Functional disorders of the sympathetic nervous system in mice lacking the alpha 1B subunit (Cav 2.2) of N-type calcium channels

Cav2-type calcium channels encoded by cac regulate AP-independent neurotransmitter release at cholinergic synapses in adult Drosophila brain

Voltage-gated calcium channels containing alpha1 subunits encoded by Ca(v)2 family genes are critical in regulating release of neurotransmitter at chemical synapses. In Drosophila, cac is the only Ca(v)2-type gene. Cacophony (CAC) channels are localized in motor neuron terminals where they have been shown to mediate evoked, but not AP-independent, release of glutamate at the larval neuromuscular junction (NMJ). Cultured embryonic neurons also express CAC channels, but there is no information about the properties of CAC-mediated currents in adult brain nor how these channels regulate transmission in central neural circuits where fast excitatory synaptic transmission is predominantly cholinergic. Here we report that wild-type neurons cultured from late stage pupal brains and antennal lobe projection neurons (PNs) examined in adult brains, express calcium currents with two components: a slow-inactivating current sensitive to the spider toxin Plectreurys toxin II (PLTXII) and a fast-inactivating PLTXII-resistant component. CAC channels are the major contributors to the slow-inactivating PLTXII-sensitive current based on selective reduction of this component in hypomorphic cac mutants (NT27 and TS3). Another characteristic of cac mutant neurons both in culture and in whole brain recordings is a reduced cholinergic miniature excitatory postsynaptic current frequency that is mimicked in wild-type neurons by acute application of PLTXII. These data demonstrate that cac encoded Ca(v)2-type calcium channels regulate action potential (AP)-independent release of neurotransmitter at excitatory cholinergic synapses in the adult brain, a function not predicted from studies at the larval NMJ.

Genesis and regulation of the heart automaticity

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.

Control of hyperactivation in sperm

BACKGROUND

Sperm hyperactivation is critical to fertilization, because it is required for penetration of the zona pellucida. Hyperactivation may also facilitate release of sperm from the oviductal storage reservoir and may propel sperm through mucus in the oviductal lumen and the matrix of the cumulus oophorus. Hyperactivation is characterized by high amplitude, asymmetrical flagellar bending.

METHODS

This is a review of the original literature on the mechanisms that regulate hyperactivation, including physiological factors and signaling pathways.

RESULTS

Computer-assisted semen analysis systems can be used to identify hyperactivated sperm by setting minimum thresholds for curvilinear velocity (VSL) and lateral head movement and a maximum threshold for path linearity. Hyperactivation is triggered by a rise in flagellar Ca(2+) resulting from influx primarily through plasma membrane CatSper channels and possibly also by release of Ca(2+) from a store in the redundant nuclear envelope. It requires increased pH and ATP production. The physiological signals that trigger the rise in Ca(2+) remain elusive, but there is evidence that the increased Ca(2+) acts through a calmodulin/calmodulin kinase pathway. Hyperactivation is considered part of the capacitation process; however, the regulatory pathway that triggers hyperactivation can operate independently from that which prepares sperm to undergo the acrosome reaction. Hyperactivation may be modulated by chemotactic signals to turn sperm toward the oocyte.

CONCLUSIONS

Little is known about exactly what triggers hyperactivation in human sperm. This information could enable clinicians to develop reliable fertility assays to assess normal hyperactivation in human sperm samples.

Modified sympathetic nerve system activity with overexpression of the voltage-dependent calcium channel beta3 subunit

N-type voltage-dependent calcium channels (VDCCs) play determining roles in calcium entry at sympathetic nerve terminals and trigger the release of the neurotransmitter norepinephrine. The accessory beta3 subunit of these channels preferentially forms N-type channels with a pore-forming CaV2.2 subunit. To examine its role in sympathetic nerve regulation, we established a beta3-overexpressing transgenic (beta3-Tg) mouse line. In these mice, we analyzed cardiovascular functions such as electrocardiography, blood pressure, echocardiography, and isovolumic contraction of the left ventricle with a Langendorff apparatus. Furthermore, we compared the cardiac function with that of beta3-null and CaV2.2 (alpha1B)-null mice. The beta3-Tg mice showed increased expression of the beta3 subunit, resulting in increased amounts of CaV2.2 in supracervical ganglion (SCG) neurons. The beta3-Tg mice had increased heart rate and enhanced sensitivity to N-type channel-specific blockers in electrocardiography, blood pressure, and echocardiography. In contrast, cardiac atria of the beta3-Tg mice revealed normal contractility to isoproterenol. Furthermore, their cardiac myocytes showed normal calcium channel currents, indicating unchanged calcium influx through VDCCs. Langendorff heart perfusion analysis revealed enhanced sensitivity to electric field stimulation in the beta3-Tg mice, whereas beta3-null and Cav2.2-null showed decreased responsiveness. The plasma epinephrine and norepinephrine levels in the beta3-Tg mice were significantly increased in the basal state, indicating enhanced sympathetic tone. Electrophysiological analysis in SCG neurons of beta3-Tg mice revealed increased calcium channel currents, especially N- and L-type currents. These results identify a determining role for the beta3 subunit in the N-type channel population in SCG and a major role in sympathetic nerve regulation.

The inhibition of release by mGlu7 receptors is independent of the Ca2+ channel type but associated to GABAB and adenosine A1 receptors

Neurotransmitter release is inhibited by G-protein coupled receptors (GPCRs) through signalling pathways that are negatively coupled to Ca2+ channels and adenylyl cyclase. Through Ca2+ imaging and immunocytochemistry, we have recently shown that adenosine A1, GABAB and the metabotropic glutamate type 7 receptors coexist in a subset of cerebrocortical nerve terminals. As these receptors inhibit glutamate release through common intracellular signalling pathways, their co-activation occluded each other responses. Here we have addressed whether the occlusion of receptor responses is restricted to the glutamate release mediated by N-type Ca2+ channels by analysing this process in nerve terminals from mice lacking the alpha1B subunit (Cav 2.2) of these channels. We found that glutamate release from cerebrocortical nerve terminals without these channels, in which release relies exclusively on P/Q type Ca2+ channels, is not modulated by mGlu7 receptors. Furthermore, there is no occlusion of the release inhibition by GABAB and adenosine A1. Hence, in the cerebrocortical preparation, these three receptors only appear to coexist in N-type channel containing nerve terminals. In contrast, in hippocampal nerve terminals lacking this subunit, where mGlu7 receptors modulate glutamate release via P/Q type channels, the occlusion of inhibitory responses by co-stimulation of adenosine A1, GABAB and mGlu7 receptors was observed. Thus, occlusion of the responses by the three GPCRs is independent of the Ca2+ channel type but rather, it is associated to functional mGlu7 receptors.

