Establishment of neck-incision pain model

The animal neck around the bilateral thyroid gland area was treated topically with depilatory cream 1 day before operation. After detecting the baseline thermal pain threshold, a 1.5-cm longitudinal incision was made along the midline of the neck under isoflurane (0.5–1% in oxygen) inhalational anesthesia using a table-top isoflurane anesthesia unit, Matrx VIP 3000 (Midmark Corporation, Orchard Park, NY). This was followed by repeated blunt dissection stimulation along the bilateral sternohyoideus around the thyroid gland regions for 30 min with a pair of forceps. The incision was then sutured in layers. Rats in the sham operation (control) group were similarly anesthetized but without incision and dissection stimulation.