Effects of dextromethorphan on rats' acquisition of responding with delayed reinforcement

Separate groups of 16 rats received 0, 40, 60, or 80 mg/kg dextromethorphan prior to a 2-h response-acquisition session during which responses on one lever produced food (reinforcement lever, RL, responses) after a 15-s resetting delay and responses on the other lever cancelled food deliveries earned by RL responses, but otherwise had no programmed consequences. When compared to the 0 mg/kg dose, the 40, 60, and 80 mg/kg doses significantly decreased the latency to the tenth RL response, which has been used previously as an index of response acquisition [Pallares, MA, Nadal, RA, Silvestro, JS, Ferre, NS. Effects of ketamine, a noncompetitive NMDA antagonist, on the acquisition of the lever-press response in rats. Physio Behav 1995; 57:389-392.]. Only the 80 mg/kg dose, however, significantly reduced the total number of food pellets earned, the total number of RL responses, or the total number of rats that met the criterion for response acquisition. The present results indicate that dextromethorphan can disrupt initial response acquisition (i.e., learning) with positive reinforcement, although the dose that did so depended on the measure used to index performance. Moreover, the effects of the drug did not appear to reflect specific learning impairment, but rather more general disruption of behavior.