Most nocturnal moths find their mating partners with the help of pheromones released by the females and attracting the males. The males perceive the
pheromone blend with a large number of specialized sensory hairs on their antennae, the sensilla trichoidea. In the sphinx moth Manduca sexta, each trichoid sensillum contains two olfactory receptor neurons, ensheathed by a
common set of accessory cells, the innermost thecogen cell, the trichogen and the tormogen cell (Fig. 1; Keil, 1989; Lee and Strausfeld, 1990). The trichogen and the tormogen cell secrete the receptor lymph or sensillum lymph, which
fills the receptor lymph cavity and the lumen of the sensory hair. The
receptor lymph contains high concentrations of potassium (up to 200 mmol l-1) and pheromonebinding protein (Kaissling et al., 1985; Vogt and Riddiford, 1986).
In extracellular tip recordings, the tip of the sensory hair is clipped off,
and the recording electrode, containing sensillum lymph ringer, is slipped over the open hair (Kaissling, 1995), while the reference electrode is inserted into
the haemolymph space. In this way, the electrical activity of the olfactory receptor neurons and the accessory cells are recorded as a composite signal.
In contrast to many other extracellular recording techniques, tip recordings thus give access to a clearly defined set of electrically active cells. The
first part of this thesis, comprising Chapters 1–4, presents results obtained in tip recordings. Chapter 1 describes the electrical activity of trichoid sensilla in the absence of pheromone. Oscillations of the transepithelial potential and their negative correlation with flight activity were characterized.
The oscillations were influenced by injections of the biogenic amines octopamine and serotonin near the antennal base. In addition, the spontaneous action potential activity of both ORNs was quantitatively investigated. There is obviously no correlation between the time course of the transepithelial
potential and the spontaneous action potential activity. Likewise, the action potential activity was not significantly influenced by amine injection. This
chapter was published in the Journal of Experimental Biology (Dolzer et al., 2001).
Pheromone receptors are made for the detection of pheromone. So Chapters 2 and 3 deal with the responses of trichoid sensilla to stimulation with bombykal, the main component in the conspecific pheromone blend (Tumlinson et al., 1989). In response to pheromone, trichoid sensilla generate a negative deflection of the transepithelial potential, the sensillar potential, and a series of action
potentials. These responses were quantitatively investigated, and their dose-dependence was determined in two different states of adaptation.
While it has long been known that two different adaptation mechanisms act on the generation of sensillar potentials and action potentials, respectively (Zack, 1979), this study suggests the presence of a third mechanism that presumably acts via stabilization of the resting potential.