The naming of voltage-gated calcium channels

Expression of the alpha(2)delta subunit interferes with prepulse facilitation in cardiac L-type calcium channels

We investigated the role of the accessory alpha(2)delta subunit on the voltage-dependent facilitation of cardiac L-type Ca(2+) channels (alpha(1C)). alpha(1C) Channels were coexpressed in Xenopus oocytes with beta(3) and alpha(2)delta calcium channel subunits. In alpha(1C) + beta(3), the amplitude of the ionic current (measured during pulses to 10 mV) was in average approximately 1.9-fold larger after the application of a 200-ms prepulse to +80 mV. This phenomenon, commonly referred to as voltage-dependent facilitation, was not observed when alpha(2)delta was coexpressed with alpha(1C) + beta(3). In alpha(1C) + beta(3), the prepulse produced a left shift ( approximately 40 mV) of the activation curve. Instead, the activation curve for alpha(1C) + beta(3) + alpha(2)delta was minimally affected by the prepulse and had a voltage dependence very similar to the G-V curve of the alpha(1C) + beta(3) channel facilitated by the prepulse. Coexpression of alpha(2)delta with alpha(1C) + beta(3) seems to mimic the prepulse effect by shifting the activation curve toward more negative potentials, leaving little room for facilitation. The facilitation of alpha(1C) + beta(3) was associated with an increase of the charge movement. In the presence of alpha(2)delta, the charge remained unaffected after the prepulse. Coexpression of alpha(2)delta seems to set all the channels in a conformational state from where the open state can be easily reached, even without prepulse.

Q- and L-type calcium channels control the development of calbindin phenotype in hippocampal pyramidal neurons in vitro

Cultured immature hippocampal neurons from embryonic 17-day-old rats were used to explore activity-dependent regulation of neuronal phenotype differentiation in the developing hippocampus. The calbindin-D28k phenotype of the pyramidal neurons appeared during the first 6 days in culture, and was expressed by 12% of the cells on day 6. Daily stimulation with 50 mM KCl during the first 5 days in vitro increased the number of calbindin-D28k-positive pyramidal neurons without affecting neuronal survival. This effect was prevented by buffering extracellular Ca2+. Omega-agatoxin-IVA-sensitive Q-type and nitrendipine-sensitive L-type voltage-gated Ca2+ channels (VGCCs) carried Ca2+ currents and Ca2+ influx in immature pyramidal neurons at somata level. Blockade of these channels inhibited calbindin-D28k phenotype induced by 50 mM KCl. Conversely, glutamate-activated Ca2+ channel antagonists did not affect the KCl-induced calbindin-D28k phenotype. Chronic blockade of Q- and/or L-type VGCCs downregulated the normal calbindin-D28k development of immature pyramidal neurons without affecting neuronal survival, the somatic area of pyramidal neurons or the number of GABAergic-positive (gamma-aminobutyric acid) interneurons. However, at later developmental stages, Q-type VGCCs lost their ability to control Ca2+ influx at somata level, and both Q- and L-type VGCCs failed to regulate calbindin-D28k phenotype. These results suggest that Q-type channels, which have been predominantly associated with neurotransmitter release in adult brain, transiently act in synergy with L-type VGCCs to direct early neuronal differentiation of hippocampal pyramidal neurons before the establishment of their synaptic circuits.

Immunohistochemical study on the distribution of neuronal voltage-gated calcium channels in the rat cerebellum

Many neuronal processes are regulated by calcium influx through voltage-gated calcium channels (VGCCs), including protein phosphorylation, gene expression, neurotransmitter release, and firing patterns of action potential. In the present study, we have used anti-peptide antibodies directed against a unique sequence in rat alpha(1A), alpha(1B), alpha(1C) and alpha(1D) subunits of VGCCs to determine their cellular distribution in normal rat cerebellum. Throughout the molecular layer, immunoreactivity for alpha(1B) and alpha(1D) subunits were found in the cell bodies of basket and stellate cells as well as in the neuropil. In the Purkinje cells, only alpha(1C)-IR was observed in the dendritic branches of Purkinje cells, whereas immunoreactivity for alpha(1B) and alpha(1D) subunits were rarely found in the cell bodies of Purkinje cells. Immunoreactivity for the alpha(1A), alpha(1B,) and alpha(1D) subunits were strong in the granule cell bodies, whereas alpha(1C)-IR was not prominent in the cell bodies. In the cerebellar nuclei, a distinct band of punctate immunoreactivity for the alpha(1A), alpha(1B), alpha(1C), and alpha(1D) subunits were observed. The overall results of the above localization study showed clearly that the alpha(1A), alpha(1B,) alpha(1C) and alpha(1D) pore forming subunits of VGCCs have differential distribution in the rat cerebellum. The present studies may provide useful data for such future investigations to understand the role of calcium channels in neurological pathways.

Calcium currents in retrogradely labeled pyramidal cells from rat sensorimotor cortex

Our previous studies of calcium (Ca(2+)) currents in cortical pyramidal cells revealed that the percentage contribution of each Ca(2+) current type to the whole cell Ca(2+) current varies from cell to cell. The extent to which these currents are modulated by neurotransmitters is also variable. This study was directed at testing the hypothesis that a major source of this variability is recording from multiple populations of pyramidal cells. We used the whole cell patch-clamp technique to record from dissociated corticocortical, corticostriatal, and corticotectal projecting pyramidal cells. There were significant differences between the three pyramidal cell types in the mean percentage of L-, P-, and N-type Ca(2+) currents. For both N- and P-type currents, the range of percentages expressed was small for corticostriatal and corticotectal cells as compared with cells which project to the corpus callosum or to the general population. The variance was significantly different between cell types for N- and P-type currents. These results suggest that an important source of the variability in the proportions of Ca(2+) current types present in neocortical pyramidal neurons is recording from multiple populations of pyramidal cells.

Ion channels in glial cells

Functional and molecular analysis of glial voltage- and ligand-gated ion channels underwent tremendous boost over the last 15 years. The traditional image of the glial cell as a passive, structural element of the nervous system was transformed into the concept of a plastic cell, capable of expressing a large variety of ion channels and neurotransmitter receptors. These molecules might enable glial cells to sense neuronal activity and to integrate it within glial networks, e.g., by means of spreading calcium waves. In this review we shall give a comprehensive summary of the main functional properties of ion channels and ionotropic receptors expressed by macroglial cells, i.e., by astrocytes, oligodendrocytes and Schwann cells. In particular we will discuss in detail glial sodium, potassium and anion channels, as well as glutamate, GABA and ATP activated ionotropic receptors. A majority of available data was obtained from primary cell culture, these results have been compared with corresponding studies that used acute tissue slices or freshly isolated cells. In view of these data, an active glial participation in information processing seems increasingly likely and a physiological role for some of the glial channels and receptors is gradually emerging.

