Mean (±1 SEM) relative choice for each alternative (upper) and the number of trials completed (lower) during saline (triangles), histamine (circles), remifentanil (diamonds), and spiradoline (squares) tests in Experiment 1. Ten rats completed tests with saline and histamine, 7 rats completed tests with spiradoline (0.056 mg/kg/infusion), and 6 rats completed the remaining tests (spiradoline [0.0056 mg/kg/infusion], remifentanil [0.001–0.01 mg/kg/infusion], and histamine redetermination). Data shown are for only the 6 rats that completed the study, with filled symbols denoting a significant (p<0.05) effect compared with saline. C: unit dose in mg/kg/infusion.