BrainResearch, 301(1984)149-152 149 Elsevier BRE20209 Preferential uptake of norepinephrine into dopaminergic terminals of a synaptosomal preparation from rat cerebral cortex MARTIN MICHEL, CHRISTOPH HIEMKE and RUDIGER GHRAF lnstitut far Physiologische Chemie, Universitiitsklinikum Essen, D-4300 Essen I (F. R. G.) (Accepted January 31st, 1984) Key words: catecholamine uptake - - norepinephrine - - dopamine - - cerebral cortex - - hypothalamus - - striatum - - desmethylimipramine In a synaptosomal preparation from male rat cerebral cortex only 34% of total norepinephrine (NE) uptake could be inhibited by nanomolar concentrations of desmethylimipramine (DMI) with an apparent IC50value of 0.37 nM. The residual uptake was efficient- ly inhibited by micromolar concentrations of DMI (ICs0 = 4.0 pM). In synaptosomes from the hypothalamus, 74% of total NE uptake could be blocked by DMI with an IC50 of 0.1 nM whereas in synaptosomes from the striatum the ICs0for DMI inhibition was 3.8 #M. It is concluded that in synaptosomes from rat cerebral cortex only 34%, and in synaptosomes from the hypothalamus 74% of total NE are taken up by noradrenergic nerve terminals whereas the residual NE uptake occurs in dopaminergic nerve endings. The high-affinity catecholamine uptake system of noradrenergic nerve terminals transports both nor- epinephrine (NE) and dopamine (DA). Similarly, dopaminergic terminals accumulate both DA and NE (for a review, see ref. 6). The tricyclic antide- pressant desmethylimipramine (DMI) is the most po- tent inhibitor of high-affinity catecholamine uptake into noradrenergic terminals so far described. In syn- aptosomal preparations rich in noradrenergic termi- nals (hypothalamus), the drug inhibits NE uptake with an effective concentration in the nanomolar range5, 7 whereas in synaptosomal preparations, rich in dopaminergic and poor in noradrenergic terminals (striatum), DMI inhibits both DA 5,7 and NE (this in- vestigation) uptake in the micromolar range. Morphological 13 and pharmacologicalll. 12 studies have demonstrated the presence of both noradrener- gic and dopaminergic nerve terminals in the cerebral cortex of the rat. Thus, in synaptosomal preparations obtained from this tissue, it should be possible to dis- criminate between two distinct populations of cate- cholamine uptake terminals which markedly differ in their sensitivity to DMI inhibition. In this investiga- tion, using a wide range of narrowly spaced DMI con- centration increments, we have tested for the pres- ence of a two-phase inhibition curve of catechola- mine (NE and DA) uptake into synaptosomal prepa- rations from cerebral cortex. IC50 values obtained from such curves were compared to those calculated from DMI inhibition of NE uptake into synapto- somes from hypothalamus and striatum. L-[ring-2,5,6-3H]norepinephrine (spec. act. 40--60 Ci/mmol) and 3,4-[ring-2,5,6-3H]dihydroxyphenyl - ethylamine (dopamine, spec. act. 40--60 Ci/mmol) were obtained from New England Nuclear. Desme- thylimipramine (mono-N-desmethyl-imipramine) hy- drochloride was a gift from Ciba-Geigy. Male rats of the strain Han: Sprague-Dawley were obtained from the Zentralinstitut fiir Versuchstiere (Hannover, F.R.G.). The hypothalamus and corpus striatum were dissected according to ref. 3. Pooled tissue from hypothalamus, striatum or cerebral (pa- rietal, occipital and temporal) cortex was homoge- nized in 0.3 M sucrose and a crude synaptosomal fraction (9000 g sediment) prepared as described previously 2. Synaptosomal NE and DA content were determined radioenzymatically 4 in the 0.3 M per- chloric acid supernatants obtained from the cerebral cortex synaptosomes. In the uptake studies synapto- somes were finally resuspended in 10 vols. of Correspondence: R. Ghraf, Institut fiir Physiologische Chemie, Universit/itsklinikum Essen, D-4300 Essen 1, F.R.G. 0006-8993/84/$03.00 © 1984 Elsevier Science Publishers B.V.