Decavel C, Van den Pol A N
Section of Neurosurgery, Yale University School of Medicine, New Haven, Connecticut 06510.
J Comp Neurol. 1990 Dec 22;302(4):1019-37. doi: 10.1002/cne.903020423.
To study the organization and distribution of the inhibitory amino acid neurotransmitter GABA in the medial hypothalamus, we used a postembedding immunocytochemical approach with colloidal gold. Quantitative analysis showed that half (49%) of all synapsing boutons studied were immunoreactive for GABA, based on immunogold staining of the suprachiasmatic, arcuate, supraoptic, and paraventricular nuclei. This was corroborated with pre-embedding peroxidase immunostaining with antisera against glutamate decarboxylase, the GABA synthetic enzyme. These data suggest that GABA is the numerically dominant neurotransmitter in the hypothalamus, and emphasize the importance of inhibitory circuits in the hypothalamus. Serial ultrathin sections were used to reconstruct GABA immunoreactive boutons and axons in three dimensions. With this type of analysis we found less morphological heterogeneity between GABA immunoreactive boutons than with single ultrathin sections. Single sections sometimes showed boutons containing only small clear vesicles, and other with both clear vesicles and small dense core vesicles. However, with serial sections through individual boutons, dense core vesicles were consistently found at the periphery of the pre-synaptic GABA immunoreactive boutons, suggesting probable co-localization of GABA with unidentified peptides in most if not all boutons throughout the hypothalamus. A positive correlation was found between the density of small clear vesicles and the intensity of immunostaining with colloidal gold particles. GABA immunoreactive axons generally made symmetrical type synaptic specializations, although a small percentage made strongly asymmetrical synaptic specializations. Vesicles in GABA immunoreactive boutons were slightly smaller than those in non-reactive boutons. Synaptic efficacy is related to the position of the synapse on the post-synaptic neuron. While the majority of GABA immunoreactive axons made synaptic contact with dendrites, the distribution of GABA immunoreactive synapses on somata and dendrites was the same as would be expected from a random distribution of all boutons. No preferential innervation of cell bodies by GABA immunoreactive terminals was found. Serial ultrathin sections showed that a GABA immunoreactive axon would sometimes make repeated synaptic contacts with a single postsynaptic neuron, indicating a high degree of direct control by the presynaptic GABAergic cell. Other immunoreactive axons made synaptic contact with a number of adjacent dendrites and cells, suggesting a role for GABA in synchronizing the activity of hypothalamic neurons. Based on the density of immunogold particles per unit area, varying concentrations of immunoreactive GABA were found in different presynaptic boutons in the hypothalamus.
为了研究内侧下丘脑抑制性氨基酸神经递质γ-氨基丁酸（GABA）的组织和分布，我们采用了胶体金后包埋免疫细胞化学方法。定量分析显示，基于对视交叉上核、弓状核、视上核和室旁核的免疫金染色，所研究的所有突触小体中有一半（49%）对GABA呈免疫反应性。这一点通过用抗谷氨酸脱羧酶（GABA合成酶）抗血清进行的包埋前过氧化物酶免疫染色得到了证实。这些数据表明，GABA是下丘脑中数量上占主导的神经递质，并强调了下丘脑抑制性回路的重要性。连续超薄切片用于三维重建GABA免疫反应性小体和轴突。通过这种分析类型，我们发现GABA免疫反应性小体之间的形态异质性比单个超薄切片时要少。单个切片有时显示小体仅含有小清亮囊泡，而其他小体则同时含有清亮囊泡和小致密核心囊泡。然而，通过对单个小体的连续切片，致密核心囊泡始终位于突触前GABA免疫反应性小体的周边，这表明在整个下丘脑中，大多数（如果不是全部）小体中GABA可能与未鉴定的肽共定位。发现小清亮囊泡的密度与胶体金颗粒免疫染色的强度之间存在正相关。GABA免疫反应性轴突通常形成对称型突触特化，尽管有一小部分形成强不对称突触特化。GABA免疫反应性小体中的囊泡略小于非反应性小体中的囊泡。突触效能与突触在突触后神经元上的位置有关。虽然大多数GABA免疫反应性轴突与树突形成突触接触，但GABA免疫反应性突触在胞体和树突上的分布与所有小体随机分布时预期的相同。未发现GABA免疫反应性终末对细胞体有优先支配。连续超薄切片显示，一条GABA免疫反应性轴突有时会与单个突触后神经元反复形成突触接触，这表明突触前GABA能细胞具有高度的直接控制作用。其他免疫反应性轴突与许多相邻的树突和细胞形成突触接触，这表明GABA在同步下丘脑神经元的活动中发挥作用。根据每单位面积免疫金颗粒的密度，在下丘脑不同的突触前小体中发现了不同浓度的免疫反应性GABA。
