nicotine

Meaning: Nicotine is a psychoactive stimulant found in tobacco. It acts as an agonist at nicotinic acetylcholine receptors, which are ligand-gated ion channels. Activating them in the ventral tegmental area increases dopamine release in the nucleus accumbens, the mesolimbic reward pathway. This feels rewarding, so the behavior is positively reinforced. Nicotine also raises alertness and heart rate through sympathetic effects.

Repeated use leads to tolerance (a larger dose is needed for the same effect), physical dependence, and withdrawal when use stops. Typical withdrawal features are irritability, anxiety, poor concentration, and craving. Relief of these symptoms by the next dose acts as negative reinforcement.

On the MCAT: Nicotine appears in passages on addiction, the reward pathway, and operant conditioning. Expect questions asking which pathway is involved (dopamine, mesolimbic), whether smoking is maintained by positive or negative reinforcement (both can apply), or how tolerance and withdrawal change behavior. Questions may also tie nicotine to health behavior models, since stimulant effects and social cues such as peer smoking can influence use.

Common confusion: Nicotine is not a depressant. It is a stimulant, even though many users say smoking relaxes them. That relaxation is largely relief from withdrawal. Do not confuse nicotinic receptors with muscarinic receptors, which are G protein-coupled and respond to muscarine rather than nicotine.