Arrhythmia in Isolated Prenatal Hearts after Ablation of the Cav2.3 (α1E) Subunit of Voltage-gated Ca2+ Channels

A voltage-gated calcium channel containing Cav2.3e (alpha1Ee) as the ion conducting pore has recently been detected in rat heart. Functional evidence for this Ca2+ channel to be involved in the regulation of heart beating, besides L- and T-type channels, was derived from murine embryos where the gene for Cav1.2 had been ablated. The remaining "L-type like" current component was not related to recombinant splice variants of Cav1.3 containing channels. As recombinant Cav2.3 channels from rat were reported to be weakly dihydropyridine sensitive, the spontaneous activity of the prenatal hearts from Cav2.3(-|-) mice was compared to that of Cav2.3(+|+) control animals to investigate if Cav2.3 could represent such a L-type like Ca(2+) channel. The spontaneous activity of murine embryonic hearts was recorded by using a multielectrode array. Between day 9.5 p.c. to 12.5 p.c., the beating frequency of isolated embryonic hearts from Cav2.3-deficient mice did not differ significantly from control mice but the coefficient of variation within individual episodes was more than four-fold increased in Cav2.3-deficient mice indicating arrhythmia. In isolated hearts from wild type mice, arrhythmia was induced by superfusion with a solution containing 200 nM SNX-482, a blocker of some R-type voltage gated Ca2+ channels, suggesting that R-type channels containing the splice variant Cav2.3e as ion conducting pore stabilize a more regular heart beat in prenatal mice.

Protein kinase C isozyme-specific potentiation of expressed Ca v 2.3 currents by acetyl-beta-methylcholine and phorbol-12-myristate, 13-acetate

Protein kinase C (PKC) is implicated in the potentiation of Ca v 2.3 currents by acetyl-beta-methylcholine (MCh), a muscarinic M1 receptor agonist or phorbol-12-myristate, 13-acetate (PMA). The PKC isozymes responsible for the action of MCh and PMA were investigated using translocation as a measure of activation and with isozyme-selective antagonists and siRNA. Ca v channels were expressed with alpha1 2.3, beta1b and alpha2delta subunits and muscarinic M1 receptors in the Xenopus oocytes and the expressed currents (I Ba) were studied using Ba2+ as the charge carrier. Translocation of PKC isozymes to the membrane studied by Western blot revealed that all eleven known PKC isozymes are present in the Xenopus oocytes. Exposure of the oocytes to MCh led to the translocation of PKC alpha whereas PMA activated PKC betaII and epsilon isozymes. The action of MCh was inhibited by Go 6976, an inhibitor of cPKC isozymes or PKC alpha siRNA. PMA-induced potentiation of Ca v 2.3 currents was inhibited by CG533 53, a PKC betaII antagonist, betaIIV5.3, a peptide translocation inhibitor of PKC betaII or PKC betaII siRNA. Similarly, epsilonV1.2, a peptide translocation inhibitor of PKC epsilon or PKC epsilon siRNA inhibited PMA action. The inhibitors of PKC increased the basal I Ba slightly. It is possible that some PKC isozymes have negative control over the I Ba. Our results implicate PKC alpha in the potentiation of Ca v 2.3 currents by MCh and PKC betaII and epsilon in the potentiation of Ca v 2.3 currents by PMA.

Alpha2-adrenoceptors couple to inhibition of R-type calcium currents in myenteric neurons

Alpha2-adrenoceptors inhibit Ca2+ influx through voltage-gated Ca2+ channels throughout the nervous system and Ca2+ channel function is modulated following activation of some G-protein coupled receptors. We studied the specific Ca2+ channel inhibited following alpha2-adrenoceptor activation in guinea-pig small intestinal myenteric neurons. Ca2+ currents (I(Ca2+)) were studied using whole-cell patch-clamp techniques. Changes in intracellular Ca2+ (delta[Ca2+]i) in nerve cell bodies and varicosities were studied using digital imaging where Ca2+ influx was evoked by KCl (60 mmol L(-1)) depolarization. The alpha2-adrenoceptor agonist, UK 14 304 (0.01-1 micromol L(-1)) inhibited I(Ca2+) and delta[Ca2+]i; maximum inhibition of I(Ca2+) was 40%. UK 14 304 did not affect I(Ca2+) in the presence of SNX-482 or NiCl2 (R-type Ca2+ channel antagonists). UK 14 304 inhibited I(Ca2+) in the presence of nifedipine, omega-agatoxin IVA or omega-conotoxin, inhibitors of L-, P/Q- and N-type Ca2+ channels. UK 14 304 induced inhibition of I(Ca2+) was blocked by pertussis toxin pretreatment (1 microg mL(-1) for 2 h). Alpha2-adrenoceptors couple to inhibition of R-type Ca2+ channels via a pertussis toxin-sensitive pathway in myenteric neurons. R-type channels may be a target for the inhibitory actions of noradrenaline released from sympathetic nerves on to myenteric neurons.

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.

