Pharmacological characteristics of the hind paw weight bearing difference induced by chronic constriction injury of the sciatic nerve in rats

AimsWe examined the possible involvement of spontaneous on-going pain in the rat chronic constriction injury (CCI) model of neuropathic pain.Main methodsThe development of weight bearing deficit, as an index of spontaneous on-going pain, was investigated in comparison to that of mechanical allodynia in CCI rats. We also examined the effects of morphine and a gabapentin analogue (1S, 3R)-3-methyl-gabapentin (3-M-gabapentin) on both the CCI-induced weight bearing deficit and mechanical allodynia.Key findingsRats with CCI demonstrated a significant reduction in weight bearing of the injured limb with a peak at a week post-operation, which was followed by a gradual recovery for over 7 weeks. The time course of development and recovery of CCI-induced weight bearing deficit appeared to follow that of foot deformation of the affected hind limb. CCI also evoked mechanical allodynia that was fully developed on a week post-operation, but showed no recovery for at least 8 weeks. 3-M-gabapentin significantly inhibited CCI-induced mechanical allodynia, but not weight bearing deficit, at 100 mg/kg p.o. Likewise, morphine was without significant effect on CCI-induced weight bearing deficit at the dose (3 mg/kg, s.c.) that could almost completely inhibit mechanical allodynia, whereas it inhibited both mechanical allodynia and weight bearing deficit at 6 mg/kg, s.c.SignificanceThe present findings suggest that CCI-induced weight bearing deficit is not a consequence of mechanical allodynia, but is attributable to spontaneous on-going pain. The rat CCI model of neuropathic pain thus represents both spontaneous on-going pain and mechanical allodynia.