha2delta-2 subunit serves in vivo as a component of P/Q-type calcium channels, is indispensable for the central nervous system function, and may be involved in hereditary cerebellar ataxias and epileptic disorders in humans.

The neurobiology of antiepileptic drugs for the treatment of nonepileptic conditions

Antiepileptic drugs (AEDs) are commonly prescribed for nonepileptic conditions, including migraine headache, chronic neuropathic pain, mood disorders, schizophrenia and various neuromuscular syndromes. In many of these conditions, as in epilepsy, the drugs act by modifying the excitability of nerve (or muscle) through effects on voltage-gated sodium and calcium channels or by promoting inhibition mediated by gamma-aminobutyric acid (GABA) A receptors. In neuropathic pain, chronic nerve injury is associated with the redistribution and altered subunit compositions of sodium and calcium channels that predispose neurons in sensory pathways to fire spontaneously or at inappropriately high frequencies, often from ectopic sites. AEDs may counteract this abnormal activity by selectively affecting pain-specific firing; for example, many AEDs suppress high-frequency action potentials by blocking voltage-activated sodium channels in a use-dependent fashion. Alternatively, AEDs may specifically target pathological channels; for example, gabapentin is a ligand of alpha2delta voltage-activated calcium channel subunits that are overexpressed in sensory neurons after nerve injury. Emerging evidence suggests that effects on signaling pathways that regulate neuronal plasticity and survival may be a factor in the delayed clinical efficacy of AEDs in some neuropsychiatric conditions, including bipolar affective disorder.

Calcium entry through L-type calcium channels is essential for neurite regeneration in cultured sympathetic neurons

Previous work showed that a post-neuritotomy rise in [Ca2+]i is required for regeneration. We tested the following hypotheses in cultured sympathetic neurons: (1) blocking L-type channels at the time of injury inhibits regeneration; (2) enhancing Ca2+ entry through L-type Ca2+ channels enhances regeneration; (3) L-type Ca2+ channel distribution is predominantly on the soma and proximal neurites of uninjured and injured neurons. To visualize L-type Ca2+ channels and block Ca2+ influx, the fluorescent dihydropyridine antagonist, DM-BODIPY, was used. Our results show that regeneration is markedly inhibited by the antagonist when administered 20 min. prior to injury, in the presence or absence of nerve growth factor (NGF) (p < 0.0001). Severe degeneration of proximal and distal neurites was seen 48 h after injury. Regeneration was minimally inhibited by the antagonist when administered 5 min after injury (p < 0.05), but not inhibited when administered 2 or 24 h after injury (p > 0.05). We found that L-type channels are distributed ubiquitously on the soma and neurites of uninjured and injured cells, and on regenerating neurites. The addition of the L-type channel agonist, BayK8644, (1 microM) 20 min prior to injury enhanced neurite length at 24 h post-injury (p = 0.002). Blocking L-type channels did not affect the viability of uninjured or injured cells. For the first time, it has been shown that Ca2+ entry through L-type Ca2+ channels is essential for post-neuritotomy sympathetic neurite regeneration, and that this effect shows a strict temporal dependency. We also demonstrated that regeneration can be enhanced by increasing Ca2+ influx through L-type channels.

3,4-Dihydroquinazoline derivatives as novel selective T-type Ca2+ channel blockers

For LVA T-type Ca2+ channel blockers, 3,4-dihydroquinazoline derivatives as new scaffolds were prepared and evaluated for the inhibitory activity against two members of the recombinant T-type Ca2+ channel family. Among them, 8a (KYS05001, IC50=0.9 microM) was nearly equipotent with mibefradil (IC50=0.84 microM) and inhibited LVA T-type Ca2+ channel with greater efficacy than HVA Ca2+ channel.

