Excitatory but not inhibitory synaptic transmission is reduced in lethargic (Cacnb4(lh)) and tottering (Cacna1atg) mouse thalami.
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Excitatory but not inhibitory synaptic transmission is reduced in lethargic (Cacnb4(lh)) and tottering (Cacna1atg) mouse thalami. Recent studies of the homozygous tottering (Cacna1atg) and lethargic mouse (Cacnb4(lh)) models of absence seizures have identified mutations in the genes encoding the alpha1A and beta4 subunits, respectively, of voltage-gated Ca2+ channels (VGCCs). beta subunits normally regulate Ca2+ currents via a direct interaction with alpha1 (pore-forming) subunits of VGCCs, and VGCCs are known to play a significant role in controlling the release of transmitter from presynaptic nerve terminals in the CNS. Because the gene mutation in Cacnb4(lh) homozygotes results in loss of the beta4 subunit's binding site for alpha1 subunits, we hypothesized that synaptic transmission would be altered in the CNS of Cacnb4(lh) homozygotes. We tested this hypothesis by using whole cell recordings of single cells in an in vitro slice preparation to investigate synaptic transmission in one of the critical neuronal populations that generate seizure activity in this strain, the somatosensory thalamus. The primary finding reported here is the observation of a significant decrease in glutamatergic synaptic transmission mediated by both N-methyl-D-aspartate (NMDA) and non-NMDA receptors in somatosensory thalamic neurons of Cacnb4(lh) homozygotes compared with matched, nonepileptic mice. In contrast, there was no significant decrease in GABAergic transmission in Cacnb4(lh) homozygotes nor was there any difference in effects mediated by presynaptic GABAB receptors. We found a similar decrease in glutamatergic but not GABAergic responses in Cacna1atg homozygotes, suggesting that the independent mutations in the two strains each affected P/Q channel function by causing defective neurotransmitter release specific to glutamatergic synapses in the somatosensory thalamus. This may be an important factor underlying the generation of seizures in these models. (+info)
Direct alteration of the P/Q-type Ca2+ channel property by polyglutamine expansion in spinocerebellar ataxia 6.
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Spinocerebellar ataxia 6 (SCA6) is caused by expansion of a polyglutamine stretch, encoded by a CAG trinucleotide repeat, in the human P/Q-type Ca(2+) channel alpha(1A) subunit. Although SCA6 shares common features with other neurodegenerative glutamine repeat disorders, the polyglutamine repeats in SCA6 are exceptionally small, ranging from 21 to 33. Because this size is too small to form insoluble aggregates that have been blamed for the cause of neurodegeneration, SCA6 is the disorder suitable for exploring the pathogenic mechanisms other than aggregate formation, whose universal role has been questioned. To characterize the pathogenic process of SCA6, we studied the effects of polyglutamine expansion on channel properties by analyzing currents flowing through the P/Q-type Ca(2+) channels with an expanded stretch of 24, 30, or 40 polyglutamines, recombinantly expressed in baby hamster kidney cells. Whereas the Ca(2+) channels with +info)
Ataxic mouse mutants and molecular mechanisms of absence epilepsy.
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Mouse genetic models for common human diseases have been studied for most of the 20th century. Although many polygenic strain differences and spontaneous single gene mutants have been extensively characterized over the years, knowing their innermost secrets ultimately requires the identity of the mutated genes. One group of neurological mutants, detected initially due to cerebellar dysfunction, was identified as models for epilepsy when they were unexpectedly found to have spike-wave seizures associated with behavioral arrest, a central feature of absence or petit-mal epilepsy. A further surprise was that recently identified defective genes encode different subunits of voltage-gated Ca(2+)channels (VGCCs), implying common seizure mechanisms. In this review we first consider these spontaneous mutants with VGCC defects in the context of other mouse models for epilepsy. Then, from the new wave of genetic and functional studies of these mutants we discuss their prospects for yielding insight into the molecular mechanisms of epilepsy. (+info)
Significant role of neuronal non-N-type calcium channels in the sympathetic neurogenic contraction of rat mesenteric artery.
