Nicotine hydrogen tartrate salt

The drugs were nicotine hydrogen tartrate salt (Sigma-Aldrich, St Louis, Missouri, USA), studied at pH 7, varenicline dihydrochloride (National Institute on Drug Abuse, Rockville, Maryland, USA), ± epibatidine dihydrochloride hydrate (Sigma-Aldrich), cytisine (Atomole Scientific, Hubei, China), cocaine hydrochloride (Sigma-Aldrich), and mecamylamine hydrochloride (Waterstone Technology LLC, Carmel, Indiana, USA). All drugs were administered subcutaneously except for cocaine and mecamylamine, which were administered intraperitoneal drugs were administered in a volume of saline equivalent to 10 ml/kg. Doses were expressed as their salt forms except for nicotine, which was expressed as the base weight.

Unless otherwise noted, all compounds were prepared in physiological saline (0.9% NaCl). Doses refer to the weight of the salt, except where noted. Drugs used and suppliers were: atomoxetine HCl (OChem Inc, Des Plaines, IL), donepezil HCl (Memory Pharmaceuticals, Montvale, NJ) and (−) nicotine hydrogen tartrate salt (base weight; Sigma-Aldrich, St Louis, MO). Saline served as the vehicle control in all behavioral tests.

(−)-Nicotine hydrogen tartrate salt (Sigma-Aldrich, St. Louis, MO) for injection was dissolved in physiological saline at a concentration of 0.1 free base mg ml⁻¹ and pH was adjusted to 6.8–7.6. Nicotine injections were administered subcutaneously at a volume of 2 ml kg⁻¹. Mice were sacrificed 48 hrs after nicotine injections (0.2 mg/kg), as our previous data show that the most robust, long-term addiction-related behavioral alteration occurs at this time point (19 (link)).

Both control and nicotine solutions contained 2% saccharin in filtered water, which was used to mask the bitter flavor of nicotine in nicotine-containing solutions (Perez et al., 2015 (link); Salas et al., 2009 (link); Zhang et al., 2012 (link)). For the majority of the experiment, nicotine bottles contained 0.1 mg/ml free-base nicotine (or 0.2804 mg/ml of nicotine hydrogen tartrate salt from Sigma Aldrich, St Louis, MO). During the “Nicotine Fading + Flavor Reintroduction” experiment, the nicotine concentration was sequentially reduced to 0.075, 0.05, 0.025, and 0.000 mg/ml (free-base). Nicotine solutions were kept in dark bottles to protect against photodegradation and were prepared fresh every 4 days. Kool-Aid ® solutions were made according to package instructions (0.14 oz powder in 2 quarts of 2% saccharin solution). Kool-Aid ® powder (Kool-Aid ® Strawberry Drink Mix Unsweetened, Kraft Foods, Chicago IL) contains flavorants, but does not contain any additional sweeteners. In other words, the only sweetener in the Kool-Aid ® solutions was the saccharin that was added during preparation of the solution.