Monoamine oxidase inhibitors inhibit [3H]quinpirole binding in rat striatal membranes

This study describes interactions of monoamine oxidase inhibitors at binding sites labeled by [3H]quinpirole, a putatively selective ligand for dopamine D2-like receptors, in in vitro binding assays in rat brain. Monoamine oxidase inhibitors potently and competitively inhibited equilibrium binding of [3H]quinpirole in homogenate binding assays with the following rank order of potencies: clorgyline > or = Ro 41-1049 > pargyline > (-)-deprenyl > (+)-deprenyl > Ro 16-6491 > iproniazid. This rank order of potencies does not correlate with the potencies of these drugs at monoamine oxidase-A or monoamine oxidase-B, sigma site(s) or dopamine receptors. Monoamine oxidase inhibitors did not alter the ability of quinpirole to compete for [3H]spiperone binding. Quinpirole did not inhibit monoamine oxidase-A or monoamine oxidase-B activity and had low affinity (200 nM) for sigma site(s). These data suggest a potential novel binding site for [3H]quinpirole in rat brain and/or an alternative site of action for the antidepressant effects of monoamine oxidase inhibitors.