β-Elemene Improves Morphine Tolerance in Bone Cancer Pain via N-Methyl-D-Aspartate Receptor 2B Subunit-Mediated μ-Opioid Receptor.

Background: Improving morphine tolerance (MT) is an urgent problem in the clinical treatment of bone cancer pain. Considering that β-Elemene is widely used in the treatment of cancer pain, we explored the effects and mechanism of β-Elemene in preventing MT of bone cancer pain. Method: Bone cancer pain and chronic MT rat model was established by injecting MADB106 cells and morphine (10 mg/kg). SH-SY5Y cells were treated with morphine (10 μg/mL) for 48 h to establish a cell model. The mechanical withdrawal threshold and thermal withdrawal latency of rats were detected by mechanical allodynia and thermal hyperalgesia tests, respectively. The protein expressions of μ-opioid receptor (MOPR), cyclic adenosine monophosphate (cAMP), N-methyl-D-aspartate receptor subunit 2B (NR2B), phosphorylated-calmodulin-dependent protein kinase II (p-CaMKII), and CaMKII were detected by western blot. The viability of SH-SY5Y cells was determined by the cell counting kit-8 assay. cAMP content in SH-SY5Y cells was measured by a LANCE cAMP kit. Result: Animal experiments showed that MT strengthened over time, while increased β-Elemene dosage alleviated MT. The viability of SH-SY5Y cells was down-regulated by high-dose β-Elemene. In the rat and cell models, long-term morphine treatment decreased the expression of MOPR and increased the cAMP and NR2B expressions and p-CaMKII/CaMKII, while β-Elemene and siNR2B counteracted the effects of morphine treatment. In addition, siNR2B reversed the effects of β-Elemene on related protein expressions and cAMP content in the cell model.
Conclusion: β-Elemene improved MT in bone cancer pain through the regulation of NR2B-mediated MOPR.

1. Introduction

Bone cancer pain is a type of refractory and complex pain associated with bone metastasis of malignant tumors, often occurring in the advanced stage of breast cancer, lung cancer, and prostate cancer [1]. Many cancer patients have pain as their first symptom in the early stages of cancer. As the progress of cancer, the degree of pain was sharply increased and severely affected the quality of life of cancer patients [2]. Cancer pain is characterized by hyperalgesia and allodynia. Hyperalgesia refers to enhanced sensitivity to noxious stimuli, while allodynia is the onset of pain by normally nonpainful stimuli such as light mechanical or cold stimuli [3]. Cancer pain seriously affects the mental and physical health of patients in advanced stages of cancer. Therefore, relieving cancer pain is extremely important to improve their quality of life and prolong survival.

Current clinical treatment options for advanced cancer pain are divided into two modalities, pharmacotherapy and non-pharmacotherapy [4, 5]. Non-pharmacological treatments for cancer pain include radiation, radioisotope therapy, and surgical treatment. The former can damage surrounding healthy tissue, and the recurrence rate is high, while surgical treatment often represents a huge blow to patients and similarly affects their quality of life [5]. Pharmacotherapy is currently the most mainstream and most accessible regimen for the treatment of cancer pain [4]. Morphine, as an opioid drug, is the first choice and the most commonly used analgesic drug for cancer pain [6]. However, the long-term use of morphine can lead to adverse events such as nausea, vomiting, vertigo, constipation, and even morphine tolerance (MT). MT is manifested by a gradual decrease or even disappearance of the analgesic effect for the same initial dose, and severe cases also develop hyperalgesia, which requires continuously increasing doses to achieve the same analgesic effect as before [7], seriously affecting the use of morphine in the clinical treatment of cancer. Therefore, it is urgent to find a method to reduce MT.

