α2-Adrenoceptor mediated inhibition of exoyctotic noradrenaline release in the absence of extracellular Ca 2+

Abstract

The effect of the α 2-adrenoceptor agonist clonidine on 3,4-diaminopyridine (3,4-DAP)-evoked [ 3H]noradrenaline ([ 3H]NA) release in rat hippocampus slices was studied in the presence or absence ( + 1 mM EGTA) of extracellular Ca 2+. 3H overflow (consisting mainly of unmetabolized [ 3H]NA) was evoked by addition of 100 μM 3.4-DAP for 10 min to the medium, which always contained 1 μM desipramine. Ligands for L-type voltage-sensitive Ca 2+ channels (VSCC) did not affect and evoked [ 3H]NA release, whereas the preferential N-type VSCC antagonist ω-conotoxin was inhibitory, both in the presence and even more potently in the absence of Ca 2+, suggesting an involvement on N-type VSCC in the mechanism of 3,4-DAP-evoked [ 3H]NA release. In the absence of extracellular Ca 2+ the initial influx, which has been previously proposed to liberate Ca 2+ from intracellular stores for the exocytotic process, most probably occurs via N-type VSCC. Clonidine inhibited the 3,4-DAP-evoked [ 3H]NA release in a concentration-dependent manner, both in the presence and even more potently in the absence of Ca 2+: its effects were antagonized by yohimbinc. In the presence of extracellular Ca 2+ the clonidine effect was not changed by addition of ω-conotoxin. Similar effects of clonidine were found in slices from the rabbit hippocampus. Since the availability of Ca 2+ from intracellular stores seems to predominate in the present model, our results lend some support to the suggestion that α 2-adrenoceptor activation might affect intracellular mechanisms of Ca 2+ homeostasis. On the other hand, however, the present data cannot completely exclude that α 2-autoreceptor activation leads to opening of presynaptic K + channels, or that modulation of Ca 2+ influx through VSCC is involved.