AGE-RELATED LOSS OF KNEE-JOINT AFFERENTS IN MICE

Previous work in our laboratory revealed markedly different rates of a ge-related death of four monoaminergic neuronal populations in the C57 BL/6 mouse. Although dorsal root ganglion neurons (DRGns) have been re ported not to suffer similar age-related death in rodents, we determin ed if there is age-related death of the subpopulation of DRGns innerva ting the knee joints of C57BL/6 mice, which are known to develop degen erative arthritis with aging. The somata of dorsal root ganglion neuro ns innervating the mouse knee joint (KJ-DRGns) were identified by retr ograde tracing with Fluoro-Gold (FG). Lumbar ganglia were serially sec tioned and the numbers of FG-labelled KJ-DRGns counted at five ages en compassing the animal's life span. Changes in size of the total popula tion of lumbar DRGns (L-DRGns) were estimated by counting nucleated so mata from every fifth toluidine blue-stained serial section from the L 3 and L4 lumbar ganglia at three different ages. Using a computer-assi sted video morphometric technique somal areas were measured from rando m sections to determine the distribution of sizes of neurons in the KJ -DRGn and general lumbar DRGn populations at different ages. Counts of FG-labelled joint afferents were 238.5 +/- 80.3 (mean +/- SD) KJ-DRGn s per knee at 2 months of age, declining to 103.2 +/- 20.1 by 24 month s, representing a 57% loss over the average life span of the C57 mice. The loss occurred in two phases, with a rapid rate over the first 8 m onths of life and a more moderate rate of loss over the remaining mont hs. L-DRGn numbers revealed a slower overall rate of loss in compariso n to the KJ-DRGn population with an average 33.7 % loss over the life span of this mouse. Somal size measurements revealed that the larger s izes of KJ-DRGns were lost over the first 8 months of life, with littl e change in the distribution of somal sizes thereafter. The distributi ons of sizes of the L-DRGn population did not change significantly ove r the life spans of the mice. The data provides evidence that the age- related loss of KJ-DRGns is significantly greater than DRGns in genera l, and may be particularly apparent in the population of larger-sized presumed mechanoreceptor neurons. The loss of the KJ-DRGns is approxim ately reciprocal to the incidence rate of knee joint osteoarthritis re ported for the C57BL/6 mice. (C) 1993 Wiley-Liss, Inc.