| Electroacupuncture(EA) has been used widely in the clinic because of its antinociceptive effect, especially for pain control. However, it has also been reported that repeated or prolonged application of EA results in a gradual fading away of its analgesic effect, termed EA tolerance. Studies have focused on EA tolerance because it results in declined EA effects. It has been demonstrated that EA exerts the antinociceptive effect by releasing the endogenous opioid peptides, such as enkephalin, endorphin. Meanwhile the anti-opioid peptides, including cholecystokinin-octopeptide-8, orphanin FQ and antiotensin II, were also mobilized during EA. Though the interactions between the opioids and the anti-opioids in EA has been described, the mechanism of EA tolerance is still unknown. Cross-tolerance has been found between EA and morphine. The involvement of micro RNA(miRNA) and excitatory amino acid transporters(EAAT) in morphine tolerance has been reported. However, the roles of miRNA and EAAT in EA tolerance remain unknown. The aims of present study was to investigate the involvement of miRNA and EAAT in EA tolerance. 1. Establishment of EA tolerance model in ratsRats with more than 50% increase in tail flick latency(TFL) were selected as responders and were used for the formal experiment. Sixty responders were randomly divided into 3 groups with 20 in each group: EA group, control group and sham group. The rats in EA group were treated with EA once daily for 8 consecutive days. Rats in the control group were restrained as those treated in EA group. Rats in the sham group were treated with needles left on acupoints but without electricity. The pain threshold were measured by TFL, which was recorded before and after EA. The change in pain threshold was calculated and was presented as the EA antinociceptive effect. As a result, there was no difference(p > 0.05) in TFL change between the control and the sham group. The changes in TFL decrased(p < 0.05) as the times of EA increased. In EA group, the TFL change was higher(p < 0.05) than that in the control group from day 1 to day 6, but was similar(p > 0.05) to the control at day 7 and day 8. These results indicated the development of EA tolerance. 2. The differential expression of hypothalamic miRNA in EA tolerance ratTo investigate the role of hypothalamic miRNA in EA tolerance, the present study performed the deep sequencing technique to explore the differentially expressed miRNAs in rats. These miRNAs were validated by qPCR and their roles in EA tolerance were further confirmed through intracerebroventricular(icv) injection technique. In the experiment of miRNA sequencing, 12 male responder rats were selected from 2 litters with 6 rats in each litter for a match-paired experiment. One of the paired rats from the same litter was treated with EA and another was used as control. The rats in EA groups were treated with EA once daily for 8 consecutive days. At day 8, immediately after EA, all rats were euthanized and their hypothalamus were collected. The hypothalamal tissues from three EA-treated or control littermates were pooled. In this way, two replicates of pooled samples from each group were obtained. The differential expression of miRNAs were analyzed using bioinformatics. The targets of these miRNAs were predicted using miRwalk and were analyzed using GO and KEGG functional annotations. In the experiment of qPCR validation, 9 male responders were selected and were allotted to 3 groups: EA group, control group and sham group, with 3 rats in each group. They were treated the same way as previously described. Upon euthanasia, their thalamus, hypothalamus and hippocampus were collected for qPCR validation. In the experiment of icv injection, 132 male responders were divided into 2 groups, i.e. EA group and control group, with 66 rats in each group. The rats in EA or the control group received icv antagomirs(antago-let-7b-5p, antago-107-3p, antago-124-3p or antago-221-3p), agomirs(ago-7a-5p, ago-204-5p, ago-148a-3p or ago-370-3p), negative antagomir, negative agomir or saline, with each reagent injected to 6 rats. Then the rats in EA group received EA once daily for 8 days. The TFL was recorded before and after EA, and the change in TFL was calculated.The results from deep sequencing showed 49 differentially expressed miRNAs, where 34 miRNAs were down-regulated and 15 miRNAs were up-regulated in EA group. A total of 5440 targets were predicted. These miRNAs may participate in EA tolerance through pathways including MAPK, neurotrophin, fatty acid metabolism, and through functional categories related to nerve impulse transmission, receptor signal pathways and gene expression