Morphine sulfate (National Institute on Drug Abuse Drug Supply Program, Bethesda, MD, USA) was dissolved in sterile water, phencyclidine hydrochloride (Sigma-Aldrich, St Louis, MO, USA) was dissolved in 0.9% saline, and 2-[(1R,2R,5R)-5-hydroxy-2-(3-hydroxypropyl) cyclohexyl]-5-(2-methyloctan-2-yl)phenol (CP55940; Sigma-Aldrich, St. Louis, MO, USA) was dissolved in pure ethanol (2 ml ethanol per bottle containing 10 mg CP55940) then diluted with a solution of 1:1:18 ethanol:emulphor:saline. Drugs were administered subcutaneously in a volume of 0.2 – 0.8 ml. Pretreatment times were 15 min for morphine and phencyclidine and 60 min for CP55940; behavioral effects of morphine and CP55940 are apparent with 15 and 60 min, respectively, and last at least 90 min in various operant tasks, including delay discounting (e.g., Maguire et al., 2012 (link), 2016 (link); Minervini and France, 2018 (link)). All doses were expressed as the weight of the forms noted above.