Review of octopamine in insect nervous systems

Abstract

Octopamine (OCT) belongs to a group of compounds known as biogenic amines. OCT, a monohydroxylic analog of norepinephrine, is found in both vertebrate and invertebrate nervous systems. OCT is present in relatively high concentrations in the neuronal and non- neuronal tissues of most invertebrate species studied. However, OCT occurs as a trace amine in vertebrates where its physiological significance remains uncertain. OCT acts as a neurotransmit - ter, neuromodulator, and neurohormone in insect nervous systems where it prominently influences multiple physiological events. In the peripheral nervous system, OCT modulates the activity of flight muscles, peripheral organs, and most sense organs. In the central nervous system, OCT is essential for the regulation of motivation, desensitization of sensory inputs, arousal, initiation, and maintenance of various rhythmic behaviors, hygiene behavior, and complex social behaviors, including establishment of labor, as well as learning and memory. As a neurotransmitter, OCT regulates endocrine gland activity and controls the emission of light in the firefly lantern. As a neurohormone, OCT is released into hemolymph, transported to target tissues, and induces mobilization of lipids and carbohydrates, preparing insects for a period of extended activity or assisting recovery from a period of increased energy demand. OCT modulates hemocytic nodulation in nonimmune larvae and enhances phagocytosis as a neurohormone. OCT exerts its effects by binding to specific receptors belonging to the superfamily of G protein-coupled receptors and shares the structural motif of seven transmembrane domains. Activation of octo- paminergic receptor types is coupled with different second messenger pathways depending on the species, tissue source, receptor type, and cell line used for expression of the cloned receptor. OCT-mediated generation of second messengers is associated with changes in cellular response, affecting insect behaviors. This review describes the roles of OCT in insect nervous systems at the behavioral and molecular levels.