Glutamate as a neurotransmitter in the brain: review of physiology and pathology

Glutamate is the principal excitatory neurotransmitter in brain. Our knowledge of the glutamatergic synapse has advanced enormously in the last 10 years, primarily through application of molecular biological techniques to the study of glutamate receptors and transporters. There are three families of ionotropic receptors with intrinsic cation permeable channels [N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate]. There are three groups of metabotropic, G protein-coupled glutamate receptors (mGluR) that modify neuronal and glial excitability through G protein subunits acting on membrane ion channels and second messengers such as diacylglycerol and cAMP. There are also two glial glutamate transporters and three neuronal transporters in the brain. Glutamate is the most abundant amino acid in the diet. There is no evidence for brain damage in humans resulting from dietary glutamate. A kainate analog, domoate, is sometimes ingested accidentally in blue mussels; this potent toxin causes limbic seizures, which can lead to hippocampal and related pathology and amnesia. Endogenous glutamate, by activating NMDA, AMPA or mGluR1 receptors, may contribute to the brain damage occurring acutely after status epilepticus, cerebral ischemia or traumatic brain injury. It may also contribute to chronic neurodegeneration in such disorders as amyotrophic lateral sclerosis and Huntington's chorea. In animal models of cerebral ischemia and traumatic brain injury, NMDA and AMPA receptor antagonists protect against acute brain damage and delayed behavioral deficits. Such compounds are undergoing testing in humans, but therapeutic efficacy has yet to be established. Other clinical conditions that may respond to drugs acting on glutamatergic transmission include epilepsy, amnesia, anxiety, hyperalgesia and psychosis.

Binding of six chimeric analogs of omega-conotoxin MVIIA and MVIIC to N- and P/Q-type calcium channels

Replacement of the N-terminal half of omega-conotoxin MVIIC, a peptide blocker of P/Q-type calcium channels, with that of omega-conotoxin MVIIA significantly increased the affinity for N-type calcium channels. To identify the residues essential for subtype selectivity, we examined single reverse mutations from MVIIA-type to MVIIC-type in this chimeric analog. A reverse mutation from Lys(7) to Pro(7) decreased the affinity for both P/Q- and N-type channels, whereas that from Leu(11) to Thr(11) increased the affinity for P/Q-type channels and decreased the affinity for N-type channels. The roles of these two residues were confirmed by synthesizing two MVIIC analogs in which Pro(7) and Thr(11) were replaced with Lys(7) and Leu(11), respectively.

Acidic motif responsible for plasma membrane association of the voltage-dependent calcium channel beta1b subunit

Voltage-dependent calcium channels consist of a pore-forming transmembrane alpha1-subunit, which is known to associate with a number of accessory subunits, including alpha2-delta- and beta-subunits. The beta-subunits, of which four have been identified (beta1-4), are intracellular proteins that have marked effects on calcium channel trafficking and function. In a previous study, we observed that the beta1b-subunit showed selective plasma membrane association when expressed alone in COS7 cells, whereas beta3 and beta4 did not. In this study, we have examined the basis for this, and have identified, by making chimeric beta-subunits, that the C-terminal region, which shows most diversity between beta-subunits, of beta1b is responsible for its plasma membrane association. Furthermore we have identified, by deletion mutations, an 11-amino acid motif present in the C terminus of beta1b but not in beta3 (amino acids 547-556 of beta1b, WEEEEDYEEE), which when deleted, reduces membrane association of beta1b. Future research aims to identify what is binding to this sequence in beta1b to promote membrane association of this calcium channel subunit. It is possible that such membrane association is important for the selective localization or clustering of particular calcium channels with which beta1b is associated.

Preferential inhibition of L- and N-type calcium channels in the rat hippocampal neurons by cilnidipine

The effect of a dihydropyridine Ca2+ antagonist, cilnidipine, on voltage-dependent Ca2+ channels was studied in acutely dissociated rat CA1 pyramidal neurons using the nystatin-perforated patch recording configuration under voltage-clamp conditions. Cilnidipine had no effect on low-voltage-activated (LVA) Ca2+ channels at the low concentrations under 10(-6) M. On the other hand, cilnidipine inhibited the high-voltage-activated (HVA) Ca2+ current (I(Ca)) in a concentration-dependent manner and the inhibition curve showed a step-wise pattern; cilnidipine selectively reduced only L-type HVA I(Ca) at the low concentrations under 10(-7) and 10(-6) M cilnidipine blocked not only L- but also N-type HVA I(Ca). At the high concentration over 10(-6) M cilnidipine non-selectively blocked the T-type LVA and P/Q- and R-type HVA Ca2+ channels. This is the first report that cilnidipine at lower concentration of 10(-6) M blocks both L-and N-type HVA I(Ca) in the hippocampal neurons.

Combinatorial synthesis of omega-conotoxin MVIIC analogues and their binding with N- and P/Q-type calcium channels

Omega-conotoxin MVIIC (MVIIC) blocks P/Q-type calcium channels with high affinity and N-type calcium channels with low affinity, while the highly homologous omega-conotoxin MVIIA blocks only N-type calcium channels. We wished to obtain MVIIC analogues more selective for P/Q-type calcium channels than MVIIC to elucidate structural differences among the channels, which discriminate the omega-conotoxins. To prepare a number of MVIIC analogues efficiently, we developed a combinatorial method which includes a random air oxidation step. Forty-seven analogues were prepared in six runs and some of them exhibited higher selectivity for P/Q-type calcium channels than MVIIC in binding assays.

Contribution of L-type calcium channels to epileptiform activity in hippocampal and neocortical slices of guinea-pigs

The aim of the present investigation was to compare the antiepileptic efficacy of the specific L-type calcium channel blocker nifedipine in hippocampal and neocortical slice preparations in the Mg2+-free model of epilepsy. The main findings were as follows. (1) In hippocampal slices, in general, nifedipine (20-80 micromol/l) exerted a suppressive effect both on repetition rate and on area under epileptiform field potentials. This effect was clearly dose dependent. In the majority of cases, this suppression was preceded by an increase, which was transient in nature. Only in the lowest concentration (20 micromol/l) used, in normal K+, instead of a depression, a persistent increase occurred. (2) In neocortical slices, in the majority of experiments, nifedipine (20-80 micromol/l) showed a depressive action only on the area under the epileptiform field potentials. The depressive effect of nifedipine on the area was dose dependent, although to a lesser extent than in the hippocampus. In nearly half of the slices this suppression was preceded by a transient increase. By contrast, the repetition rate of epileptiform field potentials increased transiently in about 20% of the slices followed by a decrease. In the remaining 80% of the slices the repetition rate increased persistently. (3) An elevation of the K+ concentration accentuated the depressive actions of nifedipine only in the hippocampus. In contrast to elevated K+, in both the hippocampus and the neocortex, epileptiform field potentials were not suppressed in all experiments in normal K+. (4) The reversibility of the depressive effects of nifedipine was differential in the two tissue types. In the hippocampus, after suppression of epileptiform field potentials they reappeared in the overwhelming majority of slices. In the neocortex, this was the case in only one experiment. These findings may indicate the existence of L-type calcium channels with a differential functional significance for epileptogenesis and/or the existence of different forms of L-type channels in hippocampal and neocortical tissue. As a whole, the differential effects of L-type calcium channel blockade in the hippocampus and neocortex point to differences in the network properties of the two tissue types.

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.

