Nicotine is an alkaloid whose structure combines a pyridine ring with a substituted pyrrolidine ring. Its neutral molecular formula is . 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]
Habenular α5 nicotinic receptor subunit signalling controls nicotine intake by Christie D. Fowler and others. Abstract; subunit loss, habenular rescue and intake-related inhibitory signalling
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. New to topics? Read the docs here!