Odorant response properties of individual neurons in an olfactory glomerular module

Abstract

Neuronal networks that are directly associated with glomeruli in the olfactory bulb are thought to comprise functional modules. However, this has not yet been experimentally proven. In this study, we explored the anatomical and functional architecture of glomerular modules using in vivo two-photon calcium imaging. Surprisingly, the deep portions of the glomerular modules showed considerable spatial overlap with other modules. Juxtaglomerular cells showed similar excitatory odorant response profiles to presynaptic olfactory sensory neuron inputs. Mitral cells exhibited a more sharply tuned molecular receptive range compared to juxtaglomerular cells, and their odorant response profiles varied depending on their interneuronal horizontal distances. These data suggest that glomerular modules are composed of functionally distinct neurons, and that homogenous odor inputs to each glomerulus may be parsed and processed in different fashions within the modules before being sent to higher olfactory centers.

Fig. 1. Multiple neuronal subtypes are associated with a single glomerulus

(A) Two-photon images showing the dye-filled glass pipette penetrating the target glomerulus. Images were captured before (left) and after (right) injection of electroporation dye. Glomerular formation was observed by green fluorescent spH signals. (B) Distribution pattern of labeled glomeruli in the dorsal OB. Imaging data were collected from the anteromedial portion of the dorsal OB (also see odor-induced spH signals in Fig. S1). (C) In vivo two-photon images showing a dye-injected glomerulus and labeled neurons in different layer and depth (50–250 μm from the surface). The dye-injected glomerulus is surrounded by a dashed line. Some neurons were observed underneath the neighboring glomeruli, but these neurons were associated with the dye-injected glomerulus. The 3D reconstruction and sequential image movie of this sample are shown in Fig. 1D–E, S1C–E, and movie S1. (D–E) Horizontal (D) and lateral (E) views of 3D reconstruction of a single glomerular module (JG cells: red, tufted cells: blue, and mitral cells: green). Arrowhead (D) indicates the view angle for lateral view (E). Post.: posterior; Lat.: lateral. F) Multiple subtypes of labeled neurons were observed. Cell body locations (layers), soma shapes and sizes, and presence of L-Dends were used for grouping. Arrowheads indicate representative examples for indicated cell types. GL: glomerular layer; EPL: external plexiform layer; MCL: mitral cell layer. Scale bars in (A) and (C–E): 50 μm; and in (F): 20 μm.

Fig. 2. Distribution patterns of labeled neurons

(A–C) Horizontal distribution patterns of JG cells (A), tufted cells (B), and mitral cells (C) in the dorsal OB. XY-axes indicate the anteroposterior and lateromedial axes. Filled gray circles indicate the mean size of a dye-filled glomerulus. Values on axes indicate distance from the center of the dye-injected glomerulus. Each point indicates one cell. w/o: without; Glom.: glomerulus; Lat.: lateral; Ant.: anterior. (D) The mean locations of each neuronal subtype from the center of the dye-injected glomerulus (red: JG cells; blue: tufted cells; green: mitral cells). The center of neuronal distributions shifted in the caudomedial direction between the GL and the MCL. (E) Upper chart: Vertical distribution patterns of labeled cells. X-axis indicates the normalized distance from the center of each layer. Y-axis indicates the normalized depth of the labeled cells from the brain surface (0) to the MCL (1.0). Mean depth of MCL was 274 ± 6.8 μm from the OB surface. The filled gray circle indicates the presumed size of the dye-injected glomerulus, and the open gray circle represents the presumed size of the adjacent glomerulus. Each point indicates one cell. Labeled cells in the deeper layers were distributed more widely than cells in the superficial layer. Lower chart: Summary of the horizontal distribution patterns of JG, tufted, and mitral cells. Red bars: JG cells; blue bars: tufted cells; green bars: mitral cells; ***p < 0.001, Tukey-Kramer test. GL: glomerular layer; EPL: external plexiform layer; MCL: mitral cell layer; glom.: glomerulus; w/o: without. Error bars indicate the standard error of the mean (s.e.m.).

