Temporal Development of Cng and Ca

A Ca-activated Cl current constitutes a large part of the transduction current in olfactory sensory neurons. The binding of odorants to olfactory receptors in the cilia produces an increase in cAMP concentration, Ca enters into the cilia through CNG channels and activates a Cl current. In intact mouse olfactory sensory neurons little is known about the kinetics of the Caactivated Cl current. Here, we directly activated CNG channels by flash photolysis of caged cAMP or 8-Br-cAMP and measured the current response with the whole-cell voltage-clamp technique in mouse neurons. We measured multiphasic currents in the rising phase of the response at -50 mV. The current rising phase became monophasic in the absence of extracellular Ca, at +50 mV, or when most of the intracellular Cl was replaced by gluconate to shift the equilibrium potential for Cl to -50 mV. These results show that the second phase of the current in mouse intact neurons is due to a Cl current activated by Ca, similarly to previous results on isolated frog cilia. The percentage of the total saturating current carried by Cl was estimated in two ways: (1) by measuring the maximum secondary current and (2) by blocking the Cl channel with niflumic acid. We estimated that in the presence of 1 mM extracellular Ca and in symmetrical Cl concentrations the Cl component can constitute up to 90% of the total current response. These data show how to unravel the CNG and Ca-activated Cl component of the current rising phase.