Dissection of placebo analgesia in mice: the conditions for activation of opioid and non-opioid systems.

Amanzio and Benedetti (J Neurosci 1999; 19: 484-494) first addressed the conditions necessary for the activation of opioid and non-opioid placebo responses in human. Here, we investigated whether placebo analgesia is subdivided into opioid and non-opioid components in mice by using the model of hot-plate test. Drug conditioning was performed by the combination of the conditioned cue stimulus with the unconditioned drug stimulus, either opioid agonist morphine hydrochloride or non-opioid aspirin. Placebo analgesic responses were evoked by an exposure to a conditioned cue previously paired with drug conditioning. Morphine conditioning produced placebo responses that were completely antagonised by naloxone. By contrast, the conditioned cue after aspirin conditioning elicited a placebo effect that was not blocked by naloxone. Therefore, we first evoked opioid and non-opioid placebo responses in mice that were either naloxone-reversible or naloxone-insensitive, depending on the drug used in conditioning procedure. These findings support that the mechanisms underlying placebo analgesia may depend on the drug conditioning that was originally performed. The present procedure of mice may serve as a model for further understanding of the opioid and non-opioid mechanisms underlying placebo responses.