Li De-Pei, Chen Shao-Rui, Pan Yu-Zhen, Levey Allan I, Pan Hui-Lin
Department of Anesthesiology, Penn State University College of Medicine, Hershey, PA 17033, USA.
J Physiol. 2002 Sep 15;543(Pt 3):807-18. doi: 10.1113/jphysiol.2002.020644.
Spinally administered muscarinic receptor agonists or acetylcholinesterase inhibitors can produce effective pain relief. However, the analgesic mechanisms and the site of actions of cholinergic agents in the spinal cord are not fully understood. In this study, we investigated the mechanisms underlying cholinergic presynaptic regulation of glutamate release onto spinal dorsal horn neurons. The role of spinal GABA(B) receptors in the antinociceptive action of muscarine was also determined. Whole-cell voltage-clamp recordings were performed on visualized dorsal horn neurons in the lamina II in the spinal cord slice preparation of rats. The miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs) were recorded in the presence of tetrodotoxin. The evoked EPSCs (eEPSCs) were obtained by electrical stimulation of the dorsal root entry zone or the attached dorsal root. Nociception in rats was measured using a radiant heat stimulus and the effect of intrathecal administration of drugs tested. Acetylcholine (10-100 microM) reduced the amplitude of monosynaptic eEPSCs in a concentration-dependent manner. Acetylcholine also significantly decreased the frequency of non-NMDA receptor-mediated mEPSCs, which was antagonized by atropine but not mecamylamine. The frequency of GABA(A) receptor-mediated mIPSCs was significantly increased by acetylcholine and this excitatory effect was abolished by atropine. Existence of presynaptic M(2) muscarinic receptors in the spinal dorsal horn was further demonstrated by immunocytochemistry staining and dorsal rhizotomy. CGP55845, a GABA(B) receptor antagonist, significantly attenuated the inhibitory effect of acetylcholine on the frequency of mEPSCs and the amplitude of monosynaptic eEPSCs in lamina II neurons. Furthermore, the antinociceptive action produced by intrathecal muscarine was significantly reduced by CGP55845 pretreatment in rats. Therefore, data from this integrated study provide new information that acetylcholine inhibits the glutamatergic synaptic input to lamina II neurons through presynaptic muscarinic receptors. Inhibition of glutamate release onto lamina II neurons by presynaptic muscarinic and GABA(B) heteroreceptors in the spinal cord probably contributes to the antinociceptive action of cholinergic agents.
脊髓给药的毒蕈碱受体激动剂或乙酰胆碱酯酶抑制剂可有效缓解疼痛。然而,胆碱能药物在脊髓中的镇痛机制和作用位点尚未完全明确。在本研究中,我们探究了胆碱能对脊髓背角神经元谷氨酸释放的突触前调节机制。还确定了脊髓GABA(B)受体在毒蕈碱镇痛作用中的作用。在大鼠脊髓切片制备的II层可视化背角神经元上进行全细胞电压钳记录。在存在河豚毒素的情况下记录微小兴奋性突触后电流(mEPSCs)和微小抑制性突触后电流(mIPSCs)。通过电刺激背根进入区或相连的背根获得诱发的EPSCs(eEPSCs)。使用辐射热刺激测量大鼠的伤害感受,并测试鞘内给药药物的效果。乙酰胆碱(10 - 100 microM)以浓度依赖的方式降低单突触eEPSCs的幅度。乙酰胆碱还显著降低非NMDA受体介导的mEPSCs的频率,阿托品可拮抗此作用,而美加明则无此作用。乙酰胆碱显著增加GABA(A)受体介导的mIPSCs的频率,阿托品可消除此兴奋作用。免疫细胞化学染色和背根切断术进一步证实脊髓背角存在突触前M(2)毒蕈碱受体。GABA(B)受体拮抗剂CGP55845显著减弱了乙酰胆碱对II层神经元mEPSCs频率和单突触eEPSCs幅度的抑制作用。此外,CGP55845预处理可显著降低大鼠鞘内注射毒蕈碱产生的镇痛作用。因此,这项综合研究的数据提供了新的信息,即乙酰胆碱通过突触前毒蕈碱受体抑制对II层神经元的谷氨酸能突触输入。脊髓中突触前毒蕈碱和GABA(B)异源受体对II层神经元谷氨酸释放的抑制可能有助于胆碱能药物的镇痛作用。