Liu W L, Shipley M T
Department of Anatomy, University of Maryland School of Medicine, Baltimore 21201-1559.
J Comp Neurol. 1994 Aug 22;346(4):541-58. doi: 10.1002/cne.903460407.
The intrabulbar associational system (IAS) originates from tufted cells whose axons terminate in the internal plexiform layer (IPL) on the opposite side of the same olfactory bulb. The postsynaptic targets of the IAS are unknown. Subpopulations of tufted cells contain different neuropeptides and transmitters but it is not known if tufted cells forming the IAS are homogeneous with respect to neurotransmitters. Therefore, the goals of the present study were to identify the postsynaptic targets of the IAS and to determine the major transmitter in this intrabulbar circuit. Biocytin anterograde tracing revealed that the axons of superficially situated tufted cells coursed directly to the IPL where they turned abruptly to run ventrally and dorsally to terminate in the IPL on the opposite side of the olfactory bulb. WGAapoHRP-Au retrograde tracing combined with immunohistochemistry for CCK revealed that all tufted cells retrogradely labeled by WGAapoHRP-Au injection in the IPL were immunoreactive for CCK. Anterograde transport of biocytin combined with postembedding immunocytochemical gold-labeling for GABA demonstrated that labeled IAS axons terminate predominantly, if not exclusively, on GABAergic granule cell dendrites in the IPL. These results confirm that the IAS arises from tufted cells and is topographically organized. We further demonstrate that tufted cells forming the IAS use the neuropeptide CCK as a transmitter. In addition, we show that the postsynaptic targets of the CCKergic IAS are the dendrites of GABAergic granule cells coursing through the IPL toward the EPL. As CCK is generally an excitatory neuropeptide, we suggest that the IAS functions to excite topographically discrete populations of granule cells. This action may lead to inhibition of equally discrete populations of mitral/tufted cells. Thus, the IAS may be an intrabulbar inhibitory circuit that coordinates topographically organized neural networks in the olfactory bulb.
球内联合系统（IAS）起源于簇状细胞，其轴突终止于同侧嗅球另一侧的内丛状层（IPL）。IAS的突触后靶点尚不清楚。簇状细胞亚群含有不同的神经肽和递质，但形成IAS的簇状细胞在神经递质方面是否同质尚不清楚。因此，本研究的目的是确定IAS的突触后靶点，并确定该球内回路中的主要递质。生物素顺行追踪显示，位于浅层的簇状细胞的轴突直接通向IPL，在那里它们突然转向腹侧和背侧，终止于嗅球另一侧的IPL。WGAapoHRP-Au逆行追踪结合CCK免疫组织化学显示，在IPL中注射WGAapoHRP-Au后逆行标记的所有簇状细胞对CCK均有免疫反应。生物素的顺行运输结合GABA的包埋后免疫细胞化学金标记显示，标记的IAS轴突主要（如果不是唯一）终止于IPL中GABA能颗粒细胞的树突上。这些结果证实IAS起源于簇状细胞并具有拓扑结构。我们进一步证明，形成IAS的簇状细胞使用神经肽CCK作为递质。此外，我们表明，CCK能IAS的突触后靶点是穿过IPL向EPL方向走行的GABA能颗粒细胞的树突。由于CCK通常是一种兴奋性神经肽，我们认为IAS的功能是兴奋拓扑学上离散的颗粒细胞群。这种作用可能导致对同样离散的二尖瓣/簇状细胞群的抑制。因此，IAS可能是一个球内抑制性回路，它协调嗅球中拓扑学上有组织的神经网络。
