Receptors, antennal

By means of intracellular recording and staining methods, we have examined the responses of AL neurons to stimulation of the ipsilateral antenna with each of the sex pheromone components as well as partial and complete blends (75). In accordance with results of behavioral and sensory-receptor studies, components A and B are the most effective and potent sex pheromone components for eliciting physiological responses in the male-specific AL neurons. On the basis of these responses, we classified the neurons into two broad categories pheromone generalists and pheromone specialists (76). Pheromone generalists are neurons that respond similarly to stimulation of either the component A input channel or the component B input channel and do not respond differently when the complete, natural pheromone blend is presented to the antenna. In contrast, pheromone specialists are neurons that can discriminate between antennal stimulation with component A and stimulation with component B. There are several types of pheromone specialists. Some... 

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... 

The distal part of these receptor cells, the dendrites (0.1-0.5 pm in diameter), extend into the hair lumen (Fig. 3), whereas their axons are connected directly to the antennal lobes in the brain where they make the first synaptic contacts. In the giant silkmoth, Antheraeapolyphemus,for example, each male... 

Differential sensory sensitivity. The insect s perception of plant odours differs essentially from their discrimination of non-volatile taste substances, as phytophagous insects may already perceive the odour at some distance from the plant. In adult phytophagous insects the antennae bear a large number of olfactory sensilla in order to detect the minute concentrations of the leaf odour components in the air downwind from a plant. The overall sensitivity of the antennal olfactory receptor system can be measured by making use of the electroantennogram technique (17). An electroantennogram (EAG) is the change in potential between the tip of an antenna and its base, in response to stimulation by an odour component. Such an EAG reflects the receptor potentials of the olfactory receptor cell population in the antenna. 

The antennal olfactory receptor system in several phytophagous insects is very sensitive in the detection of the green odour components. In the Colorado beetle Leptinotarsa decemlineata, the threshold of response for trans-2-hexen-1-ol is circa 10b molecules per ml of air (17). In comparison, at 760 mm Hg and 20 C, 1 ml of air contains about 1019 molecules. The insects tested i.e., the migratory locust Locusta migratoria, the carrot fly Psila rosae (18), the cereal aphid Sitobion avenae (19), the Colorado beetle L. decemlineata (17), Leptinotarsa... 

The sensitivity of the antennal olfactory receptor system differs even between Colorado beetle populations (see Figure 5). The beetles of the field population in Wageningen are relatively more sensitive for cis-3-hexenyl acetate when tested than those of the laboratory stock culture. Beetles of the Utah population are relatively less sensitive for trans-2-hexen-1-ol and trans-2-hexenal than the individuals of the field population in Wageningen, and those insects obtained from the laboratory stock culture. The functional significance of these differences for the geographic variation in host plant range of this insect species needs further elucidation (21,22). 

Figure 4. Sensitivity spectrum of the antennal olfactory receptor system in several phytophagous insect species to the green odor components. BAG amplitudes in response to the iruiividual components are visualized in the areas of circles. Data were derived from Refs. 18 (a), 19 (b), 17 (c), and 20 (d).

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. 

Boeckh, J., Sass, H. and Wharton, D. R. A. (1970). Antennal receptors reactions to female sex attractant in Periplaneta americana. Science 168 589. 

Schafer, R. and Sanchez, T. V. (1976). The nature and development of sex attractant specificity in cockroaches of the genus Periplaneta. II. Juvenile hormone regulates sexual dimorphism in the distribution of antennal olfactory receptors. Journal of Experimental Zoology 198 323-336. 

Selzer, R. (1981). The processing of a complex food odor by antennal olfactory receptors of Periplaneta americana. Journal of Comparative Physiology A 144 509-519. 

On the specificities of antennal olfactory receptor cells of Periplaneta americana. Chemical Senses 8 375-395. 

Washio, H., Nishino, C. and Bowers, W. S. (1976). Antennal receptor response to sex pheromone mimics in the American cockroach. Nature 262 487 189. 

Shields V. D. C. and Hildebrand J. G. (2001) Responses of a population of antennal olfactory receptor cells in the female moth Manduca sexta to plant-associated volatile organic compounds. J. Comp. Physiol. A 186, 1135-1151. 

Potential PBPs (OS-E and OS-F) U02545, U02546 identified on the third antennal segment of adult males (McKenna etal. 1994) PBP-related proteins found in different subsets of olfactory sensilla (Pikielny etal. 1994) First insect olfactory receptors (of unknown odorant selectivity) identified on the maxillary palp (Clyne etal. 1999)... 

Dickens J. C., Oliver J. E. and Mastro V. C. (1997) Response and adaptation to analogs of disparlure by specialist antennal receptor neurons of gypsy moth, Lymantria dispar. J. Chem. Ecol. 23, 2197-2210. 

Python F. and Stocker R. F. (2002) Adult-like complexity of the larval antennal lobe of D. melanogaster despite markedly low numbers of odorant receptor neurons. J. Comp. Neurol. 445, 374-387. 

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