Hard-diet feeding recovers neurogenesis in the subventricular zone and olfactory functions of mice impaired by soft-diet feeding

Abstract

The subventricular zone (SVZ) generates an immense number of neurons even during adulthood. These neurons migrate to the olfactory bulb (OB) and differentiate into granule cells and periglomerular cells. The information broadcast by general odorants is received by the olfactory sensory neurons and transmitted to the OB. Recent studies have shown that a reduction of mastication impairs both neurogenesis in the hippocampus and brain functions. To examine these effects, we first measured the difference in Fos-immunoreactivity (Fos-ir) at the principal sensory trigeminal nucleus (Pr5), which receives intraoral touch information via the trigeminal nerve, when female adult mice ingested a hard or soft diet to explore whether soft-diet feeding could mimic impaired mastication. Ingestion of a hard diet induced greater expression of Fos-ir cells at the Pr5 than did a soft diet or no diet. Bromodeoxyuridine-immunoreactive (BrdU-ir) structures in sagittal sections of the SVZ and in the OB of mice fed a soft or hard diet were studied to explore the effects of changes in mastication on newly generated neurons. After 1 month, the density of BrdU-ir cells in the SVZ and OB was lower in the soft-diet-fed mice than in the hard-diet-fed mice. The odor preferences of individual female mice to butyric acid were tested in a Y-maze apparatus. Avoidance of butyric acid was reduced by the soft-diet feeding. We then explored the effects of the hard-diet feeding on olfactory functions and neurogenesis in the SVZ of mice impaired by soft-diet feeding. At 3 months of hard-diet feeding, avoidance of butyric acid was reversed and responses to odors and neurogenesis were recovered in the SVZ. The present results suggest that feeding with a hard diet improves neurogenesis in the SVZ, which in turn enhances olfactory function at the OB.

Figure 1. BrdU-ir cells in the SVZ and OB of mice fed a hard or soft diet.

Sagittal sections of the SVZ (A–D) and OB (E–H) of mice fed a hard diet (A, B, E and F) or soft diet (C, D, G and H) for 1 month. Scale bar: 500 µm (C and G), 200 µm (D and H); I: the numbers of BrdU-ir cells at 600 µm thickness from Figure 108 of the mouse atlas (lateral 0.84 mm) of the SVZ to the lateral side of mice fed the hard diet (black column; n = 3) or soft diet (white column; n = 4). J: the number of BrdU-ir cells at 300 µm thickness from the section from Figure 108 of the mouse atlas (lateral 0.84 mm) of the OB to the lateral side of mice fed the hard diet (black column; n = 5) or soft diet (white column; n = 5). *: p<0.05; **: p<0.01.

Figure 2. Avoidance of butyric acid in mice fed a hard diet, soft diet or hard diet after soft diet.

Preferences were determined during a 4-min period in a Y-maze odor preference apparatus. Preference ratios between water placed on both sides of the Y-arm are shown in the three circles at the left in each figure (A, B, and C). When water was placed at the end of each Y-arm, the preference ratio of the mice fed the hard diet continuously, the soft diet continuously, the hard diet for 1 month after soft-diet feeding, or the hard diet for 3 months after soft-diet feeding was nearly 0.5. A: Preference ratios for 50% butyric acid in mice fed a hard diet (closed circles, n = 15) or soft diet (open circles and gray circles, n = 15 and n = 15, respectively). B: Preference ratios for 50% butyric acid in mice fed only a hard diet (closed circles, n = 15), only a soft diet (open circles, n = 15) or a hard diet after a soft diet (gray circles, n = 15) for 1 month. C: Preference ratios for 50% butyric acid in mice fed only a hard diet (closed circles, n = 10), only a soft diet (open circles, n = 10) and a hard diet after a soft diet (gray circles, n = 9) for 3 month. *: p<0.001; **: p<0.0005; ***: p<0.0001.

Figure 3. Fos-ir cells after exposure to urinary odor in the OB and Pir.

Sagittal sections of the OB and Pir after exposure of mice fed only a hard diet (A and E), only a soft diet (B and F), or a hard diet for 3 months after a soft diet for 1 month (C and G) to urinary odor, respectively. Scale bar: 200 µm (C), 500 µm (G). D: The numbers of Fos-ir cells in the mitral cell layer in 500 µm thickness from Figure 108 of the mouse atlas (lateral 0.84 mm) of the OB to the lateral side of mice fed only the hard diet (black column; n = 5), only the soft diet (white column; n = 5), or the hard diet after the soft diet (gray column; n = 5). H: The number of Fos-ir cells in 400 µm thickness from Figure 114 of the mouse atlas (lateral 1.56 mm) of the Pir of mice fed only the hard diet (black column; n = 5), only the soft diet (white column; n = 5), or the hard diet after the soft diet (gray column; n = 5). *: p<0.05; **: p<0.005.

Figure 4. BrdU-ir cells in the SVZ of mice fed a hard diet for 1 or 3 months after being fed a soft diet.

Sagittal sections of the SVZ of mice fed only a hard diet (A and D), only a soft diet (B and E), or a hard diet for 1 (C) or 3 months (F) after a soft diet for 1 month. Sagittal sections of the OB of mice fed only a hard diet (G), only a soft diet (H), and a hard diet for 3 months after a soft diet for 1 month (I). Scale bar: 200 µm (F and I); J and K: The numbers of BrdU-ir cells in 600 µm thickness from Figure 108 of the mouse atlas (lateral 0.84 mm) of the SVZ to the lateral side of mice fed only the hard diet (black column), only the soft diet (white column), or the hard diet for 1 month (J) or 3 months (K) after the soft diet (gray column). n = 4 without white column in K, n = 5 for white column in K. L: The number of BrdU-ir cells in 300 µm thickness from Figure 108 of the mouse atlas (lateral 0.84 mm) of the OB to the lateral side of mice fed only the hard diet (black column; n = 4), only the soft diet (white column; n = 5), or the hard diet for 3 months after the soft diet for 1 month (gray column; n = 4). *: p<0.05; **: p<0.01; ***: p<0.0001.

Figure 5. Effects of a hard or soft diet on the expression of Fos-ir cells in Pr5, PTg and SNc.

A: A schematic transmission pathway of oral sensation from the mouth to the SVZ. The principal sensory trigeminal nucleus (Pr5), which receive intraoral touch information via the trigeminal nerve, transmits to the pedunculopontine tegmental nucleus (PTg) via the thalamus , somatosensory cortex , and motor cortex . Neurons of the PTg innervate to the substantia nigra pars compacta (SNc) , , , in which dopaminergic neurons innervate to the SVZ , . The number of Fos-ir cells in 100 µm thickness in Figure 115 of the mouse atlas (lateral 1.68 mm) of the Pr5 (B), 100 µm thickness in Figure 111 of the mouse atlas (lateral 1.20 mm) of PTg (C), and 200 µm thickness from Figure 106 of the mouse atlas (lateral 0.60 mm) of SNc (D) to the lateral side of mice after the ingestion of a hard diet (black column), soft diet fed (white column) or no diet (gray column). n = 4 (white and gray column in B). n = 5 (C). n = 10 (black and gray column in D). n = 11 (white column in D). *: p<0.05; **: p<0.005.