Pan Yu-Zhen, Li De-Pei, Pan Hui-Lin
Department of Anesthesiology, Penn State University College of Medicine, Hershey, Pennsylvania 17033-0850, USA.
J Neurophysiol. 2002 Apr;87(4):1938-47. doi: 10.1152/jn.00575.2001.
Activation of spinal alpha(2)-adrenergic receptors by the descending noradrenergic system and alpha(2)-adrenergic agonists produces analgesia. However, the sites and mechanisms of the analgesic action of spinally administered alpha(2)-adrenergic receptor agonists such as clonidine are not fully known. The dorsal horn neurons in the outer zone of lamina II (lamina II(o)) are important for processing nociceptive information from C-fiber primary afferents. In the present study, we tested a hypothesis that activation of presynaptic alpha(2)-adrenergic receptors by clonidine inhibits the excitatory synaptic input to lamina II(o) neurons. Whole cell voltage-clamp recordings were performed on visualized lamina II(o) neurons in the spinal cord slice of rats. The miniature excitatory postsynaptic currents (mEPSCs) were recorded in the presence of tetrodotoxin, bicuculline, and strychnine. The evoked EPSCs were obtained by electrical stimulation of the dorsal root entry zone or the attached dorsal root. Both mEPSCs and evoked EPSCs were abolished by application of 6-cyano-7-nitroquinoxaline-2,3-dione. Clonidine (10 microM) significantly decreased the frequency of mEPSCs from 5.8 +/- 0.9 to 2.7 +/- 0.6 Hz (means +/- SE) without altering the amplitude and the decay time constant of mEPSCs in 25 of 27 lamina II(o) neurons. Yohimbine (2 microM, an alpha(2)-adrenergic receptor antagonist), but not prazosin (2 microM, an alpha(1)-adrenergic receptor antagonist), blocked the inhibitory effect of clonidine on the mEPSCs. Clonidine (1-20 microM, n = 8) also significantly attenuated the peak amplitude of evoked EPSCs in a concentration-dependent manner. The effect of clonidine on evoked EPSCs was abolished in the presence of yohimbine (n = 5). These data suggest that clonidine inhibits the excitatory synaptic input to lamina II(o) neurons through activation of alpha(2)-adrenergic receptors located on the glutamatergic afferent terminals. Presynaptic inhibition of glutamate release from primary afferents onto lamina II(o) neurons likely plays an important role in the analgesic action produced by activation of the descending noradrenergic system and alpha(2)-adrenergic agonists.
下行去甲肾上腺素能系统和α₂-肾上腺素能激动剂激活脊髓α₂-肾上腺素能受体会产生镇痛作用。然而，脊髓给予可乐定等α₂-肾上腺素能受体激动剂的镇痛作用位点和机制尚未完全明确。Ⅱ层外侧区（Ⅱ（o）层）的背角神经元对于处理来自C纤维初级传入的伤害性信息很重要。在本研究中，我们测试了一个假设，即可乐定激活突触前α₂-肾上腺素能受体可抑制对Ⅱ（o）层神经元的兴奋性突触输入。在大鼠脊髓切片中对可视化的Ⅱ（o）层神经元进行全细胞电压钳记录。在存在河豚毒素、荷包牡丹碱和士的宁的情况下记录微小兴奋性突触后电流（mEPSCs）。通过电刺激背根进入区或相连的背根获得诱发的EPSCs。应用6-氰基-7-硝基喹喔啉-2,3-二酮可消除mEPSCs和诱发的EPSCs。可乐定（10μM）在27个Ⅱ（o）层神经元中的25个中显著降低mEPSCs的频率，从5.8±0.9降至2.7±0.6Hz（平均值±标准误），而不改变mEPSCs的幅度和衰减时间常数。育亨宾（2μM，一种α₂-肾上腺素能受体拮抗剂）而非哌唑嗪（2μM，一种α₁-肾上腺素能受体拮抗剂）阻断了可乐定对mEPSCs的抑制作用。可乐定（1 - 20μM，n = 8）也以浓度依赖性方式显著减弱诱发的EPSCs的峰值幅度。在存在育亨宾（n = 5）的情况下，可乐定对诱发的EPSCs的作用被消除。这些数据表明，可乐定通过激活位于谷氨酸能传入终末上的α₂-肾上腺素能受体来抑制对Ⅱ（o）层神经元的兴奋性突触输入。初级传入纤维向Ⅱ（o）层神经元释放谷氨酸的突触前抑制可能在下行去甲肾上腺素能系统和α₂-肾上腺素能激动剂激活产生的镇痛作用中起重要作用。
