Characterization of Ca2+ channel currents in cultured rat cerebellar granule neurones

Regulation of Presynaptic Calcium Channels

Voltage-gated calcium channels (VGCCs), especially Cav2.1 and Cav2.2, are the major mediators of Ca2+ influx at the presynaptic membrane in response to neuron excitation, thereby exerting a predominant control on synaptic transmission. To guarantee the timely and precise release of neurotransmitters at synapses, the activity of presynaptic VGCCs is tightly regulated by a variety of factors, including auxiliary subunits, membrane potential, G protein-coupled receptors (GPCRs), calmodulin (CaM), Ca2+-binding proteins (CaBP), protein kinases, various interacting proteins, alternative splicing events, and genetic variations.

VNUT and VMAT2 segregate within sympathetic varicosities and localize near preferred Cav2 isoforms in the rat tail artery

ATP and norepinephrine (NE) are coreleased from peripheral sympathetic nerve terminals. Whether they are stored in the same vesicles has been debated for decades. Preferential dependence of NE or ATP release on Ca²⁺ influx through specific voltage-gated Ca²⁺ channel (Cav2) isoforms suggests that NE and ATP are stored in separate vesicle pools, but simultaneous imaging of NE and ATP containing vesicles within single varicosities has not been reported. We conducted an immunohistochemical study of vesicular monoamine transporter 2 (VMAT2/SLC18A2) and vesicular nucleotide translocase (VNUT/SLC17A9) as markers of vesicles containing NE and ATP in sympathetic nerves of the rat tail artery. A large fraction of varicosities exhibited neighboring, rather than overlapping, VNUT and VMAT2 fluorescent puncta. VMAT2, but not VNUT, colocalized with synaptotagmin 1. Cav2.1, Cav2.2, and Cav2.3 are expressed in nerves in the tunica adventitia. VMAT2 preferentially localized adjacent to Cav2.2 and Cav2.3 rather than Cav2.1. VNUT preferentially localized adjacent to Cav2.3 > Cav2.2 >> Cav2.1. With the use of wire myography, inhibition of field-stimulated vasoconstriction with the Cav2.3 blocker SNX-482 (0.25 µM) mimicked the effects of the P2X inhibitor suramin (100 µM) rather than the α-adrenergic inhibitor phentolamine (10 µM). Variable sensitivity to SNX-482 and suramin between animals closely correlated with Cav2.3 staining. We concluded that a majority of ATP and NE stores localize to separate vesicle pools that use different synaptotagmin isoforms and that localize near different Cav2 isoforms to mediate vesicle release. Cav2.3 appears to play a previously unrecognized role in mediating ATP release in the rat tail artery. NEW & NOTEWORTHY Immunofluorescence imaging of vesicular nucleotide translocase and vesicular monoamine transporter 2 in rat tail arteries revealed that ATP and norepinephrine, classical cotransmitters, localize to well-segregated vesicle pools. Furthermore, vesicular nucleotide translocase and vesicular monoamine transporter 2 exhibit preferential localization with specific Cav2 isoforms. These novel observations address long-standing debates regarding the mechanism(s) of sympathetic neurotransmitter corelease.

Voltage-gated calcium channels: their discovery, function and importance as drug targets

This review will first describe the importance of Ca²⁺ entry for function of excitable cells, and the subsequent discovery of voltage-activated calcium conductances in these cells. This finding was rapidly followed by the identification of multiple subtypes of calcium conductance in different tissues. These were initially termed low- and high-voltage activated currents, but were then further subdivided into L-, N-, PQ-, R- and T-type calcium currents on the basis of differing pharmacology, voltage-dependent and kinetic properties, and single channel conductance. Purification of skeletal muscle calcium channels allowed the molecular identification of the pore-forming and auxiliary α₂δ, β and ϒ subunits present in these calcium channel complexes. These advances then led to the cloning of the different subunits, which permitted molecular characterisation, to match the cloned channels with physiological function. Studies with knockout and other mutant mice then allowed further investigation of physiological and pathophysiological roles of calcium channels. In terms of pharmacology, cardiovascular L-type channels are targets for the widely used antihypertensive 1,4-dihydropyridines and other calcium channel blockers, N-type channels are a drug target in pain, and α₂δ-1 is the therapeutic target of the gabapentinoid drugs, used in neuropathic pain. Recent structural advances have allowed a deeper understanding of Ca²⁺ permeation through the channel pore and the structure of both the pore-forming and auxiliary subunits. Voltage-gated calcium channels are subject to multiple pathways of modulation by G-protein and second messenger regulation. Furthermore, their trafficking pathways, subcellular localisation and functional specificity are the subjects of active investigation.

Postnatal loss of P/Q-type channels confined to rhombic-lip-derived neurons alters synaptic transmission at the parallel fiber to purkinje cell synapse and replicates genomic Cacna1a mutation phenotype of ataxia and seizures in mice

Ataxia, episodic dyskinesia, and thalamocortical seizures are associated with an inherited loss of P/Q-type voltage-gated Ca(2+) channel function. P/Q-type channels are widely expressed throughout the neuraxis, obscuring identification of the critical networks underlying these complex neurological disorders. We showed recently that the conditional postnatal loss of P/Q-type channels in cerebellar Purkinje cells (PCs) in mice (purky) leads to these aberrant phenotypes, suggesting that intrinsic alteration in PC output is a sufficient pathogenic factor for disease initiation. The question arises whether P/Q-type channel deletion confined to a single upstream cerebellar synapse might induce the pathophysiological abnormality of genomically inherited P/Q-type channel disorders. PCs integrate two excitatory inputs, climbing fibers from inferior olive and parallel fibers (PFs) from granule cells (GCs) that receive mossy fiber (MF) input derived from precerebellar nuclei. In this study, we introduce a new mouse model with a selective knock-out of P/Q-type channels in rhombic-lip-derived neurons including the PF and MF pathways (quirky). We found that in quirky mice, PF-PC synaptic transmission is reduced during low-frequency stimulation. Using focal light stimulation of GCs that express optogenetic light-sensitive channels, channelrhodopsin-2, we found that modulation of PC firing via GC input is reduced in quirky mice. Phenotypic analysis revealed that quirky mice display ataxia, dyskinesia, and absence epilepsy. These results suggest that developmental alteration of patterned input confined to only one of the main afferent cerebellar excitatory synaptic pathways has a significant role in generating the neurological phenotype associated with the global genomic loss of P/Q-type channel function.

Transient reversal of the sodium/calcium exchanger boosts presynaptic calcium and synaptic transmission at a cerebellar synapse

The sodium/calcium exchanger (NCX) is a widespread transporter that exchanges sodium and calcium ions across excitable membranes. Normally, NCX mainly operates in its "forward" mode, harnessing the electrochemical gradient of sodium ions to expel calcium. During membrane depolarization or elevated internal sodium levels, NCX can instead switch the direction of net flux to expel sodium and allow calcium entry. Such "reverse"-mode NCX operation is frequently implicated during pathological or artificially extended periods of depolarization, not during normal activity. We have used fast calcium imaging, mathematical simulation, and whole cell electrophysiology to study the role of NCX at the parallel fiber-to-Purkinje neuron synapse in the mouse cerebellum. We show that nontraditional, reverse-mode NCX activity boosts the amplitude and duration of parallel fiber calcium transients during short bursts of high-frequency action potentials typical of their behavior in vivo. Simulations, supported by experimental manipulations, showed that accumulation of intracellular sodium drove NCX into reverse mode. This mechanism fueled additional calcium influx into the parallel fibers that promoted synaptic transmission to Purkinje neurons for up to 400 ms after the burst. Thus we provide the first functional demonstration of transient and fast NCX-mediated calcium entry at a major central synapse. This unexpected contribution from reverse-mode NCX appears critical for shaping presynaptic calcium dynamics and transiently boosting synaptic transmission, and is likely to optimize the accuracy of cerebellar information transfer.

Deletion of Cav2.1(alpha1(A)) subunit of Ca2+-channels impairs synaptic GABA and glutamate release in the mouse cerebellar cortex in cultured slices

Deletion of both alleles of the P/Q-type Ca(2+)-channel Ca(v)2.1(alpha(1A)) subunit gene in mouse leads to severe ataxia and early death. Using cerebellar slices obtained from 10 to 15 postnatal days mice and cultured for at least 3 weeks in vitro, we have analysed the synaptic alterations produced by genetically ablating the P/Q-type Ca(2+)-channels, and compared them with the effect of pharmacological inhibition of the P/Q- or N-type channels on wild-type littermate mice. Analysis of spontaneous synaptic currents recorded in Purkinje cells (PCs) indicated that the P/Q-type channels play a prominent role at the inhibitory synapses afferent onto the PCs, with the effect of deleting Ca(v)2.1(alpha(1A)) partially compensated. At the granule cell (GC) to PC synapses, both N- and P/Q-type Ca(2+)-channels were found playing a role in glutamate exocytosis, but with no significant phenotypic compensation of the Ca(v)2.1(alpha(1A)) deletion. We also found that the P/Q- but not N-type Ca(2+)-channel is indispensable at the autaptic contacts between PCs. Tuning of the GC activity implicates both synaptic and sustained extrasynaptic gamma-aminobutyric acid (GABA) release, only the former was greatly impaired in the absence of P/Q-type Ca(2+)-channels. Overall, our data demonstrate that both P/Q- and N-type Ca(2+)-channels play a role in glutamate release, while the P/Q-type is essential in GABA exocytosis in the cerebellum. Contrary to the other regions of the CNS, the effect of deleting the Ca(v)2.1(alpha(1A)) subunit is partially or not compensated at the inhibitory synapses. This may explain why cerebellar ataxia is observed at the mice lacking functional P/Q-type channels.

