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

Abstract

This study describes interactions of monoamine oxidase inhibitors at binding sites labeled by [ 3H]quinpirole, a putatively selective ligand for dopamine D 2-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 ⩾ 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, σ 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 σ 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.