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1 Neurobiology Sector, SISSA, Trieste, Italy
* To whom correspondence should be addressed. E-mail: aboccac{at}sissa.it.
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 due 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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