Arachidonic acid induces both Na+ and Ca2+ entry resulting in apoptosis

Marked accumulation of arachidonic acid (AA) and intracellular Ca2+ and Na+ overloads are seen during brain ischemia. In this study, we show that, in neurons, AA induces cytosolic Na+ ([Na+](cyt)) and Ca2+ ([Ca2+](cyt)) overload via a non-selective cation conductance (NSCC), resulting in mitochondrial [Na+](m) and [Ca2+](m) overload. Another two types of free fatty acids, including oleic acid and eicosapentaenoic acid, induced a smaller increase in the [Ca2+](i) and [Na+](i). RU360, a selective inhibitor of the mitochondrial Ca2+ uniporter, inhibited the AA-induced [Ca2+](m) and [Na+](m) overload, but not the [Ca2+](cyt) and [Na+](cyt) overload. The [Na+](m) overload was also markedly inhibited by either Ca2+-free medium or CGP3715, a selective inhibitor of the mitochondrial Na+(cyt)-Ca2+(m) exchanger. Moreover, RU360, Ca2+-free medium, Na+-free medium, or cyclosporin A (CsA) largely prevented AA-induced opening of the mitochondrial permeability transition pore, cytochrome c release, and caspase 3-dependent neuronal apoptosis. Importantly, Na+-ionophore/Ca2+-free medium, which induced [Na+](m) overload, but not [Ca2+](m) overload, also caused cyclosporin A-sensitive mitochondrial permeability transition pore opening, resulting in caspase 3-dependent apoptosis, indicating that [Na+](m) overload per se induced apoptosis. Our results therefore suggest that AA-induced [Na+](m) overload, acting via activation of the NSCC, is an important upstream signal in the mitochondrial-mediated apoptotic pathway. The NSCC may therefore act as a potential neuronal death pore which is activated by AA accumulation under pathological conditions.

Binomial parameters differ across neocortical layers and with different classes of connections in adult rat and cat neocortex

Binomial model-based analysis compared excitatory connections involving different classes of neurons in different neocortical layers. Single-sweep excitatory postsynaptic potentials (EPSPs) from dual intracellular recordings in adult cat and rat slices were measured. For data subsets corresponding to first EPSPs exhibiting different degrees of posttetanic potentiation and second, third etc. EPSPs in trains at different interspike intervals, coefficient of variation (CV), transmission failure rates (F), variance (V), and V/M were plotted against mean EPSP amplitude (M). Curves derived from binomial models in which subsets varied only in p (release probability) were fit and parameters q (quantal amplitude), and n (number of release sites) were estimated. Estimates for q and n were similar for control subsets and subsets recorded during Ca(2+) channel blockade, only p varied. Estimates from the four methods were powerfully correlated, but when CV, F, V, and V/M were plotted against M, different types of connections occupied different regions of parameter space. Comparisons of linear fits to V/M against M plots and of parameter estimates indicated that these differences were significant. Connections between pyramids in different layers and inputs to different cell classes in the same layer differed markedly. Monte Carlo simulations of more complex models subjected to simple binomial model-based analysis confirmed the significance of these differences. Binomial models, either simple, in which p and q are identical at all terminals involved, or more complex, in which they differ, adequately describe many neocortical connections, but each class uses different combinations of n, mean p, and mean q.

Maturation of rat cerebellar Purkinje cells reveals an atypical Ca2+ channel current that is inhibited by omega-agatoxin IVA and the dihydropyridine (-)-(S)-Bay K8644

To determine if the properties of Ca2+ channels in cerebellar Purkinje cells change during postnatal development, we recorded Ca2+ channel currents from Purkinje cells in cerebellar slices of mature (postnatal days (P) 40-50) and immature (P13-20) rats. We found that at P40-50, the somatic Ca2+ channel current was inhibited by omega-agatoxin IVA at concentrations selective for P-type Ca2+ channels (approximately 85%; IC50, <1 nM) and by the dihydropyridine (-)-(S)-Bay K8644 (approximately 70%; IC50, approximately 40 nM). (-)-(S)-Bay K8644 is known to activate L-type Ca2+ channels, but the decrease in current was not secondary to the activation of L-type channels because inhibition by (-)-(S)-Bay K8644 persisted in the presence of the L-type channel blocker (R,S)-nimodipine. By contrast, at P13-20, the current was inhibited by omega-agatoxin IVA (approximately 86%; IC50, approximately 1 nM) and a minor component was inhibited by (R,S)-nimodipine (approximately 8%). The dihydropyridine (-)-(S)-Bay K8644 had no clear effect when applied alone, but in the presence of (R,S)-nimodipine it reduced the current (approximately 40%), suggesting that activation of L-type channels by (-)-(S)-Bay K8644 masks its inhibition of non-L-type channels. Our findings indicate that Purkinje neurons express a previously unrecognized type of Ca2+ channel that is inhibited by omega-agatoxin IVA, like prototypical P-type channels, and by (-)-(S)-Bay K8644, unlike classical P-type or L-type channels. During maturation, there is a decrease in the size of the L-type current and an increase in the size of the atypical Ca2+ channel current. These changes may contribute to the maturation of the electrical properties of Purkinje cells.

High-voltage-activated Ca2+ currents show similar patterns of expression in stellate and pyramidal cells from rat entorhinal cortex layer II

High-voltage-activated (HVA) Ca2+ currents were studied in acutely isolated neurons from rat entorhinal cortex (EC) layer II. Stellate and pyramidal cells, the two main neuronal types of this structure, were visually identified based on morphological criteria. HVA currents were recorded by applying the whole-cell, patch-clamp technique, using 5-mM Ba2+ as the charge carrier. In both neuronal types, the amplitude of total HVA Ba2+ currents (IBas) showed a significant tendency to increase with postnatal age in the time window considered [postnatal day 15 (P15) to P28-29]. At P20-P29, when IBa expression reached stable levels, IBa density per unit of membrane area was not different in stellate versus pyramidal cells. The same was also observed when Ca2+, instead of Ba2+, was used as the charge carrier. The pharmacological current subtypes composing total HVA currents were characterized using selective blockers. Again, no significant differences were found between stellate and pyramidal cells with respect to the total-current fractions attributable to specific pharmacological Ca2+ channel subtypes. In both cell types, approximately 52-55% of total IBas was abolished by the L-type channel blocker, nifedipine (10 microM), approximately 23-30% by the N-type channel blocker, omega-conotoxin GVIA (1 microM), approximately 22-24% by the P/Q-type channel blocker, omega-agatoxin IVA (100 nM), and approximately 11-13% remained unblocked (R-type current) after simultaneous application of L-, N-, and P/Q-type channel blockers. The Cav 2.3 (alpha1E) channel blocker, SNX-482 (100 nM), abolished approximately 57-62% of total R-type current. We conclude that HVA Ca2+ currents are expressed according to similar patterns in the somata and proximal dendrites of stellate and pyramidal cells of rat EC layer II.

Amyloid peptides mediate hypoxic increase of L-type Ca2+ channels in central neurones

Prolonged hypoxia, encountered in individuals suffering from various cardiorespiratory diseases, enhances the likelihood of subsequently developing Alzheimer's disease (AD). However, the underlying mechanisms are unknown, as are the mechanisms of neurodegeneration of amyloid beta peptides (AbetaPs), although the latter involves disruption of Ca2+ homeostasis. Here, immunohistochemistry demonstrated that hypoxia increased production of AbetaPs, an effect which was prevented by inhibition of either beta or gamma secretase, the enzymes required for liberation of AbetaP from its precursor protein. Whole-cell patch clamp recordings showed that hypoxia selectively increased functional expression of L-type Ca2+ channels. This was prevented by inhibition of either beta or gamma secretase, indicating that hypoxic channel up-regulation is dependent upon AbetaP formation. Our results indicate for the first time that hypoxia promotes AbetaP formation in central neurons, and show that this leads to abnormally high selective expression of L-type Ca2+ channels whose blockade has previously been shown to be neuroprotective in AD models. These findings provide a cellular basis for understanding the increased incidence of AD following prolonged hypoxia.

Differential targeting and functional specialization of sodium channels in cultured cerebellar granule cells

The ion channel dynamics that underlie the complex firing patterns of cerebellar granule (CG) cells are still largely unknown. Here, we have characterized the subcellular localization and functional properties of Na+ channels that regulate the excitability of CG cells in culture. As evidenced by RT-PCR and immunocytochemical analysis, morphologically differentiated CG cells expressed Nav1.2 and Nav1.6, though both subunits appeared to be differentially regulated. Nav1.2 was localized at most axon initial segments (AIS) of CG cells from 8 days in vitro DIV 8 to DIV 15. At DIV 8, Nav1.6 was found uniformly throughout somata, dendrites and axons with occasional clustering in a subset of AIS. Accumulation of Nav1.6 at most AIS was evident by DIV 13-14, suggesting it is developmentally regulated at AIS. The specific contribution of these differentially distributed Na+ channels has been assessed using a combination of methods that allowed discrimination between functionally compartmentalized Na+ currents. In agreement with immunolocalization, we found that fast activating-fully inactivating Na+ currents predominate at the AIS membrane and in the somatic plasma membrane.

