Paw withdrawal thresholds and persistent hindlimb flexion in experimental mononeuropathies.

Hypersensitivity of the foot produced by a number of sciatic mononeuropathies was assessed and compared. A new tool was used, the strain-gauge algometer, that delivers a noxious stimulus and gives a direct measurement of the force for paw withdrawal. In addition, we report observations of another alteration of the flexion reflex, persistent hindlimb flexion. The mean mechanical threshold for naive rats was 5.9 +/- 0.97 centinewton (standard deviation). A superficial surgical procedure had no effect on mechanical sensitivity. Sham surgeries and a surgery in which a silicone pellet was glued to the sciatic nerve produced moderate increases in mechanical sensitivity. Interventions that produced the greatest reductions in thresholds were carrageenan neuritis, complete Freund's adjuvant neuritis, and the chronic constriction injury (CCI) model. Mechanical thresholds returned to baseline in 2 weeks in all groups. Neuropathic behaviors (licking and holding the paw after the stimulus) were observed more frequently in the CCI group. Persistent hindlimb flexion was only observed in the CCI group. The results support that midaxonal inflammation is sufficient to induce hyperalgesia. The strain-gauge algometer proved to be efficient and reliable, and calculations support that used as described in this report one can demonstrate changes in paw withdrawal thresholds as small as 15%.