延髓后角脑啡肽超微结构定位和非突触部位释放——脑啡肽突触前抑制痛觉传递的形态学基础

阿片肽在后角镇痛的作用机理,被认为是通过突触前抑制一级传入纤维P物质释放的结果,然而始终未获得形态学的证实。鉴于一级传入纤维存在大量阿片受体的事实,曾提出阿片肽突触前抑制可能是通过非突触的轴-轴作用。为了验证这一设想,本文用免疫组化方法,详细观察了大鼠延髓后角浅层亮氨酸脑啡肽(L-ENK)轴突终末的突触结构和胞吐释放。电镜观察显示,延髓后角ENK终末可分为两类,第一类终末除了含圆形小清亮囊泡外,还有较多的大颗粒小泡(一般7个以上),主要分布于Ⅰ层,很少看到此类终末形成突触;第二类终末,一般含较多圆形清亮小泡和少量大颗粒小泡(一般不超过3个),它们分布于Ⅰ层和Ⅱ层,此类终末主要形成轴-树突触和少量的轴-体突触。只见到一例轴-轴突触,其突触后成分为未标记的R型终末,此外还见到ENK阳性树突成为中央终末的突触后成分。在去传入神经条件下,上述各类终末皆可见到ENK阳性大颗粒小泡的胞吐形成,它们皆位于非突触区,而在突触部位可见到清亮小泡胞吐像,上述结果提示后角ENK非突触部位释放可能是哭触后抑制一级传入纤维P物质释放的形态学基础。

ULTRASTRUCTURAL LOCALIZATION AND NONSYNAPTIC RELEASE OF ENKEPHALIN IN THE MEDULLARY DORSAL HORN

It has been suggested that the analgesic effect of opioid peptides in the dorsal horn is mediated by presynaptic mechanism to inhibit the release of substance P. However, ultrastructural data indicated that axo-axonal synapses between enkephalin and substance P terminals are unlikely. In fact, a high density of opioid receptors are present on primary sensory fibers in this region. It has been suggested that enkephalin may be involved in presynaptic mechanism via nonsynaptie axo-axonal interactivity. The present investigation was designed to determine in further details of the synaptic relation and nonsynaptic exocytotic release of profiles with Leu-enkephalin immunoreactivity in the medullary dorsal horn of rat.Ultrastructural studies indicated that two classes of ENK containing axonal terminals appeared in the superficial medullary dorsal horn.The first class were found in lamina I which contained more LGV(more than 7 per terminal), they were devoid of synaptic specialization; and the second class were found both in laminae Ⅰ and Ⅱ, and contained mostly small round clear vesicles but a few LGV(mostly less than 3 per terminal). These profiles most commonly formed axo-dendritic synapses, and only one axo-axonal synapse was seen to contact with an unlabelled postsynaptic R-type terminal. In addition, enkephalin dendrites were also seen to be postsynaptic to unlabelled central endings. The exocytotic figures of enkephalin LGV were found in the terminals of both classes after an unilateral skin lesioning in the vibrissa area. All examples of exocytotic figures took place at nonsynaptic sites. These results confirmed that ENK are released at nonsynaptic sites. This allowed diffusion of ENK to act on SP receptors located on the primary sensory fibers to inhibit the release of SP.