Alteration of cytosolic free calcium homeostasis by SIN-1: high sensitivity of L-type Ca2+ channels to extracellular oxidative/nitrosative stress in cerebellar granule cells

Exposure of cerebellar granule neurones in 25 mm KCl HEPES-containing Locke's buffer (pH 7.4) to 50-100 microm SIN-1 during 2 h decreased the steady-state free cytosolic Ca2+ concentration ([Ca2+]i) from 168 +/- 33 nm to 60 +/- 10 nm, whereas exposure to > or = 0.3 mm SIN-1 produced biphasic kinetics: (i) decrease of [Ca2+]i during the first 30 min, reaching a limiting value of 75 +/- 10 nm (due to inactivation of L-type Ca2+ channels) and (ii) a delayed increase of [Ca2+]i at longer exposures, which correlated with SIN-1-induced necrotic cell death. Both effects of SIN-1 on [Ca2+]i are blocked by superoxide dismutase plus catalase and by Mn(III)tetrakis(4-benzoic acid)porphyrin chloride. Supplementation of Locke's buffer with catalase before addition of 0.5-1 mm SIN-1 had no effect on the decrease of [Ca2+]i but further delayed and attenuated the increase of [Ca2+]i observed after 60-120 min exposure to SIN-1 and also protected against SIN-1-induced necrotic cell death. alpha-Tocopherol, the potent NMDA receptor antagonist (+)-MK-801 and the N- and P-type Ca2+ channels blocker omega-conotoxin MVIIC had no effect on the alterations of [Ca2+]i upon exposure to SIN-1. However, inhibition of the plasma membrane Ca2+ ATPase can account for the increase of [Ca2+]i observed after 60-120 min exposure to 0.5-1 mm SIN-1. It is concluded that L-type Ca2+ channels are a primary target of SIN-1-induced extracellular nitrosative/oxidative stress, being inactivated by chronic exposure to fluxes of peroxynitrite of 0.5-1 microm/min, while higher concentrations of peroxynitrite and hydrogen peroxide are required for the inhibition of the plasma membrane Ca2+ ATPase and induction of necrotic cell death, respectively.

Mechanisms underlying cannabinoid inhibition of presynaptic Ca2+ influx at parallel fibre synapses of the rat cerebellum

Activation of CB1 cannabinoid receptors in the cerebellum acutely depresses excitatory synaptic transmission at parallel fibre-Purkinje cell synapses by decreasing the probability of glutamate release. This depression involves the activation of presynaptic 4-aminopyridine-sensitive K(+) channels by CB1 receptors, which in turn inhibits presynaptic Ca(2+) influx controlling glutamate release at these synapses. Using rat cerebellar frontal slices and fluorometric measures of presynaptic Ca(2+) influx evoked by stimulation of parallel fibres with the fluorescent dye fluo-4FF, we tested whether the CB1 receptor-mediated inhibition of this influx also involves a direct inhibition of presynaptic voltage-gated calcium channels. Since various physiological effects of CB1 receptors appear to be mediated through the activation of PTX-sensitive proteins, including inhibition of adenylate cyclases, activation of mitogen-activated protein kinases (MAPK) and activation of G protein-gated inwardly rectifying K(+) channels, we also studied the potential involvement of these intracellular signal transduction pathways in the cannabinoid-mediated depression of presynaptic Ca(2+) influx. The present study demonstrates that the molecular mechanisms underlying the CB1 inhibitory effect involve the activation of the PTX-sensitive G(i)/G(o) subclass of G proteins, independently of any direct effect on presynaptic Ca(2+) channels (N, P/Q and R (SNX-482-sensitive) types) or on adenylate cyclase or MAPK activity, but do require the activation of G protein-gated inwardly rectifying (Ba(2+)- and tertiapin Q-sensitive) K(+) channels, in addition to 4-aminopyridine-sensitive K(+) channels.

Human neuronal stargazin-like proteins, gamma2, gamma3 and gamma4; an investigation of their specific localization in human brain and their influence on CaV2.1 voltage-dependent calcium channels expressed in Xenopus oocytes

BACKGROUND

Stargazin (gamma2) and the closely related gamma3, and gamma4 transmembrane proteins are part of a family of proteins that may act as both neuronal voltage-dependent calcium channel (VDCC) gamma subunits and transmembrane alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproponinc (AMPA) receptor regulatory proteins (TARPs). In this investigation, we examined the distribution patterns of the stargazin-like proteins gamma2, gamma3, and gamma4 in the human central nervous system (CNS). In addition, we investigated whether human gamma2 or gamma4 could modulate the electrophysiological properties of a neuronal VDCC complex transiently expressed in Xenopus oocytes.

RESULTS

The mRNA encoding human gamma2 is highly expressed in cerebellum, cerebral cortex, hippocampus and thalamus, whereas gamma3 is abundant in cerebral cortex and amygdala and gamma4 in the basal ganglia. Immunohistochemical analysis of the cerebellum determined that both gamma2 and gamma4 are present in the molecular layer, particularly in Purkinje cell bodies and dendrites, but have an inverse expression pattern to one another in the dentate cerebellar nucleus. They are also detected in the interneurons of the granule cell layer though only gamma2 is clearly detected in granule cells. The hippocampus stains for gamma2 and gamma4 throughout the layers of the every CA region and the dentate gyrus, whilst gamma3 appears to be localized particularly to the pyramidal and granule cell bodies. When co-expressed in Xenopus oocytes with a CaV2.1/beta4 VDCC complex, either in the absence or presence of an alpha2delta2 subunit, neither gamma2 nor gamma4 significantly modulated the VDCC peak current amplitude, voltage-dependence of activation or voltage-dependence of steady-state inactivation.

CONCLUSION

The human gamma2, gamma3 and gamma4 stargazin-like proteins are detected only in the CNS and display differential distributions among brain regions and several cell types in found in the cerebellum and hippocampus. These distribution patterns closely resemble those reported by other laboratories for the rodent orthologues of each protein. Whilst the fact that neither gamma2 nor gamma4 modulated the properties of a VDCC complex with which they could associate in vivo in Purkinje cells adds weight to the hypothesis that the principal role of these proteins is not as auxiliary subunits of VDCCs, it does not exclude the possibility that they play another role in VDCC function.

In depolarized and glucose-deprived neurons, Na+ influx reverses plasmalemmal K+-dependent and K+-independent Na+/Ca2+ exchangers and contributes to NMDA excitotoxicity

Cerebellar granule cells (CGCs) express K+-dependent (NCKX) and K+-independent (NCX) plasmalemmal Na+/Ca2+ exchangers which, under plasma membrane-depolarizing conditions and high cytosolic [Na+], may reverse and mediate potentially toxic Ca2+ influx. To examine this possibility, we inhibited NCX or NCKX with KB-R7943 or K+-free medium, respectively, and studied how gramicidin affects cytosolic [Ca2+] and 45Ca2+ accumulation. Gramicidin forms pores permeable to alkali cations but not Ca2+. Therefore, gramicidin-induced Ca2+ influx is indirect; it results from fluxes of monovalent cations. In the presence of Na+, but not Li+ or Cs+, gramicidin induced Ca2+ influx that was inhibited by simultaneous application of KB-R7943 and K+-free medium. The data indicate that gramicidin-induced Na+ influx reverses NCX and NCKX. To test the role of NCX and/or NCKX in excitotoxicity, we studied how NMDA affects the viability of glucose-deprived and depolarized CGCs. To assure depolarization of the plasma membrane, we inhibited Na+,K+-ATPase with ouabain. Although inhibition of NCX or NCKX reversal failed to significantly limit 45Ca2+ accumulation and excitotoxicity, simultaneously inhibiting NCX and NCKX reversal was neuroprotective and significantly decreased NMDA-induced 45Ca2+ accumulation. Our data suggest that NMDA-induced Na+ influx reverses NCX and NCKX and leads to the death of depolarized and glucose-deprived neurons.

Intrinsic membrane properties and synaptic inputs regulating the firing activity of the dopamine neurons

Dopamine (DA) neurones of the ventral mesencephalon are involved in the control of reward related behaviour, cognitive functions and motor performances, and provide a critical site of action for major categories of neuropsychiatric drugs, such as antipsychotic agents, dependence producing drugs and anti-Parkinson medication. The midbrain DA neurones are mainly located in the substantia nigra pars compacta (SNPC) and the ventral tegmental area (VTA). Intrinsic membrane properties regulate the activity of these neurones. In fact, they possess several conductances that allow them to fire in a slow pacemaker-like mode. The internal set of membrane currents interact with afferent synaptic inputs which, especially in in vivo conditions, contribute to accelerate or decelerate the firing activity of the cells in accordance with the necessity to optimise the release of dopamine in the terminal fields. In particular, discrete excitatory and inhibitory inputs transform the firing from a low regular into a bursting pattern. The bursting activity promotes dopamine release being very important in cognition and motor performances. In the present paper we review electrophysiological data regarding the role of glutamatergic and cholinergic and GABAergic afferent inputs in regulating the midbrain DAergic neuronal activity.