In recent years, studies have demonstrated that the occurrence of MT was closely related to the number and activity of μ-opioid receptor (MOPR), the main receptor of morphine [8]. Previous studies have shown that MOPR positive rate and its protein expression were downregulated in the dorsal root ganglia of rats with bone cancer pain [9]. In addition, the expression of MOPR was significantly downregulated in the rat's locus coeruleus neurons after prolonged morphine treatment, suggesting that the mechanism of MT may be related to the downregulation of MOPR expression [8]. N-methyl-D-aspartate (NMDA) receptors, the glutamate-specific ion channel receptors, play an important role in transmitting nociceptive information and are widely distributed in the spinal cord, especially in the superficial dorsal horn [10]. Some studies have found that NMDA receptors are involved in the development of MT. NMDA receptors are composed of NR1, NR2 (A, B, C, and D), and NR3 (A and B) [11]. Bajo et al. found that NMDA receptors were involved in the tolerance process caused by long-term morphine action, but the subunits had different roles, and NR2B was influential in the occurrence and maintenance of injury induced by morphine. They also discovered that in the MT rats, NR2B expression in the nucleus accumbens and central amygdala increased [12]. Moreover, other scholars proved that suppressing NR2B improved analgesic tolerance to morphine [13].

β-Elemene is a compound found in Curcuma wenyujin, a Chinese herbal medicine, which has shown good anticancer activity in many tumor treatments [14]. Studies have shown that β-Elemene exerts an antitumor effect by inhibiting the proliferation and inducing apoptosis of tumor cells [15]. For example, β-Elemene induces apoptosis in ovarian cancer cells by activating the STAT-NF-κB pathway [16]. Moreover, the available literature demonstrates that β-Elemene has been proved to alleviate bone cancer pain in rats by regulating the N-methyl-D-aspartate receptor 2B subunit (NR2B) [17], which plays a key role in the progression of MT [18]. However, the role of β-Elemene in MT of bone cancer pain has not been clarified and needs further exploration.

In this study, we detected the MT-related protein expressions in bone cancer pain-MT model rats and cells and explored the effects of β-Elemene on MT of bone cancer pain and its possible molecular mechanism associated with NR2B, hoping to provide a new basis for clinical bone cancer pain-MT research and treatment.

2. Materials and Methods

2.1. Animals and Ethics Statement

A total of 30 SPF grade Wistar rats (250 g ± 15 g) were purchased from Vital river, China, and raised in normal conditions. The light duration was provided at a normal circadian rhythm, the room temperature was maintained at 21 ± 0.5°C, and humidity at 45-50%. The rats were allowed to eat and drink freely, and the experiment began about 3 days after adaptation. All animal experiments were performed at the Zhejiang Cancer Hospital. The study protocol was approved by the Committee of Experimental Animals of Zhejiang Cancer Hospital (LY19H290001) and was performed in accordance with the guidelines of the China Council on Animal Care and Use. Special care was given to minimize the pain and discomfort of the animals.

2.2. Cell Culture

Rat breast cancer cell line MADB106 (CL-0148) was obtained from Procell (China) and was cultured in DMEM medium (PM150210, Procell) with 10% fetal bovine serum (FBS, 164210-500, Procell) and 1% penicillin-streptomycin solution (P/S, PB180120, Procell). SH-SY5Y human neuroblastoma cells (CL-0208, Procell, China) were cultured in MEM/F12 medium (PM151220, Procell) supplemented with 15% FBS and 1% P/S. All cells were cultured at 37 °C with 5% CO2.

2.3. Bone Cancer Pain-Chronic MT Model

A bone cancer pain model was established using the rats following previously described methodologies [19, 20]. Briefly, pentobarbital sodium (P-010, Supelco, USA) at a dose of 60 mg/kg was intraperitoneally injected into rats. After successful anesthesia, with their abdomen downward, the left hind limb was punctured with a 10 mL syringe; then a 10 μL micropipette was used to enter the tibial bone marrow cavity, and 3 μL of MADB106 cells containing 5 × 103 cells were slowly injected. Thereafter, the pinhole was sealed with bone wax, the wound was cleaned, and the skin was sutured.

