Cav2.2 Channels Sustain Vesicle Recruitment at a Mature Glutamatergic Synapse

The composition of voltage-gated Ca2+ channel (Cav) subtypes that gate action potential (AP)-evoked release changes during the development of mammalian CNS synapses. Cav2.2 and Cav2.3 lose their function in gating-evoked release during postnatal synapse maturation. In mature boutons, Cav2.1 currents provide the almost exclusive trigger for evoked release, and Cav2.3 currents are required for the induction of presynaptic long-term potentiation. However, the functional significance of Cav2.2 remained elusive in mature boutons, although they remain present at active zones and continue contributing significantly to presynaptic Ca2+ influx. Here, we addressed the functional significance of Cav2.2 and Cav2.3 at mature parallel-fiber (PF) to Purkinje neuron synapses of mice of either sex. These synapses are known to exhibit the corresponding developmental Cav subtype changes in gating release. We addressed two hypotheses, namely that Cav2.2 and Cav2.3 are involved in triggering spontaneous glutamate release and that they are engaged in vesicle recruitment during repetitive evoked release. We found that spontaneous miniature release is Ca2+ dependent. However, experiments with Cav subtype-specific blockers excluded the spontaneous opening of Cavs as the Ca2+ source for spontaneous glutamate release. Thus, neither Cav2.2 nor Cav2.3 controls spontaneous release from PF boutons. Furthermore, vesicle recruitment during brief bursts of APs was also independent of Ca2+ influx through Cav2.2 and Cav2.3. However, Cav2.2, but not Cav2.3, currents significantly boosted vesicle recruitment during sustained high-frequency synaptic transmission. Thus, in mature PF boutons Cav2.2 channels are specifically required to sustain synaptic transmission during prolonged neuronal activity.SIGNIFICANCE STATEMENT At young CNS synapses, action potential-evoked release is gated via three subtypes of voltage-gated Ca2+ channels: Cav2.1, Cav2.2, and Cav2.3. During postnatal maturation, Cav2.2 and Cav2.3 lose their function in gating evoked release, such that at mature synapses Cav2.1 provides the almost exclusive source for triggering evoked release. Cav2.3 currents are required for the induction of presynaptic long-term potentiation. However, the function of the still abundant Cav2.2 in mature boutons remained largely elusive. Here, we studied mature cerebellar parallel-fiber synapses and found that Cav2.2 does not control spontaneous release. However, Ca2+ influx through Cav2.2 significantly boosted vesicle recruitment during trains of action potentials. Thus, Cav2.2 in mature parallel-fiber boutons participate in sustaining synaptic transmission during prolonged activity.

N-type calcium channel and renal injury

Accumulating evidences indicated that voltage-gated calcium channels (VDCC), including L-, T-, N-, and P/Q-type, are present in kidney and contribute to renal injury during various chronic diseases trough different mechanisms. As a voltage-gated calcium channel, N-type calcium channel was firstly been founded predominately distributed on nerve endings which control neurotransmitter releases. Since sympathetic nerve is distributed along renal afferent and efferent arterioles, N-type calcium channel blockade on sympathetic nerve terminals would bring renal dynamic improvement by dilating both arterioles and reducing glomerular pressure. In addition, large body of scientific research indicated that neurotransmitters, such as norepinephrine, releases by activating N-type calcium channel can trigger inflammatory and fibrotic signaling pathways in kidney. Interestingly, we recently demonstrated that N-type calcium channel is also expressed on podocytes and may directly contribute to podocyte injury in denervated animal models. In this paper, we will summarize our current knowledge regarding renal N-type calcium channels, and discuss how they might contribute to the river that terminates in renal injury.

Neuregulin-1-dependent control of amygdala microcircuits is critical for fear extinction

The posttraumatic stress disorder is marked by an impaired ability to extinct fear memory acquired in trauma. Although previous studies suggest that fear extinction depends on the function of the amygdala, the underlying mechanisms are unclear. We found that NRG1 receptors (ErbB4) were abundantly expressed in the intercalated cells mass of amygdala (ITC). The NRG1-ErbB4 pathway in the ITC promotes fear extinction. The NRG1-ErbB4 pathway in the ITC did not affect excitatory input to ITC neurons from BLA neurons but increased feed-forward inhibition of (the central medial nucleus of the amygdala) CeM neurons through increased GABAergic neurotransmission of ITC neurons. We also found that the NRG1-ErbB4 signaling pathway in ITC might regulate fear extinction through P/Q-type voltage-activated Ca2+ channels (VACCs) but not through L- or N-type VACCs. Overall, our results suggest that the NRG1-ErbB4 signaling pathway in the ITC might represent a potential target for the treatment of anxiety disorders.

Nociceptin/orphanin FQ peptide receptor mediates inhibition of N-type calcium currents in vestibular afferent neurons of the rat

The vestibular system is modulated by various neuromodulators including opioid peptides. The current study was conducted to determine whether activation of nociceptin/orphanin FQ peptide (NOP) receptors modulates voltage-gated calcium currents and action potential discharge of rat vestibular afferent neurons. We performed whole cell patch-clamp recordings on cultured vestibular afferent neurons from P7-P10 Long-Evans rats. Application of nociceptin/orphanin FQ (N/OFQ), a 17-amino acid neuropeptide that is the endogenous ligand for NOP receptor, inhibits the high-voltage activated (HVA) component of the calcium current in a concentration-dependent manner with a half inhibitory concentration of 26 nM. Said inhibitory action on the calcium current is voltage-dependent, which was made clear by the fact that it was reverted in 80% by a depolarizing prepulse. Furthermore, the effect of N/OFQ was blocked by application of the specific NOP-antagonist UFP101, by preincubation with G-protein blocker pertussis toxin, and by coapplication of the specific N-type calcium-current blocker ω-conotoxin-MVIIA. N/OFQ application causes an increase in the duration and maximum rate of repolarization of action potentials. It also decreases repetitive discharge and discharge elicited by sinusoidal stimulation. These results show that in vestibular afferents, NOP receptor activation inhibits N-type calcium current by activating G proteins, mostly through the Gβγ subunit. This suggests that NOP activation produces a presynaptic modulation of primary vestibular afferent neurons' output into the vestibular nuclei, thus taking part in the integration and gain setting of vestibular information in second-order vestibular nucleus neurons.NEW & NOTEWORTHY Our results show that in primary vestibular afferent neurons, activation of the nociceptin/orphanin FQ peptide receptor inhibits the N-type calcium current by a mechanism mediated by G proteins. We propose that calcium current inhibition modulates neurotransmitter release from vestibular afferents, producing a presynaptic modulation of vestibular input to vestibular nuclei, thus contributing to gain control in the vestibular afferent input.

Magnetic Nanoparticle-Based Mechanical Stimulation for Restoration of Mechano-Sensitive Ion Channel Equilibrium in Neural Networks

Techniques offering remote control of neural activity with high spatiotemporal resolution and specificity are invaluable for deciphering the physiological roles of different classes of neurons in brain development and disease. Here, we first confirm that microfabricated substrates with enhanced magnetic field gradients allow for wireless stimulation of neural circuits dosed with magnetic nanoparticles using calcium indicator dyes. We also investigate the mechanism of mechano-transduction in this system and identify that N-type mechano-sensitive calcium ion channels play a key role in signal generation in response to magnetic force. We next applied this method for chronic stimulation of a fragile X syndrome (FXS) neural network model and found that magnetic force-based stimulation modulated the expression of mechano-sensitive ion channels which are out of equilibrium in a number of neurological diseases including FXS. This technique can serve as a tool for acute and chronic modulation of endogenous ion channel expression in neural circuits in a spatially localized manner to investigate a number of disease processes in the future.

In vivo impact of presynaptic calcium channel dysfunction on motor axons in episodic ataxia type 2

Ion channel dysfunction causes a range of neurological disorders by altering transmembrane ion fluxes, neuronal or muscle excitability, and neurotransmitter release. Genetic neuronal channelopathies affecting peripheral axons provide a unique opportunity to examine the impact of dysfunction of a single channel subtype in detail in vivo. Episodic ataxia type 2 is caused by mutations in CACNA1A, which encodes the pore-forming subunit of the neuronal voltage-gated calcium channel Cav2.1. In peripheral motor axons, this channel is highly expressed at the presynaptic neuromuscular junction where it contributes to action potential-evoked neurotransmitter release, but it is not expressed mid-axon or thought to contribute to action potential generation. Eight patients from five families with genetically confirmed episodic ataxia type 2 underwent neurophysiological assessment to determine whether axonal excitability was normal and, if not, whether changes could be explained by Cav2.1 dysfunction. New mutations in the CACNA1A gene were identified in two families. Nerve conduction studies were normal, but increased jitter in single-fibre EMG studies indicated unstable neuromuscular transmission in two patients. Excitability properties of median motor axons were compared with those in 30 age-matched healthy control subjects. All patients had similar excitability abnormalities, including a high electrical threshold and increased responses to hyperpolarizing (P < 0.00007) and depolarizing currents (P < 0.001) in threshold electrotonus. In the recovery cycle, refractoriness (P < 0.0002) and superexcitability (P < 0.006) were increased. Cav2.1 dysfunction in episodic ataxia type 2 thus has unexpected effects on axon excitability, which may reflect an indirect effect of abnormal calcium current fluxes during development.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

Synaptic Multivesicular Release in the Cerebellar Cortex: Its Mechanism and Role in Neural Encoding and Processing

The number of synaptic vesicles released during fast release plays a major role in determining the strength of postsynaptic response. However, it remains unresolved how the number of vesicles released in response to action potentials is controlled at a single synapse. Recent findings suggest that the Cav2.1 subtype (P/Q-type) of voltage-gated calcium channels is responsible for inducing presynaptic multivesicular release (MVR) at rat cerebellar glutamatergic synapses from granule cells to molecular layer interneurons. The topographical distance from Cav2.1 channels to exocytotic Ca(2+) sensors is a critical determinant of MVR. In physiological trains of presynaptic neurons, MVR significantly impacts the excitability of postsynaptic neurons, not only by increasing peak amplitude but also by prolonging decay time of the postsynaptic currents. Therefore, MVR contributes additional complexity to neural encoding and processing in the cerebellar cortex.

