Healthy, specific pathogen free (SPF) grade male Wistar rats (2 months old; 250±20 g) were purchased from the experimental animal center and maintained in the SPF Xi'an Medical University Animal Experimental Center. The animals were maintained at 21±1°C, at a relative humidity of 50–70% and a 12 h day/night cycle. All procedures were approved by the Animal Ethics Committee of The First Affiliated Hospital of Xi'an Medical University.

Pentobarbital sodium was purchased from Zhpharma Ltd. PVDF membranes were purchased from Pall Life Sciences. Western blotting-related chemical reagents were purchased from the Beyotime Institute of Biotechnology and enhanced chemiluminescence (ECL) reagents were purchased from GE Healthcare. Rabbit anti-BDNF and rabbit anti-NF-κB antibodies, as well as sheep anti-rabbit horseradish peroxidase (HRP)-labeled IgG secondary antibodies were purchased from Abcam, Inc. Methylprednisolone was purchased from Sigma-Aldrich (Merck KGaA). Tumor necrosis factor (TNF)-α and IL-2 ELISA kits were purchased from R&D Systems, Inc., and the Caspase 3 Activity Assay kit was purchased from Cell Signaling Technology, Inc. Microsurgical instruments were purchased from Suzhou Medical Instrument Factory. The RNA extraction kit and reverse transcription kit were purchased from Applied Biosystems (Thermo Fisher Scientific, Inc.). The Multi-Parameter Monitor Small Animal Physiology Monitor was purchased from Shanghai Yuken Instrument Company, and the Amp PCR System 2400 DNA Amplification System was purchased from PerkinElmer, Inc. The Imark microplate reader was purchased from BD Biosciences. Key-point evoked potential meters, electromyographs and Magpro magnetic stimulators were purchased from Dantec Dynamics. The UWM-02 Ultrashort Wave Therapy Instrument was purchased from Marubeni Corporation.

The rats were randomly divided into four groups: SCI; methylprednisolone (300 mg/kg); high-frequency electrotherapy (treated with microwave irradiation from 7 cm at 10 w for 10 min per day); and combination (treated with electrotherapy combined with 300 mg/kg methylprednisolone). N=10 for each group.

According to current literature, the rat SCI model was established using the modified ALLEN struck method (14). Following anesthetization using an intraperitoneal injection of 30 mg/kg sodium pentobarbital, the rats were immobilized on the operating table. The vertebral plates and spinous processes of the thoracic vertebrae (T9-T11) were removed. The center was set at the T10 spinal cord and a circular area with a diameter of approximately 4 mm was determined as the lesion area. According to the physiological curvature of the dorsal spinal cord of rats, pre-bent plastic flexion pads of 3 mm in length, 2 mm in width, and 1 mm thick were prepared. The pads were placed outside of the spinal cord in the T10 area. Using the modified ALLEN's striking device, the center was set in the median posterior of the spinal cord. The striker rod moves freely through the sleeve at a height of 5 cm and directly hits the plastic gasket. Signs of successful modeling include the body and lower extremities retracting and flapping, in additional to tail swing. The surgical incision was closed and antibiotics (Baytril^®; Bayer AG; 4 mg/kg subcutaneous) were routinely administered.

Spinal cord tissues were collected as previously described (15). The rats were sacrificed using sodium pentobarbital to effect, and transcardially perfused with 0.1 M phosphate-buffered saline (PBS, pH 7.4) followed by 4% paraformaldehyde (in PBS). To maintain consistency, each spinal cord was rostrally transected at the T6 spinal root, and a 3 cm segment of spinal cord was immediately dissected and fixed in paraformaldehyde for 2 h at 4°C, followed by the relevant analysis.

On the 20th day after surgery, the BBB score was measured to evaluate the recovery ability of the joints and lower limbs. The score was 0–21 points; 0 points indicated no visible hind limb movement, and 21 points indicated continuous palmar movement, continuous coordination gait, continuous toe grip, the parallel position of the active paw to the body, continued trunk stability and tail tilt. A higher BBB score indicates improved recovery (12).

At 20 days post-surgery, evoked potentials and electromyography were used to detect SEP and MEP. The rats were anesthetized and stabilized at the right iliac and the Achilles tendon. A DC square wave pulse current with a wave width of 0.2 msec and a frequency of 2 Hz was selected. The test time was 20 msec, and a superposition range of ±200 msec was recorded. The current waveform P1 and an incubation period of N1 were recorded. The main electrode was placed in the muscle belly of the right gastrocnemius and the action potential was recorded using a magnetic stimulator with a test time of 0.2 msec.

On the 20th day after surgery, a total of 5 ml blood was collected via the tail vein. The blood was centrifuged at 1,200 × g rpm for 15 min, and the serum was collected and stored at −80°C.

