Nepetalactone: a new opioid analgesic from Nepeta caesarea Boiss.

The essential oils of Nepeta species including Nepeta phyllochlamys P. H. Davis, N. nuda L. ssp. nuda, and N. caesarea Boiss. have been screened by use of the tail-flick and tail immersion (52.5 degrees C) methods. Of the species studied, only N. caesarea showed significant analgesic activity, besides marked sedation, which was also blocked by naloxone, indicating involvement of opioid receptors. Moreover, it was only active on mechanical, not thermal, algesic response which suggests specificity for specific opioid receptor subtypes, excluding mu-opioid receptors. Because 4a alpha,7alpha,7a alpha-nepetalactone is the main component of the essential oil of N. caesarea, and is present at very high levels (92-95%), it is concluded that 4a alpha,7alpha,7a alpha-nepetalactone is the active principle and has a specific opioid receptor subtype agonistic activity.