Establishment of chronic MT model: 10 days after the inoculation of tumor cells in rats, a subcutaneous injection of morphine (10 mg/kg, M9524, Supelco, USA) was given once a day for 10 days [21]. The SH-SY5Y cells were treated with 10 μg/mL morphine for 48 h to establish the MT cell model [22].

2.4. Drug Administration and Grouping

At the same time as morphine administration from the tenth day after surgery, β-Elemene (63965, Supelco, USA) was injected into the rats intraperitoneally [23], dissolved in 200 μL of a 1 : 1 volume ratio mixture of DMSO and PBS. The rats were randomly divided into 5 groups, with 6 mice in each group. The rats in H-β-Elemene, M-β-Elemene, and L-β-Elemene group were separately treated with 2.8 mg/kg, 1.4 mg/kg, and 0.7 mg/kg β-Elemene, once a day for ten consecutive days. For the control group, the rats were treated with saline tibial injections and intraperitoneal injections using equal volumes of DMSO. In the MT group, the model rats were treated with an equal volume of DMSO intraperitoneally.

The SH-SY5Y cells were divided into 5 groups, and part of the groups was treated with β-Elemene (5 μg/mL) [24]: control group, the cells were cultured as usual; MT group, the cells were treated with 10 μg/mL morphine for 48 h; β-Elemene group, morphine and β-Elemene were added at final concentrations of 10 μmol/L and 5 μg/mL to act on the cells for 48 h, respectively; β-Elemene+NMDA group, after the cells were treated with 10 μmol/L morphine and 5 μg/mL β-Elemene, N-methyl-D-aspartic acid (NMDA, M3262, Sigma-Aldrich, USA) was added as an NR2B agonist at the intervention concentration of 10 μg/mL for 24 h; and siNR2B group, after the transfection of siNR2B, the cells were treated with 10 μg/mL morphine for 48 h.

2.5. Mechanical Allodynia Test

All rats were acclimatized to the new environment for 30 min before the tests. The mechanical withdrawal threshold of the rats was analyzed before modeling and after drug administration each day in the morning during the drug treatment period. The rats were placed on a wired mesh inside a Plexiglas compartment (20 × 20 × 30 cm), and different Von Frey filaments (Stoelting, USA) in scores from 0-80 g were applied to the plantar soft tissue of the hind paw to determine the withdrawal threshold. Each filament was pressed to the rat's left plantar 3 times at intervals of 5 s, each time for 1 s, and if the rat responded 2 consecutive times (paw withdrawal), the pressure was determined as the response threshold, and the test stopped. If a rat did not respond to the filament (80 g), it was considered the response threshold.

2.6. Thermal Hyperalgesia Test

Thermal hyperalgesia was detected by thermal withdrawal latencies of rats using a hot plate apparatus in a plastic cylinder (Technology & Market CORP, China). The rats were individually placed on the hot plate (52 °C), and latency was defined as the time elapsed before the rat licked a hindpaw or jumped. A threshold time of 50 s was set to prevent tissue damage.

2.7. Transfection

SiRNA targeted NR2B (siNR2B, A09001, target sequence: 5′-CAGAAGAATGGTACAAATCCAAG-3′) was purchased from GenePharma Company (China); then, siNR2B was transfected into the SH-SY5Y cells, which was achieved using Lipo6000 transfection reagent (C0526, Beyotime, China). Briefly, about 3 × 105 cells/well were seeded onto 6-well plates one day before transfection to make the cell density reach 70%-90% the next day. After adding Lipo6000 transfection reagent (5 μL), 100 pmol of siNR2B was diluted in 125 μL DEME medium without antibiotics, and serum was used to transfect the cells of each well at room temperature. Then, 250 μL mixtures of Lipo6000 and RNA were added to the cells and cultured at 37 °C for 24 h.