TNF-α and IL-2 expression levels were detected by ELISA according to the manufacturer's protocol. A total of 50 µl diluted standard or sample was added in triplicate to a 96-well plate and incubated at 37°C for 1 h. After washing five times, 50 µl enzyme-labeled reagent was added to each well and incubated at 37°C for 30 min. The plate was subsequently treated with 50 µl each of color agent A and B at 37°C for 10 min. Finally, 50 µl stop solution was added and the absorption at 450 nm was recorded using a microplate reader. The concentration of the sample was calculated and linear regression was compared with the absorbance value of the standard.

Caspase 3 activity in spinal cord tissue was determined using the Caspase 3 Activity Assay kit according to the manufacturer's protocol. Briefly, the cells were enzymatically digested and centrifuged at 600 × g for 5 min (4°C). The cells were subsequently treated with RIPA lysis buffer (Thermo Fisher Scientific, Inc.) on ice for 15 min and centrifuged at 20,000 × g for 5 min (4°C). Absorbance was detected at 405 nm following the addition of 2 mM Ac-DEVD-pNA to each test sample.

Total RNA was extracted from the tissue samples using TRIzol^® reagent (Thermo Fisher Scientific, Inc.) and reverse transcribed using a High-Capacity cDNA Reverse Transcription kit (Thermo Fisher Scientific, Inc.) per the manufacturer's protocol. The primers for qPCR were designed by Primer Premier 6.0 (Premier Biosoft International) and synthetized by Invitrogen; Thermo Fisher Scientific, Inc. (Table I). qPCR was performed using SYBR Green (Thermo Fisher Scientific, Inc.) and the thermocycling conditions were as follows: 35 cycles at 92°C for 30 sec, 58°C for 40 sec, and 72°C for 35 sec. The 2^−DDCq method (16) was used to calculate relative expression levels in reference to GAPDH.

The sample tissues were lysed using RIPA buffer and quantified using a bicinchoninic acid assay. The proteins (40 µg/lane) were separated by SDS-PAGE using a 10% gel, and transferred to a PVDF membrane at 160 mA for 1.5 h. The membrane was blocked with 5% skim milk at room temperature for 2 h, and subsequently incubated with primary antibodies against BDNF (1:1,000; cat. no. ab226843), NF-κB (1:1,500; cat. no. ab16502) and β-actin (1:2,000; cat. no. ab8227) at 4°C overnight. After washing three times with PBS-Tween, the membrane was incubated with a secondary horseradish peroxidase-conjugated antibody (1:5,000; cat. no. ab6721) at room temperature for 30 min. The proteins were then visualized using ECL reagent. Each experiment was repeated four times and protein expression was quantified using Quantity One software (version 4.6.8; Bio-Rad Laboratories, Inc.).

All data analysis was performed using SPSS 19.0 software (IBM Corp.). The data are presented as the mean ± standard deviation and compared by one-way ANOVA and Newman-Keuls multiple comparisons analysis. P<0.05 was considered to indicate a statistically significant difference.

BBB score analysis was performed to evaluate the impact of methylprednisolone combined with high frequency electrotherapy on SCI rats. All treatment groups exhibited significantly elevated BBB scores compared with the untreated, SCI group (P<0.05). However, the combination group exhibited more significant effects on SCI (P<0.05; Fig. 1). This suggested that methylprednisolone combined with high frequency electrotherapy improved the pathological process of SCI and promoted recovery.

SEPs and MEPs were analyzed to assess the influence of methylprednisolone combined with high frequency electrotherapy on SCI rats. All treatment groups displayed obviously improved SEPs and MEPs compared with the SCI group (P<0.05), though the combination group exhibited the most significant effects on SCI (P<0.05; Table II). These results indicated that methylprednisolone combined with high frequency electrotherapy alleviated the pathological effects of SCI and promoted recovery.

RT-qPCR and western blotting were used to determine BDNF mRNA and protein expressions levels in the spinal cord tissues of rats. All treatment groups presented with markedly increased BDNF mRNA and protein expression levels compared with the SCI group (P<0.05). The combination group exhibited more significant effects on SCI (P<0.05) (Fig. 2 and 3).

RT-qPCR and western blot analysis were used to evaluate the expression levels of NF-κB mRNA and protein in rat spinal cord tissues. The treatment groups showed considerably reduced expression levels of NF-κB mRNA and protein compared with the untreated, SCI group (P<0.05), and the combination group exhibited the most significant effects on SCI (P<0.05; Fig. 4 and 5).

The effects of methylprednisolone combined with high frequency electrotherapy on serum TNF-α and IL-2 expression level were assessed by ELISA. The treatment groups revealed a reduction in serum TNF-α and IL-2 expression levels compared with the SCI group (P<0.05); furthermore, the combination group exhibited a more significant effect on SCI (P<0.05; Fig. 6) revealing that methylprednisolone combined with high frequency electrotherapy inhibited cytokine release, alleviating inflammatory damage.

Caspase 3 activity was detected in the spinal cord tissues. Each treatment group displayed significantly decreased caspase 3 activity compared with the SCI group (P<0.05). The combination group exhibited more significant effects on SCI (P<0.05; Fig. 7), which collectively indicated that methylprednisolone combined with high frequency electrotherapy suppressed caspase 3 activity to regulate apoptosis.