l-DOPA promotes striatal dopamine release through D1 receptors and reversal of dopamine transporter.

Previous studies have pointed out that l-DOPA can interact with D1 or D2 receptors independent of its conversion to endogenous dopamine. The present study was set to investigate whether l-DOPA modulates dopamine release from striatal nerve terminals, using a preparation of synaptosomes preloaded with [ ³ H]DA. Levodopa (1 µM) doubled the K ⁺ -induced [ ³ H]DA release whereas the D2/D3 receptor agonist pramipexole (100 nM) inhibited it. The l-DOPA-evoked facilitation was mimicked by the D1 receptor agonist SKF38393 (30-300 nM) and prevented by the D1/D5 antagonist SCH23390 (100 nM) but not the DA transporter inhibitor GBR12783 (300 nM) or the aromatic l-amino acid decarboxylase inhibitor benserazide (1 µM). Higher l-DOPA concentrations (10 and 100 µM) elevated spontaneous [ ³ H]DA efflux. This effect was counteracted by GBR12783 but not SCH23390. Binding of [ ³ H]SCH23390 in synaptosomes (in test tubes) revealed a dense population of D1 receptors (2105 fmol/mg protein). Both SCH23390 and SKF38393 fully inhibited [ ³ H]SCH23390 binding (Ki 0.42 nM and 29 nM, respectively). l-DOPA displaced [ ³ H]SCH23390 binding maximally by 44% at 1 mM. This effect was halved by addition of GBR12935 and benserazide. We conclude that l-DOPA facilitates exocytotic [ ³ H]DA release through SCH23390-sensitive D1 receptors, independent of its conversion to DA. It also promotes non-exocytotic [ ³ H]DA release, possibly via conversion to DA and reversal of DA transporter. These data confirm that l-DOPA can directly interact with dopamine D1 receptors and might extend our knowledge of the neurobiological mechanisms underlying l-DOPA clinical effects.
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