How Brain Drugs Work

Agonists, antagonists, reuptake inhibitors and enzyme blockers: the basic ways medicines and recreational drugs change brain signalling — plus the blood-brain barrier problem and the surprising role of serendipity.

The nervous system communicates chemically, so chemicals can change how it communicates. Almost all psychoactive drugs — medicines and recreational substances alike — act on synaptic signalling in a handful of ways.

The main mechanisms

Agonists mimic a transmitter and activate its receptors. Morphine is an agonist at mu-opioid receptors; nicotine at nicotinic acetylcholine receptors; psilocybin (via psilocin) at serotonin 5-HT2A receptors. Antagonists block receptors without activating them: most antipsychotics block dopamine D2 receptors; naloxone blocks opioid receptors, reversing overdoses; caffeine blocks adenosine receptors. Positive allosteric modulators bind elsewhere on a receptor and boost its response to the natural transmitter — benzodiazepines do this at GABA-A receptors.

Reuptake inhibitors block transporter proteins that clear transmitter from the synapse, so it lingers longer. SSRIs block the serotonin transporter; cocaine blocks dopamine, noradrenaline and serotonin transporters; methylphenidate blocks dopamine and noradrenaline transporters. Releasers such as amphetamine and MDMA reverse the transporters, pushing transmitter out. Enzyme inhibitors stop transmitters being broken down: MAO inhibitors (monoamines) and acetylcholinesterase inhibitors (donepezil). Precursors provide raw material: L-dopa becomes dopamine. Ion channel blockers act directly on channels: lidocaine and several anti-seizure drugs block sodium channels.

Getting in

To act on the brain, a drug must cross the blood-brain barrier — generally requiring small, fat-soluble molecules or a hitch on a transporter — and reach effective concentrations without too many side effects elsewhere. Many promising compounds fail at this hurdle.

Fast effects, slow benefits

Drugs change synapses within minutes to hours, but therapeutic effects can take weeks, as with antidepressants. That delay suggests the brain's adaptive responses — receptor changes, gene expression, plasticity — matter more than the immediate chemical change. Adaptation also underlies tolerance, dependence and withdrawal.

The role of luck

Many of the first psychiatric drugs were found by serendipity in the 1940s–60s: lithium for mania (1949), chlorpromazine — developed as an anaesthesia aid — for psychosis (1952), iproniazid — a tuberculosis drug that made patients euphoric — and imipramine for depression (late 1950s), and chlordiazepoxide, the first benzodiazepine (approved 1960). Understanding of their mechanisms came later and shaped neurotransmitter theories of mental illness.