The Ca2+-activated Cl− channel TMEM16B shapes the response time course of olfactory sensory neurons

Mammalian olfactory sensory neurons (OSNs) generate an odorant-induced response by sequentially activating two ion channels, which are in their ciliary membranes. First, a cationic, Ca²⁺-permeable cyclic nucleotide-gated channel is opened following odorant stimulation via a G protein-coupled transduction cascade and an ensuing rise in cAMP. Second, the increase in ciliary Ca²⁺ opens the excitatory Ca²⁺-activated Cl⁻ channel TMEM16B, which carries most of the odorant-induced receptor current. While the role of TMEM16B in amplifying the response has been well established, it is less understood how this secondary ion channel contributes to response kinetics and action potential generation during single as well as repeated stimulation and, on the other hand, which response properties the cyclic nucleotide-gated (CNG) channel determines. We first demonstrate that basic membrane properties such as input resistance, resting potential and voltage-gated currents remained unchanged in OSNs that lack TMEM16B. The CNG channel predominantly determines the response delay and adaptation during odorant exposure, while the absence of the Cl⁻ channels shortens both the time the response requires to reach its maximum and the time to terminate after odorant stimulation. This faster response termination in Tmem16b knockout OSNs allows them, somewhat counterintuitively despite the large reduction in receptor current, to fire action potentials more reliably when stimulated repeatedly in rapid succession, a phenomenon that occurs both in isolated OSNs and in OSNs within epithelial slices. Thus, while the two olfactory ion channels act in concert to generate the overall response, each one controls specific aspects of the odorant-induced response.

Key points

• Mammalian olfactory sensory neurons (OSNs) generate odorant-induced responses by activating two ion channels sequentially in their ciliary membranes: a Na⁺, Ca²⁺-permeable cyclic nucleotide-gated (CNG) channel and the Ca²⁺-activated Cl⁻ channel TMEM16B.
• The CNG channel controls response delay and adaptation during odorant exposure, while TMEM16B amplifies the response and influences the time required for the response to reach its peak and terminate.
• OSNs lacking TMEM16B display faster response termination, allowing them to fire action potentials more reliably during rapid repeated stimulation.
• The CNG and TMEM16B channels have distinct and complementary roles in shaping the kinetics and reliability of odorant-induced responses in OSNs.