Evolutionary aspects of octopaminergic systems with emphasis on arthropods

We review current knowledge on octopaminergic systems in all major phyla with emphasis on arthropods. Octopaminergic systems occur in all triploblastic animals investigated. Close relationships of the octopamine-receptors in protostomes to vertebrate alpha-adrenergic receptors suggest an ancient common origin. Some evidence suggests that the octopaminergic system may be younger than the vertebrate adrenergic system. All octopaminergic systems are constructed from comparatively few neurons, and the cell populations in different representatives of a given phylum are clearly similar. Current data do not allow any conclusions on the relationships between molluscs and annelids (Lophotrochozoa) to nematodes and arthropods (Ecdysozoa). In chelicerates, including Limulus as a remaining xiphosuran, and crustaceans, octopaminergic neurons occur in pairs. All investigated winged insects (Pterygota) possess similar arrangements of octopaminergic cell populations, suggesting that their octopaminergic systems have been largely conserved during evolution. Unpaired octopaminergic neurons, with symmetrical, bilaterally projecting efferent axons in insects do not appear to have counterparts in other arthropods. Unpaired-octopaminergic neurons may thus be an autapomorphic feature of winged insects. Octopamine acts as an inhibitory neurotransmitter in gastropods, and as an excitatory transmitter controlling bioluminescence in fireflies. Octopamine is also implicated in controlling bioluminescence in other phyla. All critically examined triploblastic invertebrates release octopamine as a hormone, as a peripheral modulator and as a central neuromodulator in the nervous system, which exerts its action via evolutionary related G-protein-coupled receptors that activate cAMP. The evolution of the octopaminergic system seems fundamental for the evolution of efficient locomotory mechanisms, complex social interactions, and cognitive abilities of arthropods.