Muscarinic enhancement of R-type calcium currents in hippocampal CA1 pyramidal neurons

The "toxin-resistant" R-type Ca2+ channels are expressed widely in the CNS and distributed mainly in apical dendrites and spines. They play important roles in regulating signal transduction and intrinsic properties of neurons, but the modulation of these channels in the mammalian CNS has not been studied. In this study we used whole-cell patch-clamp recordings and found that muscarinic activation enhances R-type, but does not affect T-type, Ca2+ currents in hippocampal CA1 pyramidal neurons after N, P/Q, and L-type Ca2+ currents selectively were blocked. M1/M3 cholinergic receptors mediated the muscarinic stimulation of R-type Ca2+ channels. The signaling pathway underlying the R-type enhancement was independent of intracellular [Ca2+] changes and required the activation of a Ca(2+)-independent PKC pathway. Furthermore, we found that the enhancement of R-type Ca2+ currents resulted in the de novo appearance of Ca2+ spikes and in remarkable changes in the firing pattern of R-type Ca2+ spikes, which could fire repetitively in the theta frequency. Therefore, muscarinic enhancement of R-type Ca2+ channels could play an important role in modifying the dendritic response to synaptic inputs and in the intrinsic resonance properties of neurons.

Cannabinoids modulate the P-type high-voltage-activated calcium currents in purkinje neurons

Endocannabinoids released by postsynaptic cells inhibit neurotransmitter release in many central synapses by activating presynaptic cannabinoid CB1 receptors. In particular, in the cerebellum, endocannabinoids inhibit synaptic transmission at granule cell to Purkinje cell synapses by modulating presynaptic calcium influx via N-, P/Q-, and R-type calcium channels. Using whole cell patch-clamp techniques, we show that in addition to this presynaptic action, both synthetic and endogenous cannabinoids inhibit P-type calcium currents in isolated rat Purkinje neurons independent of CB1 receptor activation. The IC50 for the anandamide (AEA)-induced inhibition of P-current peak amplitude was 1.04 +/- 0.04 microM. In addition, we demonstrate that all the tested cannabinoids in a physiologically relevant range of concentrations strongly accelerate inactivation of P currents. The effects of AEA cannot be attributed to the metabolism of AEA because a nonhydrolyzing analogue of AEA, methanandamide inhibited P-type currents with a similar efficacy. All effects of cannabinoids on P-type Ca2+ currents were insensitive to antagonists of CB1 cannabinoid or vanilloid TRPV1 receptors. In cerebellar slices, WIN 55,212-2 significantly affected spontaneous firing of Purkinje neurons in the presence of CB1 receptor antagonist, in a manner similar to that of a specific P-type channel antagonist, indicating a possible functional implication of the direct effects of cannabinoids on P current. Taken together these findings demonstrate a functionally important direct action of cannabinoids on P-type calcium currents.

Micro-opioid receptor preferentially inhibits oxytocin release from neurohypophysial terminals by blocking R-type Ca2+ channels

Oxytocin release from neurophypophysial terminals is particularly sensitive to inhibition by the micro-opioid receptor agonist, DAMGO. Because the R-type component of the neurophypophysial terminal Ca2+ current (ICa) mediates exclusively oxytocin release, we hypothesised that micro-opioids could preferentially inhibit oxytocin release by blocking this channel subtype. Whole-terminal recordings showed that DAMGO and the R-type selective blocker SNX-482 inhibit a similar ICa component. Measurements of [Ca2+]i levels and oxytocin release confirmed that the effects of DAMGO and SNX-482 are not additive. Finally, isolation of the R-type component and its associated rise in [Ca2+]i and oxytocin release allowed us to demonstrate the selective inhibition by DAMGO of this channel subtype. Thus, micro-opioid agonists modulate specifically oxytocin release in neurophypophysial terminals by selectively targeting R-type Ca2+ channels. Modulation of Ca2+ channel subtypes could be a general mechanism for drugs of abuse to regulate the release of specific neurotransmitters at central nervous system synapses.

Topiramate Inhibits the Initiation of Plateau Potentials in CA1 Neurons by Depressing R-type Calcium Channels

PURPOSE

Cholinergic-dependent plateau potentials (PPs) are intrinsically generated conductances that can elicit ictal-type seizure activity. The aim of this study was to investigate the actions of topiramate (TPM) on the generation of PPs.

METHODS

We used whole-cell patch-clamp recordings from CA1 pyramidal neurons in rat hippocampal slices to examine the effects of TPM on the PPs.

RESULTS

In current-clamp mode, action potentials evoked PPs after cholinergic receptor stimulation. Therapeutically relevant concentrations of TPM (50 microM) depressed the PPs evoked by action potentials. Surprisingly, in voltage-clamp mode, we discovered that the cyclic nucleotide-gated (CNG) current that underlies PP generation (denoted as I(tail)) was not depressed. However, significantly longer depolarizing voltage steps were required to elicit I(tail). This suggested that the calcium entry trigger for evoking PPs was depressed by TPM and not I(tail) itself. TPM had no effect on calcium spikes in control conditions; however, TPM did reduce calcium spikes after cholinergic-receptor stimulation. We recently found that R-type calcium spikes are enhanced by cholinergic-receptor stimulation. Therefore we isolated R-type calcium spikes with a cocktail containing tetrodotoxin, omega-conotoxin MVIIC, omega-conotoxin-GVIA, omega-agatoxin IVA, and nifedipine. R-type calcium spikes were significantly depressed by TPM. We also examined the effects of TPM on recombinant Ca(V)2.3 calcium channels expressed in tsA-201 cells. TPM depressed currents mediated by Ca(V)2.3 subunits by a hyperpolarizing shift in steady-state inactivation.

CONCLUSIONS

We have found that TPM reduces ictal-like activity in CA1 hippocampal neurons through a novel inhibitory action of R-type calcium channels.

