Aging in the rat olfactory system: correlation of changes in the olfactory epithelium and olfactory bulb

Two regions closely linked synaptically (olfactory epithelium and olfactory bulb) have been compared in an age-graded series of rats. Previous findings of growth and atrophy of constituent elements in Sprague-Dawley Wisconsin (SD) rats have now been confirmed in Charles River (Crl) rats. In the olfactory bulbs of Charles River (Crl) rats, the volume of layers, the number of olfactory axodendritic synapses in the glomeruli, the total volume of glomerular dendrites, and the size of mitral cell bodies all approximately double between 3 and 24--27 months, and then all decrease by 36 months. Unlike SD rats, however, no loss in the number of mitral cells occurs in Crl rats, and the increase in volume of the olfactory bulbs from 3 to 24 months is approximately double that of SD rats. In the olfactory epithelium the total number of septal olfactory receptors more than doubles between 3 and 18--24 months and then declines markedly, as does the volume of olfactory axons in olfactory bulb glomeruli. Comparison of the regression lines for change in number of septal receptors with that of the size of mitral cell bodies discloses that the decline in number of receptors begins several months earlier than the decline in mitral cell size. This suggests that the atrophic changes in the olfactory bulb may in part be secondary to changes in the receptors of the olfactory epithelium. Numbers of synapses in the glomeruli appear to decline less markedly with age than the number of receptors, and a significant increase in number of synapses per receptor occurs in the oldest group studied (33 months), suggesting a compensatory increase in the relative number of synapses per receptor in the surviving receptors.