[Olfactory signal transduction: quantification of molecular chains held in a nano-scale structure]

Olfactory transduction is achieved by the G-protein-mediated molecular system. For a long period of time, research on the olfactory transduction system has been limited to qualitative descriptions, because of experimental difficulties accompanying use of diverse ligands, multiple receptors and the fine structure of the sensory cilia (200 nm diameter). However, recent works gradually overcame those difficulties to reveal the molecular systems within nano-scale biological structure. When the cAMP dynamics were estimated, the cAMP-production rate was on the order of 20,000/s/cilium at the maximum odorant stimulus. This number is much smaller than that known for other systems, and therefore it is suggested that the olfactory enzymes use low signal amplification. Apparently, signal transduction with a small number of molecules is achieved by the fine ciliary structure that has a high surface-volume ratio in which even a small change in the absolute number of molecules is reflected in a big change in concentration. In addition, the olfactory receptor cell has a unique and strong non-linear amplification detecting a slight change in the odorant dose, which is regulated by Ca2- that flows through the CNG channel; a cytoplasmic increase of Ca2- in turn activates excitatory Cl current to boost the net transduction current.