Signaling by olfactory receptor neurons near threshold

Abstract

An important contributing factor for the high sensitivity of sensory systems is the exquisite sensitivity of the sensory receptor cells. We report here the signaling threshold of the olfactory receptor neuron (ORN). We first obtained a best estimate of the size of the physiological electrical response successfully triggered by a single odorantbinding event on a frog ORN, which was ∼0.034 pA and had an associated transduction domain spanning only a tiny fraction of the length of an ORN cilium. We also estimated the receptor-current threshold for an ORN to fire action potentials in response to an odorant pulse, which was ∼1.2 pA. Thus, it takes about 35 odorantbinding events successfully triggering transduction during a brief odorant pulse in order for an ORN to signal to the brain.olfaction | olfactory transduction | sensory transduction O ur visual system has pushed to the physical limit of sensitivity. Thus, a dark-adapted human subject can report light when just a few photons are absorbed in a retinal area spanning hundreds of rod photoreceptors, suggesting that each dark-adapted rod can signal the absorption of a single photon (1). What about the olfactory system? How many odorant molecules have to bind to an olfactory receptor neuron (ORN) to trigger an output signal? Retinal rods lack axons, so a graded, light-induced change in membrane potential directly modulates synaptic transmission. In contrast, ORNs have axons and require action potentials to convey olfactory signals to the brain. Hence, the question becomes: How many odorant-binding events successfully triggering transduction are required for changing an ORN's firing?The canonical mechanism of olfactory transduction in the main olfactory epithelium is now quite well understood (2-5). Odorant receptors (ORs) on ORN cilia are G protein-coupled-receptors (6) that, upon binding specific odorants, activate the adenylyl cyclase type-III via the G protein, G olf . The ensuing rise in cAMP opens a cyclic-nucleotide-gated (CNG), nonselective cation channel to produce a membrane depolarization. Additionally, the Ca 2+ influx through the CNG channels opens a Ca 2+ -activated Cl channel on the ciliary membrane (7,8). The resulting Cl − flux is, however, outward (i.e., inward Cl current) because of a high intracellular Cl − concentration maintained by steady Cl − uptake via a Na/K/Cl cotransporter, NKCC1 (9-12, but see 13), thus further depolarizing the cell to provide amplification (9, 10). Simultaneously, the Ca 2+ influx triggers, via Ca 2+ -calmodulin, multiple negative-feedback pathways that lead to adaptation (2-5). Finally, action-potential generation in ORNs involves voltage-gated Na channels and lowvoltage-activated (T-type) Ca channels, at least near threshold (14). Previously, we have found surprisingly that most odorant-binding events on an ORN are actually "inconsequential": that is, unsuccessful in triggering transduction, apparently because the odorant-OR complex typically exists too briefly to activate the downstream pathway (15). The events descr...

Interplay among cGMP, cAMP, and Ca2+ in Living Olfactory Sensory Neurons In Vitro and In Vivo

“…These odor concentrations are well within the range (1 nM-1 mM) of those used in previous studies (Bozza and Kauer, 1998;Bhandawat et al, 2005Bhandawat et al, , 2010 on dissociated OSNs. The stimuli baths applied were carefully and slowly delivered via an application pipette positioned far away (ϳ3 mm) from the cell to obtain a homogeneous distribution of the stimulus in the bath, capable of stimulating the entire cells and not a specific compartment.…”

“…1 E), as observed in previous studies (Bozza and Kauer, 1998;Bozza et al, 2002). At all concentrations tested (1, 50, and 200 M), well within the range (1 nM-1 mM) of those used in previous experiments on isolated OSNs (Bozza and Kauer, 1998;Bhandawat et al, 2005Bhandawat et al, , 2010, odors elicited Ca 2ϩ responses ( Fig. 1) indistinguishable from those obtained in freshly plated neurons.…”

Distinct signaling of Drosophila chemoreceptors in olfactory sensory neurons

“…The dose-response relationship was approximately linear at its foot (Fig. 1D, Left, Inset) but was supralinear overall (Hill coefficient, 1.7 for this cell), broadly similar to the behavior of vertebrate OSNs (27,28). As expected from OSNs expressing different Ors, the sensitivity to butyl acetate varied widely across randomly recorded Or-expressing OSNs, with a half-saturating concentration of 0.6-17 mM (10 cells; 3.7 mM in Fig.…”

Scaffolding by MUPP1 regulates odorant-mediated signaling in olfactory sensory neurons

“…In the case of unitary responses that are triggered by individual odorant-binding events, spatially and functionally segregated ciliary transduction domains have been described (Bhandawat et al, 2005;Bhandawat et al, 2010). Additionally, recordings of Ca 2+ changes in individual cilia using fluorescent Ca 2+ indicator dyes have shown that Ca 2+ microdomains exist, which contain one to three CNG channels (Castillo et al, 2010).…”