Laminar distributions of interneurons in the main olfactory bulb of the adult hamster

Abstract

The laminar distributions of intemeurons in the hamster olfactory bulb were studied with rapid Golgi techniques. Eight morphologically distinct classes of intemeurons were characterized according to their somal locations and their dendritic and axonal properties. Two of these classes, Blanes and Golgi cells, had somata restricted to the deeper layers of the bulb and were not observed more superficially than the mitral body layer (MBL). Their dendrites and axons were predominantly situated within the granule cell layer (GRL) and internal plexiform layer (IPL) but occassionally could be traced as superficially as the deeper portion of the external plexiform layer (EPL). Neither their dendrites nor their axons were oriented consistently with respect to the layers of the bulb. Blanes cells had numerous dendritic spines whereas Golgi cells were relatively spine-poor. Two other classes had somata that were restricted to the IPL and MBL, and had dendrites that exhibited clear orientations with respect to these layers. One class, horizontal cells, had dendrites that ran tangentially within the IPL and MBL. The other class, Cajal cells, had radially oriented dendrites that extended peripherally into the superficial region of the EPL and centrally for greater distances into the GRL. Both classes had axons that projected superficially into the EPL. The granule cells in our material were similar to those described in other species. The sixth class of intemeurons was designated as Van Gehuchten cells. The somata of Van Gehuchten cells were restricted to the EPL and MBL. Their processes branched elaborately within the EPL and MBL but were not oriented consistently with respect to these layers. It is unclear from our material whether these cells bear axons or whether they are amacrine intemeurons. Another class, superficial short axon cells, had somata that were predominantly located at the boundary between the EPL and the glomerular layer (GL) but could also be found throughout the periglomerular region and the superficial half of the EPL. These cells had dendritic and axonal processes that extended predominantly into the GL and appeared to branch around and between individual glomeruli. The eighth class, periglomerular cells, had somata located throughout the GL. Most of these cells gave rise to one dendritic trunk that arborized within a single glomerulus. Occassionally a second, less elaborate dendritic process emerged from the main trunk or from the soma and extended into the periglomerular space or encroached upon an adjacent glomerulus. However, we did not find any periglomerular cells with extensive dendritic arbors in more than one glomerulus. These distributions are discussed in relation to the notion that olfactory bulb interneurons may be assigned to two functional groups associated with two distinct levels of integration.