[Study on analgesia of oxymatrine and its relation to calcium channels]

OBJECTIVE

To study whether the analgesis of oxymatrine (OMT) affects N-type voltage-gated calcium channels (VGCCs).

METHODS

Totally 45 mice were randomly divided into the sham-operation group, the model group [established by partial sciatic nerve ligation (PSNL)] , and the OMT treatment group according to random digit table, 15 in each group. The dorsal root ganglions (DRG) were separated in PSNL pain model mice. Intracellular calcium concentration ([Ca2+]i) was determined with Fluo-3 AM immunofluorescent probe in cultured DRG neurons. Different protein expression levels of N-type (Cav2. 2) and L-type ( Cav1. 3) among VGCCs from brain and DRG tissues were detected with Western blot.

RESULTS

Compared with the sham-operation group, [Ca2+]i, increased in cultured DRG neurons (P <0. 05) , protein expression levels of Cav2. 2 in the brain tissue increased (P <0. 05), protein expression levels of Cav2. 2 in DRG tissues decreased in the model group (P <0. 01). Compared with the model group, [Ca2+]i, decreased in cultured DRG neurons (P < 0. 05), protein expression levels of Cav2. 2 in the brain tissue decreased (P <0. 01), protein expression levels of Cav2. 2 in DRG tissues increased in the OMT treatment group (P <0. 01). There was no statistical difference in Cav1. 3 expressions in cultured DRG neurons and the brain (P >0. 05).

CONCLUSION

Analgesic effect of OMT might be related to Cav2. 2 channel mediated calcium ion flux.

Neuronal calcium wave propagation varies with changes in endoplasmic reticulum parameters: a computer model

Calcium (Ca²⁺) waves provide a complement to neuronal electrical signaling, forming a key part of a neuron's second messenger system. We developed a reaction-diffusion model of an apical dendrite with diffusible inositol triphosphate (IP₃), diffusible Ca²⁺, IP₃ receptors (IP₃Rs), endoplasmic reticulum (ER) Ca²⁺ leak, and ER pump (SERCA) on ER. Ca²⁺ is released from ER stores via IP₃Rs upon binding of IP₃ and Ca²⁺. This results in Ca²⁺-induced-Ca²⁺-release (CICR) and increases Ca²⁺ spread. At least two modes of Ca²⁺ wave spread have been suggested: a continuous mode based on presumed relative homogeneity of ER within the cell and a pseudo-saltatory model where Ca²⁺ regeneration occurs at discrete points with diffusion between them. We compared the effects of three patterns of hypothesized IP₃R distribution: (1) continuous homogeneous ER, (2) hotspots with increased IP₃R density (IP₃R hotspots), and (3) areas of increased ER density (ER stacks). All three modes produced Ca²⁺ waves with velocities similar to those measured in vitro (approximately 50-90 μm /sec). Continuous ER showed high sensitivity to IP₃R density increases, with time to onset reduced and speed increased. Increases in SERCA density resulted in opposite effects. The measures were sensitive to changes in density and spacing of IP₃R hotspots and stacks. Increasing the apparent diffusion coefficient of Ca²⁺ substantially increased wave speed. An extended electrochemical model, including voltage-gated calcium channels and AMPA synapses, demonstrated that membrane priming via AMPA stimulation enhances subsequent Ca²⁺ wave amplitude and duration. Our modeling suggests that pharmacological targeting of IP₃Rs and SERCA could allow modulation of Ca²⁺ wave propagation in diseases where Ca²⁺ dysregulation has been implicated.

Gamma band activity in the RAS-intracellular mechanisms

Gamma band activity participates in sensory perception, problem solving, and memory. This review considers recent evidence showing that cells in the reticular activating system (RAS) exhibit gamma band activity, and describes the intrinsic membrane properties behind such manifestation. Specifically, we discuss how cells in the mesopontine pedunculopontine nucleus, intralaminar parafascicular nucleus, and pontine SubCoeruleus nucleus dorsalis all fire in the gamma band range when maximally activated, but no higher. The mechanisms involve high-threshold, voltage-dependent P/Q-type calcium channels, or sodium-dependent subthreshold oscillations. Rather than participating in the temporal binding of sensory events as in the cortex, gamma band activity in the RAS may participate in the processes of preconscious awareness and provide the essential stream of information for the formulation of many of our actions. We address three necessary next steps resulting from these discoveries: an intracellular mechanism responsible for maintaining gamma band activity based on persistent G-protein activation, separate intracellular pathways that differentiate between gamma band activity during waking versus during REM sleep, and an intracellular mechanism responsible for the dysregulation in gamma band activity in schizophrenia. These findings open several promising research avenues that have not been thoroughly explored. What are the effects of sleep or REM sleep deprivation on these RAS mechanisms? Are these mechanisms involved in memory processing during waking and/or during REM sleep? Does gamma band processing differ during waking versus REM sleep after sleep or REM sleep deprivation?

Opioid-related (ORL1) receptors are enriched in a subpopulation of sensory neurons and prolonged activation produces no functional loss of surface N-type calcium channels

The opioid-related receptor, ORL1, is activated by the neuropeptide nociceptin/orphanin FQ (N/OFQ) and inhibits high-voltage-activated (HVA) calcium channel currents (I(Ca)) via a G-protein-coupled mechanism. Endocytosis of ORL1 receptor during prolonged N/OFQ exposure was proposed to cause N-type voltage-gated calcium channel (VGCC) internalization via physical interaction between ORL1 and the N-type channel. However, there is no direct electrophysiological evidence for this mechanism in dorsal root ganglion (DRG) neurons or their central nerve terminals. The present study tested this using whole-cell patch-clamp recordings of HVA I(Ca) in rat DRG neurons and primary afferent excitatory synaptic currents (eEPSCs) in spinal cord slices. DRG neurons were classified on the basis of diameter, isolectin-B4 (IB4) binding and responses to capsaicin, N/OFQ and a μ-opioid agonist, DAMGO. IB4-negative neurons less than 20 μm diameter were selectively responsive to N/OFQ as well as DAMGO. In these neurons, ORL1 desensitization by a supramaximal concentration of N/OFQ was not followed by a decrease in HVA I(Ca) current density or proportion of whole-cell HVA I(Ca) contributed by N-type VGCC as determined using the N-type channel selective blocker, ω-conotoxin CVID. There was also no decrease in the proportion of N-type I(Ca) when neurons were incubated at 37°C with N/OFQ for 30 min prior to recording. In spinal cord slices, N/OFQ consistently inhibited eEPSCs onto dorsal horn neurons. As observed in DRG neurons, preincubation of slices in N/OFQ for 30 min produced no decrease in the proportion of eEPSCs inhibited by CVID. In conclusion, no internalization of the N-type VGCC occurs in either the soma or central nerve terminals of DRG neurons following prolonged exposure to high, desensitizing concentrations of N/OFQ.

[Effects of gabapentin on high-voltage-activated calcium current in dorsal root ganglion neurons in rats]

OBJECTIVE

To compare the effects of gabapentin on high-voltage-activated calcium (HVA) current in dorsal root ganglion (DRG) neurons in normal and nerve-injured rats and understand the reasons of their differences.

METHODS

Pathogen-free male SD rats (weight 180 - 220 mg) aged 4 - 6 weeks were used. The animals were anesthetized with intraperitoneal pentobarbital sodium 50 mg/kg. L(5) spinal nerve was ligated between DRG and sciatic nerve and cut distal to the ligature. The animals were decapitated at Day 14 post-operation. L(5) (SNL-L(5) group) and L(4) DRGs (SNL-L(4) group) were respectively isolated and the ganglionic neurons enzymatically dissociated. The control group of rats was not operated. The lumbar DRG neurons of normal rats were treated similarly. The HVA-Ca(2+) current was recorded by the technique of whole cell patch clamp.