Calcium channels--basic aspects of their structure, function and gene encoding; anesthetic action on the channels--a review

PURPOSE

To review recent findings concerning Ca(2+) channel subtype/structure/function from electrophysiological and molecular biological studies and to explain Ca(2+) channel diseases and the actions of anesthetics on Ca(2+) channels.

SOURCE

The information was obtained from articles published recently and from our published work.

PRINCIPAL FINDINGS

Voltage-dependent Ca(2+) channels serve as one of the important mechanisms for Ca(2+) influx into the cells, enabling the regulation of intracellular concentration of free Ca(2+). Recent advances both in electrophysiology and in molecular biology have made it possible to observe channel activity directly and to investigate channel functions at molecular levels. The Ca(2+) channel can be divided into subtypes according to electrophysiological characteristics, and each subtype has its own gene. The L-type Ca(2+) channel is the target of a large number of clinically important drugs, especially dihydropyridines, and binding sites of Ca(2+) antagonists have been clarified. The effects of various kinds of anesthetics in a variety of cell types have been demonstrated, and some clinical effects of anesthetics can be explained by the effects on Ca(2+) channels. It has recently become apparent that some hereditary diseases such as hypokalemic periodic paralysis result from calcium channelopathies.

CONCLUSION

Recent advances both in electrophysiology and in molecular biology have made it possible to clarify the Ca(2+) channel structures, functions, genes, and the anesthetic actions on the channels in detail. The effects of anesthetics on the Ca(2+) channels either of patients with hereditary channelopathies or using gene mutation techniques are left to be discovered.

Contribution of L-type Ca(2+) channels to evoked transmitter release in cultured Xenopus nerve-muscle synapses

1. Simultaneous pre- and postsynaptic patch recordings were obtained from the varicosity synapses formed by Xenopus motoneurons on muscle cells in embryonic cultures, in order to elucidate the contribution of N- and L-type Ca(2+) channels to the varicosity Ca(2+) current (I(Ca)) and evoked transmitter release. 2. Although N-type channels are predominant in the varicosities and generally thought to be responsible for all evoked release, in most synapses a fraction of I(Ca) and release could be reversibly blocked by the L-type channel antagonist nifedipine, and enhanced by the agonist Bay K8644. Up to 50 % (mean, 21 %) of the I(Ca) evoked by a voltage clamp waveform mimicking a normal presynaptic action potential (APWF) is composed of L-type current. 3. Surprisingly, the nifedipine-sensitive (L) channels activated more rapidly (time-constant, 0.46 ms at +30 mV) than the nifedipine-insensitive (N) channels (time constant, 1.42 ms). Thus the L-type current would play a disproportionate role in the I(Ca) linked to a normal action potential. 4. The relationship between I(Ca) and release was the same for nifedipine-sensitive and -resistant components. The N- and L-components of I(Ca) are thus equally potent in evoking release. This may represent an immature stage before N-type channels become predominant. 5. Replacing Ca(2+) in the medium with Ba(2+) strongly enhanced the L-type component, suggesting that L-type channels may be inactivated at Ca(2+) levels close to those at rest. 6. We speculate that populations of L-type channels in different parts of the neuron may be recruited or inactivated by fluctuations of the cytosolic Ca(2+) concentration within the physiological range.

Venom from Anemesia species of spider modulates high voltage-activated Ca(2+) currents from rat cultured sensory neurones and excitatory post synaptic currents from rat hippocampal slices

The actions of crude venom from Anemesia species of spider were investigated in cultured dorsal root ganglion neurones from neonatal rats and hippocampal slices. Using mass spectrometry (MALDI-TOF MS), 10-12 distinct peptides with masses between about 3 and 10kDa were identified in the crude spider venom. At a concentration of 5 microg/ml crude Anemesia venom transiently enhanced the mean peak whole cell voltage-activated Ca(2+) current in a voltage-dependent manner and potentiated transient increases in intracellular Ca(2+) triggered by 30mM KCI as measured using Fura-2 fluorescence imaging. Additionally, 5-8 microg/ml Anemesia venom increased the amplitude of glutamatergic excitatory postsynaptic currents evoked in hippocampal slices. Omega-Conotoxin GVIA (1 microM) prevented the increase in voltage-activated Ca(2+) currents produced by Anemesia venom. This attenuation occurred when the cone shell toxin was applied before or after the spider venom. Anemesia venom (5 microg/ml) created no significant change in evoked action potentials but produced modest but significant inhibition of voltage-activated K(+) currents. At a concentration of 50 microg/ml Anemesia venom only produced reversible inhibitory effects, decreasing voltage-activated Ca(2+) currents. However, no significant effects on Ca(2+) currents were observed with a concentration of 0.5 microg/ml. The toxin(s) in the venom that enhanced Ca(2+) influx into sensory neurones was heat-sensitive and was made inactive by boiling or repetitive freeze-thawing. Boiled venom (5 microg/ml) produced significant inhibition of voltage-activated Ca(2+) currents and freeze-thawed venom inhibited Ca(2+) transients measured using Fura-2 fluorescence. Our data suggest that crude Anemesia venom contains components, which increased neuronal excitability and neurotransmission, at least in part this was mediated by enhancing Ca(2+) influx through N-type voltage-activated Ca(2+) channels.

Ducky mouse phenotype of epilepsy and ataxia is associated with mutations in the Cacna2d2 gene and decreased calcium channel current in cerebellar Purkinje cells

The mouse mutant ducky, a model for absence epilepsy, is characterized by spike-wave seizures and ataxia. The ducky gene was mapped previously to distal mouse chromosome 9. High-resolution genetic and physical mapping has resulted in the identification of the Cacna2d2 gene encoding the alpha2delta2 voltage-dependent calcium channel subunit. Mutations in Cacna2d2 were found to underlie the ducky phenotype in the original ducky (du) strain and in a newly identified strain (du(2J)). Both mutations are predicted to result in loss of the full-length alpha2delta2 protein. Functional analysis shows that the alpha2delta2 subunit increases the maximum conductance of the alpha1A/beta4 channel combination when coexpressed in vitro in Xenopus oocytes. The Ca(2+) channel current in acutely dissociated du/du cerebellar Purkinje cells was reduced, with no change in single-channel conductance. In contrast, no effect on Ca(2+) channel current was seen in cerebellar granule cells, results consistent with the high level of expression of the Cacna2d2 gene in Purkinje, but not granule, neurons. Our observations document the first mammalian alpha2delta mutation and complete the association of each of the major classes of voltage-dependent Ca(2+) channel subunits with a phenotype of ataxia and epilepsy in the mouse.

Ducky mouse phenotype of epilepsy and ataxia is associated with mutations in the Cacna2d2 gene and decreased calcium channel current in cerebellar Purkinje cells

The mouse mutant ducky, a model for absence epilepsy, is characterized by spike-wave seizures and ataxia. The ducky gene was mapped previously to distal mouse chromosome 9. High-resolution genetic and physical mapping has resulted in the identification of the Cacna2d2 gene encoding the alpha2delta2 voltage-dependent calcium channel subunit. Mutations in Cacna2d2 were found to underlie the ducky phenotype in the original ducky (du) strain and in a newly identified strain (du(2J)). Both mutations are predicted to result in loss of the full-length alpha2delta2 protein. Functional analysis shows that the alpha2delta2 subunit increases the maximum conductance of the alpha1A/beta4 channel combination when coexpressed in vitro in Xenopus oocytes. The Ca(2+) channel current in acutely dissociated du/du cerebellar Purkinje cells was reduced, with no change in single-channel conductance. In contrast, no effect on Ca(2+) channel current was seen in cerebellar granule cells, results consistent with the high level of expression of the Cacna2d2 gene in Purkinje, but not granule, neurons. Our observations document the first mammalian alpha2delta mutation and complete the association of each of the major classes of voltage-dependent Ca(2+) channel subunits with a phenotype of ataxia and epilepsy in the mouse.

Roles of high-voltage-activated calcium channel subtypes in a vertebrate spinal locomotor network

Lamprey spinal cord neurons possess N-, L-, and P/Q-type high-voltage-activated (HVA) calcium channels. We have analyzed the role of the different HVA calcium channels subtypes in the overall functioning of the spinal locomotor network by monitoring the influence of their specific agonists and antagonists on synaptic transmission and on N-methyl-D-aspartate (NMDA)-elicited fictive locomotion. The N-type calcium channel blocker omega-conotoxin GVIA (omega-CgTx) depressed synaptic transmission from excitatory and inhibitory interneurons. Blocking L-type and P/Q-type calcium channels with nimodipine and omega-agatoxin, respectively, did not affect synaptic transmission. Application of omega-CgTx initially decreased the frequency of the locomotor rhythm, increased the burst duration, and subsequently increased the coefficient of variation and disrupted the motor pattern. These effects were accompanied by a depression of the synaptic drive between neurons in the locomotor network. Blockade of L-type channels by nimodipine also decreased the frequency and increased the duration of the locomotor bursts. Conversely, potentiation of L-type channels increased the frequency of the locomotor activity and decreased the duration of the ventral root bursts. In contrast to blockade of N-type channels, blockade or potentiation of L-type calcium channels had no effect on the stability of the locomotor pattern. The P/Q-type calcium channel blocker omega-agatoxin IVA had little effect on the locomotor frequency or burst duration. The results indicate that rhythm generation in the spinal locomotor network of the lamprey relies on calcium influx through L-type and N-type calcium channels.