Inhibition of α1E Ca2+ Channels by Carbonic Anhydrase Inhibitors

We examined if a range of carbonic anhydrase inhibitors (CAIs) interacted with the high-voltage activated voltage-sensitive calcium channels (VSCCs) encoded by the human alpha(1E) subunit. Whole-cell recordings were made from HEK293 cells stably expressing human alpha(1E)beta(3)-mediated calcium channels. SNX-482 (an alpha(1E) inhibitor) blocked alpha(1E)-mediated VSCCs with an IC(50) close to 10 nM. The anticonvulsant CAI ethoxyzolamide also inhibited these currents, with an IC(50) close to 1 microM, and produced an accompanying 20-mV hyperpolarizing shift in the steady-state inactivation profile. Other structurally diverse CAIs (e.g., acetazolamide and benzolamide) produced approximately 30 - 40% inhibition of alpha(1E)beta(3)-mediated Ca(2+) currents at 10 microM. Topiramate (10 microM), an anticonvulsant with CAI activity, inhibited these currents by 68 +/- 7%. This off-target activity of CAIs at VSCCs may contribute to some of the effects they produce both in vitro and in vivo.

Cholinergic and dopaminergic amacrine cells differentially express calcium channel subunits in the rat retina

Immunofluorescence labeling was performed to study the expression of high voltage-activated Ca(2+) channel subunits on rat retinal cholinergic and dopaminergic amacrine cells, which were double labeled with antibodies against choline acetyltransferase and tyrosine hydroxylase, respectively. The alpha(1A) subunit was predominantly expressed on the processes but not on the somata of cholinergic amacrine cells, whereas staining for alpha(1B) and alpha(1E) was observed in both structures of the cells. Immunoreactivity of alpha(1C) and alpha(1D) was not found in the cholinergic amacrine cells. Dopaminergic amacrine cells, on the other hand, exhibited a differential expression pattern of the Ca(2+) channel subunits, with alpha(1A), alpha(1C) and alpha(1E) being expressed on both somata and processes and alpha(1B) predominantly on the processes of the cells. No alpha(1D) labeling was seen. These results suggest that Ca(2+) channel subunits differentially expressed on the cholinergic and dopaminergic amacrine cells may endow these two cell types with different physiological properties.

Permeation and selectivity in calcium channels

Recent advances-both experimental and theoretical-provide a tentative image of the structures in Ca channels that make them exceptionally selective. The image is very different from K channels, which obtain high selectivity with a rigid pore that tightly fits K(+) ions and is lined by carbonyl oxygens of the polypeptide backbone. Ca channels rely on four glutamate residues (the EEEE locus), whose carboxyl side chains likely reach into the pore lumen to interact with passing Ca(2+) ions. The structure is thought to be flexible, tightly binding a single Ca(2+) ion in order to block Na(+) flux but rearranging to interact with multiple Ca(2+) ions to allow Ca(2+) flux. The four glutamates are not equivalent, a fact that seems important for Ca(2+) permeation. This review describes the experimental evidence that leads to these conclusions and the attempts by theorists to explain the combination of high selectivity and high flux that characterizes Ca channels.

Pathophysiology, epidemiology, and impact of migraine

Despite a decade of progress, migraine headache remains prevalent, disabling, underdiagnosed, and undertreated in the United States. Migraine affects approximately 12% of the population, and the economic burden in terms of annual cost of labor lost to migraine disability is between $5.6 and $17.2 billion. The threshold for migraine may be genetically determined, although recent genetic and neurophysiologic studies point to migraine as possibly a channelopathy. Cerebral cortical and brain stem changes occur in migraine. Head pain and associated symptoms of migraine can be explained by activation of the trigeminal vascular system. Evidence has also been accumulated that suggests the release of nitric oxide is an important trigger mechanism. Introduction of the triptans has dramatically advanced acute migraine pharmacotherapy, and preventive therapy has greatly improved; however, public health initiatives may be needed to further advance diagnosis and treatment of this common and disabling disorder.

