Contribution of L-type Ca2+ channels to evoked transmitter release in cultured Xenopus nerve-muscle synapses (original) (raw)
Related papers
The Journal of Physiology, 1995
The effects of the calcium channel blockers, funnel-web spider toxin (FTX), w-agatoxin IVA (w-Aga IVA) and w-conotoxin GVIA (w-CgTX), were tested on transmitter release and presynaptic currents in frog motor nerve endings. 2. Evoked transmitter release was blocked by FTX (IC50= 0o02 #l ml-') and w-CgTX (1 #M) but was not affected by w-Aga IVA (0'5/M). When FTX (0 1 #l ml-') was assayed on spontaneous release either in normal Ringer solution or in low Ca2+-high Mg2+ solution, it was found not to affect miniature endplate potential (MEPP) amplitude but to increase MEPP frequency by-2-fold in both conditions. 3. Presynaptic calcium currents (Ica), measured by the perineurial technique in the presence of 10 mm tetraethylammonium chloride (TEA) and 200 juM BaCl2 to block K+ currents, were blocked by w-CgTX (5 ,UM), partially blocked by FTX (1 1d ml-') and not affected by w-Aga IVA (0 5,UM). 4. The presynaptic calcium-activated potassium current (IK(ca)) measured by the perineurial technique in the presence of 0 F5jM 3,4-aminopyridine (DAP) to block voltage-dependent K+ currents, was strongly affected by charybdotoxin (ChTX) (300 nM) and completely abolished by BaCl2 (200 juM). This current was also blocked by w-CgTX (5 FM) and by CdC12 (200 FM) but was not affected by FTX (1 Fl ml-'). The blockade by w-CgTX could not be reversed by elevating [Ca]o to 10 mM. 5. The results suggest that in frog synaptic terminals two w-CgTX-sensitive populations might coexist. The transmitter release process seems to be mediated by calcium influx through a w-CgTXand FTX-sensitive population.
British Journal of Pharmacology, 1997
1. The effects of the voltage-dependent calcium channel (VDCC) blockers omega-agatoxin IVA (omega-AgaIVA), omega-conotoxin GVIA (omega-CgTx), omega-conotoxin MVIIC (omega-MVIIC) and omega-conotoxin MVIID (omega-MVIID) were evaluated on transmitter release in the mouse diaphragm preparation. The effects of omega-AgaIVA and omega-MVIIC were also evaluated on the perineurial calcium and calcium-dependent potassium currents, ICa and IK(Ca), respectively, in the mouse levator auris preparation. 2. The P- and Q-type VDCC blocker omega-AgaIVA (100 nM) and P- Q- and N-type channel blockers omega-MVIIC (1 microM) and omega-MVIID (3 microM) strongly reduced transmitter release (> 80-90% blockade) whereas the selective N-type channel blocker omega-CgTx (5 microM) was ineffective. 3. The process of release was much more sensitive to omega-MVIIC (IC50 = 39 nM) than to omega-MVIID (IC50 = 1.4 microM). After almost completely blocking transmitter release (quantal content approximately 0.3% of its control value) with 3 microM omega-MVIIC, elevating the external [Ca2+] from 2 to 10 mM induced an increase of approximately 20 fold on the quantal content of the endplate potential (e.p.p.) (from 0.2 +/- 0.04 to 4.8 +/- 1.4). 4. Nerve-evoked transmitter release in a low Ca(2+)-high Mg2+ medium (low release probability, quantal content = 2 +/- 0.1) had the same sensitivity to omega-AgaIVA (IC50 = 16.8 nM) as that in normal saline solutions. In addition, K(+)-evoked transmitter release was also highly sensitive to the action of this toxin (IC50 = 11.5 nM; 100 nM > 95% blockade). The action of omega-AgaIVA on transmitter release could be reversed by toxin washout if the experiments were carried out at 31-33 degrees C. Conversely, the effect of omega-AgaIVA persisted even after two hours of toxin washout at room temperature. 5. Both the calcium and calcium-dependent potassium presynaptic currents, ICa and IK(Ca), respectively, were highly sensitive to low concentrations (10-30 nM) of omega-AgaIVA. The ICa and the IK(Ca) were also strongly reduced by 1 microM omega-MVIIC. The most marked difference between the action of these two toxins was the long incubation times required to achieve maximal effects with omega-MVIIC. 6. In summary these results provide more evidence that synaptic transmission at the mammalian neuromuscular junction is mediated by Ca2+ entry through P- and/or Q-type calcium channels.
Modulation of Voltage-Dependent Calcium Channels in Cultured Neuronsa
Annals of the New York Academy of Sciences, 2006
In many types of excitable cells there are several classes of voltage-dependent calcium channels (VDCC) as determined by the characteristic properties of single channel activity. A clear distinction exists between low conductance channels, activated by moderate depolarizations (low voltage-activated, LVA), and high conductance channels activated by large depolarizations (high voltage-activated, HVA).'" In cardiac tissue these were termed T and L channels, respectively, because they were found to be pharmacologically as well as biophysically distinct.' Because of this, they can also be clearly differentiated in whole-cell current recordings.'" Evidence also exists for a third class of single channel conductance (N type) whose biophysical properties were originally described as being intermediate between T and L and which appears to be expressed only in cells of neuronal origin.ss The sensitivity of N-type channels to irreversible block by w-conotoxin GVIA (w-CgTx)' and the large number of w-CgTx binding sites in neuronal tissue indicate that they are likely to be important for neuronal function! High threshold Caz+ currents insensitive to both w-CgTx and dihydropyridines have been reported:' and a selective blocker for at least part of this current is the peptide toxin from Agelenopsis aperta, w-agatoxin IVA (w-aga IVA). The current inhibited by this toxin has been termed P current, because Purkinje cells express calcium channel currents that are largely resistant to w-CgTx and 1P-dihydropyridines (DHPs): and these currents are sensitive to w-aga IVA.* It is difficult to distinguish these currents by biophysical means at the whole cell l e~e l , 5 "~ and although estimates of their single channel conductances indicate differences, this is complicated by the existence of subconductance states.'O However, prolongation of single channel open times by DHP agonists remains diagnostic of L channels.'' aThe work in this laboratory was supported by the Wellcome Trust and MRC.
