neurotransmitter

Nerve cells, or neurons, do not touch each other. Between one cell and the next there is a narrow gap called a synapse, and a signal has to cross it chemically. The sending cell releases a neurotransmitter into the gap; the molecule drifts across, attaches to a receptor on the receiving cell the way a key fits a lock, and that binding changes what the receiving cell does next. The US National Institute on Drug Abuse describes the sequence in exactly these terms: a neuron releases a neurotransmitter into the synapse, it crosses and attaches to receptors on the receiving neuron, and this causes changes in the receiving cell.

The brain uses dozens of them. Dopamine, serotonin and norepinephrine are three of the best studied, and each one operates across many different circuits rather than doing a single job, which is why it is misleading to call any of them the chemical for one mood or behaviour.

The signal has to stop as well as start. Proteins called transporters pull neurotransmitters back into the cell that released them, which limits or shuts off the message. That recycling step is called reuptake, and it is the step many psychiatric and neurological medicines act on: block the transporter and more of the neurotransmitter stays in the gap for longer.