Functional and pharmacological characterization of a β-adrenergic-like octopamine receptor from the silkworm Bombyx mori

A cDNA encoding a seven-transmembrane receptor was cloned from the nervous tissues of silkworm (Bombyx mori) larvae. Sequence analysis indicated that the gene is an ortholog of CG6989, which encodes a Drosophila β-adrenergic-like octopamine (OA) receptor (DmOctβ2R). As very little information is available regarding this class of receptors, we generated a cell line that stably expressed the gene in HEK-293 cells and we then performed functional and pharmacological studies of this receptor. [3H]OA-binding assays using membrane preparations of this cell line showed that the receptor possesses a higher affinity for OA than for tyramine (TA) or dopamine (DA). The cell line elicited a bell-shaped, OA concentration-dependent increase in intracellular cAMP levels, with a maximum at 100 nM. (R)-OA was more potent than (S)-OA. TA and DA had weak or marginal effects on cAMP production. The OA receptor agonist demethylchlordimeform elicited a similar biphasic response, although the maximum response was attained at a concentration as low as 1 nM. The rank order of potency of other agonists was as follows: naphazoline > tolazoline, clonidine. Among the antagonists tested, only chlorpromazine significantly attenuated the OA-induced increase in cAMP levels. No increase in intracellular Ca2+ levels was observed with OA at concentrations up to 100 μM. These findings indicate that the cloned receptor is a β-adrenergic-like OA receptor with unique functional and pharmacological properties.