Cloning and functional characterization of squid voltage-dependent Ca2+ channel beta subunits: involvement of N-terminal sequences in differential modulation of the current

cDNAs that encode beta subunits of voltage-dependent Ca(2+) channel were cloned from the optic lobe of the squid Loligo bleekeri. The subunits, LoCa(v)beta(1a) and LoCa(v)beta(1b) are 96% identical in amino acid sequence. The sole sequence differences are in the N-terminal region and in a five amino acid insertion in the central region of LoCa(v)beta(1b). RT-PCR revealed that LoCa(v)beta(1a) and LoCa(v)beta(1b) transcripts were expressed mainly in the optic lobe and stellate ganglion, and more weakly in mantle muscle, systemic heart, gill, branchial heart, stomach and liver. Coexpression of LoCa(v)beta(1a) or LoCa(v)beta(1b) with mammalian Ca(v)2.3 and alpha(2)/delta subunits in the Xenopus oocyte resulted in high-voltage-activated currents, and showed slow current inactivation and moderate steady-state inactivation. Comparison of the squid subunits with four mammalian beta subunits, beta(1b), beta(2a), beta(3) and beta(4), demonstrated that the modulatory effects of the beta subunits on steady-state inactivation kinetics were beta(3)<beta(4) approximately beta(1b)<LoCa(v)beta(1a) approximately LoCa(v)beta(1b)<beta(2a). LoCa(v)beta(1a)-induced current amplitude was about two to four times higher than that of LoCa(v)beta(1b). Experiments with point mutants and chimeras suggest that potential PKC and CK2 phosphorylation sites in the N-terminal region of LoCa(v)beta(1b) affect the current amplitude reciprocally, and may be responsible for regulating current amplitude.

Actions of sipatrigine, 202W92 and lamotrigine on R-type and T-type Ca2+ channel currents

Relatively little has been published on the pharmacology of R-type and T-type Ca(2+) channels. Here, whole-cell Ca(2+) channel currents were recorded from human embryonic kidney 293 cell-lines transfected with either alpha1E subunits (R-type currents) or alpha1G or alpha1I subunits (T-type currents). R-type currents were inhibited by sipatrigine and the related compound 202W92 (R-(-)-2,4-diamino-6-(fluromethyl)-5-(2,3,5-trichlorophenyl)pyrimidine) with IC(50) 10 and 56 microM, respectively. A therapeutic concentration of lamotrigine (10 microM) inhibited R-type currents (30%) but was without effect on alpha1I-mediated T-type currents. Lamotrigine was also a weak inhibitor of T-type currents mediated by alpha1G subunits (<10% inhibition by 100 microM).

Potential of mean force calculations on an L-type calcium channel model

To understand the mechanisms of Na(+)/Li(+) permeation at submicromolar Ca(2+) concentrations, Na(+)/Li(+) blocking at higher Ca(2+) concentrations (10(-6)-10(-4) M) and Ca(2+) permeation at millimolar Ca(2+) concentrations, we used our recently described L-type calcium channel model. For this purpose, we obtained potential of mean force (pmf) curves for the position change of one Na(+) and one Ca(2+) ion inside the channel and for the position change of a second Ca(2+) ion when the EEEE locus is coordinated to Ca(2+). The pmf curves suggest that (i) at submicromolar Ca(2+) concentrations, because of the low velocity of Ca(2+) entry in the channel, monovalent ion flux occurs; (ii) at Ca(2+) concentrations between 10(-6) and 10(-4) M, thermodynamic equilibrium between the channel and Ca(2+) is achieved; as the coordination of Ca(2+) with the locus is more favorable than the coordination of Na(+), the monovalent ion flux is blocked; and (iii) to put a second Ca(2+) ion inside the channel at an appropriate rate, the Ca(2+) concentration should reach millimolar levels. Nevertheless, the entry of a second Ca(2+) is thermodynamically unfavorable, indicating that the competition of two Ca(2+) ions for the locus leads to Ca(2+) permeation.

