EFFECTS OF THE PUTATIVE P-TYPE CALCIUM-CHANNEL BLOCKER, R,R-(-)-DAURISOLINE ON NEUROTRANSMITTER RELEASE

The alkaloid and medicinal herb constituent, R,R-(-)-daurisoline, was originally reported to be a N-type Ca2+ channel blocker, but newer evi dence indicates that it is a blocker of P-type Ca2+ channels. To clari fy its specificity with respect to N- and P-channels, we compared its effects on the electrically induced release of endogenous glutamate, H -3-GABA and H-3-noradrenaline, from brain slices with those of omega-a gatoxin IVA and omega-conotoxin GVIA. Like omega-agatoxin IVA (but wit h about 1000-fold lower potency), and unlike omega-conotoxin GVIA, R,R -(-)-daurisoline inhibited the release of H-3-GABA and glutamate, with IC50 values of 8 and 18 mu M. However, inhibition particularly of H-3 -GABA release was more complete than by omega-agatoxin IVA, indicating interaction with one or more additional voltage-sensitive Ca2+ channe ls, possibly the Q-type. Its potency to inhibit glutamate release elic ited either electrically, by veratrine or by high concentrations of K was similar, in contrast to sodium channel blockers. The effects of R ,R-(-)-daurisoline on the release of H-3-noradrenaline, H-3-dopamine a nd H-3-acetylcholine were in agreement with previous knowledge from ex periments with omega-agatoxin IVA suggesting an involvement of P-chann els. A weak inhibition of H-3-noradrenaline release at 10 mu M, simila r to that by omega-agatoxin IVA at 0.03 mu M, was occluded by alpha(2) -antagonistic properties and could be unmasked in presence of rauwolsc ine. At 10 mu M, it also inhibited electrically evoked H-3-dopamine an d H-3-5-hydroxytryptamine release and caused a marked spontaneous rele ase of all three monoamines in a reserpine-like manner. Spontaneous an d evoked release of H-3-acetylcholine was inhibited by about 25% at 10 mu M. In radioligand binding studies, R,R-(-)-daurisoline interacted with alpha(1)- and alpha(2)-adrenoceptors, 5-HT2 and muscarinic cholin ergic receptors with IC50 values close to 1 mu M, and with mu opiate r eceptors even with 0.18 mu M. Atropine reduced the weak inhibitory eff ect of R,R-(-)-daurisoline on H-3-acetylcholine release somewhat, sugg esting that it was brought about by both P channel blockade and cholin ergic agonist activity, The effect on H-3-GABA release was unaffected by naloxone, indicating that the interaction of R,R-(-)-daurisoline wi th mu opiate receptors is antagonistic. The pattern of effects on neur otransmitter release observed with R,R-(-)-daurisoline resembles that of omega-agatoxin IVA and supports previous electrophysiological data suggesting that the compound blocks P-type voltage-sensitive Ca2+ chan nels, However, the more complete blockade of amino acid release by R,R -(-)-daurisoline suggests interaction with additional Ca2+ channel sub types. Although it does also possess other pharmacological properties, we think that the compound is suitable to test whether blockade of gl utamate release via voltage-sensitive Ca2+ channels is a viable concep t to obtain novel neuroprotective and/or anticonvulsant compounds.