N-type voltage-dependent calcium channels mediate the nicotinic enhancement of GABA release in chick brain. (1/34)

The role of voltage-dependent calcium channels (VDCCs) in the nicotinic acetylcholine receptor (nAChR)-mediated enhancement of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) was investigated in chick brain slices. Whole cell recordings of neurons in the lateral spiriform (SpL) and ventral lateral geniculate (LGNv) nuclei showed that cadmium chloride (CdCl2) blocked the carbachol-induced increase of spontaneous GABAergic IPSCs, indicating that VDCCs might be involved. To conclusively show a role for VDCCs, the presynaptic effect of carbachol on SpL and LGNv neurons was examined in the presence of selective blockers of VDCC subtypes. omega-Conotoxin GVIA, a selective antagonist of N-type channels, significantly reduced the nAChR-mediated enhancement of gamma-aminobutyric acid (GABA) release in the SpL by 78% compared with control responses. Nifedipine, an L-type channel blocker, and omega-Agatoxin-TK, a P/Q-type channel blocker, did not inhibit the enhancement of GABAergic IPSCs. In the LGNv, omega-Conotoxin GVIA also significantly reduced the nAChR-mediated enhancement of GABA release by 71% from control values. Although omega-Agatoxin-TK did not block the nicotinic enhancement, L-type channel blockers showed complex effects on the nAChR-mediated enhancement. These results indicate that the nAChR-mediated enhancement of spontaneous GABAergic IPSCs requires activation of N-type channels in both the SpL and LGNv.  (+info)

N-Type Ca(2+) channels trigger release of excitatory and inhibitory neurotransmitter from nerve endings in canine bronchi. (2/34)

We set out to characterize the types of Ca(2+) channels that mediate release of the predominant excitatory (acetylcholine) and inhibitory (norepinephrine) neurotransmitters in canine bronchi, using electrically evoked contractions and relaxations, respectively, as indicators of this release. We found that the selective N-type Ca(2+) channel blocker (omega-conotoxin GVIA) eliminated electrically evoked contractions in a dose-dependent fashion (half-maximal inhibition in the presence of 1-5 nM) but had no significant effect on those evoked by exogenously added acetylcholine. Selective blockers of P-type Ca(2+) channels (omega-agatoxin TK; 10(-8) to 10(-7) M) or of L-type Ca(2+) channels (nifedipine; 10(-8) to 10(-6) M) had no significant effect on the responses to neurally released or exogenously added acetylcholine. Likewise, electrically evoked relaxations were blocked by omega-conotoxin GVIA (10(-7) M) but not by omega-agatoxin TK (10(-7) M) or nifedipine (10(-7) M); none of these Ca(2+) channel blockers had a significant inhibitory effect on isoproterenol-triggered relaxations. We conclude that excitatory and inhibitory neurotransmission in canine bronchi is mediated predominantly by N-type Ca(2+) channels, with little or no contribution from L-, P-, Q-, or T-type channels.  (+info)

Role of glutamate receptors and voltage-dependent calcium channels in glutamate toxicity in energy-compromised cortical neurons. (3/34)

We have examined the effect of glutamate receptor antagonists and voltage-dependent calcium channel blockers on the neuronal injury induced by the combination of a low concentration of N-methyl-D-aspartate (NMDA) or kainate and energy compromise resulting from the use of glucose-free incubation buffer. Toxicity induced by NMDA or kainate was enhanced in the glucose-free buffer. NMDA-or non-NMDA-receptor antagonists added to the glucose-free buffer at the same time inhibited the neuronal cell death induced by each agonist. An NMDA-receptor antagonist, MK-801, but not non-NMDA-receptor antagonists, inhibited the toxicity when added to the culture medium after exposure of the cells to the agonists. P/Q-type calcium channel blockers, omega-agatoxin IVA and omega-agatoxin TK, and an N-type calcium channel blocker, omega-conotoxin GVIA, significantly attenuated the neuronal injury, although an L-type calcium channel blocker, nifedipine, showed little neuroprotective effect. A combination of calcium channel blockers of the three subtypes showed the most prominent neuroprotective effect. These observations suggest that the overactivation of NMDA and non-NMDA receptors and consequent activation of the voltage-dependent calcium channels lead to neuronal cell death in energy-compromised cortical neurons.  (+info)

The spider toxin omega-Aga IIIA defines a high affinity site on neuronal high voltage-activated calcium channels. (4/34)

