[Reduction of the behavioral response to a viscero-peritoneal nociceptive stimulation during the development of experimental polyarthritis in the rat].

Writhing behaviour induced by a viscero-peritoneal nociceptive stimulus (i.p. acetic acid) has been investigated during the development of arthritis experimentally induced in rats by s. c. injection of Mycobacterium butyricum; experiments were carried out at various (1-9 weeks) post-inoculation periods. In the most severe phase (2-5 weeks post-inoculation), when arthritis-induced hyperaesthesia was most pronounced, writhing behaviour was strongly reduced (80%). In the following period, the progressive decrease of hyperaesthesia was contemporaneous 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. 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 (i. e. counter-irritation phenomenon).