Neurotransmitter
A chemical messenger carrying signals across the gap between nerve cells.
A neurotransmitter is a chemical released by one neuron to signal to the next across the synapse, the tiny gap between them. It binds receptors on the receiving cell and makes it more or less likely to fire.
The familiar names each cover a range of jobs rather than a single mood. Acetylcholine drives muscle contraction at the neuromuscular junction and is central to attention. Glutamate and GABA are the main excitatory and inhibitory transmitters in the brain. Dopamine is involved in motivation and movement, serotonin in mood, appetite and sleep, and noradrenaline in arousal.
Popular writing tends to flatten this into a chemical for each feeling, which is where most of the nonsense begins. Dopamine is not the pleasure chemical, and dopamine detox is not a thing your brain can do. Reducing constant novelty may genuinely make ordinary tasks more appealing; the mechanism offered for it is fiction.
The serotonin story shows the same pattern. The simple chemical imbalance account of depression is not how researchers describe it now, even though drugs acting on serotonin help many people, which tells you the relationship between a transmitter and an experience is not one to one.
Exercise affects several of these systems, and the outcome evidence is better than any mechanistic claim about it. A 2024 network meta-analysis of 218 studies found walking or jogging, yoga and strength training produced moderate reductions in depressive symptoms.
Supplements sold as precursors, such as 5-HTP or tyrosine, are on much weaker ground: precursor availability is rarely the rate limiting step, and the human evidence is thin.
In practice, treat single transmitter explanations of behaviour with suspicion, and note that the interventions with real effects are sleep, exercise and, where needed, proper treatment.