Suppression of potassium channels elicits calcium-dependent plateau potentials in suprachiasmatic neurons of the rat

By using whole-cell recordings in acute and organotypic hypothalamic slices, we found that following K+ channel blockade, sustained plateau potentials can be elicited by current injection in suprachiasmatic neurons. In an attempt to determine the ionic basis of these potentials, ion-substitution experiments were carried out. It appeared that to generate plateau potentials, calcium influx was required. Plateau potentials were also present when extracellular calcium was replaced by barium, but were independent upon an increase in the intracellular free calcium concentration. Substitution of extracellular sodium by the impermeant cation N-methyl-D-glucamine indicated that sodium influx could also contribute to plateau potentials. To gain some information on the pharmacological profile of the Ca++ channels responsible for plateau potentials, selective blocker of various types of Ca++ channel were tested. Plateau potentials were unaffected by isradipine, an L-type Ca++ channel blocker. However, they were slightly reduced by omega-conotoxin GVIA and omega-agatoxin TK, blockers of N-type and P/Q-type Ca++ channels, respectively. These data suggest that R-type Ca++ channels probably play a major role in the genesis of plateau potentials. We speculate that neurotransmitters/neuromodulators capable of reducing or suppressing potassium conductance(s) may elicit a Ca++-dependent plateau potential in suprachiasmatic neurons, thus promoting sustained firing activity and neuropeptide release.

Emergence of a R-type Ca2+ channel (CaV 2.3) contributes to cerebral artery constriction after subarachnoid hemorrhage

Cerebral aneurysm rupture and subarachnoid hemorrhage (SAH) inflict disability and death on thousands of individuals each year. In addition to vasospasm in large diameter arteries, enhanced constriction of resistance arteries within the cerebral vasculature may contribute to decreased cerebral blood flow and the development of delayed neurological deficits after SAH. In this study, we provide novel evidence that SAH leads to enhanced Ca2+ entry in myocytes of small diameter cerebral arteries through the emergence of R-type voltage-dependent Ca2+ channels (VDCCs) encoded by the gene CaV 2.3. Using in vitro diameter measurements and patch clamp electrophysiology, we have found that L-type VDCC antagonists abolish cerebral artery constriction and block VDCC currents in cerebral artery myocytes from healthy animals. However, 5 days after the intracisternal injection of blood into rabbits to mimic SAH, cerebral artery constriction and VDCC currents were enhanced and partially resistant to L-type VDCC blockers. Further, SNX-482, a blocker of R-type Ca2+ channels, reduced constriction and membrane currents in cerebral arteries from SAH animals, but was without effect on cerebral arteries of healthy animals. Consistent with our biophysical and functional data, cerebral arteries from healthy animals were found to express only L-type VDCCs (CaV 1.2), whereas after SAH, cerebral arteries were found to express both CaV 1.2 and CaV 2.3. We propose that R-type VDCCs may contribute to enhanced cerebral artery constriction after SAH and may represent a novel therapeutic target in the treatment of neurological deficits after SAH.

Insulin-like growth factor-1 increases intracellular calcium concentration in human primary neuroendocrine pancreatic tumor cells and a pancreatic neuroendocrine tumor cell line (BON-1) via R-type Ca2+ channels and regulates chromogranin a secretion in BON-1 cells

Insulin-like growth factor 1 (IGF-1) is a potent mitogenic and secretory factor that acts on voltage operated Ca(2+) channels (VOCCs). VOCCs are categorized into L-type channels (Ca(V)1.1-1.4), P/Q-type channels (Ca(V)2.1), N-type channels (Ca(V)2.2), R-type channels (Ca(V)2.3), and T-type channels (Ca(V)3.1-3.3). Aside from regulating membrane excitability, VOCCs influence chromogranin A (CgA) secretion in neuroendocrine tumor (NET) cells. It is not known, whether VOCCs play a role in the IGF-1-dependent regulation of CgA secretion in NET cells. We therefore studied the effects of IGF-1 on individual VOCC subtypes and characterized their role in mediating IGF-1-dependent regulation of CgA secretion in NET cells. Using specific modulators of VOCC subtypes, we identified the functional expression of L-, N-, P/Q- and R-type channels in primary as well as permanent models of NET. The IGF-1-induced intracellular Ca(2+) increase in NET cells was mainly due to the activation of R-type channel activity. The effects on intracellular calcium, observed in whole-cell patch-clamp recordings and fluorescence imaging, were partially blocked by the specific R-type channel blocker SNX-482 and antisense oligonucleotides against the alpha(1) subunit of this channel. IGF-1 potently induced CgA secretion. The effect of IGF-1 was reduced by both, inhibition of R-type channel activity and a reduction of R-type channel expression using antisense oligonucleotides. Since R-type channels exist in NET cells and couple to both, IGF-1 receptor signaling as well as CgA secretion, pharmacological interference with R-type channels may represent a new therapeutic option by blocking Ca(2+) signaling thereby abrogating IGF-1-dependent hypersecretion in NET disease.