Defective Ca2+ channel clustering in axon terminals disturbs excitability in motoneurons in spinal muscular atrophy

Proximal spinal muscular atrophy (SMA) is a motoneuron disease for which there is currently no effective treatment. In animal models of SMA, spinal motoneurons exhibit reduced axon elongation and growth cone size. These defects correlate with reduced beta-actin messenger RNA and protein levels in distal axons. We show that survival motoneuron gene (Smn)-deficient motoneurons exhibit severe defects in clustering Cav2.2 channels in axonal growth cones. These defects also correlate with a reduced frequency of local Ca2+ transients. In contrast, global spontaneous excitability measured in cell bodies and proximal axons is not reduced. Stimulation of Smn production from the transgenic SMN2 gene by cyclic adenosine monophosphate restores Cav2.2 accumulation and excitability. This may lead to the development of new therapies for SMA that are not focused on enhancing motoneuron survival but instead investigate restoration of growth cone excitability and function.

Modified behavioral characteristics following ablation of the voltage-dependent calcium channel beta3 subunit

Voltage-dependent calcium channels are important for calcium influx and the ensuing intracellular calcium signal in various excitable membranes. The beta subunits of these channels modify calcium currents through pore-forming alpha1 subunits of the high-voltage- activated calcium channels. In the present study, beta3 subunit-null mice were used to investigate the importance of the beta3 subunit of the voltage-dependent calcium channel, which couples with the CaV2.2 (alpha1B) subunit to form the major component of neuronal N-type calcium channels in the brain. Western blot analysis revealed a significant decrease in N-type calcium channels in beta3 subunit-null mice, while protein levels of other high-voltage-activated calcium channel alpha1 subunits were unchanged. Immunoprecipitation analysis with an anti-CaV2.2 antibody showed that reshuffling of the assembly of N-type channels had occurred in the beta3 subunit-null mice. Ablation of this subunit resulted in modified nociception, decreased anxiety, and increased aggression. The beta3 subunit-null mice also showed impaired learning ability. These results suggest the importance of voltage-dependent calcium channels and the key role of the beta3 subunit in memory formation, nociceptive sensory transduction, and various neurological signal transduction pathways.

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

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

A CaV2.1 calcium channel mutationrockerreduces the number of postsynaptic AMPA receptors in parallel fiber-Purkinje cell synapses

The rocker mice are hereditary ataxic mutants that carry a point mutation in the gene encoding the CaV2.1 (P/Q-type) Ca2+ channel alpha1 subunit, and show the mildest symptoms among the reported CaV2.1 mutant mice. We studied the basic characteristics of the rocker mutant Ca2+ channel and their impacts on excitatory synaptic transmission in cerebellar Purkinje cells (PCs). In acutely dissociated PC somas, the rocker mutant channel showed a moderate reduction in Ca2+ channel current density, whereas its kinetics and voltage dependency of gating remained nearly normal. Despite the small changes in channel function, synaptic transmission in the parallel fiber (PF)-PC synapses was severely impaired. The climbing fiber inputs onto PCs showed a moderate impairment but could elicit normal complex spikes. Presynaptic function of the PF-PC synapses, however, was unexpectedly almost normal in terms of paired-pulse facilitation, sensitivity to extracellular Ca2+ concentration and glutamate concentration in synaptic clefts. Electron microscopic analyses including freeze-fracture replica labeling revealed that both the number and density of postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors substantially decreased without gross structural changes of the PF-PC synapses. We also observed an abnormal arborization of PC dendrites in young adult rocker mice (approximately 1 month old). These lines of evidence suggest that even a moderate dysfunction of CaV2.1 Ca2+ channel can cause substantial changes in postsynaptic molecular composition of the PF-PC synapses and dendritic structure of PCs.

Alternative splicing in the C-terminus of CaV2.2 controls expression and gating of N-type calcium channels

N-type Ca(V)2.2 calcium channels localize to presynaptic nerve terminals of nociceptors where they control neurotransmitter release. Nociceptive neurons express a unique set of ion channels and receptors important for optimizing their role in transmission of noxious stimuli. Included among these is a structurally and functionally distinct N-type calcium channel splice isoform, Ca(V)2.2e[37a], expressed in a subset of nociceptors and with limited expression in other parts of the nervous system. Ca(V)2.2[e37a] arises from the mutually exclusive replacement of e37a for e37b in the C-terminus of Ca(V)2.2 mRNA. N-type current densities in nociceptors that express a combination of Ca(V)2.2e[37a] and Ca(V)2.2e[37b] mRNAs are significantly larger compared to cells that express only Ca(V)2.2e[37b]. Here we show that e37a supports increased expression of functional N-type channels and an increase in channel open time as compared to Ca(V)2.2 channels that contain e37b. To understand how e37a affects N-type currents we compared macroscopic and single-channel ionic currents as well as gating currents in tsA201 cells expressing Ca(V)2.2e[37a] and Ca(V)2.2e[37b]. When activated, Ca(V)2.2e[37a] channels remain open for longer and are expressed at higher density than Ca(V)2.2e[37b] channels. These unique features of the Ca(V)2.2e[37a] isoform combine to augment substantially the amount of calcium that enters cells in response to action potentials. Our studies of the e37a/e37b splice site reveal a multifunctional domain in the C-terminus of Ca(V)2.2 that regulates the overall activity of N-type calcium channels in nociceptors.

Blocker-resistant presynaptic voltage-dependent Ca2+ channels underlying glutamate release in mice nucleus tractus solitarii

The visceral sensory information from the internal organs is conveyed via the vagus and glossopharyngeal primary afferent fibers and transmitted to the second-order neurons in the nucleus of the solitary tract (NTS). The glutamate release from the solitary tract (TS) axons to the second-order NTS neurons remains even in the presence of toxins that block N- and P/Q-type voltage-dependent Ca(2+) channels (VDCCs). The presynaptic VDCC playing the major role at this synapse remains unidentified. To address this issue, we examined two hypotheses in this study. First, we examined whether the remaining large component occurs through activation of a omega-conotoxin GVIA (omega-CgTX)-insensitive variant of N-type VDCC by using the mice genetically lacking its pore-forming subunit alpha(1B). Second, we examined whether R-type VDCCs are involved in transmitter release at the TS-NTS synapse. The EPSCs evoked by stimulation of the TS were recorded in medullary slices from young mice. omega-Agatoxin IVA (omega-AgaIVA; 200 nM) did not significantly affect the EPSC amplitude in the mice genetically lacking N-type VDCC. SNX-482 (500 nM) and Ni(2+) (100 microM) did not significantly reduce EPSC amplitude in ICR mice. These results indicate that, unlike in most of the brain synapses identified to date, the largest part of the glutamate release at the TS-NTS synapse in mice occurs through activation of non-L, non-P/Q, non-R, non-T and non-N (including its posttranslational variants) VDCCs at least according to their pharmacological properties identified to date.

Voltage-Gated Calcium Channels and Idiopathic Generalized Epilepsies

The idiopathic generalized epilepsies encompass a class of epileptic seizure types that exhibit a polygenic and heritable etiology. Advances in molecular biology and genetics have implicated defects in certain types of voltage-gated calcium channels and their ancillary subunits as important players in this form of epilepsy. Both T-type and P/Q-type channels appear to mediate important contributions to seizure genesis, modulation of network activity, and genetic seizure susceptibility. Here, we provide a comprehensive overview of the roles of these channels and associated subunits in normal and pathological brain activity within the context of idiopathic generalized epilepsy.

