The failure of cocaine to serve as an orally self-administered reinforcer in Lewis rats

The aim of this study was to develop a procedure to establish orally delivered cocaine as a reinforcer for rats. Several procedures that have proven reliable in other studies were instituted. (1) Food-induced drinking: food was presented to engender high rates of drinking, then cocaine solutions replaced water, and finally the food was removed. Peak drug intakes ranged from 9.4 to 13.8mg/kg. (2) Ethanol-fade: ethanol was established as a reinforcer, cocaine was gradually added to the ethanol, and the ethanol was gradually removed. Peak cocaine intakes ranged from 3.6 to 5.2mg/kg. (3) Modified food-induced drinking: food was presented, but was followed by a timeout period, allowing digestion to progress prior to cocaine drinking. Peak drug intakes ranged from 12.9 to 18.6mg/kg. However, cocaine was not established as a reinforcer with any of these methods. High cocaine doses may be necessary, but are not sufficient, to establish oral cocaine self-administration in rats.