The contribution of a Ca2+-activated Cl- conductance to amino-acid-induced inward current responses of ciliated olfactory neurons of the rainbow trout

摘要

To determine whether amino-acid-induced inward currents of ciliated olfactory receptor neurons (ORNs) in rainbow trout (Oncorhynchus mykiss) include a Ca2+-activated Cl- conductance, we first studied changes in reversal potential and the current/voltage relationships of the responses of ORNs to an amino acid mixture (L-alanine, L-arginine, L-glutamate and L-norvaline; all 10 mmol l(-1)) with different concentrations of Na+ and Cl- in the perfusion and recording pipette solutions. We also examined the effects of six different Cl- channel blockers on the responses of ORNs using a conventional whole-cell voltage-clamp technique, The amino acid mixture and one blocker were applied focally to the cilia of ORNs using a double-barrelled micropipette and a pressure ejection system, The expected shifts in reversal potential, indicating the contribution of the Ca2+-activated Cl- conductance, occurred in both positive and negative directions depending on the external and internal Na+ and Cl(-)concentrations, Niflumic acid, flufenamic acid, NPPB [5-nitro-2-(3-phenylpropylamino)-benzonate] and DCDPC (3',5-dichlorodiphenylamine-2-carboxylate), at 0.5 mmol l(-1), reversibly blocked both the amino-acid-induced inward currents and the background activity in most ORNs, The effectiveness of these blocking agents varied from 77 to 91 % for ORNs perfused externally with standard Ringer's solution, SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonate), at 5.0 mmol l(-1), irreversibly inhibited the physiological response (100 % inhibition), whereas DIDS(4,4'-diisothiocyanatostilbene-2,2'-disuphonate) at 5.0 mmol l(-1), had the smallest effect (45 %) of the inhibitors tested. The dose of niflumic acid inducing 50 % inhibition (IC50), determined specifically for the current component of the Ca2+-activated Cl- channels, was 70 mu mol l(-1). Our results suggest that these blockers are not specific for Ca2+-activated Cl- channels and that the density of these channels varies between individual ORNs, Our results also show that the Ca2+-activated Cl- conductance plays an important role in olfactory transduction and allows fishes to adapt to various ionic environments. [References: 47]