Methylmercury affects multiple subtypes of calcium channels in rat cerebellar granule cells

We tested the ability of methylmercury (MeHg) to block calcium channel current in cultures of neonatal cerebellar granule cells using whole-cell patch clamp techniques and Ba(2+) as charge carrier. Low micromolar concentrations of MeHg (0.25-1 microM) reduced the amplitude of whole cell Ba(2+) current in a concentration- and time-dependent fashion; however, this effect was not voltage-dependent and the current-voltage relationship was not altered. Increasing the stimulation frequency hastened the onset and increased the magnitude of block at both 0.25 and 0.5 microM MeHg but not at 1 microM. In the absence of stimulation, all concentrations of MeHg were able to decrease current amplitude. The ability of several Ca(2+) channel antagonists (omega-conotoxin GVIA, omega-conotoxin MVIIC, omega-agatoxin IVA, calcicludine, and nimodipine) to alter the MeHg-induced effect was tested in an effort to determine if MeHg targets a specific subtype of Ca(2+) channel. Each of the antagonists tested was able to decrease a portion of whole cell Ba(2+) current under control conditions. However, none were able to attenuate the MeHg-induced block of whole cell Ba(2+) current, suggesting either that the mechanism of MeHg-induced block involves sites other than those influenced specifically by Ca(2+) channel antagonists or that MeHg was able to "outcompete" these toxins for their binding sites. These results show that acute exposure to submicromolar concentrations of MeHg can block Ba(2+) currents carried through multiple Ca(2+) channel subtypes in primary cultures of cerebellar granule cells. However, it is unlikely that the presence of a specific Ca(2+) channel subtype is able to render granule cells more susceptible to the neurotoxicologic actions of MeHg.

Overlapping selectivity of neurotoxin and dihydropyridine calcium channel blockers in cerebellar granule neurones

Calcium (Ca(2+)) currents have been studied extensively in cerebellar granule neurones, but much of the whole-cell pharmacology is inconsistent. Ca(2+) channel currents were recorded from granule neurones to investigate whether the commonly used Ca(2+) channel blockers show overlapping selectivity. Using combinations of toxin channel blockers, 45% of the total current was shown to be carried by Ca(2+) channels susceptible to block by the combined, or cumulative application of, omega-agatoxin IVA, omega-conotoxin GVIA and omega-conotoxin MVIIC, thus representing P/Q- and N-type channel currents. However, sequential application of these toxins showed that substantial overlap occurred in the proportions of current sensitive to individual toxins. Application of the 1, 4-dihydropyridine nicardipine at 1 microM, a concentration reported to be selective for L-type channels, blocked 16% of the total current, without reducing the current sensitive to the toxins used. However, greater concentrations of nicardipine (>10 microM) blocked a proportion of the total current that could not be accounted for by L-type channels alone. These results demonstrate that a pharmacological approach based on the L, N, P/Q, and R classification does not adequately describe the Ca(2+) channel subtypes found in cerebellar granule neurones due to substantial cross-selectivity to the drugs and toxins used.

Inhibition of neuronal high-voltage activated calcium channels by the omega-phoneutria nigriventer Tx3-3 peptide toxin

We have investigated the effect of omega-PnTx3-3 (referred to in previous papers simply as Tx3-3), a peptide toxin from the venom of the spider Phoneutria nigriventer, on neuronal high-voltage activated (HVA) Ca(2+) channels, using whole-cell patch-clamp. omega-PnTx3-3 (120 nM) blocked 74+/-8% of the total HVA Ca(2+) currents of cerebellar granule neurones, without affecting the low-voltage activated (LVA) current. P/Q/R-type currents in cerebellar granule neurones, isolated using 4 microM nicardipine and 100 nM omega-conotoxin GVIA, were markedly (79+/-6%) inhibited by 60 nM omega-PnTx3-3. R-type currents, isolated either by additional application of 0.5-1 microM of omega-agatoxin IVA or by pre-incubation with 5 microM omega-conotoxin MVIIC were inhibited almost totally by 120 nM of omega-PnTx3-3. omega-PnTx3-3 reversibly altered the kinetics of the P/Q/R current, increasing the degree of inactivation that occurred during a 50 ms pulse from 20% to 40%. N-type currents, recorded from neuroblastoma N18 cells, were partially (34+/-2%) inhibited by 320 nM omega-PnTx3-3. L-type currents, recorded from GH3 cells, were partially (45+/-12%) inhibited by 80 nM omega-PnTx3-3. We conclude that omega-PnTx3-3 inhibits all known HVA Ca(2+) channels, and most effectively the P/Q- and R-type currents.

Voltage- and gamma-aminobutyric acid-activated membrane currents in the human medulloblastoma cell line MHH-MED-3

The whole-cell patch clamp technique was used to characterize voltage- and neurotransmitter-activated currents in the medulloblastoma cell line MHH-MED-3 and cells from tissue slices and primary cultures of two medulloblastoma biopsies. These preparations revealed similar electrophysiological properties. All tested cells displayed 4-aminopyridine-sensitive delayed rectifying K(+) currents, gamma-aminobutyric acid(A) receptor-mediated Cl(-) currents and most of them inward rectifier K(+) currents. Transient inward currents were mainly carried by low-voltage activated T-type Ca(2+) channels in MHH-MED-3 cells, and tetrodotoxin-sensitive Na(+) channels in cells from the primary culture. From these characteristics we conclude that medulloblastoma cells share physiological features with developing cerebellar granule cells at an immature stage.

The development and pharmacological characterization of calcium channel currents in cultured embryonic rat septal cells

We characterized the development and pharmacology of Ca(2+) channel currents in NGF-treated embryonic day 21 cultured rat septal cells. Using standard whole-cell voltage clamp techniques, cells were held at -80 mV and depolarized to construct current-voltage relations in conditions that eliminated Na(+) or K(+) currents. Barium (10 mM) was used as the charge carrier. Maximum current was produced when cells were depolarized to 0 or +10 mV. Recordings from 77 cells revealed that Ca(2+) channel current density increases over time in culture from nearly 0 pA/pF on day 2 in vitro (0.65+/-0.65 pA/pF) to (6.95+/-1.59 pA/pF) on days 6-8. This was followed by a period where currents became nearly 3 times more dense (21.05+/-7.16 pA/pF) at days 9-17. There was little or no evidence for low voltage activated currents. Bath application of 50-100 microM CdCl(2) abolished approximately 95% of the current. Application of 10 microM nimodipine produced a 50.5+/-3.22% reduction in current, 2 microM omega-CTx-GVIA produced a 32.4+/-7.3% reduction, and application of 4 microM omega-Aga-IVA produced a 29.5+/-5.73% reduction in current. When all three inhibitors (10 microM nimodipine, 2 microM omega-CTx-GVIA, and 4 microM omega-Aga-IVA) were applied simultaneously, a residual current remained that was 18.0+/-4.9% of the total current and was completely abolished by application of CdCl(2). This is the first report to characterize Ca(2+) channel currents in cultured embryonic septal cells. These data indicate that there is a steady increase in Ca(2+) channel expression over time in vitro, and show that like other cultured neuronal cells, septal cells express multiple Ca(2+) channel types including L, N, P/Q and R-type channels.

Possible modulatory role of voltage-activated Ca(2+) currents determining the membrane properties of isolated pyramidal neurones of the rat dorsal cochlear nucleus

Voltage-activated Ca(2+) currents have been studied in pyramidal cells isolated enzymatically from the dorsal cochlear nuclei of 6-11-day-old Wistar rats, using whole-cell voltage-clamp. From hyperpolarized membrane potentials, the neurones exhibited a T-type Ca(2+) current on depolarizations positive to -90 mV (the maximum occurred at about -40 mV). The magnitude of the T-current varied considerably from cell to cell (-56 to -852 pA) while its steady-state inactivation was consistent (E(50)=-88.2+/-1.7 mV, s=-6. 0+/-0.4 mV). The maximum of high-voltage activated (HVA) Ca(2+) currents was observed at about -15 mV. At a membrane potential of -10 mV the L-type Ca(2+) channel blocker nifedipine (10 microM) inhibited approximately 60% of the HVA current, the N-type channel inhibitor omega-Conotoxin GVIA (2 microM) reduced the current by 25% while the P/Q-type channel blocker omega-Agatoxin IVA (200 nM) blocked a further 10%. The presence of the N- and P/Q-type Ca(2+) channels was confirmed by immunochemical methods. The metabotropic glutamate receptor agonist (+/-)-1-aminocyclopentane-trans-1, 3-dicarboxylic acid (200 microM) depressed the HVA current in every cell studied (a block of approximately 7% on an average). The GABA(B) receptor agonist baclofen (100 microM) reversibly inhibited 25% of the HVA current. Simultaneous application of omega-Conotoxin GVIA and baclofen suggested that this inhibition could be attributed to the nearly complete blockade of the N-type channels. Possible physiological functions of the voltage-activated Ca(2+) currents reported in this work are discussed.

