Perception of noxious compounds by contact chemoreceptors of the blowfly, Phormia regina:: Putative role of an odorant-binding protein

The blowfly, Phormia regina, has sensilla with four contact-chemoreceptor cells and one mechanoreceptor cell on its labellum. Three of the four chemoreceptor cells are called the sugar, the salt and the water receptor cells, respectively. However, the specificity of the remaining chemoreceptor cell, traditionally called the `fifth cell', has not yet been clarified. Referring to behavioral evaluation of the oral toxicity of monoterpenes, we measured the electrophysiological response of the `fifth cell' to these compounds. Of all the monoterpenes examined, d-limonene exhibited the strongest oral toxicity and induced the severest aversive behavior with vomiting and/or excretion in the fly. d-Limonene, when dispersed in an aqueous stimulus solution including dimethyl sulfoxide or an odorant-binding protein (OBP) found in the contact-chemoreceptor sensillum, the chemical sense-related lipophilic ligand-binding protein (CRLBP), evoked impulses from the `fifth cell'. Considering the relationship between the aversive effects of monoterpenes and the response of the `fifth cell' to these effects, we propose that the `fifth cell' is a warning cell that has been differentiated as a taste system for detecting and avoiding dangerous foods. Here we suggest that in the insect contact-chemoreceptor sensillum, CRLBP carries lipophilic members of the noxious taste substances to the `fifth cell' through the aqueous sensillum lymph. This insect OBP may functionally be analogous to the von Ebner's grand protein in taste organs of mammals.