Decreased G-protein-mediated regulation and shift in calcium channel types with age in hippocampal cultures

The membrane density of L-type voltage-sensitive Ca(2+) channels (L-VSCCs) of rat hippocampal neurons increases over age [days in vitro (DIV)] in long-term primary cultures, apparently contributing both to spontaneous cell death and to enhanced excitotoxic vulnerability. Similar increases in L-VSCCs occur during brain aging in vivo in rat and rabbit hippocampal neurons. However, unraveling both the molecular basis and the functional implications of these age changes in VSCC density will require determining whether the other types of high-threshold VSCCs (e.g., N, P/Q, and R) also exhibit altered density and/or changes in regulation, for example, by the important G-protein-coupled, membrane-delimited inhibitory pathway. These possibilities were tested here in long-term hippocampal cultures. Pharmacologically defined whole-cell currents were corrected for cell size differences over age by normalization with whole-cell capacitance. The Ca(2+) channel current density (picoamperes per picofarad), mediated by each Ca(2+) channel type studied here (L, N, and a combined P/Q + R component), increased through 7 DIV. Thereafter, however, only L-type current density continued to increase, at least through 21 DIV. Concurrently, pertussis toxin-sensitive G-protein-coupled inhibition of non-L-type Ca(2+) channel current induced by the GABA(B) receptor agonist baclofen or by guanosine 5'-3-O-(thio)triphosphate declined dramatically with age in culture. Thus, the present studies identify selective and novel parallel mechanisms for the time-dependent alteration of Ca(2+) influx, which could importantly influence function and vulnerability during development and/or aging.

The monoamine neurons of the rat brain preferentially express a splice variant of alpha1B subunit of the N-type calcium channel

The N-type voltage-dependent calcium channels play a significant role in neurotransmitter release. The alpha1B subunit of the N-type calcium channel functions as the primary subunit that forms the pore and contains the structural motifs that mediate the pharmacological and gating properties of the channel. We report on an isoform of the alpha1B subunit that is preferentially expressed by the monoaminergic neurons of the rat brain. This isoform contains a 21-amino acid cassette in the synprint site present in the cytoplasmic loop between domains IIS6 and IIIS1. RT-PCR of micropunched tissue was used to show preferential expression of this isoform in regions of the brain containing monoaminergic neurons and to a lesser extent in the cerebellum. Double-label in situ hybridization was used to show expression of this isoform mRNA in dopaminergic neurons of the ventral mesencephalon. The expression of two distinct N-type calcium channels containing these alpha1B subunit isoforms by the monoaminergic neurons may provide for synapse-specific regulation of neurotransmitter release.

Multiple calcium channels regulate neurotransmitter release from vagus nerve terminals in the cat bronchiole

1. Twitch-like contractions and non-adrenergic non-cholinergic (NANC) relaxations evoked by electrical field stimulation (EFS) of the cat bronchiole were used to examine the voltage-activated calcium channels involved in excitatory and inhibitory neurotransmission in the cat bronchiole. 2. Nifedipine (50 microM), the L-type calcium channel antagonist, did not affect the twitch-like contraction and NANC relaxations. However, low concentrations of the N-type calcium channel blocker omega-conotoxin GVIA (omega-CgTX GVIA) (0.1 microM) irreversibly abolished twitch-like contractions evoked by trains of EFS </=10 stimuli at 20 Hz. 3. After the prolonged treatment with 0.1 microM omega-CgTX GVIA, EFS evoked initial fast and later slow NANC relaxations in the presence of 5-HT (10 microM), atropine and guanethidine (1 microM each). However increased concentration of omega-CgTX GVIA (1 microM) completely suppressed the slow NANC relaxation without affecting the initial fast component. 4. omega-agatoxin IVA (100 nM), the P- and Q-type calcium channel inhibitor, and nimodipine (10 microM), the L- and T-type calcium channel blocker, did not affect the amplitude of the initial fast NANC relaxation in the absence or presence of omega-CgTX GVIA (1 microM). 5. The contraction or relaxation induced by exogenous acetylcholine (ACh) (0.5 microM) or the nitric oxide donor, s-nitroso-N-acetyl penicillamine (SNAP) (1 microM) were not affected by omega-CgTX GVIA (1 microM). 6. Taken together, these results suggest that generation of twitch-like contraction and later slow NANC relaxation are regulated by N-type calcium channels, whereas generation of the initial fast NANC relaxation possibly involves R-type calcium channel.

Current modulation and membrane targeting of the calcium channel alpha1C subunit are independent functions of the beta subunit

1. The beta subunits of voltage-sensitive calcium channels facilitate the incorporation of channels into the plasma membrane and modulate calcium currents. In order to determine whether these two effects of the beta subunit are interdependent or independent of each other we studied plasma membrane incorporation of the channel subunits with green fluorescent protein and immunofluorescence labelling, and current modulation with whole-cell and single-channel patch-clamp recordings in transiently transfected human embryonic kidney tsA201 cells. 2. Coexpression of rabbit cardiac muscle alpha1C with rabbit skeletal muscle beta1a, rabbit heart/brain beta2a or rat brain beta3 subunits resulted in the colocalization of alpha1C with beta and in a marked translocation of the channel complexes into the plasma membrane. In parallel, the whole-cell current density and single-channel open probability were increased. Furthermore, the beta2a isoform specifically altered the voltage dependence of current activation and the inactivation kinetics. 3. A single amino acid substitution in the beta subunit interaction domain of alpha1C (alpha1CY467S) disrupted the colocalization and plasma membrane targeting of both subunits without affecting the beta subunit-induced modulation of whole-cell currents and single-channel properties. 4. These results show that the modulation of calcium currents by beta subunits can be explained by beta subunit-induced changes of single-channel properties, but the formation of stable alpha1C-beta complexes and their increased incorporation into the plasma membrane appear not to be necessary for functional modulation.

Immunohistochemical detection of alpha1E voltage-gated Ca(2+) channel isoforms in cerebellum, INS-1 cells, and neuroendocrine cells of the digestive system

Polyclonal antibodies were raised against a common and a specific epitope present only in longer alpha1E isoforms of voltage-gated Ca(2+) channels, yielding an "anti-E-com" and an "anti-E-spec" serum, respectively. The specificity of both sera was established by immunocytochemistry and immunoblotting using stably transfected HEK-293 cells or membrane proteins derived from them. Cells from the insulinoma cell line INS-1, tissue sections from cerebellum, and representative regions of gastrointestinal tract were stained immunocytochemically. INS-1 cells expressed an alpha1E splice variant with a longer carboxy terminus, the so-called alpha1Ee isoform. Similarily, in rat cerebellum, which was used as a reference system, the anti-E-spec serum stained somata and dendrites of Purkinje cells. Only faint staining was seen throughout the cerebellar granule cell layer. After prolonged incubation times, neurons of the molecular layer were stained by anti-E-com, suggesting that a shorter alpha1E isoform is expressed at a lower protein density. In human gastrointestinal tract, endocrine cells of the antral mucosa (stomach), small and large intestine, and islets of Langerhans were stained by the anti-E-spec serum. In addition, staining by the anti-E-spec serum was observed in Paneth cells and in the smooth muscle cell layer of the lamina muscularis mucosae. We conclude that the longer alpha1Ee isoform is expressed in neuroendocrine cells of the digestive system and that, in pancreas, alpha1Ee expression is restricted to the neuroendocrine part, the islets of Langerhans. alpha1E therefore appears to be a common voltage-gated Ca(2+) channel linked to neuroendocrine and related systems of the body.

