To evaluate the efficacy of gene therapy using a gene gun or direct injection for the transfer of human preproenkephalin (PPE) plasmid cDNA using a capsaicin-induced bladder pain model in rats. Opioid peptides play an essential role in the modulation of micturition reflex and control of inflammatory pain. PPE is one such precursor molecule.

Human PPE cDNA was cloned into a modified pCMV plasmid and delivered into the bladder wall of adult female rats by direct injection or gene gun. At 4 and 7 days after gene therapy, continuous cystometrograms were performed under urethane anesthesia by filling the bladder (0.08 mL/min) with saline, followed by 15 μM capsaicin. Immunohistochemical staining was used to detect enkephalins in the bladder after PPE cDNA transfer.

The intercontraction interval was decreased after intravesical instillation of capsaicin (65.0% and 63.1% decrease) in the control group or direct PPE gene injection group, respectively. However, the gene gun-treated group showed a significantly reduced response to capsaicin instillation at day 4 and day 7 (intercontraction interval 16.2% and 42.8% decrease, respectively). This analgesic effect was reversed by intravenous naloxone, an opioid antagonist (5 mg/kg). Increased enkephalin immunoreactivity in the bladder was observed in the gene gun-treated group at day 4, which was reduced at day 7.

The PPE gene can be effectively transferred and suppress the nociceptive response in the bladder using the gene gun method. These results support potential clinical application of PPE gene gun delivery system for the treatment of bladder pain and other types of visceral pain.