Figure 1 shows the chemical structure of each drug, except nicotine, included in the study. (–)-Nicotine hydrogen tartrate salt, cytisine and epibatidine dihydrochloride were obtained from Sigma Chemical (St. Louis, MO). Varenicline dihydrochloride was obtained from the Research Technology Branch of the National Institute on Drug Abuse (Rockville, MD), mecamylamine from Waterstone Technology (Camel, IN) and dihydro-β-erythroidine hydrobromide from Tocris (Minneapolis, MN). 2′-Fluorodeschloroepibatidine (RTI-7527-36), 3′-(3″-dimethylaminophenyl) epibatidine (RTI-7527-76), and 2′-fluoro-(4-nitrophenyl) deschloroepibatidine (RTI-7527-102) were synthesized at the Center for Organic and Medicinal Chemistry, Research Triangle Institute (Research Triangle Park, NC) according to methods previously described (Carroll et al., 2004 (link), 2005 (link), 2010 (link)). Drugs were dissolved in 0.9% physiological saline and administered s.c. (except mecamylamine which was administered i.p.) in a volume of 10 ml/kg. Nicotine dose is expressed in terms of the weight of the free base; dose of other drugs is expressed as the weight of the base and salt.