Regulation of calcium channel alpha(1A) subunit splice variant mRNAs in kainate-induced temporal lobe epilepsy

P/Q-type voltage-gated Ca(2+) channels (VGCC) regulate neurotransmitter release in the hippocampus and molecular alterations of their alpha(1A) pore-forming subunits are involved in various animal and human CNS diseases. We evaluated, using RT-PCR and in situ hybridization, the spatio-temporal activation of two alpha(1A) subunits splice variants (alpha(1A-a) and alpha(1A-b)) in control and kainic acid (KA)-treated rats. Six hours after KA treatment, alpha(1A-a) and alpha(1A-b) mRNAs increased, decreased or remained unchanged with area specific patterns. These changes were evidenced in the hippocampus and the dentatus gyrus and absent in the cerebellum. The alpha(1A) mRNA upregulation lasted for at least 7 days after KA treatment. Altogether, these results indicate that alpha(1A-a) and alpha(1A-b) mRNAs following seizure onset exhibit a complex and specific spatio-temporal pattern. The long-lasting changes in alpha(1A) subunit mRNA contents suggests that VGCC may be involved in the mechanisms generating chronic focal hyperexcitability and/or cellular damage in temporal lobe epilepsy.

Spontaneous action potentials initiate rhythmic intercellular calcium waves in immortalized hypothalamic (GT1-1) neurons

GT1-1 cells exhibit spontaneous action potentials and transient increases in intracellular calcium concentration ([Ca2+]i) that occur in individual cells and as spatially propagated intercellular Ca2+ waves. In this study, simultaneous cell-attached patch-clamp recording of action currents (indicative of action potentials) and fluorescence imaging of [Ca2+]i revealed that Ca2+ transients in GT1-1 cells were preceded by a single action current or a burst of action currents. Action currents preceded Ca2+ transients in a similar pattern regardless of whether the Ca2+ transients were limited to the individual cell or occurred as part of an intercellular Ca2+ wave. Both the action currents and Ca2+ transients were abolished by 1 microM tetrodotoxin. Removal of extracellular Ca2+ abolished all spontaneous Ca2+ transients without inhibiting the firing of action currents. Nimodipine, which blocks L-type Ca2+ currents in GT1-1 cells, also abolished all spontaneous Ca2+ signaling. Delivery of small voltage steps to the patch pipette in the cell-attached configuration elicited action currents the latency to firing of which decreased with increasing amplitude of the voltage step. These results indicate that spontaneous intercellular Ca2+ waves are generated by a propagated depolarization, the firing of action potentials in individual cells, and the resulting influx of Ca2+ through L-type Ca2+ channels. These patterns of spontaneous activity may be important in driving the pulsatile release of GnRH from networks of cells.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

The pharmacology of ion channels: with particular reference to voltage-gated Ca2+ channels

Ion channels are molecular machines that serve as principal integrating and regulatory devices for the control of cellular excitability. They are also major targets for drug action. The basic aspects of ion channel structure and pharmacological control are reviewed and illustrated with specific reference to a major class of therapeutic agents and molecular tools--the clinically available Ca2+ channel antagonists.

Stereoselective characterization of the 1,4-dihydropyridine binding site at L-type calcium channels in the resting state and the opened/inactivated state

Via a 3D-QSAR pseudoreceptor modeling approach, atomistic binding site models for pharmacologically active 1,4-dihydropyridines (DHPs) were developed. Applying a training set of pure DHP enantiomers a pseudoreceptor model representing the resting state of voltage-gated calcium channels (VGCCs) was generated by correlating experimental versus predicted free energies of binding (DeltaG degrees). For validation further test set derivatives-not used for receptor generation-were predicted yielding root-mean-square (rms) deviation of 0.532 kcal/mol. Selectivity of the resting state model was checked by using the same DHP training set compounds but experimental data for the inactivated channel mode. Although there was found an almost perfect correlation for the training set, the following free relaxation of the corresponding test set applying a Monte Carlo protocol showed rms of 2.033 kcal/mol, clearly demonstrating the lack of any predicting character of the hybrid model. Taking into consideration 19 additional nifedipine analogues, a further verification of the model was performed. This yielded a good correlation for the 12 training set compounds and a satisfactory prediction for the test set molecules with rms of 0.409 kcal/mol. The generation of a pseudoreceptor model depicting the opened/inactivated state of VGCCs required one single additional residue to achieve a rms of 0.848 kcal/mol for the prediction of the test set derivatives. Since all pseudoreceptor models are composed of the same six amino acid residues-Thr, Phe, Gly, Met, Tyr, Tyr-transition from resting to open/inactivated state may be described by one additional hydrogen bond donor interaction (Thr) at the left-hand side of DHPs. Furthermore, a potential charge-transfer interaction for all electron-deficient 4-phenyl DHPs is postulated, because significant correlation between quantum chemically AM1 (R = 0.91) and RHF 6-31G (R = 0.84) computed LUMO energies and experimentally detected DeltaG degrees exp values was found.

Tonic dopamine inhibition of L-type Ca2+ channel activity reduces alpha1D Ca2+ channel gene expression

Hormones and neurotransmitters have both short-term and long-term modulatory effects on the activity of voltage-gated Ca2+ channels. Although much is known about the signal transduction underlying short-term modulation, there is far less information on mechanisms that produce long-term effects. Here, the molecular basis of long-lasting suppression of Ca2+ channel current in pituitary melanotropes by chronic dopamine exposure is examined. Experiments involving in vivo and in vitro treatments with the dopaminergic drugs haloperidol, bromocriptine, and quinpirole show that D2 receptors persistently decrease alpha1D L-type Ca2+ channel mRNA and L-type Ca2+ channel current without altering channel gating properties. In contrast, another L-channel (alpha1C) mRNA and P/Q-channel (alpha1A) mRNA are unaffected. The downregulation of alpha1D mRNA does not require decreases in cAMP levels or P/Q-channel activity. However, it is mimicked and occluded by inhibition of L-type channels. Thus, interruption of the positive feedback between L-type Ca2+ channel activity and alpha1D gene expression can account for the long-lasting regulation of L-current produced by chronic activation of D2 dopamine receptors.

