= Nicotine
<Nicotine> is an <alkaloid> whose structure combines a <pyridine> ring with a substituted <pyrrolidine> ring. Its neutral <molecular formula> is $\mathrm{C_{10}H_{14}N_2}$. The naturally occurring form specified by ChEBI has S <stereochemistry>, so connectivity and atom count alone do not state the complete identity.<Nicotine in ChEBI>[CHEBI:17688; natural(S) form, structure, formula and charge]{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.<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]{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.
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