2.8. Western Blot

The rats were euthanized by intraperitoneal injecting an overdose of pentobarbital sodium (200 mg/kg) after completing the behavioral tests. Then, an 18-gauge needle was inserted into the caudal end of the vertebral column, and the spinal cord tissue was expelled with cold phosphate-buffered saline. L3-L5 of spinal cord tissues were used to detect the gene expressions of the μ-opioid receptor (MOPR), cyclic adenosine monophosphate (cAMP), NR2B, phosphorylated-calmodulin-dependent protein kinase II (p-CaMKII), CaMKII, and β-actin. Radio-immunoprecipitation assay lysis buffer (P0013K, Beyotime, China) was used to extract total protein from cells and tissues. Protein content was qualified with the BCA kit (C503021, Sangon, China). Then, 30 μg of protein samples were separated by SDS-PAGE and transferred to the PVDF membrane (IPFL00010, Millipore, USA). After blocking the protein for 2 h, the membrane was cultured with primary antibodies and then washed by TBST and incubated with Goat Anti-Rabbit IgG H&L (1 : 2000, ab7090, Abcam, UK) and Goat Anti-Mouse IgG H&L (1 : 3000, ab205719, Abcam, UK). After the color was developed in the dark, the gray value was digitized by ImageJ software V1.8 (National Institute of Health). The following primary antibodies were obtained from Abcam: MOPR (1 : 1000; Rabbit; ab134054, 75 kDa), cAMP (1 : 20000; Rabbit; ab76238, 42 kDa), NR2B (1 : 1000; Rabbit; ab254356, 166 kDa), CaMKII (1 : 1000; Rabbit; ab134041, 54 kDa), p-CaMKII (1 : 1000; Rabbit; ab32678, 50 kDa), and β-actin (1 : 1000; Mouse; ab8226, 42 kDa).

2.9. Cell Counting Kit-8 (CCK-8) Assay

CCK-8 assay (C0037, Beyotime, China) was conducted to determine the viability of SH-SY5Y cells following the manufacturer's instructions. When the confluence reached 70% -80%, the cells were digested by trypsin and planted at 96-well plates at a density of 5 × 103 cells/well. After incubation for 24 h, 20 μL CCK-8 reagent was added to each well. A microplate reader (VL0000D2, ThermoFisher, USA) was used to detect the viability of cells at 450 nm.

2.10. Determination of cAMP Content

The cAMP content in SH-SY5Y cells was detected using a LANCE cAMP kit (TRF0262, PerkinElmer). The cells were harvested using the Versene solution (15040066, Gibco, USA) followed by washing with HBSS buffer (24020117, Gibco, USA). The cells were then resuspended at a concentration of 2 × 106 cells/mL in stimulation buffer (HBSS 1×, containing 5 mM HEPES, 0.1% BSA, 0.05 mM IBMX). The cAMP-specific monoclonal antibodies labeled with the ULight™ dye (TRF0262, PerkinElmer) were added to the final cell suspension, and then naloxone was added to the cell suspension to precipitate the cAMP overshoot. After cells were incubated at 37 °C for 15 min, the LANCE signal, which can be used to analyze the cAMP levels, was obtained at 665 nM by a TECAN instrument (Infinite F200, Tecan, Austria).

2.11. Statistical Analysis

SPSS 20.0 was used for statistical analysis. The measurement data are expressed by mean ± standard deviation. One-way ANOVA was used for multiple group comparisons, and the Bonferroni test was adopted for further analysis of the difference between the two groups. p < 0.05 was used to demarcate statistical significance.