A new beta subtype-specific interaction in alpha1A subunit controls P/Q-type Ca2+ channel activation

The cytoplasmic beta subunit of voltage-dependent calcium channels modulates channel properties in a subtype-specific manner and is important in channel targeting. A high affinity interaction site between the alpha1 interaction domain (AID) in the I-II cytoplasmic loop of alpha1 and the beta interaction domain (BID) of the beta subunit is highly conserved among subunit subtypes. We describe a new subtype-specific interaction (Ss1) between the amino-terminal cytoplasmic domain of alpha1A (BI-2) and the carboxyl terminus of beta4. Like the interaction identified previously () between the carboxyl termini of alpha1A and beta4 (Ss2), the affinity of this interaction is lower than AID-BID, suggesting that these are secondary interactions. Ss1 and Ss2 involve overlapping sites on beta4 and are competitive, but neither inhibits the interaction with AID. The interaction with the amino terminus of alpha1 is isoform-dependent, suggesting a role in the specificity of alpha1-beta pairing. Coexpression of beta4 in Xenopus oocytes produces a reduced hyperpolarizing shift in the I-V curve of the alpha1A channel compared with beta3 (not exhibiting this interaction). Replacing the amino terminus of alpha1A with that of alpha1C abolishes this difference. Our data contribute to our understanding of the molecular organization of calcium channels, providing a functional basis for variation in subunit composition of native P/Q-type channels.

Neuronal voltage-activated calcium channels: on the roles of the alpha 1E and beta 3 subunits

Many neurons of the central and peripheral nervous systems display multiple high voltage-activated (HVA) Ca2+ currents, often classified as L-, N-, P-, Q, and R-type. The heterogeneous properties of these channels have been attributed to diversity in their pore-forming alpha 1, subunits, in association with various beta subunits. However, there are large gaps in understanding how individual subunits contribute to Ca2+ channel diversity. Here we describe experiments to investigate the roles of alpha 1E and beta 3 subunits in mammalian neurons. The alpha 1E subunit is the leading candidate to account for the R-type channel, the least understood of the various types of high voltage-activated Ca2+ channels. Incubation with alpha 1E antisense oligonucleotide caused a 53% decrease in the peak R-type current density, while no significant changes in the current expression were seen in sense oligonucleotide-treated cells. The specificity of the alpha 1E antisense oligonucleotides was supported by the lack of change in the amplitude of P/Q current. These results upheld the hypothesis that members of the E class of alpha 1 subunits support the high voltage-activated R-type current in cerebellar granule cells. We studied the role of the Ca2+ channel beta 3 subunit using a gene targeting strategy. In sympathetic beta 3-/- neurons, the L-type current was significantly reduced relative to wild type (wt). In addition, N-type Ca2+ channels made up a smaller proportion of the total Ca2+ current than in wt due to a lower N-type current density in a group of neurons with small total currents. Voltage-dependent activation of P/Q-type Ca2+ channels was described by two Boltzmann components with different voltage dependence. The absence of the beta 3 subunit was associated with a shift in the more depolarized component of the activation along the voltage axis toward more negative potentials. The overall conclusion is that deletion of the beta 3 subunit affects at least three distinct types of HVA Ca2+ channel, but no single type of channel is solely dependent on beta 3.

Molecular and functional diversity of voltage-gated calcium channels

The contributing roles of voltage-gated calcium channels (VGCC) to the generation of electrical signaling are well documented. VGCCs open in response to depolarization of the plasma membrane and mediate the flux of calcium into excitable cells, which further depolarizes the membrane. But a more relevant role of VGCCs is to serve as highly regulated mechanisms to deliver calcium ions into specific intracellular locales for a variety of calcium-dependent processes including neurotransmitter release, hormone secretion, neuronal survival, and muscle contraction. Recent biochemical and molecular biological studies have demonstrated that the calcium channel pore-forming subunit (alpha 1) is not an isolated entity, but in fact interacts physically with a variety of strategically localized proteins. The functional consequences of such interactions as well as other molecular aspects of VGCC will be discussed. Finally, although far from a final conclusion, what is currently known about the molecular composition of native calcium channels will be summarized.

Calcium influx is required for endocytotic membrane retrieval

Cells use endocytotic membrane retrieval to compensate for excess surface membrane after exocytosis. Retrieval is thought to be calcium-dependent, but the source of this calcium is not known. We found that, in sea urchin eggs, endocytotic membrane retrieval required extracellular calcium. Inhibitors of P-type calcium channels-cadmium, omega-conotoxin MVIIC, omega-agatoxin IVA, and omega-agatoxin TK-blocked membrane retrieval; selective inhibitors of N-type and L-type channels did not. Treatment with calcium ionophores overcame agatoxin inhibition in a calcium-dependent manner. Cadmium blocked membrane retrieval when applied during the first 5 minutes after fertilization, the period when the membrane potential is depolarized. We conclude that calcium influx through omega-agatoxin-sensitive channels plays a key role in signaling for endocytotic membrane retrieval.

Antibodies against the beta subunit of voltage-dependent calcium channels in Lambert-Eaton myasthenic syndrome

Lambert-Eaton myasthenic syndrome is an autoimmune disease that impairs neuromuscular transmission. Several studies suggest that neurotransmitter release is reduced by an immune response directed against the calcium channel complex of nerve terminals. The immunoglobulin G fractions from Lambert-Eaton myasthenic syndrome patients immunoprecipitate solubilized neuronal N- and P/Q-type channels and in certain cases brain, skeletal and cardiac muscle L-type channels [El Far O. et al. (1995) J. Neurochem. 64, 1696-1702; Lennon V. A. and Lambert E. H. (1989) Mayo Clin. Proc. 64, 1498-1504; Sher E. et al. (1989) Lancet ii, 640-643; Suenaga A. et al. (1996) Muscle Nerve 19, 1166-1168]. These channel immunoprecipitation assays are considered as useful for the diagnosis of this syndrome. In this study, we demonstrate that two predominant neuronal voltage-dependent calcium channel beta subunits (beta3 and beta4, of mol. wt 58,000) are general targets of Lambert-Eaton myasthenic syndrome autoantibodies. Of 20 disease sera tested, 55% were able to immunoprecipitate 35S-labeled beta subunits. All five patients affected with small-cell lung carcinoma were positive for the beta-subunit immunoprecipitation assay. Interestingly, only a fraction of the beta-subunit-positive sera was also able to immunoprecipitate N- and P/Q-type channels, suggesting that several of the beta-subunit epitopes are masked in native channels. In accordance with this observation, we found that several beta-positive sera were able to prevent the interaction between calcium channel alpha1 and beta subunits in vitro. In cases where sera were able to immunoprecipitate beta subunits, N- and P/Q-type channels, the immunoprecipitation of both channel types was either partially or entirely mediated by beta-subunit antibodies. Our results suggest that assays based on the immunoprecipitation of beta subunits can be used as an additional test to assist in the diagnosis of Lambert-Eaton myasthenic syndrome.

Cellular and molecular basis for electrical rhythmicity in gastrointestinal muscles

Regulation of gastrointestinal (GI) motility is intimately coordinated with the modulation of ionic conductance expressed in GI smooth muscle and nonmuscle cells. Interstitial cells of Cajal (ICC) act as pacemaker cells and possess unique ionic conductances that trigger slow wave activity in these cells. The slow wave mechanism is an exclusive feature of ICC: Smooth muscle cells may lack the basic ionic mechanisms necessary to generate or regenerate slow waves. The molecular identification of the components for these conductances provides the foundation for a complete understanding of the ionic basis for GI motility. In addition, this information will provide a basis for the identification or development of therapeutics that might act on these channels. It is much easier to study these conductances and develop blocking drugs in expression systems than in native GI muscle cells. This review focuses on the relationship between ionic currents in native GI smooth muscle cells and ICC and their molecular counterparts.

