Sensillum recordings

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)...

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... 

A Recording from a basiconic sensillum on the palp classified as pb1. 

A tungsten electrode inserted at the sensillum base records action potentials fired by two neurons, which can be reliably separated based on amplitude and shape of spikes. Neuron A fires the large spike and responds to stimulation with ethyl acetate. Neuron B (small spikes labeled with dots) does not respond to this odorant. For odor stimulation, air was expelled from a 5 ml syringe laden with 20 pi of odorant, diluted 10-2 in paraffin oil. B The palp carries six classes of ORN with different response spectra. They are consistently housed together as stereotypical pairs in three functional types of sensilla. 

Nagai T. (1983) On the relationship between the electroantennogram and simultaneously recorded single sensillum response of the European com borer, Ostrinia nubialis. Arch. Insect Biochem. Physiol. 1, 85-91. 

White R R. (1991) The electroantennogram response effects of varying sensillum numbers and recording electrode position in a clubbed antenna. J. Insect Physiol. 37, 145-152. 

Figure 10.3 Electrophysiological response of the CHC sensillum to nestmate or non-nestmate CHCs with or without CjapCSP. Left column Recordings to stimulus solutions of 10 mM NaCI, 10 mM NaCI plus CSP and 10 mM NaCI plus bovine serum albumin (BSA). Middle column Recordings to non-nestmate CHCs dissolved in the same stimulus solutions. Right column Recordings to nestmate CHCs dissolved in the same stimulus solutions.

While mammalian ORNs are densely clustered in an epithelium deep inside the nose, in insects one to four ORNs are housed in hairlike structures on the body surface, called sensilla (Fig. 2). The dendrites of the ORNs, expressing the ORs, extend into the lymph of the sensillum shaft. At least for certain pheromones, ORN activation requires the presence of an odorant-binding protein in the lymph (Ha and Smith 2006). The stereotyped assembly of ORNs in sensilla has proven useful to record from identified neurons and to define the range of odours to which they respond. 

Figure 41. Electrical recording from a contact chemoreceptor sensillum of a fly. The dendrite of each neuron is sensitive to a different class (sugars, cations, anions, water) of substances. Electrical responses (right) are recorded through a crack made in the side of the sensillum. (Reference 193, p. 204).

Fig. 1.1(a) Sidewall recordings from a lateral sensillum styloconicum of a larva of Manduca sexta (tobacco homworm). Stimulation with an airstream (0.8 m s ) carrying water vapor. 

Some of the recording and stimulating difficulties can be avoided by the side-wall recording technique (Dethier, 1976 Hanson, 1970 Morita and Yamashita, 1959 Rees, 1968). Examples of such recordings are given in Fig. 1.1. The problem with this method, which has prevented its wider use, is a technical one (i) since normally only one sensillum of a preparation can be investigated, the variability between insects and sensilla cannot be separated (ii) the technique itself creates inherent variability (Blaney, 1974). 

Because most receptor cells are not narrowly responsive to specific chemicals and more than one cell is located in a sensillum, the interpretation of recorded... 

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). 

Figure 3.2 (See color insert.) Schematic overview of insect antennal structures and possible modes of elec-trophysioiogicai measurements. Antennae are adorned with many sensillum types, each housing olfactory receptor neuron (ORNs) of various sensitivities as defined by the olfactory receptors they express. Approximate summated responses from many/all sensilla can be recorded in the fonn of voltage deflections, termed electroantennogram (right), or individual ORN responses can be measured by penetrating a single sensillum (bottom).

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