Short small-interfering RNAs produce interferon--mediated analgesia

There is increasing interest in RNA interference in pain research using the intrathecal route to deliver small-interfering RNA (siRNA). An interferon (IFN) response is a common side-effect of siRNA. However, the IFN response in the spinal cord after intrathecal administration of siRNA remains unknown. We hypothesized that high doses of siRNAs can elicit off-target analgesia via releasing IFN-. We investigated the IFN response and its role in regulating pain sensitivity in the spinal cords after intrathecal administration of siRNAs. Male SpragueDawley rats were given intrathecal injections of non-targeting (NT) siRNAs or IFN- and tested for complete Freunds adjuvant (CFA)-induced mechanical allodynia and heat hyperalgesia. IFN- in the spinal cord after injection of NT siRNAs was measured by western blotting and immunohistochemical staining. IFN- was up-regulated in the spinal cord after intrathecal treatment of NT siRNAs. Intrathecal injection of NT siRNAs, at high doses of 10 or 20 g, reduced CFA-induced inflammatory pain (P0.05). Intrathecal application of IFN- inhibited pain hypersensitivity in inflamed rats and produced analgesia in nave rats (P0.05). Notably, the anti-nociceptive effects elicited by NT siRNAs and IFN- were reversed by IFN- neutralizing antibody and naloxone. Our data suggest that (i) intrathecal administration of high doses of siRNA (epsilon 10 g) induced up-regulation of IFN- in the spinal cord and produced analgesic effects through IFN-, and (ii) IFN-s analgesic effect is mediated via opioid receptors. Caution must be taken to avoid IFN--mediated analgesic effects of siRNAs in pain research.