Schedule-induced polydipsic choice between cocaine and lidocaine solutions: critical importance of associative history

Two groups of rats given daily, 3-h, intermittent food-pellet sessions, which induced polydipsia, drank either cocaine (COC) or lidocaine (LIDO) solutions. Then water was made available concurrently, but neither group preferred drug solution. When drug was presented in a glucose-saccharin vehicle, which was subsequently faded out, both the COC and LIDO groups acquired a strong preference for drug solution rather than water. Half of each group was continued, and half was directly faded to a preference for the other drug solution rather than water. Then COC and LIDO solutions were available concurrently and animals overwhelmingly preferred the drug solution that historically had been associated with the glucose-saccharin vehicle. Finally, the COC-preferring and LIDO-preferring groups had the concentrations of their preferred drug increased step-wise several times and preferences were maintained. Thus, acquired drug preferences remained stable as preferred drug concentration was increased, making it more bitter than the nonpreferred drug. The abuse potential of COC affords it little advantage over LIDO in the institution and maintenance of oral drug preference, suggesting that the post-ingestive pharmacological action of a drug is less important for acquiring drug-taking than is a history of associating the drug with another reinforcing event.