Reciprocal inhibitory influences of heterotopic nociceptive stimuli have been known for centuries (counter-irritation phenomena). To assess whether such phenomena could be measured objectively in a chronic pain model, we have investigated the writhing behaviour induced by a viscero-peritoneal nociceptive stimulus (i.p. acetic acid) in arthritic rats. The latter was induced by s.c. injection of mycobacterium butyricum suspended in oil into the base of the tail and experiments carried out at various (1-9 weeks) post-inoculation periods. To assess the arthritis-induced hyperaesthesia, the threshold for struggle triggered by a calibrated pressure applied on an inflamed hindpaw (Randall-Selitto test) was measured before each experiment. A clear relationship was observed between the two parameters during the evolution of the disease. In the most severe phase (2nd to 5th week post-inoculation), when arthritis-induced hyperaesthesia was most pronounced (50% reduction of the threshold for struggle), writhing behaviour was strongly reduced (80%). At that time, writhing behaviour was enhanced by naloxone (0.4 mg/kg i.v.). In the following period (5-9 weeks), the progressive decrease of hyperaesthesia was correlated with a recovery of the writhing behaviour. We conclude that in this chronic pain model, the behavioural reaction to a viscero-peritoneal nociceptive stimulus is impaired. Endogenous opioid systems could be involved in this phenomenon. Such heterotopic inhibitory processes could be relevant to some paradoxical clinical observations such as the masking of a pain by the experience of pain at another locus. © 1986.
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