[3H]DAMGO (56.0 Ci/mmol) and tritium-sensitive storage phosphor screens were purchased from Perkin Elmer (Boston, MA, USA). Naloxone was kindly provided by the National Institute on Drug Abuse (NIDA). [3H]microscales were obtained from Amersham Co. (Arlington Heights, IL, USA). All other reagent-grade chemicals were obtained from Sigma Chemical Co (St. Louis, MO, USA) or Fisher Scientific (Pittsburgh, PA, USA).