Endocannabinoids inhibit transmission at granule cell to Purkinje cell synapses by modulating three types of presynaptic calcium channels

At many central synapses, endocannabinoids released by postsynaptic cells inhibit neurotransmitter release by activating presynaptic cannabinoid receptors. The mechanisms underlying this important means of synaptic regulation are not fully understood. It has been shown at several synapses that endocannabinoids inhibit neurotransmitter release by reducing calcium influx into presynaptic terminals. One hypothesis maintains that endocannabinoids indirectly reduce calcium influx by modulating potassium channels and narrowing the presynaptic action potential. An alternative hypothesis is that endocannabinoids directly and selectively inhibit N-type calcium channels in presynaptic terminals. Here we test these hypotheses at the granule cell to Purkinje cell synapse in cerebellar brain slices. By monitoring optically the presynaptic calcium influx (Ca(influx)) and measuring the EPSC amplitudes, we found that cannabinoid-mediated inhibition arises solely from reduced presynaptic Ca(influx). Next we found that cannabinoid receptor activation does not affect the time course of presynaptic calcium entry, indicating that the reduced Ca(influx) reflects inhibition of presynaptic calcium channels. Finally, we assessed the classes of presynaptic calcium channels inhibited by cannabinoid receptor activation via peptide calcium channel antagonists. Previous studies established that N-type, P/Q-type, and R-type calcium channels are all present in granule cell presynaptic boutons. We found that cannabinoid activation reduced Ca(influx) through N-type, P/Q-type, and R-type calcium channels to 29, 60, and 55% of control, respectively. Thus, rather than narrowing the presynaptic action potential or exclusively modulating N-type calcium channels, CB1 receptor activation inhibits synaptic transmission by modulating all classes of calcium channels present in the presynaptic terminal of the granule cell to Purkinje cell synapse.

R-type calcium channels in myenteric neurons of guinea pig small intestine

Currents carried by L-, N-, and P/Q-type calcium channels do not account for the total calcium current in myenteric neurons. This study identified all calcium channels expressed by guinea pig small intestinal myenteric neurons maintained in primary culture. Calcium currents were recorded using whole cell techniques. Depolarizations (holding potential = -70 mV) elicited inward currents that were blocked by CdCl(2) (100 microM). Combined application of nifedipine (blocks L-type channels), Omega-conotoxin GVIA (blocks N-type channels), and Omega-agatoxin IVA (blocks P/Q-type channels) inhibited calcium currents by 56%. Subsequent addition of the R-type calcium channel antagonists, NiCl(2) (50 microM) or SNX-482 (0.1 microM), abolished the residual calcium current. NiCl(2) or SNX-482 alone inhibited calcium currents by 46%. The activation threshold for R-type calcium currents was -30 mV, the half-activation voltage was -5.2 +/- 5 mV, and the voltage sensitivity was 17 +/- 3 mV. R-type currents activated fully in 10 ms at 10 mV. R-type calcium currents inactivated in 1 s at 10 mV, and they inactivated (voltage sensitivity of 16 +/- 1 mV) with a half-inactivation voltage of -76 +/- 5 mV. These studies have accounted for all of the calcium channels in myenteric neurons. The data indicate that R-type calcium channels make the largest contribution to the total calcium current in myenteric neurons. The relatively positive half-activation voltage and rapid activation kinetics suggest that R-type channels could contribute to calcium entry during somal action potentials or during action potential-induced neurotransmitter release.

Capacitance measurements of exocytosis in mouse pancreatic alpha-, beta- and delta-cells within intact islets of Langerhans

Capacitance measurements of exocytosis were applied to functionally identified alpha-, beta- and delta-cells in intact mouse pancreatic islets. The maximum rate of capacitance increase in beta-cells during a depolarization to 0 mV was equivalent to 14 granules s(-1), <5% of that observed in isolated beta-cells. Beta-cell secretion exhibited bell-shaped voltage dependence and peaked at +20 mV. At physiological membrane potentials (up to approximately -20 mV) the maximum rate of release was approximately 4 granules s(-1). Both exocytosis (measured by capacitance measurements) and insulin release (detected by radioimmunoassay) were strongly inhibited by the L-type Ca(2+) channel blocker nifedipine (25 microm) but only marginally (<20%) affected by the R-type Ca(2+) channel blocker SNX482 (100 nm). Exocytosis in the glucagon-producing alpha-cells peaked at +20 mV. The capacitance increases elicited by pulses to 0 mV exhibited biphasic kinetics and consisted of an initial transient (150 granules s(-1)) and a sustained late component (30 granules s(-1)). Whereas addition of the N-type Ca(2+) channel blocker omega-conotoxin GVIA (0.1 microm) inhibited glucagon secretion measured in the presence of 1 mm glucose to the same extent as an elevation of glucose to 20 mm, the L-type Ca(2+) channel blocker nifedipine (25 microm) had no effect. Thus, glucagon release during hyperglycaemic conditions depends principally on Ca(2+)-influx through N-type rather than L-type Ca(2+) channels. Exocytosis in the somatostatin-secreting delta-cells likewise exhibited two kinetically separable phases of capacitance increase and consisted of an early rapid (600 granules s(-1)) component followed by a sustained slower (60 granules s(-1)) component. We conclude that (1) capacitance measurements in intact pancreatic islets are feasible; (2) exocytosis measured in beta-cells in situ is significantly slower than that of isolated cells; and (3) the different types of islet cells exhibit distinct exocytotic features.