The spider toxin omega-agatoxin IIIA (omega-Aga-IIIA) is a potent inhibitor of high voltage-activated calcium currents in the mammalian brain. To establish the biochemical parameters governing its action, we radiolabeled the toxin and examined its binding to native and recombinant calcium channels. In experiments with purified rat synaptosomal membranes, both kinetic and equilibrium data demonstrate one-to-one binding of omega-Aga-IIIA to a single population of high affinity sites, with K(d) = approximately 9 pm and B(max) = approximately 1.4 pmol/mg protein. Partial inhibition of omega-Aga-IIIA binding by omega-conotoxins GVIA, MVIIA, and MVIIC identifies N and P/Q channels as components of this population. omega-Aga-IIIA binds to recombinant alpha(1B) and alpha(1E) calcium channels with a similar high affinity (K(d) = approximately 5-9 pm) in apparent one-to-one fashion. Results from recombinant alpha(1B) binding experiments demonstrate virtually identical B(max) values for omega-Aga-IIIA and omega-conotoxin MVIIA, providing further evidence for a one-to-one stoichiometry of agatoxin binding to calcium channels. The combined evidence suggests that omega-Aga-IIIA defines a unique, high affinity binding site on N-, P/Q-, and R-type calcium channels.  (+info)

All classes of calcium channel couple with equal efficiency to exocytosis in rat melanotropes, inducing linear stimulus-secretion coupling. (5/34)

1. The contribution of low voltage-activated (LVA) T-type Ca2+ channels and four different types of high voltage-activated (HVA) Ca2+ channel to exocytosis, and the relationship between calcium influx and exocytosis during action potentials (APs) were studied in pituitary melanotropes. 2. Selective HVA Ca2+ channel blockers reduced exocytosis, monitored by membrane capacitance measurements, proportional to the reduction in Ca2+ influx. The efficacy of Ca2+ in stimulating exocytosis did not change in the presence of the Ca2+ channel blockers, indicating that all HVA Ca2+ channels act together in stimulating exocytosis. 3. The relationship between Ca2+ influx and exocytosis during the AP was examined using APs recorded from spontaneously active melanotropes as command templates under voltage clamp. Under voltage clamp, multiphasic Ca2+ currents were activated over the entire duration of the APs, i.e. during the rising phase as well as the plateau phase. The maximum amplitude of the Ca2+ current coincided with the peak of the AP. 4. The relationship between Ca2+ entry and exocytosis was linear for the different phases of the AP. Also, the influx of Ca2+ through LVA T-type channels stimulated exocytosis with the same efficacy as through the HVA channels. 5. APs of increasing duration ( approximately 50 to approximately 300 ms) evoked increasing amounts of exocytosis. The number of entering Ca2+ ions and the capacitance change were linearly related to AP duration, resulting in a fixed relationship between Ca2+ entry and exocytosis. 6. The results show that Ca2+ ions, entering a melanotrope, couple with equal strength to exocytosis regardless of the channel type involved. We suggest that the linear relationship between Ca2+ entry and secretion observed under physiological conditions (during APs), results from the equal strength with which LVA and HVA channels in melanotropes couple to exocytosis. This guarantees that secretion takes place over the entire duration of the AP.  (+info)

Dendro-somatic distribution of calcium-mediated electrogenesis in purkinje cells from rat cerebellar slice cultures. (6/34)

The role of Ca2+ entry in determining the electrical properties of cerebellar Purkinje cell (PC) dendrites and somata was investigated in cerebellar slice cultures. Immunohistofluorescence demonstrated the presence of at least three distinct types of Ca2+ channel proteins in PCs: the alpha1A subunit (P/Q type Ca2+ channel), the alpha1G subunit (T type) and the alpha1E subunit (R type). In PC dendrites, the response started in 66 % of cases with a slow depolarization (50 +/- 15 ms) triggering one or two fast (approximately 1 ms) action potentials (APs). The slow depolarization was identified as a low-threshold non-P/Q Ca2+ AP initiated, most probably, in the dendrites. In 16 % of cases, this response propagated to the soma to elicit an initial burst of fast APs. Somatic recordings revealed three modes of discharge. In mode 1, PCs display a single or a short burst of fast APs. In contrast, PCs fire repetitively in mode 2 and 3, with a sustained discharge of APs in mode 2, and bursts of APs in mode 3. Removal of external Ca2+ or bath applications of a membrane-permeable Ca2+ chelator abolished repetitive firing. Tetraethylammonium (TEA) prolonged dendritic and somatic fast APs by a depolarizing plateau sensitive to Cd2+ and to omega-conotoxin MVII C or omega-agatoxin TK. Therefore, the role of Ca2+ channels in determining somatic PC firing has been investigated. Cd2+ or P/Q type Ca2+ channel-specific toxins reduced the duration of the discharge and occasionallyinduced the appearance of oscillations in the membrane potential associated with bursts of APs. In summary, we demonstrate that Ca2+ entry through low-voltage gated Ca2+ channels, not yet identified, underlies a dendritic AP rarelyeliciting a somatic burst of APs whereas Ca2+ entry through P/Q type Ca2+ channels allowed a repetitive firing mainly by inducing a Ca2+-dependent hyperpolarization.  (+info)

