The main olfactory bulb

The olfactory bulb is an allocortex that, like other cortical structures, has a characteristic laminar organization. The layers of the main olfactory bulb and their principal cell types are discussed next (Fig. 2). 

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In its central projections the vomeronasal pathway, distinguished by a unique lectin-affinity, ascends to an accessory olfactory bulb, while dorsal and ventral pathways supply the dorsal and ventral regions of the main olfactory bulb (Saito and Taniguchi, 2000). The AOS (but not the MOS) of salamanders displays considerable diversity in the... 

Giannetti N., Saucier D. and Astic L. (1992). Organization of the septal organ projection to the main olfactory bulb in adult and newborn rats. J Comp Neurol 323, 288-298. 

Brennan, P.A., Schellinck, H.M., De La Riva, C., Kendrick, K.M. and Keverne, K.B. (1998) Changes in neurotransmitter release in the main olfactory bulb following an olfactory conditioning procedure in mice. Neurosci. 87, 583-590. 

Two olfactory systems have evolved in terrestrial vertebrates which differ in both their peripheral anatomy and central projections. The main olfactory system is usually conceived as a general analyzer that detects and differentiates among complex chemosignals of the environment (Firestein 2001). Odors are detected by olfactory sensory neurons located in the main olfactory epithelium (MOE) these neurons project to glomeruli in the main olfactory bulb (MOB). The mitral and tufted neurons abutting these MOB glomeruli then transmit olfactory signals to various... 

As with many macrosmatic mammals, rodents have two separate chemosensory systems, the main olfactory system (MOS) and accessory olfactory system (AOS), which respond to social odors. Importantly, these sensory systems differ not only in their peripheral morphology and central projections, but also in the types of chemosignals that they process (Meredith 1991). Sensory neurons of the MOS, which are located in the main olfactory epithelium and project to the main olfactory bulbs, process volatile chemicals and can detect odors at a distance. In contrast, sensory neurons of the AOS, which are located in the vomeronasal organs (VNO) and project to the accessory olfactory bulbs, primarily process large, non-volatile chemicals and require contact for stimulation (Meredith 1991). 

FIGURE 5.7 Projection ofreceptor input from olfactory epithelium onto glomeruli in the main olfactory bulb in mice. The epithelium is organized into four zones defined by expression of odorant receptors. Olfactory neurons of a particular zone project to a corresponding zone in the bulb. Axons of these olfactory neurons that express the same odorant receptor (such as those shown in black) converge to a small number of glomeruli. AOB, accessory olfactory bulbs, NC, nucleus coeruleus. (From Mori etal, 1999.)... 

The septal organ is a small patch of sensory epithelium on the wall of the septum, in the anterior part of the nasal cavity, and ventral to the olfactory epithelium. It is found primarily in rodents, has chemical receptors similar to olfactory receptors, and is sensitive to volatile odorants. It projects into the main olfactory bulb, but not into the accessory olfactoiy bulb (Pedersen and Benson, 1986). Because of its forward location, the septal organ may serve as an early-warning system that arouses resting or sleeping animals when volatiles are present (Wysocki, 1989). 

Nerve transection experiments have shown that normal estrus cyclicity and behavioral estrus in mice relies on sensory input through the main olfactory bulbs and does not require the accessory olfactory system (Rajendren and Dominic, 1986). 

Johnson, B. A. and Leon, M. (2001). Spatial representations of odorant chemistry in the main olfactory bulb of the rat. In Chemical Signals in Vertebrates, vol. 9, ed. A. Marchlewska-Koj, J. J. Lepri, and D. Miiller-Schwarze, pp. 85-91. New York Kluwer Academic/Plenum. 

Figure 7 Schematic diagram demonstrating the connection system between the nasal odor receptors and the (main) olfactory bulb. Sensory neurons expressing identical odorant receptors converge their axons to a limited number of defined glomeruli. AOB, accessory olfactory bulb NC, neocortex. Reproduced from K. Mori H. Nagao Y. Yoshihara, Science 1999, 286, 711-715, with permission from AAAS.

Cho JH, Lepine M, Andrews W, Pamavelas J, Cloutier JF (2007) Requirement for Slit-1 and Robo-2 in zonal segregation of olfactory sensory neuron axons in the main olfactory bulb.J Neurosci 27(34) 9094-9104... 

CastiEoPE,Carleton A, Vincent JD, Lledo PM. 1999. Multiple and opposing roles of cholinergic transmission in the main olfactory bulb. J Neurosci 19 9180-9191. 

Davis BJ, Burd GD, Macrides F. 1982. Localization of methionine-enkephalin, substance P, and somatostatin immunor-eactivities in the main olfactory bulb of the hamster. J Comp Neurol 204 377-383. 

Giustetto M, Kirsch J, Fritschy JM, Cantino D, Sassoe-Pognetto M. 1998. Localization of the clustering protein gephyrin at GABAergic synapses in the main olfactory bulb of the cat. J Comp Neurol 395 231-244. 

Kakuta S, Oda S, Takayanagi M, Kishi K. 1998. Parvalbumin immunoreactive neurons in the main olfactory bulb ofthe house musk shrew, Suncus murinus. Brain Behav Evol 52 285-291. 

