Odor information processing by the olfactory bulb analyzed in gene-targeted mice

Abstract

In mammals, olfactory sensory neurons (OSNs) expressing a specific odorant receptor (OR) gene project with precise stereotypy onto mitral/tufted (M/T) cells in the main olfactory bulb (MOB). It remains challenging to understand how incoming olfactory signals are transformed into outputs of M/T cells. By recording from OSNs expressing mouse I7 receptor and their postsynaptic neurons in the bulb, we found that I7 OSNs and their corresponding M/T cells exhibit similarly selective tuning profiles at low concentrations. Increasing the concentration significantly reduces response selectivity for both OSNs and M/T cells, although the tuning curve of M/T cells remains comparatively narrow. By contrast, interneurons in the MOB are broadly tuned, and blocking GABAergic neurotransmission reduces selectivity of M/T cells at high odorant concentrations. Our results indicate that olfactory information carried by an OR is channeled to its corresponding M/T cells and support the role of lateral inhibition via interneurons in sharpening the tuning of M/T cells.

Figure 1. I7 OSNs Respond ~10 Times More Sensitively to Heptanal than to Octanal

(A) Schematic representation of experimental strategy. I7 OSNs (green) were identified by green fluorescence in an intact epithelial preparation. Their firing of action potentials was recorded extracellularly from dendritic knobs in cell-attached mode. Odorants were applied by pressure through a seven-barrel pipette. (B) Strong green fluorescence in the dendritic knob and cilia of a GFP⁺ I7 OSN. (C) Traces showing the responses evoked by heptanal at different concentrations as indicated by the numbers to the left (in μM). Horizontal bars indicate the timing of odorant application (300 ms). In this and the following figures, heptanal is abbreviated as 7al and octanal as 8al. (D) Reponses of another I7 OSN to octanal at different concentrations. (E) Dose-response plots and fitted sigmoid functions of I7 OSNs to heptanal (n = 10 cells) and octanal (n = 10 cells). The response strength was normalized to the mean saturated firing frequency to heptanal. Error bars indicate SEM in this and the following figures. (F) Mean and individual EC₅₀ values of different I7 OSNs to heptanal and octanal. ***p < 0.001 (between-group t test).

Figure 2. I7 OSNs Respond Selectively to Heptanal at a Low Concentration, but Are Activated by Several Other Aldehydes at a Higher Concentration

(A) Traces showing the responses of a single I7 OSN to six different aldehydes at 1 μM. In this and the following figures, odorants are abbreviated as follows: trans-2-octenal, t2-8al; cis-6-nonenal, c6-9al; 2,6-dimethyl-5-heptenal, melonal; cis-4-heptenal, c4-7al. (B) Mean response strengths of I7 OSNs to the six aldehydes at 1 μM (n = 5 cells). (C) Traces showing responses of another I7 OSN to the same set of odorants as in (A) but at 10 μM. (D) Mean response strengths of I7 OSNs to the six aldehydes at 10 μM (n = 5 cells).

Figure 3. I7 M/T Cells Respond Sensitively to Heptanal, whereas Sensitivity to Octanal Is Significantly Lower

