Experimental animals
Adult male Sprague-Dawley rats (200–250g) were procured from the Shanghai Laboratory Animal Center at the Chinese Academy of Science. Rats were initially housed in cages within the number upper limit and had free access to food and water. The cages were 47.5cm in length, 35.0cm in width, and 22.0cm in height. Rats were maintained on a 12/12 h light/dark cycle (lights on 7:30 AM–7:30 PM). Animals were placed in individual cages after surgery. This study was carried out in compliance with the ARRIVE guidelines. All experimental protocols were approved by the Animal Care and Use Committee of Shanghai Sixth People’s Hospital affiliated with Shanghai Jiao Tong University (ethics certificate number:20160226) and performed as per the National Institutes of Health guidelines for the Care and Use of Laboratory Animals.
Animal model
The animal model of tibial neuroma transposition (TNT) was adopted as described by Dorsi et al 12. Rats were deeply anesthetized using sodium pentobarbital via intraperitoneal injection (50mg/kg). The tibial nerve of the rat was dissociated from the proximal 8 mm of the heel bifurcate to the distal 1mm of the foot bifurcate, then ligated at the proximity of the plantar bifurcation with a 6-0 silk suture. Through a subcutaneous tunnel, the transected nerve terminal was gently pulled to a part 2 cm superior to the lateral malleolus and fixed beneath the skin 8-10mm superior to the lateral malleolus. In the sham-operated rats, only the tibial nerve was dissociated and kept intact. A small piece of connective tissue was ligated and passed through the subcutaneous tunnel as per the method described above.
Experiment 1: To evaluate the effect of sound stimulation on neuroma pain, rats were randomly assigned to one of the three groups (n=6 for each group): Group 1 (GC), TNT with white noise (n = 6); group 2(GM), TNT with music (n = 6); and group 3(GH), TNT (n = 6).
White noise and music stimulation were administered once a day after surgery until the end of the study (42nd day). Pain behavioral tests were carried out before surgery and on the 3rd, 10th, 14th, 21st, 28th, 35th, and 42nd days after surgery.
Experiment 2: To investigate the role of leptin in pain behaviors modulated by white noise and sound stimulation.
At the end of the observation period, we analyzed the histological samples of blood, spinal cord, and prefrontal cortex.
Music stimuli
A piece of traditional Chinese music “LiangZhu” played by erhu with about 1000Hz and 50dB was stored in an audio player positioned in the middle of the cage and left a quiet surrounding. After the operation, the music was daily played in the room, where the GM group TNT rats lived for 30 minutes at noon. GC was played white noise, and other operating conditions were similar to those of GM. GH did not play any sound, and the ambient noise was approximately 45dB.
Evaluation of pain behaviors
1. Von Frey's up-down method
The behavioral tests were performed in a quiet and appropriate room by an experienced observer blinded to the different treatments. The rats were allowed to adjust to the testing environment for 20 minutes before testing. The evaluation of the neuroma pain approach described by Dorsi et al was used with minor modifications. The trial comprised 10 repetitive applications of a von Frey filament (15g for 1-2 s) to the neuroma position at intervals of 1 to 2 minutes. The paw withdrawal threshold was measured using the von Frey up-down method described by Chaplan et al13. The test began with a middle filament in the series (2.0g). An ascending series of von Frey filaments of logarithmically incremental force (0.6, 1.0, 1.4, 2.0, 4.0, 6.0, 8.0, 10.0, and 15.0g; Stoelting) were applied to the lateral plantar surface of the hind paw. A positive response was a rapid withdrawal and/or licking of the paw immediately after stimulus application. Whenever a positive or negative response occurred, the testing proceeded for five more stimuli after the first change in response, and the pattern of responses was converted to a 50% paw withdrawal threshold (50% PWT) as described by Sun et al 4.
2. Paw withdrawal latency to thermal heat
According to the Hargreaves method, the rats were placed in plexiglass chambers, and a radiant beam of light was applied to the hind paw through a glass floor 14. Paw withdrawal latencies were recorded in duplicate per paw. A third measurement would be taken if the second latency recorded was not within 64 seconds of the first. The values of the two closest latencies were averaged to establish the overall latency to withdrawal.
Histology analysis
Rats were deeply anesthetized using an intraperitoneal injection of 100mg/kg sodium pentobarbital and blood was collected by cardiac puncture. Subsequently, the spinal cord and prefrontal cortex of all rats were harvested and immediately stored at -80℃, to be processed for use in real-time quantitative PCR or Western blotting. The same was done with blood samples.
1. Enzyme-linked immunosorbent assay
After the pain behavior test, blood was collected 42 days after the operation. The plasma was separated by centrifugation at 5000g for 5 min and the supernatants were obtained for subsequent protein analysis. Rat Leptin ELISA kit (Crystal Chem Inc., Chicago, IL, USA) was used to analyze the leptin levels in plasma.
2.Western blotting
In tissue protein extraction buffer (Thermo Fisher), the obtained spinal cord or prefrontal cortex was homogenized with a protease inhibitor cocktail (Roche). The BCA Protein Assay Kit was used to establish the protein concentrations of samples. Thereafter, the proteins were separated using the SDS-PAGE, then transferred to PVDF membranes (Merck Millipore). After blocking with 5% bovine serum albumin at room temperature for 1h, the membranes were incubated overnight at 4℃ with the following primary antibodies; anti-leptin receptor (1:2000, rabbit polyclonal IgG; Invitrogen), anti-β actin (1:1000, mouse monoclonal IgG; Santa Cruz) and anti-β tubulin (1:2000, mouse monoclonal IgG, HuaAn Biotechnology) antibodies. Afterward, the membranes were incubated with HRP-conjugated goat anti-rabbit or goat anti-mouse secondary antibodies (1:5000; HuaAn Biotechnology) at room temperature for 1h. The membranes were rinsed using Tris-buffered saline and 0.1% Tween 20 between the above steps. Eventually, blots were detected via enhanced chemiluminescence substrate solution (ThermoFisher) using the ImageQuant Ai600 (General Electric Co., Boston, MA, USA) and quantified using the ImageJ software (version 2.0.0, National Institutes of Health, Rockville, MA, USA).
Statistical analysis
Statistical analyses were performed using statistical packages (GraphPad Prism 5, GraphPad, San Diego, CA). T-test was used to compare the two groups, whereas a two-way ANOVA test was used to compare the three groups. A P value of less than 0.05 (P<0.05) was considered statistically significant.