Carlton S M, Hayes E S
Marine Biomedical Institute, University of Texas Medical Branch, Galveston 77550.
J Comp Neurol. 1990 Oct 8;300(2):162-82. doi: 10.1002/cne.903000203.
It is hypothesized that terminals containing gamma-aminobutyric acid (GABA) participate in presynaptic inhibition of primary afferents. To date, few convincing GABA-immunoreactive (GABA-IR) axo-axonic synapses have been demonstrated in support of this theory. The goal of this study is to document the relationship between GABA-IR profiles and central terminals in glomerular complexes in lumbar cord of the monkey (Macaca fascicularis). In addition, the relationship between GABA-IR profiles and other neural elements are analyzed in order to better understand the processing of sensory input in the spinal cord. GABA-IR cell bodies were present in Lissauer's tract (LT) and in all laminae in the spinal gray matter except lamina IX. GABA-IR fibers and terminals were heavily concentrated in LT; laminae I, II, and III; and present in moderate concentration in the deeper laminae of the dorsal horn, ventral horn (especially in association with presumed motor neurons), and lamina X. Electron microscopic analysis confined to LT and laminae I, II, and III demonstrated GABA-IR cell bodies, dendrites, and myelinated and unmyelinated fibers. GABA-IR cell bodies received sparse synaptic input, some of which was immunoreactive for GABA. The majority of the synaptic input to GABA-IR neurons occurred at the dendritic level. Furthermore, the presence of numerous vesicle-containing GABA-IR dendrites making synaptic interactions indicated that GABA-IR dendrites also provided a major site of output. Two consistent arrangements were observed in laminae I-III concerning vesicle-containing GABA-IR dendrites: 1) they were often postsynaptic to central terminals and 2) they participated in reciprocal synapses. The majority of GABA-IR axon terminals observed contained round clear vesicles and varying numbers of dense core vesicles. Only on rare occasions were GABA-IR terminals with flattened vesicles observed. GABA-IR terminals were not observed as presynaptic elements in axo-axonic synapses; however, on some occasions, GABA-IR profiles presumed to be axon terminals were observed postsynaptic to large glomerular type terminals. Our findings suggest that a frequent synaptic arrangement exists in which primary afferent terminals relay sensory information into a GABAergic system for further processing. Furthermore, GABA-IR dendrites appear to be the major source of input and output for this inhibitory system. The implications of this GABAergic neurocircuitry are discussed in relation to the processing of sensory input in the superficial dorsal horn and in terms of mechanisms of primary afferent depolarization (PAD).
据推测，含有γ-氨基丁酸（GABA）的终末参与初级传入纤维的突触前抑制。迄今为止，很少有令人信服的GABA免疫反应性（GABA-IR）轴-轴突触被证实支持这一理论。本研究的目的是记录猴（食蟹猴）腰髓肾小球复合体中GABA-IR结构与中枢终末之间的关系。此外，分析GABA-IR结构与其他神经成分之间的关系，以便更好地理解脊髓中感觉输入的处理过程。GABA-IR细胞体存在于Lissauer束（LT）以及脊髓灰质除第IX层以外的所有层中。GABA-IR纤维和终末在LT、第I、II和III层中高度集中；在背角、腹角（特别是与假定的运动神经元相关处）和第X层的较深层中浓度适中。局限于LT以及第I、II和III层的电子显微镜分析显示了GABA-IR细胞体、树突以及有髓和无髓纤维。GABA-IR细胞体接受稀疏的突触输入，其中一些对GABA有免疫反应性。对GABA-IR神经元的大多数突触输入发生在树突水平。此外，大量含有囊泡的GABA-IR树突进行突触相互作用，这表明GABA-IR树突也是一个主要的输出位点。在第I-III层中观察到含有囊泡的GABA-IR树突有两种一致的排列方式：1）它们通常是中枢终末的突触后成分；2）它们参与交互突触。观察到的大多数GABA-IR轴突终末含有圆形清亮囊泡和数量不等的致密核心囊泡。仅在极少数情况下观察到含有扁平囊泡的GABA-IR终末。未观察到GABA-IR终末作为轴-轴突触中的突触前成分；然而，在某些情况下，推测为轴突终末的GABA-IR结构在大型肾小球型终末的突触后被观察到。我们的研究结果表明存在一种常见的突触排列方式，即初级传入终末将感觉信息传递到一个GABA能系统中进行进一步处理。此外，GABA-IR树突似乎是这个抑制系统的主要输入和输出来源。本文讨论了这种GABA能神经回路在浅背角感觉输入处理以及初级传入去极化（PAD）机制方面的意义。
