Transgene-mediated enkephalin release enhances the effect of morphine and evades tolerance to produce a sustained antiallodynic effect in neuropathic pain

We examined the pharmacologic characteristics of herpes simplex virus (HSV) vector-mediated expression of proenkephalin in the dorsal root ganglion in a rodent model of neuropathic pain. We found that: (i) vector-mediated enkephalin produced an antiallodynic effect that was reversed by naloxone; (ii) vector-mediated enkephalin production in animals with spinal nerve ligation prevented the induction of c-fos expression in second order sensory neurons in the dorsal horn of spinal cord; (iii) the effect of vector-mediated enkephalin enhanced the effect of morphine, reducing the ED₅₀ of morphine 10-fold; (iv) animals did not develop tolerance to the continued production of vector-mediated enkephalin over a period of several weeks; and, (v) vector transduction continued to provide an analgesic effect despite the induction of tolerance to morphine. This is the first demonstration of gene transfer to provide an analgesic effect in neuropathic pain. The pharmacologic analysis demonstrates that transgene-mediated expression and local release of opioid peptides produce some effects that are distinct from peptide analogues delivered pharmacologically.