Chem. Senses 25: 293-311,
2000
© Oxford University Press 2000
Perireceptor and Receptor Events in Olfaction. Comparison of Concentration and Flux Detectors: a Modeling Study
ivan1
2
Unité de Biométrie, INRA, 78026 Versailles Cedex, France,
1 Institute of Entomology, Academy of Sciences of Czech Republic, Brani
ovská 31, 370 05 
eské Bud
jovice and
2 Institute of Physiology, Academy of Sciences of Czech Republic, Víde
ská 1083, 142 20 Prague 4, Czech Republic
Correspondence to be sent to: Jean-Pierre Rospars, Unité de Biométrie, INRA, 78026 Versailles Cedex, France. e-mail: rospars{at}versailles.inra.fr
Transduction in chemosensory cells begins with the association of ligand molecules to receptor proteins borne by the cell membrane. The receptor–ligand complexes formed act as signaling compounds that trigger a G-protein cascade. This receptor–ligand interaction, described here by a single-step or double-step reaction, depends on factors controlling the access of the ligand molecules to the cell membrane. Two basic mechanisms can be distinguished: concentration detectors (CD), in which the ligand can freely diffuse to the membrane, and flux detectors (FD), in which it accumulates irreversibly in a distinct perireceptor space where it is chemically deactivated. These two systems, plus their generalization, are investigated and compared. The transient and steady-state numbers of complexes are studied as a function of the external ligand concentration. The biological significance of the results is shown in a well-studied example of FD, the insect sex-pheromone olfactory receptor neuron. How the number of complexes can code for the intensity of stimulation is analyzed using the size, dynamic range and sensitivity of the steady-state responses, and the time needed to reach a predefined level of the transient responses. It is shown that the FD design affords a large increase in sensitivity (a shift of the threshold response towards low concentration) with respect to the CD design, which is paid for by a lesser ability to follow fast changes in stimulus intensity.
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