Levey A I, Edmunds S M, Hersch S M, Wiley R G, Heilman C J
Department of Neurology, Emory University School of Medicine, Atlanta, Georgia 30322.
J Comp Neurol. 1995 Jan 16;351(3):339-56. doi: 10.1002/cne.903510303.
The m2 muscarinic acetylcholine receptor gene is expressed at high levels in basal forebrain, but the paucity of information about localization of the encoded receptor protein has limited the understanding of cellular and subcellular mechanisms involved in cholinergic actions in this region. The present study sought to determine the cellular localization of m2 protein, its relationship to cholinergic neurons, and its pre- and postsynaptic distribution in the rat medial septum-diagonal band complex using immunocytochemistry with polyclonal rabbit antibodies and a newly developed rat monoclonal antibody specific to the m2 receptor. Light microscopic colocalization studies demonstrated that m2 was present in a subset of choline acetyltransferase immunoreactive neurons, in choline acetyltransferase-negative neurons, and in more neuropil elements than was choline acetyltransferase. Intraventricular injections of 192 IgG-saporin, an immunotoxin directed to the low-affinity nerve growth factor receptor, resulted in depletion of choline acetyltransferase-immunoreactive neurons in the medial septum-diagonal band complex, whereas m2 immunoreactivity in neurons and in the neuropil was unchanged. By electron microscopy, m2 receptor in medial septum-diagonal band complex was localized to the plasmalemma of a small population of small to medium-sized neurons, and it was also found in dendrites, axons, and axon terminals in the neuropil. Neurons expressing m2 immunoreactivity received synaptic contacts from unlabelled axon terminals. A small distinct subpopulation of large neurons, unlabelled by m2 immunoreactivity, received synaptic contacts from m2-immunoreactive terminals. Thus, m2 receptor is situated to mediate the local effects of acetylcholine on basal forebrain cholinergic and noncholinergic neurons and, also, at both pre- and postsynaptic sites.
M2毒蕈碱型乙酰胆碱受体基因在前脑基底核中高水平表达，但关于编码受体蛋白定位的信息匮乏，限制了对该区域胆碱能作用所涉及的细胞和亚细胞机制的理解。本研究旨在利用兔多克隆抗体和新开发的针对M2受体的大鼠单克隆抗体进行免疫细胞化学，确定大鼠内侧隔-斜角带复合体中M2蛋白的细胞定位、其与胆碱能神经元的关系及其突触前和突触后的分布。光学显微镜共定位研究表明，M2存在于胆碱乙酰转移酶免疫反应性神经元的一个亚群、胆碱乙酰转移酶阴性神经元以及比胆碱乙酰转移酶更多的神经毡成分中。脑室内注射192 IgG-皂草素（一种针对低亲和力神经生长因子受体的免疫毒素）导致内侧隔-斜角带复合体中胆碱乙酰转移酶免疫反应性神经元减少，而神经元和神经毡中的M2免疫反应性未改变。通过电子显微镜观察，内侧隔-斜角带复合体中的M2受体定位于一小部分中小型神经元的质膜上，并且在神经毡中的树突、轴突和轴突终末中也有发现。表达M2免疫反应性的神经元接受来自未标记轴突终末的突触联系。一个未被M2免疫反应性标记的大神经元的小而独特的亚群接受来自M2免疫反应性终末的突触联系。因此，M2受体位于可介导乙酰胆碱对基底前脑胆碱能和非胆碱能神经元的局部作用，并且在突触前和突触后位点均有作用。
