Octopamine (OA), also known as para-octopamine and norsynephrine among synonyms, is an organic chemical closely related to norepinephrine, and synthesized biologically by a homologous pathway. Octopamine is often considered the major "fight-or-flight" neurohormone of invertebrates. Its name is derived from the fact that it was first identified in the salivary glands of the octopus. In many types of invertebrates, octopamine is an important neurotransmitter and hormone. In protostomes—arthropods, molluscs, and several types of worms—it substitutes for norepinephrine and performs functions apparently similar to those of norepinephrine in mammals, functions that have been described as mobilizing the body and nervous system for action. In mammals, octopamine is found only in trace amounts (i.e., it is a trace amine), and no biological function has been solidly established for it. It is also found naturally in numerous plants, including bitter orange. Octopamine has been sold under trade names such as Epirenor, Norden, and Norfen for use as a sympathomimetic drug, available by prescription.
Functions
Cellular effects Octopamine exerts its effects by binding to and activating receptors located on the surface of cells. These receptors have mainly been studied in insects, where they can be divided into distinct types:
OctαR (alpha-adrenergic-like), are structurally and functionally similar to noradrenergic alpha-1 receptors in mammals. There are multiple subtypes of the OctαR receptor. For example, the kissing bug (Rhodnius prolixus) has Octα1-R, Octα2R. OctβR (beta-adrenergic-like), are structurally and functionally similar to noradrenergic beta receptors in mammals. There are multiple subtypes of the OctβR receptor. For example, the fruit fly (Drosophila melanogaster) has DmOctβ1R, DmOctβ2R, and DmOctβ3R. OAMB. The diversity of this receptor is relatively unknown. The fruit fly (Drosophila melanogaster) has two distinct isoforms which are functionally distinct: OambK3 and OambAS. TyrR (mixed octopamine/tyramine receptors), which are structurally and functionally similar to noradrenergic alpha-2 receptors in mammals. Receptors in the TyrR class, however, are generally more strongly activated by tyramine than by octopamine. Phylogenetic studies claim that in ancient bilaterians such as Platynereis dumerilii there is a co-existence of norepinephrine, tyramine and octopamine receptor signaling. However, due to partial overlapping in their signalling functionality tyramine and octopamine receptors have been lost in vertebrates. In vertebrates no octopamine-specific receptors have been identified. Octopamine binds weakly to receptors for norepinephrine and epinephrine, but it is not clear whether this has any functional significance. It binds more strongly to trace amine-associated receptors (TAARs), especially TAAR1.
Invertebrates Octopamine was first discovered by Italian scientist Vittorio Erspamer in 1948 in the salivary glands of the octopus and has since been found to act as a neurotransmitter, neurohormone and neuromodulator in invertebrates. Although Erspamer discovered its natural occurrence and named it, octopamine had actually existed for many years as a pharmaceutical product. It is widely used in energy-demanding behaviors by all insects, crustaceans (crabs, lobsters, crayfish), and spiders. Such behaviors include modulating muscle tension, flying, ovulation and egg-laying, and jumping.
In non-insect invertebrates In lobsters, octopamine seems to direct and coordinate neurohormones to some extent in the central nervous system, and it was observed that injecting octopamine into a lobster and crayfish resulted in limb and abdomen extension. In the nematode, octopamine is found in high concentrations in adults, decreasing egg-laying and pharyngeal pumping behaviors with an antagonistic effect to serotonin. Octopaminergic nerves in the mollusc may be present in the heart, with high concentrations in the nervous system.
In non-Drosophila insects In insects, octopamine is released by a select number of neurons, but acts broadly throughout the central brain, on all sense organs, and on several non-neuronal tissues. In the thoracic ganglia, octopamine is primarily released by DUM (dorsal unpaired median) and VUM (ventral unpaired median) neurons, which release octopamine onto neural, muscular, and peripheral targets. These neurons are important for mediating energy-demanding motor behaviors, such as escape-induced jumping and flight. For example, the locust DUMeti neuron releases octopamine onto the extensor tibia muscle to increase muscle tension and increase relaxation rate. These actions promote efficient leg muscle contraction for jumping. During flight, DUM neurons are also active and release octopamine throughout the body to synchronize energy metabolism, respiration, muscle activity and flight interneuron activity. Octopamine in locusts is four times more concentrated in the axon than in the soma, and decreases the locust's myogenic rhythm. In the honey bee, octopamine has a major role in learning and memory. In the firefly, octopamine release leads to light production in the lantern. In larvae of the oriental armyworm, octopamine is immunologically beneficial, increasing survival rates in high-density populations. The emerald cockroach wasp stings the host for its larvae (a cockroach) in the head ganglion (brain). The venom blocks octopamine receptors and the cockroach fails to show normal escape responses, grooming itself excessively. It becomes docile and the wasp leads it to the wasp's den by pulling its antenna like a leash.
In Drosophila Octopamine affects almost every process of the fruit fly and is widely present in both the adult and larval fly. A non-exhaustive list of some of the areas in which Octopamine modulates:
Learning and memory Ovulation and Egg-Laying Locomotion Muscle Physiology Aggression Alcohol and drug tolerance Feeding Microbiome and gut physiology Sleep Modulating effects of exercise Metabolism
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