Pickel V M, Chan J
Department of Neurology and Neuroscience, Cornell University Medical College, New York, NY 10021.
J Neurosci Res. 1994 Apr 15;37(6):735-49. doi: 10.1002/jnr.490370608.
Met5-enkephalin inhibits sodium and water excretion and antagonizes the central actions of angiotensin II in subfornical organ of rat brain. We examined the ultrastructural basis for enkephalin modulation in this circumventricular region. Additionally, we examined the possibility that there might be cellular sites for functional interactions involving Met5-enkephalin and gamma-aminobutyric acid (GABA), a known inhibitory transmitter throughout the central nervous system. Met5-enkephalin and GABA were identified in single coronal sections through the subfornical organ using immunoperoxidase and silver-enhanced immunogold labeling methods, respectively. Enkephalin-like immunoreactivity was most prominently localized within axon terminals. These were distributed primarily in the central, highly vascular, regions of the subfornical organ. Enkephalin-labeled terminals were apposed to the basement membranes of fenestrated capillaries and also formed symmetric, inhibitory type synapses with neurons. In terminals associated with either blood vessels or neurons, the enkephalin immunoreactivity was enriched in large (80-150 nm) dense core vesicles. The immunoreactive vesicles were usually located within portions of the axon in close proximity to astrocytic processes. In contrast, smaller vesicles in the same terminals were more often aggregated near the basement membrane of the capillaries and the active zone of the synapse. The targets of enkephalin-immunoreactive terminals were either unlabeled or GABA-labeled dendrites of local neurons. Enkephalin was also co-localized with GABA in perikarya and in axon terminals. Terminals containing only GABA were far more abundant than those containing enkephalin or enkephalin and GABA. GABA-immunoreactive terminals formed symmetric synapses on unlabeled dendrites some of which also received convergent input from terminals containing enkephalin. Additionally, the enkephalin-immunoreactive terminals were closely apposed to GABA-labeled and unlabeled terminals. These results suggest sites for nonsynaptic release of Met5-enkephalin from dense core vesicles in contact with astrocytes near blood vessels and synaptic complexes in the rat subfornical organ. Moreover, the observed dual localization and pre- and postsynaptic associations between neurons containing Met5-enkephalin and GABA indicate that inhibitory effects of opioids in the subfornical organ may be mediated or potentiated by GABA.
甲硫氨酸脑啡肽抑制钠和水的排泄，并拮抗大鼠脑穹窿下器官中血管紧张素II的中枢作用。我们研究了这个脑室周围区域脑啡肽调节的超微结构基础。此外，我们还研究了是否可能存在涉及甲硫氨酸脑啡肽和γ-氨基丁酸（GABA，一种已知的在整个中枢神经系统中起抑制作用的神经递质）的功能相互作用的细胞位点。分别使用免疫过氧化物酶和银增强免疫金标记方法，在通过穹窿下器官的单个冠状切片中鉴定出甲硫氨酸脑啡肽和GABA。脑啡肽样免疫反应最显著地定位于轴突终末。这些轴突终末主要分布在穹窿下器官的中央、血管丰富的区域。脑啡肽标记的终末与有孔毛细血管的基底膜相邻，并与神经元形成对称的抑制性突触。在与血管或神经元相关的终末中，脑啡肽免疫反应性在大的（80 - 150纳米）致密核心囊泡中富集。免疫反应性囊泡通常位于轴突中靠近星形胶质细胞突起的部分。相比之下，同一终末中的较小囊泡更常聚集在毛细血管基底膜和突触活动区附近。脑啡肽免疫反应性终末的靶标是局部神经元未标记或GABA标记的树突。脑啡肽还与GABA在神经元胞体和轴突终末中共定位。仅含有GABA的终末比含有脑啡肽或同时含有脑啡肽和GABA的终末丰富得多。GABA免疫反应性终末在未标记的树突上形成对称突触，其中一些树突也接受来自含有脑啡肽的终末的汇聚输入。此外，脑啡肽免疫反应性终末与GABA标记和未标记的终末紧密相邻。这些结果表明，在大鼠穹窿下器官中，甲硫氨酸脑啡肽从与血管附近星形胶质细胞接触的致密核心囊泡以及突触复合体中非突触性释放的位点。此外，观察到的含有甲硫氨酸脑啡肽和GABA的神经元之间的双重定位以及突触前和突触后关联表明，阿片类药物在穹窿下器官中的抑制作用可能由GABA介导或增强。