TheDrosophila cacts2mutation reduces presynaptic Ca2+entry and defines an important element in Cav2.1 channel inactivation

Voltage-gated Ca2+ channels in nerve terminals open in response to action potentials and admit Ca2+, the trigger for neurotransmitter release. The cacophony gene encodes the primary presynaptic voltage-gated Ca2+ channel in Drosophila motor-nerve terminals. The cac(ts2) mutant allele of cacophony is associated with paralysis and reduced neurotransmission at non-permissive temperatures but the basis for the neurotransmission deficit has not been established. The cac(ts2) mutation occurs in the cytoplasmic carboxyl tail of the alpha1-subunit, not within the pore-forming trans-membrane domains, making it difficult to predict the mutation's impact. We applied a Ca2+-imaging technique at motor-nerve terminals of mutant larvae to test the hypothesis that the neurotransmission deficit is a result of impaired Ca2+ entry. Presynaptic Ca2+ signals evoked by single and multiple action potentials showed a temperature-dependent reduction. The amplitude of the reduction was sufficient to account for the neurotransmission deficit, indicating that the site of the cac(ts2) mutation plays a role in Ca2+ channel activity. As the mutation occurs in a motif conserved in mammalian high-voltage-activated Ca2+ channels, we used a heterologous expression system to probe the effect of this mutation on channel function. The mutation was introduced into rat Ca(v)2.1 channels expressed in human embryonic kidney cells. Patch-clamp analysis of mutant channels at the physiological temperature of 37 degrees C showed much faster inactivation rates than for wild-type channels, demonstrating that the integrity of this motif is critical for normal Ca(v)2.1 channel inactivation.

New Conotoxin SO-3 Targeting N-type Voltage-Sensitive Calcium Channels

Selective blockers of the N-type voltage-sensitive calcium (Ca_V) channels are useful in the management of severe chronic pain. Here, the structure and function characteristics of a novel N-type Ca_V channel blocker, SO-3, are reviewed. SO-3 is a 25- amino acid conopeptide originally derived from the venom of Conus striatus, and contains the same 4-loop, 6-cysteine framework (C-C-CC-C-C) as O-superfamily conotoxins. The synthetic SO-3 has high analgesic activity similar to ω-conotoxin MVIIA (MVIIA), a selective N-type Ca_V channel blocker approved in the USA and Europe for the alleviation of persistent pain states. In electrophysiological studies, SO-3 shows more selectivity towards the N-type Ca_V channels than MVIIA. The dissimilarity between SO-3 and MVIIA in the primary and tertiary structures is further discussed in an attempt to illustrate the difference in selectivity of SO-3 and MVIIA towards N-type Ca_V channels.

Targeting chronic and neuropathic pain: the N-type calcium channel comes of age

The rapid entry of calcium into cells through activation of voltage-gated calcium channels directly affects membrane potential and contributes to electrical excitability, repetitive firing patterns, excitation-contraction coupling, and gene expression. At presynaptic nerve terminals, calcium entry is the initial trigger mediating the release of neurotransmitters via the calcium-dependent fusion of synaptic vesicles and involves interactions with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex of synaptic release proteins. Physiological factors or drugs that affect either presynaptic calcium channel activity or the efficacy of calcium-dependent vesicle fusion have dramatic consequences on synaptic transmission, including that mediating pain signaling. The N-type calcium channel exhibits a number of characteristics that make it an attractive target for therapeutic intervention concerning chronic and neuropathic pain conditions. Within the past year, both U.S. and European regulatory agencies have approved the use of the cationic peptide Prialt for the treatment of intractable pain. Prialt is the first N-type calcium channel blocker approved for clinical use and represents the first new proven mechanism of action for chronic pain intervention in many years. The present review discusses the rationale behind targeting the N-type calcium channel, some of the limitations confronting the widespread clinical application of Prialt, and outlines possible strategies to improve upon Prialt's relatively narrow therapeutic window.

Deficits in spatial learning and motor coordination in ADAM11-deficient mice

BACKGROUND

ADAM11 is a member of the ADAM gene family and is mainly expressed in the nervous system. It is thought to be an adhesion molecule, since it has a disintegrin-like domain related to cell-cell or cell-matrix interactions. To elucidate the physiological functions of ADAM11, we generated ADAM11-deficient mice by means of gene targeting.

RESULTS

ADAM11-deficient mice were apparently normal, and survived more than one year with no major histological abnormalities in the brain or spinal cord. Because ADAM11 is highly expressed in the hippocampus and cerebellum, we have examined ADAM11 mutant mice for learning using visual and hidden water maze tasks, and their motor coordination using a rotating rod task. Our results showed that their visual water maze task results are normal, but the hidden water maze and rotating rod task skills are impaired in ADAM11-deficient mice.

CONCLUSION

Our results indicate that ADAM11 mutation does not affect cell migration and differentiation during development, but affects learning and motor coordination. Thus, ADAM11 might play an important signalling or structural role as a cell adhesion molecule at the synapse, and may thus participate in synaptic regulation underlying behavioural changes.

Genetic background influences P/Q-type Ca2+ channel alpha1A subunit mRNA expression in olfactory bulb and reproductive ability of N-type Ca2+ channel alpha1B subunit-deficient mice

The Ca2+ channel alpha1B subunit is a pore-forming component capable of generating N-type Ca2+ channel activity. Although the N-type Ca2+ channel plays a role in a variety of neuronal functions, alpha1B-deficient mice did not show apparent behavioral abnormality. In a previous study, we observed a compensatory increase of mRNA expression of the P/Q-type Ca2+ channel alpha1A subunit gene in olfactory bulb of alpha1B-deficient mice with a CBA x C57BL/6 background; these mice showed a normal reproductive ability. In this study, we found that the mRNA expression level of the alpha1A subunit was the same in olfactory bulb of wild, heterozygous, and homozygous alpha1B-deficient mice with a CBA/JN background, and the homozygous male mice produced no offspring. These results suggest that the genetic background influences alpha1A subunit mRNA expression and reproductive ability in alpha1B-deficient mice.

Cell-specific alternative splicing increases calcium channel current density in the pain pathway

N-type calcium channels are critical for pain transduction. Inhibitors of these channels are powerful analgesics, but clinical use of current N-type blockers remains limited by undesirable actions in other regions of the nervous system. We now demonstrate that a unique splice isoform of the N-type channel is restricted exclusively to dorsal root ganglia. By a combination of functional and molecular analyses at the single-cell level, we show that the DRG-specific exon, e37a, is preferentially present in Ca(V)2.2 mRNAs expressed in neurons that contain nociceptive markers, VR1 and Na(V)1.8. Cell-specific inclusion of exon 37a correlates closely with significantly larger N-type currents in nociceptive neurons. This unique splice isoform of the N-type channel could represent a novel target for pain management.

