Olfactory system specialized

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). 

Pheromones are powerful modulators of insect behavior. Since the isolation and identification of the first pheromone, (10E, 12Z)-hexadec-10,12-dien-l-ol, the sex attractant of the silk moth Bombyx mori, thousands of other insect pheromones have been identified. Our understanding of the sensory apparatus required for pheromone detection has also increased significantly. Coincidentally, B. mori was instrumental in many of these advances (see below). Volatile pheromones are detected by a specialized olfactory system localized on the antennae. The precise recognition of species-specific nuances in the structure and composition of pheromone components is essential for effective pheromone-based communication. The pheromone olfactory system of species studied so far exhibits remarkable selectivity towards the species-specific pheromone blend. Pheromones are emitted in low (fg-pg) quantities and are dispersed and greatly diluted in air plumes. Thus, pheromone olfaction systems are among the most sensitive chemosensory systems known. (Schneider et al., 1968). This chapter summarizes efforts (particularly over the past 10 years) to understand the molecular basis for the remarkable selectivity and sensitivity of the pheromone olfactory system in insects. The chapter will also outline efforts to design compounds that interfere with one or more of the early events in olfaction. 

Diversity in the structure and proportion of pheromone components is mirrored in the diversity of the proteins from the olfactory system. A specialized olfactory system is responsible for distinguishing the pheromone from other odorants in the environment. The high precision of the pheromone olfactory system becomes apparent when we compare closely related species whose pheromones differ in subtle ways. For example, Heliothis species have the same unsaturated aldehyde as the major pheromone component, but their pheromone signals differ in the structure and proportion of minor components (Table 16.1). Another example is seen with the gypsy moth (Lymantria dispar) and nun moth (Lymantria monacha), both of which respond to la. The blend produced by the nun moth consists mostly of lb, which is a powerful behavioral antagonist in the gypsy moth and is behaviorally inactive in the nun moth (Hansen, 1984). Stereochemical features play an important role in the molecular recognition of pheromone components. 

The olfactory system may not be designed to supply the brain with a neutral representation of all odors in the environment. Mechanisms of sensory representations have evolved under constant pressure to capture what is relevant. Genetic pathways that shape its physiology are likely to reflect both conserved and highly specialized elements. An exaggerated focus on the adaptation of olfactory acuity to dietary needs would be counterproductive. Just as the fly s visual system does not represent an elaborate apple detector, its olfactory system... 

In addition to the main olfactory system, most mammals possess a well-developed vomeronasal system that is commonly regarded to be specialized for the transmission of pheromonal information [241]. 

We believe that the hormonal stimuli which have been found to affect fish physiology and behavior should be considered pheromonal in nature because they stimulate the olfactory system in highly specific manners at low concentrations and, when tested on the whole organism, often evoke specific, adaptive responses which do not appear to be learned (Sorensen, 1996). At least in fishes, hormonal products can exert special pheromonal actions on conspecifics without being specialized. This being the case, we define a sex pheromone as a substance, or mixture of substances which is released by an individual and which evokes in conspecifics a specific and adaptive reproductive response, the expression of which does not require specific learning. ... 

Shapiro L. and Halpem M. (1995). Lectin histochemical identification of carbohydrate moieties in opossum chemosensory systems during development, with special emphasis on VVA-identified subdivisions in the accessory olfactory bulb. J Morphol 224, 331-349. 

Fig-1 Schematic view of the overall olfactory processing in insects. Pheromones and other semiochemicals are detected by specialized sensilla on the antennae, where the chemical signal is transduced into nervous activity. The olfactory receptor neurons in the semiochemi-cal-detecting sensilla are connected directly to the antennal lobe. Here the semiochemical-derived electrical signals are processed and sent out (through projection neurons) to the protocerebrum. Olfactory information is then integrated with other stimulus modalities, a decision is made, and the motor system is told what to do... 

Although pheromones can be considered as a special form of odorants (scents), their actions, effects and functions have similarities to those of hormones. They bind to a specific receptor which then activates an effector system, which initiates an action potential. They bind to specific sensory cells, the neurones, in the olfactory epithelium, which is located on the roof of the nasal cavities. The epithelium consists of three types of cells, basal, supporting and sensory cells (neurones). The neurones are bipolar, that is they possess a single dendrite, which extends from the cell body to the surface of the olfactory epithelium, and an axon that forms a synapse with a nerve that transfers information to the olfactory centre in the brain. The epithelium is covered with a thick layer of mucus, in which the pheromones dissolve. The mucus contains proteins that bind the pheromone(s) for delivery to the olfactory receptors and then to remove them once they have been detected. 

Fritschy JM, Johnson DK, Mohler H, Rudolph U (1998b) Independent assembly and subcel-lular targeting of GABAa receptor subtypes demonstrated in mouse hippocampal and olfactory neurons in vivo. Neurosci Lett 249 99-102 Fritschy JM, Crestani F, Rudolph U, Mdhler H (2004) GABAa receptor subtypes with special reference to memory function and neurological disorders. In Hensch T (ed) Excitatory inhibitory balance synapses, circuits and systems plasticity. Kluver Academic Press, pp 215-228... 

Figure 20.1 Olfactory sensory cells of both insects and vertebrates are primary sensory cells, i.e. they are bipolar neurons extending a sensory dendritic process towards the odorous environment and projecting an unbranched axon directly to specialized target regions in the central nervous system.

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