Comparative Exploration of the Structure-Activity Space of Cloned α-Like Octopamine Receptors from a Marine and a Terrestrial Arthropod

Structure-dependent receptor subtype selectivity and G protein subtype preference of heterocyclic agonists in heterologously expressed silkworm octopamine receptors

(R)-Octopamine (OA), a major invertebrate biogenic amine, plays an important role in a wide variety of physiological processes as a neurohormone, neuromodulator, and neurotransmitter in insects. OA receptors (OARs) are class A G protein-coupled receptors that specifically bind OA to activate downstream signaling cascades by coupling to G proteins and presumably other regulatory proteins. These receptors are broadly classified as α- and β-adrenergic-like OARs (α- and β-ALOARs). OARs are considered important targets of insecticides and acaricides. In the present study, we examined the actions of an array of 13 heterocyclic OAR agonists with the moieties that correspond to the phenyl group and the basic nitrogen atom of OA on α- and β-ALOARs from the silkworm (Bombyx mori) and the signaling pathways activated through these actions. The results indicated that these compounds display structure-dependent receptor subtype selectivity and G protein subtype preference, underscoring the need to determine which subtype and signaling pathway mediates toxicologically relevant effects for the efficient discovery of novel pest control chemicals. The results of insecticidal assays using B. mori larvae suggested that the activation of signal transduction pathways via α-ALOARs might be mainly responsible for the toxicological effects of the heterocycles.

Molecular and pharmacological characterization of a β-adrenergic-like octopamine receptor from the green rice leafhopper Nephotettix cincticeps

As the counterparts of noradrenaline and adrenaline in vertebrates, octopamine (OA) regulates multiple physiological and behavioral processes in invertebrate. OA mediates its effects via binding to specific octopamine receptors (OARs). Functional and pharmacological characterization of OARs have been reported in several insects. However, little work was documented in hemipteran insects. We cloned a β-adrenergic-like OAR (NcOA2B2) from Nephotettix cincticeps. NcOA2B2 shares high similarity with members of the OA2B2 receptor class. Transcript level of NcOA2B2 varied in various tissues and was highly expressed in the leg. After heterologous expression in CHO-K1 cells, NcOA2B2 was dose-dependently activated by OA (EC₅₀ = 2.56 nM) and tyramine (TA) (EC₅₀ = 149 nM). Besides putative octopaminergic agonists, dopaminergic agonists and amitraz and DPMF potently activated NcOA2B2 in a dose-dependent manner. Receptor activity was blocked by potential antagonists and was most efficiently antagonized by asenapine. Phentolamine showed both antagonist and agonist effects on NcOA2B2. Our results offer the important information about molecular and pharmacological characterization of an OAR from N. cincticeps that will provide the basis for forthcoming studies on its roles in physiological processes and behaviors, and facilitate the design of novel insecticides for pest control.