AmOctα2R: Functional Characterization of a Honeybee Octopamine Receptor Inhibiting Adenylyl Cyclase Activity

The catecholamines norepinephrine and epinephrine are important regulators of vertebrate physiology. Insects such as honeybees do not synthesize these neuroactive substances. Instead, they use the phenolamines tyramine and octopamine for similar physiological functions. These biogenic amines activate specific members of the large protein family of G protein-coupled receptors (GPCRs). Based on molecular and pharmacological data, insect octopamine receptors were classified as either alpha- or beta-adrenergic-like octopamine receptors. Currently, one alpha- and four beta-receptors have been molecularly and pharmacologically characterized in the honeybee. Recently, an alpha(2)-adrenergic-like octopamine receptor was identified in Drosophila melanogaster (DmOct alpha 2R). This receptor is activated by octopamine and other biogenic amines and causes a decrease in intracellular cAMP ([cAMP](i)). Here, we show that the orthologous receptor of the honeybee (AmOct alpha 2R), phylogenetically groups in a clade closely related to human alpha(2)-adrenergic receptors. When heterologously expressed in an eukaryotic cell line, AmOct alpha 2R causes a decrease in [cAMP](i). The receptor displays a pronounced preference for octopamine over tyramine. In contrast to DmOct alpha 2R, the honeybee receptor is not activated by serotonin. Its activity can be blocked efficiently by 5-carboxamidotryptamine and phentolamine. The functional characterization of AmOct alpha 2R now adds a sixth member to this subfamily of monoaminergic receptors in the honeybee and is an important step towards understanding the actions of octopamine in honeybee behavior and physiology.