MOB and AOB

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. 

There are major differences between centrifugal inputs to MOB and AOB. First, centrifugal inputs to AOB arise from far fewer brain regions than inputs to MOB. The major afferents to AOB are from the bed nucleus of the stria terminalis, the nucleus of the accessory olfactory tract, the medial amygdala nucleus and the posteromedial cortical amygdala nucleus (De Olmos et al. 1978 Shipley and Adamek, 1984). A restricted part of the medial division of AON sends a dense projection to the granule cell layer of AOB (Rizvi et al. 1992), but all other divisions of AON lack connections with AOB. 

Snakes were anaesthetized with urethane (2 g/kg) injected i.p. and secured in a Kopf sterotaxic instrument equipped with a specially designed snake restraint. The vomeronasal epithelium was exposed by removing the medial palate and mushroom body. The nasal cavity was exposed by removing part of the mucosa of the roof of the mouth. For recording single units in the MOB and AOB, the cranium was opened dorsally and the bulb exposed (Figure 1). 

The responses of neurons to activating stimuli were concentration-dependent firing fiiequency increased with increased concentration for both airborne odorants and liquid stimuli delivered to the MOB and AOB, respectively (Figure 11). The firing rates to the lowest concentrations of stimulating substances approached firing rates to control stimuli and to nonstimulating substances (liquid stimuli for the MOB and airborne stimuli for the AOB). 

Female hamsters with transections of the lateral olfactory tract (LOT efferents and afferents of MOB and AOB) at the level of the anterior olfactory nucleus (AON) are not anosmic, but do show very little interest in male and female odors (Petrulis et al., 1999). This result indicates that chemosensory structures caudal to the cut, such as Pir, MeA and LEnt, are critical for female hamster sex odor preferences and interest in conspecific odors. Removing the VNO has no effect on a female s preferential investigation of male volatile odorants from a male versus a female s odor but did reduce the overall amount of investigation of male flank scent when direct contact was allowed (Petrulis et al., 1999). It appears that, similar to male hamsters, preferential attraction by females toward opposite sex odors at a distance is mediated by the MOS but that the VNS may play a role in chemoinvestigation in circumstances where direct contact with odorants is allowed. 

PI. 2.4 Bulbar layers horizontal section of MOB + AOB (outlined) in Mouse Lemur (Microcebus murinus). g = Glomerular m = Mitral and gr = Granular layers (courtesy of Alain Schilling ). 

When combined with VN-x it provides a useful parallel approach, as AOB-x leaves other nasal afferents untouched. However, in some species it may also damage MOB efferents (Beltramino and Taleisnik, 1983 Meredith, 1988). Systematic and sequential deafferentation by VN nerve section can yield additional information by examination of the consequences for local degeneration in the AOB (Roland et al.,... 

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...

Fig. 3 Vomeronasal system. Schematic representation of a rodent nasal cavity and brain (lateral view). Accessory olfactory bulb (AOB) mitral cells project to vomeronasal and extended amygdala. Inset The VNO is a bilateral tubular structure located at the base of the nasal septum. VSNs that express the same V1R or V2R converge on a small number of glomeruli in the AOB. Sensory neurons located in the apical layer of the epithelium project to the anterior part of the AOB, whereas those present in the basal layer project to the posterior part. MOE main olfactory epithelium, MOB main olfactory bulb, BSTMPM posteromedial bed nucleus of the stria terminalis, MEA medial amygdaloid nucleus, BACfF bed nucleus of the accessory olfactory tract, PMCO posteromedial cortical amygdaloid area...

Figure 2. Recording sites in the AOB and MOB. GL=gloraemlar layer, MCL=mitral cell layer.

Twenty-two cells, one from each snake, were monitored for activation by prey stimuli. All cells responded to at least one stimulus. We report here the results on nine of these neurons. These cells were systematically tested with different concentrations of stimulating substances and with appropriate control stimuli. In all cases, air delivery of prey odors activated MOB mitral cells but not AOB mitral/tufled cells. Conversely, liquid delivery of prey odors activated AOB mitral/tufred cells, but not MOB mitral cells. Control substances, pure air and snake Ringer s solution, failed to activate any cells (Figures 5-10). 

Figure 11. Concentration-response curves for responses of AOB and MOB mitral cells to EWW, GW, EWV and GFWV.

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