3. Results

3.1. β-Elemene Alleviated MT and Reversed the Effect of Morphine Treatment on Related Protein Expressions

First, we performed a series of animal experiments to measure the analgesic effect of morphine on the rat bone cancer pain models. We observed that in the mechanical allodynia and thermal hyperalgesia tests, before modeling (day 0), there was no obvious difference in each group. On day 1 (the first day of drug administration), morphine administration produced a significant analgesic effect on the model rats (p < 0.001, Figures 1(a)–1(b)), and there was no difference between the drug administration groups. However, from day 2, the analgesia level of morphine in the morphine administration groups gradually decreased, which was lower than on day 1 (Figures 1(a)–1(b)). On day 10, there was no significant difference between the MT group and the control group, but the mechanical withdrawal threshold and thermal withdrawal latency in the β-Elemene groups were largely higher than those in the MT group (p < 0.01, Figures 1(a)–1(b)), in which the H-β-Elemene group demonstrated the most significant effects (p < 0.01, Figures 1(a)–1(b)). It indicated that the rats injected with morphine had tolerance to the analgesic effect of tumor-induced pain, while β-Elemene alleviated the tolerance induced by morphine, and the high-dose β-Elemene (2.8 mg/kg) was more effective.

Figure 1

The effects of β-Elemene on the morphine-induced tolerance and the related protein expressions. (a) The mechanical withdrawal threshold of rats was detected by mechanical allodynia test. (b) The thermal withdrawal latency of rats was detected by thermal hyperalgesia test. (c-e) The expressions of MOPR, cAMP and NR2B and the ratio of p-CaMKII to CaMKII were determined by western blot, with β-actin as the internal reference. All experiments were repeated three times to obtain average values. The data are presented as the mean ± SD of three independent experiments; ∗p < 0.05; ∗∗p < 0.01; ∗∗∗p < 0.001vs. Control; ∧p < 0.05; ∧∧p < 0.01; ∧∧∧p < 0.001vs. MT. Abbreviations: MOPR: μ-opioid receptor; cAMP: cyclic adenosine monophosphate; NR2B: N-methyl-D-aspartate receptor subunit 2B; p-CaMKII: phosphorylated-calmodulin-dependent protein kinase II.

Subsequently, the related protein expressions in rat spinal cord were detected to investigate the underlying mechanism of the effect of β-Elemene on morphine-induced tolerance. We found that in the MT group, the expression of MOPR was significantly reduced (p < 0.001, Figures 1(c)–1(d)), and β-Elemene rescued the expression of MOPR (p < 0.001, Figures 1(c)–1(d)). In addition, the cAMP and NR2B expressions were notably increased in the MT group (p < 0.001, Figures 1(c)–1(d)), while β-Elemene inhibited the expressions of cAMP and NR2B (p < 0.05, Figures 1(c)–1(d)), and the high-dose β-Elemene showed greatest inhibitory effects (p < 0.001, Figures 1(c)–1(d)). Besides, the ratio of p-CaMKII to CaMKII was highly elevated in the MT group (p < 0.001, Figures 1(c) and 1(e)), but the ratio was down-regulated with increased β-Elemene dosage (p < 0.001, Figures 1(c) and 1(e)). It could be concluded that increasing dosage of β-Elemene partially counteracted the effects of morphine based on the expressions of these proteins.

3.2. High-Dose β-Elemene Could Inhibit the Viability of SH-SY5Y Cells

After the animal experiments, we studied the effects of β-Elemene at the cellular level. First, we analyzed the cytotoxicity of β-Elemene on SH-SY5Y cells at different concentrations (0, 5, 10, 20, and 25 μg/mL) and time courses of action (24, 48, and 72 h). The results showed that when the action time was 24 and 48 h, β-Elemene inhibited cell viability starting from a concentration of 10 μg/mL (p < 0.01, Figure 2(a)), and the higher the concentration, the more obvious the inhibitory effect (p < 0.05, Figure 2(a)). In addition, when the action time was 72 h, the cytotoxicity of β-Elemene began to appear from 5 μg/mL, so we chose the β-Elemene of 5 μg/mL for the following experiments under the action time of 48 h. Moreover, we found that the increased dosage of β-Elemene had no significant effect on intracellular cAMP content (Figure 2(b)).