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1. The possible involvement of pre-junctional non-N-type Ca2+ channels in noradrenaline (NA)-mediated neurogenic contraction by electrical field stimulation (EFS) was examined pharmacomechanically in the isolated rat mesenteric artery. 2. EFS-generated contraction of endothelium-denuded mesenteric artery was frequency-dependent (2 - 32 Hz) and was abolished by tetrodotoxin (TTX, 1 microM), guanethidine (5 microM) or prazosin (100 nM), indicating that NA released from sympathetic nerve endings mediates the contractile response. 3. NA-mediated neurogenic contractions to lower frequency stimulations (2 - 8 Hz) were almost abolished by an N-type Ca2+ channel blocker, omega-conotoxin-GVIA (1 microM) whereas the responses to higher frequency stimulations (12 - 32 Hz) were less sensitive to omega-conotoxin-GVIA. The omega-conotoxin-GVIA-resistant component of the contractile response to 32 Hz stimulation was inhibited partly (10 - 20%) by omega-agatoxin-IVA (10 - 100 nM; concentrations which are relatively selective for P-type channels) and to a greater extent by omega-agatoxin-IVA (1 microM) and omega-conotoxin-MVIIC (3 microM), both of which block Q-type channels at the concentrations used. 4. omega-Agatoxin-IVA (10 - 100 nM) alone inhibited 32 Hz EFS-induced contraction by 10 approximately 20% whereas omega-conotoxin-MVIIC (3 microM) alone inhibited the response by approximately 60%. 5. These omega-toxin treatments did not affect the contractions evoked by exogenously applied NA. 6. These findings show that P- and Q-type as well as N-type Ca2+ channels are involved in the sympathetic neurogenic vascular contraction, and suggest the significant role of non-N-type Ca2+ channels in NA release from adrenergic nerve endings when higher frequency stimulations are applied to the nerve. (+info)
Ablation of P/Q-type Ca(2+) channel currents, altered synaptic transmission, and progressive ataxia in mice lacking the alpha(1A)-subunit.
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The Ca(2+) channel alpha(1A)-subunit is a voltage-gated, pore-forming membrane protein positioned at the intersection of two important lines of research: one exploring the diversity of Ca(2+) channels and their physiological roles, and the other pursuing mechanisms of ataxia, dystonia, epilepsy, and migraine. alpha(1A)-Subunits are thought to support both P- and Q-type Ca(2+) channel currents, but the most direct test, a null mutant, has not been described, nor is it known which changes in neurotransmission might arise from elimination of the predominant Ca(2+) delivery system at excitatory nerve terminals. We generated alpha(1A)-deficient mice (alpha(1A)(-/-)) and found that they developed a rapidly progressive neurological deficit with specific characteristics of ataxia and dystonia before dying approximately 3-4 weeks after birth. P-type currents in Purkinje neurons and P- and Q-type currents in cerebellar granule cells were eliminated completely whereas other Ca(2+) channel types, including those involved in triggering transmitter release, also underwent concomitant changes in density. Synaptic transmission in alpha(1A)(-/-) hippocampal slices persisted despite the lack of P/Q-type channels but showed enhanced reliance on N-type and R-type Ca(2+) entry. The alpha(1A)(-/-) mice provide a starting point for unraveling neuropathological mechanisms of human diseases generated by mutations in alpha(1A). (+info)
Presynaptic Ca(2+) influx at a mouse central synapse with Ca(2+) channel subunit mutations.
