06. Noradrenaline and Octopamine: The Alarm Twins

At the Maudsley Hospital in south London, in the early 1960s, patients taking a new class of antidepressant kept returning with the same account: a headache arriving within an hour of a meal, felt at the back of the skull as something being forced open, and behind it a blood pressure high enough to rupture a vessel in the brain. Blackwell collected twelve such cases — eleven of the patients on tranylcypromine, one on phenelzine, all of them monoamine-oxidase inhibitors, drugs that work by disabling the enzyme that strips small amine messengers out of circulation. His report reached The Lancet in October 1963 under a title that named the drug class but not the trigger. The trigger was cheese.

The chemistry underneath is exact and slightly absurd. Ripening bacteria decarboxylate the amino acid tyrosine — snip a carbon dioxide off one end — leaving tyramine, a small amine otherwise of almost no consequence to a human being. In an unmedicated person it barely exists as a signal: monoamine oxidase in the gut wall and the liver destroys nearly all of it before it reaches the bloodstream. Block that enzyme and the tyramine arrives intact, is carried into sympathetic nerve endings by the very transporter that normally recycles noradrenaline, and displaces the stored noradrenaline there in one mass release. The result is every effect of a maximal fright — vessels clamped, pressure spiking, heart driven hard — delivered at once and for no reason at all.

What makes this more than a pharmacological footnote is which molecule did it. In a honeybee or a fruit fly, tyramine is a signalling molecule in its own right, with dedicated receptors and behavioural jobs; in a human it is a trace amine — a category defined by scarcity, cleared so fast it was long treated as metabolic litter. The cheese reaction is the one everyday place where an invertebrate's messenger turns acutely, dangerously human.

That crossover is not a coincidence, and the reason is that the two branches were never separate inventions. One assembly line runs from a common amino acid: tyrosine, hydroxylated by tyrosine hydroxylase into L-DOPA, decarboxylated by DOPA decarboxylase into dopamine — the two steps the previous chapter follows to their own conclusions. Add one more enzyme, dopamine β-hydroxylase, and dopamine becomes noradrenaline. Add one after that, phenylethanolamine N-methyltransferase, and noradrenaline becomes adrenaline. Each addition is a separate commit with its own date, and the dates are not close together.

The parallel branch does the same two operations in the opposite order. Decarboxylate tyrosine first and you get tyramine — the cheese molecule. Hydroxylate that, and you get octopamine, the amine that runs much of an insect's arousal and energy mobilisation. Vol. I's chapter on rank showed what octopamine does to a lobster's posture with nothing but a syringe; what it did not do was date the machinery. The order of operations is the only real difference between the branches, and for most of the twentieth century that difference was taken to mean two enzyme families, one vertebrate and one invertebrate, descended from an ancient split.

That reading is now wrong, and the correction is unusually clean. In 2023, Matthew Goulty, Gaelle Botton-Amiot, Ezio Rosato, Simon Sprecher and Roberto Feuda published a formal phylogenetic reconciliation of the monoaminergic gene families in Nature Communications, and found that dopamine β-hydroxylase and tyramine β-hydroxylase are not two clades at all. The fruit fly's TBH gene is a one-to-one ortholog of the human DBH gene — the same gene, inherited from the same ancestral copy, at strong branch support. In the authors' own summary, the name distinction is purely semantic. One enzyme lineage, continuously present since close to the split between cnidarians and bilaterians, performs whichever final hydroxylation the local chemistry offers it: dopamine to noradrenaline where dopamine is the abundant substrate, tyramine to octopamine where the decarboxylation ran first. The same study places the lineage's sister paralog, MOXD — still functionally uncharacterised in any species — at the last common ancestor of Cnidaria and Bilateria, which puts the family's root deeper than crown Bilateria even though the surviving DBH and TBH genes themselves are almost entirely bilaterian.

So the naming was the accident. Two labels, coined in two research communities that rarely read each other, made the invertebrate version sound like a lesser, earlier draft of the vertebrate one. It was never a draft. It was the same gene, handed a different substrate.

