[Behavioral functions of serotonin and octopamine: some paradoxes of comparative physiology]

Evaluation of recent data on monoaminergic control of invertebrate behavior suggests that behavioral functions of serotonin in molluscs and octopamine in insects are remarkably similar. Specifically, in the respective taxa, these monoamines are responsible for activation of food searching and intense locomotion, increase in food consumption and general activity, enhancement of cardial and respiratory rhythms, facilitation of learning, sensitization of sensory circuits. At the same time, in insects, behavioral effects of serotonin are opposite to those of octopamine. It seems thus that two monoamines have exchanged their behavioral roles in the two major invertebrate taxa. Possible reasons of this paradoxical inversion touches inevitably upon basic questions of signal molecular evolution.