Expression

Gene expression is the process by which the information encoded in a gene is converted into a functional protein. In neurons, this most often means producing receptors, transporters, or enzymes — the molecular machinery that determines how sensitive a cell is and how it communicates.

A cell does not express every gene at full intensity all the time. It continuously adjusts which genes are active and at what level, making expression a dial, not a switch. That dial responds to the cell's chemical environment, which means repeated substance exposure can physically reshape a neuron's sensitivity by shifting which proteins it makes and how many.

How it works · its role

Each gene carries a regulatory region called a promoter. When the right molecular signals arrive — triggered by a neurotransmitter or a drug's downstream effects — proteins called transcription factors attach to the promoter and initiate gene reading. The resulting messenger RNA is translated into protein by ribosomes in the cell body.

Because this chain of events takes hours to days, expression-level changes unfold far more slowly than immediate synaptic signalling. A substance can reach the nucleus via second-messenger cascades, or alter the chemical tags on DNA-packaging proteins — a process called epigenetic regulation — changing how readily a gene can be transcribed at all.

Relevance to substances & effects

Expression changes are a core mechanism by which repeated substance use reshapes neural circuits. When a neuron is persistently activated — by opioids binding their receptors, for instance — it typically responds by producing fewer of those receptors. Fewer binding sites on the surface means a weaker response to the same dose: a molecular basis of tolerance.

The reverse can also occur. Stimulants such as amphetamines and cocaine alter dopamine receptor and transporter expression over time, contributing to sensitisation and to craving that persist well after the drug has cleared the body.

Classic psychedelics acting at 5-HT₂A receptors trigger downstream expression changes — including shifts in proteins involved in neuroplasticity — which may partly explain effects that outlast the drug's half-life. Cannabis, benzodiazepines, and alcohol each produce characteristic expression shifts with heavy use, shifts that can persist for weeks and shape the heightened-vulnerability window after stopping.