Receptor

A receptor is a protein — almost always embedded in a cell membrane — that detects and responds to specific chemical signals. When the right molecule binds to it, the protein changes shape, and that physical change alters what the cell does: a nerve may fire, a muscle may contract, or a cascade of reactions may begin inside the cell.

The body maintains a large vocabulary of receptors, each tuned to particular endogenous signalling molecules — neurotransmitters, hormones, and neuropeptides. Psychoactive substances enter this system by binding to the same receptors those endogenous molecules use, either mimicking their action, blocking it, or distorting it.

How it works · its role

Receptors fall into two broad families that work on different timescales. Ionotropic receptors are ion channels: ligand binding opens a pore in the membrane, ions rush through, and the electrical charge of the cell shifts within milliseconds. Metabotropic receptors — the largest family being G protein-coupled receptors (GPCRs) — work more slowly, triggering internal signalling cascades that can alter the cell for seconds to minutes.

What a substance does at a receptor matters as much as which receptor it reaches. An agonist activates the receptor, producing the same downstream effect as the natural ligand. An antagonist occupies the receptor without activating it, blocking the natural signal. A partial agonist activates the receptor less fully than the natural ligand — which can mean it acts as an agonist when the natural signal is low and as a partial blocker when it is high.

Receptors also come in subtypes. Dopamine signals through at least five subtypes (D1 through D5), each linked to different circuits and effects. A drug's selectivity — which subtypes it binds, and how strongly — shapes the character of its action far more than the neurotransmitter system alone.

Relevance to substances & effects

Receptor pharmacology explains much of why substances feel distinct from one another even when they act on the same neurotransmitter system. Classic psychedelics such as LSD and psilocin are potent agonists at the 5-HT₂A serotonin receptor subtype; this binding is thought to drive most of their perceptual and cognitive effects.

Opioids bind mu-opioid receptors, producing pain suppression, euphoria, and respiratory depression. Cannabis compounds act on cannabinoid CB1 receptors concentrated in the brain and CB2 receptors more prevalent in immune tissue. Benzodiazepines do not directly activate the GABA-A receptor but bind a distinct modulatory site on it, amplifying the channel's response to its natural ligand.

Because receptors are the mechanism by which most psychoactive effects are produced, a compound's receptor profile — the pattern of subtypes it reaches, and whether it acts as agonist, antagonist, or partial agonist at each — is the foundational layer for understanding what a substance will do, how it interacts with other drugs, and why its effects differ from superficially similar compounds.