Differential plasma membrane targeting of voltage-dependent calcium channel subunits expressed in a polarized epithelial cell line

1. Voltage-dependent calcium channels (VDCCs) show a highly non-uniform distribution in many cell types, including neurons and other polarized secretory cells. We have examined whether this can be mimicked in a polarized epithelial cell line (Madin-Darby canine kidney), which has been used extensively to study the targeting of proteins. 2. We expressed the VDCC alpha1A, alpha1B or alpha1C subunits either alone or in combination with accessory subunits alpha2-delta and the different beta subunits, and examined their localization immunocytochemically. An alpha1 subunit was only targeted to the plasma membrane if co-expressed with the accessory subunits. 3. The combination alpha1C/alpha2-delta and all beta subunits was always localized predominantly to the basolateral membrane. It has been suggested that this is equivalent to somatodendritic targeting in neurons. 4. In contrast, the alpha1B subunit was expressed at the apical membrane with all the accessory subunit combinations, by 24 h after microinjection. This membrane destination shows some parallels with axonal targeting in neurons. 5. The alpha1A subunit was consistently observed at the apical membrane in the combinations alpha1A/alpha2-delta/beta1b or beta4. In contrast, when co-expressed with alpha2-delta/beta2a, alpha1A was clearly targeted to the basolateral membrane. 6. In conclusion, the VDCC alpha1 subunit appears to be the primary determinant for targeting the VDCC complex, but the beta subunit can modify this destination, particularly for alpha1A.

Sequence differences between alpha1C and alpha1S Ca2+ channel subunits reveal structural determinants of a guarded and modulated benzothiazepine receptor

The molecular basis of the Ca2+ channel block by (+)-cis-diltiazem was studied in class A/L-type chimeras and mutant alpha1C-a Ca2+ channels. Chimeras consisted of either rabbit heart (alpha1C-a) or carp skeletal muscle (alpha1S) sequence in transmembrane segments IIIS6, IVS6, and adjacent S5-S6 linkers. Only chimeras containing sequences from alpha1C-a were efficiently blocked by (+)-cis-diltiazem, whereas the phenylalkylamine (-)-gallopamil efficiently blocked both constructs. Carp skeletal muscle and rabbit heart Ca2+ channel alpha1 subunits differ with respect to two nonconserved amino acids in segments IVS6. Transfer of a single leucine (Leu1383, located at the extracellular mouth of the pore) from IVS6 alpha1C-a to IVS6 of alpha1S significantly increased the (+)-cis-diltiazem sensitivity of the corresponding mutant L1383I. An analysis of the role of the two heterologous amino acids in a L-type alpha1 subunit revealed that corresponding amino acids in position 1487 (outer channel mouth) determine recovery of resting Ca2+ channels from block by (+)-cis-diltiazem. The second heterologous amino acid in position 1504 of segment IVS6 (inner channel mouth) was identified as crucial inactivation determinant of L-type Ca2+ channels. This residue simultaneously modulates drug binding during membrane depolarization. Our study provides the first evidence for a guarded and modulated benzothiazepine receptor on L-type channels.

Structural elements in domain IV that influence biophysical and pharmacological properties of human alpha1A-containing high-voltage-activated calcium channels

We have cloned two splice variants of the human homolog of the alpha1A subunit of voltage-gated Ca2+ channels. The sequences of human alpha1A-1 and alpha1A-2 code for proteins of 2510 and 2662 amino acids, respectively. Human alpha1A-2alpha2bdeltabeta1b Ca2+ channels expressed in HEK293 cells activate rapidly (tau+10mV = 2.2 ms), deactivate rapidly (tau-90mV = 148 micros), inactivate slowly (tau+10mV = 690 ms), and have peak currents at a potential of +10 mV with 15 mM Ba2+ as charge carrier. In HEK293 cells transient expression of Ca2+ channels containing alpha1A/B(f), an alpha1A subunit containing a 112 amino acid segment of alpha1B-1 sequence in the IVS3-IVSS1 region, resulted in Ba2+ currents that were 30-fold larger compared to wild-type (wt) alpha1A-2-containing Ca2+ channels, and had inactivation kinetics similar to those of alpha1B-1-containing Ca2+ channels. Cells transiently transfected with alpha1A/B(f)alpha2bdeltabeta1b expressed higher levels of the alpha1, alpha2bdelta, and beta1b subunit polypeptides as detected by immunoblot analysis. By mutation analysis we identified two locations in domain IV within the extracellular loops S3-S4 (N1655P1656) and S5-SS1 (E1740) that influence the biophysical properties of alpha1A. alpha1AE1740R resulted in a threefold increase in current magnitude, a -10 mV shift in steady-state inactivation, and an altered Ba2+ current inactivation, but did not affect ion selectivity. The deletion mutant alpha1ADeltaNP shifted steady-state inactivation by -20 mV and increased the fast component of current inactivation twofold. The potency and rate of block by omega-Aga IVA was increased with alpha1ADeltaNP. These results demonstrate that the IVS3-S4 and IVS5-SS1 linkers play an essential role in determining multiple biophysical and pharmacological properties of alpha1A-containing Ca2+ channels.

Voltage and calcium use the same molecular determinants to inactivate calcium channels

During sustained depolarization, voltage-gated Ca2+ channels progressively undergo a transition to a nonconducting, inactivated state, preventing Ca2+ overload of the cell. This transition can be triggered either by the membrane potential (voltage-dependent inactivation) or by the consecutive entry of Ca2+ (Ca2+-dependent inactivation), depending on the type of Ca2+ channel. These two types of inactivation are suspected to arise from distinct underlying mechanisms, relying on specific molecular sequences of the different pore-forming Ca2+ channel subunits. Here we report that the voltage-dependent inactivation (of the alpha1A Ca2+ channel) and the Ca2+-dependent inactivation (of the alpha1C Ca2+ channel) are similarly influenced by Ca2+ channel beta subunits. The same molecular determinants of the beta subunit, and therefore the same subunit interactions, influence both types of inactivation. These results strongly suggest that the voltage and the Ca2+-dependent transitions leading to channel inactivation use homologous structures of the different alpha1 subunits and occur through the same molecular process. A model of inactivation taking into account these new data is presented.

