Photoaffinity Labeling of a Neuronal Octopamine Receptor

The invertebrate aminergic neurotransmitter and neuromodulator octopamine (OA) acts at both neuronal and nonneuronal receptors that appear to have distinct pharmacological characteristics. The current work uses a potent and specific OA photoaffinity ligand, tritiated 2(2,6‐diethyl‐4‐azidophenylimino)imidazolidine ([ 3 H]NC‐5Z), to identify and characterize a putative neuronal OA receptor protein in membranes from nerve tissue of the desert locust, Schistocerca gregaria . Under nonphotolyzing conditions, [ 3 H]NC‐5Z demonstrated high‐affinity binding ( K D = 2.5 ± 0.3 n M ; B max = 702 fmol/mg of protein) to a single class of noninteracting sites. The absolute and rank order potency of binding of both agonists and antagonists was highly correlated ( r = 0.99) with their known ability to displace [ 3 H]OA binding to locust neuronal membranes and was consistent with the labeling of a class 3 OA receptor. Under photolyzing conditions, [ 3 H]NC‐5Z demonstrated irreversible binding that was resistant to trichloroacetic acid and methanol, displaceable by OA and other octopaminergic agonists and antagonists, soluble in sodium dodecyl sulfate, and only sparingly soluble in nonionic detergents. Membrane‐bound [ 3 H]NC‐5Z, solubilized with Nonidet P‐40, bound specifically only to immobilized concanavalin A or lentil lectin. Sodium dodecyl sulfate‐polyacrylamide gel electrophoresis of photolyzed proteins under reducing conditions revealed a single peak of radioactivity with a molecular mass of 53 ± 5 kDa. Taken together, these biochemical and pharmacological results support the identity of this protein peak as that of the neuronal OA3 receptor.