Behavioral, pharmacological and molecular characterization of the saphenous nerve partial ligation: A new model of neuropathic pain

The saphenous partial ligation (SPL) model is a new, easily performed, rodent model of neuropathic pain that consists of a unilateral partial injury to the saphenous nerve. The present study describes behavioral, pharmacological and molecular properties of this model. Starting between 3 and 5 days after surgery, depending on the modality tested, animals developed clear behaviors indicative of neuropathic pain such as cold and mechanical allodynia, and thermal and mechanical hyperalgesia compared with naive and sham animals. These pain behaviors were still present at 1 month. Signs of allodynia also extended to the sciatic nerve territory. No evidence of autotomy or bodyweight loss was observed. Cold and mechanical allodynia but not thermal and mechanical hyperalgesia was reversed by morphine (4 mg/kg i.p.). The cannabinoid receptor agonist WIN 55,212-2 (5 mg/kg i.p.) improved signs of allodynia and hyperalgesia tested except for mechanical hyperalgesia. Gabapentin (50 mg/kg i.p.) was effective against cold and mechanical allodynia but not hyperalgesia. Finally, amitriptyline (10 mg/kg i.p.) failed to reverse allodynia and hyperalgesia and its administration even led to hyperesthesia. Neurobiological studies looking at the expression of μ opioid receptor (MOR), cannabinoid CB1 and CB2 receptors showed a significant increase for all three receptors in ipsilateral paw skin, L3-L4 dorsal root ganglia and spinal cord of neuropathic rats compared with naive and sham animals. These changes in MOR, CB1 and CB2 receptor expression are compatible with what is observed in other neuropathic pain models and may explain the analgesia produced by morphine and WIN 55,212-2 administrations. In conclusion, we have shown that the SPL is an adequate model that will provide a new tool for clarifying peripheral mechanisms of neuropathic pain in an exclusive sensory nerve.