Olfactory disturbance in aged rats in association with mitochondrial changes in the olfactory bulb neurons

Zhimin DING; Shumin ZHAO

ABSTRACT

BACKGROUND:

Mutation of the mitochondrial DNA may occur during the aging process of organisms, which is especially likely in the central nervous system. Evidences have been obtained that mitochondrial dysfunction may ensue from genetic impairment involved in oxidative phosphorylation, which is accompanied by corresponding morphological changes.

OBJECTIVE:

To investigate the association between olfactory disturbance and ultrastructural mitochondrial changes in olfactory bulb neurons of aged rats in comparison with young rats.

DESIGN:

Randomized controlled experiment.

SETTING:

Department of Internal Medicine, Affiliated Hospital of Chende Medical College and Department of Electron Microscopy of Chende Medical College.MATERIALS This experiment was conducted in the Department of Electron Microscopy of Chende Medical College between April and December 2002. Sixteen male Wistar rats were divided equally into aged group (＞ 24 months) with body mass of 300-350 g and young group (6 months) with body mass of 180-220 g.

METHODS:

The rats in the two groups were anaesthetized by intraperitoneal injection of 10 g/L urethane (1 g/kg) and the chest was opened to insert a tube into the ascending aorta for perfusion with 200 mL of the mixture containing glutaric dialdehyde and paraformaldehyde for fixation.The olfactory bulb was then obtained and sliced, fixed in perosmic acid and embedded. Each layer of the olfactory bulb was observed under optical microscope and ultra-thin sections were prepared for observation under transmission electron microscope.MAIN OUTCOME

MEASURES:

The stratification of rat olfactory bulb and ultrastructural changes of the mitochondria in the major neurons in the olfactory bulb.REULSTS No obvious changes were found in the stratification of the olfactory bulb in the two groups. From the exterior to the interior of the olfactory bulb, the olfactory nerve fiber layer, glomerular layer, external plexiform layer, mitral cell lalyer, internal plexiform layer and olfactory bundle nerve fiber layer were observed. Ultrastructurally, the mitochondria were more numerous in the brush cells in aged rat olfactory bulb, appearing rodshaped or ovoid with irregular cristae which were arranged in local disorder and some fragmented into small granules to form small bodies of high electron density. A few mitochondria became myeliuoid body with irregular or concentric circular morphologies. In the younger rats, most of the mitochondria in the neurons were ovoid with distinct outer and inner membrane and regular arrangement of the mitochondrial cristae, and irregular arrangement of the cristae was seen only in rare cases.

CONCLUSION:

The stratification of the olfactory bulb is basically the same in aged and young rats, but the ultrastructural changes in the mitochondria in the major neurons in aged rat olfactory bulb can be obvious,which is consistent with the reduced mitochondrial oxidative phosphorylation function. This might be one of the major causes for olfactory disturbance related to aging.