Main olfactory system

The extent of compensatory changes, if any, within the main olfactory system could reveal something of the specificity or generality of... 

Lin D.M. and Ngai J. (1999). Development of the vertebrate main olfactory system. Curr Opin Neurobiol 9, 74-78. 

Swann J. and Fiber J. (1997). Sex differences in function of a pheromonally stimulated pathway role of steroids and the main olfactory system. Brain Res Bull 44, 409-413. 

Spehr, M., Kelliher, K., Li, X.-H., Boehm, T., Leinders-Zufall, T. and Zufall, F. (2006) Essential role of the main olfactory system in social recognition of major histocompatibility complex peptide ligands. J. Neurosci 26, 1961-1970. 

Abstract In this chapter, we review recent data about the involvement of both the main and the accessory olfactory system in mate recognition and the control of sexual behavior in mice. Whereas the main olfactory system seems to play a central role in mate recognition in both male and female mice, clear sex differences emerge with regard to which olfactory system plays a more important role in the control of sexual behavior. Indeed, the main but not the accessory olfactory system seems to be more important in regulating sexual behavior in male mice, whereas in female mice, the accessory olfactory system seems to play a critical role in the control of mating. 

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... 

Although the case is not unequivocal, evidence suggests that the main olfactory system plays a more important role in sex discrimination than the accessory olfactory... 

Which olfactory system mediates sexual behavior in mice has also been a matter of debate. Early studies (Thompson and Edwards 1972 Edwards and Burge 1973) suggested a role for the main olfactory system in the display of female sexual receptivity, since destructions of the MOE by intranasal infusion with a zinc sulfate solution (ZnS04) attenuated lordosis behavior in estrogen-progesterone-treated mice. However, the VNO may be also involved as a reduction in female sexual receptivity by VNO removal has been reported in various other rodent species (rats Rajendren,... 

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). 

Swann, Rahaman, F., Bijak, T. and Fiber, J. (2001) The main olfactory system mediates pheromone-induced fos expression in the extended amygdala and preoptic area of the male Syrian hamster. Neuroscience 105, 695-706. 

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. 

Apart from taste, vertebrates have five different chemoreceptor systems for airborne chemosignals the main olfactory system, the vomeronasal organ (VNO), the trigeminal nerve, the septal organ of Masera, and the nervus termi-nalis. They each will be discussed in turn. All five are fully functional in most mammals (Fig. 5.1). 

In mammals, the main olfactory system is the work horse in the perception of odors. Excellent detailed reviews of the mammalian olfactory system are available elsewhere. In brief, the olfactory epithelium is located on a portion of the scroll bones (endoturbinales and posterior part of nasoturbinales F ig. 5.4), in humans it is located about 1 cm beneath the bridge of the nose. Olfactory reception is affected by several factors, such as the size, shape, or wetness of the nasal passages. In the dog, the olfactory membrane extends over 75-150 cm depending on body size, while in humans it is only 2-4cm. It consists of three... 

The main olfactory system is designed with considerable redundancy. Rats that have been bulbectomized on one side still have the same absolute threshold and intensity difference threshold as intact rats (Slotnick and Schoonover, 1984). 

The main olfactory system mediates munerous behaviors that will be discussed in the section on signaling and priming pheromones, and also... 

From the accessory bulb, projections lead to the medial nuclei and posteri-omedial portion of the cortical nuclei of the amygdala. The amygdala in turn is connected to higher centers via the stria terminalis, and hypothalamic structures. Thus, the accessory olfactory system represents a neural pathway separate from the main olfactory system. Both project into absolutely different parts of the amygdaloids (Powers and Winans, 1975 Powers etal, 1979). 

Species recognition Responses to vaginal secretion marks Main olfactory system... 

In the golden hamster, the VNO is essential for certain reproductive behaviors, while the main olfactory system mediates responses that involve species recognition (Johnston, 1992 Table 5.2). As in mice, removal of the VNO impairs sexual behavior in male golden hamsters, but only if carried out before the animal had had sexual experience (Meredith, 1986). The same is true for ultrasonic vocalizations. 

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]). 

Male mice learn about female cues. In the presence of females, a male emits ultrasonic vocalizations. These vocalizations become less frequent after extirpation of the VNO. The more experience a male has had with females, the smaller the deficit he suffers. The learning is mediated by interaction between the main olfactory system and the accessory olfactory system after vomeronasal deprivation, olfactory cues maintain the behavior (Wysocki etal., 1986). 

In mice, strange females or their urine increase the levels of plasma testosterone in males (Macrides etal., 1975). Experienced male golden hamsters, on the other hand, do not depend on specific pheromonal odors for this testosterone surge induced by estrous females. Other cues, possibly learned odors from a female, perceived via the main olfactory system, appear to activate the neuroendocrine refiex that results in increased testosterone release (Johnston, 2001). 

Although the vomeronasal system is specialized to detect stimuli in a liquid environment, it probably is not functional in utero, at least in mice. Fluorescent microspheres were not taken up by the vomeronasal organ as the access canal is not open yet in utero. In rats, by contrast, the canal is open before birth and the microspheres can be taken up. The olfactory epithelium of the main olfactory system plays a greater role prenatally, as evidenced by the uptake of radiolabeled 2-deoxyglucose (Coppola and Coltrane 1994). Fetal mice respond to amyl acetate and isovaleric acid delivered into the nasal cavity through a tiny cannula (Coppola, 2001). In both rats and mice, the main olfactory system, and not the vomeronasal system, appears to mediate prenatal olfaction (Coppola, 2001). 

Sensory development points to the importance of chemical cues veiy early in life after birth. From the fourth day of life, mouse pups respond to the odor of their nest (Schmidt etal., 1986). In rats, the main olfactory system processes odor responses during the first days of life. 

---