The receptors run on a separate clock from the enzymes, and they are where the twins story actually lives. Vol. I established, through Gáspár Jékely's work, that noradrenaline-type signalling coexisted with octopamine and tyramine in the last bilaterian common ancestor rather than being replaced by them. Unpacked past that summary, the 2017 reconstruction by Thomas Bauknecht and Jékely in BMC Biology resolves not one shared inheritance but a whole panel of them — adrenergic α1, α2 and β; octopamine α and β; tyramine type-1 and type-2 — every family traceable to the same bilaterian node, and every family carrying its own separate history of loss. Adrenergic receptors are gone from most ecdysozoans, including most insects, yet kept in the water flea Daphnia pulex and in a priapulid worm; octopamine and tyramine receptors are gone from most deuterostomes, yet kept in the hemichordate Saccoglossus kowalevskii; the adrenergic-β family turns up only in chordates and one xenacoelomorph, the marine worm Meara stichopi. There is no relay anywhere in the tree — no baton pass from an old system to a new one, only lineages independently dropping whichever half of an ancestral pair they had stopped using.

Dating that node means leaning on the anchor Vol. I already handled honestly: the protostome–deuterostome split, conventionally placed beyond 600 million years ago, given as 642–761 Mya by molecular clock and roughly 558–575 Mya when calibrated to fossils. Rocks and clocks disagree, and this book cites the disagreement rather than a single number.

Why two systems survived six hundred million years apiece, rather than one absorbing the other, is where the book's three standing hypotheses — frozen accident, active selection, chemical constraint — pull against each other. Chemical constraint has the strongest showing, and its clearest statement is old: R. A. Jensen's 1976 argument in the Annual Review of Microbiology that new functions arise mostly by recruiting existing enzymes to nearby substrates rather than by inventing machinery. But selection has real evidence on the octopamine side. Herman Lehman, David Schulz, Andrew Barron, Gene Robinson and colleagues showed in 2006 that honeybee foraging onset tracks brain octopamine and TBH transcript levels as workers shift from hive duties to flight; Paul Stevenson, Varya Dyakonova, Jan Rillich and Klaus Schildberger showed in 2005 that cricket aggression escalates under octopamine agonists and collapses under antagonists, with serotonin pulling the opposite way — the same opposed-amine architecture Vol. I found in the lobster, in an animal built on a different nervous-system plan. An insect with no discrete endocrine gland needs one fast, cheap, whole-body mobilisation signal; vertebrates, once glands were available, ran the same job down two channels — a fast neural one through the sympathetic chain, a slow hormonal one through chromaffin tissue. Octopamine was not retained by inertia.

The younger twin

Adrenaline is the newcomer, and the gap is larger than most readers expect. PNMT — the enzyme that methylates noradrenaline into adrenaline — is vertebrate-only, and the 2023 reconciliation dates its origin, through a duplication it shares with two other vertebrate-exclusive methyltransferases, to the ancestor of jawed vertebrates specifically rather than vertebrates broadly. Crown Gnathostomata runs to roughly 450 million years by molecular clock, with a hard fossil floor supplied in 2022 when You-an Zhu, Per Ahlberg, Min Zhu and colleagues described complete jawed fishes from the early Silurian of China in Nature, at about 439 million years. Lampreys and hagfish, which left the jawed lineage around 560 Mya, carry no PNMT ortholog in this analysis. What they do carry is the gland-side hardware: Jacques Paiement and Donald McMillan described extracardiac chromaffin cells in larval lampreys in 1975 — neural-crest-derived, noradrenaline-making, structurally the tissue a mammalian adrenal medulla is built from. The organ was there without the last enzyme. If the gnathostome-only result holds, true adrenaline is about a hundred million years younger than the noradrenaline it is made from.

The wiring is older still, and that is the part a fifty-year textbook line got wrong. Following Botar in 1974 and Burnstock in 1969, the sympathetic chain — the paired ganglia running alongside the spine that carry the fight-or-flight command — was held to be an innovation of jawed vertebrates, absent from cyclostomes. In 2024, Brittany Edens, Jan Stundl, Hugo Urrutia and Marianne Bronner reported in Nature that sea lamprey embryos do build trunk sympathetic neurons: sympathoblasts near the dorsal aorta, traced to trunk neural crest by dye lineage labelling, expressing both tyrosine hydroxylase and DBH, and running a transcriptional program homologous to the gnathostome one. A rudimentary sympathetic nervous system therefore dates to the base of Vertebrata, around 560 million years ago — more than a hundred million years before the molecule that now amplifies it existed. The alarm circuit was built first, and adrenaline was added to a system already running.

