Behavioral mechanisms of oxycodone's effects in female and male rats: II. Reinforcement magnitude and implications for impulsive/risky choice.

Rats responded under a concurrent-chains procedure wherein reinforcement magnitude was varied within sessions and oxycodone's effects on sensitivity to magnitude were evaluated in two experiments. In Experiment 1, the alternative providing the larger magnitude was signaled and effects of acute (0.1-1.0 mg/kg) and chronic (1.0 mg/kg, twice daily) oxycodone administration were examined in female and male rats. Under baseline, sensitivity was slightly higher for females than males. Acute oxycodone decreased sensitivity in both sexes, but females were more susceptible to this effect. Effects of chronic administration on sensitivity were somewhat variable; on average, females showed slight tolerance and males showed slight sensitization to this effect. No physical dependence was noted during withdrawal probes. In Experiment 2, the alternative providing the larger magnitude was not signaled and effects of acute oxycodone were evaluated in a separate group of male rats. Sensitivity was higher under baseline, and larger doses reduced sensitivity to a greater extent in Experiment 2 than in Experiment 1. Taken with previous data on oxycodone's effects on sensitivity to reinforcement delay, oxycodone would be expected to leave impulsive choice unchanged in both sexes. Additional analyses revealed that oxycodone's effects on sensitivity in both experiments were baseline dependent: higher sensitivities were reduced to a greater extent than lower sensitivities.