Octopamine (β,4-dihydroxyphenethylamine) is an endogenous biogenic amine that is closely related to norepinephrine, and has effects on the adrenergic and dopaminergic systems. It is also found naturally in numerous plants, including bitter orange. Biosynthesis of the D-(–)-enantiomer of octopamine is by β-hydroxylation of tyramine via the enzyme dopamine β-hydroxylase. Under the trade names Epirenor, Norden, and Norfen, octopamine is also used clinically as a sympathomimetic agent.
Role in 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 flying, egg-laying, and jumping.
Octopamine acts as the insect equivalent of norepinephrine and has been implicated in regulating aggression in invertebrates, with different effects on different species. Studies have shown that reducing the neurotransmitter octopamine and preventing coding of tyramine beta hydroxylase (an enzyme that converts tyramine to octopamine) decreases aggression in Drosophila without influencing other behaviors.
The best-understood role for octopamine is in the locust jump. Here it modulates muscle activity, making the leg muscles contract more effectively. This is at least in part due to an increase in the rate of contraction and of relaxation.
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.
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.
Role in vertebrates
In vertebrates, octopamine replaces norepinephrine in sympathetic neurons with chronic use of monoamine oxidase inhibitors. It may be responsible for the common side effect of orthostatic hypotension with these agents, though there is also evidence that it is actually mediated by increased levels of N-acetylserotonin.
One study noted that octopamine might be an important amine that influences the therapeutic effects of inhibitors such as monoamine oxidase inhibitors, especially because a large increase in octopamine levels was observed when animals were treated with this inhibitor. Octopamine was positively identified in the urine samples of mammals such as humans, rats, and rabbits treated with monoamine oxidase inhibitors. Very small amounts of octopamine were also found in certain animal tissues. It was observed that within a rabbit's body, the heart and kidney held the highest concentrations of octopamine.
In mammals, octopamine may mobilize the release of fat from adipocytes (fat cells), which has led to its promotion on the internet as a slimming aid. However, the released fat is likely to be promptly taken up into other cells, and there is no evidence that octopamine facilitates weight loss. Octopamine may also increase blood pressure significantly when combined with other stimulants, as in some weight loss supplements.
Owing to lack of research, much is not known about octopamine or its role in humans.
- P.D. Evans, "Octopamine", in Comprehensive Insect Physiology, 11, 499, Oxford University Press 1985.