The octopamine and tyramine, as the invertebrate counterparts of the vertebrate adrenergic transmitters, control and modulate many physiological and behavioral processes. Both molecules mediate their effects by binding to specific receptors belonging to the superfamily of G-protein-coupled receptors. So far, four families of octopamine and tyramine receptors have been reported. Here, we described the functional characterization of one putative octopamine/tyramine receptor gene from the rice stem borer, Chilo suppressalis. By a mechanism of alternative splicing, this receptor gene (CsOA3) encodes two molecularly distinct transcripts, CsOA3S and CsOA3L. CsOA3L differs from CsOA3S on account of the presence of an additional 30 amino acids within the third intracellular loop. When heterologously expressed, both receptors cause increases of intracellular Ca 2+ concentration. The short form, CsOA3S, was activated by both octopamine and tyramine, resulting in decreased intracellular cAMP levels ([cAMP] i ) in a dose-dependent manner, whereas dopamine and serotonin are not effective. However, CsOA3L did not show any impact on [cAMP] i . Studies with series of agonists and antagonists confirmed that CsOA3 has a different pharmacological profile from that of other octopamine receptor families. The CsOA3 is, to our knowledge, a novel family of insect octopamine receptors.

Serotonin (5-HT) modulates both neural and immune responses in vertebrates, but its role in insect immunity remains uncertain. We report that hemocytes in the caterpillar, Pieris rapae are able to synthesize 5-HT following activation by lipopolysaccharide. The inhibition of a serotonin-generating enzyme with either pharmacological blockade or RNAi knock-down impaired hemocyte phagocytosis. Biochemical and functional experiments showed that naive hemocytes primarily express 5-HT1B and 5-HT2B receptors. The blockade of 5-HT1B significantly reduced phagocytic ability; however, the blockade of 5-HT2B increased hemocyte phagocytosis. The 5-HT1B-null Drosophila melanogaster mutants showed higher mortality than controls when infected with bacteria, due to their decreased phagocytotic ability. Flies expressing 5-HT1B or 5-HT2B RNAi in hemocytes also showed similar sensitivity to infection. Combined, these data demonstrate that 5-HT mediates hemocyte phagocytosis through 5-HT1B and 5-HT2B receptors and serotonergic signaling performs critical modulatory functions in immune systems of animals separated by 500 million years of evolution.DOI: http://dx.doi.org/10.7554/eLife.12241.001

Most currently used insecticides are neurotoxic chemicals that target a limited number of sites and insect cholinergic neurotransmission is the major target. A potential target for insecticide development is the muscarinic acetylcholine receptor (mAChR), which is a metabotropic G-protein-coupled receptor. Insects have A- and B-type mAChRs and the five mammalian mAChRs are close to the A-type. We isolated a cDNA (CG12796) from the fruit fly, Drosophila melanogaster. After heterologous expression in Chinese hamster ovary K1 cells, CG12796 could be activated by acetylcholine [EC50 (half maximal effective concentration), 73 nM] and the mAChR agonist oxotremorine M (EC50 , 48.2 nM) to increase intracellular Ca(2+) levels. Thus, the new mAChR is coupled to Gq/11 but not Gs and Gi/o . The classical mAChR antagonists atropine and scopolamine N-butylbromide at 100 μM completely blocked the acetylcholine-induced responses. The orthologues of CG12796 can also be found in the genomes of other insects, but not in the genomes of the honeybee or parasitoid wasps. Knockdown of CG12796 in the central nervous system had no effect on male courtship behaviours. We suggest that CG12796 represents the first recognized member of a novel mAChR class.

The biogenic amine serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter in vertebrates and invertebrates. It acts in regulation and modulation of many physiological and behavioral processes through G-protein-coupled receptors. Five 5-HT receptor subtypes have been reported in Drosophila that share high similarity with mammalian 5-HT 1A , 5-HT 1B , 5-HT 2A , 5-HT 2B , and 5-HT 7 receptors. We isolated a cDNA (Pr5-HT 8 ) from larval Pieris rapae, which shares relatively low similarity to the known 5-HT receptor classes. After heterologous expression in HEK293 cells, Pr5-HT 8 mediated increased [Ca 2+ ] i in response to low concentrations (< 10 nM) of 5-HT. The receptor did not affect [cAMP] i even at high concentrations (> 10 lM) of 5-HT. Dopamine, octopamine, and tyramine did not influence receptor signaling. Pr5-HT 8 was also activated by various 5-HT receptor agonists including 5-methoxytryptamine, (AE)-8-Hydroxy-2-(dipropylamino) tetralin, and 5-carboxamidotryptamine. Methiothepin, a non-selective 5-HT receptor antagonist, activated Pr5-HT 8 . WAY 10635, a 5-HT 1A antagonist, but not SB-269970, SB-216641, or RS-127445, inhibited 5-HT-induced [Ca 2+ ] i increases. We infer that Pr5-HT 8 represents the first recognized member of a novel 5-HT receptor class with a unique pharmacological profile. We found orthologs of Pr5-HT 8 in some insect pests and vectors such as beetles and mosquitoes, but not in the genomes of honeybee or parasitoid wasps. This is likely to be an invertebrate-specific receptor because there were no similar receptors in mammals.

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