Identification of Sites Responsible for Potentiation of Type 2.3 Calcium Currents by Acetyl-β-methylcholine

To address mechanisms for the differential sensitivity of voltage-gated Ca2+ channels (Cav) to agonists, channel activity was compared in Xenopus oocytes coexpressing muscarinic M(1) receptors and different Cav alpha1 subunits, all with beta1B,alpha2/delta subunits. Acetyl-beta-methylcholine (MCh) decreased Cav 1.2c currents, did not affect 2.1 or 2.2 currents, but potentiated Cav 2.3 currents. Phorbol 12-myristate 13-acetate (PMA) did not affect Cav 1.2c or 2.1 currents but potentiated 2.2 and 2.3 currents. Comparison of the amino acid sequences of the alpha1 subunits revealed a set of potential protein kinase C phosphorylation sites in common between the 2.2 and 2.3 channels that respond to PMA and a set of potential sites unique to the alpha1 2.3 subunits that respond to MCh. Quadruple Ser --> Ala mutation of the predicted MCh sites in the alpha1 2.3 subunit (Ser-888, Ser-892, and Ser-894 in the II-III linker and Ser-1987 in the C terminus) caused loss of the MCh response but not the PMA response. Triple Ser --> Ala mutation of just the II-III linker sites gave similar results. Ser-888 or Ser-892 was sufficient for the MCh responsiveness, whereas Ser-894 required the presence of Ser-1987. Ser --> Asp substitution of Ser-888, Ser-892, Ser-1987, and Ser-892/Ser-1987 increased the basal current and decreased the MCh response but did not alter the PMA response. These results reveal that sites unique to the II-III linker of alpha1 2.3 subunits mediate the responsiveness of Cav 2.3 channels to MCh. Because Cav 2.3 channels contribute to action potential-induced Ca2+ influx, these sites may account for M1 receptor-mediated regulation of neurotransmission at some synapses.

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.

Differential expression of calcium channels in sympathetic and parasympathetic preganglionic inputs to neurons in paracervical ganglia of guinea-pigs

Neurons in pelvic ganglia receive nicotinic excitatory post-synaptic potentials (EPSPs) from sacral preganglionic neurons via the pelvic nerve, lumbar preganglionic neurons via the hypogastric nerve or both. We tested the effect of a range of calcium channel antagonists on EPSPs evoked in paracervical ganglia of female guinea-pigs after pelvic or hypogastric nerve stimulation. omega-Conotoxin GVIA (CTX GVIA, 100 nM) or the novel N-type calcium channel antagonist, CTX CVID (100 nM) reduced the amplitude of EPSPs evoked after pelvic nerve stimulation by 50-75% but had no effect on EPSPs evoked by hypogastric nerve stimulation. Combined addition of CTX GVIA and CTX CVID was no more effective than either antagonist alone. EPSPs evoked by stimulating either nerve trunk were not inhibited by the P/Q calcium channel antagonist, omega-agatoxin IVA (100 nM), nor the L-type calcium channel antagonist, nifedipine (30 microM). SNX 482 (300 nM), an antagonist at some R-type calcium channels, inhibited EPSPs after hypogastric nerve stimulation by 20% but had little effect on EPSPs after pelvic nerve stimulation. Amiloride (100 microM) inhibited EPSPs after stimulation of either trunk by 40%, while nickel (100 microM) was ineffective. CTX GVIA or CTX CVID (100 nM) also slowed the rate of action potential repolarization and reduced afterhyperpolarization amplitude in paracervical neurons. Thus, release of transmitter from the terminals of sacral preganglionic neurons is largely dependent on calcium influx through N-type calcium channels, although an unknown calcium channel which is resistant to selective antagonists also contributes to release. Release of transmitter from lumbar preganglionic neurons does not require calcium entry through either conventional N-type calcium channels or the variant CTX CVID-sensitive N-type calcium channel and seems to be mediated largely by a novel calcium channel.

Adenosine modulation of [Ca2+]i in cerebellar granular cells: multiple adenosine receptors involved

Elimination of adenosine by addition of adenosine deaminase (ADA) to the media leads to alterations in intracellular free calcium concentration ([Ca(2+)](i)) in cerebellar granular cells. Adenosine deaminase brings about increases or decreases in [Ca(2+)](i) depending on the previous activation state of the cell. These effects are dependent on the catalytic activity of adenosine deaminase, since its previous catalytic inactivation with Hg(2+) prevents the above-mentioned changes in intracellular calcium. Extracellular calcium is required for the increase in [Ca(2+)](i) promoted by ADA. This rise is insensitive to thapsigargin, but sensitive to micromolar concentrations of Ni(2+). Toxins specific for L, N and P/Q calcium channels do not overtly reduce this effect. N(6)-Cyclopentyl adenosine (CPA), an A(1) receptor agonist, produces a partial reversion of ADA effects, while CGS21680, A(2A)/A(2B) receptor agonist, slightly enhances them. Expression of A(1), A(2A), A(2B) and A(3) adenosine receptor mRNAs was detected in cerebellar granular cell cultures. These results suggest that adenosine modulate [Ca(2+)](i) in cerebellar granule cells through different adenosine receptor subtypes which, at least in part, seem to act through R-type calcium channels.

Comparative effects of methylmercury and Hg(2+) on human neuronal N- and R-type high-voltage activated calcium channels transiently expressed in human embryonic kidney 293 cells

