Calcium Activates a Chloride Conductance Likely Involved in Olfactory Receptor Neuron Repolarization in the Moth Spodoptera littoralis

The response of insect olfactory receptor neurons (ORNs) to odorants involves the opening of Ca 2+ -permeable channels, generating an increase in intracellular Ca 2+ concentration. Here, we studied the downstream effect of this Ca 2+ rise in cultured ORNs of the moth Spodoptera littoralis . Intracellular dialysis of Ca 2+ from the patch pipette in whole-cell patch-clamp configuration activated a conductance with a K 1/2 of 2.8 μm. Intracellular and extracellular anionic and cationic substitutions demonstrated that Cl − carries this current. The anion permeability sequence I − > NO 3 − > Br − > Cl − > CH 3 SO 3 − ≫ gluconate − of the Ca 2+ -activated Cl − channel suggests a weak electrical field pore of the channel. The Ca 2+ -activated current partly inactivated over time and did not depend on protein kinase C (PKC) and CaMKII activity or on calmodulin. Application of Cl − channel blockers, flufenamic acid, 5-nitro-2-(3-phenylpropylamino) benzoic acid, or niflumic acid reversibly blocked the Ca 2+ -activated current. In addition, lowering Cl − concentration in the sensillar lymph bathing the ORN outer dendrites caused a significant delay in pheromone response termination in vivo . The present work identifies a new Cl − conductance activated by Ca 2+ in insect ORNs presumably required for ORN repolarization.