Recent advances in insect olfaction, specifically regarding the morphology and sensory physiology of antennal sensilla of the female sphinx moth Manduca sexta

Abstract

The antennal flagellum of female Manduca sexta bears eight sensillum types: two trichoid, two basiconic, one auriculate, two coeloconic, and one styliform complex sensilla. The first type of trichoid sensillum averages 34 μm in length and is innervated by two sensory cells. The second type averages 26 μm in length and is innervated by either one or three sensory cells. The first type of basiconic sensillum averages 22 μm in length, while the second type averages 15 μm in length. Both types are innervated by three bipolar sensory cells. The auriculate sensillum averages 4 μm in length and is innervated by two bipolar sensory cells. The coeloconic type-A and type-B both average 2 μm in length. The former type is innervated by five bipolar sensory cells, while the latter type, by three bipolar sensory cells. The styliform complex sensillum occurs singly on each annulus and averages 38-40 μm in length. It is formed by several contiguous sensilla. Each unit is innervated by three bipolar sensory cells. A total of 2,216 sensilla were found on a single annulus (annulus 21) of the flagellum. Electrophysiological responses from type-A trichoid sensilla to a large panel of volatile odorants revealed three different subsets of olfactory receptor cells (ORCs). Two subsets responded strongly to only a narrow range of odorants, while the third responded strongly to a broad range of odorants. Anterograde labeling of ORCs from type-A trichoid sensilla revealed that their axons projected mainly to two large female glomeruli of the antennal lobe.

Dynamic Characterization ofDrosophilaAntennal Olfactory Neurons Indicates Multiple Opponent Signaling Pathways in Odor Discrimination

“…Both inhibitory and excitatory responses have been reported in insect primary olfactory neurons (de Bruyne et al, 2001;Shields and Hildebrand, 2001;Hallem et al, 2004;Tichy et al, 2005;Yao et al, 2005) and their odorant receptors . The frequency response approach allowed us to detect simultaneous excitation and inhibition in multiple neurons within a sensillum during the same stimulation by fluctuating concentrations of odorants.…”

“…Inhibitory responses in insect olfaction include cockroach antennal sensilla responding with opposite polarity to lemon oil (Tichy et al, 2005), Manduca sexta olfactory sensilla giving inhibitory responses to a range of odorants (Shields and Hildebrand, 2001), Drosophila basiconic and coeloconic sensilla inhibited by some food odorants and terpenes (de Bruyne et al, 2001;Yao et al, 2005), and Drosophila trichoid sensilla inhibited by fruit odorants (Hallem et al, 2004). A survey of some Drosophila olfactory receptor molecules to a wide range of odorants found 17% excitatory versus 11% inhibitory responses .…”

Olfaction in vector-host interactions

“…Many recent studies, however, have clarified that broad or general tuning, as indicated by previous studies, is a result of two common experimental practices: the lack of screens identifying the actual key ligands for these ORNs; and the concentrations at which kairomones were applied were many hundreds of times higher than environmentally and physiologically relevant (e.g. , Dickens 1990, Heinbockel and Kaissling 1996, Larsson et al 2001, Priesner 1979, Shields and Hildebrand 2001, Skiri et al 2004. Kazama and Wilson (2008), working with D. melanogaster, have determined that high volatile concentrations, which result in high ORN spike frequencies, dampen AL input receptivity and thus, high frequency ORN responses are not the optimal characteristics by which to identify key ligands.…”

Identification and Characterization of Receptors for Insecticidal Toxins from Bacillus thuringiensis

“…sexta is a moth that belongs to the order Lepidoptera; defoliators of tobacco, tomato, and other plants of the Solanaceae family characterize this group of insects and are present over much of the American continent. Because of its large size, M. sexta has served as a model organism to study the mechanism of action of Cry toxins from Bt (Shields and Hildebrand 2001;Kanost et al 2004;Pigott and Ellar 2007). Even for this important model organism, there was very little genetic information available before 2008, when Zou et al through a transcriptome analysis using the first generation of pyrosequencing technology of 454 identified a large number of ESTs associated with hemolymph and immune system which considerably increased the genomic information available for this organism (Zou et al 2008).…”