Chemosensory

Taste-active chemicals react with receptors on the surface of sensory cells in the papillae causing electrical depolarization, ie, drop in the voltage across the sensory cell membrane. The collection of biochemical events that are involved in this process is called transduction (15,16). Not all the chemical steps involved in transduction are known however, it is clear that different transduction mechanisms are involved in different taste quaUties different transduction mechanisms exist for the same chemical in different species (15). Thus the specificity of chemosensory processes, ie, taste and smell, to different chemicals is caused by differences in the sensory cell membrane, the transduction mechanisms, and the central nervous system (14). 

The vomeronasal organ (VNO), located in the nose, is a small chemical sensing stmcture associated with odors and behavioral effects. The vomeronasal system, which is made up of the VNO and a portion of the brain s limbic system, is stmcturaHy independent of the olfactory and nervous terminalis systems in the nose. It may, however, interact with these systems in a manner dependent on prior experience or learning, and therefore be direcdy related to the association of smells and experiences. This independent chemosensory system in the nose may prove to open doors to new learning associated with the sense of smell and human behavior. 

Liberies SD, Buck LB (2006) A second class of chemosensory receptors in the olfactory epithelium. Nature 442 645-650... 

Rodentia (4/30) Of the 2000 or so species, none is suspected of loss. Even a highly fossorial form, the murine Mole Rat, maintains an AOS capacity (Heth, 1995 Zuri, 1998). A promising candidate, the subterranean and eusocial Naked Mole Rat (Batherygidae), shows little or no chemosensory contribution to reproductive regulation (Lacey, 1991 Faulkes et al., 1993). The role of its AOS remains to be discovered, possibly the outbreeding (dispersive) morph is the best candidate type (O Riain et al., 1996). 

The preceding examples may impact upon the chemosensory organisation of one or both systems, but are unstudied. 

A possible alternative is the microvillous part of the chemosensory epithelium in the Organ of Rodolfo-Masera (Septal Organ), which is VN-like (Taniguchi etal., 1993). This mixed receptor population requires much further study since it could prove to have intermediate odourant sensitivities (Giannetti et al., 1995). Its distribution and function(s) are still incompletely known however it is sufficiently widespread, from Opossums to Rodents, to warrant an intensive survey (Rodolfo-Masera, 1943 Kratzing, 1978 Giannetti et al., 1992 and 1995). 

PI. 2.1B Amphibian AOS, surface of accessory neuroepithelium chemosensory strips divided by non-sensory ridges in lateral nasal sinus of Japanese Red-bellied Newt (iCyaops pyrrhogaster) SEM X 742, R = ridge with microvillous cells and G = groove with ciliated cells and cellular protrusions (from Jones et at., 1994). 

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).

In the squamous/stratified epithelium covering, the palatal aperture of the N-P canals and the dorso-lateral surfaces of the papilla, there are occasional clusters of taste buds. These non-olfactory chemosensory elements are positioned at or near to the entrance to the AOS, suggesting that some initial chemosensation may arise from the sampling of material... 

Fig. 2.14 Cellular morphology of neuroepithelium chemosensory groove in lamellae of the African lungfish Protopterus annectens (Dipnoan) basal, receptor and supporting cells (CB, CRe and CS) MB basement membrane. Owen, x400 (from Derivot, 1984).

Fig. 2.18 Chemosensory pathways, Bony Fish, (a) Peripheral ON = Olfactory nv. (input axons) GL = glomerular layer ICL = internal cell layer MCI = mitral cell layer RC = ruffed cell and CF = centrifugal fibre (from Sorensen, 1998). (b) CNS interconnections (from Bardach and Villars, 1968).

Flight by group-living species is likely to improve survival if coordinated by mutual chemosensory arousal, with the advantage of producing predator confusion. Whether such examples require a degree of altruistic selection within the social group is unresolved. 

The vertebrates show many morpho-functional variants on a basic theme (Chap. 2). Some of these, such as the pattern of distribution of the genetically distinct chemosensory neurones within die VN epithelium, will be related to the level of complexity of the animal. In some groups, the VNO can be equally complex, whilst the accessory areas of the brain will differ in complexity, as in the advanced reptiles and mammals. Eventually, detailed comparisons of the genomic repertoire of the various accessory systems should reveal the extent of the operational distinctions amongst them. Of particular interest would be the events which account for the suppression of AOS morphogenesis, and those which compensate for its absence. 

The chemosensory stem cells give rise to several types of neuronal and non-neuronal cell lines under the influence of multiple organisers. From a ventro-lateral infolding, the olfactory pit is produced and this invagination soon becomes separated into two areas which will produce the main and accessory olfactory neurones [Figs. 4.2(a)-(d)]. 

Later in intra-uterine life, the human infant is susceptible to early chemical prompting, but again the affector route is not known with certainty. Neonatal discrimination in favour of familiar (maternal) amniotic fluid is demonstrable, suggesting that the foetus already has active chemosensory capacities (Schaal, 1998). Smell and taste are operative in the near full-term foetus since it shows detection of about 120 mg/day maternal intake of anethole (as anise condiments) within a few days before parturition this exposure induced subsequent preferential responses by babies to anethole (Schaal et ai, 2000). The human neonate is not likely to have its organ as a fully functioning chemosensor,... 

The developmental changes seen in the immediate postnatal period in altricial rodents and especially in the early stages of marsupials, are an expected outcome of their shortened gestational period, early parturition and consequential dependent status. Regrettably, the relative contribution of the main and accessory chemosensory route(s) cannot be fully assessed. The lesser importance of the AOS (by some tests)... 

In support of this contention, the carrier protein Aphrodisin makes an early appearance in vaginal secretions. In pre-pubertal hamsters, it thus indicates chemosensory preparation for the onset of female maturity (Magert, 1999). The proven ability of the AOS to modulate the CNS-pituitary-gonadal axis by advancing or retarding endocrine activity (Chap. 5), underlines its role as primarily the chemosensor of the reproductive system. The adaptive consequence of responses, which allows an avoidance of premature breeding, or of a postponement of puberty, would seem to be advantageous. 

The balance of the evidence at present inclines against any major chemosensory role (Monti-Bloch et al., 1998 Trotier et al., 2000 Meredith, 2001). As noted, evidence of pre-natal, even if transient, functionality (Chap. 4) needs expansion not neglect (Yukimatsu et al., 2000). Its existence into adult hood is at least anatomically admitted, while the degree of variability uncovered in recent surveys of occurrence and of basic morphology (Table 5.1), suggest that an absolute functional disregard is premature. 

The heterogeneity of the VN primary neurones is reflected in their modes of chemosensory preferences. The relative binding efficiencies for distinct odourant types onto the membrane sites is indeed functionally partitioned. When urinary fractions from male mice were applied to VN cells of females, stimulation by a lipophilic and volatile odourant fraction activated only the Gi protein-expressing cells. In contrast, Go activation was elicited by one of the lipocalin superfamily the MUP fraction containing an a-2-globulin (Krieger, 1999). This observation... 

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