Expression of voltage-dependent calcium channel subunits in the rat retina

The expression patterns of different Ca(2+) channel alpha(1) subunits (alpha(1A-E)) were immunohistochemically studied in the rat retina. Intense immunoreactivity (IR) for alpha(1A) (P/Q-type) and alpha(1B) (N-type) Ca(2+) channels was observed in both the outer and inner plexiform layers (OPL and IPL). In addition, alpha(1B)-IR was found in the outer and inner nuclear layers. Staining for alpha(1E) (R-type) was diffusely distributed in all three nuclear layers and in the IPL. The alpha(1C) and alpha(1D), two L-type Ca(2+) channel subunits, exhibited distinct expression patterns, with alpha(1C) being almost exclusively expressed on bipolar cells, and alpha(1D) mainly on photoreceptor cell bodies and in the OPL. Staining for alpha(1D) was also observed on Müller cells. The differential expression pattern of the alpha(1) subunits suggests that these Ca(2+) channel subtypes may be associated with different retinal functions.

An osmotically induced cytosolic Ca2+ transient activates calcineurin signaling to mediate ion homeostasis and salt tolerance of Saccharomyces cerevisiae

Hyperosmotic stress caused by NaCl, LiCl, or sorbitol induces an immediate and short duration ( approximately 1 min) transient cytosolic Ca(2+) ([Ca(2+)](cyt)) increase (Ca(2+)-dependent aequorin luminescence) in Saccharomyces cerevisiae cells. The amplitude of the osmotically induced [Ca(2+)](cyt) transient was attenuated by the addition of chelating agents EGTA or BAPTA, cation channel pore blockers, competitive inhibitors of Ca(2+) transport, or mutations (cch1Delta or mid1Delta) that reduce Ca(2+) influx, indicating that Ca(ext)(2+) is a source for the transient. An osmotic pretreatment (30 min) administered by inoculating cells into media supplemented with either NaCl (0.4 or 0.5 m) or sorbitol (0.8 or 1.0 m) enhanced the subsequent growth of these cells in media containing 1 m NaCl or 2 m sorbitol. Inclusion of EGTA in the osmotic pretreatment media or the cch1Delta mutation reduced cellular capacity for NaCl but not hyperosmotic adaptation. The stress-adaptive effect of hyperosmotic pretreatment was mimicked by exposing cells briefly to 20 mm CaCl(2). Thus, NaCl- or sorbitol-induced hyperosmotic shock causes a [Ca(2+)](cyt) transient that is facilitated by Ca(2+) influx, which enhances ionic but not osmotic stress adaptation. NaCl-induced ENA1 expression was inhibited by EGTA, cch1Delta mutation, and FK506, indicating that the [Ca(2+)](cyt) transient activates calcineurin signaling to mediate ion homeostasis and salt tolerance.

Differential discrimination of fast and slow synaptic waveforms by two low-voltage-activated calcium channels

Electrophysiological analysis of human embryonic kidney 293 cells stably expressing recombinant channels was used to compare how the biophysical properties of the low-voltage-activated Ca(2+) channels encoded by the alpha(1G) (Ca(V)3.1) or alpha(1I) (Ca(V)3.3) subunits shape their responses to excitatory synaptic potentials. In medium containing 2 mM extracellular Ca(2+) standard current-voltage relationships demonstrated both channel types to be clearly low-voltage activated with significant slowly activating current responses being observed at -66 mV. At all test potentials examined, activation of Ca(V)3.3 was substantially slower than that of Ca(V)3.1. To probe how these different T-type channels might respond to excitatory postsynaptic potentials (EPSPs), mock EPSPs with different kinetic profiles were created from the sum of exponentials. These waveforms were then used as command templates in voltage-clamp experiments. Ca(V)3.1-mediated channels responded effectively to both rapidly decaying mock EPSPs and slowly decaying EPSPs. In contrast, Ca(V)3.3-mediated channels were poorly gated by rapidly decaying EPSPs but were effectively activated by the more prolonged synaptic response. When activated with mock EPSPs Ca(V)3.3-mediated currents were more resistant to steady-state depolarisation of the pre-stimulus holding potential. Ca(V)3.3 currents were also more resistant to repetitive application of prolonged EPSPs, which caused substantial inactivation of Ca(V)3.1-mediated currents. The addition of a single mock action potential to the peak of a rapidly decaying EPSP voltage-clamp template greatly enhanced the currents produced by either Ca(V)3.1 or Ca(V)3.3-expressing cells. This facilitatory effect was considerably greater for Ca(V)3.3-mediated channels. From these data we suggest that the slow activation kinetics of Ca(V)3.3-mediated T-type channels enable them to respond selectively to either slow or suprathreshold synaptic potentials.