Expression cDNA clones of alpha1B-1 or alpha1E-3 subunits coding for human neuronal N-(Cav2.2) or R-subtype (Cav2.3) Ca2+ channels, respectively, was combined with alpha2-bdelta and beta3-a Ca2+ channel subunits, and transfected into human embryonic kidney cells for transient expression to determine whether specific types of neuronal voltage-sensitive Ca2+ channels are affected differentially by methylmercury (MeHg) and Hg2+. For both Ca2+ channel subtypes, MeHg (0.125-5.0 microM) or Hg2+ (0.1-5 microM) caused a time- and concentration-dependent reduction of current. MeHg caused an initial, rapid component and a subsequent more gradual component of inhibition. The rapid component of block was completed between 100 and 150 s after beginning treatment. At 0.125 to 1.25 microM, MeHg caused a more gradual decline in current. Apparent IC50 values were 1.3 and 1.1 microM for MeHg, and 2.2 and 0.7 microM for Hg2+ on N- and R-types, respectively. For N-type current, effects of Hg2+ were initially greater on the peak current than on the sustained current remaining at the end of a test pulse; subsequently, Hg2+ blocked both components of current. For R-type current, Hg2+ affected peak and sustained current approximately equally. Kinetics of inactivation also seemed to be affected by Hg2+ in cells expressing N-type but not R-type current. Washing with MeHg-free solution could not reverse effects of MeHg on either type of current. The effect of Hg2+ on N- but not R-type current was partially reversed by Hg2+-free wash solution. Therefore, different types of Ca2+ channels have differential susceptibility to neurotoxic mercurials even when expressed in the same cell type.

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).

Ontogeny of voltage-sensitive calcium channel alpha(1A) and alpha(1E) subunit expression and synaptic function in rat central nervous system

Immunohistochemical expression in the neocortex, hippocampus and cerebellum of the alpha(1A) or alpha(1E) subunit of the voltage-sensitive Ca(2+) channel was examined in Long-Evans hooded rats on gestational day 18 and postnatal days 1, 4, 7, 10, 14, 21, 90, 360 and 720. On gestational day 18 and postnatal day 1, alpha(1A) immunoreactivity was more dense in the neocortex and hippocampus than the cerebellum. By postnatal day 7, levels of alpha(1A) immunoreactivity increased dramatically in the cerebellum, while in neocortex, alpha(1A) immunoreactivity became more sparse, which approached the more diffuse pattern of cellular staining in the mature brain. Expression of alpha(1E) in the neocortex, hippocampus and cerebellum was much less dense than alpha(1A) between gestational day 18 and postnatal day 4. There was also significant alpha(1E) immunoreactivity in the mossy fibers of the hippocampus and in dendrites of Purkinje cells of the cerebellum. Depolarization-dependent 45Ca(2+) influx was examined in rat brain synaptosomes on postnatal days 4, 7, 10, 14, 21 and >60. In neocortical and hippocampal synaptosomes, 45Ca(2+) influx increased steadily with age and reached adult levels by postnatal day 10. In cerebellar synaptosomes, 45Ca(2+) influx was constant across all ages, except for a spike in activity which was observed on postnatal day 21. In neocortical and hippocampal synaptosomes, 100 nM omega-conotoxin MVIIC significantly inhibited 45Ca(2+) influx on postnatal day 10 and 14, respectively, or after. In cerebellar synaptosomes, influx was inhibited by omega-conotoxin MVIIC only on postnatal day 10 or prior. On postnatal day 7, 45Ca(2+) influx was not inhibited in neocortical and hippocampal synaptosomes by a combination of 10 microM nifedipine, 1 microM omega-conotoxin GVIA and 1 microM omega-conotoxin MVIIC, suggesting that an 'insensitive' flux predominates at this age. Overall, the results suggest that expression of voltage-sensitive Ca(2+) channels during development is dynamic and is important in central nervous system development.

Calcium channel subtypes contributing to acetylcholine release from normal, 4-aminopyridine-treated and myasthenic syndrome auto-antibodies-affected neuromuscular junctions

1 Acetylcholine release at the neuromuscular junction relies on rapid, local and transient calcium increase at presynaptic active zones, triggered by the ion influx through voltage-dependent calcium channels (VDCCs) clustered on the presynaptic membrane. Pharmacological investigation of the role of different VDCC subtypes (L-, N-, P/Q- and R-type) in spontaneous and evoked acetylcholine (ACh) release was carried out in adult mouse neuromuscular junctions (NMJs) under normal and pathological conditions. 2 omega-Agatoxin IVA (500 nM), a specific P/Q-type VDCC blocker, abolished end plate potentials (EPPs) in normal NMJs. However, when neurotransmitter release was potentiated by the presence of the K(+) channel blocker 4-aminopyridine (4-AP), an omega-agatoxin IVA- and omega-conotoxin MVIIC-resistant component was detected. This resistant component was only partially sensitive to 1 micro M omega-conotoxin GVIA (N-type VDCC blocker), but insensitive to any other known VDCC blockers. Spontaneous release was dependent only on P/Q-type VDCC in normal NMJs. However, in the presence of 4-AP, it relied on L-type VDCCs too. 3 ACh release from normal NMJs was compared with that of NMJs of mice passively injected with IgGs obtained from patients with Lambert-Eaton myasthenic syndrome (LEMS), a disorder characterized by a compromised neurotransmitter release. Differently from normal NMJs, in LEMS IgGs-treated NMJs an omega-agatoxin IVA-resistant EPP component was detected, which was only partially blocked by calciseptine (1 micro M), a specific L-type VDCC blocker. 4 Altogether, these data demonstrate that multiple VDCC subtypes are present at the mouse NMJ and that a resistant component can be identified under 'pharmacological' and/or 'pathological' conditions.