Calcium channels in the GABAergic presynaptic nerve terminals projecting to meynert neurons of the rat

Effects of selective Ca2+ channel blockers on GABAergic inhibitory postsynaptic currents (IPSCs) were studied in the acutely dissociated rat nucleus basalis of Meynert (nBM) neurons attached with nerve endings, namely, the "synaptic bouton" preparation, and in the thin slices of nBM, using nystatin perforated and conventional whole-cell patch recording modes, respectively. In the synaptic bouton preparation, nicardipine (3 x 10(-6) M) and omega-conotoxin-MVIIC (3 x 10(-6) M) reduced the frequency of spontaneous postsynaptic currents by 37 and 22%, respectively, whereas omega-conotoxin-GVIA had no effect. After blockade of L- and P/Q-type Ca2+ channels, successive removal of Ca2+ from external solution had no significant effect on the residual spontaneous activities, indicating that N-, R-, and T-type Ca2+ channels are not involved in the spontaneous GABA release. Thapsigargin, but not ryanodine, increased the frequency of spontaneous IPSCs in both the synaptic bouton and slice preparations, suggesting the partial contribution of the intracellular Ca2+ storage site to the spontaneous GABA release. In contrast, omega-conotoxin-GVIA (3 x 10(-6) M) and omega-conotoxin-MVIIC (3 x 10(-6) M) suppressed the evoked IPSCs by 31 and 37%, respectively, but nicardipine produced no significant effect. The residual evoked currents were abolished in Ca2+-free external solution but not in the external solution containing 10(-5) M Ni2+, suggesting the involvement of N-, P/Q-, and R-type Ca2+ channels but not L- and T-type ones in the evoked IPSCs. Neither thapsigargin nor ryanodine had any significant effects on the evoked IPSCs. It was concluded that Ca2+ channel subtypes responsible for spontaneous transmitter release are different from those mediating the transmitter release evoked by nerve stimulation.

P/Q-type voltage-gated calcium channel antibodies in paraneoplastic disorders of the central nervous system

Whether P/Q-type voltage-gated calcium channel (VGCC) antibodies are present in the serum of patients with paraneoplastic syndromes other than the Lambert-Eaton myasthenic syndrome (LEMS) and tumors other than small-cell lung cancer (SCLC) is controversial. Using a commercially available radioimmunoprecipitation assay kit, we examined the sera of 93 patients with paraneoplastic syndromes of the central nervous system (CNS), including 27 patients with paraneoplastic cerebellar degeneration (PCD) associated with tumors other than SCLC and 66 SCLC patients with paraneoplastic encephalomyelitis and sensory neuronopathy (PEM/SN). All PCD sera from patients with tumors other than SCLC were negative for P/Q-type VGCC antibodies. Eight of 66 (12%) SCLC patients with PEM/SN had P/Q-type VGCC antibodies; 4 had LEMS and the other 4 had no symptoms of LEMS or they were overlooked and, therefore, not examined electrophysiologically. In patients with paraneoplastic syndromes of the CNS, the detection of P/Q-type VGCC antibodies supports the diagnosis of LEMS; in our series, only 6% of patients with SCLC and PEM/SN may have had a false positive antibody result, or undiagnosed LEMS.

Muscarine modulates Ca2+ channel currents in rat sensorimotor pyramidal cells via two distinct pathways

We used the whole cell patch-clamp technique and single-cell reverse transcription-polymerase chain reaction (RT-PCR) to study the muscarinic receptor-mediated modulation of calcium channel currents in both acutely isolated and cultured pyramidal neurons from rat sensorimotor cortex. Single-cell RT-PCR profiling for muscarinic receptor mRNAs revealed the expression of m1, m2, m3, and m4 subtypes in these cells. Muscarine reversibly reduced Ca2+ currents in a dose-dependent manner. The modulation was blocked by the muscarinic antagonist atropine. When the internal recording solution included 10 mM ethylene glycol-bis(beta-aminoethyl ether)-N, N,N',N'-tetraacetic acid (EGTA) or 10 mM bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA), the modulation was rapid (tauonset approximately 1.2 s). Under conditions where intracellular calcium levels were less controlled (0.0-0.1 mM BAPTA), a slowly developing component of the modulation also was observed (tauonset approximately 17 s). Both fast and slow components also were observed in recordings with 10 mM EGTA or 20 mM BAPTA when Ca2+ was added to elevate internal [Ca2+] ( approximately 150 nM). The fast component was due to a reduction in both N- and P-type calcium currents, whereas the slow component involved L-type current. N-ethylmaleimide blocked the fast component but not the slow component of the modulation. Preincubation of cultured neurons with pertussis toxin (PTX) also greatly reduced the fast portion of the modulation. These results suggest a role for both PTX-sensitive G proteins as well as PTX-insensitive G proteins in the muscarinic modulation. The fast component of the modulation was reversed by strong depolarization, whereas the slow component was not. Reblock of the calcium channels by G proteins (at -90 mV) occurred with a median tau of 68 ms. We conclude that activation of muscarinic receptors results in modulation of N- and P-type channels by a rapid, voltage-dependent pathway and of L-type current by a slow, voltage-independent pathway.

Single tottering mutations responsible for the neuropathic phenotype of the P-type calcium channel

Recent genetic and molecular biological analyses have revealed many forms of inherited channelopathies. Homozygous ataxic mice, tottering (tg) and leaner (tgla) mice, have mutations in the P/Q-type Ca2+ channel alpha1A subunit gene. Although their clinical phenotypes, histological changes, and locations of gene mutations are known, it remains unclear what phenotypes the mutant Ca2+ channels manifest, or whether the altered channel properties are the primary consequence of the mutations. To address these questions, we have characterized the electrophysiological properties of Ca2+ channels in cerebellar Purkinje cells, where the P-type is the dominant Ca2+ channel, dissociated from the normal, tg, and tgla mice, and compared them with the properties of the wild-type and mutant alpha1A channels recombinantly expressed with the alpha2 and beta subunits in baby hamster kidney cells. The most striking feature of Ca2+ channel currents of mutant Purkinje cells was a marked reduction in current density, being reduced to approximately 60 and approximately 40% of control in tg and tgla mice, respectively, without changes of cell size. The Ca2+ channel currents in the tg Purkinje cells showed a relative increase in non-inactivating component in voltage-dependent inactivation. Besides the same change, those of the tgla mice showed a more distinct change in voltage dependence of activation and inactivation, being shifted in the depolarizing direction by approximately 10 mV, with a broader voltage dependence of inactivation. In the recombinant expression system, the tg channel with a missense mutation (P601L) and one form of the two possible tgla aberrant splicing products, tgla (short) channel, showed a significant reduction in current density, while the other form of the tgla channels, tgla (long), had a current density comparable to the normal control. On the other hand, the shift in voltage dependence of activation and inactivation was observed only for the tgla (long) channel. Comparison of properties of the native and recombinant mutant channels suggests that single tottering mutations are directly responsible for the neuropathic phenotypes of reduction in current density and deviations in gating behavior, which lead to neuronal death and cerebellar atrophy.

Rat embryonic hippocampal neurons express a new class A calcium channel variant

The aim of this study was to investigate, using a RT-PCR strategy, rat voltage-gated class A calcium channel (alpha1A) splice variants during rat hippocampus development. Results demonstrate the presence of multiple alpha1A mRNAs with the hippocampus formation and revealed a new variant of the rat alpha1A subunit (alpha1A-EFe) that diverges from alpha1A-a in the EF-hand domain. alpha1A-EFe expression in hippocampal neurons is restricted to the embryonic period. This in vivo developmental program is recapitulated in dissociated cultures of E17 embryonic hippocampal neurons. These data demonstrate that rat hippocampus neurons express a unique alpha1A splice variant during the embryonic period and suggest that alternative RNA splicing may modulate neuronal calcium channel properties during development.

