Chemoreception

We might fairly gauge the future of biological sciences, centuries ahead, by estimating the time it will take to reach a complete, comprehensive understanding of odor. 

LEWIS THOMAS,in Lflte Night Thoughts on Listening to MahW s Ninth 

Mice are able actively to seek or avoid priming pheromones that modulate their ovarian cycle and onset of puberty. Peripubertal female mice avoid the urine odor of adult males, known to accelerate puberty in females, and are more attracted to the odor of grouped adult females. This behavior is particularly effective because the active space of the (almost) non-volatile male pheromone is small, and prolonged exposure is required for the effect to occur (Coppola and O Connell, 1988). Likewise, prepubertal female mice do not urinate near urine marks of adult males, while grouped, estrous, and diestrous adult females do. Such behavior may help young females to avoid exposure to male odors until they reach puberty. This way they would be protected from mating too early, and their eventual reproductive success would be enhanced (Drickamer, 1989a). 

Various specialized structures may aid in sampling air or improving directional smelling. The elephant s tnmk can be seen as an olfactory periscope tube-nosed bats Murina and Harpiocephalus spp., Vespertilionidae) and hammerhead sharks (Sphyrnidae) represent bizarre examples of specialized olfactoiy anatomy. Nostrils of lower vertebrates tend to be farther apart than those of higher vertebrates that have more flexible heads and necks (Stoddart, 1983). 

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M. G. J. Beets, SAR Structure—Activity Relationships in Human Chemoreception, AppHed Science Pubhshers, London, 1978. 

M. Mathlouthi, J. A. Kanters, and G. G. Birch, Sweet-Taste Chemoreception Elsevier Science Publishing Co., Inc., New York, 1993. 

Fig. 47. Three Possible Courses of Chemoreceptive Events in Sweet-taste Chemoreception. ...

Insectivora (4/6) Poorly studied but comparable to other primitive eutherians (Broom, 1915 Hofer, 1982 Stephan, 1982 Matsuzaki et al., 1993) the variety of ecotypes represented, from aquatic to burrowing, are unknown for chemoreceptive adaptations. 

The higher vertebrates retained the basic Nervus termitialis, a VN-associated chemoreceptive neurai network (Fig. 2.9) with some cognate functions (Chap. 5 and Murphy, 1998). In mammals, an apparently unique and enigmatic OR cell cluster with a separable MOB input occurs. This minor structure (Fig. 2.10), the Septal Organ (of Masera), is possibly adaptive, has no peripheral connection to the VNO, and its capacity for central modulation of MOS and or AOS input is unknown (Naguro, 1984 Marshall, 1986). 

The chemoreceptive systems built up a capacity to discriminate many thousands of chemicals by expanding the range of receptor types. The alterations to the MOS were more varied from reduction and elimination, to enhancement and expansion. In the AOS genome, the latter trend appears (Chap. 6.4) to be less extensive, perhaps being stabilised to maintain the benefits derived from the linkage between the exocrine and endocrine systems. 

Analysis of the relative distribution and expression of OR/VNOR genes would clarify the variety of functional chemoreceptive divisions amongst this transitional group. 

Fig. 2.9 Nasal chemoreceptive systems (Terminalis MOS and AOS) in neonate Rabbit. CP = cribriform plate F = forebrain GT = ganglion terminale NT(SNT) = Nervus terminalis ON = olfactory nvs. and F = forebrain (after Huber and Guild, 1913).

Fig. 2.10 Nasal chemoreceptive systems (Rodent) — chemosensory and autonomic fibres Masera s organ (= Septal Organ) and NT, and vasomotor (NP and Ethmoidal) in adult hamster (from Meredith, 1983).

A wider variety of rearing strategies across representatives of all diosmic taxa would extend assessments of the relative chemoreceptive contributions to parental-offspring interactions. 

The anatomical separation achieved by reptiles parallels the divergence of the main/accessory receptor categories. AOS chemoreception is assumed to elaborate its semiochemical responses by selective gains in ligand-capture efficiency and by alterations of threshold values. Once... 

The chemoreceptive mechanisms in amphibia are undoubtedly worthy of further analysis, not only for their own sake, but to provide clues as to the origination of advanced chemosignal systems. As noted above, a pheromonal signal from the mental gland acts as a courtship/ receptivity inducer. The plethodontid receptivity factor (PRF) (Chap. 3) despite its size (22 kD), seems to have been converted from its internal role as an inter-cellular cytokine, to an inter-individual coordinator of reproductive activity (Rollmann et al., 1999). Endocrine or... 

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