RESULTS

Compared with the SNL-L(4) group [(16.0 ± 1.9)%, (26.9 ± 2.0)%, (27.4 ± 2.3)%] and the control group, gabapentin inhibited the peak calcium current highlier at 10, 100 and 300 µmol/L in the SNL-L(5) group [(18.5 ± 1.7)%, (32.0 ± 2.6)%, (32.7 ± 2.8)%] (P < 0.05). The steady-state inactivation curves shifted to more hyperpolarized potentials in the SNL-L(5) group. The N-type relative contribution to the gabapentin-sensitive HVA-Ca(2+) current was markedly elevated in the SNL-L(5) group compared with that in other two groups (P < 0.05).

CONCLUSION

Gabapentin enhances the inhibition of HVA-Ca(2+) current in injured DRG neurons following spinal nerve ligation in rats. The alteration in the activation of electrophysiological properties and the increase of N-type relative contribution to the total HVA-Ca(2+) current may be involved in the mechanism.

Insights into migraine mechanisms and CaV2.1 calcium channel function from mouse models of familial hemiplegic migraine

Migraine is a very common disabling brain disorder with unclear pathogenesis. A subtype of migraine with aura (familial hemiplegic migraine type 1: FHM1) is caused by mutations in CaV2.1 (P/Q-type) Ca2+ channels. This review describes the functional consequences of FHM1 mutations in knockin mouse models carrying the mild R192Q or severe S218L mutations in the orthologous gene. The FHM1 knockin mice show allele dosage-dependent gain-of-function of neuronal P/Q-type Ca2+ current, reflecting activation of mutant channels at lower voltages, and allele dosage- and sex-dependent facilitation of induction and propagation of cortical spreading depression (CSD), the phenomenon that underlies migraine aura. Gain-of-function of neuronal Ca2+ current, facilitation of CSD and post-CSD motor deficits were larger in S218L than R192Q knockin mice, in correlation with the more severe human S218L phenotype. Enhanced cortical excitatory neurotransmission, due to increased action potential-evoked Ca2+ influx and increased probability of glutamate release at pyramidal cell synapses, but unaltered inhibitory neurotransmission at fast-spiking interneuron synapses, were demonstrated in R192Q knockin mice. Evidence for a causative link between enhanced glutamate release and CSD facilitation was obtained. The data from FHM1 mice strengthen the view of CSD as a key player in the pathogenesis of migraine, give insight into CSD mechanisms and point to episodic disruption of excitation-inhibition balance and neuronal hyperactivity as the basis for vulnerability to CSD ignition in migraine.

Long-term blockade of L/N-type Ca(2+) channels by cilnidipine ameliorates repolarization abnormality of the canine hypertrophied heart

BACKGROUND AND PURPOSE

The heart of the canine model of chronic atrioventricular block is known to have a ventricular electrical remodelling, which mimics the pathophysiology of long QT syndrome. Using this model, we explored a new pharmacological therapeutic strategy for the prevention of cardiac sudden death.

EXPERIMENTAL APPROACH

The L-type Ca(2+) channel blocker amlodipine (2.5 mg.day(-1)), L/N-type Ca(2+) channel blocker cilnidipine (5 mg.day(-1)), or the angiotensin II receptor blocker candesartan (12 mg.day(-1)) was administered orally to the dogs with chronic atrioventricular block for 4 weeks. Electropharmacological assessments with the monophasic action potential (MAP) recordings and blood sample analyses were performed before and 4 weeks after the start of drug administration.

KEY RESULTS

Amlodipine and cilnidipine decreased the blood pressure, while candesartan hardly affected it. The QT interval, MAP duration and beat-to-beat variability of the ventricular repolarization period were shortened only in the cilnidipine group, but such effects were not observed in the amlodipine or candesartan group. Plasma concentrations of adrenaline, angiotensin II and aldosterone decreased in the cilnidipine group. In contrast, plasma concentrations of angiotensin II and aldosterone were elevated in the amlodipine group, whereas in the candesartan group an increase in plasma levels of angiotensin II and a decrease in noradrenaline and adrenaline concentrations were observed.

CONCLUSIONS AND IMPLICATIONS

Long-term blockade of L/N-type Ca(2+) channels ameliorated the ventricular electrical remodelling in the hypertrophied heart which causes the prolongation of the QT interval. This could provide a novel therapeutic strategy for the treatment of cardiovascular diseases.

A presynaptic homeostatic signaling system composed of the Eph receptor, ephexin, Cdc42, and CaV2.1 calcium channels

The molecular mechanisms underlying the homeostatic modulation of presynaptic neurotransmitter release remain largely unknown. In a screen, we isolated mutations in Drosophila ephexin (Rho-type guanine nucleotide exchange factor) that disrupt the homeostatic enhancement of presynaptic release following impairment of postsynaptic glutamate receptor function at the Drosophila neuromuscular junction. We show that Ephexin is sufficient presynaptically for synaptic homeostasis and localizes in puncta throughout the nerve terminal. However, ephexin mutations do not alter other aspects of neuromuscular development, including morphology or active zone number. We then show that, during synaptic homeostasis, Ephexin functions primarily with Cdc42 in a signaling system that converges upon the presynaptic CaV2.1 calcium channel. Finally, we show that Ephexin binds the Drosophila Eph receptor (Eph) and Eph mutants disrupt synaptic homeostasis. Based on these data, we propose that Ephexin/Cdc42 couples synaptic Eph signaling to the modulation of presynaptic CaV2.1 channels during the homeostatic enhancement of presynaptic release.

[The role of N-type high-voltage-activated calcium channels in plasticity regulation of inhibitory synaptic transmission in cultured hippocampal neurons]

Now it is clear that N-type Ca2+ channels contribute to synaptic transmission at many of CNS synapses. However, it is not known whether presynaptic N-type Ca2+ channels contribute to short-term synaptic plasticity (STP) mediated by GABA release at inhibitory synapses of cultured hippocampal neurons. We studied the sensitivity of GABAergic paired pulse depression (PPD) as a common form of STP to selective N-type high-voltage-activated Ca2+ channels blocker omega-conotoxin (omegaCgTx). Evoked inhibitory postsynaptic currents (eIPSCs) were studied using patch-clamp technique in whole-cell configuration in postsynaptic neuron and local exteracellular paired pulse stimulation of single presynaptic axon by rectangular pulse with 0.4 ms duration, the interpulse interval in pair was 150 ms. CgTx (200 nM; 1 microM) in a dose-dependent manner irreversibly reduced the amplitude of paired eIPSCs by 25-49% and decreased PPD by 11-22% compared with control. These results confirm that N-type Ca2+ channels are highly involved in inhibitory synaptic transmission and short-term synaptic plasticity in cultured hippocampal neurons.

Origin of the voltage dependence of G-protein regulation of P/Q-type Ca2+ channels

G-protein (Gbetagamma)-mediated voltage-dependent inhibition of N- and P/Q-type Ca(2+) channels contributes to presynaptic inhibition and short-term synaptic plasticity. The voltage dependence derives from the dissociation of Gbetagamma from the inhibited channels, but the underlying molecular and biophysical mechanisms remain largely unclear. In this study we investigated the role in this process of Ca(2+) channel beta subunit (Ca(v)beta) and a rigid alpha-helical structure between the alpha-interacting domain (AID), the primary Ca(v)beta docking site on the channel alpha(1) subunit, and the pore-lining IS6 segment. Gbetagamma inhibition of P/Q-type channels was reconstituted in giant inside-out membrane patches from Xenopus oocytes. Large populations of channels devoid of Ca(v)beta were produced by washing out a mutant Ca(v)beta with a reduced affinity for the AID. These beta-less channels were still inhibited by Gbetagamma, but without any voltage dependence, indicating that Ca(v)beta is indispensable for voltage-dependent Gbetagamma inhibition. A truncated Ca(v)beta containing only the AID-binding guanylate kinase (GK) domain could fully confer voltage dependence to Gbetagamma inhibition. Gbetagamma did not alter inactivation properties, and channels recovered from Gbetagamma inhibition exhibited the same activation property as un-inhibited channels, indicating that Gbetagamma does not dislodge Ca(v)beta from the inhibited channel. Furthermore, voltage-dependent Gbetagamma inhibition was abolished when the rigid alpha-helix between the AID and IS6 was disrupted by insertion of multiple glycines, which also eliminated Ca(v)beta regulation of channel gating, revealing a pivotal role of this rigid alpha-helix in both processes. These results suggest that depolarization-triggered movement of IS6, coupled to the subsequent conformational change of the Gbetagamma-binding pocket through a rigid alpha-helix induced partly by the Ca(v)beta GK domain, causes the dissociation of Gbetagamma and is fundamental to voltage-dependent Gbetagamma inhibition.

