DAMGO, GDP and GTPγS were purchased from Sigma Chemical Co (St. Louis, MO). [35S]GTPγS (1250 Ci/mmole) and phosphor screens were obtained from Perkin Elmer (Boston, MA). [14C]microscales were purchased from GE healthcare, formerly Amersham Biosciences (Arlington Heights, IL). All other reagent-grade chemicals were obtained from Sigma Chemical Co (St. Louis, MO) or Fisher Scientific (Pittsburgh, PA).
