Remote nerve injection of mu opioid receptor adeno-associated viral vector increases antinociception of intrathecal morphine

We have shown previously that using recombinant adeno-associated viral vector (rAAV) to up-regulate mu opioid receptors (μORs) in dorsal root ganglia (DRGs) increases the potency of subcutaneous morphine. Here we report an improved method of introducing rAAV-μOR viral vectors into DRGs. Instead of injecting the rAAV-μOR gene directly into DRGs as shown before, the vector was introduced into the sciatic nerve of rats. Changes in μOR expression and antinociceptive effects of intrathecal morphine in rAAV-μOR rats were examined. Immunocytochemical studies showed that the transduced μORs were expressed in all types (ie, small, medium, and large) of DRG neurons. The expression of μORs in DRG neurons, quantified by Western blotting, was increased by 1.7-fold 4 weeks after the sciatic nerve injection. The up-regulation persisted for more than 6 months. The effects of intrathecal morphine on paw withdrawal latencies to heat were studied in rats inflamed with complete Freund's adjuvant. Compared with rats injected with rAAV containing the enhanced green fluorescent protein gene (rAAV-EGFP), the antinociceptive potency of intrathecal morphine in rAAV-μOR rats was significantly increased, and the effective dose (ED50) for morphine was 5.4-fold lower (rAAV-μOR: ED50 = 0.84 μg, confidence interval, 0.70-0.99 μg; rAAV-EGFP: ED50 = 4.50 μg, confidence interval, 3.55-5.86 μg). With minimum tissue damage and a large persistent increase in the opioid potency, remote nerve injection of rAAV-μOR to up-regulate μORs could be a useful therapeutic strategy for the treatment of chronic pain.