3H-Catecholamine binding to alpha-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 nM respectively. The maximum number of 3H-catecholamine binding sites, 10--12 pmoles/g tissue, and the pharmacological characteristics of (+/-)-3H-epinephrine binding, indicate that the catecholamines label the same alpha-noradrenergic receptor in the rat as does 3H-clonidine. At 25 degrees, (+/-)-3H-epinephrine binding associates rapidly to equilibrium, and dissociates in a biphasic manner. The affinities of alpha-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 alpha-receptor binding sites respectively, and a 51% increase in the number of 3H-dihydroalprenolol beta-receptor sites, in rat forebrain. Reserpine pretreatment does not alter the affinities of either alpha- or beta-receptor 3H-ligands.