Characterization of [3H]quinpirole binding to D2-like dopamine receptors in rat brain

Abstract

The putative D2 dopamine receptor agonist quinpirole (LY 171,555) is the most widely used D2 agonist in in vivo and in vitro studies of D2 receptor- mediated effects. In addition, quinpirole may have even higher affinity for the recently described D3 dopamine receptor. The present study describes the in vitro binding properties of newly developed [3H]quinpirole in rat brain. [3H]Quinpirole binding was characterized in striatal membrane homogenate preparations using a filtration assay. Nonspecific binding was defined by 1 μM (+)-butaclamol. Specific [3H]quinpirole binding was saturable, and dependent on temperature, membrane concentration, sodium concentration and guanine nucleotides. Saturation analysis revealed high affinity binding characteristics (K(D) = 2.3 ± 0.3 nM) which were confirmed by association- dissociation kinetics. The pharmacological profile of [3H]quinpirole binding in striatum was: (-)-N-n-propylnorapomorphine (±)-2-amino-6,7-dihydroxyl- 1,2,3,4-tetrahydronaphthalene ≥ quinpirole > apomorphine > bromocriptine > dopamine > SKF 38393 >> 5-hydroxytryptamine for putative dopamine agonists; spiperone > (+)-butaclamol > haloperidol > (-)-sulpiride > clozapine > SCH 23390 >> cinanserin for antagonists. [3H]Quinpirole binding exhibited stereoselectivity: (-)-sulpiride > (+)-sulpiride and (+)-butaclamol > (-)- butaclamol. This pharmacological profile is similar, though not identical, to that observed for [3H]spiperone-labeled D2 receptors. The regional distribution of [3H]quinpirole binding sites roughly paralleled the distribution of [3H]spiperone binding sites, with greatest densities present in the striatum, nucleus accumbens and olfactory tubercles. The most marked differences between [3H]quinpirole and [3H]spiperone binding were observed in cerebral cortical brain regions. These data indicate that [3H]quinpirole is a suitable ligand for the study of D2-like receptors.