The role of MAP1A light chain 2 in synaptic surface retention of Cav2.2 channels in hippocampal neurons

Ca(v)2.2 channels are localized at nerve terminals where they play a critical role in neurotransmission. However, the determinant that controls surface retention of these channels has not been identified. Here, we report that presynaptic surface localization of Ca(v)2.2 is mediated through its interaction with light chain 2 (LC2) of microtubule-associated protein MAP1A. Deletion of a 23-residue binding domain within the Ca(v)2.2 C terminus resulted in reduced synaptic distribution of the mutant channels. Using an antibody generated against an extracellular epitope of Ca(v)2.2, we demonstrate that interfering the interaction with LC2 reduced surface expression of endogenous Ca(v)2.2 at presynaptic boutons. In addition, the disruption of LC2-Ca(v)2.2 coupling reduced Ca(2+)-influx into nerve terminals through Ca(v)2.2 and impaired activity-dependent FM4-64 uptake. The treatments of neurons with Latrunculin A to disrupt actin filaments resulted in reduced density of surface Ca(v)2.2-positive boutons. Furthermore, LC2NT, a LC2 truncated mutant lacking the actin-binding domain, could not rescue Ca(v)2.2 surface expression after suppressing LC2 expression with RNAi. Because actin filaments are major cytomatric components at the presynaptic boutons, these observations suggest a mechanism by which LC2 provides anchoring of surface Ca(v)2.2 to the actin cytoskeleton, thus contributing to presynaptic function.

Voltage-dependent calcium channels in ventromedial hypothalamic neurones of postnatal rats: modulation by oestradiol and phenylephrine

Oestradiol actions in the hypothalamus play an important role in reproductive behaviour. Oestradiol treatment in vivo induces alpha(1b)-adrenoceptor mRNA and increases the density of alpha(1B)-adrenoceptor binding in the hypothalamus. Oestradiol is also known to modulate neuronal excitability, in some cases by modulating calcium channels. We assessed the effects of phenylephrine, an alpha(1)-adrenergic agonist, on low-voltage-activated (LVA) and high-voltage-activated (HVA) calcium channels in ventromedial hypothalamic (VMN) neurones from vehicle- and oestradiol-treated female rats. Whole-cell and gramicidin perforated-patch recordings were obtained, with barium as the charge carrier. In the absence of phenylephrine, oestradiol treatment increased the magnitude of LVA currents compared to controls, but had no effect on HVA currents. Phenylephrine enhanced HVA currents in a significantly greater proportion of neurones from oestradiol-treated rats (76%) than from vehicle-treated (41%) rats. The L-channel blocker nifedipine abolished this oestradiol effect on phenylephrine-enhanced HVA currents. Preincubating slices with the N-type channel blocker omega-conotoxin GVIA completely blocked the phenylephrine response, suggesting that the N-type channel is essential. Phenylephrine also stimulated LVA currents in approximately two-thirds of neurones in slices from both vehicle- and oestradiol-treated rats. Our data show that oestradiol increases LVA currents in the VMN. Oestradiol also amplifies alpha(1)-adrenergic signalling by increasing the proportion of neurones showing phenylephrine-stimulated HVA currents mediated by N- and L-type calcium channels. In this way, oestradiol may increase excitatory responses to arousing adrenergic inputs to VMN neurones governing oestradiol-dependent reproductive behaviour.

[Pathological changes of potential-activated calcium channels in sensory neurons]

The review considers the structure and function of high-and low-voltage potential activated calcium channels in sensory neurons. A special attention is paid to compression of the function of these channels in normal conditions and during development of pathological conditions. The role of low-voltage activated T-type calcium channels during such forms of pathology as neuropathy, acidosis and alkalosis because the changes in synaptic transmission occurring during these forms of pathological changes are most intensively altered during changes in functional structures of these type of channels. During studying of high-voltage activated calcium channels main attention has been concentrated on changes in the function of N-type potential activated calcium channels.

mGluR7 inhibits glutamate release through a PKC-independent decrease in the activity of P/Q-type Ca2+ channels and by diminishing cAMP in hippocampal nerve terminals

The modulation of calcium channels by metabotropic glutamate receptors (mGluRs) is a key event in the fine-tuning of neurotransmitter release. Here we report that, in hippocampal nerve terminals from adult rats, the inhibition of glutamate release by the group III mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) is largely mediated by mGluR7. In this preparation, P/Q-type Ca(2+) channels support the major component of glutamate release while the remaining release is supported by N-type Ca(2+) channels. The release associated with P/Q channels was modulated by mGluR7, either in the presence of omega-conotoxin-GVIA or after decreasing the extracellular Ca(2+) concentration [Ca(2+)](o) to abolish the contribution of N-type Ca(2+) channels. Under these conditions, L-AP4 (1 mm) reduced the evoked glutamate release by 35 +/- 2%. This inhibition was largely prevented by pertussis toxin, but it was insensitive to inhibitors of protein kinase C (bisindolylmaleimide) and protein kinase A (H-89). Furthermore, this inhibition was associated with a reduction in the Ca(2+) influx mediated by P/Q channels in the absence of any detectable change in cAMP levels. However, L-AP4 decreased the levels of cAMP in the presence of forskolin. The activation of this additional signalling pathway was very efficient in counteracting the facilitation of glutamate release induced by forskolin. Thus, mGluR7 mediates the inhibition of glutamate release at hippocampal nerve terminals primarily by inhibiting P/Q-type Ca(2+) channels, although augmenting the levels of cAMP reveals the ability of the receptor to decrease cAMP.

Increased Ca2+ channel currents in cerebellar Purkinje cells of the ataxic groggy rat

The ataxic groggy rat (strain name; GRY) is an autosomal recessive neurological mutant found in a closed colony of Slc:Wistar rats. Recent genetic analysis has identified the missense (M251K) mutation in the alpha(1) subunit of the Ca(V)2.1 (P/Q-type) voltage-dependent Ca(2+) channel gene (Cacna1a) of GRY rat. In this study, we found that high-voltage-activated (HVA) Ca(2+) channel currents in acutely dissociated Purkinje cells of GRY rats showed increased (not decreased) current density and depolarizing shift of the activation and inactivation curves compared with those of normal Wistar rats. In contrast low-voltage-activated (LVA) Ca(2+) channel currents of GRY rats showed no significant changes. These results suggest that functional alteration of Ca(2+) channel currents in cerebellar Purkinje cells of GRY rats is attributed to the change of HVA Ca(2+) channel currents, and that increased HVA Ca(2+) channel function underlies the cerebellar dysfunction and ataxic phenotype of GRY rats.

[P/Q-type voltage-dependent calcium channels in neurological disease]

INTRODUCTION

Voltage-dependent calcium channels (VDCC) are hetero-multimeric complexes that mediate calcium influx into cells in response to changes in membrane potential. The alpha1A subunit, encoded by the CACNA1A gene, is the pore-forming structure specific to the neuronal P/Q-type voltage-dependent calcium channels (P/QCC), present exclusively in neurons. The ancillary subunits beta, alpha2delta and gamma, which are common to other VDCC, modulate alpha1A activity. P/QCC are involved in neuronal plasticity and survival, and mediate fast neurotransmission in the central and peripheral nervous system. Their highest levels of expression are found in the Purkinje cell layer of the cerebellum and in the hippocampus.

METHODS

Congenital and acquired disturbances of the P/QCCs lay behind some neurological diseases, such as spinocerebellar ataxia type 6, episodic ataxia type 2 and paraneoplastic cerebellar degeneration; familial hemiplegic migraine; generalized convulsive epilepsy, generalized absence epilepsy and myasthenic syndrome of Lambert-Eaton.

CONCLUSION

In this article, the structure and modulation of normal P/QCCs, and the neurological diseases caused by disturbances in these are reviewed. Electrophysiological characterization of mutated P/QCCs has yielded decreased calcium conductance in every case, compared with wild type channels. Research about calcium channelopathies should clarify how altered channel function produces disease and lead to new treatments for these conditions.

Myotonic dystrophy CTG repeat expansion alters Ca2+channel functional expression in PC12 cells

We previously reported that expression of myotonic dystrophy (DM1) expanded CUG repeats impedes NGF-induced differentiation in a PC12 clone (CTG90 cells). Here, we present evidence for changes in the fractional contribution of distinct voltage-gated Ca(2+) channels, key elements in neurotrophin-promoted differentiation, to the total Ca(2+) current in the CTG90 cells. Patch-clamp recordings showed that the relative proportion of pharmacologically isolated Ca(2+) channel types differed between control and CTG90 cells. Particularly, the functional expression of N-type channels was significantly reduced. Though quantitative real-time RT-PCR revealed that transcripts for the pore-forming subunit encoding the N-type channels remained unchanged, the protein level analyzed by semi-quantitative Western blotting was down-regulated in the CTG90 cells. These data suggest modifications in the processing of N-type Ca(2+) channels in PC12 cells expressing the DM1 mutation.