(A) Schematic representation of simplified circuitry in the MOB and our experimental strategy. A dye- and neurobiotin-filled glass electrode (red pipette) was targeted to a GFP⁺ I7 glomerulus to search for I7 M/T cells. Their firing of action potentials was extracellularly recorded, likely from the primary dendritic trunks of M/T cells. The recorded neurons were then juxtacellularly labeled (red), and their relationship with the I7 glomerulus was confirmed by the presence of dye-filled primary dendritic tufts exclusively in GFP⁺ glomerulus. G, glomerulus; Gr, granule cells. In this and the following figures, short axon cells are not shown for simplicity. (B) Schematic representation of the experimental setup. GFP⁺ glomeruli were visualized by two-photon microscopic imaging. Odorants were stored in headspace vials on an 8 × 8 panel and delivered with a custom-made robotic olfactometer. Two headspace vials in the panel containing pure air or air with mineral oil were used as controls. An additional set of seven aldehydes was delivered by a custom-made seven-channel olfactometer. (C) A two-photon microscopic image shows the targeting of tetramethylrhodamine dextran amine-filled recording pipette (red) into the I7 glomerulus (green). (D) During juxtacellular labeling, the firing of the recorded cell was tightly synchronized with current injection. (E–G) Three examples showing heptanal-evoked responses from different subtypes of I7 M/T cells (a mitral cell in E, a middle tufted cell in F, and an external tufted cell in G). (Top panels) Juxtacellular labeling revealed that the recorded cells (red) extended their primary dendritic tufts into dorsal I7 glomeruli labeled with GFP (green). Blue, DAPI labeling to delineate glomeruli. Below the images are physiological traces, respiratory rhythms, and mean peristimulus time histogram (PSTH) plots showing that heptanal evoked strong firing of action potentials from these cells. In all three examples, heptanal was diluted as 1% saturated vapor. In this and the following figures, horizontal bars above physiological traces represent 2 s odorant pulses and serve as timescale as well. Scale bars, 20 μm. (H) Physiological traces showing the response intensities of a mitral cell to heptanal and octanal at different concentrations. The concentrations shown on the left (in μM) were calculated from the dilutions of saturated vapor pressure of heptanal and octanal. For heptanal, 1% dilution of saturated vapor corresponds to 1.9 μM. (I) Dose-response curves of I7 M/T cells to heptanal (n = 8 cells) and octanal (n = 7 cells). The response strengths were normalized to the saturated response strength of I7 M/T cells to heptanal. (J) Mean EC₅₀ values of I7 M/T cells to heptanal were significantly lower than those to octanal (**p < 0.01; t test). The colors of circles code for different cell subtypes in this figure and Figure 4D. Red, mitral cells; blue, middle tufted cells; black, external tufted cells. See also Figure S1 and Table S1.

Figure 4. I7 M/T Cells Exhibit Sharp Tuning Curves, with Heptanal Being the Most Effective Stimulus

(A) Tuning curves of three I7 M/T cells in response to a diverse set of odorants, all at 0.1% dilutions of saturated vapor. Mean response strength to each odorant is plotted against odorant identity (Table S2). Arrow points to the strength of response to heptanal. (B) Tuning curves of three additional I7 M/T cells in response to the same set of odorants at 1% dilution. (C) Comparison of the mean tuning curves of I7 M/T cells at 0.1% and 1% dilutions (n = 9 and 10 cells, respectively). Response intensities were normalized to the maximum responses of the cells to heptanal. Odorant identities are aligned to produce a smooth tuning curve for responses to odorants at 1% dilution, with a maximum response in the middle. The alignment of odorant identities is identical for horizontal axes in (A)–(C) and is listed in Table S2. (D) The lifetime sparseness (S) value of I7 M/T cells to odorants at 0.1% dilution is significantly higher than that at 1% (*p < 0.05; t test between concentrations). Circles indicate S values of individual M/T cells. (E) The matrices of Pearson's correlation coefficients for the tuning curves across individual I7 M/T cells at concentrations of 0.1% (left-hand panel, mean coefficient = 0.72 ± 0.06, mean ± STD) and 1% (right-hand panel, 0.44 ± 0.016). Color bar indicates the range of correlation coefficients. M, mitral cells; T, middle tufted cells; E, external tufted cells. See also Figure S2 and Table S2.

Figure 5. Near-I7 M/T Cells Are Narrowly Tuned and Lack Responses to Heptanal

(A) Schematic representation of recording strategy. M/T cells associated with glomeruli adjacent to the GFP⁺ I7 glomerulus were recorded and labeled. (B) The morphology of a mitral cell (red) that extended its primary dendrite into a glomerulus adjacent to the dorsal I7 glomerulus (green). Blue, DAPI staining. Inset shows the dorsal view of the dye-filled dendritic tufts and electrode (red) near the green I7 glomerulus. The image in the inset was collected in vivo immediately after recording. (C) Firing patterns of the cell in (B) showing representative excitatory (upper trace) and inhibitory (lower trace) responses to odorant at 1% dilution. (D) Tuning curve of the cell shown in (B) in response to 69 different odorants at 1% dilution. (E) Physiological traces showing the response intensities of a near-I7 mitral cell to heptanal at different concentrations. (F) Dose-response plot and fitted sigmoid function of four near-I7 M/T cells (black) that were activated by heptanal. The red dashed curve indicates the mean dose-response plot of I7 M/T cells for heptanal.

