Clobenpropit, a histamine H3 receptor antagonist/inverse agonist, inhibits [3H]-dopamine uptake by human neuroblastoma SH-SY5Y cells and rat brain synaptosomes

Background: Clobenpropit, a potent antagonist/inverse agonist at the histamine H₃ receptor (H₃R), reduced the cytotoxic action of 6-hydroxydopamine (6-OHDA) in neuroblastoma SH-SY5Y cells transfected with the human H₃R. We therefore set out to study whether this effect involved a receptor-independent action on dopamine transport.

Methods: The uptake of [³H]-dopamine was assayed in SH-SY5Y cells and rat striatal or cerebro-cortical isolated nerve terminals (synaptosomes). Clobenpropit binding to the human norepinephrine (NET) and dopamine (DAT) transporters was analyzed by molecular modeling.

Results: In SH-SY5Y cells, [³H]-dopamine uptake was inhibited by desipramine (selective NET inhibitor), GBR-12909 (selective DAT inhibitor), and fluoxetine (selective inhibitor of the Serotonin Transporter, SERT) with IC₅₀ values 37, 537, and 2800nM, respectively. The potency rank order indicates that [³H]-dopamine uptake is primarily performed by NET. Clobenpropit inhibited [³H]-dopamine uptake (maximum inhibition 82.7±2.8%, IC₅₀ 490nM), and the effect was reproduced by the H₃R antagonist/inverse agonist iodophenpropit, but not by the agonists R-α-methylhistamine and immepip or the antagonists/inverse agonists ciproxifan and A-331440. Clobenpropit also inhibited [³H]-dopamine uptake by rat striatal and cerebro-cortical synaptosomes (-54.6±11.3% and -46.3±9.6%, respectively, at 10μM). Modeling of the human NET and DAT obtained by homology from the crystal of Drosophila melanogaster DAT showed that clobenpropit can bind to a site also recognized in both transporters by nisoxetine, a potent NET inhibitor.

Conclusion: These data indicate a direct inhibitory effect of clobenpropit on Catecholamine transport.