Differential role of N-type calcium channel splice isoforms in pain

N-type calcium channels are essential mediators of spinal nociceptive transmission. The core subunit of the N-type channel is encoded by a single gene, and multiple N-type channel isoforms can be generated by alternate splicing. In particular, cell-specific inclusion of an alternatively spliced exon 37a generates a novel form of the N-type channel that is highly enriched in nociceptive neurons and, as we show here, downregulated in a neuropathic pain model. Splice isoform-specific small interfering RNA silencing in vivo reveals that channels containing exon 37a are specifically required for mediating basal thermal nociception and for developing thermal and mechanical hyperalgesia during inflammatory and neuropathic pain. In contrast, both N-type channel isoforms (e37a- and e37b-containing) contribute to tactile neuropathic allodynia. Hence, exon 37a acts as a molecular switch that tailors the channels toward specific roles in pain.

Targeting of voltage-gated K+ and Ca2+ channels and soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins to cholesterol-rich lipid rafts in pancreatic alpha-cells: effects on glucagon stimulus-secretion coupling

Pancreatic alpha-cells secrete glucagon in response to low glucose to counter insulin actions, thereby maintaining glucose homeostasis. The molecular basis of alpha-cell stimulus-secretion coupling has not been fully elucidated. We investigated the expression of voltage-gated K(+) (K(V)) and Ca(2+) (Ca(V)) channels, and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in pancreatic alpha-cells and examined their targeting to specialized cholesterol-rich lipid rafts. In alpha-cells, we detected the expression of K(V)4.1/4.3 (A-type current), K(V)3.2/3.3 (delayed rectifier current), Ca(V)1.2 (L-type current), Ca(V)2.2 (N-type current), and the SNARE (synaptosomal-associated protein of 25 kDa, syntaxin 1A, and vesicle-associated membrane protein 2) and SNARE-associated proteins (Munc-13-1 and Munc-18a). We also detected caveolin-2, a structural protein of cholesterol-rich lipid rafts. Of these proteins, caveolin-2, K(V)4.1/4.3, Ca(V)1.2, and SNARE proteins (syntaxin 1A, synaptosomal-associated protein of 25 kDa, and vesicle-associated membrane protein 2) target to lipid raft domains on alpha-cell plasma membranes. Disruption of lipid rafts by depletion of membrane cholesterol with methyl-beta-cyclodextrin decreased the association of K(V)4.1/4.3, Ca(V)1.2, and SNARE proteins with lipid rafts. This resulted in inhibition of A-type K(V) currents and enhancement of glucagon secretion from alpha-cells. Consistently, capacitance measurements of exocytosis of single alpha-cells showed enhanced exocytosis after membrane cholesterol depletion. Taken together, our results demonstrate the association of K(V)4, Ca(V)1.2, and SNARE proteins with lipid rafts in pancreatic alpha-cells. Glucagon secretion from alpha-cells is regulated by lipid rafts, and the dissociation of SNARE proteins from cholesterol-rich lipid raft domains enhances glucagon secretion.

The cross channel activities of spider neurotoxin huwentoxin-I on rat dorsal root ganglion neurons

In this paper, we investigated the action of huwentoxin-I (HWTX-I) purified from the venom of the Chinese bird spider Ornithoctonus huwena on Ca(2+), Na(+) channels of adult rat dorsal root ganglion (DRG) neurons. The results showed that huwentoxin-I could reduce the peak currents of N-type Ca(2+) channels (IC(50) approximately 100 nM) and TTX-S Na(+) channels (IC(50) approximately 55 nM), whereas no effect was detected on TTX-R Na(+) channels. The comparative studies indicated that the selectivity of HWTX-I on Ca(2+) channels was higher that of MVIIA and approximately the same as that of GVIA. HWTX-I is the first discovered toxin with the cross channel activities from the spider O. huwena venom similar to micro O-conotoxins MrVIA and MrVIB.

Inhibitory Effects of Artemisinin on Voltage-Gated Ion Channels in Intact Nodose Ganglion Neurones of Adult Rats

Recent data show that artemisinin has anti-arrhythmic and local anaesthetic effects. To better understand the mechanisms, the effects of artemisinin on action potential discharge and voltage-gated ion channels properties were studied on nodose ganglion neurones of adult rats with known sensory afferent fibre type using whole cell patch and vagus nodose slice preparation. The present data show that both depolarization and repolarization of action potentials were markedly inhibited by artemisinin in a concentration- and time-dependent manner in either A-type or C-type nodose ganglion neurones without change in conduction velocity. Both tetrodotoxin-sensitive (TTX-S) Na+ and tetrodotoxin-resistant (TTX-R) Na+ currents were significantly reduced by micro-perfusion of artemisinin; the steady-state half-activation and half-inactivation for both TTX-S and TTX-R Na+ currents were shifted towards the right without changing slope factors. Median inhibition concentration (IC50) are 68.1 microM and 236.2 microM for TTX-S and TTX-R Na+ currents, respectively. Total outward K+ currents from C-type nodose ganglion neurones were blocked by artemisinin 30-300 microM concentration-dependently, IC50 being 104.7 microM. This effect was mimicked by tetraethylammonium 15 mM. Peak currents of N-type Ca2+ channels were also reduced significantly (IC50=344.6 microM) in the presence of artemisinin, which was less effective than that induced by 1 microM omega-conotoxin (CTX) GIVA. Our data demonstrate that depolarization and repolarization of action potentials recorded from either A- or C-type nodose ganglion neurones were inhibited by artemisinin in a concentration- and time-dependent manner, and that this inhibitory effect of artemisinin is probably due to the non-selective inhibition of all major ion channels functionally expressed in nodose ganglion neurones.

Ca2+ channel subtypes and pharmacology in the kidney

A large body of evidence has accrued indicating that voltage-gated Ca(2+) channel subtypes, including L-, T-, N-, and P/Q-type, are present within renal vascular and tubular tissues, and the blockade of these Ca(2+) channels produces diverse actions on renal microcirculation. Because nifedipine acts exclusively on L-type Ca(2+) channels, the observation that nifedipine predominantly dilates afferent arterioles implicates intrarenal heterogeneity in the distribution of L-type Ca(2+) channels and suggests that it potentially causes glomerular hypertension. In contrast, recently developed Ca(2+) channel blockers (CCBs), including mibefradil and efonidipine, exert blocking action on L-type and T-type Ca(2+) channels and elicit vasodilation of afferent and efferent arterioles, which suggests the presence of T-type Ca(2+) channels in both arterioles and the distinct impact on intraglomerular pressure. Recently, aldosterone has been established as an aggravating factor in kidney disease, and T-type Ca(2+) channels mediate aldosterone release as well as its effect on renal efferent arteriolar tone. Furthermore, T-type CCBs are reported to exert inhibitory action on inflammatory process and renin secretion. Similarly, N-type Ca(2+) channels are present in nerve terminals, and the inhibition of neurotransmitter release by N-type CCBs (eg, cilnidipine) elicits dilation of afferent and efferent arterioles and reduces glomerular pressure. Collectively, the kidney is endowed with a variety of Ca(2+) channel subtypes, and the inhibition of these channels by their specific CCBs leads to variable impact on renal microcirculation. Furthermore, multifaceted activity of CCBs on T- and N-type Ca(2+) channels may offer additive benefits through nonhemodynamic mechanisms in the progression of chronic kidney disease.

The Kelch-like protein 1 modulates P/Q-type calcium current density

The actin-binding protein Kelch-like 1 (KLHL1) is a neuronal protein that belongs to the evolutionarily-conserved Kelch protein super-family. The mammalian KLHL1 is brain-specific, cytosolic and can form multimers and bind actin filaments. KLHL1's function is likely that of an actin-organizing protein, possibly modulating neurite outgrowth, the dynamic morphology of dendritic spine heads; or anchoring proteins essential for post-synaptic function, like ion channels. Targeted deletion of the KLHL1 gene in Purkinje neurons results in dendritic deficits in these neurons, abnormal gait, and progressive loss of motor coordination in mice [He Y, Zu T, Benzow KA, Orr HT, Clark HB, Koob MD (2006) Targeted deletion of a single SCA8 ataxia locus allele in mice causes abnormal gait, progressive loss of motor coordination, and Purkinje cell dendritic deficits. J Neurosci 26:9975-9982]. Here we tested the hypothesis that KLHL1 may interact and modulate voltage-gated calcium channels by assessing the interaction of the principal subunit of P/Q-type channels, alpha(1A), with KLHL1. Experiments in human embryonic kidney line HEK 293 (HEK) cells and cerebellar primary cultures revealed co-incidence of alpha(1A) and KLHL1 immunoreactivity when testing both the endogenous or epitope-tagged versions of the proteins. Similarly, co-immunoprecipitation experiments in HEK cells and brain tissue exposed the presence of KLHL1 in protein samples immunoprecipitated with FLAG-tagged or alpha(1A) antibodies. Functional studies of KLHL1 on P/Q-type current properties probed with whole-cell patch clamp revealed a significant increase in mean current density in the presence of KLHL1 (80% increase; from -13.2+/-2.0 pA/pF to -23.7+/-4.2 pA/pF, P<0.02), as well as a shift in steady state activation V(50) of -5.5 mV (from 12.8+/-1.8 mV to 7.3+/-1.0 mV, P<0.02). Our data are consistent with a modulatory effect of KLHL1 on the P/Q-type calcium channel function and suggest a possible novel role for KLHL1 in cellular excitability.

