Neuronal calcium channels encoded by the alpha 1A subunit and their contribution to excitatory synaptic transmission in the CNS

Colonic overexpression of the T-type calcium channel Cav 3.2 in a mouse model of visceral hypersensitivity and in irritable bowel syndrome patients

BACKGROUND

Among the different mechanisms involved in irritable bowel syndrome (IBS) physiopathology, visceral hypersensitivity seems to play a key role. It involves sensitization of the colonic primary afferent fibers, especially through an overexpression of ion channels. The aims of this translational study were to investigate the colonic expression of Ca_v 3.2 calcium channels and their involvement in an animal model of colonic hypersensitivity, and to assess their expression in the colonic mucosa of symptomatic IBS patients.

METHODS

This bench-to-bed study combined a preclinical experimental study on mice and a case-control clinical study. Preclinical studies were performed on wild-type and Ca_v 3.2-KO mice. Colonic sensitivity and Ca_v 3.2 expression were studied after a low-dose treatment of dextran sodium sulfate (DSS 0.5%). Regarding the clinical study, colonic biopsies were performed in 14 IBS patients and 16 controls during a colonoscopy to analyze the mucosal Ca_v 3.2 expression.

KEY RESULTS

Wild-type, but not Ca_v 3.2-KO, mice developed visceral hypersensitivity without colonic inflammation, after 0.5% DSS treatment. A significant increase of Ca_v 3.2 mRNA (p = 0.04) was found in the colon of low-dose DSS-treated wild-type (WT) mice compared to their controls. In human colonic biopsies, the Ca_v 3.2 mRNA level was significantly higher in the IBS group compared to the control group (p = 0.01). The immunofluorescence staining revealed their protein expression in colonic mucosa, particularly in nerve fibers.

CONCLUSIONS & INFERENCES

This translational study supports the involvement of the calcium channels Ca_v 3.2 in abdominal pain, as observed in IBS patients. It opens new therapeutic perspectives based on molecules specifically blocking these channels.

Upregulation of L-type calcium channels in colonic inhibitory motoneurons of P/Q-type calcium channel-deficient mice

Enteric inhibitory motoneurons use nitric oxide and a purine neurotransmitter to relax gastrointestinal smooth muscle. Enteric P/Q-type Ca2+ channels contribute to excitatory neuromuscular transmission; their contribution to inhibitory transmission is less clear. We used the colon from tottering mice (tg/tg, loss of function mutation in the α1A pore-forming subunit of P/Q-type Ca2+ channels) to test the hypothesis that P/Q-type Ca2+ channels contribute to inhibitory neuromuscular transmission and colonic propulsive motility. Fecal pellet output in vivo and the colonic migrating motor complex (ex vivo) were measured. Neurogenic circular muscle relaxations and inhibitory junction potentials (IJPs) were also measured ex vivo. Colonic propulsive motility in vivo and ex vivo was impaired in tg/tg mice. IJPs were either unchanged or somewhat larger in tissues from tg/tg compared with wild-type (WT) mice. Nifedipine (L-type Ca2+ channel antagonist) inhibited IJPs by 35 and 14% in tissues from tg/tg and WT mice, respectively. The contribution of N- and R-type channels to neuromuscular transmission was larger in tissues from tg/tg compared with WT mice. The resting membrane potential of circular muscle cells was similar in tissues from tg/tg and WT mice. Neurogenic relaxations of circular muscle from tg/tg and WT mice were similar. These results demonstrate that a functional deficit in P/Q-type channels does not alter propulsive colonic motility. Myenteric neuron L-type Ca2+ channel function increases to compensate for loss of functional P/Q-type Ca2+ channels. This compensation maintains inhibitory neuromuscular transmission and normal colonic motility.

Calcium current homeostasis and synaptic deficits in hippocampal neurons from Kelch-like 1 knockout mice

Kelch-like 1 (KLHL1) is a neuronal actin-binding protein that modulates voltage-gated Ca_V2.1 (P/Q-type) and Ca_V3.2 (α_1H T-type) calcium channels; KLHL1 knockdown experiments (KD) cause down-regulation of both channel types and altered synaptic properties in cultured rat hippocampal neurons (Perissinotti et al., 2014). Here, we studied the effect of ablation of KLHL1 on calcium channel function and synaptic properties in cultured hippocampal neurons from KLHL1 knockout (KO) mice. Western blot data showed the P/Q-type channel α_1A subunit was less abundant in KO hippocampus compared to wildtype (WT); and P/Q-type calcium currents were smaller in KO neurons than WT during early days in vitro, although this decrease was compensated for at late stages by increases in L-type calcium current. In contrast, T-type currents did not change in culture. However, biophysical properties and western blot analysis revealed a differential contribution of T-type channel isoforms in the KO, with Ca_V3.2 α_1H subunit being down-regulated and Ca_V3.1 α_1G up-regulated. Synapsin I levels were also reduced in the KO hippocampus and cultured neurons displayed a concomitant reduction in synapsin I puncta and decreased miniature excitatory postsynaptic current (mEPSC) frequency. In summary, genetic ablation of the calcium channel modulator resulted in compensatory mechanisms to maintain calcium current homeostasis in hippocampal KO neurons; however, synaptic alterations resulted in a reduction of excitatory synapse number, causing an imbalance of the excitatory-inhibitory synaptic input ratio favoring inhibition.

Down-regulation of endogenous KLHL1 decreases voltage-gated calcium current density

The actin-binding protein Kelch-like 1 (KLHL1) can modulate voltage-gated calcium channels in vitro. KLHL1 interacts with actin and with the pore-forming subunits of Cav2.1 and CaV3.2 calcium channels, resulting in up-regulation of P/Q and T-type current density. Here we tested whether endogenous KLHL1 modulates voltage gated calcium currents in cultured hippocampal neurons by down-regulating the expression of KLHL1 via adenoviral delivery of shRNA targeted against KLHL1 (shKLHL1). Control adenoviruses did not affect any of the neuronal properties measured, yet down-regulation of KLHL1 resulted in HVA current densities ~68% smaller and LVA current densities 44% smaller than uninfected controls, with a concomitant reduction in α(1A) and α(1H) protein levels. Biophysical analysis and western blot experiments suggest Ca(V)3.1 and 3.3 currents are also present in shKLHL1-infected neurons. Synapsin I levels, miniature postsynaptic current frequency, and excitatory and inhibitory synapse number were reduced in KLHL1 knockdown. This study corroborates the physiological role of KLHL1 as a calcium channel modulator and demonstrates a novel, presynaptic role.