Distribution of the voltage-dependent calcium channel alpha1G subunit mRNA and protein throughout the mature rat brain

The molecular identity of a gene which encodes the pore-forming subunit (alpha1G) of a member of the family of low-voltage-activated, T-type, voltage-dependent calcium channels has been described recently. Although northern mRNA analyses have shown alpha1G to be expressed predominantly in the brain, the detailed cellular distribution of this protein in the central nervous system (CNS) has not yet been reported. The current study describes the preparation of a subunit specific alpha1G riboprobe and antiserum which have been used in parallel in situ mRNA hybridization and immunohistochemical studies to localize alpha1G in the mature rat brain. Both alpha1G mRNA and protein were widely distributed throughout the brain, but variations were observed in the relative level of expression in discrete nuclei. Immunoreactivity for alpha1G was typically localized in both the soma and dendrites of many neurons. Whilst alpha1G protein and mRNA expression were often observed in cells known to exhibit T-type current activity, some was also noted in regions, e.g. cerebellar granule cells, in which T-type activity has not been described. These observations may reflect differences between the subcellular distribution of channels that can be identified by immunohistochemical methods compared with electrophysiological techniques.

Isoforms of alpha1E voltage-gated calcium channels in rat cerebellar granule cells--detection of major calcium channel alpha1-transcripts by reverse transcription-polymerase chain reaction

In primary cultures of rat cerebellar granule cells, transcripts of voltage-gated Ca2+ channels have been amplified by reverse transcription-polymerase chain reaction and identified by sequencing of subcloned polymerase chain reaction products. In these neurons cultured for six to eight days in vitro, fragments of the three major transcripts alpha1C, alpha1A, and alpha1E are detected using degenerated oligonucleotide primer pairs under highly stringent conditions. Whole-cell Ca2+ current recordings from six to eight days in vitro granule cells show that most of the current is due to L-type (25%), P-type (33%) and R-type (30%) Ca2+ channels. These data support the correlation between alpha1A and P-type Ca2+ channels (G1) and between alpha1E and R-type channels (G2 and G3). By including specific primer pairs for alpha1E the complimentary DNA fragments of indicative regions of alpha1E isoforms are amplified corresponding to the three most variable regions of alpha1E, the 5'-end, the II/III-loop, and the central part of the 3'-end. Although the complementary DNA fragments of the 5'-end of rat alpha1E yield a uniform reverse transcription-polymerase chain reaction product, its structure is unusual in the sense that it is longer than in the cloned rat alpha1E complementary DNA. It corresponds to the alpha1E isoform reported for mouse and human brain and is also expressed in cerebellum and cerebrum of rat brain as the major or maybe even the only variant of alpha1E. While fragments of a new rat alpha1E isoform are amplified from the 5'-end, three known fragments of the II/III-loop and two known isoforms homologue to the 3'-coding region are detected, which in the last case are discriminated by a 129 base pair insertion. The shift of the alpha1E expression from a pattern seen in cerebellum (alpha1Ee) to a pattern identified in other regions of the brain (alpha1E-3) is discussed. These data show that: (i) alpha1E is expressed in rat brain as a structural homologue to the mouse and human alpha1E; and (ii) rat cerebellar granule cells in primary culture express a set of alpha1E isoforms, containing two different sized carboxy termini. Since no new transcripts of high-voltage-activated Ca2+ channels genes are identified using degenerate oligonucleotide primer pairs, the two isoforms differentiated by the 129 base pair insertion might correspond to the two R-type channels, G2 and G3, characterized in these neurons. Functional studies including recombinant cells with the different proposed isoforms should provide more evidence for this conclusion.

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.

Differential distribution of three members of a gene family encoding low voltage-activated (T-type) calcium channels

Low voltage-activated (T-type) calcium currents are observed in many central and peripheral neurons and display distinct physiological and functional properties. Using in situ hybridization, we have localized central and peripheral nervous system expression of three transcripts (alpha1G, alpha1H, and alpha1I) of the T-type calcium channel family (CaVT). Each mRNA demonstrated a unique distribution, and expression of the three genes was largely complementary. We found high levels of expression of these transcripts in regions associated with prominent T-type currents, including inferior olivary and thalamic relay neurons (which expressed alpha1G), sensory ganglia, pituitary, and dentate gyrus granule neurons (alpha1H), and thalamic reticular neurons (alpha1I and alpha1H). Other regions of high expression included the Purkinje cell layer of the cerebellum, the bed nucleus of the stria terminalis, the claustrum (alpha1G), the olfactory tubercles (alpha1H and alpha1I), and the subthalamic nucleus (alpha1I and alpha1G). Some neurons expressed high levels of all three genes, including hippocampal pyramidal neurons and olfactory granule cells. Many brain regions showed a predominance of labeling for alpha1G, including the amygdala, cerebral cortex, rostral hypothalamus, brainstem, and spinal cord. Exceptions included the basal ganglia, which showed more prominent labeling for alpha1H and alpha1I, and the olfactory bulb, the hippocampus, and the caudal hypothalamus, which showed more even levels of all three transcripts. Our results are consistent with the hypothesis that differential gene expression underlies pharmacological and physiological heterogeneity observed in neuronal T-type calcium currents, and they provide a molecular basis for the study of T-type channels in particular neurons.

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.

Differential distribution of three members of a gene family encoding low voltage-activated (T-type) calcium channels

Low voltage-activated (T-type) calcium currents are observed in many central and peripheral neurons and display distinct physiological and functional properties. Using in situ hybridization, we have localized central and peripheral nervous system expression of three transcripts (alpha1G, alpha1H, and alpha1I) of the T-type calcium channel family (CaVT). Each mRNA demonstrated a unique distribution, and expression of the three genes was largely complementary. We found high levels of expression of these transcripts in regions associated with prominent T-type currents, including inferior olivary and thalamic relay neurons (which expressed alpha1G), sensory ganglia, pituitary, and dentate gyrus granule neurons (alpha1H), and thalamic reticular neurons (alpha1I and alpha1H). Other regions of high expression included the Purkinje cell layer of the cerebellum, the bed nucleus of the stria terminalis, the claustrum (alpha1G), the olfactory tubercles (alpha1H and alpha1I), and the subthalamic nucleus (alpha1I and alpha1G). Some neurons expressed high levels of all three genes, including hippocampal pyramidal neurons and olfactory granule cells. Many brain regions showed a predominance of labeling for alpha1G, including the amygdala, cerebral cortex, rostral hypothalamus, brainstem, and spinal cord. Exceptions included the basal ganglia, which showed more prominent labeling for alpha1H and alpha1I, and the olfactory bulb, the hippocampus, and the caudal hypothalamus, which showed more even levels of all three transcripts. Our results are consistent with the hypothesis that differential gene expression underlies pharmacological and physiological heterogeneity observed in neuronal T-type calcium currents, and they provide a molecular basis for the study of T-type channels in particular neurons.

Early metabolic inhibition-induced intracellular sodium and calcium increase in rat cerebellar granule cells

1. Possible mechanisms responsible for the increases in intracellular calcium ([Ca2+]i) and sodium ([Na+]i) levels seen during metabolic inhibition were investigated by continuous [Ca2+]i and [Na+]i measurement in cultured rat cerebellar granule cells. An initial small mitochondrial Ca2+ release was seen, followed by a large influx of extracellular Ca2+. A large influx of extracellular Na+ was also seen. 2. The large [Ca2+]i increase was not due to opening of voltage-dependent or voltage-independent calcium channels, activation of NMDA/non-NMDA channels, activation of the Na+i-Ca2+o exchanger, or inability of plasmalemmal Ca2+-ATPase to extrude, or mitochondria to take up, calcium. 3. The large [Na+]i increase was not due to activation of the TTX-sensitive Na+ channel, the Na+i-Ca2+o exchanger, the Na+-H+ exchanger, or the Na+-K+-2Cl- cotransporter, or an inability of Na+-K+-ATPase to extrude the intracellular sodium. 4. Phospholipase A2 (PLA2) activation may be involved in the large influx, since both were completely inhibited by PLA2 inhibitors. Moreover, melittin (a PLA2 activator) or lysophosphatidylcholine or arachidonic acid (both PLA2 activation products) caused similar responses. Inhibition of PLA2 activity may help prevent the influx of these ions that may result in serious brain injury and oedema during hypoxia/ischaemia.

Dopamine D2-receptor isoforms expressed in AtT20 cells inhibit Q-type high-voltage-activated Ca2+ channels via a membrane-delimited pathway

Dopamine D2 receptors both acutely and chronically inhibit high-voltage-activated Ca2+ channels (HVA-CCs). Two alternatively spliced isoforms, D2L (long) and D2S (short), are expressed at high levels in rat pituitary intermediate lobe melanotropes but are lacking in anterior lobe corticotropes. We stably transfected D2L and D2S into corticotrope-derived AtT20 cells. Both isoforms coupled to inhibition of Q-type calcium channels through pertussis toxin-sensitive G proteins. Thus, we have created a model system in which to study the kinetics of D2-receptor regulation of Ca2+ channels. Rapid inhibition of HVA-CCs was characterized using a novel fluorescence video imaging technique for the measurement of millisecond kinetic events. We measured the time elapsed (lag time) between the arrival of depolarizing isotonic 66 mM K+, sensed by fluorescence from included carboxy-X-rhodamine (CXR), and the beginning of increased intracellular Ca2+ levels (sensed by changes in indo 1 fluorescence ratio). The lag time averaged 350-550 ms, with no significant differences among cell types. Addition of the D2-agonist quinpirole (250 microM) to the K+/CXR solution significantly increased the lag times for D2-expressing cells but did not alter the lag time for AtT20 controls. The increased lag times for D2L- and D2S-transfected cells suggest that at least a fraction of the Ca2+ channels was inhibited within the initial 350-550 ms. As this inhibition time is too fast for a multistep second messenger pathway, we conclude that inhibition occurs via a membrane-delimited diffusion mechanism.

