Noxious heat threshold measured with slowly increasing temperatures: novel rat thermal hyperalgesia models.

The conventional methods for the study of thermal pain in animals apply constant suprathreshold heat stimuli and measure the reflex latency of pain-avoiding reactions. The latency measured by these methods may greatly vary upon repeated measurements which is a major disadvantage concerning reliability. The presently introduced novel approach involves applying a slowly increasing thermal stimulus which allows determination of the noxious heat threshold i.e. the lowest temperature evoking pain-avoiding behaviour. An increasing-temperature hot plate and an increasing-temperature water bath are presented which are both suitable to determine the noxious heat threshold with high reproducibility. Acute thermal hyperalgesia models based on the drop of the heat threshold are also described for each equipment which proved to be highly sensitive to standard analgesics.