Octopamine is an organic chemical closely related to norepinephrine, and synthesized biologically by a homologous pathway. Its name derives 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 norephinephrine 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, 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 three types: alpha-adrenergic-like, which are structurally and functionally similar to noradrenergic alpha-1 receptors in mammals; beta-adrenergic-like, which are structurally and functionally similar to noradrenergic beta receptors in mammals; and 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. 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, 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, 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 decreases aggression in Drosophila without influencing other behaviors. 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 and VUM 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. In the honey bee and fruit fly, octopamine has a major role in learning and memory. In the firefly, octopamine release leads to light production in the lantern. 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. Heberlein et al. have conducted studies of alcohol tolerance in fruit flies; they found that a mutation that caused octopamine deficiency also caused lower alcohol tolerance. The emerald cockroach wasp stings the host for its larvae in the head ganglion. 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.
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. Octopamine was found to be 93% eluted by urine within 24 hours of being produced in the body as a byproduct of Iproniazid in rabbits.
Pharmacology
Octopamine has been sold under trade names such as Epirenor, Norden, and Norfen for use in medicine as a sympathomimetic drug, available by prescription. Very little information exists concerning its clinical usefulness or safety, though. In mammals, octopamine may mobilize the release of fat from adipocytes, 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. The lists octopamine as a banned substance for in competition use, as a "specified stimulant" on the 2019 Prohibited List.
Octopamine binds to its respective G-protein coupled receptors to initiate a cell signal transduction pathway. At least three groups of octopamine GPCR have been defined. OctαR are more closely related to α-adrenergic receptors, while OctβR are more closely related to β-adrenergic receptors. The Octopamine/Tyramine receptors can bind both ligands, and display agonist-specific coupling. Oct-TyrR is listed in both OCTOPAMINE and TYRAMINE RECEPTORS gene groups.