Presynaptic R-type calcium channels contribute to fast excitatory synaptic transmission in the rat hippocampus

The possibility that R-type calcium channels contribute to fast glutamatergic transmission in the hippocampus has been assessed using low concentrations of NiCl(2) and the peptide toxin SNX 482, a selective antagonist of the pore-forming alpha(1E) subunit of R-type calcium channel. EPSPs or EPSCs were recorded in the whole-cell configuration of the patch-clamp technique mainly from CA3 hippocampal neurons. Effects of both NiCl(2) and SNX 482 were tested on large (composite) EPSCs evoked by mossy and associative-commissural fiber stimulation. NiCl(2) effects were also tested on minimal EPSPs-EPSCs. Both substances reduced the amplitude of EPSPs-EPSCs. This effect was associated with an increase in the number of response failures of minimal EPSPs-EPSCs, an enhancement of the paired-pulse facilitation ratios of both minimal and composite EPSCs, and a reduction of the inverse squared coefficient of variation (CV(-2)). The reduction of CV(-2) was positively correlated with the decrease in EPSC amplitude. The inhibitory effect of NiCl(2) was occluded by SNX 482 but not by omega-conotoxin-MVIIC, a broad-spectrum antagonist thought to interact with N- and P/Q-type calcium channels, supporting a specific action of low concentrations of NiCl(2) on R-type calcium channels. Together, these observations indicate that both NiCl(2) and SNX 482 act at presynaptic sites and block R-type calcium channels with pharmacological properties similar to those encoded by the alpha(1E) gene. These channels are involved in fast glutamatergic transmission at hippocampal synapses.

Pharmacological and biophysical characterization of voltage-gated calcium currents in the endopiriform nucleus of the guinea pig

The endopiriform nucleus (EPN) is a well-defined structure that is located deeply in the piriform region at the border with the striatum and is characterized by dense intrinsic connections and prominent projections to piriform and limbic cortices. The EPN has been proposed to promote synchronization of large populations of neurons in the olfactory cortices via the activation of transient depolarizations possibly mediated by Ca(2+) spikes. It is known that principal cells in the EPN express both a low- and high-voltage-activated (HVA) Ca(2+) currents. We further characterized HVA conductances possibly related to Ca(2+)-spike generation in the EPN with a whole cell, patch-clamp study on neurons acutely dissociated from the EPN of the guinea pig. To study HVA currents in isolation, experiments were performed from a holding potential of -60 mV, using Ba(2+) as the permeant ion. Total Ba(2+) currents (I(Ba)) evoked by depolarizing square pulses peaked at 0/+10 mV and were completely abolished by 200 microM Cd(2+). The pharmacology of HVA I(Ba)s was analyzed by applying saturating concentrations of specific Ca(2+)-channel blockers. The L-type blocker nifedipine (10 microM; n = 11), the N-type-channel blocker omega-conotoxin GVIA (0.5 microM; n = 24), and the P/Q-type blocker omega-conotoxin MVIIC (1 microM; n = 16) abolished fractions of total I(Ba)s equal on average to 24.7 +/- 5.4%, 27.1 +/- 3.4%, and 22.2 +/- 2.4%, respectively (mean +/- SE). The simultaneous application of the three blockers reduced I(Ba) by 68.5 +/- 6.6% (n = 10). Nifedipine-sensitive currents and most N- and P/Q-type currents were slowly decaying, the average fractional persistence after 300 ms of steady depolarization being 0.77 +/- 0.02, 0.60 +/- 0.06, and 0.68 +/- 0.04, respectively. The residual, blocker-resistant (R-type) currents were consistently faster inactivating, with an average fractional persistence after 300 ms of 0.30 +/- 0.08. Fast-decaying R-type currents also displayed a more negative threshold of activation (by about 10 mV) than non-R-type HVA currents. These results demonstrate that EPN neurons express multiple pharmacological components of the HVA Ca(2+) currents and point to the existence of an R-type current with specific functional properties including fast inactivation kinetics and intermediate threshold of activation.

The status of voltage-dependent calcium channels in alpha 1E knock-out mice

It has been hypothesized that R-type Ca currents result from the expression of the alpha(1E) gene. To test this hypothesis we examined the properties of voltage-dependent Ca channels in mice in which the alpha(1E) Ca channel subunit had been deleted. Application of omega-conotoxin GVIA, omega-agatoxin IVA, and nimodipine to cultured cerebellar granule neurons from wild-type mice inhibited components of the whole-cell Ba current, leaving a "residual" R current with an amplitude of approximately 30% of the total Ba current. A minor portion of this R current was inhibited by the alpha(1E)-selective toxin SNX-482, indicating that it resulted from the expression of alpha(1E). However, the majority of the R current was not inhibited by SNX-482. The SNX-482-sensitive portion of the granule cell R current was absent from alpha(1E) knock-out mice. We also identified a subpopulation of dorsal root ganglion (DRG) neurons from wild-type mice that expressed an SNX-482-sensitive component of the R current. However as with granule cells, most of the DRG R current was not blocked by SNX-482. We conclude that there exists a component of the R current that results from the expression of the alpha(1E) Ca channel subunit but that the majority of R currents must result from the expression of other Ca channel alpha subunits.

R-Type Ca2+ channels are coupled to the rapid component of secretion in mouse adrenal slice chromaffin cells

Patch-clamp measurements of Ca(2+) currents and membrane capacitance were performed on slices of mouse adrenal glands, using the perforated-patch configuration of the patch-clamp technique. These recording conditions are much closer to the in vivo situation than those used so far in most electrophysiological studies in adrenal chromaffin cells (isolated cells maintained in culture and whole-cell configuration). We observed profound discrepancies in the quantities of Ca(2+) channel subtypes (P-, Q-, N-, and L-type Ca(2+) channels) described for isolated mouse chromaffin cells maintained in culture. Differences with respect to previous studies may be attributable not only to culture conditions, but also to the patch-clamp configuration used. Our experiments revealed the presence of a Ca(2+) channel subtype never before described in chromaffin cells, a toxin and dihydropyridine-resistant Ca(2+) channel with fast inactivation kinetics, similar to the R-type Ca(2+) channel described in neurons. This channel contributes 22% to the total Ca(2+) current and controls 55% of the rapid secretory response evoked by short depolarizing pulses. Our results indicate that R-type Ca(2+) channels are in close proximity with the exocytotic machinery to rapidly regulate the secretory process.

