Fig. 5.

Serotonin 5-HT_2AR in DRG neurons represses oxycodone-induced antinociception in male mice. (A) Restoration of 5-HT_2AR expression in DRG neurons of 5-HT_2AR^−/− male mice. By insertion of a “stop” cassette 5’ upstream of exon 1 (grey box) flanked by loxP sites (red arrows), we interrupted the translation of the 5-HT_2AR gene. Pirt-Cre excises the stop cassette and restores expression of 5-HT_2AR to areas where their promoter activities overlap. (B) Representative immunohistochemical images with anti-5-HT_2AR antibody and HRP-conjugated secondary antibody in DRG tissue samples of 5-HT_2AR^−/− and 5-HT_2AR^−/−:Pirt-Cre mice. Note that 5-HT_2AR immunoreactivity is rescued in the DRG of 5-HT_2AR^−/−:Pirt-Cre male mice. (C) Adjunctive effect of volinanserin (0.125 mg/kg) or vehicle on the cumulative dose-response induced by oxycodone in the TFR test in male 5-HT_2AR^−/− and 5-HT_2AR^−/−:Pirt-Cre mice (n = 10-12 mice per group) (Oxycodone effect F[7,244] = 90.34, p < 0.001, Volinanserin/Genotype effect F[4,244] = 6.90, p < 0.01). (D and E) Male 5-H_2AR^−/−:Emx1-Cre, 5-HT_2AR^−/−:DAT-Cre and 5-HT_2AR^−/− mice were given oxycodone (8 mg/kg), volinanserin (0.125 mg/kg) and oxycodone, or vehicle once a day for 5 days, after which locomotor activity was evaluated. On day 8 (3 days after the last injection), all mice received a single dose of oxycodone (8 mg/kg), and then tested for locomotor activity (n = 9-20 mice per group). Two-way ANOVA (D – days 0-5, Time effect F[5,522] = 24.44, p < 0.001, Volinanserin/Oxycodone/Genotype effect F[5,522] = 39.77, p < 0.001) or one-way ANOVA (D – day 8 and E, F[6,85] = 9.45, p < 0.001) followed by Bonferroni’s multiple comparison test. *p < 0.05, **p < 0.01, ***p < 0.001, n.s., not significant. Data are mean ± S.E.M.