ARTIFICIAL URETERAL CALCULOSIS IN RATS - BEHAVIORAL CHARACTERIZATION OF VISCERAL PAIN EPISODES AND THEIR RELATIONSHIP WITH REFERRED LUMBAR MUSCLE HYPERALGESIA

In a rat model of artificial ureteral calculosis, the aim of the study was to characterize the behavioural manifestations of direct visceral pain and to evaluate the relationship between number, duration and co mplexity of the visceral episodes and the extent of referred lumbar mu scular hyperalgesia. As evidenced by non-stop video-tape recordings ov er 4-14 days, almost 98% of stone-implanted rats showed episodes simil ar to the writhing behaviour characteristic of noxious visceral stimul ation in animals. From one rat to another, these episodes varied from very few (1-3) to a very high number (+/- 60), lasted a few minutes to over 45 min and were of variable complexity, as evaluated via an arbi trary scale on the basis of the combination of movements. Their number and duration decreased significantly, in a linear fashion, as time pa ssed after the operation, so that they were mostly concentrated during the first 3 days. Number, duration and complexity of episodes were re duced by chronic treatment with morphine in a dose-dependent fashion. Stone-implanted rats displaying visceral episodes also showed hyperalg esia of the ipsilateral oblique musculature, as evidenced by a decreas e in the vocalization threshold to electrical muscle stimulation, whic h was maximum on the first 3-4 days after implantation but lasted up t o 10 days. The visceral episodes and the muscle hyperalgesia showed a strict relationship of interdependence: a significant, direct linear c orrelation was found between number and duration of episodes and degre e of ipsilateral muscle hyperalgesia; such a correlation was also foun d between degree of complexity of episodes and tendency to also develo p a contralateral muscle hyperalgesia. By applying the results of the study to the interpretation of human pathology, referred lumbar muscle hyperalgesia from ureteral calculosis would appear to be a strict fun ction of the colic pain experienced.