Inhibition of [3H]quinpirole binding by a monoamine oxidase inhibitor in subcellular fractions of rat striatum

[H-3]Quinpirole is a dopamine agonist with high affinity for D-2-like dopamine receptors. A number of non-dopaminergic compounds, most notably monoamine oxidase inhibitors (MAOIs), inhibit the binding of [H-3]quinpirole, but not other D-2-like agonists and antagonists, in rat striatal membranes by a mechanism that does not involve the enzymatic activity of MAO. To further characterize this novel interaction, the subcellular distribution of spiperone-displaceable, "D-2-like" [H-3]quinpirole-labeled sites in rat striatum was assessed and compared with the distribution of MAOI-displaceable [H-3]quinpirole binding (MQB). "D-2-like" [H-3]quinpirole binding exhibited similar nanomolar affinity in the crude synaptosomal (P-2) crude microsomal (P-3), and ribosomal, post-microsomal (P-4) fractions. Total binding activity (fmol bound/fraction) of "D-2-like" [H-3]quinpirole binding was concentrated in the synaptosomal fraction (P-2B). The subcellular distribution of MQB paralleled that of "D-2-like" [H-3]quinpirole binding. This suggests that "D-2-like" [H-3]quinpirole binding and MQB occur at a common membrane-bound binding site.