The invention provides an expression cassette comprising a DNA sequence encoding amino acids 1-99 of human preproenkephalin, a DNA sequence encoding a precursor of a carboxy-amidated peptide flanked by dibasic cleavage sites and optionally a DNA sequence encoding a marker protein (such as Enhanced Green Fluorescent Protein (GFP)) all in operable linkage and under control of a promoter. Where the encoded precursor of a carboxy-amidated peptide is an agonist for an opioid receptor, the invention further provides a method of treating neuropathic pain by administering the gene transfer vector comprising such an expression cassette to a patient. The invention also provides a method for detecting a peptide having a desired effect comprising introducing a library of DNA sequences encoding one or more precursors of carboxy-amidated peptides into host cells; expressing the carboxy-amidated peptides encoded in the library to provide expression products; and screening from the polypeptide expression products for the desired effect.