When expressed in CHO cells, both humanized receptors showed binding of the classical OPRM1 selective ligand [3H]-DAMGO, displaced by the endogenous OPRM1 agonist β-End with expected low nanomolar IC50, and with no difference between genotypes (F[4,46]=1.3, p=0.3; Fig. 2D). Signaling, assessed as modulation of Ca2+ currents in acutely isolated superior trigeminal ganglion neurons, was also normal in response to both DAMGO and β-End, once again without any genotype differences (DAMGO: F[4,12]= 1.8, p=0.2; β-End: F[4,12]=1.2, p=0.4; Fig 2E–F). Finally, [3H]-DAMGO binding on mouse brain sections did not show genotype differences in receptor densities across a number of brain regions examined, including ventral and dorsal striatum and the ventral tegmental area (VTA; Figure S5).
