The conditional deletion of mu receptor in Nav1.8 primary neurons does not abrogate tolerance to morphine-induced antinociception.

<p>Morphine dose-dependent antinociception was measured following repeated 4-day i.p. injections of morphine or saline in mu^fl controls, mu-cKO and mu-KO animals. The shift to right for both mu^fl and mu-cKO chronic-morphine animals indicates the development of a comparable tolerance to analgesia. Total mu-KO animals show no antinociception. n=6-7/genotype/treatment, two-way ANOVA (genotype x treatment F(1,30) = 19.919, <i>P</i> <0.001 for treatment; F(2,30) = 97.039, <i>P</i> <0.001 for genotype); post-hoc Fisher test for individual morphine doses, ✰ <i>P</i> <0.05, ✰✰ <i>P</i><0.01, chronic morphine (tolerance) <i>vs</i> chronic saline in mu^fl mice; ★ <i>P</i><0.05, ★★ <i>P</i><0.00 chronic morphine (tolerance) <i>vs</i> chronic saline in mu-cKO mice.