Pathogenesis of sciatic pain - A study of spontaneous behavior in rats exposed to experimental disc herniation

Study Design. A new way to study pain in experimental animals without handling of the animals and based on registration of spontaneous behavior using video recordings. Objectives. To evaluate if experimental disc herniation in the rat may induce changes in spontaneous behavior. Summary of Background Data. The knowledge regarding the basic pathophysiologic mechanisms of sciatica has increased dramatically during the last decade. However, studies have mainly assessed nerve dysfunction rather than pain. Existing methods to study pain generally comprise a certain amount of handling and registration of changes in sensory thresholds. In the present study we introduce a new way to assess pain that focuses on changes in behavior rather than on changes in thresholds. Methods. Forty rats were divided equally into four experimental series: sham exposure of the left L4 dorsal root ganglion, exposure of the left L4 dorsal root ganglion and incision of the L4-L5 disc, exposure and slight displacement of the left L4 dorsal root ganglion, and combination of disc incision and displacement. The rats were videotaped the day before surgery and on day 1, 3, 7, 14, and 21 after surgery. Spontaneous behavior was categorized into 10 behaviors and recorded during 20 minutes of observation. Results. Disc incision and displacement per se did not induce any behaviors different from that observed in the sham-operated group. In the series with the combination of disc incision and displacement there was increased focal pain, seen as increased lifting of the hind paw on the operated side and increased rotation of the head toward the operated side. This pain pattern was most pronounced the day after surgery. Fourteen days after surgery there were no detectable differences in behavior between this group and the sham group. At day 21 after surgery, however, another picture of increased immobility and decreased locomotion was seen in this group, possibly indicating more generalized pain. Conclusions. The study demonstrates that it is possible to detect changes in spontaneous behavior after experimental disc herniation. However, such changes may only be seen if disc incision and slight mechanical deformation are combined. This is in agreement with previous clinical and experimental observations. The present model allows for convenient assessment of pain in a way that focuses on spontaneous behavior rather than changes in pain thresholds and that reduces the interference of the researcher and environment on the outcome of the assessment.