Octopamine-binding sites in the brain of Locusta migratoria

Abstract Binding sites for octopamine in the brain of Locusta migratoria were characterized using [ 3 H]octopamine as radioligand. Equilibrium binding of [ 3 H]octopamine to a membrane fraction from brains was achieved after a 60-min incubation period at 26°C. Binding of the radioligand was a function of tissue concentration, with a linear correlation over the range of 0.25–1 brain equivalents. Incubation of aliquots of a membrane fraction from brains with increasing concentrations of [ 3 H]octopamine indicated saturable binding at radioligand concentrations of 100 nM and above. Scatchard analysis of the binding data suggested the presence of a single binding site with the equilibrium dissociation constant ( K d ) being 3±0.5 × 10 −8 M, and the capacity of binding sites ( B max ) being 0.2±0.1 pmol/mg protein. The Hill number was nearly equal to one, which indicates that one molecule of octopamine binds to one octopamine-binding site. A Scatchard analysis of the displacement data using dl -octopamine, also suggested the presence of a single binding site. The rank order of potency of different octopamine agonists and antagonists in competing for binding, based on IC 50 values was: naphazoline > clonidine > metoclopramide > chlorpromazine, which indicates that the binding site for [ 3 H]octopamine is an octopamine 2 receptor. Since naphazoline > tolazoline, it is likely that the binding site for [ 3 H]cotopamine is an octopamine 2A receptor.