Fujita T, Kumamoto E
Department of Physiology, Saga Medical School, 5-1-1 Nabeshima, Saga 849-8501, Japan.
Neuroscience. 2006;139(3):1095-105. doi: 10.1016/j.neuroscience.2006.01.010. Epub 2006 Mar 3.
Intrathecally-administered endomorphin-1 and endomorphin-2 produce antinociceptive effects which are different from each other. In order to elucidate a cellular basis for this result, we examined the effects of endomorphin-1 and endomorphin-2 on holding currents and spontaneous glutamatergic excitatory transmission in substantia gelatinosa neurons of adult rat spinal cord slices by use of the whole-cell patch-clamp technique. In about half of the neurons examined, endomorphin-1 and endomorphin-2 produced an outward current having a similar amplitude (25-27 pA at 1 microM) at -70 mV with almost the same value of effective concentration producing half-maximal response (0.19-0.21 microM). Both of them reversed at a potential close to the equilibrium potential for K+, and had the slope conductance that was larger at negative (-120 to -140 mV) than positive potentials (-60 to -90 mV). The endomorphin-1 and endomorphin-2 currents were reduced in amplitude by K+-channel inhibitors, Ba2+ (100 microM) and 4-aminopyridine (1 mM), and also by mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (1 microM) to a similar extent. The endomorphin-2 but not endomorphin-1 current amplitude was increased by dipeptidyl peptidase IV inhibitor diprotin A (30 microM). One micromolar endomorphin-1 and endomorphin-2 reduced the frequency of spontaneous excitatory postsynaptic current with a similar time course and extent without altering its amplitude; these actions were not in the presence of D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (1 microM). We conclude that endomorphin-1 and endomorphin-2 hyperpolarize membranes by opening inwardly-rectifying K+ channels and attenuate the spontaneous release of L-glutamate from nerve terminals in the substantia gelatinosa, both of which are mediated by mu-opioid receptors, in a manner quantitatively similar to each other. The difference in antinociceptive effects between endomorphin-1 and endomorphin-2 could not be attributed to a distinction in their effects on excitatory transmission in substantia gelatinosa neurons, and may be explained by a difference in their enzymatic degradation.
鞘内注射内吗啡肽-1和内吗啡肽-2会产生彼此不同的抗伤害感受作用。为了阐明这一结果的细胞基础，我们运用全细胞膜片钳技术，研究了内吗啡肽-1和内吗啡肽-2对成年大鼠脊髓切片胶状质神经元的保持电流和自发性谷氨酸能兴奋性传递的影响。在所检测的约一半神经元中，内吗啡肽-1和内吗啡肽-2在-70 mV时产生了幅度相似的外向电流（1 μM时为25 - 27 pA），产生半数最大反应的有效浓度值几乎相同（0.19 - 0.21 μM）。它们均在接近钾离子平衡电位的电位处反转，且在负电位（-120至-140 mV）时的斜率电导大于正电位（-60至-90 mV）时的斜率电导。内吗啡肽-1和内吗啡肽-2的电流幅度可被钾离子通道抑制剂钡离子（100 μM）和4 - 氨基吡啶（1 mM）以及μ阿片受体拮抗剂D - Phe - Cys - Tyr - D - Trp - Arg - Thr - Pen - Thr - NH2（1 μM）以相似程度降低。二肽基肽酶IV抑制剂二丙谷酰胺（30 μM）可增加内吗啡肽-2而非内吗啡肽-1的电流幅度。1 μM的内吗啡肽-1和内吗啡肽-2以相似的时间进程和程度降低自发性兴奋性突触后电流的频率，且不改变其幅度；在存在D - Phe - Cys - Tyr - D - Trp - Arg - Thr - Pen - Thr - NH2（1 μM）时这些作用消失。我们得出结论，内吗啡肽-1和内吗啡肽-2通过开放内向整流钾离子通道使膜超极化，并减弱胶状质中神经末梢L - 谷氨酸的自发性释放，这两者均由μ阿片受体介导，且在数量上彼此相似。内吗啡肽-1和内吗啡肽-2在抗伤害感受作用上的差异不能归因于它们对胶状质神经元兴奋性传递的影响存在区别，可能是由它们酶促降解的差异所解释。
