Conditioned hyperalgesia depends on the pain sensitivity measure.

Conflicting reports about the acquisition of conditioned hyperalgesia during the development of conditioned morphine tolerance have led researchers to suggest that tolerance reflects a reduction of stimulus processing rather than a compensatory response interaction. I tested conditioned hyperalgesia on both the hot-plate and tail-flick tests in the same animals. In accordance with previous reports, the tail-flick responses in drug-free animals failed to reveal a conditioned compensatory hyperalgesia. Conditioning effects in the tail-flick test were found only when the animals were challenged with a low dose of morphine. However, the hot-plate responses in drug-free animals replicated earlier demonstrations of conditioned hyperalgesia. The results suggest that the measurement of conditioned responses in drug-free animals depends on characteristics of the assessment procedure. These findings are consistent with accounts of morphine tolerance that depend on compensatory response interactions.