Functional Specialization of Presynaptic Cav2.3 Ca2+ Channels
Neuron, 2003
Epileptology beit with a lower efficacy than N-and P/Q-type Ca 2ϩ channels . University Bonn Sigmund-Freud Str. 25 In addition to fast neurotransmitter release, some forms of synaptic plasticity also require a rise in presyn-53105 Bonn Germany aptic Ca 2ϩ . At the mossy fiber-CA3 synapse, for instance, the induction of synaptic long-term potentiation 3 Department of Neurophysiology University of Cologne (LTP) appears to be independent of Ca 2ϩ influx into the postsynaptic CA3 neurons (Zalutsky and ). Robert-Koch-Str. 39 50931 Kö ln Rather, a rise in presynaptic Ca 2ϩ is thought to be the initial step in LTP induction (Castillo et al., 1994), which Germany subsequently leads to expression of mossy fiber LTP via an increase in the probability of neurotransmitter release (Zalutsky and Nicoll, 1990; Castillo et al., 2002; Maeda et al., 1997; Xiang et al., 1994; but see Yeckel et Summary al. , 1999). The finding that both fast neurotransmitter release and the induction of mossy fiber LTP require a Ca 2؉ influx into presynaptic terminals via voltagedependent Ca 2؉ channels triggers fast neurotransmitter rise in the presynaptic Ca 2ϩ concentration raises the basic question whether both of these processes are release as well as different forms of synaptic plasticity. Using electrophysiological and genetic techniques triggered by Ca 2ϩ entry via identical Ca 2ϩ channel subtypes. In contrast to the detailed knowledge of Ca 2ϩ we demonstrate that presynaptic Ca 2؉ entry through Ca v 2.3 subunits contributes to the induction of mossy channels mediating fast neurotransmitter release at central synapses (Iwasaki and Takahashi, 1998; Qian and fiber LTP and posttetanic potentiation by brief trains of presynaptic action potentials while they do not play a role in fast synaptic transmission, paired-pulse facilitation, or frequency facilitation. This functional spe-it has remained unclear which Ca 2ϩ entry pathways are a factor in presynaptic forms of LTP. Mossy fiber LTP cialization is most likely achieved by a localization remote from the release machinery and by a Ca v 2.3 can be induced when either N-type or P/Q-type Ca 2ϩ channels are blocked (Castillo et al., 1994), and these channel-dependent facilitation of presynaptic Ca 2؉ influx. Thus, the presence of Ca v 2.3 channels boosts the experiments indicate that-aside from N-and P/Q-type Ca 2ϩ channels-additional sources of Ca 2ϩ entry could accumulation of presynaptic Ca 2؉ triggering presynaptic LTP and posttetanic potentiation without affecting contribute to the induction of LTP. R-type Ca 2ϩ channels resistant to organic Ca 2ϩ chan-the low release probability that is a prerequisite for the enormous plasticity displayed by mossy fiber syn-nel antagonists are present at certain presynaptic terminals and may contribute to intraterminal Ca 2ϩ increases apses.
Neuroscience, 2001
Xenopus nerve±muscle co-cultures, we have examined the contribution of calcium-activated potassium (K Ca) channels to the regulation of transmitter release evoked by single action potentials. The presynaptic varicosities that form on muscle cells in these cultures were studied directly using patch-clamp recording techniques. In these developing synapses, blockade of K Ca channels with iberiotoxin or charybdotoxin decreased transmitter release by an average of 35%. This effect would be expected to be caused by changes in the late phases of action potential repolarization. We hypothesize that these changes are due to a reduction in the driving force for calcium that is normally enhanced by the local hyperpolarization at the active zone caused by potassium current through the K Ca channels that co-localize with calcium channels. In support of this hypothesis, we have shown that when action potential waveforms were used as voltage-clamp commands to elicit calcium current in varicosities, peak calcium current was reduced only when these waveforms were broadened beginning when action potential repolarization was 20% complete. In contrast to peak calcium current, total calcium in¯ux was consistently increased following action potential broadening. A model, based on previously reported properties of ion channels, faithfully reproduced predicted effects on action potential repolarization and calcium currents. From these data, we suggest that the large-conductance K Ca channels expressed at presynaptic varicosities regulate transmitter release magnitude during single action potentials by altering the rate of action potential repolarization, and thus the magnitude of peak calcium current.
Presynaptic calcium channels: pharmacology and regulation
Neurochemistry international, 1995
Voltage-dependent Ca2+ channels are considered as molecular trigger elements for signal transmission at chemical synapses. Due to their central role in this fundamental process, function and pharmacology of presynaptic Ca2+ channels have recently been the subject of extensive exploration employing various experimental techniques. Several lines of evidence indicate that, at nerve terminals in higher vertebrates, the evoked influx of Ca2+ -ions is mainly mediated by Ca2+ channels of the P-type. The stringent regulation of presynaptic Ca2+ channels is supposed to be involved in fine-tuning the efficiency of synaptic transmission. Intrinsic control mechanisms, such as voltage- or Ca(2+)-dependent inactivation, or modulation of channel activity, either by G-proteins directly or via phosphorylation by protein kinases, may be of particular functional importance.