Quantitative Analysis of Signal Transduction in the Olfactory Receptor Cell
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We investigated the effect of cyclic nucleotide monophosphate (cNMP) on the cytoplasm of cilia in olfactory receptor cells. In this experiment, we applied caged cNMP to isolated living olfactory receptor cells with the whole-cell patch-clamp method. Photolysis of caged compounds was controlled quantitatively by UV light stimulation locally applied to the cilia. Light illumination induced an inward current in all tested cells, which were loaded with either 1 mM caged cyclic adenosine monophosphate (cAMP) or 1 mM caged cyclic guanosine monophosphate. The amplitude of the light-induced current was dependent on both light intensity and duration. The intensity-and duration-response relation were well fitted by the Hill equation with high cooperativity (Hill coefficient, approximately 4–5), supporting the notion that a Cl current added onto a cNMP-induced current makes a nonlinear booting of the signal transduction. To confirm that idea, we examined Hill’s fitting for the developing phase of the current at +100 mV and −50 mV. The developing phase became more slowly at +100 mV, and the time course was fitted by a smaller Hill coefficient. Furthermore, we compared the light- and odorant-induced response to estimate the activation time course of adenylyl cyclase. Adenylyl cyclase was activated about 260 ms after the onset of the odorant stimulation. When long steps (light and odorant) were applied to the cells, the odorant-induced current showed a stronger and more remarkable decay than did the light-induced response. This observation suggests that the molecular system regulating desensitization locates upstream of the cAMP production site in addition to the direct modulation of the CNG channel.