The study investigates the mechanisms underlying placebo analgesia, focusing on the roles of cognitive expectation and conditioning in activating endogenous opioid systems and specific nonopioid subsystems. Using a model of human experimental ischemic arm pain, researchers tested how different types of placebo responses—induced by cognitive expectation cues, drug conditioning, or a combination—were affected by the opioid antagonist naloxone. Expectation cues alone produced placebo responses that were completely blocked by naloxone, while morphine conditioning alone induced a naloxone-reversible effect. Ketorolac conditioning, a nonopioid drug, produced a partially naloxone-reversible effect when combined with expectation cues, but was naloxone-insensitive when used alone. The study highlights that cognitive factors like expectation trigger endogenous opioid systems, while conditioning activates specific subsystems. When conditioning is done with opioids, placebo analgesia is mediated via opioid receptors, whereas nonopioid conditioning, such as ketorolac, involves other mechanisms. These findings suggest that placebo analgesia can be divided into opioid and nonopioid components, depending on the procedure used to induce the response. The study also shows that placebo responses can occur without expectation of pain relief, indicating that previous conditioning with either opioids or nonopioids can lead to analgesic effects even in the absence of expectation. This is supported by findings that conditioned placebo responses can be achieved without expectancy, as shown by Voudouris et al. (1990). The research underscores the importance of balancing cognitive and conditioning factors in placebo analgesia. It has implications for understanding how psychological and neurochemical systems interact to produce analgesic effects. The study also emphasizes the role of conditioning in placebo responses, which is relevant for pain, the immune system, and pharmacotherapy. Overall, the findings clarify previous contradictory studies on the effects of naloxone on placebo analgesia, showing that the response depends on the strength of expectation cues and prior conditioning experiences.The study investigates the mechanisms underlying placebo analgesia, focusing on the roles of cognitive expectation and conditioning in activating endogenous opioid systems and specific nonopioid subsystems. Using a model of human experimental ischemic arm pain, researchers tested how different types of placebo responses—induced by cognitive expectation cues, drug conditioning, or a combination—were affected by the opioid antagonist naloxone. Expectation cues alone produced placebo responses that were completely blocked by naloxone, while morphine conditioning alone induced a naloxone-reversible effect. Ketorolac conditioning, a nonopioid drug, produced a partially naloxone-reversible effect when combined with expectation cues, but was naloxone-insensitive when used alone. The study highlights that cognitive factors like expectation trigger endogenous opioid systems, while conditioning activates specific subsystems. When conditioning is done with opioids, placebo analgesia is mediated via opioid receptors, whereas nonopioid conditioning, such as ketorolac, involves other mechanisms. These findings suggest that placebo analgesia can be divided into opioid and nonopioid components, depending on the procedure used to induce the response. The study also shows that placebo responses can occur without expectation of pain relief, indicating that previous conditioning with either opioids or nonopioids can lead to analgesic effects even in the absence of expectation. This is supported by findings that conditioned placebo responses can be achieved without expectancy, as shown by Voudouris et al. (1990). The research underscores the importance of balancing cognitive and conditioning factors in placebo analgesia. It has implications for understanding how psychological and neurochemical systems interact to produce analgesic effects. The study also emphasizes the role of conditioning in placebo responses, which is relevant for pain, the immune system, and pharmacotherapy. Overall, the findings clarify previous contradictory studies on the effects of naloxone on placebo analgesia, showing that the response depends on the strength of expectation cues and prior conditioning experiences.