Origin of basal activity in mammalian olfactory receptor neurons

Abstract

Mammalian odorant receptors form a large, diverse group of G protein-coupled receptors that determine the sensitivity and response profile of olfactory receptor neurons. But little is known if odorant receptors control basal and also stimulus-induced cellular properties of olfactory receptor neurons other than ligand specificity. This study demonstrates that different odorant receptors have varying degrees of basal activity, which drives concomitant receptor current fluctuations and basal action potential firing. This basal activity can be suppressed by odorants functioning as inverse agonists. Furthermore, odorant-stimulated olfactory receptor neurons expressing different odorant receptors can have strikingly different response patterns in the later phases of prolonged stimulation. Thus, the influence of odorant receptor choice on response characteristics is much more complex than previously thought, which has important consequences on odor coding and odor information transfer to the brain.

Figure 1.

Basal action potential activity in olfactory receptor neurons. A–C show responses from isolated ORNs expressing the identified odorant receptors mOR-EG, M71, and I7 stimulated with their respective ligands eugenol, acetophenone, and heptanal. (D–F) Recordings in the absence of stimulation to monitor basal spike firing activity, same three cells as above. Red, low-pass (50 Hz) filtered recordings to display the underlying fluctuations of the receptor current. The Ca²⁺-activated Cl⁻ channel blocker niflumic acid (300 µM, applied from 30 to 60 s) entirely abolished basal activity. (G–I) Expanded view of the same traces as in D–E to show current fluctuations.

Figure 2.

Kinetics of basal current fluctuations. (A–C) Power spectrum analysis of the black (filtered at DC-5000 Hz) traces in Fig. 1, D–F. Note the increase in noise in the 1–15-Hz band (shaded in gray) in the M71 and I7-positive ORNs, which is not observed in the mOR-EG–expressing ORN. (D) Average basal spike firing in the absence of stimulation as a function of basal variance (average of 9–12 ORNs). (E) Average maximal currents elicited by 100 µM of eugenol, acetophenone, and heptanal ORNs obtained from the mOR-EG, M71-GFP, and I7-GFP mouse lines, respectively. Numbers in the bars of the histograms are the numbers of cells recorded from. *, current levels that were statistically different (t test, 0.05 level). Data points are mean ± SEM.

Figure 3.

Kinetics of small odorant responses. (A) Suction pipette recordings from an I7-expressing ORN. Black trace is the average of 10 responses to 30-ms stimulations at 0.3 µM heptanal. Blue traces are the average of 10 recordings in the presence of 300 µM niflumic acid (no odorant) to suppress basal noise. The recording bandwidth of the displayed traces was 0–50 Hz. (B) Power spectrum analysis of corresponding traces (filtered at DC-5000 Hz) from A. Similar results in 12 ORNs. (C) Difference of the power spectra (odorant minus niflumic acid) in B fitted with A₂ + (A₁ − A₂)/(1 + (f/f₀)ⁿ), with A1 = 0.13 pA²Hz⁻¹, A2 = 0.001 pA²Hz⁻¹, f₀ = 6.4 Hz, and n = 3.4. (D) Difference of the power spectra (basal noise minus niflumic acid) in Fig. 2 C obtained from basal noise. Fitting parameters are A1 = 0.04 pA²Hz⁻¹, A2 = 0.001 pA²Hz⁻¹, f₀ = 5.6 Hz, and n = 3.7.

Figure 4.

(A–C) The odorant receptor determines the response size to the phosphodiesterase inhibitor IBMX. mOR-EG, M71, and I7-expressing ORNs were exposed to their respective ligands at 100 µM and to 1 mM IBMX for 1 s. (D) For each ORN, the response to IBMX was normalized to their odorant response, average of 10–25 ORNs. Data points are mean ± SEM.

Figure 5.

Suppression of basal firing by an inverse agonist. (A) Suction pipette recordings of an I7-expressing ORN. Application of cycloheptanecarbaldehyde (chca, 100 µM), previously described as an I7 receptor antagonist (Peterlin et al., 2008), suppressed all basal firing. (B) Power spectra analysis in the absence and presence of chca, 1–15 Hz frequency band shaded in gray. (C) Suppression of noise (ΔVar) is similar with chca and niflumic acid (NA), mean ± SEM, n = 14 for chca and 11 for NA. (D) chca does not reduce the response when coapplied with heptanal (both odorants at 100 µM). Only preapplication (5 s) of the antagonist leads to response reduction. (E) Black and green traces are recorded before and after chca application. All recordings are from the same ORN.

Figure 6.

The odorant receptor determines response patterns during prolonged stimulation. (A) mOR-EG, M71, and I7-expressing ORNs were exposed to their respective ligands for 8 s at concentrations indicated next to each trace. Black and red recordings were filtered at DC-5000 Hz and DC-50 Hz, respectively. (B) The traces in A were auto-correlogrammed from 1–8 s to reveal underlying preferred response periodicities.