Voltage gated sodium and calcium channel blockers for the treatment of chronic inflammatory pain

The inflammatory response is a natural response of the body that occurs immediately following tissue damage, which may be due to injury, infection or disease. The acute inflammatory response is an essential mechanism that promotes healing and a key aspect is the ensuing pain, which warns the subject to protect the site of injury. Thus, it is common to see a zone of primary sensitization as well as consequential central sensitization that generally, is maintained by a peripheral drive from the zone of tissue injury. Inflammation associated with chronic pain states, such as rheumatoid and osteoarthritis, cancer and migraine etc. is deleterious to health and often debilitating for the patient. Thus there is a large unmet clinical need. The mechanisms underlying both acute and chronic inflammatory pain are extensive and complex, involving a diversity of cell types, receptors and proteins. Among these the contribution of voltage gated sodium and calcium channels on peripheral nociceptors is critical for nociceptive transmission beyond the peripheral transducers and changes in their distribution, accumulation, clustering and functional activities have been linked to both inflammatory and neuropathic pain. The latter has been the main area for trials and use of drugs that modulate ion channels such as carbamazepine and gabapentin, but given the large peripheral drive that follows tissue damage, there is a clear rationale for blocking voltage gated sodium and calcium channels in these pain states. It has been hypothesized that pain of inflammatory origin may evolve into a condition that resembles neuropathic pain, but mixed pains such as low back pain and cancer pain often include elements of both pain states. This review considers the therapeutic potential for sodium and calcium channel blockers for the treatment of chronic inflammatory pain states.

Severely impaired neuromuscular synaptic transmission causes muscle weakness in theCacna1a-mutant mouserolling Nagoya

The ataxic mouse rolling Nagoya (RN) carries a missense mutation in the Cacna1a gene, encoding the pore-forming subunit of neuronal Ca(v)2.1 (P/Q-type) Ca2+ channels. Besides being the predominant type of Ca(v) channel in the cerebellum, Ca(v)2.1 channels mediate acetylcholine (ACh) release at the peripheral neuromuscular junction (NMJ). Therefore, Ca(v)2.1 dysfunction induced by the RN mutation may disturb ACh release at the NMJ. The dysfunction may resemble the situation in Lambert-Eaton myasthenic syndrome (LEMS), in which autoantibodies target Ca(v)2.1 channels at NMJs, inducing severely reduced ACh release and resulting in muscle weakness. We tested neuromuscular function of RN mice and characterized transmitter release properties at their NMJs in diaphragm, soleus and flexor digitorum brevis muscles. Clinical muscle weakness and fatigue were demonstrated using repetitive nerve-stimulation electromyography, grip strength testing and an inverted grid hanging test. Muscle contraction experiments showed a compromised safety factor of neuromuscular transmission. In ex vivo electrophysiological experiments we found severely impaired ACh release. Compared to wild-type, RN NMJs had 50-75% lower nerve stimulation-evoked transmitter release, explaining the observed muscle weakness. Surprisingly, the reduction in evoked release was accompanied by an approximately 3-fold increase in spontaneous ACh release. This synaptic phenotype suggests a complex effect of the RN mutation on different functional Ca(v)2.1 channel parameters, presumably with a positive shift in activation potential as a prevailing feature. Taken together, our studies indicate that the gait abnormality of RN mice is due to a combination of ataxia and muscle weakness and that RN models aspects of the NMJ dysfunction in LEMS.

Cannabinoid receptor agonists inhibit Ca(2+) influx to synaptosomes from rat brain

We examined the effects of cannabinoid receptor agonists on (45)Ca(2+) uptake in rat brain synaptosomes. A cannabinoid receptor agonist, (R)-(+)-[2,3-dihydro-5-methyl-3-[(4-merpholino)methyl]pyrrolo-[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthyl)methanone (WIN 55212-2) dose-dependently inhibited (45)Ca(2+) uptake in rat synaptosomes. Only an endogenous cannabinoid receptor agonist, anandamide, dose-dependently inhibited (45)Ca(2+) uptake in rat synaptosomes, but not an endogenous cannabinoid receptor agonist, palmitoylethanolamide. Only a cannabinoid CB1 antagonist, [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride] (SR 141716A), reversed the inhibitory effect of these WIN 55212-2 and anandamide on (45)Ca(2+) uptake in rat synaptosomes, but not a cannabinoid CB2 receptor antagonist, [N-[(1S)-endo-1,3,3-trimethylbicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)pyrazole-3-carboxamide] (SR 144528). The inhibitory effects of WIN 55212-2 and anandamide on (45)Ca(2+) uptake in rat synaptosomes were reversed by the pretreatment of a voltage-sensitive A-type K(+) channel blocker, dendrotoxin, but no other type of K(+) channel blockers, i.e. iberiotoxin, charybdotoxin or glibenclamide. These findings suggest that cannabinoid receptors inhibit Ca(2+) influx into rat brain nerves via the activation of CB1 receptors and the opening of voltage-sensitive A-type K(+) channels.

Functional compensation by other voltage-gated Ca2+ channels in mouse basal forebrain neurons with Ca(V)2.1 mutations

Tottering (tg/tg) and leaner (tg(la)/tg(la)) mutant mice exhibit distinct mutations in the gene encoding the voltage-activated Ca(2+) channel alpha(1A) subunit (CACNA1A), the pore-forming subunit of the Ca(V)2.1 (P/Q type) Ca(2+) channels. These mice exhibit absence seizures and deficiencies in motor control and other functions. Previous work in cerebellar Purkinje neurons has shown that these mutations cause dramatic reductions in calcium channel function. Because Purkinje cell somata primarily express the Ca(V)2.1 channels, the general decrease in Ca(V)2.1 channel function is observed as a profound decrease in whole-cell current. In contrast to Purkinje cells, basal forebrain (BF) neurons express all of the Ca(2+) channel alpha(1) subunits, with Ca(V)2.1 contributing approximately 30% to the whole-cell current in wild-type (+/+) mice. Here, we show that whole-cell Ba(2+) current densities in BF neurons are not reduced in the mutant genotypes despite a reduction in the Ca(V)2.1 contribution. By blocking the different Ca(2+) channel subtypes with specific pharmacological agents, we found a significant increase in the proportion of Ca(V)1 Ca(2+) current in mutant phenotypes. There was no change in tissue mRNA expression of calcium channel subtypes Ca(V)2.1, Ca(V)2.2, Ca(V)1.2, Ca(V)1.3, and Ca(V)2.3 in the tottering and leaner mutant mice. These results suggest that Ca(V)1 channels may functionally upregulate to compensate for reduced Ca(V)2.1 function in the mutants without an increase in Ca(v)1 message. Single-cell reverse transcription polymerase chain reaction (RT-PCR) experiments in a subset of sampled neurons revealed that approximately 90% of the cells could be considered cholinergic based on choline acetyltransferase (ChAT) mRNA expression.