Figure 6. I7 PG Cells Respond Broadly and Heterogeneously

(A) Schematic representation of experimental strategy for recording I7 PG cells. (B) Juxtacellular labeling revealed that a PG cell (red) extended its dendrites exclusively into an I7 glomerulus (green). Blue, DAPI staining. (C) Physiological traces of the cell in (B) showing representative excitatory responses to 2 s pulses of heptanal and octanal at 1% dilution. (D) Two examples of tuning curves of I7 PG cells to 69 odorants at 1%. Arrows point to the strengths of responses to heptanal. (E) Comparison of the mean tuning curves of I7 PG cells at the concentrations of 0.1% (n = 6 cells) and 1% (n = 9 cells). Odorant identities are aligned so that the mean tuning curve at high concentration is smooth, with maximum response in the middle. The odorant identities in horizontal axes of (D and E) are identical and are listed in Table S2. (F) The lifetime sparseness values of I7 PG cells at 0.1% and 1%. (G) The matrices of Pearson's correlation coefficients for the tuning curves across individual I7 PG cells at the concentrations of 0.1% (left-hand panel, mean coefficient = 0.13 ± 0.18) and 1% (right-hand panel, 0.20 ± 0.15). See also Figure S3 and Table S2.

Figure 7. Granule Cells in the MOB Are Broadly Tuned

(A) Schematic representation of recording strategy. Granule cells were randomly recorded and labeled by lowering electrodes into the granule cell layer. (B) The morphology of a granule cell, which belongs to the most abundant neuron type in the bulb. (C) The granule cell in (B) shows both excitatory and inhibitory responses to odorants at 1% dilution. (D) Two examples of tuning curves of granule cells to 64 odorants at 1% dilution. (Upper panel) The tuning curve of the granule cell shown in (B) at 1% dilution. (Lower panel) The tuning curve of another granule cell illustrating the lack of inhibition. (E) Mean tuning curves of granule cells to 64 odorants at 0.1% and 1% dilutions (n = 5 and 8 cells, respectively). The response strengths of granule cells to odorants at 0.1% dilutions are normalized to the mean peak responses of granule cells to odorants at 1% dilutions. See also Figure S4.

Figure 8. Inhibitory Connections in the MOB Enhance Response Selectivity of I7 M/T Cells to Odorants at High Concentrations

(A) Traces (upper panel) and PSTHs (lower panel) of an I7 M/T cell to 0.1% heptanal and octanal before (black lines) and after (gray lines) bicuculline methiodide (BMI, 200 mM) superfusion. (B) The effects of BMI application on the response strengths of I7 M/T cells to seven odorants at 0.1% dilution (n = 6 cells). Response strengths were normalized to the responses evoked by the most effective odorant before BMI application. BP, butyl propionate; other odorants were abbreviated using conventions in Figures 1 and 2. *p < 0.05 (paired t test between responses before and after BMI). (C) Traces and PSTHs of another I7 M/T cell to 1% heptanal and octanal before and after BMI application. (D) The effects of BMI application on the response strength of I7 M/T cells to seven odorants at 1% dilution (n = 6 cells). *p < 0.05; **p < 0.01 (paired t test between responses before and after BMI). (E) BMI application reduced the lifetime sparseness (S) values of I7 M/T cells in response to odorants at 1% dilution but not 0.1% dilution. n.s., not significant; ***p < 0.001 (paired t test between responses before and after BMI). The overall S values here were lower than those when tested with 69 odorants because only a small subset of aldehydes were used, resulting in a biased stimulus set. (F) Examples showing the response patterns of two I7 M/T cells to 10% heptanal before (left panel) and after (right panel) BMI application. See also Figure S5.