Nicotine

Nicotine is an alkaloid whose structure combines a pyridine ring with a substituted pyrrolidine ring. Its neutral molecular formula is ${\mathrm{C}}_{10}{\mathrm{H}}_{14}{\mathrm{N}}_2$. The naturally occurring form specified by ChEBI has S stereochemistry, so connectivity and atom count alone do not state the complete identity.^[ref]

Nicotine acts as an agonist at nicotinic acetylcholine receptors. It is a ligand acting on a receptor, not the receptor itself and not acetylcholine. Different receptor subunit combinations and the surrounding circuit can change the observed response.

A 2011 study tested this circuit dependence using loss of the α5 receptor subunit and targeted restoration in the medial habenula. The work connected the subunit to a signal limiting nicotine intake in the tested animals.^[ref] Changing a receptor component while leaving the chemical ligand identifiable helps distinguish molecular identity from biological sensitivity. It does not reduce every action of nicotine to that one circuit.