Developmental consequences of neuromuscular junctions with reduced presynaptic calcium channel function

Evoked neurotransmitter release at the Drosophila neuromuscular junction (NMJ) is regulated by the amount of calcium influx at the presynaptic nerve terminal, as for most chemical synapses. Calcium entry occurs via voltage-gated calcium channels. The temperature-sensitive Drosophila mutant, cac(TS2), has a reduced amount of calcium entry during evoked stimulation. We have used this mutation to examine homeostatic regulatory mechanisms during development of the NMJ on muscle 6 within the developing larva. The amplitude of the excitatory postsynaptic potentials are reduced for both the Ib and Is motor neurons in 3rd instar larvae which have been raised at 33 degrees C from the 1st instar stage. Larvae raised at 25 degrees C and larvae pulsed at 33 degrees C from the late 2nd instar for various lengths of time show a reduced synaptic efficacy as a 3rd instar. The results indicate that the nerve terminal cannot fully compensate physiologically in the regulation of synaptic transmission during larval life for a reduced amount of evoked calcium entry. Morphological comparisons of Ib and Is terminals in relation to length and numbers of varicosities are significantly reduced in cac(TS2), which also suggests a lack in homeostatic ability. These findings are relevant since many deficits in synaptic transmission in various systems are compensated for either physiologically or structural over development, but not in this case for reduced calcium entry during evoked transmission.

Voltage-gated cation channel modulators for the treatment of stroke

Neuronal voltage-gated cation channels regulate the transmembrane flux of calcium, sodium and potassium. Neuronal ischaemia occurring during acute ischaemic stroke results in the breakdown in the normal function of these ion channels, contributing to a series of pathological events leading to cell death. A dramatic increase in the intracellular concentration of calcium during neuronal ischaemia plays a particularly important role in the neurotoxic cascade resulting in stroke-related acute neurodegeneration. One approach to provide therapeutic benefit following ischaemic stroke has been to target neuronal voltage-gated cation channels, and particularly blockers of calcium and sodium channels, for post-stroke neuroprotection. A recent development has been the identification of openers of large-conductance calcium- and voltage-dependent potassium channels (maxi-K channels), which hyperpolarize ischaemic neurons, reduce excitatory amino acid release, and reduce ischaemic calcium entry. Thus far, targeting these voltage-gated cation channels has not yet yielded significant clinical benefit. The reasons for this may involve the lack of small-molecule blockers of many neuronal members of these ion channel families and the design of preclinical stroke models, which do not adequately emulate the clinical condition and hence lack sufficient rigor to predict efficacy in human stroke. Furthermore, there may be a need for changes in clinical trial designs to optimise the selection of patients and the course of drug treatment to protect neurons during all periods of potential neuronal sensitivity to neuro-protectants. Clinical trials may also have to be powered to detect small effect sizes or be focused on patients more likely to respond to a particular therapy. The development of future solutions to these problems should result in an improved probability of success for the treatment of stroke.

Novel spider toxin slows down the activation kinetics of P-type Ca2+ channels in Purkinje neurons of rat

We have identified a novel polypeptide toxin (Lsp-1) from the venom of the spider Lycosa (LS). Its effect has been examined on the P-type calcium channels in Purkinje neurons, using whole-cell patch-clamp. This toxin (at saturating concentration 7 nM) produces prominent (four-fold) deceleration of the activation kinetics and partial (71+/-6%) decrease of the amplitude of P-current without affecting either deactivation or inactivation kinetics. These effects are not use-dependent. They are partially reversible within a minute upon the wash-out of the toxin. Intracellular perfusion of Purkinje neurons with 100 microM of GDP or 2 microM of GTPgammaS, as well as strong depolarising pre-pulses (+100 mV), do not eliminate the action of Lsp-1 on P-channels indicating that down-modulation via guanine nucleotide-binding proteins (G-proteins) is not involved in the observed phenomenon. In view of extremely high functional significance of P-channels, the toxin can be suggested as a useful pharmacological tool.

Gene dosage-dependent transmitter release changes at neuromuscular synapses of Cacna1a R192Q knockin mice are non-progressive and do not lead to morphological changes or muscle weakness

Ca(v)2.1 channels mediate neurotransmitter release at the neuromuscular junction (NMJ) and at many central synapses. Mutations in the encoding gene, CACNA1A, are thus likely to affect neurotransmitter release. Previously, we generated mice carrying the R192Q mutation, associated with human familial hemiplegic migraine type-1, and showed first evidence of enhanced presynaptic Ca(2+) influx [Neuron 41 (2004) 701]. Here, we characterize transmitter release in detail at mouse R192Q NMJs, including possible gene-dosage dependency, progression of changes with age, and associated morphological damage and muscle weakness. We found, at low Ca(2+), decreased paired-pulse facilitation of evoked acetylcholine release, elevated release probability, and increased size of the readily releasable transmitter vesicle pool. Spontaneous release was increased over a broad range of Ca(2+) concentrations (0.2-5mM). Upon high-rate nerve stimulation we observed some extra rundown of transmitter release. However, no clinical evidence of transmission block or muscle weakness was found, assessed with electromyography, grip-strength testing and muscle contraction experiments. We studied both adult ( approximately 3-6 months-old) and aged ( approximately 21-26 months-old) R192Q knockin mice to assess effects of chronic elevation of presynaptic Ca(2+) influx, but found no additional or progressive alterations. No changes in NMJ size or relevant ultrastructural parameters were found, at either age. Our characterizations strengthen the hypothesis of increased Ca(2+) flux through R192Q-mutated presynaptic Ca(v)2.1 channels and show that the resulting altered neurotransmitter release is not associated with morphological changes at the NMJ or muscle weakness, not even in the longer term.

