Characterization of a norepinephrine binding site in bovine hypothalamus

A norepinephrine binding site in bovine hypothalamic crude synaptosomal fractions has been identified and characterized. [ 3H](−)norepinephrine binding, conducted in the presence of EDTA to eliminate norepinephrine oxidation, was complete in 6 min, had a T 1 2 for dissociation of less than 15 sec, was dependent on protein concentration, and was distinct from binding to bovine serum albumin. (−)Norepinephrine (P < 0.05) but not (+)norepinephrine at 10 −7M competed with 2 × 10 −8 M [ 3H] (−) norepinephrine for binding indicating saturability and stereospecificity of the site. In addition, the l-isomer of isoproterenol had a 20 times higher affinity for the binding site than the d-isomer. However, (+)propranolol, (−)propranolol, dichloroisoproterenol, phenylephrine, phentolamine, tyramine, serotonin, chlorpromazine and catechol at 10 −4M and desipramine at 10 −7M did not compete with 2 × 10 −8M [ 3H](−)norepinephrine for binding. In addition, norepinephrine was not metabolized nor oxidized during the reaction. These results indicate that there is a stereospecific, stereoselective norepinephrine binding site in the hypothalamus in addition to the β-receptor. In addition, the binding site does not have the characteristics of the classical peripheral α or β-receptor and is probably not monoamine oxidase, catechol- o-methyl transferase nor the Uptake 1 Site.