Olfactory sensillum

Figure 14.1 Schematic of olfactory sensillum and a generalized biochemical pathway of odor reception. A An olfactory sensillum includes 2-3 neurons surrounded by 3 support cells olfactory dendrites/cilia project up the fluid filled lumen of a cuticular hair. The sensillum lumen is isolated from hemolymph by a cellular barrier. Modified from Steinbrecht (1969) see Steinbrecht (1999) for more details. B Hydrophobic odor molecules enter the aqueous sensillum lumen via pores penetrating the cuticular hair wall. Hydrophilic OBPs are proposed to bind and transport odors to receptor proteins located in the neuronal membranes. ODEs (pathway I) in the sensellum lumen are proposed to degrade these odor molecules. Cytoplasm of support cells contain xenobiotic inactivating enzymes, such as glutathione-S-transferase (GST) (pathway I la) which may also serve to inactivate odor molecules (pathway lib). Interactions between OBPs and ORs and the function of SNMP are unclear. Modified from Rogers et al. (1999).

P americana is one of just a few species of insects in which both peripheral and central olfactory processing have been studied. In contrast to many short-lived lepidopterans, in which the male antenna is highly specialized for sex pheromone reception, the antennae of male cockroaches contain numerous food-responsive sensilla. In addition to olfactory sensilla, the antennae also house mechano-, hygro-and thermoreceptors, as well as contact chemoreceptors (Schaller, 1978 review Boeckh et al., 1984). Extensive ultrastructural and electrophysiological evidence has demonstrated that morphologically defined sensillum types house receptor cells of specific functional types (Sass, 1976, 1978, 1983 Schaller, 1978 Selzer, 1981, 1984 review Boeckh and Ernst, 1987). Boeckh and Ernst (1987) defined 25 types of cell according to their odor spectra, but of the 65 500 chemo- and mechanosensory sensilla on the antenna of adult male P. americana, an estimated 37 000 house cells that respond to periplanone-A and periplanone-B. 

Sexual dimorphism of antenna sensillum types does not become morphologically apparent before the adult stage. Antennal segments increase in length approximately three-fold during postembryonic development in both males and females (Schafer and Sanchez, 1976). In the female, the sensillar population increases 7.5-fold, whereas adult males have 12 times more sensilla than first instars the difference results from a significant proliferation of olfactory sensilla in males. 

Klein U. (1987) Sensillum-lymph proteins from antennal olfactory hairs of the moth Antheraea polyphemus (Saturniidae). Insect Biochem. 17, 1193-1204. 

Figure 16.1 The three levels of molecular recognition in the pheromone olfactory system of insects. Pheromone adsorbs on the cuticle, where it enters the sensillum lymph through pores (1). The first level of molecular recognition occurs when the PBP binds and desorbs the pheromone from the cuticle (2). PBP transports the pheromone through the lymph to the receptor, where the second level of recognition occurs (3). The third level of recognition involves the pheromone-degrading enzymes, which rapidly inactivate pheromone that has dissociated from the PBP (4). PBP-pheromone and/or pheromone alone may also be removed by an endocytotic process, possibly mediated by SNMP (5). Finally, intracellular enzymes may be involved in further removal of pheromone (6).

Generally, the taste sensillum contains only small numbers of receptor neurons for fundamental tastes. Hence, the taste sensillum might not be suitable for perception of CHC pheromones that contain many components. For such multi-component pheromone perception, olfactory sensilla with many receptor neurons might be more suitable, even though CHC contact pheromones are non-volatile in most cases. 

The diverse morphology of the olfactory organ among insect species provides a basis upon which classification has been based. Basically, the olfactory organ in the adult insect comprises two components on the head the antenna and the maxillary palp (Fig. la). Numerous sensilla cover the surface of the antennae and prevent direct contact of ORNs with the external environment. Each sensillum is filled with a potassium- and protein-rich fluid called sensillum lymph and houses one to four ORN dendrites. The small pits on the cuticle surfaces of sensilla allow contact of the ORN dendrite with volatile odorants that dissolve in the lymph. In Drosophila, the third segment of the antenna and of the maxillary palp possess approximately 1,200 and 120 ORNs, respectively. 

Fig. 1 Olfactory organ of insects. Insects have two types of olfactory organ antenna and maxillary palp. Antennae and maxillary palps both bear numerous sensilla. An individual sensillum houses between one and four dendrites of olfactory receptor neurons, a Morphology of the olfactory organ of the adult silkmoth. b Morphology of the olfactory organ of silkmoth larvae...

When a pheromone molecule interacts with the receptor protein a transduction cascade triggers the formation of a nervous signal. This signal travels to the ORN cell body, located just below the base of the sensillum. There, action potentials are triggered that travel down the ORN axon into the AL the primary olfactory centre of the moth brain. 

Insect perception of volatile semiochemicals is mediated through olfactory sensilla, located mainly on the antennae. These sensilla have a porous cuticular surface through which semiochemicals can pass and make contact with the sensillum lymph. Semiochemicals are usually hydrophobic, organic chemicals. For land-living insects, these molecules must be transferred across the aqueous lymph to membrane-bound G-protein-coupled receptors (GPCRs) on the olfactory neurones, from which signal transduction occurs.Transfer across the sensillum lymph is an evolutionary adaptation to terrestrial habitation and is mediated by odorant-binding proteins (OBPs). These are small (14-20 kDa), acidic. 

After the olfactory receptor is activated, the semiochemical signal must be destroyed to prevent continued stimulation. Esterase, dehydrogenase, aldehyde oxidase, epoxidase, and glutathione-S-transferase activities have all been detected in the sensillum and could degrade the signal. A further example is found in the pale-brown chafer. Phyllopertha diversa. which uses 1,3-dimethyl-2,4-( liT,3//)-quinazolinedione 10 as its sex pheromone. 

Mclver et al. (1980) have recently observed a unique contact chemoreceptive sensillum, the bifurcate sensilla on the tarsi of the female black fly (Simulidae). This type of sensillum has characteristics of a contact chemoreceptor, although a sieve-like structure in the pore region, which increases the absorptive surface area at the tip, suggests a secondarily acquired olfactory function. 

The generated spike activity of the-different sensory cells in response to stimuli is decoded by the central nervous system (CNS). To investigate the sensory code of the receptor cells of a sensillum or several sensilla, different approaches can be used. The most direct method is to record from central neurons. In contrast to the investigation of central neurons responding to olfactory stimulations (Chapter 2), such recordings have been performed only recently from interneurons of the thoracic ganglion of the blowfly in response to the stimulation of tarsal contact chemoreceptive sensilla (Rook et al., 1980). 

The specificity of an olfactory cell reflects the specificity of membrane receptors, also called acceptors, which are assumed to be located in the dendritic membranes. The possibility that the specificity might be confined to the conduction system, e.g., microtubules, was excluded by the observation that two sensory cells innervating the same sensillum, and sharing the same conduction system, could be activated by different key substances. In activating a cell, the odor molecule is thought to bind to a membrane receptor protein... 

Figure 2. Schematic drawing of a pheromone-detecting sensillum placodeum. In the Japanese beetle, these pheromone detectors house two olfactory receptor neurons (ORNs), one specialized for the detection of pheromone (5,7,8) and the other tuned to the behavioral antagtmist (5,7,8).

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