Modulation of voltage-dependent calcium channels by neurotransmitters and neuropeptides in parasympathetic submandibular ganglion neurons

The control of saliva secretion is mainly under parasympathetic control, although there also could be a sympathetic component. Sympathetic nerves are held to have a limited action in secretion in submandibular glands because, on electrical stimulation, only a very small increase to the normal background, basal secretion occurs. Parasympathetic stimulation, on the other hand, caused a good flow of saliva with moderate secretion of acinar mucin, plus an extensive secretion of granules from the granular tubules. The submandibular ganglion (SMG) is a parasympathetic ganglion which receives inputs from preganglionic cholinergic neurons, and innervates the submandibular salivary gland to control saliva secretion. Neurotransmitters and neuropeptides acting via G-protein coupled receptors (GPCRs) change the electrical excitability of neurons. In these neurons, many neurotransmitters and neuropeptides modulate voltage-dependent calcium channels (VDCCs). The modulation is mediated by a family of GPCRs acting either directly through the membrane delimited G-proteins or through second messengers. However, the mechanism of modulation and the signal transduction pathway linked to an individual GPCRs depend on the animal species. This review reports how neurotransmitters and neuropeptides modulate VDCCs and how these modulatory actions are integrated in SMG systems. The action of neurotransmitters and neuropeptides on VDCCs may provide a mechanism for regulating SMG excitability and also provide a cellular mechanism of a variety of neuronal Ca(2+)-dependent processes.

N- and P/Q-type Ca2+ channels regulate synaptic efficacy between spinal dorsolateral funiculus terminals and motoneurons

Ca2+ influx through voltage-gated Ca2+ channels mediates synaptic transmission at numerous central synapses. However, electrophysiological and pharmacological evidence linking Ca+ channel activity with neurotransmitter release in the vertebrate mature spinal cord is scarce. In the current report, we investigated in a slice preparation from the adult turtle spinal cord, the effects of various Ca+ channel antagonists on neurotransmission at terminals from the dorsolateral funiculus synapsing motoneurons. Bath application of tetrodotoxin or NiCl2 prevented the monosynaptic excitatory postsynaptic potentials (EPSPs), and this effect was mimicked by exposure to a zero-Ca2+ solution. Application of polypeptide toxins that block N- and P/Q-type channels (omega-CTx-GVIA and omega-Aga-IVA) reduced the EPSP amplitude in a dose-dependent manner. By analyzing the input resistance and the EPSP time course, and using a paired pulse protocol we determined that both toxins act at presynaptic level to modulate neurotransmitter release. RT-PCR studies showed the expression of N- and P/Q-type channel mRNAs in the turtle spinal cord. Together, these results indicate that N- and P/Q-type Ca2+ channels may play a central role in the regulation of neurotransmitter release in the adult turtle spinal cord.

Presynaptic homeostasis at CNS nerve terminals compensates for lack of a key Ca2+ entry pathway

At central synapses, P/Q-type Ca(2+) channels normally provide a critical Ca(2+) entry pathway for neurotransmission. Nevertheless, we found that nerve terminals lacking alpha(1A) (Ca(V)2.1), the pore-forming subunit of P/Q-type channels, displayed a remarkable preservation of synaptic function. Two consistent physiological changes reflective of synaptic homeostasis were observed in cultured hippocampal neurons derived from alpha(1A) (-/-) mice. First, the presynaptic response to an ionophore-mediated Ca(2+) elevation was 50% greater, indicating an enhanced Ca(2+) sensitivity of the release machinery. Second, basal miniature excitatory postsynaptic current frequency in alpha(1A) (-/-) neurons was increased 2-fold compared with WT neurons and occluded the normal response of presynaptic terminals to cAMP elevation, suggesting that the compensatory mechanism in alpha(1A) (-/-) synapses and the modulation of presynaptic function by PKA might share a final common pathway. We used cDNA microarray analysis to identify molecular changes underlying homeostatic regulation in the alpha(1A) (-/-) hippocampus. The 40,000 entries in our custom-made array included likely targets of presynaptic homeostasis, along with many other transcripts, allowing a wide-ranging examination of gene expression. The developmental pattern of changes in transcript levels relative to WT was striking; mRNAs at 5 and 11 days postnatal showed little deviation, but clear differences emerged by 22 days. Many of the transcripts that differed significantly in abundance corresponded to known genes that could be incorporated within a logical pattern consistent with the modulation of presynaptic function. Changes in endocytotic proteins, signal transduction kinases, and candidates for Ca(2+)-sensing molecules were consistent with implications of the direct physiological experiments.

Novel splice site CACNA1A mutation causing episodic ataxia type 2

Episodic ataxia type 2 (EA-2) is an autosomal dominant neurological disorder, characterized by episodes of ataxia, vertigo, nausea, nystagmus, and fatigue, associated with acetazolamide responsiveness. The disease is caused by mutations in the P/Q-type calcium channel Ca(v)2.1 subunit gene, CACNA1A, located on chromosome 19p13.2. We analyzed a family with 13 affected individuals for linkage to this locus and reached a two-point maximum LOD score of 4.48. A novel CACNA1A mutation, IVS36-2A>G, at the 3' acceptor splice site of intron 36 was identified by sequencing. It is the first described CACNA1A acceptor splice site mutation and the most C-terminal EA-2-causing mutation reported to date.

