Perireceptor and receptor events in olfaction. Comparison of concentrationand flux detectors: a modeling study

Transduction in chemosensory cells begins with the association of ligand mo lecules to receptor proteins borne by the cell membrane. The receptor-ligan d complexes formed act as signaling compounds that trigger a G-protein casc ade. This receptor-ligand interaction, described here by a single-step or d ouble-step reaction, depends on factors controlling the access of the ligan d molecules to the cell membrane. Two basic mechanisms can be distinguished : concentration detectors (CD), in which the ligand can freely diffuse to t he 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 o f the external ligand concentration. The biological significance of the res ults is shown in a well-studied example of FD, the insect sex-pheromone olf actory receptor neuron. How the number of complexes can code for the intens ity of stimulation is analyzed using the size, dynamic range and sensitivit y 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 lo w concentration) with respect to the CD design, which is paid for by a less er ability to follow fast changes in stimulus intensity.