Molecular Cloning and Pharmacological Characterization of a Molluscan Octopamine Receptor

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Abstract
We describe the cloning and functional expression of a cDNA encoding a novel G protein-coupled receptor, which was isolated from the central nervous system of the pond snail Lymnaea stagnalis. The amino acid sequence predicted by this cDNA shows highest similarity with the sequence of the Locusta tyramine receptor, theDrosophila tyramine/octopamine receptor, and the mammalian α-adrenergic receptors. On expression in mammalian cells, [3H]rauwolscine, an α2-adrenergic receptor antagonist, binds with high affinity (KD = 2.9 × 10−9m) to the receptor. Of several tested neurotransmitters, octopamine (which is considered to be the invertebrate counterpart of norepinephrine) showed the highest affinity (1.9 × 10−6m) for the receptor. Therefore, we consider this receptor to be the first true octopamine receptor to be cloned. The ligand binding properties of the novel receptor, designated Lym oa1, seem to be distinct from any of the binding profiles described for octopamine receptors in tissue preparations. Although the pharmacological profile of Lym oa1 shows some similarity with that of Tyr/Oct-Dro and Tyr-Loc, there are also clear differences. In particular, phentolamine, chlorpromazine, and mianserine display markedly higher affinities for Lym oa1 than for the insect receptors. As far as the vertebrate adrenergic receptors are concerned, the ligand binding properties of Lym oa1 resemble α2-adrenergic receptors more than they do α1- or β-adrenergic receptors. Octopaminergic stimulation of Lym oa1 induces an increase in both inositol phosphates and cAMP (EC50 = 9.1 × 10−7m and 5.1 × 10−6m, respectively). This is in contrast to the signal transduction pathways described for the related tyramine- and α2-adrenergic receptors, which couple in an inhibitory way to adenylyl cyclase.