Decreased gene expression of calretinin and ryanodine receptor type 1 in tottering mice

Tottering mice are a spontaneously occurring animal model of human absence epilepsy. They carry a mutation in the P/Q-type calcium channel alpha1A subunit gene which is highly expressed by cerebellar Purkinje cells. In this study, we investigated the role of calretinin and ryanodine receptor type 1 (RyR1) gene expression in the cerebellum of tottering mice. Cerebellar tissue specimens from four experimental groups were processed for in situ hybridization histochemistry (ISHH): (1) wild-type (+/+); (2) heterozygous (tg/+) and two homozygous groups; either (3) without occurrence of an episode of paroxysmal dyskinesia (tg/tg-N); or (4) after an episode of paroxysmal dyskinesia (tg/tg-P) that lasted about 45 min on average. Quantitative analysis showed a statistically significant decrease (p = 0.0001, ANOVA) of calretinin gene expression at the level of the simple lobule of the cerebellum in both homozygous groups compared to the wild-type and heterozygous groups. RyR1 was decreased in the flocculus of the cerebellum in both the tg/tg-N and tg/tg-P groups compared to wild type (p = 0.0174, ANOVA). These results suggest that calretinin gene expression, as well as other genes involved in regulation of calcium homeostasis, such as RyR1, may play a role in the biochemical functional alterations present in tottering mice.

Novel Cav2.1 splice variants isolated from Purkinje cells do not generate P-type Ca2+ current

The alpha(1)2.1 (alpha(1A)) subunits of P-type and Q-type Ca(2+) channels are encoded by a single gene, Cacna1a. Although these channels differ in the inactivation kinetics and sensitivity to omega-agatoxin IVA, the mechanism underlying these differences remains to be clarified. Alternative splicings of the Cacna1a transcript have been postulated to contribute to the respective properties, however, the splice variants responsible for P-type Ca(2+) channels have not been identified. To explore P-type-specific splice variants, we aimed at cloning alpha(1)2.1 from isolated mouse Purkinje cells using single-cell reverse transcription-PCR, because in Purkinje cells P-type currents dominate over the whole currents (>95%) with Q-type currents undetected. As a result, two novel splice variants were cloned. Compared with the previously cloned mouse alpha(1)2.1, two novel variants had additional 48 amino acids at the amino termini, six single amino acid changes, and splicing variations at the exon 46/47 boundary, which produced different carboxyl termini. Furthermore, one variant had one RNA editing site. However, electrophysiological and pharmacological studies indicated that these variants did not generate P-type current in cultured cells. These results suggest that P-type-specific splice variants may exist but that post-translational processing or modification by uncharacterized interacting proteins is also required for generating the P-type current.

The function of Ca(2+) channel subtypes in exocytotic secretion: new perspectives from synaptic and non-synaptic release

By mediating the Ca(2+) influx that triggers exocytotic fusion, Ca(2+) channels play a central role in a wide range of secretory processes. Ca(2+) channels consist of a complex of protein subunits, including an alpha(1) subunit that constitutes the voltage-dependent Ca(2+)-selective membrane pore, and a group of auxiliary subunits, including beta, gamma, and alpha(2)-delta subunits, which modulate channel properties such as inactivation and channel targeting. Subtypes of Ca(2+) channels are constituted by different combinations of alpha(1) subunits (of which 10 have been identified) and auxiliary subunits, particularly beta (of which 4 have been identified). Activity-secretion coupling is determined not only by the biophysical properties of the channels involved, but also by the relationship between channels and the exocytotic apparatus, which may differ between fast and slow types of secretion. Colocalization of Ca(2+) channels at sites of fast release may depend on biochemical interactions between channels and exocytotic proteins. The aim of this article is to review recent work on Ca(2+) channel structure and function in exocytotic secretion. We discuss Ca(2+) channel involvement in selected types of secretion, including central neurotransmission, endocrine and neuroendocrine secretion, and transmission at graded potential synapses. Several different Ca(2+) channel subtypes are involved in these types of secretion, and their function is likely to involve a variety of relationships with the exocytotic apparatus. Elucidating the relationship between Ca(2+) channel structure and function is central to our understanding of the fundamental process of exocytotic secretion.

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.

Autoradiographic localization of N-type VGCCs in gerbil hippocampus and failure of omega-conotoxin MVIIA to attenuate neuronal injury after transient cerebral ischemia

In the mammalian central nervous system, transient global ischemia of specific duration causes selective degeneration of CA1 pyramidal neurons in hippocampus. Many of the ischemia-induced pathophysiologic cascades that destroy the neurons are triggered by pre- and postsynaptic calcium entry. Consistent with this, many calcium channel blockers have been shown to be neuroprotective in global models of ischemia. omega-Conotoxin MVIIA, a selective N-type VGCC blocker isolated from the venom of Conus magus, protects CA1 neurons in the rat model of global ischemia, albeit transiently. The mechanism by which this peptide renders neuroprotection is unknown. We performed high-resolution receptor autoradiography with the radiolabeled peptide and observed highest binding in stratum lucidum of CA3 subfield, known to contain inhibitory neurons potentially important in the pathogenesis of delayed neuronal death. This finding suggested that the survival of stratum lucidum inhibitory neurons might be the primary event, leading to CA1 neuroprotection after ischemia. Testing of this hypothesis required the reproduction of its neuroprotective effects in the gerbil model of global ischemia. Surprisingly, we found that omega-MVIIA did not attenuate CA1 hippocampal injury after 5 min of cerebral ischemia in gerbil. Possible reasons are discussed. Lastly, we show that the peptide can be used as a synaptic marker in assessing short and long-term changes that occur in hippocampus after ischemic injury.

Amplitude-dependent spike-broadening and enhanced Ca(2+) signaling in GnRH-secreting neurons

In GnRH-secreting (GT1) neurons, activation of Ca(2+)-mobilizing receptors induces a sustained membrane depolarization that shifts the profile of the action potential (AP) waveform from sharp, high-amplitude to broad, low-amplitude spikes. Here we characterize this shift in the firing pattern and its impact on Ca(2+) influx experimentally by using prerecorded sharp and broad APs as the voltage-clamp command pulse. As a quantitative test of the experimental data, a mathematical model based on the membrane and ionic current properties of GT1 neurons was also used. Both experimental and modeling results indicated that inactivation of the tetrodotoxin-sensitive Na(+) channels by sustained depolarization accounted for a reduction in the amplitude of the spike upstroke. The ensuing decrease in tetraethylammonium-sensitive K(+) current activation slowed membrane repolarization, leading to AP broadening. This change in firing pattern increased the total L-type Ca(2+) current and facilitated AP-driven Ca(2+) entry. The leftward shift in the current-voltage relation of the L-type Ca(2+) channels expressed in GT1 cells allowed the depolarization-induced AP broadening to facilitate Ca(2+) entry despite a decrease in spike amplitude. Thus the gating properties of the L-type Ca(2+) channels expressed in GT1 neurons are suitable for promoting AP-driven Ca(2+) influx in receptor- and non-receptor-depolarized cells.