Expression pattern of voltage-dependent calcium channel alpha1 and beta subunits in adrenal gland of N-type Ca2+ channel alpha1B subunit gene-deficient mice

The Ca2+ channel alpha1B subunit is a pore-forming component capable of generating N-type Ca2+ channel activity. Although N-type Ca2+ channel plays a role in a variety of neuronal functions, alpha1B-deficient mice exhibit normal life span without apparent abnormalities of behavior, histology or plasma norepinephrine level, presumably owing to compensation by some other Ca2+ channel alpha1 or beta subunit. In this study, we studied the levels of alpha1A, alpha1C, alpha1D, C1E, beta1, beta2, beta3 and beta4 mRNAs in adrenal gland of alpha1B-deficient mice. The alpha1A mRNA in homozygous mice was expressed at higher level than in wild or heterozygous mice, but no difference in the expression levels of alpha1c, alpha1D, alpha1E, beta1, beta2, beta3 and beta4 was found among wild, heterozygous and homozygous mice. The protein level of alpha1A in homozygous mice was also expressed at higher level than in wild or heterozygous mice. To examine whether increased expression is induced by cis-regulatory element within 5'-upstream region of alpha1A gene, we examined lacZ expression in alpha1B-deficient x alpha1A6.3-lacZ mice (carrying a 6.3-kb 5'-upstream fragment of alpha1A gene fused to E. coli lacZ reporter gene), which express lacZ in medullar chromaffin cells, but not in cortex. The levels of lacZ expression in homozygous alpha1B-deficient x alpha1A6.3-lacZ mice were higher than in wild or heterozygous mice. Therefore, a possible explanation of the normal behavior and plasma norepinephrine level of alpha1B-deficient mice is that compensation by alpha1A subunit occurs and that 6.3-kb 5'-upstream region of alpha1A gene contains enhancer cis-element(s) for compensation in adrenal medulla chromaffin cells.

Effects of Calmodulin and Ca2+ Channel Blockers on ω-conotoxin GVI A Binding to Crude Membranes from α1B Subunit (Cav2.2) Expressed BHK Cells and Mice Brain Lacking the α1B Subunits

Characteristics for the specific binding of 125I-omega-CTX GVIA and 125I-omega-CTX MVIIC to crude membranes from BHKN101 cells expressing the alpha1B subunits of Cav2.2 channels and from mice brain lacking the alpha1B subunits of Cav2.2 channels, particularly, the effects of CaM and various Ca2+ channel blockers on these specific bindings were investigated. Specific binding of 125I-omega-CTX GVIA to the crude membranes from BHKN101 cells was observed, but not from control BHK6 cells. omega-CTX GVIA, omega-CTX MVIIC and omega-CTX SVIB inhibited the specific binding of 125I-omega-CTX GVIA to crude membranes from BHKN101 cells, and the IC50 values for omega-CTXGVIA, omega-CTX MVIIC and omega-CTX SVIB were 0.07, 8.5 and 1.7 nM, respectively. However, omega-agatoxin IVA and calciseptine at concentrations of 10(-9)-10(-6) M did not inhibit specific binding. Specific binding was also about 80% inhibited by 20 microg protein/ml CaM. The amount of 125I-omega-CTX GVIA (30 pM) specifically bound to membranes from brain of knockout mice lacking alpha1B subunits of Cav2.2 channels was about 30% of that to the crude membranes from brain of wild-type. On the other hand, specific binding of 125I-omega-CTX MVIIC (200 pM) was observed on the crude membranes of both BHKN101 and control BHK6 cells. The specific binding of 125I-omega-CTX MVIIC (200 pM) was not inhibited by omega-CTX GVIA and omega-CTX SVIB, and also omega-Aga IVA and calciseptine at concentrations of 10(-9)-10(-7) M, although specific binding was almost completely dose dependently inhibited by non-radiolabeled omega-CTX MVIIC (IC50 value was about 0.1 nM). 20 microg protein/ml CaM did not inhibit specific binding. Therefore, these results suggest that BHKN101 cells have a typical Cav2.2 channels which are also inhibited by CaM and have not specific binding sites for omega-CTX MVIIC, although omega-CTX MVIIC is a blocker for both Cav2.1 (alpha1A; P/Q-type) and Cav2.2 channels.

Use-dependent blockade of Cav2.2 voltage-gated calcium channels for neuropathic pain

The translocation of extracellular calcium (Ca(2+)) via voltage-gated Ca(2+) channels (VGCCs) in neurons is involved in triggering multiple physiological cell functions but also the abnormal, pathophysiological responses that develop as a consequence of injury. In conditions of neuropathic pain, VGCCs are involved in supplying the signal Ca(2+) important for the sustained neuronal firing and neurotransmitter release characteristic of these syndromes. Preclinical data have identified N-type VGCCs (Ca(v)2.2) as key participants in contributing to these Ca(2+) signaling events and clinical data with the peptide blocker Prialt have now validated Ca(v)2.2 as a bona fide target for future drug discovery efforts to identify new and novel therapeutics for neuropathic pain. Imperative for the success of such an endeavor will be the ability to identify compounds selective for Ca(v)2.2, versus other VGCCs, but also compounds which demonstrate effective blockade during the pathophysiological states of neuropathic pain without compromising channel activity associated with sustaining normal housekeeping cellular functions. An approach to obtain this research target profile is to identify compounds, which are more potent in blocking Ca(v)2.2 during higher frequencies of firing as compared to the slower more physiologically-relevant frequencies. This may be achieved by identifying compounds with enhanced potency for the inactivated state of Ca(v)2.2. This commentary explores the rationale and options for engineering a use-dependent blocker of Ca(v)2.2. It is anticipated that this use-dependent profile of channel blockade will result in new chemical entities with an improved therapeutic ratio for neuropathic pain.

Identical phenotypes of CatSper1 and CatSper2 null sperm

Among several candidate Ca(2+) entry channels in sperm, only CatSper1 and CatSper2 are known to have required roles in male fertility. Past work with CatSper1 null sperm indicates that a critical lesion in hyperactivated motility underlies the infertility phenotype and is associated with an absence of depolarization-evoked Ca(2+)entry. Here we show that failure of hyperactivation of CatSper2 null sperm similarly correlates with an absence of depolarization evoked Ca(2+) entry. Additional shared aspects of the phenotypes of CatSper1 and -2 null sperm include unperturbed regional distributions of conventional voltage-gated Ca(2+) channel proteins and robust acceleration of the flagellar beat by bicarbonate. Further study reveals that treatment of both wild-type and CatSper2 null sperm with procaine increases beat asymmetry, a characteristic of the flagellar waveform of hyperactivation. This partial rescue of the loss-of-hyperactivation phenotype suggests that an absence of CatSper2 precludes hyperactivation by preventing delivery of needed Ca(2+) messenger rather than by preventing flagellar responses to Ca(2+). CatSper2 null sperm also have an increased basal cAMP content and beat frequency. Protein kinase A inhibitor H89 lowers beat frequency to that of wild-type sperm, suggesting that CatSper2 is required for protein kinase A-mediated, tonic control of resting cAMP content. Relative to wild-type testis, CatSper1 and -2 null testes contain normal amounts of CatSper2 and -1 transcripts, respectively. However, CatSper1 null sperm lack CatSper2 protein and CatSper2 null sperm lack CatSper1 protein. Hence, stable expression of CatSper1 protein requires CatSper2 and vice versa. This co-dependent expression dictates identical loss-of-function sperm phenotypes for CatSper1 and -2 null mutants.

