MOLECULAR-CLONING AND PHARMACOLOGICAL CHARACTERIZATION OF A MOLLUSCANOCTOPAMINE RECEPTOR

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 se quence predicted by this cDNA shows highest similarity with the sequen ce of the Locusta tyramine receptor, the Drosophila tyramine/octopamin e receptor, and the mammalian alpha-adrenergic receptors. On expressio n in mammalian cells, [H-3]rauwolscine, an alpha(2)-adrenergic recepto r antagonist, binds with high affinity (K-D = 2.9 x 10(-9) M) to the r eceptor. Of several tested neurotransmitters, octopamine (which is con sidered to be the invertebrate counterpart of norepinephrine) showed t he highest affinity (1.9 x 10(-6) M) for the receptor. Therefore, we c onsider this receptor to be the first true octopamine receptor to be c loned. The ligand binding properties of the novel receptor, designated Lym oa(1), seem to be distinct from any of the binding profiles descr ibed for octopamine receptors in tissue preparations. Although the pha rmacological profile of Lym oa(1) shows some similarity with that of T yr/Oct-Dro and Tyr-Loc, there are also clear differences. In particula r, phentolamine, chlorpromazine, and mianserine display markedly highe r affinities for Lym oa(1) than for the insect receptors. As far as th e vertebrate adrenergic receptors are concerned, the ligand binding pr operties of Lym oa(1) resemble alpha(2)-adrenergic receptors more than they do alpha(1)- or beta-adrenergic receptors. Octopaminergic stimul ation of Lym oa(1) induces an increase in both inositol phosphates and cAMP (EC(50) = 9.1 x 10(-7) M and 5.1 x 10(-6) M, respectively). This is in contrast to the signal transduction pathways described for the related tyramine- and alpha(2)-adrenergic receptors, which couple in a n inhibitory way to adenylyl cyclase.