A temperature-sensitive paralytic mutant defines a primary synaptic calcium channel in Drosophila

Neurotransmission at chemical synapses involves regulated exocytosis of neurotransmitter from the presynaptic terminal. Neurotransmitter release is thought to be triggered by calcium influx through specific classes of voltage-gated calcium channels. Here we report genetic and functional analysis implicating a specific calcium channel gene product in neurotransmitter release. We have isolated a temperature-sensitive paralytic allele of the Drosophila calcium channel alpha1 subunit gene, cacophony (cac). This mutant, referred to as cac(TS2), allows functional analysis of synaptic transmission after acute perturbation of a specific alpha1 subunit. Electrophysiological analysis at neuromuscular synapses revealed that neurotransmitter release in cac(TS2) is markedly reduced at elevated temperatures, indicating that cac encodes a primary alpha1 subunit functioning in synaptic transmission. These observations further define the molecular basis of voltage-gated calcium entry at synapses and provide a new starting point for further genetic analysis of synaptic mechanisms.

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

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

Genetic modifiers of the Drosophila NSF mutant, comatose, include a temperature-sensitive paralytic allele of the calcium channel alpha1-subunit gene, cacophony

The N-ethylmaleimide-sensitive fusion protein (NSF) has been implicated in vesicle trafficking in perhaps all eukaryotic cells. The Drosophila comatose (comt) gene encodes an NSF homolog, dNSF1. Our previous work with temperature-sensitive (TS) paralytic alleles of comt has revealed a function for dNSF1 at synapses, where it appears to prime synaptic vesicles for neurotransmitter release. To further examine the molecular basis of dNSF1 function and to broaden our analysis of synaptic transmission to other gene products, we have performed a genetic screen for mutations that interact with comt. Here we report the isolation and analysis of four mutations that modify TS paralysis in comt, including two intragenic modifiers (one enhancer and one suppressor) and two extragenic modifiers (both enhancers). The intragenic mutations will contribute to structure-function analysis of dNSF1 and the extragenic mutations identify gene products with related functions in synaptic transmission. Both extragenic enhancers result in TS behavioral phenotypes when separated from comt, and both map to loci not previously identified in screens for TS mutants. One of these mutations is a TS paralytic allele of the calcium channel alpha1-subunit gene, cacophony (cac). Analysis of synaptic function in these mutants alone and in combination will further define the in vivo functions and interactions of specific gene products in synaptic transmission.

Impaired neurotransmitter release and elevated threshold for cortical spreading depression in mice with mutations in the alpha1A subunit of P/Q type calcium channels

The P/Q type voltage-gated Ca2+ channels are involved in membrane excitability and Ca2+-dependent neurotransmitter release within the CNS. Mutations in the CacnalA gene encoding the alpha1A subunit of the P/Q type Ca2+ channel have recently been reported in tottering mice and a more severely affected allele, leaner. Here we show using in vivo cortical microdialysis that evoked increases of extracellular glutamate levels are markedly attenuated in both mutants upon KCl-induced depolarization compared with wild-type mice. Tottering and leaner mice also show a 10-fold resistance to cortical spreading depression induced by cortical electrical stimulation or KCl application to the pial surface. A slower transcortical propagation speed and failure to sustain regenerative spread of the depolarizing wave were more pronounced in leaner neocortex. Both signaling defects appeared unrelated to the developmental history of repeated cortical spike-wave discharges, since neither were observed in the stargazer mouse, a Ca2+ channel gamma2 subunit mutant with a similar seizure phenotype. These data demonstrate two cortical excitability defects revealed by prolonged depolarization in cerebral networks expressing mutant P/Q type Ca2+ channels, and are the first to identify a gene linked to a spreading depression phenotype.

Voltage-gated ion channels and hereditary disease

By the introduction of technological advancement in methods of structural analysis, electronics, and recombinant DNA techniques, research in physiology has become molecular. Additionally, focus of interest has been moving away from classical physiology to become increasingly centered on mechanisms of disease. A wonderful example for this development, as evident by this review, is the field of ion channel research which would not be nearly as advanced had it not been for human diseases to clarify. It is for this reason that structure-function relationships and ion channel electrophysiology cannot be separated from the genetic and clinical description of ion channelopathies. Unique among reviews of this topic is that all known human hereditary diseases of voltage-gated ion channels are described covering various fields of medicine such as neurology (nocturnal frontal lobe epilepsy, benign neonatal convulsions, episodic ataxia, hemiplegic migraine, deafness, stationary night blindness), nephrology (X-linked recessive nephrolithiasis, Bartter), myology (hypokalemic and hyperkalemic periodic paralysis, myotonia congenita, paramyotonia, malignant hyperthermia), cardiology (LQT syndrome), and interesting parallels in mechanisms of disease emphasized. Likewise, all types of voltage-gated ion channels for cations (sodium, calcium, and potassium channels) and anions (chloride channels) are described together with all knowledge about pharmacology, structure, expression, isoforms, and encoding genes.

Ion channels in presynaptic nerve terminals and control of transmitter release

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

Recurrence of the T666M calcium channel CACNA1A gene mutation in familial hemiplegic migraine with progressive cerebellar ataxia

Familial hemiplegic migraine (HM) is an autosomal dominant migraine with aura. In 20% of HM families, HM is associated with a mild permanent cerebellar ataxia (PCA). The CACNA1A gene encoding the alpha1A subunit of P/Q-type voltage-gated calcium channels is involved in 50% of unselected HM families and in all families with HM/PCA. Four CACNA1A missense mutations have been identified in HM: two in pure HM and two in HM/PCA. Different CACNA1A mutations have been identified in other autosomal dominant conditions: mutations leading to a truncated protein in episodic ataxia type 2 (EA2), small expansions of a CAG trinucleotide in spinocerebellar ataxia type 6 and also in three families with EA2 features, and, finally, a missense mutation in a single family suffering from episodic ataxia and severe progressive PCA. We screened 16 families and 3 nonfamilial case patients affected by HM/PCA for specific CACNA1A mutations and found nine families and one nonfamilial case with the same T666M mutation, one new mutation (D715E) in one family, and no CAG repeat expansion. Both T666M and D715E substitutions were absent in 12 probands belonging to pure HM families whose disease appears to be linked to CACNA1A. Finally, haplotyping with neighboring markers suggested that T666M arose through recurrent mutational events. These data could indicate that the PCA observed in 20% of HM families results from specific pathophysiologic mechanisms.

