Upregulation of hMOR and Ala348,353 hMOR expression upon ligand pretreatment of the cells. Saturation experiments using [3H]diprenorphine on intact HEK 293 cells stably expressing hMOR (checked bars) or Ala348,353 hMOR (black bars) following cell treatment with naloxone, cyprodime or CTOP 1 mM for 48 h as described in the Experimental Procedures. Data are given as the mean ± S.E.M. from at least 4 independent experiments. Statistical analysis was performed using Student's t test to compare upregulation at wild-type and mutant receptors, ** p < 0.01 compared to hMOR.