Nepetalactone: A new opioid analgesic from Nepeta caesarea Boiss.

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