Mechanisms of neuronal chloride accumulation in intact mouse olfactory epithelium

When olfactory receptor neurons respond to odours, a depolarizing Cl- efflux is a substantial part of the response. This requires that the resting neuron accumulate Cl- against an electrochemical gradient. In isolated olfactory receptor neurons, the Na+-K+-2Cl(-) cotransporter NKCC1 is essential for Cl- accumulation. However, in intact epithelium, a robust electrical olfactory response persists in mice lacking NKCC1. This response is largely due to a neuronal Cl- efflux. It thus appears that NKCC1 is an important part of a more complex system of Cl- accumulation. To identify the remaining transport proteins, we first screened by RT-PCR for 21 Cl- transporters in mouse nasal tissue containing olfactory mucosa. For most of the Cl- transporters, the presence of mRNA was demonstrated. We also investigated the effects of pharmacological block or genetic ablation of Cl- transporters on the olfactory field potential, the electroolfactogram (EOG). Mice lacking the common Cl-/HCO3- exchanger AE2 had normal EOGs. Block of NKCC cotransport with bumetanide reduced the EOG in epithelia from wild-type mice but had no effect in mice lacking NKCC1. Hydrochlorothiazide, a blocker of the Na+-Cl- cotransporter, had only a small effect. DIDS, a blocker of some KCC cotransporters and Cl-/HCO3- exchangers, reduced the EOG in epithelia from both wild-type and NKCC1 knockout mice. A combination of bumetanide and DIDS decreased the response more than either drug alone. However, no combination of drugs completely abolished the Cl- component of the response. These results support the involvement of both NKCC1 and one or more DIDS-sensitive transporters in Cl- accumulation in olfactory receptor neurons.