Voltage-dependent calcium channels and cardiac pacemaker activity: from ionic currents to genes

The spontaneous activity of pacemaker cells in the sino-atrial node controls the heart rhythm and rate under physiological conditions. Compared to working myocardial cells, pacemaker cells express a specific array of ionic channels. The functional importance of different ionic channels in the generation and regulation of cardiac automaticity is currently subject of an extensive research effort and has long been controversial. Among families of ionic channels, Ca(2+) channels have been proposed to substantially contribute to pacemaking. Indeed, Ca(2+) channels are robustly expressed in pacemaker cells, and influence the cell beating rate. Furthermore, they are regulated by the activity of the autonomic nervous system in both a positive and negative way. In this manuscript, we will first discuss how the concept of the involvement of Ca(2+) channels in cardiac pacemaking has been proposed and then subsequently developed by the recent advent in the domain of cardiac physiology of gene-targeting techniques. Secondly, we will indicate how the specific profile of Ca(2+) channels expression in pacemaker tissue can help design drugs which selectively regulate the heart rhythm in the absence of concomitant negative inotropism. Finally, we will indicate how the new possibility to assign a specific gene activity to a given ionic channel involved in cardiac pacemaking could implement the current postgenomic research effort in the construction of the cardiac Physiome.

Function and dysfunction of synaptic calcium channels: insights from mouse models

In the past few years several spontaneous or engineered mouse models with mutations in Ca2+ channel genes have become available, providing a powerful approach to defining Ca2+ channel function in vivo. There have been recent advances in outlining the phenotypes and in the functional analysis of mouse models with mutations in genes encoding the pore-forming subunits of Ca(V)2.1 (P/Q-type), Ca(V)2.2 (N-type) and Ca(V)2.3 (R-type) Ca2+ channels, the channels involved in controlling neurotransmitter release at mammalian synapses. These data indicate that Ca(V)2.1 channels have a dominant and efficient specific role in initiating fast synaptic transmission at central excitatory synapses in vivo, and suggest that the Ca(V)2.1 channelopathies are primarily synaptic diseases. The different disorders probably arise from disruption of neurotransmission in specific brain regions: the cortex in the case of migraine, the thalamus in the case of absence epilepsy and the cerebellum in the case of ataxia.

Ataxia and peripheral nerve hypomyelination in ADAM22-deficient mice

Background

ADAM22 is a member of the ADAM gene family, but the fact that it is expressed only in the nervous systems makes it unique. ADAM22's sequence similarity to other ADAMs suggests it to be an integrin binder and thus to have a role in cell-cell or cell-matrix interactions. To elucidate the physiological functions of ADAM22, we employed gene targeting to generate ADAM22 knockout mice.

Results

ADAM22-deficient mice were produced in a good accordance with the Mendelian ratio and appeared normal at birth. After one week, severe ataxia was observed, and all homozygotes died before weaning, probably due to convulsions. No major histological abnormalities were detected in the cerebral cortex or cerebellum of the homozygous mutants; however, marked hypomyelination of the peripheral nerves was observed.

Conclusion

The results of our study demonstrate that ADAM22 is closely involved in the correct functioning of the nervous system. Further analysis of ADAM22 will provide clues to understanding the mechanisms of human diseases such as epileptic seizures and peripheral neuropathy.

Pattern of compensatory expression of voltage-dependent Ca2+ channel alpha1 and beta subunits in brain of N-type Ca2+ channel alpha1B subunit gene-deficient mice with a CBA/JN genetic background

The Ca(2+) channel alpha(1B) subunit is a pore-forming component capable of generating N-type Ca(2+) channel activity. Although the N-type Ca(2+) channel plays a role in a variety of neuronal functions, alpha(1B)-deficient mice with a CBA/JN genetic background show no apparent behavioral or anatomical-histological abnormality, presumably owing to compensation by other Ca(2+) channels. In this study, we examined the mRNA expression of the alpha(1A), alpha(1C), alpha(1D), alpha(1E), beta(1), beta(2), beta(3) and beta(4) subunits in the olfactory bulb, cerebral cortex, hippocampus and cerebellum of alpha(1B)-deficient mice. We found that the mRNA expression levels of the alpha(1A), alpha(1C), alpha(1D), alpha(1E), beta(1), beta(2), beta(3) and beta(4) subunits were the same in the olfactory bulbs of wild, heterozygous and homozygous alpha(1B)-deficient mice. In the cerebral cortex, alpha(1A) mRNA in homozygous alpha(1B)-deficient mice was expressed at a higher level than in wild or heterozygous mice, but no difference in the expression levels of the alpha(1C), alpha(1D), alpha(1E), beta(1), beta(2), beta(3) and beta(4) subunits was found among wild, heterozygous and homozygous mice. In hippocampus and cerebellum, beta(4) mRNA in homozygous alpha(1B)-deficient mice was expressed at a higher level than in wild or heterozygous mice, but no difference in the expression levels of the alpha(1A), alpha(1C), alpha(1D), alpha(1E), beta(1), beta(2) and beta(3) subunits was found among wild, heterozygous and homozygous mice. These results suggest that the compensatory mechanisms differ in different brain regions of alpha(1B)-deficient mice with a CBA/JN genetic background.

Inhibitory effect of pranidipine on N-type voltage-dependent Ca2+ channels in mice

We investigated the N-type voltage-dependent calcium channel blocking action of pranidipine, a novel dihydropyridine (DHP) derivative. Pranidipine significantly suppressed KCl-induced intracellular calcium changes ([Ca(2+)](i)) in a dose-dependent fashion in dorsal root ganglion neurons. A patch-clamp investigation revealed a dose-dependent blocking effect on N-type currents. Intrathecal injection of pranidipine significantly shortened the licking time in the late phase of the formalin test, as occurs with cilnidipine and amlodipine, which act on L- and N-type channels. Conversely, nicardipine, which acts exclusively on L-type channels, had no antinociceptive effect. Our results indicate that pranidipine inhibits N-type calcium channels. Furthermore, it exerts an antinociceptive effect, which might be related to an attenuation of synaptic transmission by nociceptive neurons due to the blocking effect of pranidipine on N-type calcium channels in primary nociceptive afferent fibers.

Drug targets in the voltage-gated calcium channel family: why some are and some are not

The L-type calcium channel antagonists have been, and continue to be, a very successful group of therapeutic agents targeted at cardiovascular disorders, notably angina and hypertension. The discovery that the voltage-gated calcium channels are a large and widely distributed family with important roles in both the peripheral and central nervous systems has initiated a major search for drugs active at other calcium channel types directed at disorders of the central nervous system, including pain, epilepsy, and stroke. These efforts have not been therapeutically successful thus far, and small molecule equivalents of the L-type blockers nifedipine, diltiazem, and verapamil directed at non-L-type channels have not been found. The underlying reasons for this are discussed together with suggestions for new directions, including fertility control, oxygen-sensitive channels, and calcium channel activators.

