Hayar A, Guyenet P G
Department of Pharmacology, University of Virginia, Charlottesville, Virginia 22908, USA.
J Neurophysiol. 1998 Oct;80(4):2003-14. doi: 10.1152/jn.1998.80.4.2003.
The effects of methionine-enkephalin (ME) on visualized bulbospinal neurons of the rostral ventrolateral medulla (RVL) were characterized in thin slices at 32 degrees C using the whole cell patch-clamp technique. Thirty-five percent of the recorded neurons were found to be tyrosine hydroxylase immunoreactive (C1 neurons). In voltage-clamp recordings, ME (3 microM) induced an outward current in 66% of RVL bulbospinal neurons. A similar percentage of C1 and non-C1 neurons were opioid sensitive. The current induced by ME was inwardly rectifying, reversed close to the potassium equilibrium potential, and was blocked by barium. Most spontaneous postsynaptic currents recorded in these neurons were tetrodotoxin (TTX)-resistant miniature postsynaptic currents (mPSCs). Approximately, 75% of mPSCs had rapid kinetics (decay time = 4.7 ms) and were glutamatergic [miniature excitatory postsynaptic currents (mEPSCs)] because they were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). The remaining mPSCs had much slower kinetics (decay time = 19.6 ms) and were GABAergic [miniature inhibitory postsynaptic currents (mIPSCs)] as they were blocked by gabazine (3 microM) but not by strychnine (3-10 microM). ME decreased the frequency of mEPSCs and mIPSCs by 69 and 43%, respectively. The inhibitory effects of ME were mimicked by the selective mu-opioid receptor agonist endomorphin-1 (EM, 3 microM) and were blocked by naloxone (1 microM). In the absence of TTX, excitatory PSCs evoked by focal electrical stimulation were isolated by application of gabazine and strychnine. EM reduced the amplitude of the evoked EPSCs by 41% without changing their decay time. We conclude that opioids inhibit the majority of RVL C1 and non-C1 bulbospinal neurons by activating a potassium conductance postsynaptically and by decreasing the presynaptic release of glutamate. These cellular mechanisms could explain the depressive cardiovascular effects and the sympathoinhibition produced by opioid transmitters in the RVL, in particular during hypotensive hemorrhage.
在32℃条件下，采用全细胞膜片钳技术，在薄片中研究了甲硫氨酸脑啡肽（ME）对延髓头端腹外侧区（RVL）可视化延髓脊髓神经元的影响。发现35%的记录神经元为酪氨酸羟化酶免疫反应阳性（C1神经元）。在电压钳记录中，ME（3μM）在66%的RVL延髓脊髓神经元中诱导出外向电流。C1和非C1神经元中对阿片类药物敏感的比例相似。ME诱导的电流呈内向整流，在接近钾平衡电位时反转，并被钡阻断。在这些神经元中记录到的大多数自发突触后电流是对河豚毒素（TTX）有抗性的微小突触后电流（mPSCs）。大约75%的mPSCs具有快速动力学（衰减时间 = 4.7毫秒），是谷氨酸能的[微小兴奋性突触后电流（mEPSCs）]，因为它们被6-氰基-7-硝基喹喔啉-2,3-二酮（10μM）阻断。其余的mPSCs具有慢得多的动力学（衰减时间 = 19.6毫秒），是γ-氨基丁酸能的[微小抑制性突触后电流（mIPSCs）]，因为它们被gabazine（3μM）阻断，但不被士的宁（3 - 10μM）阻断。ME分别使mEPSCs和mIPSCs的频率降低了69%和43%。ME的抑制作用被选择性μ-阿片受体激动剂内吗啡肽-1（EM，3μM）模拟，并被纳洛酮（1μM）阻断。在没有TTX的情况下，通过应用gabazine和士的宁分离出由局灶性电刺激诱发的兴奋性PSC。EM使诱发的EPSCs幅度降低了41%，而不改变其衰减时间。我们得出结论，阿片类药物通过突触后激活钾电导和减少谷氨酸的突触前释放来抑制大多数RVL C1和非C1延髓脊髓神经元。这些细胞机制可以解释阿片类递质在RVL中产生的心血管抑制作用和交感神经抑制作用，特别是在低血压性出血期间。
