DOPAMINE TRANSPORTER MEDIATED RELEASE OF DOPAMINE - ROLE OF CHLORIDE

Using a rapid (0.5 ml/min) flow rate superfusion system, the dopamine (DA) transporter mediated release of DA is further explored, and compa red to the depolarization evoked release of DA in rat striatal synapto somes preloaded with radioactive DA (H-3-DA). In this system external DA in the low mu M range efficaciously releases the preloaded transmit ter, the maximal response being reached at 3 mu M DA. The external DA stimulated release is Ca2+-independent, Cl--dependent, and blocked by both bupropion and nomifensine. The atypical antidepressant bupropion inhibits H-3-DA accumulation to rat striatal synaptosomes with a calcu lated IC50 of 1.3 x 10(-6)M. Among DA uptake blockers some are known t o act as DA releasing agents. Here we found that the DA uptake blocker nomifensine (30 mu M) is unable to modify the baseline release of H-3 -DA, whereas bupropion (10 mu M) clearly elevates the baseline release of H-3-DA in a Ca2+-independent and Cl--dependent manner. The non rel easing agent nomifensine blocks the release of H-3-DA induced by bupro pion. The Ca2+-dependent, high K+ depolarization evoked release of H-3 -DA is not modified by nomifensine and does not depend on the external Cl- concentration. When the depolarizing medium contains DA the carri er mediated release of H-3-DA induced by the external DA is additive t o the high K+ induced response. A drastic drop in the external Cl- con centration induces H-3-DA release. This release of H-3-DA induced by l ow external Cl- levels is completely blocked by nomifensine, which onl y slightly diminished the release of H-3-DA induced by the absence of external Na+. On the basis of these results, it is concluded that: 1) Rapid perfusion flow rates eliminate DA reuptake. 2) DA uptake inhibit ors either with or without DA releasing capabilities block the release of DA induced by mu M levels of external DA. 3) By preventing translo cation of the DA transporter mobile moiety, nomifensine may inhibit th e release of DA induced by external DA or bupropion and by drastic dro ps in the external Cl- concentration. 4) In the absence of nomifensine , the DA transporter works under both resting and depolarized conditio ns, but in contrast to the GABA transporter (Sitges et al.: Neurochem Res 18:1081-1087, 1993), the DA transporter does not contribute to the amount of the DA released by depolarization. 5) Reversal of the DA up take carrier is favored by conditions increasing the internal DA level s. 6) Cl- rather than Na+ is a major determinant in H-3-DA movements t hrough the DA transporter. (C) 1994 Wiley-Liss, Inc.