Age-related changes in p2 odorant receptor mapping in the olfactory bulb

Abstract

The ability to identify odors is dependent on the spatial mapping of odorant receptors onto fixed positions within the olfactory bulb. In elderly adults, odor identification and discrimination is often impaired. The objective of this study was to determine if there are age-related changes in odorant receptor mapping. We studied 8 groups of mice ranging in age from 2 weeks to 2.5 years and mapped the projection of P2 odorant receptors onto targeted glomeruli within medial and lateral domains of the olfactory bulb. A total of 60 mice were used to measure the number of P2 glomeruli, bulb length, the position of each glomerulus, and the amount of P2 axons targeting each glomerulus. We found that over 70% of olfactory bulbs contained multiple P2 glomeruli, bulb length increased 42% between the ages of 2 and 13 weeks, and the position of P2 glomeruli shifted with bulb growth. In most cases, targeted glomeruli were either completely or partially filled with P2 axons. In some cases, targeting was diffuse, with glomeruli receiving only a few stray P2-labeled axons. The frequency of diffuse targeting was rare (<4%) in adult mice 3-6 months in age. However, significant increases in diffuse targeting were observed in older mice, reaching 10% at 1 year and 22% at 2 years of age. These findings suggest that odorant receptor mapping becomes more disrupted in old age and could account for impaired olfactory function in elderly adults.

Figure 1

Diagram illustrating the method used to measure the position of P2 glomeruli. (A) Lateral view of the olfactory bulb showing lines representing coronal sections passing through the plane of the anterior reference section (A), posterior reference section (P), and section containing the glomerulus (G). The AP distance defines the olfactory bulb length and AG distance the position of the glomerulus. (B) Diagram of a coronal section of the olfactory bulb passing through a glomerulus G. A point one-third the distance along a line (D–V) passing though the dorsal and ventral glomerular layers, and a vector extending to the center of the glomerulus G, was used to measure the angle that defines the position of the glomerulus within a section. Both AG distance and angle were used to determine the coordinate position of each glomerulus (see Materials and methods for details). M, medial; L, lateral; D, dorsal; V, ventral.

Figure 2

Coronal sections of olfactory bulbs illustrating location of GF (full) glomeruli. (A1) In anterior sections, P2 glomeruli (round structures, dark blue stain) are found in lateral regions (domains) of the bulb. In more posterior sections, (B1) glomeruli are located on the medial side. Examples of single and double occurrences of GF glomeruli are shown at higher magnification, (A2 and B2). Sections stained with X-Gal and neutral red. Calibration bars: 1000 μm for A1and B1, 100 μm for A2 and B2.

Figure 3

Graphs showing age-related changes in the olfactory bulbs. (A) Olfactory bulb length (AP distance) and body weight both increase with age. (B) The AG distance of P2 glomeruli shifts as the bulb increases in length. This shift is most noticeable during a period of rapid growth (2–13 weeks).

Figure 4

Graphs illustrating the relative position of GF glomeruli for different age groups. (A) Graph shows P2 glomeruli mapped in bulb space using AG/AP (relative position in bulb) and angle (medial lateral position) measurements. P2 glomeruli in younger mice tend to be in a more caudal (posterior) location than that for older mice. (B) Graphs showing changes in relative position (AG/AP) and angle with age. Data (means ± standard deviations) for GFs in the medial (diamonds) and lateral (triangles) domains are plotted separately.

Figure 5

Images of glomeruli showing differences in P2 target specificity and amounts of innervation. Photo above shows the convergence of P2 axons (dark blue stain) onto a single glomerulus (GF) and a stray axon targeting additional glomeruli (GT). Inset (top right) shows low-power view of olfactory bulb and nasal cavity whole mount preparation. Histological sections in lower panels give examples of glomeruli filled (GF), partially filled (GP), and those with only trace (GT) amounts of P2 axons.

Figure 6

Graphs showing the percentages of GF, GP, and GT glomeruli present in the olfactory bulb for different age groups. The percentage of GF glomeruli was similar across all age groups. GP glomeruli increased during growth and maturation (2–26 weeks) and then declined with increasing age. GT glomeruli were relatively common (20%) at 2 weeks, decrease in adults (13 and 26 weeks), and then gradually increase in old age (104–130 weeks, 2–2.5 years). Error bars, ±standard deviation; * denotes P < 0.05. Multiple comparisons of the means for the different age groups were tested with an analysis of variance and the Tukey honestly significant difference post hoc test.