Given the heterogeneous nature of sensory neurons, the electrophysiological experiments were performed in cells with comparable biophysical characteristics, including cell size, Ca2+ channel current density, and peak current potential. Thus, it is unlikely that the effects observed with morphine were a result of employing different cell subpopulations. In addition, the electrophysiological data obtained from sensory neurons isolated from the parent mice were not significantly different from the 118AA group. Also differences in MOR density between 118AA and 118GG neurons are unlikely to account for our findings. Binding assays, which measured receptor density in several brain regions, showed no significant differences in our mouse model25. Moreover a recent study, also performed in postmortem human brain areas involved in nociception, found that receptor density of MOR was not different between 118AA and 118GG carriers24. However, another report found lower MOR messenger RNA in postmortem human brain tissue from 118G individuals compared to both heterozygous or 118AA subjects22. However, in this study protein levels were not assessed.
