Heterogeneity of parvalbumin-containing neurons in the mouse main olfactory bulb, with special reference to short-axon cells and betaIV-spectrin positive dendritic segments.

The structural features of parvalbumin-positive neurons were studied in the mouse main olfactory bulb (MOB). Parvalbumin-positive neurons were heterogeneous, including numerous medium-sized interneurons in the external plexiform layer (EPL), some few large short-axon cells and a few periglomerular cells. Their overall distribution pattern and structural features resembled those of the rat MOB. However, large short-axon cells were frequently encountered in the internal plexiform and granule cell layers, which were rare in the rat MOB. In addition a few large short-axon cells were also encountered throughout the EPL. These short-axon cells extended their axons mainly in the EPL, usually making columnar axonal fields. Most parvalbumin-positive cells except periglomerular cells were confirmed to be glutamic acid decarboxylase positive. We examined the immuno-localization of the markers for the axon initial segments (AISs), betaIV-spectrin and sodium channels, to determine whether or not heterogeneous parvalbumin-positive neurons have axons. We confirmed their localization on the AISs of the large short-axon cells and periglomerular cells. However, these markers were encountered on some patch-like segments on the dendritic processes instead of the thin axon-like processes of the medium-sized EPL interneurons. The present study revealed the diversity of parvalbumin-positive neurons in the mouse MOB and their particular structural properties hitherto unknown.