Main olfactory bulb cortex

Fig. 35.1 Simplified diagram of chemosensory circuit in amygdala. Vomeronasal input via accessory olfactory bulb (VNO/ AOB) is analyzed in anterior and posterior medial amygdala (MeA, MeP). MeP appears to be inhibited by intercalated nucleus (ICNc) for heterospecific and artificial stimuli. MOE/ MOB Main olfactory epithelium/Main olfactory bulb. ACN Anterior Cortical Nucleus. PC Piriform Cortex. BLA Basolateral amygdala. ICNr rostral part of medial intercalated nucleus. ICNc caudal part of ICN. MPOA Medial Preoptic Area. VMH Ventro-medial hypothalamus...

Abbreviations ACo = anterior cortical amygdaloid nucleus AOB = accessory olfactory bulb Me = medial amygdaloid nucleus AON = anterior olfactory nucleus (m = medial division) PCo = posterior cortical amygdaloid nucleus BST = bed nucleus of the stria terminalis DHR = dorsal hippocampal rudiment DPC = dorsal peduncular cortex DR = dorsal raphe nucleus Ent = entorhinal cortex LC = locus coeruleus LPO = lateral preoptic area MOB = main olfactory bulb MnR = median raphe BAOT = nucleus of the accessory olfactory tract NLOT = nucleus of the lateral olfactory tract DB = nucleus of the diagonal band PeCo = periamygdaloid cortex Pir = piriform cortex Tu = olfactory tubercle TT = taenia tecta... 

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). 

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. 

Figure 1. A. Sagittal section of the anterior telencephalon stained with antibody to olfactory marker protein. Ant=anterior AOB, AOB=accessory olfactory bulb, FCx=frontal cortex, MOB=main olfactory bulb, Post=posterior AOB. Rostral is to the left. B. Vomeronasal organ stained by the NissI method. Bv=blood vessel, Lu=lumen, S=septum arrows point to sensory epithelium.

The mRNA coding for 5-HT6 receptor has been localized in the rat brain by Northern blot, PCR, and in situ hybridization (206,207,213,214) (see also Fig. 10) and the protein by immunohistochemistry (213). The first and more detailed description on the localization of mRNA coding for 5-HT6 receptor was published by Ward and co-workers (215), reporting that the main rat brain regions where this receptor is expressed is the pyramidal layer of the olfactory tubercle, islands of Calleja, nucleus accumbens, striatum, hippocampus, and piriform cortex. At moderate levels, it is expressed in other cortical areas, the olfactory bulb, some nuclei of the hypothalamus and amygdale, the habenula, and the cerebellum. No mRNA expression is found in the raphe nucleus. These results were confirmed later (204). 

Important aspects bioassays as well as their types are discussed below. One of the main drawbacks with the bioassays is that some of them have long durations, which can delay the results. Hence, researches on new and faster bioassays are carried out. An interesting new possible bioassay involves the use of olfactory j3-waves that is, bursts of c.20 Hz fast waves that are elicited in the olfactory bulb and pyriform cortex in rats are observed.13 These fast waves are also observed in voles.14 These studies indicate that these /3-waves may provide an easy means of identifying new antifeedants in small herbivores. 

The indusium griseum (IG) or dorsal hippocampal continuation receives input from but does not project to the olfactory bulb. It is a thin layer of cortex which runs parasagit-tally just dorsal to the corpus callosum. IG has been the subject of debate as to whether it is more related to the hippocampus or olfactory bulb (cf. Wyss and Sripanidkulchai, 1983 Adamek et al. 1984 for further discussion). It now seems clear that IG receives direct inputs to its tiny molecular layer from the olfactory bulb (Wyss and Sripanidkulchai, 1983 Adamek et al. 1984 De Olmos et al. 1978). This input is mainly to the rostral IG with fewer fibers running more caudally. The molecular layer of IG also receives input from the lateral and medial entorhinal cortex (Luskin and Price, 1983b). Since the entorhinal area receives direct olfactory bulb inputs and, in turn, projects to the dentate gyrus of the hippocampus it has been suggested that IG is a phylogenetically old part of the hippocampus that receives direct olfactory information as opposed to most of the hippocampus that receives only indirect olfactory input via the entorhinal area (Adamek et al. 1984). 

The cytoarchitecture of NLOT (Fig. 17C) has been studied extensively by McDonald (1983). It is considered an anterior part of the amygdala. NLOT can be subdivided into 3 layers on the basis of Nissl preparations a superficial plexiform layer I which contains a few small and medium-sized cells, a layer II which contains many tightly packed cells, and layer III located dorsal to layer II and containing fairly large, loosely packed cells. Most cells of NLOT are medium-sized pyramidal shaped with extensive spines on secondary and distal dendrites. According to McDonald (1983), layers I and II appear similar in connections to the piriform cortex while layer III seems to be a closely related subcortical area. Many neurons of layers II and fewer neurons of layer III project to the olfactory bulb (de Olmos et al. 1978 Shipley and Adamek, 1984). In addition to olfactory bulb projections, many axons of NLOT neurons make up the stria terminalis and cross to the contralateral piriform cortex, olfactory tubercle, lateral nucleus of the amygdala, and bed nucleus of the stria terminalis (de Olmos, 1972). Afferent connections to NLOT arise mainly from olfactory related areas and the basolateral nucleus of the amygdala. 

Piriform cortex, lateral entorhinal cortex and the transitional cortical areas project heavily back to the olfactory bulb (Figs. 13,14, 18,19). The projections are heavier from the rostral than the caudal parts of primary olfactory cortex in rat and mouse (Shipley and Adamek, 1984). A few cells in the posterolateral and medial cortical amygdaloid areas may project to the MOB (Shipley and Adamek, 1984). These feedback projections to the olfactory bulb arise mainly from pyramidal neurons in layers II and III in primary olfactory cortex. 

The axons of the mitral cells give off collaterals within the bulb in the internal plexiform and granule cell layers (Mori et al. 1983). The main axons course predominantly in the lateral olfactory tract which forms at the level of the AOB. These caudally directed axons give off collaterals in the anterior olfactory nucleus (AON) and other regions of olfactory cortex (Figs. 13, 18, 19). Tufted cells collateralize to an even greater extent in the bulb than mitral cells. The intrabulbar association pathway formed by CCK-ergic tufted cells was discussed earlier. 

Fig, 14. Major connections of the main (MOB) and accessory (AOB) bulbs with cortical (gray panels) and subcortical structures (ellipses). Output projections of MOB and AOB shown by solid lines reciprocal and centrifugal projections to MOB and AOB are shown by gray lines. Cortical areas comprising the primary and accessory olfactory cortex are indicated by squares. 

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