Direct G protein modulation of Cav2 calcium channels

The regulation of presynaptic, voltage-gated calcium channels by activation of heptahelical G protein-coupled receptors exerts a crucial influence on presynaptic calcium entry and hence on neurotransmitter release. Receptor activation subjects presynaptic N- and P/Q-type calcium channels to a rapid, membrane-delimited inhibition-mediated by direct, voltage-dependent interactions between G protein betagamma subunits and the channels-and to a slower, voltage-independent modulation involving soluble second messenger molecules. In turn, the direct inhibition of the channels is regulated as a function of many factors, including channel subtype, ancillary calcium channel subunits, and the types of G proteins and G protein regulatory factors involved. Twenty-five years after this mode of physiological regulation was first described, we review the investigations that have led to our current understanding of its molecular mechanisms.

Co-assembly of N-type Ca2+ and BK channels underlies functional coupling in rat brain

Activation of large conductance Ca(2+)-activated potassium (BK) channels hastens action potential repolarisation and generates the fast afterhyperpolarisation in hippocampal pyramidal neurons. A rapid coupling of Ca(2+) entry with BK channel activation is necessary for this to occur, which might result from an identified coupling of Ca(2+) entry through N-type Ca(2+) channels to BK channel activation. This selective coupling was extremely rapid and resistant to intracellular BAPTA, suggesting that the two channel types are close. Using reciprocal co-immunoprecipitation, we found that N-type channels were more abundantly associated with BK channels than L-type channels (Ca(V)1.2) in rat brain. Expression of only the pore-forming alpha-subunits of the N-type (Ca(V)2.2) and BK (Slo(27)) channels in a non-neuronal cell-line gave robust macroscopic currents and reproduced the interaction. Co-expression of Ca(V)2.2/Ca(V)beta(3) subunits with Slo(27) channels revealed rapid functional coupling. By contrast, extremely rare examples of rapid functional coupling were observed with co-expression of Ca(V)1.2/Ca(V)beta(3) and Slo(27) channels. Action potential repolarisation in hippocampal pyramidal neurons was slowed by the N-type channel blocker omega-conotoxin GVIA, but not by the L-type channel blocker isradipine. These data showed that selective functional coupling between N-type Ca(2+) and BK channels provided rapid activation of BK channels in central neurons.

Alternative splicing controls G protein-dependent inhibition of N-type calcium channels in nociceptors

Neurotransmitter release from mammalian sensory neurons is controlled by Ca(V)2.2 N-type calcium channels. N-type channels are a major target of neurotransmitters and drugs that inhibit calcium entry, transmitter release and nociception through their specific G protein-coupled receptors. G protein-coupled receptor inhibition of these channels is typically voltage-dependent and mediated by Gbetagamma, whereas N-type channels in sensory neurons are sensitive to a second G protein-coupled receptor pathway that inhibits the channel independent of voltage. Here we show that preferential inclusion in nociceptors of exon 37a in rat Cacna1b (encoding Ca(V)2.2) creates, de novo, a C-terminal module that mediates voltage-independent inhibition. This inhibitory pathway requires tyrosine kinase activation but not Gbetagamma. A tyrosine encoded within exon 37a constitutes a critical part of a molecular switch controlling N-type current density and G protein-mediated voltage-independent inhibition. Our data define the molecular origins of voltage-independent inhibition of N-type channels in the pain pathway.

Involvement of phospholipase C in endothelin 1-induced stimulation of Ca++ channels and basilar artery contraction in rabbits

OBJECT

Endothelin 1 (ET-1) is a major cause of cerebral vasospasm after subarachnoid hemorrhage (SAH), and extracellular Cal++ influx plays an essential role in ET-1-induced vasospasm. The authors recently demonstrated that ET-1 activates two types of Ca"-permeable nonselective cation channels (designated NSCC-1 and NSCC-2) and a store-operated Cal++ channel (SOCC) in vascular smooth-muscle cells located in the basilar arteries (BAs) of rabbits. In the present study, they investigate the effects of phospholipase C (PLC) on ET-1-induced activation of these Ca++ channels and BA contraction by using the PLC inhibitor U73122. Methods. To determine which Cal++ channels are activated via a PLC-dependent pathway, these investigators monitored the intracellular free Cal++ concentration ([Ca++]i). The role of PLC in ET-1-induced vascular contraction was examined by performing a tension study of rabbit BA rings. The U73122 inhibited the ET-1-induced transient increase in [Ca++]i, which resulted from mobilization of Ca++ from the intracellular store. Phospholipase C also inhibited ET-1-induced extracellular Ca++ influx through the SOCC and NSCC-2, but not through the NSCC-1. The U73122 inhibited the ET-1-induced contraction of the rabbit BA rings, which depended on extracellular Cal++ influx through the SOCC and NSCC-2. Conclusions. These results indicate the following. (1) The SOCC and NSCC-2 are stimulated by ET-1 via a PLC-dependent cascade whereas NSCC-1 is stimulated via a PLC-independent cascade. (2) The PLC is involved in the ET-1-induced contraction of rabbit BA rings, which depends on extracellular Ca++ influx through the SOCC and NSCC-2.

Modulation of Ca2+ Channels by Heterologously Expressed Wild-Type and Mutant Human μ-Opioid Receptors (hMORs) Containing the A118G Single-Nucleotide Polymorphism

The most common single-nucleotide polymorphism (SNP) of the human μ-opioid receptor (hMOR) gene occurs at position 118 (A118G) and results in substitution of asparagine to aspartate at the N-terminus. The purpose of the present study was to compare the pharmacological profile of several opioid agonists to heterologously expressed hMOR and N-type Ca2+ channels in sympathetic neurons. cDNA constructs coding for wild-type and mutant hMOR were microinjected in rat superior cervical ganglion neurons and N-type Ca2+ channel modulation was investigated using the whole cell variant of the patch-clamp technique. Concentration–response relationships were generated with the following selective MOR agonists: DAMGO, morphine, morphine-6-glucuronide (M-6-G), and endomorphin I. The estimated maximal inhibition for the agonists ranged from 52 to 64% for neurons expressing either hMOR subtype. The rank order of potencies for estimated EC50 values (nM) in cells expressing wild-type hMOR was: DAMGO (31) ≫ morphine (76) ≅ M-6-G (77) ≅ endomorphin I (86). On the other hand, the rank order in mutant-expressing neurons was: DAMGO (14) ≫ morphine (39) ≫ endomorphin I (74) ≅ M-6-G (82), with a twofold leftward shift for both DAMGO and morphine. The DAMGO-mediated Ca2+ current inhibition was abolished by the selective MOR blocker, CTAP, and by pertussis toxin pretreatment of neurons expressing either hMOR subtype. These results suggest that the A118G variant MOR exhibits an altered signal transduction pathway and may help explain the variability of responses to opiates observed with carriers of the mutant allele.

Properties of human Cav2.1 channel with a spinocerebellar ataxia type 6 mutation expressed in Purkinje cells

Spinocerebellar ataxia type 6 (SCA6) is caused by polyglutamine expansion in P/Q-type Ca2+ channels (Ca(v)2.1) and is characterized by predominant degeneration of cerebellar Purkinje cells. To characterize the Ca(v)2.1 channel with an SCA6 mutation in cerebellar Purkinje cells, we have generated knock-in mouse models that express human Ca(v)2.1 with 28 polyglutamine repeats (disease range) and with 13 polyglutamine repeats (normal range). Patch-clamp recordings of the Purkinje cells from homozygous control or SCA6 knock-in mice revealed a non-inactivating current that is highly sensitive to a spider toxin omega-Agatoxin IVA, indicating that the human Ca(v)2.1 expressed in Purkinje cells exhibits typical P-type properties in contrast to the previous data showing Q-type properties, when it was expressed in cultured cell lines. Furthermore, the voltage dependence of activation and inactivation and current density were not different between SCA6 and control, though these properties were altered in previous reports using non-neuronal cells as expression systems. Therefore, our results do not support the notion that the alteration of the channel properties may underlie the pathogenic mechanism of SCA6.

Modified sympathetic regulation in N-type calcium channel null-mouse

To elucidate the physiological importance of neuronal (N)-type calcium channels in sympathetic controls, we analyzed N-type channel-deficient (NKO) mice. Immunoprecipitation analysis revealed increased interaction between beta3 (a major accessory subunit of N-type channels) and R-type channel-forming CaV2.3 in NKO mice. R-R intervals in NKO ECG recordings were elongated and fluctuating, suggesting disturbed sympathetic tonus. N-type channel inhibitors elongated the R-R interval in control mice, whereas R-type channel blocking with SNX-482 significantly affected NKO but not control mice, indicating a compensatory role for R-type channels. Echocardiography and Langendorff heart analysis confirmed a major role for R-type channels in NKO mice. Combined, our biochemical and physiological analyses strongly suggest that the remaining sympathetic tonus in NKO mice is dependent on R-type calcium channels.

