Orthonasal versus retronasal glomerular activity in rat olfactory bulb by fMRI

Abstract

Odorants can reach olfactory receptor neurons (ORNs) by two routes: orthonasally, when volatiles enter the nasal cavity during inhalation/sniffing, and retronasally, when food volatiles released in the mouth pass into the nasal cavity during exhalation/eating. Previous work in humans has shown that both delivery routes of the same odorant can evoke distinct perceptions and patterns of neural responses in the brain. Each delivery route is known to influence specific responses across the dorsal region of the glomerular sheet in the olfactory bulb (OB), but spatial distributions across the entire glomerular sheet throughout the whole OB remain largely unexplored. We used functional MRI (fMRI) to measure and compare activations across the entire glomerular sheet in rat OB resulting from both orthonasal and retronasal stimulations of the same odors. We observed reproducible fMRI activation maps of the whole OB during both orthonasal and retronasal stimuli. However, retronasal stimuli required double the orthonasal odor concentration for similar response amplitudes. Regardless, both the magnitude and spatial extent of activity were larger during orthonasal versus retronasal stimuli for the same odor. Orthonasal and retronasal response patterns show overlap as well as some route-specific dominance. Orthonasal maps were dominant in dorsal-medial regions, whereas retronasal maps were dominant in caudal and lateral regions. These different whole OB encodings likely underlie differences in odor perception between these biologically important routes for odorants among mammals. These results establish the relationships between orthonasal and retronasal odor representations in the rat OB.

Keywords: Glomeruli; Olfactory bulb; Orthonasal; Retronasal; fMRI.

Figure 1

(A) Process of converting 3D MRI data to a flatmap of the glomerular sheet (GS). The glomerular layer is traced on each coronal slice, the trace is cut at the bottom and flattened to a row, and then all such rows are combined to form the flatmap. Within each slice of the MRI data, the layer mask forms a ring shape, or an inverted-U shape. Thus, each row of the coronal image has either one or two sections of the layer in it. If there are two sections, these are the left and right sides of the layer. Arranging these voxels from left-bottom to left-top, concatenated with right-top to right-bottom converts the layer into a one-voxel row in the flatmap. As one row corresponds to one slice in the original MRI, combining all of the rows in slice order creates a flatmap. (B) Orientation of each flatmap. This flatmap will have a rostral-caudal dimension, assuming rows from coronal slices were concatenated. The other dimension will go ventral-medial-dorsal-lateral-ventral based on how it was unwrapped. Thus, a 2D image is produced of the layer. Code is provided in (Thompson et al., 2018).

Figure 2

(A) Glomerular sheet activation for methyl valerate (MV) and ethyl butyrate (EB) odor during orthonasal stimulation (n=6 rats). Percentage change in fMRI BOLD activation maps was calculated between baseline versus odor stimulation. (B) As A, except retronasal stimulation (n=5 rats). While all odors are similar (all are esters, fruit-like odors), there are slight differences in the activation patterns between them. Odor concentrations were different for ortho (20 %) versus (40 %) retronasal stimulation, needed to make BOLD signal amplitudes more similar. Orientation of all flat maps is shown on the right side. Reproducibility maps across trials and subjects were shown in Supplemental Figure 1. R3.4

Figure 3

(A) Activation over time in regions of the left and right bulb’s glomerular sheet which have >0% mean activation during stimulation, for the MV and EB odors during orthonasal stimulation. Thin black line is mean of all trials (n=6 rats), gray shading ±1 S.D., thick black line is time of stimulation. (B) As A, retronasal stimulation (n=5 rats). Note longer stimulation than orthonasal.

Figure 4 –

Statistical maps of differences between orthonasal and retronasal activation patterns. Percent activation maps were normalized on a per-trial basis to zero mean, unit variance. (A) Similarity map. The orthonasal maps for all trials were averaged and summed to the average of the retronasal maps for all trials under MV stimulation. High values indicate co-activation between the two methods. (B) As A, but EB stimulation similarity map. (C) Dominance map. A two-sample, equal-variance, two-tailed T-test was performed between orthonasal trials and retronasal trials for MV stimulation. Resultant T values are shown with orthonasal > retronasal as < 0 (blue) and retronasal > orthonasal as > 0 (red). (D) As C, but EB stimulation dominance map. For both odors, relatively higher activation in the orthonasal condition is seen in dorsal and medial regions, whereas relatively higher activation in the retronasal condition is seen in a small caudal and lateral region. Statistical significance was tested at p < 0.05, corrected for multiple comparisons using SGoF, with separate statistical families for each odor. Statistical significance was observed in some regions for both orthonasal > retronasal and retronasal > orthonasal, and also both MV and EB (the spatial extent of statistical significance is shown in Supplemental Figure 3).