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Genetic alterations in Ca(2+) channel subunits can be used to study the interaction among channel subunits and their roles in channel function. P/Q- and N-type Ca(2+) channels reside at the presynaptic terminal and control the release of neurotransmitter at mammalian central synapses. We used fluorescence imaging techniques to investigate presynaptic Ca(2+) currents and neurotransmitter release at hippocampal Schaffer collateral synapses in both tottering (tg, alpha(1A) subunit) and lethargic (lh, beta(4) subunit) mutant mice. Application of selective toxins revealed a large reduction in presynaptic P/Q-type Ca(2+) transients, from 39% of total in +/+ mice to 6% in tg/tg mice, whereas the proportion of N-type increased from 35 to 68%, respectively. Neurotransmitter release in the tg/tg mutant relied almost exclusively on N-type channels, as shown by the complete blockade of synaptic transmission with omega-conotoxin GVIA. Remarkably, loss of beta4, a subunit predicted to regulate the subcellular targeting and modulation of both P/Q- and N-type channels, resulted in no significant difference in the ratio of Ca(2+) channel subtypes or Ca(2+) dependence of neurotransmitter release in lethargic mice. G-protein-mediated inhibition of Ca(2+) channels was also unaltered. These results indicate that a profound decrease in presynaptic P/Q-type currents leads to dependence of neurotransmitter release on N-type channels. In contrast, absence of beta(4) appears not to compromise either P/Q- or N-type channel function at this hippocampal synapse, implicating rescue of presynaptic Ca(2+) currents by other available beta subunits. The present study reveals compensatory molecular mechanisms in the regulation of presynaptic Ca(2+) entry and neurotransmitter release. (+info)
P2Y purinoceptors inhibit exocytosis in adrenal chromaffin cells via modulation of voltage-operated calcium channels.
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We have used combined membrane capacitance measurements (C(m)) and voltage-clamp recordings to examine the mechanisms underlying modulation of stimulus-secretion coupling by a G(i/o)-coupled purinoceptor (P2Y) in adrenal chromaffin cells. P2Y purinoceptors respond to extracellular ATP and are thought to provide an important inhibitory feedback regulation of catecholamine release from central and sympathetic neurons. Inhibition of neurosecretion by other G(i/o)-protein-coupled receptors may occur by either inhibition of voltage-operated Ca(2+) channels or modulation of the exocytotic machinery itself. In this study, we show that the P2Y purinoceptor agonist 2-methylthio ATP (2-MeSATP) significantly inhibits Ca(2+) entry and changes in C(m) evoked by single 200 msec depolarizations or a train of 20 msec depolarizations (2.5 Hz). We found that P2Y modulation of secretion declines during a train such that only approximately 50% of the modulatory effect remains at the end of a train. The inhibition of both Ca(2+) entry and DeltaC(m) are also attenuated by large depolarizing prepulses and treatment with pertussis toxin. Inhibition of N-type, and to lesser extent P/Q-type, Ca(2+) channels contribute to the modulation of exocytosis by 2-MeSATP. The Ca(2+)-dependence of exocytosis triggered by either single pulses or trains of depolarizations was unaffected by 2-MeSATP. When Ca(2+) channels were bypassed and exocytosis was evoked by flash photolysis of caged Ca(2+), the inhibitory effect of 2-MeSATP was not observed. Collectively, these data suggest that inhibition of exocytosis by G(i/o)-coupled P2Y purinoceptors results from inhibition of Ca(2+) channels and the Ca(2+) signal controlling exocytosis rather than a direct effect on the secretory machinery. (+info)
Combinatorial synthesis of omega-conotoxin MVIIC analogues and their binding with N- and P/Q-type calcium channels.
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Omega-conotoxin MVIIC (MVIIC) blocks P/Q-type calcium channels with high affinity and N-type calcium channels with low affinity, while the highly homologous omega-conotoxin MVIIA blocks only N-type calcium channels. We wished to obtain MVIIC analogues more selective for P/Q-type calcium channels than MVIIC to elucidate structural differences among the channels, which discriminate the omega-conotoxins. To prepare a number of MVIIC analogues efficiently, we developed a combinatorial method which includes a random air oxidation step. Forty-seven analogues were prepared in six runs and some of them exhibited higher selectivity for P/Q-type calcium channels than MVIIC in binding assays. (+info)