Electrophysiological characterisation of the human N-type Ca2+ channel III: pH-dependent inhibition by a synthetic macrocyclic polyamine

The effects of a novel synthetic macrocyclic polyamine (LY310315) were investigated on recombinant human N-type Ca2+ channels stabley expressed in HEK293 cells. LY310315 proved to be a potent and reversible N-type Ca2+ channel antagonist. Inhibition by this compound was dose-dependent with an IC50 of approximately 0.4 microM at pH 7.35. LY310315 blocked very rapidly at all concentrations tested. Upon washout, recovery of the Ca2+ current developed with a time constant of approximately 30 s. Use-dependence in the development of block indicated that voltage-dependent transitions in the channel protein were required to permit significant inhibition. Application of > 100 times the IC50 dose of LY310315 to the interior of the cell produced no detectable Ca2+ current inhibition. LY310315 had no effects on the kinetics of channel activation or deactivation but did slightly slow the rate of macroscopic inactivation observed during a 300 ms test depolarisation. In the presence of LY310315 the activation curve was significantly shallower. This resulted in a shift in the activation midpoint voltage to a more depolarised levels. LY310315-induced inhibition of human N-type channels was strongly dependent on the extracellular pH, with increased potency seen upon extracellular acidification. Although most effective against N-type Ca2+ channels, LY310315 was also found to inhibit both P-type and L-type Ca2+ channels. LY310315 proved to be a weak blocker of Na+ currents, but produced approximately 50% of the K+ currents of AtT20 cells at a concentration of 0.5 microM.

Visualization of calcium channels involved in transmitter release from neuronal growth cones

Immunocytological localization of omega-agatoxin IVA (omega-aga IVA)-sensitive Ca2+ channels involved in glutamate release from growth cones of cultured rat dorsal root ganglion (DRG) neurons was studied with field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The omega-aga IVA-sensitive Ca2+ channels were visualized by labeling with immuno-gold particles (30 nm). FE-SEM and TEM images showed that immuno-gold particles were present in the area of growth cones as well as somata, and generally absent on neurite stem and fibroblasts. TEM images of vertical ultra-thin sections showed that the immuno-gold particles were present on the surface of the plasma membrane. Since the gold particles indicate the immunological presence of omega-aga IVA-sensitive Ca2+ channels, the Ca2+ channels involved in transmitter release are present on growth cones before making synapse formation.

Modulation of Ca2+ channel currents of acutely dissociated rat periaqueductal grey neurons

1. The actions of the neuropeptide nociceptin on the calcium channel currents (IBa) of acutely dissociated rat periaqueductal grey (PAG) neurons were examined using whole-cell patch clamp techniques. These effects were compared with those of opioid receptor agonists and the GABAB receptor agonist baclofen. 2. Neurons from young adult rats (23 to 56 days old) expressed predominantly omega-conotoxin GVIA (N-type)- and omega-agatoxin IVA (P/Q-type)-sensitive IBa, together with smaller amounts of nimodipine-sensitive current and current resistant to all three blockers. There was proportionately more N-type IBa in neurons from female rats and proportionately more resistant current in neurons from male rats. 3. Nociceptin (EC50, 5 nM) and baclofen (EC50, 0.8 microM) inhibited IBa in all PAG neurons, while the opioid agonist methionine enkephalin (met-enkephalin; 300 nM-10 microM) inhibited IBa in 40 % of neurons. The effects of met-enkephalin were reversed by the mu-opioid antagonist CTAP, and mimicked by the mu-opioid agonist DAMGO (300 nM-3 microM). The delta-opioid agonists DPDPE and deltorphin II, and the kappa-opioid agonist U69593, did not affect IBa in any neuron. The actions of nociceptin were not mimicked or blocked by the opioid antagonist naloxone or the nociceptin analogue [desPhe1]-nociceptin. 4. The effects of nociceptin and baclofen on IBa were blocked by pretreatment of the neurons with pertussis toxin (500 ng ml-1, 8 h). 5. Nociceptin predominantly inhibited the N-type (EC50, 2 nM; maximum inhibition, 50 %) and P/Q-type (EC50, 7 nM; maximum inhibition, 33 %) IBa while having little effect on the L-type and R-type IBa. 6. These results are consistent with the previously described actions of nociceptin, baclofen and micro-opioids in PAG slices, whereby they couple to increases in an inwardly rectifying K+ conductance. These agonists thus have the potential to modulate the function of PAG neurons via a number of different cellular effectors.

Differential roles of two types of voltage-gated Ca2+ channels in the dendrites of rat cerebellar Purkinje neurons

The distribution and function of voltage-gated Ca2+ channels in Purkinje neurons in rat cerebellar slices were studied using simultaneous Ca2+ imaging and whole-cell patch clamp recording techniques. Voltage-gated Ca2+ channels were activated by applying depolarizing voltage steps through the pipette attached at the soma in a voltage-clamp mode in the presence of tetrodotoxin. Poor space clamp due to extensive arborization of the dendrites allowed the dendrites to fire Ca2+ spikes. Ca2+ imaging with Fura-2 injected through the pipette, showed a steady [Ca2+]i increase at the soma and transient, spike-linked [Ca2+]i jumps in the dendrites. omega-Agatoxin-IVA (200 nM) abolished the depolarization-induced Ca2+ spikes, the spike-linked [Ca2+]i increase in the dendrites, and the steady [Ca2+]i increase at the soma. omega-Conotoxin-GVIA (5 microM) and nifedipine (3 microM) had no significant effect on the depolarization-induced responses. In the presence of 4-aminopyridine(2 mM) and omega-Agatoxin-IVA, transient [Ca2+]i increases remained in the dendrites. Low concentrations of Ni2+(100 microM) reversibly suppressed this [Ca2+]i increase. The voltage for half-maximal activation and inactivation of this component were lower than -50 mV and -31 mV, respectively. In normal conditions, low concentration of Ni2+ slowed the onset of the Ca2+ spike without changing the time course of the spikes or the amplitude of the accompanying [Ca2+]i increase. These results show that omega-Agatoxin-IVA-sensitive Ca2+ channels are distributed both in the soma and the dendrites, and are responsible for dendritic Ca2+ spikes, whereas low-voltage activated, Ni2+-sensitive Ca2+ channels are distributed in the whole dendrites including both thick and fine branches, and provide boosting current for spike generation.

Calcium influx through N- and P/Q-type channels activate apamin-sensitive calcium-dependent potassium channels generating the late afterhyperpolarization in lamprey spinal neurons

Lamprey spinal neurons exhibit a fast afterhyperpolarization and a late afterhyperpolarization (AHP) which is due to the activation of apamin-sensitive SK Ca2+-dependent K+ channels (KCa) activated by calcium influx through voltage-dependent channels during the action potential (Hill et al. 1992, Neuroreport, 3, 943-945). In this study we have investigated which calcium channel subtypes are responsible for the activation of the KCa channels underlying the AHP. The effects of applying specific calcium channel blockers and agonists were analysed with regard to their effects on the AHP. Blockade of N-type calcium channels by omega-conotoxin GVIA resulted in a significant decrease in the amplitude of the AHP by 76.2+/-14.9% (mean +/- SD). Application of the P/Q-type calcium channel blocker omega-agatoxin IVA reduced the amplitude of the AHP by 20.3+/-10.4%. The amplitude of the AHP was unchanged during application of the L-type calcium channel antagonist nimodipine or the agonist (+/-)-BAY K 8644, as was the compound afterhyperpolarization after a train of 10 spikes at 100 Hz. The effects of calcium channel blockers were also tested on the spike frequency adaptation during a train of action potentials induced by a 100-200 ms depolarizing pulse. The N- and P/Q-type calcium channel antagonists decreased the spike frequency adaptation, whereas blockade of L-type channels had no effect. Thus in lamprey spinal cord motor- and interneurons, apamin-sensitive KCa channels underlying the AHP are activated primarily by calcium entering through N-type channels, and to a lesser extent through P/Q-type channels.

