The effect of acetylcholine on pain-related electric activities in the hippocampal CA3 of rats

Electroacupuncture Reduces the Effects of Acute Noxious Stimulation on the Electrical Activity of Pain-Related Neurons in the Hippocampus of Control and Neuropathic Pain Rats

To study the effects of acupuncture analgesia on the hippocampus, we observed the effects of electroacupuncture (EA) and mitogen-activated protein kinase (MEK) inhibitor on pain-excited neurons (PENs) and pain-inhibited neurons (PINs) in the hippocampal area CA1 of sham or chronic constrictive injury (CCI) rats. The animals were randomly divided into a control, a CCI, and a U0126 (MEK1/2 inhibitor) group. In all experiments, we briefly (10-second duration) stimulated the sciatic nerve electrically and recorded the firing rates of PENs and PINs. The results showed that in both sham and CCI rats brief sciatic nerve stimulation significantly increased the electrical activity of PENs and markedly decreased the electrical activity of PINs. These effects were significantly greater in CCI rats compared to sham rats. EA treatment reduced the effects of the noxious stimulus on PENs and PINs in both sham and CCI rats. The effects of EA treatment could be inhibited by U0126 in sham-operated rats. The results suggest that EA reduces effects of acute sciatic nerve stimulation on PENs and PINs in the CA1 region of the hippocampus of both sham and CCI rats and that the ERK (extracellular regulated kinase) signaling pathway is involved in the modulation of EA analgesia.

Involvement of hippocampal acetylcholinergic receptors in electroacupuncture analgesia in neuropathic pain rats

BACKGROUND

Cumulating 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.

METHODS

Forty 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.

RESULTS

Following 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.

CONCLUSIONS

EAS 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.

Neuroanatomical and neuropharmacological approaches to postictal antinociception-related prosencephalic neurons: the role of muscarinic and nicotinic cholinergic receptors

Several studies have suggested the involvement of the hippocampus in the elaboration of epilepsy. There is evidence that suggests the hippocampus plays an important role in the affective and motivational components of nociceptive perception. However, the exact nature of this involvement remains unclear. Therefore, the aim of this study was to determine the role of muscarinic and nicotinic cholinergic receptors in the dorsal hippocampus (dH) in the organization of postictal analgesia. In a neuroanatomical study, afferent connections were found from the somatosensory cortex, the medial septal area, the lateral septal area, the diagonal band of Broca, and the dentate gyrus to the dH; all these areas have been suggested to modulate convulsive activity. Outputs to the dH were also identified from the linear raphe nucleus, the median raphe nucleus (MdRN), the dorsal raphe nucleus, and the locus coeruleus. All these structures comprise the endogenous pain modulatory system and may be involved either in postictal pronociception or antinociception that is commonly reported by epileptic patients. dH-pretreatment with cobalt chloride (1.0 mmol/L CoCl2/0.2 μL) to transiently inhibit local synapses decreased postictal analgesia 10 min after the end of seizures. Pretreatment of the dH with either atropine or mecamylamine (1.0 μg/0.2 μL) attenuated the postictal antinociception 30 min after seizures, while the higher dose (5.0 μg/0.2 μL) decreased postictal analgesia immediately after the end of seizures. These findings suggest that the dH exerts a critical role in the organization of postictal analgesia and that muscarinic and nicotinic cholinergic receptor-mediated mechanisms in the dH are involved in the elaboration of antinociceptive processes induced by generalized tonic-clonic seizures.

Modulatory role of glutamic acid on the electrical activities of pain-related neurons in the hippocampal CA3 region

Glutamic acid (Glu) participates in pain modulation of the central nervous system. The CA3 region of the hippocampal formation has been suggested to be involved in nociceptive perception. However, it is unknown whether Glu could modulate the electrical activities of pain-related neurons in the hippocampal CA3 region. The present study aimed to determine the effects of Glu and its receptor antagonist MK-801 in the pain-evoked response of both pain-excited neurons (PENs) and pain-inhibited neurons (PINs) in the hippocampal CA3 region of normal rats. We used a train of electric impulses applied to the sciatic nerve as noxious stimulation. The electrical activities of either PENs or PINs in the hippocampal CA3 region were recorded by a glass microelectrode. The results revealed that intra-CA3 region microinjection of Glu (0.5 μg/1 μl) increased the evoked firing frequency and shortened the firing latency of PEN, while decreased the evoked firing frequency and prolonged the inhibitory duration of PIN in the hippocampal CA3 region of rat evoked by the noxious stimulation. Intra-CA3 region administration of MK-801 (0.25 μg/1 μl) produced the opposite response. These results suggest that Glu and its receptors in hippocampal CA3 region are involved in the modulation of nociceptive information transmission by affecting the electric activities of PENs and PINs.