Artificial ureteral calculosis in rats: behavioural characterization of visceral pain episodes and their relationship with referred lumbar muscle hyperalgesia

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Abstract
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 complexity of the visceral episodes and the extent of referred lumbar muscular hyperalgesia. As evidenced by non-stop video-tape recordings over 4-14 days, almost 98% of stone-implanted rats showed episodes similar to the writhing behaviour characteristic of noxious visceral stimulation 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 arbitrary scale on the basis of the combination of movements. Their number and duration decreased significantly, in a linear fashion, as time passed after the operation, so that they were mostly concentrated during the first 3 days. Number, duration and complexity of episodes were reduced by chronic treatment with morphine in a dose-dependent fashion. Stone-implanted rats displaying visceral episodes also showed hyperalgesia of the ipsilateral oblique musculature, as evidenced by a decrease in the vocalization threshold to electrical muscle stimulation, which was maximum on the first 3-4 days after implantation but lasted up to 10 days. The visceral episodes and the muscle hyperalgesia showed a strict relationship of interdependence: a significant, direct linear correlation was found between number and duration of episodes and degree of ipsilateral muscle hyperalgesia; such a correlation was also found between degree of complexity of episodes and tendency to also develop 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 function of the colic pain experienced.