Calcium currents in hair cells isolated from semicircular canals of the frog

L-type and R-type Ca(2+) currents were detected in frog semicircular canal hair cells. The former was noninactivating and nifedipine-sensitive (5 microM); the latter, partially inactivated, was resistant to omega-conotoxin GVIA (5 microM), omega-conotoxin MVIIC (5 microM), and omega-agatoxin IVA (0.4 microM), but was sensitive to mibefradil (10 microM). Both currents were sensitive to Ni(2+) and Cd(2+) (>10 microM). In some cells the L-type current amplitude increased almost twofold upon repetitive stimulation, whereas the R-type current remained unaffected. Eventually, run-down occurred for both currents, but was prevented by the protease inhibitor calpastatin. The R-type current peak component ran down first, without changing its plateau, suggesting that two channel types generate the R-type current. This peak component appeared at -40 mV, reached a maximal value at -30 mV, and became undetectable for voltages > or =0 mV, suggestive of a novel transient current: its inactivation was indeed reversibly removed when Ba(2+) was the charge carrier. The L-type current and the R-type current plateau were appreciable at -60 mV and peaked at -20 mV: the former current did not reverse for voltages up to +60 mV, the latter reversed between +30 and +60 mV due to an outward Cs(+) current flowing through the same Ca(2+) channel. The physiological role of these currents on hair cell function is discussed.

The prostaglandin E series modulates high-voltage-activated calcium channels probably through the EP3 receptor in rat paratracheal ganglia

The modulation of high-voltage-activated (HVA) Ca2+ channels by the prostaglandin E series (PGE1 and PGE2) was studied in the paratracheal ganglion cells. Prostaglandin E1, E2, STA2 (a stable analogue of thromboxane A2), 17-phenyl-trinor-PGE2 (an EP1-selective agonist) and sulprostone (an EP3-selective agonist) inhibited the HVA Ca2+ current (HVA ICa) dose-dependently, and the rank order of potency to inhibit HVA Ca2+ channels was sulprostone>PGE2, PGE1>STA2>>17-phenyl-trinor-PGE2. SC-51089 (10(-5) M), a selective EP1-receptor antagonist, showed no effect on the PGE1- or PGE2-induced inhibition of the HVA ICa, thereby indicating that PGE1- and PGE2-induced inhibition of the HVA Ca2+ channels is possibly mediated by the EP3 receptor. The PGE1-sensitive component of the current was markedly reduced in the presence of omega-conotoxin-GVIA (3x10(-6) M), but not with nifedipine (3x10(-6) M). PGE1 and PGE2 also inhibited the remaining ICa in a saturating concentration of nifedipine, omega-conotoxin-GVIA and omega-conotoxin-MVIIC, suggesting that R-type Ca2+ channels are involved. The inhibitory effect of PGE1 or sulprostone was prevented by pretreatment with pertussis toxin [islet activating protein (IAP)] or phorbol-12-myristate-13-acetate (PMA), and the protein kinase C (PKC) inhibitor chelerythrine blocked the action of PMA. It was concluded that PGE1 selectively reduces both N- and R-type Ca2+ currents by activating a G-protein probably through the EP3 receptor in paratracheal ganglion cells.

An R-type Ca(2+) current in neurohypophysial terminals preferentially regulates oxytocin secretion

Multiple types of voltage-dependent Ca(2+) channels are involved in the regulation of neurotransmitter release (Tsien et al., 1991; Dunlap et al., 1995). In the nerve terminals of the neurohypophysis, the roles of L-, N-, and P/Q-type Ca(2+) channels in neuropeptide release have been identified previously (Wang et al., 1997a). Although the L- and N-type Ca(2+) currents play equivalent roles in both vasopressin and oxytocin release, the P/Q-type Ca(2+) current only regulates vasopressin release. An oxytocin-release and Ca(2+) current component is resistant to the L-, N-, and P/Q-type Ca(2+) channel blockers but is inhibited by Ni(2+). A new polypeptide toxin, SNX-482, which is a specific alpha(1E)-type Ca(2+) channel blocker (Newcomb et al., 1998), was used to characterize the biophysical properties of this resistant Ca(2+) current component and its role in neuropeptide release. This resistant component was dose dependently inhibited by SNX-482, with an IC(50) of 4.1 nM. Furthermore, SNX-482 did not affect the other Ca(2+) current types in these CNS terminals. Like the N- and P/Q-type Ca(2+) currents, this SNX-482-sensitive transient Ca(2+) current is high-threshold activated and shows moderate steady-state inactivation. At the same concentrations, SNX-482 blocked the component of oxytocin, but not of vasopressin, release that was resistant to the other channel blockers, indicating a preferential role for this type of Ca(2+) current in oxytocin release from neurohypophysial terminals. Our results suggest that an alpha(1E) or "R"-type Ca(2+) channel exists in oxytocinergic nerve terminals and, thus, functions in controlling only oxytocin release from the rat neurohypophysis.