Expression analysis of P/Q-type Ca2+ channel α1A subunit mRNA in olfactory mitral cell in N-type Ca2+ channel α1B subunit gene-deficient mice

N-type and P/Q-type Ca2+ channels play an important role in the processing of olfactory information. However, N-type Ca2+ channel alpha1B-deficient mice show normal behavior, presumably owing to compensation by other Ca2+ channels. P/Q-type Ca2+ channel alpha1A mRNA was expressed at a higher level in olfactory bulb of homozygous alpha1B-deficient mice than wild-type or heterozygous mice. LacZ expression in olfactory mitral cells of homozygous alpha1B-deficient x alpha1A1.5-lacZ mice, carrying a 1.5-kb 5'-upstream fragment of the alpha1A gene fused to the lacZ reporter gene, was increased compared to that in wild-type or heterozygous mice. Therefore, a possible explanation for the normal behavior of alpha1B-deficient mice is compensation by the alpha1A gene and that the 1.5-kb 5'-upstream region of this gene contains an enhancer cis-element for compensation in olfactory mitral cells.

Increased expression of P/Q-type Ca2+ channel α1A subunit mRNA in cerebellum of N-type Ca2+ channel α1B subunit gene-deficient mice

The Ca(2+) channel alpha(1B) subunit is a pore-forming component capable of generating N-type Ca(2+) channel activity. Although the N-type Ca(2+) channel plays a role in a variety of neuronal functions, alpha(1B)-deficient mice show normal behavior, presumably owing to compensation by the other Ca(2+) channels. In this study, we examined the mRNA expression of the P/Q-type Ca(2+) channel alpha(1A) subunit in cerebellum of alpha(1B)-deficient mice. The alpha(1A) subunit mRNA in homozygous alpha(1B)-deficient mice was expressed at a significantly higher level than in wild or heterozygous mice. To examine whether the increased expression is induced by a cis-regulatory element within the 5'-upstream region of the alpha(1A) subunit gene, we examined lacZ expression in alpha(1B)-deficient x alpha(1A)3.0-lacZ mice (carrying a 3.0-kb 5'-upstream fragment of the alpha(1A) subunit gene fused to Escherichia coli lacZ reporter gene), which express lacZ in granule but not in Purkinje cells, and in alpha(1B)-deficient x alpha(1A)6.3-lacZ mice (carrying a 6.3-kb 5'-upstream region fused to lacZ gene), which express lacZ in Purkinje but not in granule cells. The levels of lacZ expression in homozygous alpha(1B)-deficient x alpha(1A)3.0-lacZ mice were significantly higher than in wild or heterozygous mice, but no difference in lacZ expression level was found among wild, heterozygous and homozygous alpha(1B)-deficient x alpha(1A)6.3-lacZ mice. Therefore, a possible explanation of the normal behavior of alpha(1B)-deficient mice is that compensation by alpha(1A) subunit gene occurs and that the 3.0-kb 5'-upstream region of alpha(1A) subunit gene contains an enhancer cis-element(s) for compensation in cerebellar granule cells.

Differential involvement of N-type calcium channels in transmitter release from vasoconstrictor and vasodilator neurons

1. The effects of calcium channel blockers on co-transmission from different populations of autonomic vasomotor neurons were studied on isolated segments of uterine artery and vena cava from guinea-pigs. 2. Sympathetic, noradrenergic contractions of the uterine artery (produced by 200 pulses at 1 or 10 Hz; 600 pulses at 20 Hz) were abolished by the N-type calcium channel blocker omega-conotoxin (CTX) GVIA at 1-10 nm. 3. Biphasic sympathetic contractions of the vena cava (600 pulses at 20 Hz) mediated by noradrenaline and neuropeptide Y were abolished by 10 nm CTX GVIA. 4. Neurogenic relaxations of the uterine artery (200 pulses at 10 Hz) mediated by neuronal nitric oxide and neuropeptides were reduced <50% by CTX GVIA 10-100 nm. 5. Capsaicin (3 microm) did not affect the CTX GVIA-sensitive or CTX GVIA-resistant neurogenic relaxations of the uterine artery. 6. The novel N-type blocker CTX CVID (100-300 nm), P/Q-type blockers agatoxin IVA (10-100 nm) or CTX CVIB (100 nm), the L-type blocker nifedipine (10 microm) or the 'R-type' blocker SNX-482 (100 nm), all failed to reduce CTX GVIA-resistant relaxations. The T-type channel blocker NiCl(2) (100-300 microm) reduced but did not abolish the remaining neurogenic dilations. 7. Release of different neurotransmitters from the same autonomic vasomotor axon depends on similar subtypes of calcium channels. N-type channels are responsible for transmitter release from vasoconstrictor neurons innervating a muscular artery and capacitance vein, but only partly mediate release of nitric oxide and neuropeptides from pelvic vasodilator neurons.

Voltage-dependent calcium channels are involved in neurogenic dural vasodilatation via a presynaptic transmitter release mechanism

Amissense mutation of the CACNA1A gene that encodes the alpha1A subunit of the voltage-dependent P/Q-type calcium channel has been discovered in patients suffering from familial hemiplegic migraine. This suggested that calcium channelopathies may be involved in migraine more broadly, and established the importance of genetic mechanisms in migraine. Channelopathies share many clinical characteristics with migraine, and thus exploring calcium channel functions in the trigeminovascular system may give insights into migraine pathophysiology. It is also known that drugs blocking the P/Q- and N-type calcium channels have been successful in other animal models of trigeminovascular activation and head pain. In the present study, we used intravital microscopy to examine the effects of specific calcium channel blockers on neurogenic dural vasodilatation and calcitonin gene-related peptide (CGRP)-induced dilation. The L-type voltage-dependent calcium channel blocker calciseptine significantly attenuated (20 microg kg(-1), n=7) the dilation brought about by electrical stimulation, but did not effect CGRP-induced dural dilation. The P/Q-type voltage-dependent calcium channel blocker omega-agatoxin-IVA (20 microg kg-1, n=7) significantly attenuated the dilation brought about by electrical stimulation, but did not effect CGRP-induced dural dilation. The N-type voltage-dependent calcium channel blocker omega-conotoxin-GVIA (20 microg kg(-1), n=8 and 40 microg kg(-1), n=7) significantly attenuated the dilation brought about by electrical stimulation, but did not effect CGRP-induced dural dilation. It is thought that the P/Q-, N- and L-type calcium channels all exist presynaptically on trigeminovascular neurons, and blockade of these channels prevents CGRP release, and, therefore, dural blood vessel dilation. These data suggest that the P/Q-, N- and L-type calcium channels may be involved in trigeminovascular nociception.

