Mechanism of aminopyridine-induced release of [3H]dopamine from rat brain synaptosomes.

1. Aminopyridines (APs) induced the release of [3H]dopamine (3H-DA) from rat synaptosomal preparations. 2. 4-AP and 3,4-DAP were of equal efficacy in inducing release of 3H-DA; 3-AP, 2-AP and 2,6-AP were less active; pyridine and pyridine-4-carboxylamide were inactive. 3. Cd2+ was more effective in inhibiting 4-AP-induced release of 3H-DA (IC50 approximately 4 microM) than Co2+ and Ni2+ (IC50s approximately 500 microM). 4. While 4-AP increased the 45Ca2+ content of whole synaptosomal preparations, no effect of 4-AP on 45Ca2+ content was observed in lysed synaptosomal preparations. 5. 4-AP-induced 45Ca2+ uptake was inhibited by Cd2+, Ni2+ and Co2+ in concentration ranges similar to those inhibiting 3H-DA release.

Effects of ?-latrotoxin, lanthanides and elevated potassium concentration on acetylcholine release and free intracellular calcium levels in rat cerebral cortex synaptosomes.

At peripheral cholinergic nerve terminals, lanthanum (La(3+)) increases acetylcholine (ACh) release and supports transmitter release induced by ?-latrotoxin (?-LTx). The present experiments investigate these interactions at central cholinergic nerve terminals by measuring the effects of La(3+) and gadolinium (Gd(3+)), another element of the lathanide (Ln(3+)) series, on ACh release and free intracellular calcium concentrations ([Ca(2+)](i)) in rat cortical synaptosomal preparations. Ln(3+) had no measurable effect on basal synaptosomal ACh release, ACh contents or [Ca(2+)](i). Release of ACh induced by ?-LTx was inhibited by the presence of Ln(3+), as were ACh release responses to elevated K(+). The rise in [Ca(2+)](i) induced by ?-LTx was inhibited by Ln(3+) at concentrations similar to those that inhibited ACh release. Addition of Ln(3+) subsequent to an ?-LTx-evoked increase in [Ca(2+)](i) resulted in an immediate (within 4 s) cessation of calcium entry. Gd(3+) binding studies using the radioisotope (153)Gd(3+) revealed that Gd(3+) bound extremely rapidly to synaptosomes and that under the experimental conditions used to measure either ?-LTx- or K(+)-induced ACh release, little or no free Gd(3+) was likely to exist in the medium. We conclude that at central cholinergic neurons 1-Ln(3+) do not affect basal ACh release and 2-Ln(3+) are effective inhibitors for ?-LTx-induced ACh release and changes in [Ca(2+)](i).

Interactions between the presynaptically active neurotoxins alpha-latrotoxin and omega-conotoxin GVIA: studies on calcium fluxes and binding parameters in rat and chicken synaptosomes.

Possible interactions between alpha-latrotoxin, an activator of synaptosomal calcium uptake, and omega-conotoxin GVIA, an inhibitor of voltage-sensitive calcium channels of the N-type, were investigated in rat and chicken synaptosomal preparations. While omega-conotoxin GVIA potently and effectively inhibited calcium uptake induced by elevated potassium in chick synaptosomes, little or no effect of omega-conotoxin GVIA was observed either in potassium-treated rat synaptosomes or in alpha-latrotoxin-exposed synaptosomes of either spaces. In contrast to the lack of effect of omega-conotoxin on stimulated calcium uptake in rat synaptosomes, cadmium effectively inhibited calcium uptake induced by either potassium or alpha-latrotoxin. Synaptosomal calcium transport induced by alpha-latrotoxin can be bidirectional, since alpha-latrotoxin also induced efflux of preaccumulated calcium. Competition experiments revealed that binding of 125I-labelled omega-conotoxin and 125I-labelled alpha-latrotoxin was similar in either chicken or rat synaptosomes. Neither alpha-latrotoxin nor omega-conotoxin competed with the binding of the other ligand in either species. The results reported here show that (1) elevated potassium evokes calcium uptake principally through N-channels in avian but not in rat synaptosomes; (2) alpha-latrotoxin-activated calcium fluxes are omega-conotoxin insensitive but cadmium sensitive; (3) the molecular acceptors for the two ligands are likely to be unrelated synaptic membrane constituents.

Interactions between alpha-latrotoxin and trivalent cations in rat striatal synaptosomal preparations.

The interactions between alpha-latrotoxin (alpha-LTx), a neurosecretagogue purified from the venom of the black widow spider, and the trivalent cations Al3+, Y3+, La3+, Gd3+, and Yb3+ were investigated in rat striatal synaptosomal preparations. All trivalent cations tested were inhibitors of alpha-LTx-induced [3H]dopamine [( 3H]DA) release (order of potency: Yb3+ greater than Gd3+ approximately Y3+ greater than La3+ greater than Al3+). Only with Al3+ could inhibition of [3H]DA release be attributed to a block of 125I-alpha-LTx specific binding to synaptosomal preparations. The inhibitory effect of trivalent ions was reversible provided synaptosomes were washed with buffer containing EDTA. Trivalent ions also inhibited alpha-LTx-induced [3H]DA release at times when alpha-LTx-stimulated release was already evident. alpha-LTx-induced synaptosomal membrane depolarization was blocked by La3+, but not affected by Gd3+, Y3+, and Yb3+. alpha-LTx-stimulated uptake of 45Ca2+ was inhibited by all trivalent cations tested. These results demonstrate that there exist at least three means by which trivalent cations can inhibit alpha-LTx action in rat striatal synaptosomal preparations: (1) inhibition of alpha-LTx binding (Al3+); (2) inhibition of alpha-LTx-induced depolarization (La3+); and (3) inhibition of alpha-LTx-induced 45Ca2+ uptake (Gd3+, Y3+, Yb3+, La3+).