Fig. 3. Direct comparisons of presynaptic OSN terminal activities and postsynaptic activities of individual cells

(A) A dye-injected glomerulus is shown (the bright spot in the center of the white square). Outline indicates the estimation of the entire dorsal OB. Ant.: anterior; Lat.: lateral. Scale bar: 500 μm. (B) CCD images of odorant-evoked presynaptic spH signals around the target glomerulus. The odorants that were used to evoke signals are indicated for each image (3-9CHO). White arrows indicate the target glomeruli in which calcium indicator dye was injected after the CCD imaging experiment. SpH signals are scaled to the maximum ΔF/F (4%) in all images as indicated in the heat map on the right. Scale bar: 100 μm. (C) In vivo two-photon image of JG cells (arrow) associated with the target glomerulus. GL: glomerular layer. Scale bar: 50 μm. (D) Odorant-evoked presynaptic spH signals in the target glomerulus. The odorants that were used to evoke signals are indicated for each trace (3-9CHO). Black bars indicate timing of odorant delivery (3 sec). (E) Odorant-evoked postsynaptic Ca²⁺ signaling in JG cells. The odorants that were used to evoke signals are indicated for each trace (3-9CHO). Black bars indicate timing of odorant delivery (3 sec). (F) MRR differences between presynaptic OSNs and postsynaptic neurons in other glomeruli/neurons. Cells in the same layer are indicated (red: JG cells; blue: tufted cells; green: mitral cells). Y-axes: odorants used for stimulation; X-axes: fluorescent changes normalized to maximum odorant responses. Bars extending right/left from the centerline indicate fluorescent increases/decreases for each cell, respectively. Filled/empty bars represent significant/non-significant responses, respectively. Glom: glomerulus (G) eMRR widths in presynaptic OSNs (black dots) compared to postsynaptic JG cells (red dots). Vertical axis indicates the number of excitatory odorants (eMRR widths). Glomeruli and their associated cells are connected with lines. No significant changes between OSNs and JG cells were observed (eight pairs of OSN and JG cells, p=0.45, Wilcoxon t-test). Error bars indicate the standard error of the mean (s.e.m.).

Fig. 4. Odorant-evoked Ca²⁺ responses of neurons in different layers

(A) Two-photon images of target JG (cell a) and mitral (cell b) cells associated with the same glomerulus (left image: GL; right image: MCL). Scale bars: 20 μm. (B) Horizontal and vertical locations of labeled cells. Red (a) and green (b) diamonds correspond with the JG and mitral cells in (C), respectively. Other labeled neurons are shown as black open diamonds. (C) Odor-evoked Ca²⁺ responses of JG (a) and mitral cells (b). Black bars under each trace indicate the timing of odor delivery (3 sec). JG cell (red) showed excitatory responses with 5-9CHO stimulations, whereas the mitral cell (green) was excited by only 6CHO. (D) MRRs of cells in different layers in other glomeruli. Bars extending right/left from the centerline indicate fluorescent increases/decreases for each cell, respectively. Cells in the same layers are surrounded with lines (red: JG cells; blue: tufted cells; green: mitral cells). X- and Y-axes represent Ca²⁺ responses and odorants used for stimulation, respectively. JG cells had wide eMRRs while mitral cells had narrow eMRRs. Glom.: glomerulus. (E) Summary of the direct comparisons of eMRR widths within the same glomerular module. Cells associated with the same glomerulus are connected with lines (***p < 0.001, **p < 0.01, *p < 0.05, Steel-Dwass test, based on 14 JG, 20 tufted, and 24 mitral cells). Error bars indicate the standard error of the mean (s.e.m.). The data sets that include all three types of neurons (JG, tufted, and mitral cells) are highlighted by a dagger mark (†) and are connected by bold lines. glom.: glomerulus

Fig. 5. Differential odorant sensitivities of different neuronal subtypes in the same glomerulus module

(A) Two-photon images of tufted (a) and mitral (b) cells associated with the same glomerulus (left image: EPL; right image: MCL). Scale bars: 20 μm. (B) Horizontal and vertical locations of labeled neurons. Blue (a) and green (b) diamonds correspond with the tufted and mitral cells in (C), respectively. Other labeled neurons are shown as black open diamonds. (C) Odorant-evoked Ca²⁺ responses of the tufted (a) and mitral cells (b) shown in (A). Each trace indicates odor-evoked Ca²⁺ response at different concentrations of 5CHO (0.002–2%; upper trace: tufted cell; lower trace: mitral cell). Black bar under each trace shows the timing of odor delivery (3 sec). (D) Concentration response curves for JG, tufted, and mitral cells associated with the same glomerulus (Glom. #1–6). Red, blue, and green lines indicate normalized response magnitude curves for JG, tufted, and mitral cells, respectively. Filled/empty circle points represent significant/non-significant odorant responses, respectively. Glom.: glomerulus. (E) Summary of the differential odorant sensitivities for the cell types. Each colored dot indicates the minimum concentration for inducing an excitatory odorant response in the target neurons. Neurons associated with the same glomerulus are connected with lines (eight glomeruli). The data sets that include all three types of neurons (JG, tufted, and mitral cells) are highlighted by a dagger mark (†) and are connected by bold lines. Glom.: glomerulus.

