Small molecules can be great for sending biochemical signals between cells, or between different parts of a cell, because they can diffuse rapidly in water – notably, the water in your body. Big, multicellular organisms rely on hundreds of such molecules to get on with the business of living.

To be effective as a signaller, a molecule must:

• Be easy to produce (or convert)
• Be small enough to diffuse readily
• Have a unique shape

Furthermore, as one can’t control where such a molecule goes, it should not be of any other use in the body. It would be a disaster, for example, if you were to try out a molecule for sending a specific signal, and it turned out to also be very much at home producing energy.

Serotonin and tryptophan

Serotonin is a good example of an effective signalling molecule. A close chemical relative of the amino acid tryptophan, it is responsible for our overall happiness and mood. Serotonin is certainly not specific to humans – it is a very ancient signalling molecule, used by even the most basic of animals.

Amino acids, like tryptophan, are the building blocks of proteins. Each one is like a tiny Lego brick in one of those complicated Lego kits, with its own, unique ‘side chain’. Many amino acids are manufactured in the body, but some cannot be made by animals (humans can’t make nine of them). Across all animals, a subset of five or so must come from a plant (or bacterium).

Tryptophan, one of only a handful of essential amino acids, is the most chemically distinctive of the lot, with a chunky, two-ring side chain. It can’t be made internally, yet all animals must ingest it to survive. That could explain why serotonin – a derivative of tryptophan – indicates the presence of food in the body.

‘Good food’ and happiness

When food enters the gut of every animal – from the simplest, not-much-more-than-gut-and-tentacles miniature animal to the mighty blue whale – serotonin (made from tryptophan, which can’t be made in the body) is released, signalling ‘good food’ and activating digestion.

But giving the go-ahead for digestion is just one of serotonin’s happy signals – there are many. For example, to no one’s surprise, its signals branched out from ‘food ok’ in all creatures to ‘happiness’ in more sophisticated animals, which have more concentrated neural circuits in the brain for performing more complex actions.

Many neurons have serotonin receptors, and parts of the mammalian brain are assigned to pump out serotonin, which transmits signals between neurons. Without the direct link to digestion, serotonin is now a ‘pure’ signalling molecule in the brain, released by some neurons, taken up by others, and recaptured to return to baseline for future use.

That point of recapture is the site of action of some antidepressant drugs – though no one is entirely sure how precisely the blocking of serotonin uptake – which increases the amount of signalling serotonin between neurons – really changes mood. But it definitely makes us happier.