regulation. Among the top 12 abundantly differentially expressed miRNAs, 4 up-regulated(p < 0.05) miRNAs(miR-124-3p, miR-221-3p, miR-let-7b-5p and miR-107-3p) and 6 down-regulated(p < 0.05) miRNAs(miR-7a-5p, miR-148a-3p, miR-204-5p, miR-370-3, miR-434-5p and miR-344b-3p) were validated in hypothalamus using qPCR. In thalamus, 5 miRNAs were up-regulated(p < 0.05) and 4 were down-regulated(p < 0.05). In hippocampus, 5 miRNAs were up-regulated(p < 0.05) and 2 were down-regulated(p < 0.05). These results indicated the reliability of the sequencing data, and the site-specific expression patterns of some differentially expressed miRNAs in EA tolerance. The results of icv injection experiment showed that, compared with the EA + saline group, the TFL change was higher(p < 0.05) in EA + agomir-148a-3p from day 4 to day 8, in EA + agomir-370-3p form day 6 to day 8, in EA + antagomir-let-7b and EA + antagomir-107-3p from day 3 to day 8 and in EA + agomir-124-3p at day 2 and from day 6 to day 8. These results suggested the involvement of miR-148a-3p, miR-370-3p, let-7b-5p, miR-107-3p and miR-124-5p in EA tolerance. 3. The dynamic expression of spinal EAATs in EA toleranceTo investigate the role of spinal EAAT in EA tolerance, the present study determined the dynamic expression of spinal EAATs using q PCR and western blot technique. Then riluzole, a positive EAAT regulator, was intrathecally injected to observe its effect on EA tolerance. To select the optimal sampling time points for repeated EA, the time-course expression of spinal EAATs after single EA was determined. Thirty-six responders were treated with EA, and were euthanized before(0 h) and at 0.5, 1, 2, 4 and 8 h after EA, with 6 rats at each time point. The L4-5 spinal cord was collected. The expression of the three spinal EAATs were detected, including L-glutamate-L-aspartate transporter(GLAST), the glutamate transporter 1(GLT-1) and the excitatory amino-acid carrier 1(EAAC1). The time points where the highest expression levels of spinal EAATs appeared were determined as the optimal sampling points. To determine the expression patterns of the spinal EAATs in rats treated with repeated EA, 120 rats were randomly classified into two treatments, i.e. EA treatment and sham treatment with 60 rats per treatment. The rats in EA group were treated with EA once per day for 8 days consecutively. TFL was measured every day immediately before and after EA, respectively, and the changes in TFL were calculated. Twelve rats from each group were euthanized at days 0, 2, 4, 6 and 8, respectively. Since the optimal sampling time points after single EA were 2 h for EAAC1 and 4 h for GLAST and GLT-1, six out of the twelve rats were euthanized at 2 or 4 h after EA or sham treatment. To explore the effect of riluzole, a positive EAAT regulator, on EA tolerance, Thirty-six rats were allotted to six groups: EA + vehicle, EA + 5 μg riluzole, EA + 10 μg riluzole, EA + 20 μg riluzole, vehicle group and 20 μg riluzole group. EA was given once per day for 8 days. Riluzole was administered each day before EA. The TFL was examined every day before and after EA. The change in TFL was calculated.The results showed that, single EA induced no change(p > 0.05) in GLAST and GLT-1 mRNA expression, but an increase(p < 0.05) in EAAC1 mRNA expression from 1 to 8 h, with the peak at 2 h. Both GLAST and GLT-1 levels increased(p < 0.05) from 2 to 8 h, with their peak levels at 4 h, whereas EAAC1 level increased(p < 0.05) from 2 to 4 h, with the peak at 2 h after single EA. Therefore, for the experiment of repeated EA, the samples for spinal GLAST and GLT-1 expressions were taken at 4 h while samples for spinal EAAC1 expression were taken at 2 h, after each EA. For the repeated EA experiments, mRNA expressions of GLAST and GLT-1 were unchanged(p > 0.05), whereas the EAAC1 increased(p < 0.05) at day 2 and day 4 and decreased(p < 0.05) at day 8 compared with the sham group. The protein expression of the three EAATs decreased(p < 0.05) as the times of EA increased, and were approximated(p > 0.05) to those in the sham group at day 8. Statistical analysis showed that the TFL change had a positive correlation(p < 0.05) with GLAST, GLT-1 and EAAC1 levels. The experiment of intrathecal riluzole showed that, compared with the EA + vehicle group, the TFL change was higher(p < 0.05) in EA + 10 μg riluzole group from day 4 to day 8, and in EA + 20 μg riluzole group from day 5 to day 8, but showed no difference(p > 0.05) in EA + 5 μg riluzole, suggesting that riluzole dose-dependently attenuated the EA tolerance. These results indicated the involvement of the spinal EAATs in EA tolerance. |
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