8.16—Brain chemicals, ill health and drugs

Some disorders are associated with altered neurotransmitter levels or signalling. Parkinson’s involves reduced dopamine from lost neurones; antidepressant drugs can increase serotonin or noradrenaline signalling.

A drug may mimic a transmitter, supply a precursor, block breakdown or inhibit reuptake. These mechanisms change synaptic transmission, but symptoms usually involve networks and multiple contributing factors.

L-dopa is converted into dopamine in the brain, while an SSRI reduces serotonin reuptake so more remains in the synaptic cleft.

“Low neurotransmitter causes the disorder” is stronger than the evidence usually supports. Distinguish an association, a treatment mechanism and a complete explanation.