1. A method of treating neuropathic or chronic pain in a subject, said method comprising: administering a nucleic acid construct encoding an opioid receptor gene to the sciatic nerve or dorsal root ganglion of a subject, wherein the opioid receptor gene is operably linked to a promoter, and wherein the opioid receptor gene encoded by the construct is expressed in the dorsal root ganglion and expression of said opioid receptor gene increases the number of opioid receptors in the dorsal root ganglion of the subject; and administering an opioid to the subject.