Stimulation-induced changes in NADH fluorescence and mitochondrial membrane potential in lizard motor nerve terminals

To investigate mitochondrial responses to repetitive stimulation, we measured changes in NADH fluorescence and mitochondrial membrane potential (Psi(m)) produced by trains of action potentials (50 Hz for 10-50 s) delivered to motor nerve terminals innervating external intercostal muscles. Stimulation produced a rapid decrease in NADH fluorescence and partial depolarization of Psi(m). These changes were blocked when Ca2+ was removed from the bath or when N-type Ca2+ channels were inhibited with omega-conotoxin GVIA, but were not blocked when bath Ca2+ was replaced by Sr2+, or when vesicular release was inhibited with botulinum toxin A. When stimulation stopped, NADH fluorescence and Psi(m) returned to baseline values much faster than mitochondrial [Ca2+]. In contrast to findings in other tissues, there was usually little or no poststimulation overshoot of NADH fluorescence. These findings suggest that the major change in motor terminal mitochondrial function brought about by repetitive stimulation is a rapid acceleration of electron transport chain (ETC) activity due to the Psi(m) depolarization produced by mitochondrial Ca2+ (or Sr2+) influx. After partial inhibition of complex I of the ETC with amytal, stimulation produced greater Psi(m) depolarization and a greater elevation of cytosolic [Ca2+]. These results suggest that the ability to accelerate ETC activity is important for normal mitochondrial sequestration of stimulation-induced Ca2+ loads.

G-proteins modulate cumulative inactivation of N-type (Cav2.2) calcium channels

Precise regulation of N-type (Ca(V)2.2) voltage-gated calcium channels (Ca-channels) controls many cellular functions including neurotransmitter and hormone release. One important mechanism that inhibits Ca2+ entry involves binding of G-protein betagamma subunits (Gbetagamma) to the Ca-channels. This shifts the Ca-channels from "willing" to "reluctant" gating states and slows activation. Voltage-dependent reversal of the inhibition (facilitation) is thought to reflect transient dissociation of Gbetagamma from the Ca-channels and can occur during high-frequency bursts of action potential-like waveforms (APW). Inactivation of Ca-channels will also limit Ca2+ entry, but it remains unclear whether G-proteins can modulate inactivation. In part this is because of the complex nature of inactivation, and because facilitation of Ca-channel currents (I(Ca)) masks the extent and kinetics of inactivation during typical stimulation protocols. We used low-frequency trains of APW to activate I(Ca). This more closely mimics physiological stimuli and circumvents the problem of facilitation which does not occur at < or = 5 Hz. Activation of endogenous G-proteins reduced both Ca2+-dependent, and voltage-dependent inactivation of recombinant I(Ca) in human embryonic kidney 293 cells. This was mimicked by expression of wild-type Gbetagamma, but not by a point mutant of Gbetagamma with reduced affinity for Ca-channels. A similar decrease in the inactivation of I(Ca) was produced by P2Y receptors in adrenal chromaffin cells. Overall, our data identify and characterize a novel effect of G-proteins on I(Ca), and could have important implications for understanding how G-protein-coupled receptors control Ca2+ entry and Ca2+-dependent events such as neurotransmitter and hormone release.

?-Conotoxin CVIB differentially inhibits native and recombinant N- and P/Q-type calcium channels

Omega-conotoxins are routinely used as selective inhibitors of different classes of voltage-gated calcium channels (VGCCs) in excitable cells. In the present study, we examined the potent N-type VGCC antagonist omega-conotoxin CVID and non-selective N- and P/Q-type antagonist CVIB for their ability to block native VGCCs in rat dorsal root ganglion (DRG) neurons and recombinant VGCCs expressed in Xenopus oocytes. Omega-conotoxins CVID and CVIB inhibited depolarization-activated whole-cell VGCC currents in DRG neurons with pIC50 values of 8.12 +/- 0.05 and 7.64 +/- 0.08, respectively. Inhibition of Ba2+ currents in DRG neurons by CVID (approximately 66% of total) appeared to be irreversible for > 30 min washout, whereas Ba2+ currents exhibited rapid recovery from block by CVIB (> or = 80% within 3 min). The recoverable component of the Ba2+ current inhibited by CVIB was mediated by the N-type VGCC, whereas the irreversibly blocked current (approximately 22% of total) was attributable to P/Q-type VGCCs. Omega-conotoxin CVIB reversibly inhibited Ba2+ currents mediated by N- (Ca(V)2.2) and P/Q- (Ca(V)2.1), but not R- (Ca(V)2.3) type VGCCs expressed in Xenopus oocytes. The alpha2delta1 auxiliary subunit co-expressed with Ca(V)2.2 and Ca(V)2.1 reduced the sensitivity of VGCCs to CVIB but had no effect on reversibility of block. Determination of the NMR structure of CVIB identified structural differences to CVID that may underlie differences in selectivity of these closely related conotoxins. Omega-conotoxins CVIB and CVID may be useful as antagonists of N- and P/Q-type VGCCs, particularly in sensory neurons involved in processing primary nociceptive information.

NO-1886, a lipoprotein lipase activator, attenuates vascular smooth muscle contraction in rat aorta

The chemical compound [4-(4-bromo-2-cyano-phenylcarbamoyl)-benzyl]-phosphonic acid diethyl ester (NO-1886) is a lipoprotein lipase activator having beneficial effects on both diabetes control and the cardiovascular system. Preventing accumulation of lipids in the cell wall, in addition to improving insulin actions on vasculature, may indirectly contribute to the reducing effect of NO-1886 on vascular resistance. However, the direct effect of NO-1886 on vascular resistance, i.e., whether NO-1886 directly modulates the function of vascular endothelium and/or smooth muscle cells has not been investigated. In this study we therefore investigated the direct effect of NO-1886 on vascular contractility using rat aortic rings and cultured smooth muscle cell-line A10. The results show that administration of NO-1886 attenuated aortic contraction induced by phenylephrine and/or a high K(+) environment, in both the presence and absence of aortic endothelium. 1-(5-Chloronaphthalene-1-sulfonyl)homopiperazine hydrochloride (ML-9), a myosin light chain kinase (MLCK) inhibitor, blocked this inhibitory effect of NO-1886, whereas inhibitors of other signaling molecules such as calmodulin, protein kinase C and Rho-kinase had no effect. The vasorelaxant effect of NO-1886 was blocked in the absence of extracellular Ca(2+), or in the presence of the Ca(2+) channel inhibitor, verapamil. NO-1886 attenuated smooth muscle contraction induced by the cumulative addition of CaCl(2). In A10 cells, NO-1886 inhibited the membrane depolarization-induced initial peak of [Ca(2+)](i) in the presence of extracellular Ca(2+). This inhibition did not occur in the absence of extracellular Ca(2+). Taken together these results demonstrate that NO-1886 attenuates smooth muscle contraction and causes vasorelaxation by an extracellular Ca(2+)- and MLCK-dependent mechanism.

Importance of voltage-dependent inactivation in N-type calcium channel regulation by G-proteins

Direct regulation of N-type calcium channels by G-proteins is essential to control neuronal excitability and neurotransmitter release. Binding of the G(betagamma) dimer directly onto the channel is characterized by a marked current inhibition ("ON" effect), whereas the pore opening- and time-dependent dissociation of this complex from the channel produce a characteristic set of biophysical modifications ("OFF" effects). Although G-protein dissociation is linked to channel opening, the contribution of channel inactivation to G-protein regulation has been poorly studied. Here, the role of channel inactivation was assessed by examining time-dependent G-protein de-inhibition of Ca(v)2.2 channels in the presence of various inactivation-altering beta subunit constructs. G-protein activation was produced via mu-opioid receptor activation using the DAMGO agonist. Whereas the "ON" effect of G-protein regulation is independent of the type of beta subunit, the "OFF" effects were critically affected by channel inactivation. Channel inactivation acts as a synergistic factor to channel activation for the speed of G-protein dissociation. However, fast inactivating channels also reduce the temporal window of opportunity for G-protein dissociation, resulting in a reduced extent of current recovery, whereas slow inactivating channels undergo a far more complete recovery from inhibition. Taken together, these results provide novel insights on the role of channel inactivation in N-type channel regulation by G-proteins and contribute to the understanding of the physiological consequence of channel inactivation in the modulation of synaptic activity by G-protein coupled receptors.

