QUANTITATIVE AUTORADIOGRAPHY OF GPP(NH)P SENSITIVE AND INSENSITIVE [H-3] QUINPIROLE BINDING-SITES IN THE RAT-BRAIN

Recent advances in the cloning of dopamine receptor subtypes have resu lted in the detection of at least 5 genetically distinct subtypes of t he dopamine receptor. The dopaminergic agonist, quinpirole, has relati vely high affinity for the cloned D-2, D-3, and D-4 receptor subtypes. The D-3 receptor is unique within these 3 subtypes in that it does no t appear to have sensitivity to guanine nucleotides. In order to local ize the brain regions containing these subtypes of the dopamine recept or, the distribution of [H-3]quinpirole binding sites in the rat brain was mapped by autoradiography. Labelling in the absence and presence of the nonhydrolyzable GTP analog, guanylyl-5'-imidodiphosphate (Gpp(N H)p) allowed differentiation between the high affinity agonist conform ation of the D-2 receptor and the D-3 receptor. The highest densities of [H-3]quinpirole binding were found in the caudate-putamen, nucleus accumbens, islands of Calleja, and olfactory tubercle which is consist ent with the distribution of dopamine receptors seen with ''classical D-2'' receptor agonist and antagonist radioligands. In the presence of 10 muM Gpp(NH)p, binding was reduced in all these areas with the exce ption of the islands of Calleja. Additional areas which exhibited no c hange in binding when incubated with Gpp(NH)p included the nodulus and floculus of the cerebellum. Several areas exhibited a partial reducti on in binding including the glomerular layer of the olfactory bulb, nu cleus accumbens, and the superior colliculus. The distribution of thes e subtypes is consistent with the distribution of mRNA for the D-2 and D-3 receptor in the brain. Therefore, [H-3]quinpirole binds to the hi gh affinity agonist conformation of the D-2 and the D-3 receptor.