Computer imaging and analysis of dopamine (D2) and serotonin (S2) binding sites in rat basal ganglia or neocortex labeled by [3H]spiroperidol.

The equilibrium properties, pharmacological specificity and regional distribution of serotonin (S2) and dopamine (D2) binding sites in coronal or horizontal rat brain sections labeled by [3H] spiroperidol were investigated by computer analysis of digitized autoradiographs. Domperidone or (+)-butaclamol displaced [3H] spiroperidol from the anterior caudate-putamen, nucleus accumbens, olfactory tubercle, claustrum, layer 5A of motor cortex and layer 1 of the anterior cingulate cortex (IC50 = 2-80 nM). Equilibrium saturation analysis of the (+)-butaclamol-displaced binding of [3H]spiroperidol revealed higher binding affinities in the anterior caudate-putamen, nucleus accumbens and olfactory tubercle (Kd = 0.16-0.32 nM) than in the claustrum or layer 5A of motor cortex (Kd = 1.5-1.9 nM). The [3H]spiroperidol binding displaced by (+)-butaclamol was resolved into a dopaminergic (D2) component, displaced by 100 microM 2-amino-6,7-dihydroxytetrahydronapthalene or 10 microM (-)-sulpiride and a serotonergic (S2) component, displaced by 40 nM ketanserin or 100 nM methysergide. Methysergide or ketanserin displaced [3H]spiroperidol only from the caudal (peripallidal) caudate-putamen, claustrum or layer 5A of motor cortex (IC50 = 2-14 nM), whereas the D2 agonist 2-amino-6,7-dihydroxytetrahydronapthalene displaced [3H]spiroperidol from the caudate-putamen but not from cortex. The competition curve for 2-amino-6,7-dihydroxytetrahydronapthalene displacement of [3H]spiroperidol binding to D2 sites in the caudate-putamen was markedly biphasic and shifted to the right by the GTP analog, guanylimidodiphosphate. The D2 antagonist (+/-)- or (-)-sulpiride also displaced [3H]spiroperidol from the nucleus accumbens and olfactory tubercle as well as from the caudate-putamen (IC50 values = 0.14 microM). In contrast, the butyrophenone derivative, spirodecanone, displaced with equal potency all [3H]spiroperidol from each of the six brain regions. Neither the alpha-1 receptor ligand prazosin nor the excitatory amino acid receptor ligands l-aspartate, l-glutamate or dl-homocysteic acid displaced [3H]spiroperidol from any brain area. A 5-fold rostral-to-caudal gradient of decreasing S2 concentration was observed in neocortical layer 1 of horizontal sections. In the caudate-putamen, D2 density decreased by 30% rostrocaudally, whereas S2 sites were located mostly in the peripallidal caudate-putamen. The rostral-to-caudal gradients of D2 or S2 sites in the caudate-putamen correspond remarkably well with previously reported caudate-putamen concentration gradients for dopamine or serotonin, respectively.