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The catecholaminergic neurons of senescence-accelerated mice (SAM-P8) were analyzed by immunohistochemical microphotometry in terms of immunoreactivities to aromatic L-amino acid decarboxylase (AADC), dopamine (DA), or noradrenaline (NA). Accelerated senescence-resistant mice (SAM-R1) were used as control mice. The immunoreactivities to AADC, DA, and NA of the catecholaminergic neurons of the SAM-P8 mice were weaker than those of the SAM-R1 mice in all the brain regions. Immunoelectron microscopy revealed progressive degeneration of dopaminergic neurons and their terminal fibers in the substantia nigra as well as in noradrenergic neurons and their proximal dendrites in the locus coeruleus of the SAM-P8 mice. In contrast, there was no difference between the SAM-P8 and SAM-R1 mice in the distribution of AADC-only positive neurons (designated as D neurons in the rat brain by Jaeger et al.) nor in their immunoreactivities. These results may indicate that DA neurons in the substantia nigra and NA neurons in the locus coeruleus degenarate more rapidly during aging in SAM-P8 mice than in control SAM-R1 mice and that D neurons may function as a part of a compensatory system for the decreases in catecholaminergic neurons during aging.
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