N-type calcium channel alpha1B subunit (Cav2.2) knock-out mice display hyperactivity and vigilance state differences

Differential properties of voltage-dependent Ca2+ channels have been primarily ascribed to the alpha1 subunit, of which 10 different subtypes are currently known. For example, channels that conduct the N-type Ca2+ current possess the alpha1B subunit (Cav2.2), which has been localized, inter alia, to the piriform cortex, hippocampus, hypothalamus, locus coeruleus, dorsal raphe, thalamic nuclei, and granular layer of the cortex. Some of these regions have been previously implicated in metabolic and vigilance state control, and selective block of the N-type Ca2+ channel causes circadian rhythm disruption. In this study of Cav2.2-/- knock-out mice, we examined potential differences in feeding behavior, spontaneous locomotion, and the sleep-wake cycle. Cav2.2-/- mice did not display an overt metabolic phenotype but were hyperactive, demonstrating a 20% increase in activity under novel conditions and a 95% increase in activity under habituated conditions during the dark phase, compared with wild-type littermates. Cav2.2-/- mice also displayed vigilance state differences during the light phase, including increased consolidation of rapid-eye movement (REM) sleep and increased intervals between non-REM (NREM) and wakefulness episodes. EEG spectral power was increased during wakefulness and REM sleep and was decreased during NREM sleep in Cav2.2-/- mice. These results indicate a role of the N-type Ca2+ channel in activity and vigilance state control, which we interpret in terms of effects on neurotransmitter release.

N-type calcium channel alpha1B subunit (Cav2.2) knock-out mice display hyperactivity and vigilance state differences

Differential properties of voltage-dependent Ca2+ channels have been primarily ascribed to the alpha1 subunit, of which 10 different subtypes are currently known. For example, channels that conduct the N-type Ca2+ current possess the alpha1B subunit (Cav2.2), which has been localized, inter alia, to the piriform cortex, hippocampus, hypothalamus, locus coeruleus, dorsal raphe, thalamic nuclei, and granular layer of the cortex. Some of these regions have been previously implicated in metabolic and vigilance state control, and selective block of the N-type Ca2+ channel causes circadian rhythm disruption. In this study of Cav2.2-/- knock-out mice, we examined potential differences in feeding behavior, spontaneous locomotion, and the sleep-wake cycle. Cav2.2-/- mice did not display an overt metabolic phenotype but were hyperactive, demonstrating a 20% increase in activity under novel conditions and a 95% increase in activity under habituated conditions during the dark phase, compared with wild-type littermates. Cav2.2-/- mice also displayed vigilance state differences during the light phase, including increased consolidation of rapid-eye movement (REM) sleep and increased intervals between non-REM (NREM) and wakefulness episodes. EEG spectral power was increased during wakefulness and REM sleep and was decreased during NREM sleep in Cav2.2-/- mice. These results indicate a role of the N-type Ca2+ channel in activity and vigilance state control, which we interpret in terms of effects on neurotransmitter release.

Alpha-neurexins couple Ca2+ channels to synaptic vesicle exocytosis

Synapses are specialized intercellular junctions in which cell adhesion molecules connect the presynaptic machinery for neurotransmitter release to the postsynaptic machinery for receptor signalling. Neurotransmitter release requires the presynaptic co-assembly of Ca2+ channels with the secretory apparatus, but little is known about how synaptic components are organized. Alpha-neurexins, a family of >1,000 presynaptic cell-surface proteins encoded by three genes, link the pre- and postsynaptic compartments of synapses by binding extracellularly to postsynaptic cell adhesion molecules and intracellularly to presynaptic PDZ domain proteins. Using triple-knockout mice, we show that alpha-neurexins are not required for synapse formation, but are essential for Ca2+-triggered neurotransmitter release. Neurotransmitter release is impaired because synaptic Ca2+ channel function is markedly reduced, although the number of cell-surface Ca2+ channels appears normal. These data suggest that alpha-neurexins organize presynaptic terminals by functionally coupling Ca2+ channels to the presynaptic machinery.

Genetic analysis of a synaptic calcium channel in Drosophila: intragenic modifiers of a temperature-sensitive paralytic mutant of cacophony

Our previous genetic analysis of synaptic mechanisms in Drosophila identified a temperature-sensitive paralytic mutant of the voltage-gated calcium channel alpha1 subunit gene, cacophony (cac). Electrophysiological studies in this mutant, designated cac(TS2), indicated cac encodes a primary calcium channel alpha1 subunit functioning in neurotransmitter release. To further examine the functions and interactions of cac-encoded calcium channels, a genetic screen was performed to isolate new mutations that modify the cac(TS2) paralytic phenotype. The screen recovered 10 mutations that enhance or suppress cac(TS2), including second-site mutations in cac (intragenic modifiers) as well as mutations mapping to other genes (extragenic modifiers). Here we report molecular characterization of three intragenic modifiers and examine the consequences of these mutations for temperature-sensitive behavior, synaptic function, and processing of cac pre-mRNAs. These mutations may further define the structural basis of calcium channel alpha1 subunit function in neurotransmitter release.

Altered properties of quantal neurotransmitter release at endplates of mice lacking P/Q-type Ca2+ channels

Transmission at the mouse neuromuscular junction normally relies on P/Q-type channels, but became jointly dependent on both N- and R-type Ca(2+) channels when the PQ-type channel alpha(1A) subunit was deleted. R-type channels lay close to Ca(2+) sensors for exocytosis and I(K(Ca)) channel activation, like the P/Q-type channels they replaced. In contrast, N-type channels were less well localized, but abundant enough to influence secretion strongly, particularly when action potentials were prolonged. Our data suggested that active zone structures may select among multiple Ca(2+) channels in the hierarchy P/Q >R >N. The alpha(1A)-/- neuromuscular junction displayed several other differences from wild-type: lowered quantal content but greater ability to withstand reductions in the Ca(2+)/Mg(2+) ratio, and little or no paired-pulse facilitation, the latter findings possibly reflecting compensatory mechanisms at individual release sites. Changes in presynaptic function were also associated with a significant reduction in the size of postsynaptic acetylcholine receptor clusters.

Synaptic calcium-channel function in Drosophila: analysis and transformation rescue of temperature-sensitive paralytic and lethal mutations of cacophony

Voltage-gated calcium channels play a key role in chemical synaptic transmission by providing the calcium trigger for regulated neurotransmitter release. Genes encoding the primary structural subunit, alpha1, as well as accessory subunits of presynaptic calcium channels have now been identified in a variety of organisms. The cacophony (cac) gene in Drosophila, also known as nightblind A, encodes a voltage-gated calcium-channel alpha1 subunit homologous to vertebrate alpha1 subunits implicated in neurotransmitter release. A recent genetic screen in our laboratory isolated cac(TS2), a conditional cac mutant exhibiting rapid paralysis at elevated temperatures. This mutant has allowed synaptic electrophysiology after acute perturbation of a specific calcium-channel gene product, demonstrating that cac encodes a primary calcium channel functioning in neurotransmitter release. Here we report the molecular lesion in cac(TS2), a missense mutation within a calcium-dependent regulatory domain of the alpha1 subunit, as well as phenotypic rescue of temperature-sensitive and lethal cac mutations by transgenic expression of a wild-type cac cDNA. Notably, rescue of rapid, calcium-triggered neurotransmitter release was achieved by neural expression of a single cDNA containing a subset of alternative exons and lacking any conserved synaptic-protein interaction sequence. Possible implications of these findings are discussed in the context of structure-function studies of synaptic calcium channels, as well as alternative splicing and mRNA editing of the cac transcript.

