LOSS OF PRIMARY SENSORY NEURONS IN THE VERY OLD RAT - NEURON NUMBER ESTIMATES USING THE DISECTOR METHOD AND CONFOCAL OPTICAL SECTIONING

Loss of neurons has been considered to be a prime cause of nervous dis turbances that occur with advancing age. However, the notion of a cons titutive aging-related loss of neurons has been challenged recently in several studies that used up-to-date methods for counting neurons. In this study we have applied stereological techniques with the objectiv e of obtaining quantitative data on total neuron numbers and the distr ibution of neuron cross-sectional areas in the fifth cervical (C-5) an d fourth lumbar (L-4) dorsal root ganglion (DRG) of 3- and 30-month-ol d Sprague-Dawley rats. Tissue data were recorded on a confocal laser-s canning microscope with the use of the optical-disector technique and random, systematic sampling. Aged rats of both sexes disclosed only a small decrease (approximate to 12%) in the number of cervical and lumb ar DRG neurons. Furthermore, there was no significant correlation betw een the degree of neuron loss and the extent of behavioral deficits am ong the aged individuals. The DRG neurons of aged rats had a smaller m ean cross-sectional area (approximate to 15%; P < 0.001) at both DRG l evels. Further analysis of the male cohorts was carried out by using i solectin B4 and neurofilament subunit (phosphorylated 200 kDa; RT97) i mmunoreactivity (IR) as selective markers for unmyelinated and myelina ted axons, respectively, and disclosed no significant change in the re lative frequencies of immunoreactive neuron profiles in the old rats. However, RT97-IR DRG neurons of the aged rats had significantly smalle r cross-sectional areas (approximate to 9% in C-5; approximate to 16% in L-4; P < 0.001) than the young adult rats, indicating a selective c ell body atrophy among myelinated primary afferents during aging. The results indicate that loss of primary sensory neurons cannot exclusive ly explain the functional deficits in sensory perception among senesce nt individuals. It seems likely that other factors at the subcellular level and/or target interaction(s) contribute substantially to the sen sory impairments observed with advancing age. (C) 1998 Wiley-Liss, Inc .