a,b, Representative intensity distribution analyses based on TIRF images of wild-type µOR at different time points, with naloxone and DAMGO (10 µM each) added as indicated: naloxone first (a) or DAMGO first (b). When naloxone is applied first, the µOR intensity distribution remains monomeric even after subsequent addition of DAMGO, indicating that naloxone prevents activation-dependent dimerization. When DAMGO is given first, the µOR intensity distribution indicates dimerization after 15 min, and addition of naloxone results in disruption of dimers as indicated by a progressive return to a rather monomeric distribution over the next 15 min. [Data were generated as in Fig. 2 and are from 314 (a) or 470 (b) detected PSFs from 7 (a) or 6 (b) cells. Experiments were repeated independently 3 times with similar results].