μ Opioid receptor-mediated G-protein activation by heroin metabolites: evidence for greater efficacy of 6-monoacetylmorphine compared with morphine 11Abbreviations: 6-MAM, 6-monoacetylmorphine; GTPγS, guanosine-5′-O-(γ-thio)-triphosphate; DAMGO, [d-Ala2,(N-Me)Phe4,Gly5(OH)]enkephalin; M-6-G, morphin