Isolation, synthesis and pharmacological characterization of delta-palutoxins IT, novel insecticidal toxins from the spider Paracoelotes luctuosus (Amaurobiidae). (7/34)

Four novel insecticidal toxins were isolated from the venom of the spider Paracoelotes luctuosus (Araneae: Amaurobiidae) and named delta-palutoxins IT1 to IT4. The four toxins are homologous 36-37 amino acid peptides reticulated by four disulfide bridges and three have amidated C-terminal residues. The delta-palutoxins are highly homologous with the previously described mu-agatoxins and curtatoxins (77-97%). The four peptides demonstrated significant toxicity against larvae of the crop pest Spodoptera litura (Lepidoptera: Noctuidae) in a microinjection bioassay, with LD50 values in the 9-50 microg per g of insect range. This level of toxicity is equivalent to that of several of the most active scorpion toxins used in the development of recombinant baculoviruses, and the delta-palutoxins appear to be insect specific. Electrophysiological experiments demonstrated that delta-palutoxin IT1, the most active toxin acts by affecting insect sodium channel inactivation, resulting in the appearance of a late-maintained sodium current, in a similar fashion to insecticidal scorpion alpha and alpha-like toxins and is thus likely to bind to channel receptor site 3. However, delta-palutoxin IT1 was distinguished by its lack of effect on peak sodium conductance, on the early phase of sodium current inactivation and the absence of a shift in the activation voltage of the sodium channels. delta-Palutoxins are thus proposed as new insecticidal toxins related to the alpha and alpha-like scorpion toxins. They will be useful both in the development of recombinant baculoviruses in agrochemical applications and also as molecular probes for the investigation of molecular mechanisms of insect selectivity and structure and function of sodium channels.  (+info)

Oxytocin retrogradely inhibits evoked, but not miniature, EPSCs in the rat supraoptic nucleus: role of N- and P/Q-type calcium channels. (8/34)

We previously reported that oxytocin (OXT), released from the dendrites of magnocellular neurons in the supraoptic nucleus (SON), acts retrogradely on presynaptic terminals to inhibit glutamatergic transmission. Here we test the hypothesis that oxytocin reduces calcium influx into the presynaptic terminal. We used nystatin perforated-patch recording in vitro to first identify the calcium channels involved in glutamatergic transmission in the SON. [omega]-Conotoxin GVIA ([omega]-CTx) and [omega]-Agatoxin TK ([omega]-Aga) both reduced evoked EPSC amplitude, while nicardipine and nickel had no effect. A combination of [omega]-CTx and [omega]-Aga completely abolished the evoked EPSCs. This depressant effect was accompanied by an increase in the paired pulse ratio with no change in the kinetics of the evoked EPSCs, AMPA currents or postsynaptic cell properties. These results suggest that presynaptic N- and P/Q-type calcium channels mediate glutamate release in the SON while L-, T- and R-type channels make little or no contribution. Oxytocin-induced reduction of the evoked EPSC was substantially occluded in the presence of [omega]-CTx but only partially in the presence of [omega]-Aga. Amastatin, an endopeptidase inhibitor that increases the level of endogenous OXT, also reduced the evoked EPSC. This amastatin effect was also occluded by [omega]-CTx and [omega]-Aga. Miniature EPSCs, which are independent of extracellular calcium, were unaffected by either [omega]-CTx or by OXT, thus further substantiating an action of both compounds on calcium channels. Therefore, dendritically released oxytocin acts mainly via a mechanism involving the N-type channel, and to a lesser extent the P/Q-type channel, to decrease excitatory transmission.  (+info)

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