[3H]quinpirole binding to putative D2 and D3 dopamine receptors in rat brain and pituitary gland: a quantitative autoradiographic study.

The putative D2 dopamine receptor agonist quinpirole (LY 171,555) has been extensively used in a variety of in vivo and in vitro studies of D2 receptor-mediated effects and may have even higher affinity for the recently described D3 dopamine receptor. In the present study, conditions for autoradiographic visualization of [3H]quinpirole-labeled D2-like dopamine receptors were optimized and binding to slide-mounted sections was characterized with respect to pharmacology, guanine nucleotide sensitivity and regional distribution. The pharmacological profile of [3H]quinpirole binding in slide-mounted brain sections was: (+/-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene > or = quinpirole > dopamine for putative dopamine agonists; spiperone > (+)-butaclamol > (-)-sulpiride > SCH 23390 >> cinanserin > (-)-butaclamol for antagonists. [3H]Quinpirole binding was decreased in the presence of guanine nucleotides in most brain regions except in the islands of Calleja and the molecular layer of cerebellar lobules 9 and 10. The regional distribution of [3H]quinpirole binding sites roughly paralleled the distribution of [3H]-(-)-sulpiride binding sites, with greatest densities present in the olfactory bulb glomerular layer, islands of Calleja, pituitary intermediate lobe, caudate/putamen, olfactory tubercles and nucleus accumbens. However, significantly greater densities of [3H]quinpirole binding than [3H]-(-)-sulpiride binding were observed in the molecular layer of cerebellar lobules 9 and 10, the islands of Calleja and olfactory bulb glomerular layer in concordance with the recently reported distribution of D3 receptor mRNA in these brain regions. Higher concentrations of [3H]quinpirole binding were also observed in the dorsomedial caudate and pituitary intermediate lobe. These data indicate the utility of [3H]quinpirole to label D3 as well as D2 dopamine receptors.