ATYPICAL OLFACTORY GLOMERULI SUBSET OF THE RAT - QUANTITATIVE STUDY AND ORGANIZATION OF THE PERIPHERAL AFFERENTS

The atypical glomeruli constitute a particular subset of olfactory glo meruli in the rat olfactory bulb which is mainly characterized by a st rong centrifugal cholinergic innervation. In the present study, the to pographical organization of the mucose-bulbar projection of these glom eruli was analysed using small injections of WCA-HRP into the anterior nasal cavity of adult rats. The atypical olfactory glomeruli were vis ualized on adjacent bulbar sections using acetylcholinesterase histoch emistry. A mean of 29 atypical glomeruli per bulb was observed in seve ral areas of the posterior half of the olfactory bulb. Following the r ostro-caudal axis of the olfactory bulb, the first atypical glomeruli were located in lateral positions, then in dorsal and ventral ones. Th e most posterior atypical glomeruli were located in the bulbar medial side. Concerning the projections from the periphery to the atypical gl omeruli, 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 mm(2), half of the atypical population wa s labelled with the atypical glomeruli being heavily labelled. All sit es of distribution previously described were represented. When the sur face area of injection sites was inferior to 20 mm(2), only some posit ions distributed along the bulbar antero-posterior axis were represent ed. 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 pr ojections diffusely organized along the antero-posterior axis of the o lfactory mucosa. This profile was compared with that of other classica l olfactory glomeruli.