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Temporal Development of Cyclic Nucleotide-Gated and Ca²⁺-Activated Cl^– Currents in Isolated Mouse Olfactory Sensory Neurons

Sector of Neurobiology, International School for Advanced Studies, and Italian Institute of Technology, Trieste, Italy

Submitted 9 March 2007; accepted in final form 24 April 2007

A Ca²⁺-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; Ca²⁺ 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 Ca²⁺-activated 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 Ca²⁺, 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 attributed to a Cl^– current activated by Ca²⁺, 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 Ca²⁺ 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 Ca²⁺-activated Cl^– component of the current rising phase.

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