Which leaves the question the cheese opened. If humans keep no octopamine system, why does tyramine do anything to us at all? Because vertebrates built their own receptors for these molecules, separately. Seong-il Eyun, Hideaki Moriyama, Federico Hoffmann and Etsuko Moriyama surveyed 493 trace amine-associated receptor genes across 26 vertebrate genomes in 2016 and found the family absent from every non-vertebrate genome sampled, with the earliest recognisable member appearing in lamprey — a family roughly as old as vertebrates themselves. TAAR1, its one confirmed non-olfactory member, binds tyramine and octopamine directly. It is tempting to call it the octopamine receptor humans kept, and it is not one: as Lothar Lindemann and Marius Hoener set out in 2005, the vertebrate TAARs bear no phylogenetic relation to invertebrate octopamine and tyramine receptors. Two unrelated receptor families converged on the same small ligands. The cheese finds a lock in us that our ancestors cut independently.

Two of these dates are solid and one is fragile. The bilaterian node for the receptor families rests on a reconstruction nobody has contradicted in nine years, and the base-of-Vertebrata date for the sympathetic chain now rests on direct embryological evidence rather than on an absence of it. The gnathostome-only date for adrenaline rests on one gene family in one analysis, and a jawless-vertebrate genome carrying a PNMT-like sequence would collapse the gap tomorrow. What survives either way is the shape of the thing: an assembly line extended one enzyme at a time across six hundred million years, two branches that never competed and never merged, and alarm wiring laid down long before the molecule that now screams down it. A dose of that molecule sits in every auto-injector carried by someone whose immune system overreacts to shellfish — and the messenger doing the overreacting is another old amine holding down two unrelated jobs, met next in allergy season.

Commit Ledger

Overturned if: a PNMT ortholog, or a convergent adrenaline-synthesising route, is confirmed in a lamprey or hagfish — erasing the roughly hundred-million-year gap this chapter draws between the noradrenaline commit and the adrenaline one.

Sources

• 1963 · Blackwell, "Hypertensive Crisis Due to Monoamine-Oxidase Inhibitors" — The Lancet 282:849–85192743-0)
• 1975 · Paiement & McMillan, "The extracardiac chromaffin cells of larval lampreys" — General and Comparative Endocrinology 27:495–50890070-2)
• 1976 · Jensen, "Enzyme Recruitment in Evolution of New Function" — Annual Review of Microbiology 30:409–425
• 2005 · Lindemann & Hoener, "A renaissance in trace amines inspired by a novel GPCR family" — Trends in Pharmacological Sciences 26:274–281
• 2005 · Stevenson, Dyakonova, Rillich & Schildberger, "Octopamine and Experience-Dependent Modulation of Aggression in Crickets" — The Journal of Neuroscience 25:1431–1441
• 2006 · Lehman, Schulz, Barron, Wraight, Hardison, Whitney, Takeuchi, Paul & Robinson, "Division of labor in the honey bee (Apis mellifera): the role of tyramine β-hydroxylase" — Journal of Experimental Biology 209:2774–2784
• 2016 · Eyun, Moriyama, Hoffmann & Moriyama, "Molecular Evolution and Functional Divergence of Trace Amine–Associated Receptors" — PLOS ONE 11:e0151023
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• 2023 · Goulty, Botton-Amiot, Rosato, Sprecher & Feuda, "The monoaminergic system is a bilaterian innovation" — Nature Communications 14:3284
• 2024 · Edens, Stundl, Urrutia & Bronner, "Neural crest origin of sympathetic neurons at the dawn of vertebrates" — Nature 629:121–126
• 2024 · Kelberman, Rodberg, Munn et al., "Diverse ancestral brainstem noradrenergic neuron activity across species and biological factors" — bioRxiv preprint, not yet peer-reviewed