Olfactory system development

Col JA, Matsuo T, Storm DR, Rodriguez I (2007) Adenylyl cyclase-dependent axonal targeting in the olfactory system.Development 134(13) 2481-2489... 

Miyasaka N, Sato Y, Yeo SY, Hutson LD, Chien CB, Okamoto H, Yoshihara Y (2005) Robo2 is required for establishment of a precise glomerular map in the zebrafish olfactory system. Development 132 1283-1293... 

Miyasaka N, Knaut H, Yoshihara Y (2007) Cxcll2/Cxcr4 chemokine signaling is required for placode assembly and sensory axon pathfinding in the zebrafish olfactory system. Development... 

Astic L., Le Pendu J., Mollicone R., Saucier D. and Oriol R. (1989). Cellular expression of H B antigens in the rat olfactory system during development. J Comp Neurol 289, 386-394. 

Holtzman D.A. and Halpem M. (1990). Embryonic and neonatal development of the vomeronasal and olfactory systems in garter snakes. J Morphol 203, 123-140. 

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

Oelschlaeger H.A. and Buhl E.H. (1985). Development and rudimentation of the peripheral olfactory system in the harbor porpoise Phacoena phacoena, Mammalia, Cetacea. J Morphol 184, 351-360. 

Plendl J. and Schmahl W. (1988). Dolichos biflorus agglutinin a marker of the developing olfactory system in a NMRI-mouse strain. Anat Embryol 177, 439-444. 

The demonstration of this behavioural response to a male pheromone signal is significant because of the manner in which the pheromone was delivered. Most other vertebrate examples of reproductive pheromones involve reception via the olfactory system(s). In contrast, D. ocoee females received the pheromone via diffusion through the dorsal skin. We assume that the well developed superficial capillary system of these lungless salamanders is the route by which the male pheromone was transported to whatever target tissue(s) initiated responses that affected female reproductive behaviour. 

If adult odor preferences are in fact influenced by early olfactory experience, then chemosensory processing must be functional early in rodent development. Indeed, the chemosensory systems develop very early in rodents (Alberts 1976 Astic and Saucier 1981), and chemosensory processing appears to be functional both prena-tally (Pedersen and Blass 1982 Stickrod, Kimble and Smotherman 1982) and peri-natally, as evidenced by behavioral responses to odors (Devor and Schneider 1974 Gregory and Bishop 1975 Leon and Moltz 1971 Porter and Etscorn 1974). For example, Syrian hamster pups display behavioral preferences for different types of artificial odorants as early as postnatal day 4 (Devor and Schneider 1974). These data suggest that young rodents are able to both process and respond to odors present in their early environment. 

Taken together, this body of work demonstrates that adult behavioral responses to social odors are shaped by early olfactory experience. Indeed, heterospecific or artificial odor cues associated with the rearing environment acquire attractive properties that can last into adulthood in many rodent species. Furthermore, early experience with opposite-sex odors appears to be critical for the normal development of appropriate behavioral responses to sexual odors in mice and hamsters. Importantly, the behavioral plasticity observed using these different experimental approaches may all be mediated by a classical conditioning model of olfactory learning. The experience-dependent development of odor preference in rodents therefore provides a powerful model for understanding how the olfactory system recognizes and learns the salience of social odors, a function that is critical for the appropriate expression of reproductive behavior. 

The ability of olfactory systems to cope with this plenitude of stimuli together with the fact that specific volatile compounds became associated with different plants and animals and different body sites, glands, and metabolites, provided exquisitely sensitive and accurate cues to the identities of places, trails, individuals, prey, predators, mates, social groups, and food. Olfaction permitted the development of a heretofore unparalleled perceptual talent. 

The response of vertebrates to olfactory stimulation is affected by previous experience but behaviour can be specifically affected by odours (pheromones) (4). The olfactory system has been shown to detect specific components within complex mixtures and analytical chemistry techniques have been used to identify these active components (5). We have assessed the application of these methods to the problems of agricultural odours in an attempt to develop techniques applicable to both slurries and air samples. The identification of the odorous components might allow specific treatment methods to be developed. In addition, the designation of a range of indicator compounds might be useful in practice for monitoring abatement of odour nuisances. 

The vomeronasal system, also known as the accessory olfactory system, consists of chemoreceptors, organized into the VNO, the vomeronasal nerve, its terminal, the accessory olfactory bulb, and more central pathways. First described by Jacobson in 1811, the VNO has been studied intensely. We now know how stimuli reach it and what behaviors it mediates. The VNO occurs in amphibians, reptiles, and mammals. Among mammals, it is best developed in marsupials and monotremes. In birds it only appears during embryogenesis. The VNO and its function are best known for squamate reptiles, particularly snakes, and rodents and ungulates among the mammals. 

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). 

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. 

One perceives from the world only what one has been prepared to perceive. In humans and in most mammals, different senses are used to make sense of life. In contrast, in insects, chemical senses involving odorants and contact chemosensory molecules play a vital role. The olfactory system is the primary sense insects use in analyzing the environment, in crucial tasks such as finding food, nesting, mating and in conspecifics. Contact chemosensation is used especially to analyze specific substrates to assist in the identification of suitable oviposition sites, the recognition of host plants, the selection of tastants and the search for further nutrient chemicals. Dedicated to survival, both olfactory and contact chemosensory systems in insects have developed to extremely high levels of sensitivity and selectivity. 

Binding of hydrophobic molecules by specific protein carriers appears to be a very efficient mechanism to increase both solubility and transport of these molecular messengers in a hydrophilic medium. OBPs and CSPs may represent a successful application of this principle. In particular, the molecular mechanisms of transport of hydrophobic molecules may be more ancient than that most ancient of senses, olfaction. The olfactory system may have developed to extract the hydrophobic odorants from the air environment and optimize their transport and delivery to sensory cells. 

This chapter will discuss the isolation of Drosophila odorant receptor (DOR) genes, how these genes have expanded our understanding of the development and functional anatomy of the olfactory system, how the odor response profiles of OSNs respond to odorants, and the mechanisms by which odor-specific activity is relayed to the brain. 

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