Effect of Desmethylimipramine on the Release of [3H]Norepinephrine Induced by Various Agents in Hypothalamic Synaptosomes

In order to discriminate between carrier-mediated and non-carrier-mediated mechanisms of norepinephrine release, hypothalamic synaptosomes labeled with [ 3 H]norepinephrine were treated with desmethylimipramine, a blocker of the norepinephrine transport system, and then superfused under various conditions stimulating the release of the labeled amine. The stimulatory effect on release elicited by unlabeled norepinephrine, p -tyramine, and β-phenylethylamine was blocked in desmethylimipramine-treated synaptosomes. The effect of β-phenylethylamine, but not that of norepinephrine, was largely maintained in synaptosomes superfused with a sodium-free medium, in the absence of desmethylimipramine; thus β-phenylethylamine could enter synaptosomes without utilizing the norepinephrine carrier and displace the catecholamine, which would then be transported out of the particles through a desmethylimipramine-sensitive carrier. The release of [ 3 H]norepinephrine was increased upon superfusion with a sodium-free medium, and desmethylimipramine largely prevented this release. The release of [ 3 H]norepinephrine from reserpine-treated synaptosomes induced by lack of sodium was totally blocked by desmethylimipramine. Superfusion with a potassium-free medium stimulated [ 3 H]norepinephrine release, which was affected only slightly by prior treatment with desmethylimipramine. Depolarization of synaptosomes by 56 mM KCl, in the presence of calcium, on addition of the ionophore A23187 to the superfusion medium caused a desmethylimipramine-insensitive release of labeled amine, a finding compatible with exocytotic release. The release of 3 H-labeled deaminated metabolites of [ 3 H]norepinephrine induced by reserpine was not affected by desmethylimipramine.