Le Jeune H, Aubert I, Jourdan F, Quirion R
Douglas Hospital Research Center, Verdun, Québec, Canada.
J Chem Neuroanat. 1995 Aug;9(2):99-112. doi: 10.1016/0891-0618(95)00070-n.
To provide anatomical information on the complex effects of acetylcholine (ACh) in the olfactory bulb (OB), the distribution of different cholinergic muscarinic and nicotinic receptor sub-types was studied by quantitative in vitro autoradiography. The muscarinic M1-like and M2-like sub-types, as well as the nicotinic bungarotoxin-insensitive (alpha 4 beta 2-like) and bungarotoxin-sensitive (alpha 7-like) receptors were visualized using [3H]pirenzepine, [3H]AF-DX 384, [3H]cytisine and [125I] alpha-bungarotoxin (BTX), respectively. In parallel, labelling patterns of [3H]vesamicol (vesicular acetylcholine transport sites) and [3H]hemicholinium-3 (high-affinity choline uptake sites), two putative markers of cholinergic nerve terminals, were investigated. Specific labelling for each cholinergic radioligand is distributed according to a characteristic laminar and regional pattern within the OB revealing the lack of a clear overlap between cholinergic afferents and receptors. The presynaptic markers, [3H]vesamicol and [3H]hemicholinium-3, demonstrated similar laminar pattern of distribution with two strongly labelled bands corresponding to the glomerular layer and the area around the mitral cell layer. Muscarinic M1-like and M2-like receptor sub-types exhibited unique distribution with their highest levels seen in the external plexiform layer (EPL). Intermediate M1-like and M2-like binding densities were found throughout the deeper bulbar layers. In the glomerular layer, the levels of muscarinic receptor subtypes were low, the level of M2-like sites being higher than M1. Both types of nicotinic receptor sub-types displayed distinct distribution pattern. Whereas [125I] alpha-BTX binding sites were mostly concentrated in the superficial bulbar layers, [3H]cytisine binding was found in the glomerular layers, as well as the mitral cell layer and the underlying laminae. An interesting feature of the present study is the visualization of two distinct cholinoceptive glomerular subsets in the posterior OB. The first one exhibited high levels of both [3H]vesamicol and [3H]hemicholinium-3 sites. It corresponds to the previously identified atypical glomeruli and apparently failed to express any of the cholinergic receptors under study. In contrast, the second subset of glomeruli is not enriched with cholinergic nerve terminal markers but displayed high amounts of [3H]cytisine/nicotinic binding sites. Taken together, these results suggest that although muscarinic receptors have been hypothesized to be mostly involved in cholinergic olfactory processing and short-term memory in the OB, nicotinic receptors, especially of the cytisine/ alpha 4 beta 2 sub-type, may have important roles in mediating olfactory transmission of efferent neurons as well as in a subset of olfactory glomeruli.
为了提供有关乙酰胆碱（ACh）在嗅球（OB）中复杂作用的解剖学信息，通过定量体外放射自显影研究了不同胆碱能毒蕈碱和烟碱样受体亚型的分布。分别使用[3H]哌仑西平、[3H]AF-DX 384、[3H]金雀花碱和[125I]α-银环蛇毒素（BTX）来显示毒蕈碱M1样和M2样亚型以及烟碱样对银环蛇毒素不敏感（α4β2样）和对银环蛇毒素敏感（α7样）的受体。同时，研究了[3H]vesamicol（囊泡乙酰胆碱转运位点）和[3H]半胱氨酸-3（高亲和力胆碱摄取位点）这两种假定的胆碱能神经末梢标记物的标记模式。每种胆碱能放射性配体的特异性标记根据嗅球内特征性的分层和区域模式分布，揭示了胆碱能传入纤维和受体之间缺乏明显的重叠。突触前标记物[3H]vesamicol和[3H]半胱氨酸-3表现出相似的分层分布模式，有两条强标记带分别对应于肾小球层和二尖瓣细胞层周围区域。毒蕈碱M1样和M2样受体亚型表现出独特的分布，在外部丛状层（EPL）中含量最高。在嗅球较深的层中发现中等水平的M1样和M2样结合密度。在肾小球层，毒蕈碱受体亚型的水平较低，M2样位点的水平高于M1。两种烟碱样受体亚型都显示出明显不同的分布模式。虽然[125I]α-BTX结合位点大多集中在嗅球浅层，但[3H]金雀花碱结合在肾小球层以及二尖瓣细胞层和其下方的层中都有发现。本研究的一个有趣特征是在嗅球后部可视化了两个不同的胆碱能感受性肾小球亚群。第一个亚群同时表现出高水平的[3H]vesamicol和[3H]半胱氨酸-3位点。它对应于先前鉴定的非典型肾小球，显然未能表达所研究的任何胆碱能受体。相反，第二个肾小球亚群没有富含胆碱能神经末梢标记物，但显示出大量的[3H]金雀花碱/烟碱样结合位点。综上所述，这些结果表明，尽管毒蕈碱受体被认为主要参与嗅球中的胆碱能嗅觉处理和短期记忆，但烟碱样受体，尤其是金雀花碱/α4β2亚型，可能在介导传出神经元的嗅觉传递以及一部分嗅觉肾小球中发挥重要作用。
