Sodium channel cluster, βIV-spectrin and ankyrinG positive “hot spots” on dendritic segments of parvalbumin-containing neurons and some other neurons in the mouse and rat main olfactory bulbs

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Axon initial segments (AISs) and nodes of Ranvier are considered as the sites for spike generation, which are highly enriched in sodium channels and some cytoskeletal molecules such as ankyrinG, βIV-spectrin. Previously, we showed that most parvalbumin positive cells in the external plexiform layer (EPL) of the mouse main olfactory bulb (MOB) were anaxonic but displayed some patch-like βIV-spectrin and sodium channel cluster positive segments on their dendrites. In this study we further characterized those particular dendritic segments. AnkyrinG was also located there, whereas phospho-IκBα was not. Electron-microscopically those dendritic segments displayed the membrane undercoating characteristic to the AISs and nodes of Ranvier, further confirming their resemblance to the spike generation sites, “hot spots”. Three-dimensional analysis revealed that each parvalbumin positive EPL neuron had 2–7 hot spots, 3–28 μm in length and located 7–50 μm from the somata. Similar “hot spots” were also encountered on a few calretinin positive granule cells and nitric oxide synthase positive periglomerular cells in the mouse MOB. In addition parvalbumin positive EPL cells in the rat MOB displayed similar multiple dendritic “hot spots”. Our study suggested that these morphologically identified dendritic “hot spots” might correspond to dendritic spike generation sites of those neurons.