Evolutionary aspects of octopaminergic systems with emphasis on arthropods

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.