Accessory olfactory bulb function is modulated by input from the main olfactory epithelium

Abstract

Although it is now established that sensory neurons in both the main olfactory epithelium and the vomeronasal organ may be activated by both general and pheromonal odorants, it remains unclear what initiates sampling by the vomeronasal organ. Anterograde transport of wheat germ agglutinin-horseradish peroxidase was used to determine that adequate intranasal syringing with zinc sulfate interrupted all inputs to the main olfactory bulb but left intact those to the accessory olfactory bulb. Adult male treated mice were frankly anosmic when tested with pheromonal and non-pheromonal odors and failed to engage in aggressive behavior. Treated juvenile females failed to show puberty acceleration subsequent to exposure to bedding from adult males. Activation of the immediate early gene c-Fos and electrovomeronasogram recording confirmed the integrity of the vomeronasal system in zinc sulfate-treated mice. These results support the hypothesis that odor detection by the main olfactory epithelium is required to initiate sampling by the vomeronasal system.

Figure 1

Photomicrographs of frontal sections through the olfactory bulb of a saline treated control (A, A1) and a ZnSO₄ treated mouse (B, B1, C, C1) showing anterograde transport of WGA-HRP. Medial is to the left and dorsal is to the top. As shown in A, all glomeruli in saline treated mice were filled with dense WGA-HRP reaction product. In mice of the Zn- group WGA-HRP reaction product was observed only in the vomeronasal nerve (labeled N in B1 and C1) and the AOB (C, C1). A1, B1, and C1 show details of the areas outlined in the A, B, and C figures.

Figure 2

Mean correct responses in the first 60 post-treatment odor detection tasks for saline treated mice, ZnSO₄ treated mice that had some remaining input to MOB glomeruli (Group Zn+), and ZnSO₄ treated mice that had WGA-HRP reaction product only in the AOB (Group Zn−). EA: ethyl acetate. 2-H: 2-heptanone. U: urine. MB: methyl benzoate. Percent values are the percent dilutions of odorants.

Figure 3

Peak mean EVG response and range of responses to different stimuli in experimental (open bars) and control mice (shaded bars). Urine was C57BL/6 female urine diluted 1:200 in Ringers. All other odorants were dissolved in Ringers at 100 µM. EA: ethyl acetate. MB: methyl benzoate. 2-Hep: 2-heptanone. DMP: 3,5-dimethylpyrazine. Control scores based on 3 mice and numbers in open bars represent the number of experimental mice used.

Figure 4

Photomicrographs showing c-Fos expression in the glomerular region from the ventromedial area of the MOB in a ZnSO4 treated (A) and saline treated mouse (B). Little or no Fos expression was found in the MOB in Zn- mice. G: glomerular layer. EP: external plexiform layer. M: mitral cell layer. GR: granule cell layer. Calibration bar: 50 microns.

Figure 5

Representative sagittal sections showing c-Fos expression in mitral and granule cells of the accessory olfactory bulb. A and B, saline control mice exposed to female urine placed on external nares (A) or only to urinary vapor (B). C and D, ZnSO₄ treated mice exposed to female urine placed on external nares (C) or only to urinary vapor (D). GR: granule cell layer. M: mitral cell layer. GL: glomerular cell layer. Calibration bar: 100 microns.

Figure 6

Uteri from zinc sulfate treated (A) and control (B) juvenile mice after exposure to sexually mature male mice. Scale bar: 1mm