On age related changes in axons and glia of the central nervous system

Abstract

A growing body of evidence shows that phenotypic changes including axon aberrations, rather than loss of neurons, account for behavioral impairments during aging. The present thesis was undertaken to investigate the occurrence of axon aberrations in relation to transmitter identity, glial reaction and sensorimotor disturbances. To shed light on possible underlying mechanisms, signs of oxidative stress and inflammation were also examined.

The studies were performed on behaviorally defined aged (30 months old) and young adult (2-3 months old) Sprague Dawley rats, by using electron microscopy, immunohistochemistry, in situ hybridization and reverse transcriptase-polymerase chain reaction. The results show that many aged motoneurons lose a significant portion of their bouton covering, due to a decreased number of apposing boutons. Consistent with the more pronounced sensorimotor disturbances observed in the hind- in comparison with the forelimbs, lumbar motoneurons appeared more severely affected than cervical motoneurons. In the neuropil of the motor nucleus, aberrant axons were encountered. Ultrastructural analysis of aberrant axons in relation to content of amino acid neurotransmitters and the free radical scavenger glutathione (GSH) revealed that many of the aberrant axons contained high levels of glutamate-immunoreactivity (-IR) and were often enriched with GSH-IR.

Increased levels of GSH-IR were also encountered in glutamate-IR terminals with a preserved ultrastructure, suggesting that a changed redox status may be mechanistic in the development of axon aberrations. GABA- and glycine-IR terminals were more rarely affected, suggesting that excitatory and inhibitory pathways are differentially affected. In the aged rats, immunohistochemistry showed a reduced fiber density and axon aberrations of cholinergic and monoaminergic axons in both the spinal cord and the hippocampus. In contrast, the innervation of alpha-motonearons by C boutons was preserved in senescence. However, the C boutons showed a decreased labeling for cholinergic markers.

Regions disclosing axon terminal loss and aberrations showed increased expression of glial fibrillary acidic protein (GFAP, the main intermediate filament of astrocytes). Using Marchi staining on spinal cord sections, the outer parts of the white matter showed signs of a changed myelin metabolism and/or dysmyelination in aged rats. In the same regions, astro- and microglial cells showed conspicuous signs of activation, most pronounced in rats disclosing the most severe sensorimotor disturbances. The glial reaction appeared less pronounced in brain white matter compared to the spinal cord white matter.

The spinal cord white matter of aged rats also disclosed a changed expression of several cytokines, while the majority of investigated cytokines were unaltered in the hippocampus. One of the most prominent changes was an upregulation of the proinflammatory cytokine IFN-gamma, encountered in both the hippocampus and the spinal cord. There was a robust upregulation of TGFbeta-1 and IL1-beta in astroglia of spinal cord white matter, while no change was evident in the hippocampus. CNTF levels were unaltered in aged rats, however, IR appeared reduced in oligodendroglia-like cells, while it seemed increased in astroglia of the spinal cord white matter. IGF-1, a molecule with similar effects as CNTF, was upregulated in hippocampus but not in die spinal cord.