³H-catecholamine binding to α-receptors in rat brain: Enhancement by reserpine

(±)-³H-Epinephrine and (-)-³H-norepinephrine bind to rat cortex membranes in a saturable manner with dissociation constants of 16.7 and 27 nM0 respectively. The maximum number f ³H-catecholamine binding sites, 10-12 pmoles/g tissue, and the pharmacological characteristics of (±)-³H-epinephrine binding, indicate that the catecholamines label the same α-noradrenergic receptor in the rat as does ³H-clonidine. At 25°, (±)-³H-epinephrine binding associates rapidly to equilibrium, and dissociates in a biphasic manner. The affinities of α-agonists at the ³H-catecholamine binding site are 2-4 fold weaker in the rat than in the calf cortex under the same experimental conditions. Ergot alkaloids and phenoxybenzamine have similar affinities in the two tissues, whereas phentolamine and WB-4101 are 8-10 times weaker in the rat. Reserpine (0.25 mg/kg s.c. per day for 3 weeks) causes 25 and 46% increases in the numbers of (±)-³H-epinephrine and ³H-WB-4101 α-receptor binding sites respectively, and a 51% increase in the number of ³H-dihydroalprenolol β-receptor sites, in rat forebrain. Reserpine pretreatment does not alter the affinities of either α-or β-receptor ³H-ligands.