Behavioral Manifestation of Neuropathic Pain Following Sciatic Nerve Section in Rats

The present study was performed to observe the time course of behavioral signs of painful sensations in sciatic neurectomy animal model and to test the effects of sympathectomy and saphenous nerve section on these behavioral signs. Sciatic nerve was ligated and cut at the mid-thigh level under gaseous anesthesia. The application of von Frey filaments to the medial plantar surface of foot revealed weak and long-lasting mechanical allodynia (until end of test period, 20 weeks PO). Acetone application to the plantar surface of foot was used ti measure the sensitivity to cold stimulation. Cold allodynia which is interpreted as increased response to acetone application developed fairly well and lasted the end of test period (20 weeks PO). The cumulative duration of foot lifts off neutral or cold plate was used to test spontaneous, ongoing pain and was increased until 16 weeks PO and 20 weeks PO respectively. These results suggest that sciatic neurectomy which has been widely used as chronic pain model shows behavioral signs suggsting painful sensations except autotomy, which has been used as index of pain in experimental animal. Surgical sympathectomy performed 1 week after sciatic neurectomy partially reduced the behavioral signs of mechanical allodynia and cold allodynia, suggesting behavioral changes developed following section of sciatic nerve was partially sympathetic dependent. Saphenous nerve section 1 week after sciatic neurectomy almost completely reduced mechanical allodynia and cold allodynia, but did not change spontaneous, ongoing pain. These results suggest that evoked responses such as mechanical and cold allodynia are mediated by saphenous nerve activity and activating and/or maintaining mechanisms of spontanous, ongoing pain and evoked pain may be different.