Two olfactory systems

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... 

Our work clearly demonstrates that the main as opposed to the accessory olfactory system plays a central role in mate recognition in both male and female mice. By contrast, clear sex differences emerge in the relative roles of these two olfactory systems in the regulation of mating. Thus, the main olfactory system seems to be more important in regulating sexual behavior in the male, whereas in the female, the accessory olfactory system seems to predominate. 

Odors are perceived via the olfactory system, which is composed of two organs in the nose the olfactory epithelium, a very small area in the nasal system, and the trigeminal nerve endings, which are much more widely distributed in the nasal cavity (11). The olfactory epithelium is extremely sensitive, and humans often sniff to bring more odorant in contact with this area. The trigeminal nerves initiate protective reflexes, such as sneezing or interruption of irrhalation, with exposure to noxious odorants. 

Hamada, S., Senzaki, K., Hamaguchi-Hamada, K. et al. (1998). Localization of 5-HT2a receptor in rat cerebral cortex and olfactory system revealed by immuno-histochemistry using two antibodies raised in rabbit and chicken. Molec. Brain Res. 54, 199-211. 

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). 

In this model, OBPs participate in the selective transport of pheromone and other semiochemicals to their olfactory receptors. The selectivity of the system is likely to be achieved by layers of filters [ 16], i.e., by the participation of compartmentalized OBPs and olfactory receptors. It seems that OBPs transport only a subset of compounds that reach the pore tubules. Some of these compounds may not bind to the receptors compartmentalized in the particular sensilla. The odorant receptors, on the other hand, are activated by a subset of compounds, as indicated by studies in Drosophila, showing that a single OR is activated by multiple compounds [66]. If some potential receptor ligand reaches the pore tubules but are not transported by OBPs, receptor firing is prevented because the receptors are protected by the sensillar lymph. In other words, even if neither OBPs nor odorant receptors (ORs) are extremely specific, the detectors (olfactory system) can show remarkable selectivity if they function in a two-step filter. 

It is not always clear how pheromone signals are detected in mammals. Most vertebrates, mice for example, have a VNO in addition to the main olfactory system. The VNO has two separate families of olfactory receptors Vlr, 137 functional receptors in mice V2r, 60 functional receptors in mice. The genes for these are only distantly related to those for the main olfactory receptors, suggesting that these systems evolved independently. As a general rule, it is the VNO and not the olfactory epithelium that is responsible for detecting pheromone molecules. However, it has been demonstrated that mice whose VNO has been surgically removed can discriminate MHC-determined odor types. This finding clearly implicates the main olfactory system in the detection of pheromones. 

In the frog Rana temporaria, the VNO shares the nasal compartments with the main olfactory organ. The VNO has three cavities, and water enters from the external nares via two fissures. The VNO is used to sample water while the frog is submerged, while above water air is inhaled and the olfactory system stimulated (D0ving etal., 1993). 

Some sensory neurons of the VNO express two gene superfamilies, termed Vlr and V2r, that encode over 240 proteins of the seven-transmembrane type (Matsunami and Buck, 1997). These G-protein-linked putative pheromone receptors are distantly related to the main olfactory system s receptors. Receptors of the VNO are linked to different G-proteins, and their extracellular N-terminal domains are longer than those of the receptors in the main olfactory system. (Vi receptors are linked to Gi-proteins and V2 receptors to Go-proteins). The intracellular excitation mechanism in VNO sensory neurons also differs from that in the main olfactory systems instead of linking to adenylyl cyclase, the VNO receptors activate the phosphoinositol second messenger system. This has been demonstrated in several mammalian species. In hamsters, aphrodisin increases inositol 1,4,5-trisphosphate (IP3) levels in VNO membranes. Boar seminal fluid and urine stimulate increases of IP3 in the VNO of the female pig. (However, in the pig, the VNO is not necessarily essential for responses to pheromones [Dorries etal., 1997]). 

Estradiol subsequently builds up in the blood and first (within 2 to 12 hours) reduces the levels of follicle-stimulating hormone (FSH) and the amplitude of LH pulses, then (within 12 to 48 hours) causes preovulatory surges of LH and FSH. The former promotes ovulation and development of a corpus luteum (reviewed in Martin et ah, 1986). Two compounds have been indicated in the effect of the odor of ram s fleece on LH secretion in anestrous ewes. These are 1,2-hexadecanediol and 1,2-octadecanediol. In Merino sheep at least, maximum stimulation of ovulation requires full exposure to a ram, such as fenceline contact in pastures. Olfactory cues from the ram s wool, presented in a facemask for the ewe, are ineffective by themselves visual and tactile stimuli are also important. The Merino breed does not rely as much on olfactory cues as other breeds of sheep (Pearce and Oldham, 1988). The effect is not necessarily species specific hair extract from male goats stimulates LH release in ewes. For this effect, the accessory olfactory system is not necessary (Signoret etah, 1989). 

Hardin There are two ways to explain that. First, in the middle of the night it is dark and you can t use your major sensory system, and if you need to detect something such as a predator, this would be one way of achieving this — through a heightened sense in the olfactory system. Another possibihty is that... 

Figure 16.3 Recognition of conformation by the pheromone olfactory system of insects. A Rotamers around the single C-C bond once removed from the Z-double bond fall into two groups. Some rotamers are mimicked by the R-cyclopentene analog and others by the S-analogue. B Steric interactions between the adjacent methyl groups impose a subtle twist on 9, the pheromone of the dried fruit beetle. The insects have adapted their olfaction precisely to the shape of 9 an increase or decrease of the twist results in loss of activity.

The powerful analysis that is possible with the detailed description of the Drosophila olfactory system may well be hampered by a lack of knowledge in two areas of research that need more attention the chemistry and behavioral ecology of the adequate stimuli for this system. 

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