The Contribution of a Ca²⁺-Activated Cl⁻ Conductance to Amino-Acid-induced Inward Current Responses of Ciliated Olfactory Neurons of the Rainbow Trout

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′-disulphonate), 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 µmol 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.