IMMUNOHISTOCHEMICAL IDENTIFICATION OF DISCRETE SUBSETS OF RAT OLFACTORY NEURONS AND THE GLOMERULI THAT THEY INNERVATE

Glomeruli at the posterior margin of the main olfactory bulb differ in several respects from those located in the remainder of the bulb; e.g ., the olfactory sensory neurons (OSNs) that project here exhibit a di stinct biochemical phenotype and signal transduction pathway, the micr ocircuitry of the glomeruli is substantially altered, and the glomerul i are activated by unconventional odorants. In the present work, we re port that the monoclonal antibodies 2C6 and MAb213 label distinct subs ets of OSNs in the olfactory epithelium (OE), including their axons to their terminations in the main olfactory bulb (MOB). Neurons immunopo sitive with 2C6 are concentrated in the cul-de-sacs of ectoturbinates 1 and 2 and of endoturbinate IV. Unlike the vast majority of OSNs, 2C6 (+) neurons express olfactory marker protein (OMP) at a low level, but their failure to stain with anti-GAP-43 labeling indicates that the O MP ''weak'' neurons are nonetheless mature. Glomeruli positive for 2C6 are bilaterally symmetrical and occupy reproducible positions along t he posterior margin of the MOB. Three of these are very large, and we refer to them as the lateral, posterior ventral, and anterior ventral 2C6(+) necklace glomeruli. MAb213(+) neurons are concentrated in the p osteriormost tips of the cul-de-sacs and recesses at the reflection of the OE at the cribriform plate. Like 2C6(+) neurons, MAb213(+) OSNs a re weakly labeled with anti-OMP but are fully mature. MAb213(+) glomer uli are also bilaterally symmetrical; they occupy reproducible positio ns along the posterior margin of the MOB. The three largest glomeruli occupy lateral, posterior ventral, and posterior positions; the first two are found close to the aforementioned 2C6(+) glomeruli. MAb213 als o intensely labels one of the glomeruli of the modified glomerular com plex, a string of small glomeruli ventrally, and another string dorsal to the accessory olfactory bulb. Acetylcholinesterase (AChE) histoche mical staining of adjacent sections showed that many, but not all, MAb 213(+) glomeruli colocalize with dense or moderate AChE staining. Thus , it is likely that the ''necklace olfactory glomeruli'' (Shinoda et a l., 1990, 1993) and the phosphodiesterase (PDE2)(+) glomeruli (Juilfs et al., 1997) are a subset(s) of the MAb213(+) glomeruli. On the other hand, 2C6(+) glomeruli are not associated with AChE staining. These d ata indicate that the 2C6(+) glomeruli comprise a novel subset in the posterior MOB. In addition to the 2C6(+) and MAb213(+) necklace glomer uli, there is another distinct set of glomeruli at the posterior margi n of the bulb that are OMP(-), 2C6(-), and MAb213(-). In summary, the current work indicates that glomeruli at the posterior margin of the b ulb, which are necklace glomeruli in terms of location and appearance, are actually heterogeneous and may subserve specialized functions wit hin the olfactory system. (C) 1997 Wiley-Liss, Inc.