Kratskin IL, Rio JP, Kenigfest NB, Doty RL, Reperant J. 2000. A light and electron micro scopicstudy of taurine-like immunoreacriAry in the main olfactory bulb of frogs. J Chem Neuroanat 18 87-101. 

Lopez-Mascaraque L, Villalba RM, de Carlos JA. 1989. Vasoactive intestinal polypeptide-immunoreactive neurons in the main olfactory bulb of the hedgehog (Erinaceus euro-paeus). Neurosci Lett 98 19-21. 

Mugnaini E, Wouterlood FG, Dahl A-L, Oertel WH. 1984b. Immunocytochemical identification of GABAergic neurons in the main olfactory bulb of the rat. Archives Italiennes de Biologic 122 83-113. 

Puopolo M, Bean BP, Raviola E. 2005. Spontaneous activity of isolated dopaminergic perigiomeruiar cells of the main olfactory bulb, J Neurophysiol 94 3618-3627,... 

Schneider SP, Macrides F. 1978. Laminar distributions of intemeurons in the main olfactory bulb of the adult hamster. Brain Res Bull 3 73-82. 

Schoenfeld TA, Macrides F. 1984. Topographic organization of connections between the main olfactory bulb and pars externa of the anterior olfactory nucleus in the hamster. J Comp Neurol 227 121-135. 

Schoenfeld TA, Clancy AN, Forbes WB, Macrides E 1994. The spatial organization of rhe peripheral olfactory system of the hamster. Part I Receptor neuron projections to the main olfactory bulb. Brain Res Bull 34 183-210. 

Fig. 2. Cytoarchitecture of the main olfactory bulb. Coronal section of the olfactory bulb stained with cresyl violet at low magnification in A and higher magnification in B. Bar in A, 1 mm.

Fig. 3. Glomeruli. Photomicrographs of the main olfactory bulb showing the glomeruli that are especially well visualized by A. immunohistochemical staining with an antibody to olfactory marker protein (OMP) or B. histochemistry using cytochrome oxidase.

TABLE 1. Neuron types in the main olfactory bulb... 

Fig. 5. Basic circuitry of the main olfactory bulb. Axons of ORNs form the olfactory nerve (ON). These axons terminate in the glomeruli onto mitral (M) and tufted cells (external tufted cell, ET middle tufted cell, MT) and onto juxtaglomerular neurons including periglomerular cells (PG), ET cells and short axon cells (SA). There are one way and reciprocal synapses between the apical dendritic branches of mitral and tufted cells and the dendrites of juxtaglomerular neurons (upper inset - glomerular synapses). The lateral dendrites of mitral and tufted cells form one way and reciprocal synapses with the apical dendrites of granule cells (lower inset - dendrodendritic synapses).

Fig. 8. CCK in the MOB. A-C. Silver-intensified CCK-immunohistoehemical staining in the main olfactory bulb. CCK-immunoreactive neurons are located mainly in the superficial one-third of the EPL the majority of these CCK-positive neurons are tufted cells. The apical and secondary dendrites of these cells are well delineated in the higher-power micrographs of (B) and (C). Thin axon-like processes course toward the ll L. In addition to the staining of cell bodies and dendrites, there is a dense, uniform CCK-like immunoreactive band consisting of terminal-lie puncta restricted to the IPL. Calibration bar in A = 500 fira, bar in B = 100 /tm, and bar in C = 60 /mi.

Fig. 10. Associational system in the MOB. Biocytin anterograde labeling of the inlrabulbar association system (IAS) shown in dorsal (A) to ventral (F) horizontal sections. A. The injection site (asterisk) is located in the superficial half of the EPL on the lateral side of the main olfactory bulb. B-F, Biocytin-labeled tufted cells are located in the superficial part of the EPL and the deep part of the GCL. Axons and collaterals (open arrows) are densely labeled in the IPL of the medial side of the olfactory bulb ventral to the injection site. Bar in F = 800 fim and applied to all panels.

The main olfactory bulb projects to a collection of structures referred to collectively as primary olfactory cortex (De Olmos et al. 1978). These structures may be usefully divided into three groups (A) the anterior olfactory nucleus (Fig. 15) (B) rostral olfac-... 

The main olfactory bulb sends a projection to the entire extent of piriform, peri-amygdaloid and lateral entorhinal cortex (see above. Outputs of MOB). This projection terminates in the superficial half of layer I, layer la. Both mitral and tufted cells project to the rostral parts of AON and piriform cortex while the projection to more caudal parts of olfactory cortex becomes progressively dominated by mitral cells (Schoenfeld and Macrides, 1984). 

Fig. 18. Connections of the MOB. A-E. A rostral to caudal series of coronal sections showing the patterns of anterograde and retrograde labeling produced by an injection of WGA-HRP in the main olfactory bulb.

Davis, B.J. and Macrides, F. (1981) The organization of centrifugal projections from the anterior olfactory nucleus, ventral hippocampal rudiment, and piriform cortex to the main olfactory bulb in the hamster An autoradiographic study, J. Comp. Neurol. 203, 475-493. 

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