Age-related changes in noradrenergic sympathetic innervation of the rat spleen is strain dependent

Previous findings from our laboratory revealed an age-related decline in noradrenergic (NA) sympathetic innervation of the spleen in male Fischer 344 (F344) rats. The purpose of this study was to determine whether other rat strains also progressively lose NA sympathetic nerves in the aging spleen. Sympathetic innervation of spleens from 3- and 21-month-old male F344, Brown Norway (BN), BN X F344 (BNF(1)), and Lewis rats was examined using fluorescence histochemistry to localize catecholamines combined with morphometric analysis and using high-performance liquid chromatography with electrochemical detection for measuring norepinephrine (NE). Neurochemistry revealed a significant age-related decline in NE concentrations in spleens from F344 and Lewis rats. In contrast, there was no effect of age on splenic NE concentrations in BN or BNF(1) rats. Consistent with neurochemical analysis, fluorescence histochemistry revealed a striking decline in NA innervation of spleens from old F344 and Lewis rats not observed in the other two strains. However, in BN and BNF(1) rats, nerve fibers were diminished in distal portions of the spleen but not in the hilar regions. Morphometric analysis confirmed neurochemical and histological findings, revealing approximately 65-70% loss in NA nerve density in spleens from F344 and Lewis rats. These findings indicate that age-related changes in sympathetic innervation of the rat spleen are strain-dependent. Whether the loss of sympathetic nerves in spleens from F344 and Lewis rats is associated with age-related changes in the splenic microenvironment remains to be determined. The functional significance of altered sympathetic innervation of the spleen with advancing age is discussed.