M1 and M2 Muscarinic Acetylcholine Receptor Subtypes Mediate Ca2+ Channel Current Inhibition in Rat Sympathetic Stellate Ganglion Neurons

Muscarinic acetylcholine receptors (mAChRs) are known to mediate the acetylcholine inhibition of Ca2+ channels in central and peripheral neurons. Stellate ganglion (SG) neurons provide the main sympathetic input to the heart and contribute to the regulation of heart rate and myocardial contractility. Little information is available regarding mAChR regulation of Ca2+ channels in SG neurons. The purpose of this study was to identify the mAChR subtypes that modulate Ca2+ channel currents in rat SG neurons innervating heart muscle. Accordingly, the modulation of Ca2+ channel currents by the muscarinic cholinergic agonist, oxotremorine-methiodide (Oxo-M), and mAChR blockers was examined. Oxo-M–mediated mAChR stimulation led to inhibition of Ca2+ currents through voltage-dependent (VD) and voltage-independent (VI) pathways. Pre-exposure of SG neurons to the M1 receptor blocker, M1-toxin, resulted in VD inhibition of Ca2+ currents after Oxo-M application. On the other hand, VI modulation of Ca2+ currents was observed after pretreatment of cells with methoctramine (M2 mAChR blocker). The Oxo-M–mediated inhibition was nearly eliminated in the presence of both M1 and M2 mAChR blockers but was unaltered when SG neurons were exposed to the M4 mAChR toxin, M4-toxin. Finally, the results from single-cell RT-PCR and immunofluorescence assays indicated that M1 and M2 receptors are expressed and located on the surface of SG neurons. Overall, the results indicate that SG neurons that innervate cardiac muscle express M1 and M2 mAChR, and activation of these receptors leads to inhibition of Ca2+ channel currents through VI and VD pathways, respectively.

Imaging single-channel calcium microdomains

The Ca(2+) microdomains generated around the mouth of open ion channels represent the basic building blocks from which cytosolic Ca(2+) signals are constructed. Recent improvements in optical imaging techniques now allow these microdomains to be visualized as single channel calcium fluorescence transients (SCCaFTs), providing information about channel properties that was previously accessible only by electrophysiological patch-clamp recordings. We review recent advances in single channel Ca(2+) imaging methodologies, with emphasis on total internal reflection fluorescence microscopy (TIRFM) as the technique of choice for recording SCCaFTs from voltage- and ligand-gated plasmalemmal ion channels. This technique of 'optical patch-clamp recording' is massively parallel, permitting simultaneous imaging of hundreds of channels; provides millisecond resolution of gating kinetics together with sub-micron spatial resolution of channel locations; and is applicable to diverse families of membrane channels that display partial permeability to Ca(2+) ions.

The calcium channel alpha2delta-2 subunit partitions with CaV2.1 into lipid rafts in cerebellum: implications for localization and function

The accessory alpha2delta subunits of voltage-gated calcium channels are highly glycosylated transmembrane proteins that interact with calcium channel alpha1 subunits to enhance calcium currents. We compared the membrane localization and processing of native cerebellar alpha2delta-2 subunits with alpha2delta-2 stably expressed in tsA-201 cells. We identified that alpha2delta-2 is completely concentrated in cholesterol-rich microdomains (lipid rafts) in cerebellum, in which it substantially colocalizes with the calcium channel alpha1 subunit CaV2.1, although CaV2.1 is also present in the Triton X-100-soluble fraction. In tsA-201 cells, unlike cerebellum, alpha2delta-2 is not completely proteolytically processed into alpha2-2 and delta-2. However, this processing is more complete in the lipid raft fraction of tsA-201 cells, in which alpha2delta-2 also colocalizes with CaV2.1. Cholesterol depletion of intact cells disrupted their lipid rafts and enhanced CaV2.1/alpha2delta-2/beta4 currents. Furthermore, alpha2delta-2 coimmunoprecipitates with lipid raft-associated proteins of the stomatin family. The apparent affinity of alpha2delta-2 for its ligand gabapentin is increased markedly in the cholesterol-rich microdomain fractions, in both cerebellum and the stable alpha2delta-2 cell line. In contrast, alpha2delta-2 containing a point mutation (R282A) has a much lower affinity for gabapentin, and this is not enhanced in the lipid raft fraction. This R282A mutant alpha2delta-2 shows reduced functionality in terms of enhancement of CaV2.1/beta4 calcium currents, suggesting that the integrity of the gabapentin binding site may be important for normal functioning of alpha2delta-2. Together, these results indicate that both alpha2delta-2 and CaV2.1 are normally associated with cholesterol-rich microdomains, and this influences their functionality.

Acetylcholine release at neuromuscular junctions of adult tottering mice is controlled by N-(cav2.2) and R-type (cav2.3) but not L-type (cav1.2) Ca2+ channels

The mutation in the alpha(1A) subunit gene of the P/Q-type (Ca(v)2.1) Ca(2+) channel present in tottering (tg) mice causes ataxia and motor seizures that resemble absence epilepsy in humans. P/Q-type Ca(2+)channels are primarily involved in acetylcholine (ACh) release at mammalian neuromuscular junctions. Unmasking of L-type (Ca(v)1.1-1.2) Ca(2+) channels occurs in cerebellar Purkinje cells of tg mice. However, whether L-type Ca(2+) channels are also up-regulated at neuromuscular junctions of tg mice is unknown. We characterized thoroughly the pharmacological sensitivity of the Ca(2+) channels, which control ACh release at adult tg neuromuscular junctions. Block of N- and R-type (Ca(v)2.2-2.3), but not L-type Ca(2+) channels, significantly reduced quantal content of end-plate potentials in tg preparations. Neither resting nor KCl-evoked miniature end-plate potential frequency differed significantly between tg and wild type (WT). Immunolabeling of Ca(2+) channel subunits alpha(1A), alpha(1B), alpha(1C), and alpha(1E) revealed an apparent increase of alpha(1B), and alpha(1E) staining, at tg but not WT neuromuscular junctions. This presumably compensates for the deficit of P/Q-type Ca(2+)channels, which localized presynaptically at WT neuromuscular junctions. No alpha(1C) subunits juxtaposed with pre- or postsynaptic markers at either WT or tg neuromuscular junctions. Thus, in adult tg mice, immunocytochemical and electrophysiological data indicate that N- and R-type channels both assume control of ACh release at motor nerve terminals. Recruitment of alternate subtypes of Ca(2+) channels to control transmitter release seems to represent a commonly occurring method of neuronal plasticity. However, it is unclear which conditions underlie recruitment of Ca(v)2 as opposed to Ca(v)1-type Ca(2+) channels.

Contribution of the kinetics of G protein dissociation to the characteristic modifications of N-type calcium channel activity

Direct G protein inhibition of N-type calcium channels is recognized by characteristic biophysical modifications. In this study, we quantify and simulate the importance of G protein dissociation on the phenotype of G protein-regulated whole-cell currents. Based on the observation that the voltage-dependence of the time constant of recovery from G protein inhibition is correlated with the voltage-dependence of channel opening, we depict all G protein effects by a simple kinetic scheme. All landmark modifications in calcium currents, except inhibition, can be successfully described using three simple biophysical parameters (extent of block, extent of recovery, and time constant of recovery). Modifications of these parameters by auxiliary beta subunits are at the origin of differences in N-type channel regulation by G proteins. The simulation data illustrate that channel reluctance can occur as the result of an experimental bias linked to the variable extent of G protein dissociation when peak currents are measured at various membrane potentials. To produce alterations in channel kinetics, the two most important parameters are the extents of initial block and recovery. These data emphasize the contribution of the degree and kinetics of G protein dissociation in the modification of N-type currents.

Fluorophore-assisted light inactivation produces both targeted and collateral effects on N-type calcium channel modulation in rat sympathetic neurons

Fluorophore-assisted light inactivation (FALI) is a method to inactivate specific proteins on a time scale of seconds to minutes using either diffuse or coherent light. Here we examine a novel FALI modality that utilizes a fluorescein-conjugated polypeptide, alpha-bungarotoxin (BTX) and a 13 amino acid BTX-binding site engineered into the N-terminus of metabotropic glutamate receptor 8a (mGluR8a), a class C G-protein-coupled receptor (GPCR). The tagged mGluR8a was expressed in rat sympathetic neurons and labelled with fluorescein-conjugated BTX (FL-BTX). The efficacy of FALI was evaluated by monitoring mGluR8a-mediated inhibition of calcium currents (I(Ca)) using whole-cell voltage-clamp techniques. Following either wide-field or laser illumination of FL-BTX-labelled neurons, mGluR8a-mediated I(Ca) inhibition was greatly attenuated whereas holding current and basal I(Ca), measures of non-specific effects, were minimally affected. Sodium azide, a collision quencher of singlet oxygen, reduced the magnitude of FALI-mediated effects supporting a role for reactive oxygen species in the process. Although these results were consistent with an acute inactivation of mGluR8a, the intended target, two findings confounded this interpretation. First, effects on a natively expressed signalling pathway, alpha(2)-adrenergic receptor-mediated I(Ca) modulation, were observed following illumination of neurons expressing FL-BTX-labelled sodium channel beta2 subunits or ionotropic 5-HT(3) receptors, proteins with no overt relationship to GPCR signalling pathways. Second, GPCR-independent I(Ca) modulation induced with intracellular guanylyl imidophosphate was also attenuated by FALI. These data challenge the assumption that the fluorophore-tagged protein is the sole target of FALI and provide evidence that collateral damage to proximal proteins occurs following fluorophore illumination.