Dose effect of intrathecal siRNA to DOR on the antinociceptive effect of the delta opioid agonist [D-Ala2
]deltrophin II (DELT). DELT was given intrathecally (30 μg). The antinociceptive efficacy of DELT is defined as % antinociception measured 30 min after drug administration (n = 6 rats/group). Intrathecal DELT antinociception was significantly blocked after 3 consecutive once daily dose of siRNA at a daily dose of 4 μg (2 μg b.i.d.) or 2 μg (*p < 0.05), when compared with that observed in the vehicle control group. Mismatch RNA at the same doses had no effect on DELT antinociception. The 2 μg daily dose of siRNA, when administered without i-Fect™, did not block intrathecal DELT antinociception. The 0.5 μg dose of the siRNA in i-Fect™ did not have any effect on intrathecal DELT antinociception. Statistical analysis was carried out by one-way ANOVA following Dunnett's multiple comparisons.