mu2 is the receptor that cause euphoria btw mu1 is the cause of respiratory depression, physical dependence
OPIATE Mu 1 = 0.400000 ki nm
OPIATE Mu 1B4 = 0.800000 ki nm (if you know you have this version of OPIATE Mu 1)
there is no other info in first 100 google result unfortunately or ki databases. about affinity of subs on mu opiates receptors