The vasorelaxant effect of pituitary adenylate cyclase activating polypeptide and vasoactive intestinal polypeptide in isolated rat basilar arteries is partially mediated by activation of nitrergic neurons

The structurally related neuropeptides pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are recognised by two G protein-coupled receptors, termed VPAC(1)-R and VPAC(2)-R, with equal affinity. PACAP and VIP have previously been shown to relax cerebral arteries in an endothelium-independent manner. The aim of the present study was to test if intramural neurons are involved in the mediation of PACAP/VIP-induced vasodilatory responses. Therefore, the vascular tone of isolated rat basilar arteries was measured by means of a myograph. The vasorelaxing effect of PACAP was assessed in arteries precontracted by serotonin in the absence or presence of different test compounds known to selectively inhibit certain signaling proteins. The vasorelaxant effect of PACAP could be significantly reduced by the inhibitor of neuronal N-type calcium channels omega-conotoxin GVIA (omega-CgTx), as well as by 3-bromo-7-nitroindazole (3Br-7-Ni), an inhibitor of the neuronal nitric oxide-synthase (nNOS). The localization of N-type calcium channels and VPAC-Rs within the rat basilar artery was investigated by confocal laser scanning microscopy using omega-CgTx- and VIP-analogs labelled with fluorescent dyes. These findings suggest that activation of intramural neurons may represent an important effector mechanism for mediation of the vasorelaxant PACAP-response.

Bidirectional alterations in cerebellar synaptic transmission of tottering and rolling Ca2+ channel mutant mice

Hereditary ataxic mice, tottering (tg) and rolling Nagoya (tg(rol)), carry mutations in the P/Q-type Ca(2+) channel alpha(1A) subunit gene. The positions of the mutations and the neurological phenotypes are known, but the mechanisms of how the mutations cause the symptoms and how the different mutations lead to various onset and severity have remained unsolved. Here we compared fundamental properties of excitatory synaptic transmission in the cerebellum and roles of Ca(2+) channel subtypes therein among wild-type control, tg, and tg(rol) mice. The amplitude of EPSC of the parallel fiber-Purkinje cell (PF-PC) synapses was considerably reduced in ataxic tg(rol). Although the amplitude of the parallel fiber-mediated EPSC was only mildly decreased in young non-ataxic tg mice, it was drastically diminished in adult ataxic tg mice of postnatal day 28-35, showing a good correlation between the impairment of the PF-PC synaptic transmission and manifestation of ataxia. In contrast, the EPSC amplitude of the climbing fiber-Purkinje cell (CF-PC) synapses was preserved in tg, and it was even increased in tg(rol), which was associated with altered properties of the postsynaptic glutamate receptors. The climbing fiber-mediated EPSC was more dependent on other Ca(2+) channel subtypes in mutant mice, suggesting that such compensatory mechanisms contribute to maintaining the CF-PC synaptic transmission virtually intact. The results indicate that different mutations of the P/Q-type Ca(2+) channel not only cause the primary effect of different severity but also lead to diverse additional secondary effects, resulting in disruption of well balanced neural networks.

Effects of ablation of N- and R-type Ca(2+) channels on pain transmission

Recently several mutant mouse lines lacking neuronal voltage-dependent Ca(2+) channels (VDCCs) have been established by the use of gene targeting in embryonic stem cells. Pain-related behaviors in Ca(v)2.2 (alpha(1B)) and Ca(v)2.3 (alpha(1E)) knockout mice were studied to gain further insight into the mechanism of pain transmission, where VDCCs are thought to play important roles. We review here the data from these recent studies. Ca(v)2.3-/- mice showed normal responses to acute painful stimuli, and reduced responses to the somatic inflammatory pain stimuli. Ca(v)2.3+/- mice exhibited reduced symptoms of visceral inflammatory pain. Ca(v)2.3-/- mice showed abnormal behavior related to the descending antinociceptive mechanism activated by the intraperitoneal injection of acetic acid. Ca(v)2.2-/- mice showed variable acute nociceptive responses depending on the mutant lines. However, all the lines of Ca(v)2.2-/- mice exhibited reduced responses in the phase 2 of the formalin test, suggesting a suppression of inflammatory pain. Furthermore Ca(v)2.2-/- mice showed markedly reduced neuropathic pain symptoms after spinal nerve ligation. Impaired antinociception, similar to that seen in the Ca(v)2.3-/- mice, was also observed in the Ca(v)2.2-/- mice. Therefore, it is suggested that these mutant mice could provide novel models to delineate the nociceptive and antinociceptive mechanisms.

Use of transgenic mice to study voltage-dependent Ca2+ channels

During the past decade a great number of genes encoding high- and low-voltage-dependent Ca(2+) channels and their accessory subunits have been cloned. Studies of Ca(2+) channel structure-function relationships and channel regulation using cDNA expression in heterologous expression systems have revealed intricate details of subunit interaction, regulation of channels by protein kinase A (PKA) and protein kinase C (PKC), drug binding sites, mechanisms of drug action, the ion conduction pathway and other aspects of channel function. In recent years, however, we have arrived at the brink of an entirely new strategy to study Ca(2+) channels by overexpressing or knocking out genes encoding these channels in transgenic mice. In this article, various models of gene knockout or gene overexpression will be discussed. This new approach will reveal many secrets regarding Ca(2+) channel regulation and the control of Ca(2+)-dependent cellular processes.

Differential nociceptive responses in mice lacking the alpha(1B) subunit of N-type Ca(2+) channels

The role of N-type Ca(2+) channels in nociceptive transmission was examined in genetically engineered mice lacking the alpha(1B) subunit of N-type channels and in their heterozygote and wild-type littermates. In alpha(1B)-deficient mice, N-type channel activities in dorsal root ganglion neurons and spinal synaptoneurosomes were eliminated without compensation by other types of voltage-dependent Ca(2+) channels. The alpha(1B)-deficient mice showed a diminution in the phase 2 nociceptive responses more extensively than in the phase 1 nociceptive responses of the formalin test. The alpha(1B)-deficient mice exhibited significantly increased thermal nociceptive thresholds in the hot plate test, but failed to increase mechanical nociceptive thresholds in the tail pinch test. These results suggest a crucial role of N-type channels in nociceptive transmission, especially for persistent pain like phase 2 of the formalin test and for nociception induced by thermal stimuli.

Rocking and rolling with Ca2+ channels

Voltage-sensitive Ca2+ channels are a large family of related heterooligomers that couple cell excitability to intracellular signalling. Recent studies on mice carrying Ca2+ channel mutations, or in which Ca2+ channel subunits have been deleted, have provided important information about the roles carried out by these molecules in cardiovascular function, pain, epilepsy, migraine and deafness, in addition to further defining how Ca2+ channels regulate the physiology of excitable cells.