Cerebellar circuitry is activated during convulsive episodes in the tottering (tg/tg) mutant mouse

Tottering (tg) is an autosomal recessive mutation of the calcium channel alpha1A subunit in the mouse that results in epileptic spike and wave discharges, mild ataxia and paroxysmal episodes of involuntary spasms of the limbs, trunk and face. These convulsions have been especially difficult to characterize because of their unpredictable occurrence and lack of electroencephalographic correlates. However, it is, in fact, possible to induce these convulsions, making this facet of the tottering phenotype amenable to controlled experimentation for the first time. Here, the neuroanatomical basis of the convulsions in tottering mice has been identified using in situ hybridization for c-fos messenger RNA to chart abnormal neuronal activity. Convulsion-induced c-fos messenger RNA expression was most prominent in the cerebellum of convulsing tottering mice. Additionally, cerebral cortex and principal cerebellar relay nuclei were also activated during a convulsion. The c-fos activation in the cerebellum temporally preceded expression in cerebral cortex, suggesting that cerebral cortex is not driving the expression of convulsions. These results suggest that the cerebellum, a region not classically associated with paroxysmal events, is important in the generation and/or maintenance of the intermittent convulsions in tottering mutant mice.

Muscarinic receptor activation modulates Ca2+ channels in rat intracardiac neurons via a PTX- and voltage-sensitive pathway

With use of the whole cell patch-clamp technique, effects of the potent muscarinic agonist oxotremorine methiodide (oxo-M) on voltage-activated Ca2+ channel currents were investigated in acutely dissociated adult rat intracardiac neurons. In all tested neurons oxo-M reversibly inhibited the peak Ba2+ current. Inhibition of the peak Ba2+ current by oxo-M was associated with slowing of activation kinetics and was concentration dependent. The concentration of oxo-M necessary to produce a half-maximal inhibition of current and the maximal inhibition were 40.8 nM and 75.9%, respectively. Inhibitory effect of oxo-M was completely abolished by atropine. Among different muscarinic receptor antagonists, methoctramine (100 and 300 nM) significantly antagonized the current inhibition by oxo-M, with a negative logarithm of dissociation constant of 8.3 in adult rat intracardiac neurons. Internal dialysis of neurons with guanosine 5'-(thio)triphosphate (GTPgammaS, 0.5 mM) could mimic the muscarinic inhibition of the peak Ba2+ current and significantly occlude inhibitory effects of oxo-M. In addition, the internal dialysis of guanosine-5'-O-(2-thiodiphosphate) (GDPbetaS, 2 mM) also significantly reduced the muscarinic inhibition of the peak Ba2+ current by oxo-M. Inhibitory effects of oxo-M were significantly abolished by pertussis toxin (PTX, 200 and 400 ng/ml) but not by cholera toxin (400 ng/ml). Furthermore, the bath application of N-ethylmaleimide (50 microM) significantly reduced the inhibition of the peak Ba2+ current by oxo-M. The oxo-M shifted the activation curve derived from measurments of tail currents toward more positive potentials. A strong conditioning prepulse to +100 mV significantly relieved the muscarinic inhibition of peak Ba2+ currents by oxo-M and the GTPgammaS-induced current inhibition. In a series of experiments, changes in intracellular concentration of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid and protein kinase activities failed to mimic or occlude the current inhibition by oxo-M. The dihydropyridine antagonist nifedipine (10 microM) was not able to occlude any of the inhibitory effects of oxo-M, and oxo-M (3 microM) failed to reduce the slow tail currents induced by the L-type agonist methyl 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylate (FPL 64176; 2 microM). However, omega-conotoxin (omega-CgTX) GVIA (1 microM) significantly occluded the muscarinic inhibition of the Ba2+ currents. In the presence of omega-CgTX GVIA (1 microM) and nifedipine (10 microM), oxo-M could further inhibit approximately 20% of the total Ca2+ current. After complete removal of N-, Q-, and L-type currents with use of omega-CgTX GVIA, omega-agatoxin IVA, and nifedipine, 70% of the R-type current (approximately 6-7% of the total current) was inhibited by oxo-M (3 microM). In conclusion, the M2 muscarinic receptor activation selectively inhibits N-, Q-, and R-type Ca2+ channel currents, sparing L-type Ca2+ channel currents mainly via a PTX- and voltage-sensitive pathway in adult rat intracardiac neurons.

Intermolecular tuning of calmodulin by target peptides and proteins: differential effects on Ca2+ binding and implications for kinase activation

Ca(2+)-activated calmodulin (CaM) regulates many target enzymes by docking to an amphiphilic target helix of variable sequence. This study compares the equilibrium Ca2+ binding and Ca2+ dissociation kinetics of CaM complexed to target peptides derived from five different CaM-regulated proteins: phosphorylase kinase. CaM-dependent protein kinase II, skeletal and smooth myosin light chain kinases, and the plasma membrane Ca(2+)-ATPase. The results reveal that different target peptides can tune the Ca2+ binding affinities and kinetics of the two CaM domains over a wide range of Ca2+ concentrations and time scales. The five peptides increase the Ca2+ affinity of the N-terminal regulatory domain from 14- to 350-fold and slow its Ca2+ dissociation kinetics from 60- to 140-fold. Smaller effects are observed for the C-terminal domain, where peptides increase the apparent Ca2+ affinity 8- to 100-fold and slow dissociation kinetics 13- to 132-fold. In full-length skeletal myosin light chain kinase the inter-molecular tuning provided by the isolated target peptide is further modulated by other tuning interactions, resulting in a CaM-protein complex that has a 10-fold lower Ca2+ affinity than the analogous CaM-peptide complex. Unlike the CaM-peptide complexes, Ca2+ dissociation from the protein complex follows monoexponential kinetics in which all four Ca2+ ions dissociate at a rate comparable to the slow rate observed in the peptide complex. The two Ca2+ ions bound to the CaM N-terminal domain are substantially occluded in the CaM-protein complex. Overall, the results indicate that the cellular activation of myosin light chain kinase is likely to be triggered by the binding of free Ca2(2+)-CaM or Ca4(2+)-CaM after a Ca2+ signal has begun and that inactivation of the complex is initiated by a single rate-limiting event, which is proposed to be either the direct dissociation of Ca2+ ions from the bound C-terminal domain or the dissociation of Ca2+ loaded C-terminal domain from skMLCK. The observed target-induced variations in Ca2+ affinities and dissociation rates could serve to tune CaM activation and inactivation for different cellular pathways, and also must counterbalance the variable energetic costs of driving the activating conformational change in different target enzymes.

