2.4 Neuropathic pain model
The spared nerve injury (SNI) model of neuropathic pain was used. This
partial denervation model involves ligation and excision of two of the
three distal branches of the sciatic nerve. Traditional SNI involves
ligating the common peroneal and tibial nerves and sparing the sural
nerve (Decosterd & Woolf, 2000). We used
a variation in which the tibial nerve was spared (SNIt)
and the common peroneal and sural nerves were ligated and cut. Baseline
responses to mechanical stimuli were determined before surgery. Then
mice were deeply anesthetized with isoflurane 1.5-4.0% (Abbott
laboratories, North Chicago, IL), and toe pinch was used to verify that
mice were under the surgical plane of anesthesia. Under aseptic
conditions, the sciatic nerve was exposed at mid-thigh level through the
biceps femoris muscle. The common peroneal and sural nerves were tightly
ligated with 6.0 silk and axotomized distal to the ligation. Care was
taken to avoid any contact with or stretching of the intact tibial
nerve. Muscle and skin were closed in two layers with 4.0 silk and
staples, respectively. In sham surgeries, the branches of the sciatic
nerve were exposed but not ligated. The animals were returned to their
cages after surgery and monitored during recovery. SNItproduces early (<24 hr) and prolonged (>12 weeks)
mechanical hypersensitivity and neuropathic pain predominantly in the
area innervated by the tibial nerve
(Tallarida, 2006). Behavioral experiments
were initiated 2 days after surgery.