Vanilloid and TRP channels: a family of lipid-gated cation channels

The emergence of the TRP (C) and vanilloid (TRPV) receptor family of Ca(2+) permeable channels has started to provide molecular focus to a linked group of ion channels whose common feature is activation primarily by intracellular ligands. These channels have a central role in Ca(2+) homeostasis in virtually all cells and in particular those that lack voltage-gated Ca(2+) channels. We will discuss recent work that is more precisely defining both molecular form and physiological function of this important group of Ca(2+) permeable channels with particular focus on the intracellular ligands that gate and modulate channel activity.

Failure to replicate an allelic association between an exon 8 polymorphism of the human alpha(1A) calcium channel gene and common syndromes of idiopathic generalized epilepsy

The present replication study tested the validity of a previously reported allelic association between a single nucleotide polymorphism in exon 8 (SNP8) of the gene encoding the alpha(1A)-calcium channel subunit (CACNA1A) and common subtypes of idiopathic generalized epilepsy (IGE). Pyrosequencing was applied to assess the SNP8 genotypes in 354 unrelated German IGE probands, both parents of 118 IGE probands, and 186 healthy control subjects of German descent. Our population-based association analysis did not provide evidence for an allelic association of SNP8 with either IGE or two phenotypically more homogeneous IGE subtypes, consisting of either 139 probands with juvenile myoclonic epilepsy or 207 probands whose IGE started with typical absence seizures (P>0.72). In addition, the transmission disequilibrium test did not indicate a preferential transmission of SNP8 alleles in 97 informative parent-child transmissions (McNemar chi(2)=0.093, df=1, P=0.76). Accordingly, we failed to confirm previous evidence that genetic variation of the CACNA1A gene confers susceptibility to common IGE syndromes.

The diversity in the vanilloid (TRPV) receptor family of ion channels

Following cloning of the vanilloid receptor 1 (VR1) at least four other related proteins have been identified. Together, these form a distinct subgroup of the transient receptor potential (TRP) family of ion channels. Members of the vanilloid receptor family (TRPV) are activated by a diverse range of stimuli, including heat, protons, lipids, phorbols, phosphorylation, changes in extracellular osmolarity and/or pressure, and depletion of intracellular Ca2+ stores. However, VR1 remains the only channel activated by vanilloids such as capsaicin. These channels are excellent molecular candidates to fulfil a range of sensory and/or cellular roles that are well characterized physiologically. Furthermore, as novel pharmacological targets, the vanilloid receptors have potential for the development of many future disease treatments.

A molecular dynamics study of an L-type calcium channel model

In this work, we propose a molecular model of the L-type calcium channel pore from the human cardiac alpha1 subunit. Four glutamic acid residues, the EEEE locus, located at highly conserved P loops (also called SS1-SS2 segments) of the alpha1 subunit, molecularly express the calcium channel selectivity. The proposed alpha-helix structure for the SS1 segment, analyzed through molecular dynamics simulations in aqueous-phase, was validated by the plotting of Ramachandran diagrams for the averaged structures and by the analysis of i and i + 4 helical hydrogen bonding between the amino acid residues. The results of the simulation of the calcium channel model with one and two Ca2+ ions at the binding site are in accordance with mutation studies which suggest that the EEEE locus in the L-type calcium channel must form a single high-affinity binding site. These results suggest that the Ca2+ permeation through the channel would be derived from competition between two ions for the only high-affinity binding site. Furthermore, the experimentally observed blocking of the Na+ flux at micromolar Ca2+ concentrations, probably due to the occupancy of the single high-affinity binding site for one Ca2+, was also reproduced by our model.

