The contribution of a Ca(2+)-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 Ca(2+)-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 Ca(2+)-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 (IC(50)), determined specifically for the current component of the Ca(2+)-activated Cl(-) channels, was 70 micromol l(-)(1). Our results suggest that these blockers are not specific for Ca(2+)-activated Cl(-) channels and that the density of these channels varies between individual ORNs. Our results also show that the Ca(2+)-activated Cl(-) conductance plays an important role in olfactory transduction and allows fishes to adapt to various ionic environments.
Sato
K
K
Animal Behavior and Intelligence, Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan. suzuki@sci.hokudai.ac.jp
Suzuki
N
N
D000596:Amino Acids / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D015220:Calcium Channels / Q000502:physiology; D002462:Cell Membrane / Q000502:physiology; D018118:Chloride Channels / Q000037:antagonists & inhibitors / Q000502:physiology*; D008564:Membrane Potentials; D018034:Olfactory Receptor Neurons / Q000502:physiology*; D017686:Oncorhynchus mykiss / Q000502:physiology*; D018408:Patch-Clamp Techniques; D015222:Sodium Channels / Q000502:physiology
D000596:Amino Acids; D015220:Calcium Channels; D018118:Chloride Channels; D015222:Sodium Channels; D002118:Calcium