Dynorphin A is a heptadecapeptide released from the cleavage of prodynorphin and found widely distributed in the central nervous system, where it is implicated in antinociceptive functions.1,2,3 It acts as an opioid receptor agonist with nanomolar binding affinity to the three major isotypes κ, μ, and δ (Kis = 0.5-1 nM) and has also been shown to activate human ORL1, a novel opioid receptor-like receptor (Ki = 386 nM).4