Subcellular distribution of high-voltage-activated calcium channel subtypes in rat globus pallidus neurons

Globus pallidus (GP) neurons receive dense inhibitory synaptic inputs interspersed with sparse excitatory inputs distributed across the entire extent of their somata and dendrites. Yet, despite this predominance of inhibitory influence, GP neurons fire at a high tonic rate, suggesting that intrinsic properties play an important role in determining the physiological characteristics of these neurons. High-voltage-activated (HVA) calcium channels represent an important class of conductances that plays roles in controlling neurotransmitter release, postsynaptic excitability, and intracellular calcium signaling. To better understand the intrinsic properties of GP neurons, we examined the subcellular localization of HVA calcium channels by using immunocytochemistry at the electron microscopic level. Peroxidase labeling with antibodies against P/Q-, N-, and R-type HVA calcium channels demonstrated the presence of these channels in both proximal and distal dendrites of GP neurons. P/Q-, N-, and R-type channels were also found in presynaptic terminals, whereas L-type channels were found exclusively postsynaptically in neuronal elements. Immunogold labeling demonstrated that, although the density of intracellular L-type calcium channel labeling remains constant throughout the proximal-distal extent of the dendritic tree of GP neurons, the density of plasma membrane-bound channels is greater in distal dendrites. The finding of HVA calcium channels distributed throughout the whole dendritic tree of GP neurons indicates that these channels may interact with synaptic inputs to allow rich processing possibilities for GP neuron dendrites. Furthermore, the finding of a greater density of plasma membrane-bound L-type channels in distal dendrites expands the view that L-type channels are important only in somatic and proximal locations.

Changes in voltage-gated calcium channel alpha(1) gene expression in rat dorsal root ganglia following peripheral nerve injury

Although an increase in the excitability and ectopic spontaneous discharge (ESD) of primary sensory neurons can lead to abnormal burst activity, which is associated with neuropathic pain, the underlying molecular mechanisms are not fully understood. To investigate the relationship between these electrical abnormalities in injured neurons and voltage-gated calcium channel (VGCC) gene expression, reverse transcription-polymerase chain reaction (RT-PCR) was used to monitor the expression of the VGCC alpha(1) gene in the dorsal root ganglion (DRG) following chronic constriction injury (CCI) and axotomy of the rat sciatic nerve. Electrophoresis of the RT-PCR products showed the presence of multiple types of VGCC alpha(1) transcripts with various levels of basal expression in lumbar 4, 5, and 6 DRGs. CCI decreased alpha(1C), alpha(1D), alpha(1H), and alpha(1I) mRNA expression at 7 days in the ipsilateral DRG, to approximately 34-50% of the contralateral side. The same transcripts were repressed 7 days after sciatic axotomy and their reduction levels proved similar to those of CCI. Considering that changes of the intracellular calcium concentration modify the maintenance of ESD in injured DRG, these results suggest that the downregulation of alpha(1C), alpha(1D), alpha(1H) and alpha(1I) subunit gene expression in the rat DRG following peripheral nerve injury may contribute to the production of ESD associated with damaged nerves.

