Yasuyama Kouji, Meinertzhagen Ian A, Schürmann Friedrich-Wilhelm
Neuroscience Institute, Life Sciences Centre, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1.
J Comp Neurol. 2002 Apr 8;445(3):211-26. doi: 10.1002/cne.10155.
The calyx neuropil of the mushroom body in adult Drosophila melanogaster contains three major neuronal elements: extrinsic projection neurons, presumed cholinergic, immunoreactive to choline acetyltransferase (ChAT-ir) and vesicular acetylcholine transporter (VAChT-ir) antisera; presumed gamma-aminobutyric acid (GABA)ergic extrinsic neurons with GABA-like immunoreactivity; and local intrinsic Kenyon cells. The projection neurons connecting the calyx with the antennal lobe via the antennocerebral tract are the only source of cholinergic elements in the calyces. Their terminals establish an array of large boutons 2-7 microm in diameter throughout all calycal subdivisions. The GABA-ir extrinsic neurons, different in origin, form a network of fine fibers and boutons codistributed in all calycal regions with the cholinergic terminals and with tiny profiles, mainly Kenyon cell dendrites. We have investigated the synaptic circuits of these three neuron types using preembedding immuno-electron microscopy. All ChAT/VAChT-ir boutons form divergent synapses upon multitudinous surrounding Kenyon cell dendrites. GABA-ir elements also regularly contribute divergent synaptic input onto these dendrites, as well as occasional inputs to boutons of projection neurons. The same synaptic microcircuits involving these three neuron types are repeatedly established in glomeruli in all calycal regions. Each glomerulus comprises a large cholinergic bouton at its core, encircled by tiny vesicle-free Kenyon cell dendrites as well as by a number of GABAergic terminals. A single dendritic profile may thereby receive synaptic input from both cholinergic and GABAergic elements in close vicinity at presynaptic sites with T-bars typical of fly synapses. ChAT-ir boutons regularly have large extensions of the active zones. Thus, Kenyon cells may receive major excitatory input from cholinergic boutons and considerable postsynaptic inhibition from GABAergic terminals, as well as, more rarely, presynaptic inhibitory signaling. The calycal glomeruli of Drosophila are compared with the cerebellar glomeruli of vertebrates. The cholinergic boutons are the largest identified cholinergic synapses in the Drosophila brain and an eligible prospect for studying the genetic regulation of excitatory presynaptic function.
外在投射神经元，推测为胆碱能神经元，对胆碱乙酰转移酶（ChAT免疫反应阳性，ChAT-ir）和囊泡乙酰胆碱转运体（VAChT免疫反应阳性，VAChT-ir）抗血清呈免疫反应；推测为γ-氨基丁酸（GABA）能的外在神经元，具有GABA样免疫反应性；以及局部内在肯扬细胞。通过触角脑束将萼与触角叶相连的投射神经元是萼中胆碱能成分的唯一来源。它们的终末在所有萼的亚区形成一系列直径为2 - 7微米的大型终扣。起源不同的GABA免疫反应阳性外在神经元形成一个由细纤维和终扣组成的网络，与胆碱能终末以及微小结构（主要是肯扬细胞树突）共同分布在所有萼区。我们使用包埋前免疫电子显微镜研究了这三种神经元类型的突触回路。所有ChAT/VAChT免疫反应阳性终扣在众多周围的肯扬细胞树突上形成发散性突触。GABA免疫反应阳性成分也经常向这些树突提供发散性突触输入，以及偶尔向投射神经元的终扣提供输入。涉及这三种神经元类型的相同突触微回路在所有萼区的小球中反复建立。每个小球在其核心包含一个大型胆碱能终扣，周围环绕着无小泡的微小肯扬细胞树突以及一些GABA能终末。单个树突结构可能因此在突触前位点从紧邻的胆碱能和GABA能成分接收突触输入，这些位点具有典型的果蝇突触的T形杆。ChAT免疫反应阳性终扣的活性区通常有大的延伸。因此，肯扬细胞可能从胆碱能终扣接收主要的兴奋性输入，并从GABA能终末接收相当数量的突触后抑制，以及更罕见的突触前抑制信号。将果蝇的萼小球与脊椎动物的小脑小球进行了比较。胆碱能终扣是果蝇大脑中已确定的最大胆碱能突触，是研究兴奋性突触前功能的遗传调控的合适对象。
