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.
Animals Conditioning, Classical