Sensillum

Fig. 3 Diagrammatic representation of a pheromone-detecting sensillum trichodeum of a moth antenna. Note the compartmentalization of the lymph and particularly its isolation from the hemolymph...

Fig. 5 Single sensillum recordings from the pheromone-detecting sensilla placodea on P. di-versa male antennae. Note a dose-dependent increase in spike frequency after stimulus application for 300 ms (bar)...

Lepidopterous larvae bear on their mouthparts two pair of styloconic sensilla (see Figure 1). The papilla of each sensillum possesses one terminal pore which gives entrance to the dendritic regions of four gustatory receptor cells. Besides, a fifth cell in each sensillum acts as a mechanoreceptor in detecting positional changes of the papilla (7, ) ... 

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. 

The density of antennal sensilla in males rises sharply away from the basal segment for about 1 cm then declines over the next 4 cm to the tip of the antenna (Schaller, 1978 Hosl, 1990). The two receptor cells that are tuned to each of the two periplanones are housed within the same sensillum, the basiconic single-walled type , along with two other cells that respond to terpenes and alcohols (Boeckh and Ernst, 1987). However, unlike the highly specialized receptor cells of male moths, the periplanone-A and periplanone-B cells have overlapping response spectra to these two compounds. Also, it is not known how responsiveness of pheromone-sensitive sensilla to food odorants (terpenes and alcohols) affects behavior of the male cockroach. 

A number of chemo- and mechanoreceptors participate in the male behaviors. The female contact pheromone is detected by chemosensilla on the antennae and labial and maxillary palps (Ramaswamy and Gupta, 1981). The number of these sensilla increases dramatically during the metamorphic molt, and much more so in males than in females. Unfortunately, no electrophysiological recordings have been conducted, and the specific sensillum type that responds to the contact pheromone... 

Bernays, E. A., Chapman, R. F. and Hartmann, T. (2002). A highly sensitive taste receptor cell for pyrrolizidine alkaloids in the lateral galeal sensillum of a polyphagous caterpillar, Estigmene acrea. Journal of Comparative Physiology A 188 715-723. 

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).

The first identified insect OBP was the pheromone binding protein (PBP) of the silk moth A. polyphemus (Vogt and Riddiford, 1981). This 14 kDa protein appeared to be specific to the male antenna, was perhaps the most abundant soluble protein in the antenna, was located in the aqueous extracellular fluid that bathed the pheromone sensitive neurons, and could bind sex pheromone. The concentration of ApolPBP within the sensillum fluid was estimated to be about... 

The following are discussions of four categories of ODEs, based on their location in the animal. To some degree this has functional significance, distinguishing between ODEs of the sensillum lumen (soluble or membrane bound), support cell cytosol and body surface. 

Category 1. Soluble extracellular ODEs of the sensillum lumen... 

An antennal-specific aldehyde oxidase (AOX) of M. sexta (MsexAOX) was the next identified pheromone-degrading enzyme (Rybczynski el al., 1989). The activity of MsexAOX was visualized on non-denaturing PAGE, and was shown to be antennal specific but present in sensilla of both male and female antennae. MsexAOX was observed as a dimer with a combined estimated molecular mass of 295 kDa. M. sexta uses a multicomponent pheromone consisting exclusively of aldehydes including bombykal (Starratt el al., 1979 Tumlinson el al., 1989, 1994) MsexAOX was shown to degrade bombykal to its carboxylic acid. Both TLC and spectrophotometric assays were established and a variety of substrates and inhibitors were characterized. Making adjustments for the concentrations and volumes within a sensillum lumen, the in vivo half-life of pheromone was estimated at 0.6 msec in the presence of this enzyme (Rybczynski el al., 1989). 

ApolAOX, as well as the other AOXs described above, function without the assistance of any cofactors, enabling them to act autonomously in the extracellular environment of the sensillum lumen where access to cellular metabolites might be limited. The ALDH requirement for cofactors may suggest these enzymes are located in the cytoplasm of support cells where cofactors would be available. Antennal ALDH may serve as a biotransformation enzyme for pheromones and possibly xenobiotics in a manner similar to the antennal specific GST of M. sexta (Rogers et al, 1999 see below). 

Figure 14.5 SNMP immunolabeling of EM sections of pheromone-sensitive trichoid sensilla. SNMP antibodies were visualized using secondary antibody conjugated to 10 nm colloidal gold particles. A and B include sensillum cuticle C and D show only dendrites. Sensilla contained two neurons one neuron consistently showed significantly greater labeling. Scale bar. 1.25 pm (A),...

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

Expression of PBPs starts 3 days before adult eclosion in L. dispar (Vogt et al., 1989) and 35 10 h before adult emergence in M. sexta (Vogt et al., 1993), with the expression being induced by the decline in ecdysteroid levels. It has been estimated that in L. dispar PBPs undergo a combined steady-state turnover of 8 x 107 molecules per hour per sensillum (Vogt et al., 1989). 

Kim J.-Y. and Leal W. S. (2000) Ultrastructure of pheromone-detecting sensillum placodeum of the Japanse beetle, Popillia japonica Newmann (Coleoptera Scarabaeidae). Arthropod Struct. Dev. 29, 121-128. 

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).

Figure 17.5 Expression pattern of MbraCSP-A6 isoform in M. brassicae male antennae revealed by in situ hybridization to mRNA. A expression of MbraCSP-A6 isoform at the base of a sensillum trichodium. B section through the cuticle showing labeled sensilla trichodea (s.t.) bases long ones (arranged in a row) and short ones (randomly distributed) a sensillum coeloconicum (s.c.), visible on the same section, is not labeled. Bars 10 pm (adapted from Jacquin-Joly eta ., 2001, by permission of Oxford University Press).

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