Nicotine is a potent parasympathomimetic stimulant and an alkaloid found in the nightshade family of plants. Nicotine acts as an agonist at most nicotinic acetylcholine receptors (nAChRs), except at two nicotinic receptor subunits (nAChRα9 and nAChRα10) where it acts as a receptor antagonist. Nicotine is found in the leaves of Nicotiana rustica, in amounts of 2–14%; in the tobacco plant, Nicotiana tabacum; in Duboisia hopwoodii; and in Asclepias syriaca.
Nicotine constitutes approximately 0.6–3.0% of the dry weight of tobacco. It also occurs in edible plants, such as those in the Solanaceae family, which include eggplants, potatoes, and tomatoes for example, but at trace levels generally under 200 nanograms per gram, dry weight (less than .00002%). Nicotine functions as an antiherbivore chemical; consequently, nicotine was widely used as an insecticide in the past, and neonicotinoids, such as imidacloprid, are widely used.