Mechanisms of chloride uptake in frog olfactory receptor neurons
Odorant stimulation of olfactory receptor neurons (ORNs) leads to the activation of a Ca2+ permeable cyclic nucleotide-gated (CNG) channel followed by opening of an excitatory Ca2+-activated Cl− channel, which carries about 70% of the odorant-induced receptor current. This requires ORNs to have a [Cl−]i above the electrochemical equilibrium to render this anionic current excitatory. In mammalian ORNs, the Na+-K+-2Cl− co-transporter 1 (NKCC1) has been characterized as the principal mechanism by which these neurons actively accumulate Cl−. To determine if NKCC activity is needed in amphibian olfactory transduction, and to characterize its cellular location, we used the suction pipette technique to record from Rana pipiens ORNs. Application of bumetanide, an NKCC blocker, produced a 50% decrease of the odorant-induced current. Similar effects were observed when [Cl−]i was decreased by bathing ORNs in low Cl− solution. Both manipulations reduced only the Cl− component of the current. Application of bumetanide only to the ORN cell body and not to the cilia decreased the current by again about 50%. The results show that NKCC is required for amphibian olfactory transduction, and suggest that the co-transporter is located basolaterally at the cell body although its presence at the cilia could not be discarded.