Fig. 6. Excitatory odorant selectivities of JG cells

(A) Two-photon image showing three JG cells in the same focal plane. Scale bar: 20 μm. (B) Horizontal and vertical locations of the labeled JG cells. Red diamonds (a–c) correspond with the neurons in (C). Other labeled neurons are shown as black open diamonds. (C) Odor-evoked Ca²⁺ responses for the three indicated JG cells in (A, B). Black bars under each trace show the timing of odor delivery (3 sec). All of these neurons show similar excitatory odorant response profiles. (D) MRRs for JG cells in (a–c) (Glom. #1) and other glomeruli. JG cells with and without L-Dends are surrounded by solid and broken outlines, respectively. X- and Y-axes represent Ca²⁺ responses and odorants used for stimulation, respectively. Bars extending right/left from the centerline indicate fluorescent increases/decreases for each cell, respectively. Glom.: glomerulus (E) Summary of the similarities in excitatory odorant selectivities of JG, tufted, and mitral cells. JG cells show significantly higher similarities in odorant selectivity than tufted and mitral cells (***p < 0.001, Steel-Dwass test, based on 40 JG, 22 tufted, and 48 mitral cell pairs). Red bar: JG cell pairs; blue bar: tufted cell pairs; green bar: mitral cell pairs. Error bars indicate the standard error of the mean (s.e.m.).

Fig. 7. Excitatory odorant selectivities of mitral cells

(A) Two-photon image of mitral cells associated with the same glomerulus on bottom left (cells a–b) and upper right (cells c–d). Scale bar: 50 μm. (B) Horizontal and vertical locations of labeled neurons. Green diamonds (a–d) correspond with neurons in (C). Other labeled neurons are shown as black open diamonds. (C) Odor-evoked Ca²⁺ responses for cells in (A, and B). Black bar under each trace indicates timing of odor delivery (3 sec). Mitral cells a and b show strong responses to 6-7CHO stimulation and similar excitatory odorant selectivities. Cells c and d showed excitatory responses to 3CHO stimulation. The glomerulus associated with all the cells in (a) was activated by 3-7CHO odorants (Fig. 3F, Glom. #6). (D) MRRs for mitral cells associated with the same glomeruli and their horizontal distribution patterns. Cells surrounded by lines originate from the same glomerulus. X- and Y-axes represent Ca²⁺ responses and odorants used for stimulation, respectively. Bars extending right/left from the centerline indicate fluorescent increases/decreases for each cell, respectively. The horizontal distribution patterns for the cells are shown in the charts to the right of the indicated glomeruli. X- and Y-axes for the horizontal plots indicate the anteroposterior axis. The center points for the horizontal plots indicate the centers of the dye-injected glomeruli. (E–F) Correlations of interneuronal distances and the similarity of excitatory odorant selectivities between mitral (E) and JG cells (F). (G–H) Correlation coefficient-based analysis between odor-induced fluorescent changes (increase/decrease) and interneuronal distances for each pair of mitral (G) and JG cells (H). (mitral cells: R=−0.77, p < 0.001; JG cells: R=0.01, p=0.93). Each dot indicates mitral (E and G) or JG (F and H) cell pairs. Adjacent pairs of mitral cells show similar odorant selectivities, while separated pairs do not (48 pairs, R=−0.76, p < 0.001, Pearson correlation coefficient). By contrast, interneuronal distances of JG cells did not correlate with excitatory odorant selectivities (40 pairs, R=0.11, p=0.49, Pearson correlation coefficient).

Fig. 8. Schematic diagram of odorant representations in multiple neurons of a glomerular module

Each colored circle represents distinct odorant (blue, red, green, yellow). In this model, OSN and JG cells handle three odorants information, while tufted cells handle two, and mitral cells handle one. The odorant tuning specificities of the mitral cells depend on the locations of the cell bodies. Neighboring mitral cells likely receive similar inhibitory control from the same subset of granule cells. Thus, neighboring mitral cells may show similar odorant responses profiles while distant mitral cells show different ones. However, if neighboring mitral cells receive excitatory inputs from different glomeruli (⊕ mitral cell), their odorant selectivities would due to disparate excitatory inputs from their associating glomeruli. Right: Schematic diagram of eMRR sharpening. From the GL to the MCL, eMRR widths become more narrowly tuned. However, the preferred odorants resulting from the narrowed tuning differ depending on the location of each neuron. OSN: olfactory sensory neuron; ONL: olfactory nerve layer; GL: glomerular layer; EPL: external plexiform layer; MCL: mitral cell layer; IPL: internal plexiform layer; GCL: granule cell layer, Glom.: glomerulus; JG cell: juxtaglomerular cell.