What is nicotine addiction?

Nicotine is the tobacco plant's natural protection from being eaten by insects. Drop for drop it's more lethal than strychnine and three times deadlier than arsenic. Yet, amazingly, by chance, this natural insecticide's chemical structure is so similar to the neurotransmitter acetylcholine that once inside the brain it fits a host of chemical locks permitting it direct and indirect control over Sharing is sexy