Calcium channel currents in acutely dissociated intracardiac neurons from adult rats

With the use of the whole cell patch-clamp technique, multiple subtypes of voltage-activated calcium channels, as indicated by measuring Ba2+ currents, were pharmacologically identified in acutely dissociated intracardiac neurons from adult rats. All tested neurons that were held at -80 mV displayed only high-voltage-activated (HVA) Ca2+ channel currents that were completely blocked by 100 microM CdCl2. The current density of HVA Ca2+ currents was dependent on the external Ca2+ concentration. The Ba2+ (5 mM) currents were half-activated at -16.3 mV with a slope of 5.6 mV per e-fold change. The steady-state inactivation was also voltage dependent with half-inactivation at -33.7 mV and a slope of -12.1 mV per e-fold change. The most effective L-type channel activator, FPL 64176 (2 microM), enhanced the Ba2+ current in a voltage-dependent manner. When cells were held at -80 mV, the saturating concentration (10 microM) of nifedipine blocked approximately 11% of the control Ba2+ current. The major component of the Ca2+ channels was N type (63%), which was blocked by a saturating concentration (1 microM) of omega-conotoxin GVIA. Approximately 19% of the control Ba2+ current was sensitive to omega-conotoxin MVIIC (5 microM) but insensitive to low concentrations (30 and 100 nM) of omega-agatoxin IVA (omega-Aga IVA). In addition, a high concentration (1 microM) of omega-Aga IVA occluded the effect of omega-conotoxin MVIIC. Taken together, these results indicate that the omega-conotoxin MVIIC-sensitive current represents only the Q type of Ca2+ channels. The current that was insensitive to nifedipine and various toxins represents the R-type current (7%), which was sensitive to 100 microM NiCl2. In conclusion, the intracardiac neurons from adult rats express at least four different subtypes (L, N, Q, and R) of HVA Ca2+ channels. This information is essential for understanding the regulation of synaptic transmission and excitability of intracardiac neurons by different neurotransmitters and neural regulation of cardiac functions.

Absence epilepsy in tottering mutant mice is associated with calcium channel defects

Mutations at the mouse tottering (tg) locus cause a delayed-onset, recessive neurological disorder resulting in ataxia, motor seizures, and behavioral absence seizures resembling petit mal epilepsy in humans. A more severe allele, leaner (tg(la)), also shows a slow, selective degeneration of cerebellar neurons. By positional cloning, we have identified an alpha1A voltage-sensitive calcium channel gene that is mutated in tg and tg(la) mice. The alpha1A gene is widely expressed in the central nervous system with prominent, uniform expression in the cerebellum. alpha1A expression does not mirror the localized pattern of cerebellar degeneration observed in tg(la) mice, providing evidence for regional differences in biological function of alpha1A channels. These studies define the first mutations in a mammalian central nervous system-specific voltage-sensitive calcium channel and identify the first gene involved in absence epilepsy.

Selective G-protein regulation of neuronal calcium channels

We examined the properties and regulation of Ca channels resulting from the expression of human alpha1B and alpha1E subunits stably expressed in KEK293 cells. The ancillary subunits beta1B and alpha2/delta were also stably expressed in these cell lines. Ca currents in alpha1B-expressing cells had the properties of N-type currents. Ca currents in cells expressing alpha1E exhibited a novel profile that was similar to the properties of the "R type" Ca current. Introduction of GTP-gamma-S into alpha1B cells greatly enhanced the extent of prepulse facilitation of the Ca current, whereas it had only a very small effect in alpha1E-expressing cells. Activation of somatostatin receptors endogenous to HEK293 cells or kappa opioid receptors, expressed in the cells after transfection, inhibited Ca currents in alpha1B-expressing cells. This inhibition was blocked by pertussis toxin and was partially relieved by a depolarizing prepulse. In contrast, no inhibitory effects were noted in cells expressing alpha1E channels under the same circumstances. HEK293 cells normally contained G-proteins from all of the four major families. Inhibition of Ca currents by kappa agonists in alpha1B-expressing cells was enhanced slightly by the cotransfection of several G-protein alpha subunits. kappa agonists, however, had no effect in alpha1E-containing cells, even after overexpression of different G-protein alpha-subunits. In summary, these results demonstrate that there is a large difference in the susceptibility of alpha1B- and alpha1E-based Ca channels to regulation by G-proteins. This is so despite the fact that the two types of Ca channels show substantial similarities in their primary sequences.

Serotonin modulates high-voltage-activated Ca2+ channels in rat ventromedial hypothalamic neurons

The modulation of high-voltage-activated (HVA) Ca2+ channels by serotonin (5-HT) was studied in ventromedial hypothalamic (VMH) neurons acutely dissociated from 12-14-day-old Wistar rats using the whole-cell and nystatin perforated-patch recording configurations. 5-HT inhibited the HVA Ca2+ channels in a concentration-, time- and voltage-dependent manner. This inhibition was mimicked by the 5-HT1A agonist 8-hydroxy-dipropylaminotetralin and was prevented by pretreatment with pertussis toxin (PTX). omega-Conotoxin-GVIA, omega-agatoxin-IVA, nicardipine and omega-conotoxin-MVIIC blocked each fraction of HVA Ca2+ channel currents, suggesting the existence of N-, P-, L- and Q-types of HVA Ca2+ channels. A component of the current resistant to these Ca2+ channel antagonists also existed in the VMH neurons. Among these five components of HVA Ca2+ channel currents, the N- and Q-type currents were significantly inhibited by 5-HT. These findings suggest that the activation of 5-HT1A receptors produces the selective inhibition of N- and Q-type Ca2+ channels through a PTX-sensitive G-protein in rat VMH neurons.