Abstract

The possibility of interaction between neurotransmitter uptake mechanisms and presynaptic receptors regulating transmitter release was investigated using rat brain synaptosomes in superfusion. Various conditions were considered including: absence of substrate for the uptake with uptake potentially operative; absence of substrate and presence of uptake inhibitors; and uptake activated by added substrate, with or without uptake inhibitors. The release of [3H]-5-hydroxytryptamine ([3H]-5-HT) evoked by 15 mM KCl from cerebral cortex synaptosomes was inhibited by lysergic acid diethylamide. The 5-HT uptake inhibitors citalopram and chlorimipramine did not affect the inhibitory action of lysergic acid diethylamide. Clonidine decreased both the K+-evoked release of [3H]norepinephrine and that of [3H]-5-HT in cortical synaptosomes through the activation of presynaptic alpha-2 adrenoceptors. In superfusion conditions, the action of clonidine on [3H]norepinephrine release was not antagonized by the norepinephrine uptake inhibitors desipramine or cocaine; similarly, the inhibition of [3H]-5-HT release was unaffected when 5-HT uptake was blocked. The K+-evoked release of [3H]dopamine from striatal nerve terminals was potentiated by acetylcholine (ACh) through the activation of muscarinic presynaptic receptors. The action of ACh was not modified by the presence of nomifensine, a dopamine uptake inhibitor. Finally, in superfused cortical synaptosomes, the block of the high-affinity uptake of choline by hemicholinium-3 had no effect on the muscarinic autoreceptor-mediated inhibition of [3H]ACh release by ACh. Altogether the present results do not support the previously proposed idea that in nerve terminals a functional coupling may exist between uptake mechanisms and presynaptic receptors.