Objective: 

Pain threshold measurement is one of the most frequent methods to evaluate pain in experimental animals. In plantar test, pain threshold is measured as a latency to withdraw a limb in response to heat stimulation. Decrease of the threshold is traditionally interpreted as allodynia/hyperalgezia, while increase as analgesia. The aim of the present study was to demonstrate other possible interpretations.

Methods: 

The results of plantar test were analyzed under different pain states (neuropathic pain before and after the treatment, and visceral pain). Neuropathic pain was induced either by chronic constriction injury, or dorsal rhizotomy. The treatment of neuropathic pain consisted of electrical stimulation of the motor cortex. Visceral pain was induced by colorectal distension, and paw withdrawal latencies were analyzed alone and in correlation to behavioral pain score of visceral pain.

Results: 

It was shown (1) that both neuropathic and visceral pain evoked increase of pain thresholds in unaffected limbs; (2) that the increase of pain threshold was diminished after the treatment, and (3) that the pain score negatively correlated to changes of pain thresholds of the limbs.

Conclusions: 

It is concluded that increase in pain threshold of unaffected limb could be interpreted as the presence of heterotopic pain of neuropathic and/or visceral origin. Subsequent decrease of the thresholds after the treatment supports this interpretation. Further, the increase of pain thresholds of unaffected limb could be attributed to activation of diffuse noxious inhibitory control, since pain thresholds change negatively correlated to visceral pain score in colorectal distension model. Supported by SVV 262708/2011 and RG 0021620816.