Neuroprotective actions in vivo and electrophysiological actions in vitro of 202W92

202W92 (R-(-)-2,4-diamino-6-(fluromethyl)-5-(2,3,5-trichlorophenyl)pyrimidine) is a novel compound in the same chemical series as the antiepileptic drug lamotrigine and the neuroprotective sipatrigine. Here 202W92 was quantitatively assessed as a neuroprotective agent in focal cerebral ischaemia, and as an inhibitor of sodium and calcium channels and of synaptic transmission. In the rat permanent middle cerebral artery occlusion (MCAO) model of acute focal ischaemia, 202W92 reduced infarct volume by 75% in cortex and by 80% in basal ganglia, with ED(50) approximately 2 mg/kg (single i.v. dose, 10 min post-occlusion). In whole-cell current recordings from single cells, 202W92 completely and reversibly inhibited voltage gated sodium channels (IC(50) 3 x 10(-6) M) in rat freshly-isolated cortical neurons and in the GH(3) pituitary cell line. 202W92 also inhibited a nifedipine-sensitive fraction (approximately 35%) of native high-voltage-activated (HVA) calcium current in rat cortical neurons (IC(50) 15 x 10(-6) M) and weakly inhibited low-voltage-activated (LVA) calcium currents of the recombinant alpha1I-mediated T-type (IC(50)>100 x 10(-6) M). The drug inhibited the amplitude and frequency of 4-aminopyridine-evoked glutamatergic excitatory post-synaptic currents (EPSCs). In conclusion, 202W92 is an effective neuroprotective agent when administered post-ischaemia and a potent sodium channel inhibitor in vitro.

High-affinity inhibition of glutamate release from corticostriatal synapses by omega-agatoxin TK

To know which Ca(2+) channel type is the most important for neurotransmitter release at corticostriatal synapses of the rat, we tested Ca(2+) channel antagonists on the paired pulse ratio. omega-Agatoxin TK was the most effective Ca(2+) channel antagonist (IC(50)=127 nM; maximal effect=211% (with >1 microM) and Hill coefficient=1.2), suggesting a single site of action and a Q-type channel profile. Corresponding parameters for Cd(2+) were 13 microM, 178% and 1.2. The block of L-type Ca(2+) channels had little impact on transmission, but we also tested facilitation of L-type Ca(2+) channels. The L-type Ca(2+) channel agonist, s-(-)-1,4 dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-3-pyridine carboxylic acid methyl ester (Bay K 8644 (5 microM)), produced a 45% reduction of the paired pulse ratio, suggesting that even if L-type channels do not participate in the release process, they may participate in its modulation.

Extracellular signal-regulated kinases and calcium channels are involved in the proliferative effect of bisphosphonates on osteoblastic cells in vitro

Bisphosphonates (BPs) are analogues of pyrophosphate, which are widely used for the treatment of different pathologies associated with imbalances in bone turnover. Recent evidence suggested that cells of the osteoblastic lineage might be targets of the action of BPs. The objective of this work was to determine whether BPs induce proliferation of osteoblasts and whether this action involves activation of the extracellular signal-regulated kinases (ERKs). We have shown that three different BPs (olpadronate, pamidronate, and etidronate) induce proliferation in calvaria-derived osteoblasts and ROS 17/2.8 as measured by cell count and by [3H]thymidine uptake. Osteoblast proliferation induced by all BPs diminished to control levels in the presence of U0126, a specific inhibitor of the upstream kinase MEK 1 responsible for ERK phosphorylation. Consistent with this, BPs induced ERK activation as assessed by in-gel kinase assays. Phosphorylation of ERK1/2 was induced by the BPs olpadronate and pamidronate within 30 s, followed by rapid dephosphorylation, whereas etidronate induced phosphorylation of ERKs only after 90 s of incubation and returned to basal levels within 15-30 minutes. In addition, both BP-induced cell proliferation and ERK phosphorylation were reduced to basal levels in the presence of nifedipine, an L-type voltage-sensitive calcium channel (VSCC) inhibitor. These results show that BP-induced proliferation of osteoblastic cells is mediated by activation of ERKs and suggest that this effect requires influx of Ca2+ from the extracellular space through calcium channels.