Calcium channel subtypes responsible for voltage-gated intracellular calcium elevations in embryonic rat motoneurons

The central role of electrical activity and Ca2+ influx in motoneuron development raises important questions about the regulation of Ca2+ signalling induced by voltage-dependent Ca2+ influx. In the purified embryonic rat motoneuron preparation, we recorded barium currents through voltage-activated Ca2+ channels using the whole-cell configuration of the patch-clamp technique. We found that motoneurons express at least four types of high-voltage-activated Ca2+ channels, based on their kinetics, voltage-dependences and pharmacological properties. Of the sustained Ca2+ current activated at 0 mV from a holding potential of -100 mV, approximately 45% was omega-conotoxin-GVIA (1 microM) sensitive, 25% was omega-agatoxin-IVA (30 nM) sensitive and 20% was nitrendipine (250 nM) sensitive. The residual current, after applying these three antagonists, was an inactivating current that differs from classical T-type Ca2+ currents. Based on this pharmacology, changes in intracellular free Ca2+ concentrations were then monitored by Fura 2 digital imaging microspectrofluorimetry. Upon K+ depolarization, the intracellular Ca2+ transient induced by the activation of each type of Ca2+ channel appeared to be quantitatively proportional to their Ca2+ influx. The existence of a calcium-induced calcium release mechanism through activation of caffeine-, ryanodine-sensitive intracellular stores was then investigated. High doses of caffeine and low doses of ryanodine failed to increase intracellular free calcium concentrations and low concentrations of caffeine and high concentrations of ryanodine did not affect K+-induced intracellular free calcium concentration transients indicating both the absence of Ca2+-gated Ca2+-release channels and of a Ca2+-induced Ca2+ release mechanism. Together, these data provide evidence that embryonic motoneurons express multiple Ca2+ channels that function as important regulators of intracellular Ca2+ signalling and may be involved in their development.

Regulation of ion channel expression in neural cells by hormones and growth factors

Voltage-and ligand-gated ion channels are key players in synaptic transmission and neuron-glia communication in the nervous system. Expression of these proteins can be regulated at several levels (transcriptional, translational, or posttranslational) and by multiple extracellular factors in the developing and mature nervous system. A wide variety of hormones and growth factors have been identified as important in neural cell differentiation, which is a complex process involving the acquisition of cell-type-specific ion channel phenotypes. Much literature has already accumulated describing the structural and functional characteristics of ion channels, but relatively little is known about the factors that influence their synthesis and cell surface expression, although this area has generated considerable interest in the context of neural cell development. This article reviews several examples of regulated expression of these channels by cellular factors, namely peptide growth factors and steroid hormones, and discusses, where applicable, current understanding of molecular mechanisms underlying such regulation of voltage-and neurotransmitter-gated ion channels.

Diversity and properties of calcium channel types in NG108-15 hybrid cells

We used an integral of the current-voltage relation as a new evaluation of Ca2+ current component composition in NG108-15 hybrid cells. We determined significant changes in the values and composition of Ca2+ currents during cell differentiation. Only low-voltage-activated Ca2+ currents could be observed in undifferentiated cells; after cell differentiation, high-voltage-activated currents appeared and the total Ca2+ current was increased about 30-fold. By pharmacological and biophysical separation, we determined four main types of Ca2+ channels in differentiated cells: approximately 50%, 20% and 17% of N, T and L types, respectively, and 12% of residual current, which is insensitive to classical blockers of low- and high-voltage-activated currents, with the exception of (omega-conotoxin GVIA. All current components displayed kinetics and pharmacological properties similar to neuronal ones. We also established a significant Ca2+ dependence of omega-conotoxin GVIA to inhibit N-type Ca2+ channels: 10 mM Ca2+ in bath solution reduced the toxin efficacy to block N channels three-fold. The residual component fitted the properties of Q-type Ca2+ channels: it was sensitive to (omega-conotoxin GVIA and very similar to the T-type channel with respect to its kinetics; however, the threshold of its activation was closer to the high-voltage-activated component (- 40 mV). Our results show the functional diversity of Ca2+ channels and demonstrate, for the first time, that presumably the Q type of an alpha1A family, which has biophysical and pharmacological properties distinct from the previously described T, L and N types in these cells, is co-expressed in NG108-15 cells.

Whole-cell and single-channel analysis of P-type calcium currents in cerebellar Purkinje cells of leaner mutant mice

The leaner (tgla) mutation in mice results in severe ataxia and an overt neurodegeneration of the cerebellum. Positional cloning has revealed that the tgla mutation occurs in a gene encoding the voltage-activated calcium channel alpha1A subunit. The alpha1A subunit is highly expressed in the cerebellum and is thought to be the pore-forming subunit of P- and Q-type calcium channels. In this study we used both whole-cell and single-channel patch-clamp recordings to examine the functional consequences of the tgla mutation on P-type calcium currents. High-voltage-activated (HVA) calcium currents were recorded from acutely dissociated cerebellar Purkinje cells of homozygous leaner (tgla/tgla) and age-matched wild-type (+/+) mice. In whole cell recordings, we observed a marked reduction of peak current density in tgla/tgla Purkinje cells (-35.0 +/- 1.8 pA/pF) relative to that in +/+ (-103.1 +/- 5.9 pA/pF). The reduced whole-cell current in tgla/tgla cells was accompanied by little to no alteration in the voltage dependence of channel gating. In both genotypes, HVA currents were predominantly of the omega-agatoxin-IVA-sensitive P-type. Cell-attached patch-clamp recordings revealed no differences in single-channel conductance between the two genotypes and confirmed the presence of three distinct conductance levels (9, 13-14, and 17-18 pS) in cerebellar Purkinje cells. Analysis of patch open-probability (NPo) revealed a threefold reduction in the open-probability of channels in tgla/tgla patches (0.04 +/- 0.01) relative to that in +/+ (0.13 +/- 0.02), which may account for the reduced whole-cell current in tgla/tgla Purkinje cells. These results suggest that the tgla mutation can alter native P-type calcium channels at the single-channel level and that these alterations may contribute to the neuropathology of the leaner phenotype.

Modulation of calcium current in arteriolar smooth muscle by alphav beta3 and alpha5 beta1 integrin ligands

Vasoactive effects of soluble matrix proteins and integrin-binding peptides on arterioles are mediated by alphav beta3 and alpha5 beta1 integrins. To examine the underlying mechanisms, we measured L-type Ca2+ channel current in arteriolar smooth muscle cells in response to integrin ligands. Whole-cell, inward Ba2+ currents were inhibited after application of soluble cyclic RGD peptide, vitronectin (VN), fibronectin (FN), either of two anti-beta3 integrin antibodies, or monovalent beta3 antibody. With VN or beta3 antibody coated onto microbeads and presented as an insoluble ligand, current was also inhibited. In contrast, beads coated with FN or alpha5 antibody produced significant enhancement of current after bead attachment. Soluble alpha5 antibody had no effect on current but blocked the increase in current evoked by FN-coated beads and enhanced current when applied in combination with an appropriate IgG. The data suggest that alphavbeta3 and alpha5 beta1 integrins are differentially linked through intracellular signaling pathways to the L-type Ca2+ channel and thereby alter control of Ca2+ influx in vascular smooth muscle. This would account for the vasoactive effects of integrin ligands on arterioles and provide a potential mechanism for wound recognition during tissue injury.