Atypical olfactory glomeruli subset of the rat: quantitative study and organization of the peripheral afferents.

The atypical glomeruli constitute a particular subset of olfactory glomeruli in the rat olfactory bulb which is mainly characterized by a strong centrifugal cholinergic innervation. In the present study, the topographical organization of the mucoso-bulbar projection of these glomeruli was analysed using small injections of WGA-HRP into the anterior nasal cavity of adult rats. The atypical olfactory glomeruli were visualized on adjacent bulbar sections using acetylcholinesterase histochemistry. A mean of 29 atypical glomeruli per bulb was observed in several areas of the posterior half of the olfactory bulb. Following the rostro-caudal axis of the olfactory bulb, the first atypical glomeruli were located in lateral positions, then in dorsal and ventral ones. The most posterior atypical glomeruli were located in the bulbar medial side. Concerning the projections from the periphery to the atypical glomeruli, various WGA-HRP patterns of labelling were observed. When the surface area of injection sites in the anterior part of the olfactory sheet was between 30 and 40 mm2, half of the atypical population was labelled with the atypical glomeruli being heavily labelled. All sites of distribution previously described were represented. When the surface area of injection sites was inferior to 20 mm2, only some positions distributed along the bulbar antero-posterior axis were represented. These atypical glomeruli were generally partially labelled. Taken together, these results suggest that, although atypical glomeruli are restricted in the posterior olfactory bulb, they receive peripheral projections diffusely organized along the antero-posterior axis of the olfactory mucosa. This profile was compared with that of other classical olfactory glomeruli.