Honey Bee Dopamine and Octopamine Receptors Linked to Intracellular Calcium Signaling Have a Close Phylogenetic and Pharmacological Relationship

Background Three dopamine receptor genes have been identified that are highly conserved among arthropod species. One of these genes, referred to in honey bees as Amdop2 , shows a close phylogenetic relationship to the a-adrenergic-like octopamine receptor family. In this study we examined in parallel the functional and pharmacological properties of Am DOP2 and the honey bee octopamine receptor, Am OA1. For comparison, pharmacological properties of the honey bee dopamine receptors Am DOP1 and Am DOP3, and the tyramine receptor Am TYR1, were also examined. Methodology/Principal Findings Using HEK293 cells heterologously expressing honey bee biogenic amine receptors, we found that activation of Am DOP2 receptors, like Am OA1 receptors, initiates a rapid increase in intracellular calcium levels. We found no evidence of calcium signaling via Am DOP1, Am DOP3 or Am TYR1 receptors. Am DOP2- and Am OA1-mediated increases in intracellular calcium were inhibited by 10 µM edelfosine indicating a requirement for phospholipase C-β activity in this signaling pathway. Edelfosine treatment had no effect on Am DOP2- or Am OA1-mediated increases in intracellular cAMP. The synthetic compounds mianserin and epinastine, like cis -(Z)-flupentixol and spiperone, were found to have significant antagonist activity on Am DOP2 receptors. All 4 compounds were effective antagonists also on Am OA1 receptors. Analysis of putative ligand binding sites offers a possible explanation for why epinastine acts as an antagonist at Am DOP2 receptors, but fails to block responses mediated via Am DOP1. Conclusions/Significance Our results indicate that Am DOP2, like Am OA1, is coupled not only to cAMP, but also to calcium-signalling and moreover, that the two signalling pathways are independent upstream of phospholipase C-β activity. The striking similarity between the pharmacological properties of these 2 receptors suggests an underlying conservation of structural properties related to receptor function. Taken together, these results strongly support phylogenetic analyses indicating that the Am DOP2 and Am OA1 receptor genes are immediate paralogs.