Figure 2

The effects of β-Elemene on the viability and intracellular cAMP content of SH-SY5Y cells. (a) The viability of SH-SY5Y cells was detected by CCK-8 assay. (b) The intracellular cAMP content was measured by the LANCE cAMP kit. All experiments were repeated three times to obtain average values. The data are presented as the mean ± SD of three independent experiments; ∗p < 0.05; ∗∗p < 0.01; ∗∗∗p < 0.001vs. 0. Abbreviation: CCK-8, cell counting kit-8.

3.3. NR2B Agonist Reversed the Effect of β-Elemene on cAMP Content and Related Protein Expressions in SH-SY5Y Cells

Here, we established the MT cell model and administrated β-Elemene on SH-SY5Y cells. The results showed that after morphine acted on the cells, the intracellular cAMP content was markedly upregulated (p < 0.001, Figure 3(a)), while siNR2B and β-Elemene inhibited the intracellular cAMP content (p < 0.001, Figure 3(a)), but NMDA reversed the effects of β-Elemene and increased cAMP content (p < 0.001, Figure 3(a)). As for the related protein expressions in SH-SY5Y cells, the MOPR expression was reduced, and the ratio of p-CaMKII to p-CaMKI and the expressions of cAMP and NR2B were induced in the MT group (p < 0.001, Figures 3(b)–3(d)). siNR2B and β-Elemene showed the reverse effects with morphine; they increased the expression of MOPR and decreased the ratio of p-CaMKII to p-CaMKI and the expressions of cAMP and NR2B (p < 0.001, Figures 3(b)–3(d)). Additionally, the β-Elemene+NMDA group had a lower level of MOPR expression and higher levels of p-CaMKII/p-CaMKI, cAMP and NR2B compared with the β-Elemene group (p < 0.001, Figures 3(b)–3(d)), indicating that NMDA, the NR2B agonist, could reverse the effects of β-Elemene on SH-SY5Y cells.

Figure 3

The effects of NMDA on the cAMP content and related protein expressions in SH-SY5Y cells. (a) The intracellular cAMP content was measured by the LANCE cAMP kit. (b–d) The ratio of p-CaMKII to CaMKII, and the expressions of MOPR, cAMP, and NR2B were determined by western blot, with β-actin as the internal reference. All experiments were repeated three times to obtain average values. The data are presented as the mean ± SD of three independent experiments; ∗∗∗p < 0.001vs. Control; ∧∧∧p < 0.001vs. MT; ###p < 0.001vs. β-Elemene. Abbreviations: NMDA: N-methyl-D-aspartic acid; MOPR: μ-opioid receptor; cAMP: cyclic adenosine monophosphate; NR2B: N-methyl-D-aspartate receptor subunit 2B; p-CaMKII: phosphorylated-calmodulin-dependent protein kinase II.

4. Discussion

The commonly used bone cancer pain-MT models are mainly femoral or tibial cancer pain-MT models [25]. Tumor cells inoculated into bone cancer pain models include osteolytic NCTC2472 fibrosarcoma cells [26], AT-3.1 prostate cancer cells [27], MADB106 breast cancer cells, and Lewis lung cancer cells [20, 28]. In this study, the injection of MADB106 breast cancer cells and morphine (10 mg/kg) was used to establish a metastatic bone cancer pain model. The mechanical withdrawal threshold and thermal withdrawal latency of the MT group decreased to the contemporaneous level of the bone cancer pain group, marking the successful establishment of bone cancer pain-MT rat model. It is generally accepted that morphine produces analgesic effects mainly by inhibiting the adenylyl cyclase (AC)/cAMP signal transduction pathway through the binding of opioid receptors. Long-term treatment with morphine leads to inhibited analgesic effects, causing adaptive upregulation of the intracellular AC/cAMP signaling pathway and increasing cAMP content [29]. Therefore, the increase of cAMP content is usually used as the assessment criteria for the establishment of MT cell model [29, 30]. In this study, we have found that cAMP content in SH-SY5Y cells was significantly increased after treated with 10 μg/mL morphine for 48 h, suggesting that the successful establishment of cell model of MT.

