3.3. Motor Performance
There was a significant reduction of the time to fall in a Rota-rod apparatus in all groups 24
h following the second injection of acidic saline (P
< 0.05), which was kept over the time of experimentation through five days of treatment ().
Figure 5 Line graph representing the motor performance (in seconds) of the animals in groups 15Hz and 100Hz electroacupuncture, acupuncture, and control. Values are presented as mean ± standard error of mean. Inj: injection; 1: before; (more ...)
The results of this study demonstrate that both electroacupuncture and acupuncture reduced the mechanical hyperalgesia and thermal hyperalgesia following administration of associated double intramuscular injection of acidic saline which generates muscle hyperalgesia that simulated experimentally fibromyalgia syndrome. In parallel, the motor performance was optimized in groups that received electrical stimulation.
To our knowledge, this is the first work of experimental animal study in rats investigating the effects of both electroacupuncture and acupuncture on an animal model of fibromyalgia. However, similar to our findings, some previous experimental studies have demonstrated the EA effect using different animal models: reduction of mechanical hyperalgesia, such as in models of inflammatory pain induced by carrageenan [20
]; nerve growth factor [17
] or Freund's adjuvant [14
], in models of neuropathic pain after spinal section between S3 and S4 [13
] and ligation of the anterior tibial and sural nerve [34
]; pain ankle sprain [15
]; ovariectomy in female dogs [24
] and in rats [27
] and in animal models of cancer induction [19
]. Acupuncture has shown antihyperalgesic effect in neuropathic model [35
] of pain induced by formalin injection [36
]; however, there was no significant difference between acupuncture and EA in inflammatory pain model induced by Freund's adjuvant [37
] and carrageenan [38
In the present study, EA and acupuncture were able to promote reduction of the thermal hyperalgesia in this model of noninflammatory muscle pain. Some studies have demonstrated the EA-induced antihyperalgesia caused by EA in models of inflammatory pain by injection of Freund's adjuvant [16
] or carrageenan [39
], neuropathic pain by section between S1 and S2 [28
], and through the tail flick test in mice to verify the effectiveness of different frequencies of EA [40
]. However, in studies of inflammatory pain induced by administration of Freund's adjuvant [14
] and neuropathic pain [39
], there were no significant changes in thermal hyperalgesia between groups treated with control group.
Our findings showed that in the present study, the motor performance of rats showed no change in the latency to fall compared to control animals. Except for the decrease 24 hours after the second injection, when the muscle hyperalgesia is supposed to be maximum and motor performance fell. Firstly, This suggests that reversal of hyperalgesia produced by EA in both frequency bandies, as well as acupuncture, is actually assigned antihyperalgesic action of stimulation, as sedative or muscle relaxants could change motor performance after treatment. Moreover, it seems that the development of muscle widespread hyperalgesia in this experimental model was responsible for impairing studies that correlate the EA and motor performance in models of hyperalgesia in rats, which are scarce. On the other hand Jia et al. [41
] showed significant improvement in motor efficiency and coordination in mice after induction of Parkinson's disease by unilateral section of the medial forebrain bundle treated with high- and low-frequency electroacupuncture, but only the high frequency (100
Hz) demonstrated an improvement in this model for coordination and motor performance, which was not observed in our study.
After repeated use of electroacupuncture, we observed maintenance of analgesic efficacy of both low and high frequency, although some studies suggest the development of analgesic tolerance and the possible involvement of the opioidergic in mediating the effect of EA in both frequency bands. Similar to our findings, consecutive applications of EA for three days in a model of visceral hypersensitivity in mice did not develop tolerance to the analgesic treatment with electroacupuncture [42
]. However, some studies have demonstrated the participation of opioid system in the antihyperalgesia of electroacupuncture promoted by the application through the administration of blockers of μ
opioid receptors [8
]. Other studies also confirmed the action of β
-endorphin as evidenced, an increase in plasma after application of electroacupuncture in the postoperative ovariectomy in female dogs [24
] and rats [27
]. The increase in blood and brain β
-endorphin has been demonstrated in a model of cancer induction in rats [19
]. Although with these data we can not affirm the involvement of the opioid system, we believe this possibility, and yet we did not observe the effect of electroacupuncture reduced after 5 days of treatment in this animal model.
The involvement of the GABAergic system was demonstrated by Park et al. [28
] in a model of neuropathic pain in rats; the blockade of GABA (A) and GABA (B) reversed the antihyperalgesic effect after stimulation of low-frequency EA (2
Hz). In parallel, 2
Hz EA operates in spinal muscarinic receptors, after administration of atropine that reversed the analgesic effects produced by EA [29
]. The serotonergic and glutamatergic systems also had their shares in mediating the effect of EA evidenced in previous studies. In animal model of hyperalgesia induced with Freund's adjuvant, after the application of EA was producing of catecholamines and serotonin, demonstrating the activation of these pathways in controlling pain [16
]. Also, EA showed a decrease in phosphorylation of the subunits of spinal NMDA (N-methyl-D-aspartate), also demonstrating the involvement of these structures in the process of stimulation of EA analgesia [42
]. Our data do not allow us to say exactly which pain inhibition system is acting.
Only five previous studies have investigated the anti-hyperalgesic effects of therapeutic strategies in experimental model of fibromyalgia in rats, one with pharmacological treatment and the others with non-pharmacological techniques. The reduction of hyperalgesia produced by morphine, SNC80, damge, and selective opioid receptor agonists was prevented by blocking opioid receptors μ
but not κ
. Therefore, activation of spinal μ
opioid receptors and δ
reduces mechanical hyperalgesia following repeated intramuscular injection of acidic saline [7
]. Pregabalin reduces mechanical hyperalgesia, but there was motor impairment in higher dosages [4
] and the combination of tramadol and milnacipran enhances the antihyperalgesia in this animal model of FM [8
]. The reversal of secondary mechanical hyperalgesia after physical exercises of low intensity [5
] was proven in the same animal model used by us and the action of the opioid system stimulated by exercise was shown when receptor blockade by naloxone interrupted the analgesic effect.
Clinically, the use of EA in patients with fibromyalgia was investigated in only one controlled clinical trial. In this study, fibromyalgia patients were treated with six sessions of EA for three weeks, using frequency that varied between 1 and 99
Hz, with intensity-level motor contraction. Compared to the control group, subjects treated showed significant analgesic effects, reducing the intensity and distribution of pain and analgesic consumption, increased pressure pain threshold, and improved quality of sleep [46
Our work got some similar results in behavioral tests to those found in previous studies that showed the involvement of the opioid system in the reversal of hyperalgesia in animal models of pain through the use of electroacupuncture, which for us is a possibility, but the effect of treatment developed in our work did not lose its effectiveness even after 5 consecutive days, so we did not develop tolerance to treatment. We are doing some work to elucidate the possible mechanisms of acupuncture and electroacupuncture analgesia by this animal model both intrathecal and intracerebral level through blockers naloxone and naltrindole.