Abstract
BackgroundCumulating evidence has shown a close correlation between electroacupuncture stimulation (EAS) frequency-specific analgesic effect and central opioid peptides. However, the actions of hippocampal acetylcholinergic receptors have not been determined. This study aims to observe the effect of different frequencies of EAS on the expression of hippocampal muscarinic and nicotinic acetylcholinergic receptors (mAChRs, nAChRs) in neuropathic pain rats for revealing their relationship.MethodsForty male Wistar rats were randomly and equally divided into sham, CCI model, 2, 2/15 and 100 HzEA groups. The neuropathic pain model was established by ligature of the left sciatic nerve to induce chronic constriction injury (CCI). EAS was applied to bilateral Zusanli (ST36) and Yanglingquan (GB34) for 30 min, once daily for 14 days except weekends. The mechanical pain thresholds (withdrawal latencies, PWLs) of bilateral hindpaws were measured. The expression levels of hippocampal M1 and M2 mAChR, and α4 and β2 nAChR genes and proteins were detected by quantitative RT-PCR and Western blot, separately. The involvement of mAChR and nAChR in the analgesic effect of EAS was confirmed by intra-hippocampal microinjection of M1mAChR antagonist (Pirenzepine) and α4β2 nAChR antagonist (dihydro-beta-erythroidine) respectively.ResultsFollowing EAS, the CCI-induced increase of difference values of bilateral PWLs on day 6 and 14 was significantly reduced (P < 0.05), with 2/15 Hz being greater than 100 Hz EAS on day 14 (P < 0.05). After 2 weeks’ EAS, the decreased expression levels of M1 mAChR mRNA of both 2 and 2/15 Hz groups and M1 mAChR protein of the three EAS groups, α4 AChR mRNA of the 2/15 Hz group and β2 nAChR protein of the three EAS groups were considerably increased (P < 0.05), suggesting an involvement of M1 mAChR and β2 nAChR proteins in EAS-induced pain relief. No significant changes were found in the expression of M2 mAChR mRNA and protein, α4 nAChR protein and β2 nAChR mRNA after CCI and EAS (P > 0.05). The analgesic effect of EAS was abolished by intra-hippocampal microinjection of M1mAChR and α4β2 nAChR antagonists respectively.ConclusionsEAS of ST36-GB34 produces a cumulative analgesic effect in neuropathic pain rats, which is frequency-dependent and probably mediated by hippocampal M1 mAChR and β2 nAChR proteins.
Highlights
Cumulating evidence has shown a close correlation between electroacupuncture stimulation (EAS) frequency-specific analgesic effect and central opioid peptides
Related researches are relatively fewer. We demonstrated that both hippocampal and hypothalamic cholinergic activities were involved in the cumulative analgesia induced by repeated electroacupuncture stimulation (EAS) of “Zusanli” (ST36) and “Yanglingquan” (GB34) in rats with chronic constrictive injury (CCI) of the sciatic nerve [23, 24]
In comparison with the CCI model group, the difference value of PWL (PWLD) were pronouncedly decreased in rats of the CCI + EA2 Hz group and CCI + EA2/15 Hz group on day 6 and day 14 after CCI-operation (P < 0.05), and 2 Hz and 2/15 Hz EAS were notably superior to that of 100 Hz EAS in reducing PWLDS (P < 0.05, Fig. 1)
Summary
Cumulating evidence has shown a close correlation between electroacupuncture stimulation (EAS) frequency-specific analgesic effect and central opioid peptides. This study aims to observe the effect of different frequencies of EAS on the expression of hippocampal muscarinic and nicotinic acetylcholinergic receptors (mAChRs, nAChRs) in neuropathic pain rats for revealing their relationship. The hippocampus, one of the limbic structures for antinociception [3], has been shown to undergo significant changes including reduction of hippocampal volume, learning and emotional deficits, sustained endocrine stress response, etc., in chronic pain syndrome patients [4,5,6,7,8], and abnormal expression of cytokine, extracellular signal-regulated kinase, neurokinin-1 (NK1) receptor, etc., in experimental chronic neuropathic pain animals [9,10,11]. We demonstrated that both hippocampal and hypothalamic cholinergic activities were involved in the cumulative analgesia induced by repeated electroacupuncture stimulation (EAS) of “Zusanli” (ST36) and “Yanglingquan” (GB34) in rats with chronic constrictive injury (CCI) of the sciatic nerve [23, 24]. The detailed mechanisms underlying involvement of ACh in analgesia are still not clear
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