β-Elemene is an adjuvant drug in clinical practice. Relevant studies have shown that the combination of β-Elemene and first-line chemotherapy drugs for lung cancer, such as paclitaxel, improved the chemotherapy efficacy of lung cancer, prolonged the survival time of patients, and improved their quality of life [31]. Elemene injection combined with oxycodone hydrochloride could effectively relieve the pain of patients with severe cancer in clinic; the total pain relief rate was 91.67% [32]. In a study by Ma et al. [33], the authors investigated the analgesic effect of β-Elemene and explored the mechanisms at the spinal dorsal horn (SDH) level under neuropathic pain. They found that SNI induced strong astrocytic ERK activation within the SDH in the late phase of neuropathic pain and β-Elemene exerted remarkable analgesic effects on neuropathic pain by inhibiting spinal astrocytic ERK activation and subsequent neuroinflammatory processes, indicating β-Elemene as a promising analgesic for treating chronic pain. Dai et al. [34] reported that β-Elemene could alleviate neuropathic pain in mice by regulating C-X-C motif chemokine receptor 3 and GABAA receptor. However, the effects of β-Elemene in MT have not yet been investigated. This present study identified for the first time that β-Elemene could increase the low levels of mechanical withdrawal threshold and thermal withdrawal latency induced by MT, indicating that it may ameliorate MT. In addition, high-dose β-Elemene was cytotoxic to SH-SY5Y cells but did not affect cAMP content. Next, we performed mechanistic analysis on the effects of β-Elemene.

It has been reported that the expression of MOPR in rat locus coeruleus neurons was down-regulated after prolonged morphine treatment, suggesting that morphine treatment was closely related to the number and activity of MOPR [8]. Meanwhile, as previously stated, prolonged morphine treatment can lead to an increase in cAMP [29]. In addition, the gradual increase of cAMP expression level in the cell will increase Ca2+ influx and then promote Ca2+ and calmodulin (CAM) binding, thus upregulating the expression of CaMKII [35]. Numerous evidence show that CaMKII is a key and direct factor in promoting morphine tolerance and dependence [36] and inhibiting the CaMKII/NMDA receptor pathway alleviates MT [37]. For this study, our results showed that long-term morphine treatment significantly decreased MOPR expressions and increased cAMP expressions and p-CaMKII/CaMKII in MT rat and cell models, which consistent with previous studies. Moreover, we found that low dosage of β-Elemene significantly reversed the effect of MT and increased MOPR expressions and decreased cAMP expressions and p-CaMKII/CaMKII in cell and animal models.

NMDA receptors are involved in the development of MT. Bajo et al. [12] found that NMDA receptors were involved in the tolerance process caused by long-term morphine action. The NR2B expression in the nucleus accumbens and central amygdala was increased in the MT rat model. Moreover, other scholars proved that suppressing NR2B improved analgesic tolerance to morphine [13]. In our study, we discovered that the protein expression of NR2B in the cell and rat models were upregulated after long-term morphine treatment, whereas low-dose β-Elemene and siNR2B counteracted this effect and downregulated NR2B expressions. Besides, NMDA, the agonist of NR2B, had reverse effects with β-Elemene, which led to the upregulation of NR2B.

In summary, MT in the bone cancer pain model was improved at low doses of β-Elemene by modulating NR2B to promote the MOPR expression, thereby inhibiting the expressions of cAMP, p-CaMKII, and CaMKII, which might be a new therapeutic target in the future. There are some shortcomings in this study. First, more validation experiments should be considered, such as overexpression and other MT models. Secondly, the changes of cAMP, p-CaMKII and CaMKII are associated with extracellular calcium influx and the changes of neural circuits; we will investigate deeply in the future research.