On the usefulness of Fura‐2 measurements of intrasynaptosomal calcium levels in rat cortical synaptosomes to study mechanisms of presynaptic function

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Abstract
Levels of [Ca2+]iin rat cortex synaptosomes were measured using the Ca2+indicator Fura‐2. Ca2+influx was induced by veratridine in a concentration‐dependent manner (1–10μm). The resulting increase in [Ca2+]iwas inhibited by tetrodotoxin (TTX). K+(18 mm) increased the [Ca2+]iwhich was not influenced by TTX. K+‐channel blockers such as 4‐aminopyridine,α‐andδ‐dendrotoxinper sewere ineffective. The veratridineinduced Ca2+influx in synaptosomes was reduced by L‐type Ca2+‐channel blockers, such as felodipine, nifedipine and PN‐200‐110, verapamil and diltiazem.ω‐Conotoxin, an N‐type Ca2+‐channel blocker, did not inhibit the veratridine‐stimulated [Ca2+]iincrease. Bay K 8644, an L‐channel agonist, stimulated an increase of [Ca2+]iin synaptosomes which was not sensitive to TTX.R‐N6‐Phenyl‐isopropyl‐adenosine (R‐PIA) and clonidine, agonists at adenosine A1‐receptors and α2‐adrenoceptors, respectively, did not influence the veratridinestimulated [Ca2+]iincrease. R‐PIA did not interact with Bay K 8644‐stimulated [Ca2+]iincrease in synaptosomes.The results for all the substances used show major differences between the effects on Ca2+influx in synaptosomes and on the electrically evoked neurotransmitter release in slice preparations. Thus, the synaptosome preparation is not a generally applicable experimental model for the study of Ca2+mechanisms of presynaptic neuromodulation.