Biophysical and pharmacological characterization of voltage-dependent Ca2+ channels in neurons isolated from rat nucleus accumbens

The nucleus accumbens (NA) has an integrative role in behavior and may mediate addictive and psychotherapeutic drug action. Whole cell recording techniques were used to characterize electrophysiologically and pharmacologically high- and low-threshold voltage-dependent Ca2+ currents in isolated NA neurons. High-threshold Ca2+ currents, which were found in all neurons studied and include both sustained and inactivating components, activated at potentials greater than -50 mV and reached maximal activation at approximately 0 mV. In contrast, low-threshold Ca2+ currents activated at voltages greater than -64 mV with maximal activation occurring at -30 mV. These were observed in 42% of acutely isolated neurons. Further pharmacological characterization of high-threshold Ca2+ currents was attempted using nimodipine (Nim), omega-conotoxin-GVIA (omega-CgTx) and omega-agatoxin-IVA (omegaAga), which are thought to identify the L, N, and P/Q subtypes of Ca2+ currents, respectively. Nim (5-10 muM) blocked 18%, omegaCgTx (1-2 muM) blocked 25%, and omegaAga (200 nM) blocked 17% of total Ca2+ current. Nim primarily blocked a sustained high-threshold Ca2+ current in a partially reversible manner. In contrast, omegaCgTx irreversibly blocked both sustained and inactivating components. omegaAga irreversibly blocked only a sustained component. In all three of these Ca2+ channel blockers, plus 5 muM omega-conotoxin-MVIIC to eliminate a small unblocked Q-type Ca2+ current (7%), a toxin-resistant high-threshold Ca2+ current remained that was 32% of total Ca2+ current. This current inactivated much more rapidly than the other high-threshold Ca2+ currents, was depressed in 50 muM Ni2+ and reached maximal activation 5-10 mV negative to the toxin-sensitive high-threshold Ca2+ currents. Thus NA neurons have multiple types of high-threshold Ca2+ currents with a large component being the toxin-resistant "R" component.

beta subunits influence the biophysical and pharmacological differences between P- and Q-type calcium currents expressed in a mammalian cell line

Human epithelial kidney cells (HEK) were prepared to coexpress alpha1A, alpha2delta with different beta calcium channel subunits and green fluorescence protein. To compare the calcium currents observed in these cells with the native neuronal currents, electrophysiological and pharmacological tools were used conjointly. Whole-cell current recordings of human epithelial kidney alpha1A-transfected cells showed small inactivating currents in 80 mM Ba2+ that were relatively insensitive to calcium blockers. Coexpression of alpha1A, betaIb, and alpha2delta produced a robust inactivating current detected in 10 mM Ba2+, reversibly blockable with low concentration of omega-agatoxin IVA (omega-Aga IVA) or synthetic funnel-web spider toxin (sFTX). Barium currents were also supported by alpha1A, beta2a, alpha2delta subunits, which demonstrated the slowest inactivation and were relatively insensitive to omega-Aga IVA and sFTX. Coexpression of beta3 with the same combination as above produced inactivating currents also insensitive to low concentration of omega-Aga IVA and sFTX. These data indicate that the combination alpha1A, betaIb, alpha2delta best resembles P-type channels given the rate of inactivation and the high sensitivity to omega-Aga IVA and sFTX. More importantly, the specificity of the channel blocker is highly influenced by the beta subunit associated with the alpha1A subunit.

Activation of brain nitric oxide synthase in depolarized human temporal cortex slices: differential role of voltage-sensitive calcium channels

1. Nitric oxide (NO) synthase activity was studied in slices of human temporal cortex samples obtained in neurosurgery by measuring the conversion of L-[3H]-arginine to L-[3H]-citrulline. 2. Elevation of extracellular K+ to 20, 35 or 60 mM concentration-dependently augmented L-[3H]-citrulline production. The response to 35 mM KCl was abolished by N(G)-nitro-L-arginine (100 microM) demonstrating NO synthase specific conversion of L-arginine to L-citrulline. Increasing extracellular MgCl2 concentration up to 10 mM also prevented the K+ (35 mM)-induced NO synthase activation, suggesting the absolute requirement of external calcium ions for enzyme activity. 3. However, the effect of high K+ (35 mM) on citrulline synthesis was insensitive to the antagonists of ionotropic and metabotropic glutamate receptors dizocilpine (MK-801), 6-nitro-7-sulphamoylbenzo(f)-quinoxaline-2-3-dione (NBQX) or L-2-amino-3-phosphonopropionic acid (L-AP3) as well as to the nicotinic receptor antagonist, mecamylamine. 4. The 35 mM K+ response was insensitive to omega-conotoxin GVIA (1 microM) and nifedipine (100 microM), but could be prevented in part by omega-agatoxin IVA (0.1 and 1 microM). The inhibition caused by 0.1 microM omega-agatoxin IVA (approximately 30%) was enhanced by adding omega-conotoxin GVIA (1 microM) or nifedipine (100 microM). Further inhibition (up to above 70%) could be observed when the three Ca2+ channel blockers were added together. Similarly, synthetic FTX 3.3 arginine polyamine (sFTX) prevented (50% at 100 microM) the K+-evoked NO synthase activation. This effect of sFTX was further enhanced (up to 70%) by adding 1 microM omega-conotoxin GVIA plus 100 microM nifedipine. No further inhibition could be observed upon addition of MK-801 or/and NBQX. 5. It was concluded that elevation of extracellular [K+] causes NO synthase activation by external Ca2+ entering cells mainly through channels of the P/Q-type. Other Ca2+ channels (L- and N-type) appear to contribute when P/Q-channels are blocked.

Presynaptic calcium channels and field-evoked transmitter exocytosis from cultured cerebellar granule cells

Regulated exocytosis from cultured rat cerebellar granule cells can be localized by the vesicle specific marker FM2-10 to specific sites, the highest density of which are at visible varicosities coinciding with neurite-neurite contacts. Exocytosis can be evoked by uniform electrical field pulses, which initiate tetrodotoxin-sensitive action potentials, or by elevated KCl. [3H]D-Aspartate is an authentic false transmitter in this preparation, judged by sensitivity of release to bafilomycin A1 and tetanus toxin. The coupling of presynaptic voltage-activated Ca2+ channels to [3H]D-aspartate exocytosis was determined during field stimulation. The peak cytoplasmic free Ca2+ concentration achieved in the varicosities was proportional to Ca2+ entry during a 10 strain of pulses. L-type Ca2+ channels did not contribute to either Ca2+ entry or [3H]D-aspartate exocytosis. The P-type Ca2+ channel antagonist omega-agatoxin-IVA (30 nM) only inhibited at 75% of the varicosities, although a mean 15% inhibition of Ca2+ entry caused a 39% inhibition of exocytosis. In contrast the N-type Ca2+ channel inhibitor omega-conotoxin-GVIA (1 microM), which inhibited at virtually all varicosities, caused mean inhibitions of Ca2+ entry and exocytosis of 26% and 24% respectively. The toxin omega-conotoxin-MVIIC (5 microM), which inhibits N-, P- and Q-type Ca2+ channels, was effective at all varicosities. The Q-type component of Ca2+ entry was calculated to be only 5-10%; however, the additional inhibition of exocytosis was 30%. Thus P-type and particularly Q-type channels appear to be more closely coupled to exocytosis than N-type Ca2+ channels. The residual Ca2+ entry following 5 microM omega-conotoxin-MVIIC is scarcely coupled to release. The omega-agatoxin-IVA and omega-conotoxin-GVIA inhibitions of both Ca2+ entry and exocytosis were additive and varied stochastically between individual varicosities. These results demonstrate that both Q- and P-type Ca2+ channels are highly efficient in their coupling to amino acid exocytosis, with N-type less efficient, and L-type channels not at all. The Ca2+ channel types coupled to exocytosis are also able to support exocytosis when evoked by either brief field-evoked action potentials or prolonged depolarization with KCl, indicating that these presynaptic channels, in contrast to those on the somata of the cells, can respond to widely different patterns of activation.

Identifying neuronal non-L Ca2+ channels--more than stamp collecting?

The pharmacology of the majority of Ca2+ channels in the nervous system is very limited. Although attempts have been made to constrain native Ca2+ channels into the framework provided by the six pore-forming molecules cloned to date, refined biophysical analysis of Ca2+ currents, expression techniques and the use of selective toxins have helped to identify unambiguously only a limited number of Ca2+ channels. In fact, many native Ca2+ channel activities remain as 'orphans', waiting for their molecular counterparts to be defined. In this article, Janet Nooney, Régis Lambert and Anne Feltz systematically delineate the well characterized non-L Ca2+ channel activities and the missing elements in our knowledge of the Ca2+ channel family.

Calcium channel subtypes in lamprey sensory and motor neurons

Pharmacologically distinct calcium channels have been characterized in dissociated cutaneous sensory neurons and motoneurons of the larval lamprey spinal cord. To enable cell identification, sensory dorsal cells and motoneurons were selectively labeled with fluorescein-coupled dextran amine in the intact spinal cord in vitro before dissociation. Calcium channels present in sensory dorsal cells, motoneurons, and other spinal cord neurons were characterized with the use of whole cell voltage-clamp recordings and specific calcium channel agonist and antagonists. The results show that a transient low-voltage-activated (LVA) calcium current was present in a proportion of sensory dorsal cells but not in motoneurons, whereas high-voltage-activated (HVA) calcium currents were seen in all neurons recorded. The different components of HVA current were dissected pharmacologically and similar results were obtained for both dorsal cells and motoneurons. The N-type calcium channel antagonist omega-conotoxin-GVIA (omega-CgTx) blocked >70% of the HVA current. A large part of the omega-CgTx block was reversed after washout of the toxin. The L-type calcium channel antagonist nimodipine blocked approximately 15% of the total HVA current. The dihydropyridine agonist (+/-)-BayK 8644 markedly increased the amplitude of the calcium channel current. The BayK-potentiated current was not affected by omega-CgTx, indicating that the reversibility of the omega-CgTx effect is not due to a blockade of L-type channels. Simultaneous application of omega-CgTx and nimodipine left approximately 15% of the HVA calcium channel current, a small part of which was blocked by the P/Q-type channel antagonist omega-agatoxin-IVA. In the presence of the three antagonists, the persistent residual current (approximately 10%) was completely blocked by cadmium. Our results provide evidence for the existence of HVA calcium channels of the N, L, and P/Q types and other HVA calcium channels in lamprey sensory neurons and motoneurons. In addition, certain types of neurons express LVA calcium channels.