Decreased axosomatic input to motoneurons and astrogliosis in the spinal cord of aged rats.

An increasing body of evidence indicates that aging-related impairments of nervous functions are caused by damage to neuron integrity rather than by loss of neurons. By using electron microscopy, we have examined axosomatic boutons on spinal cord motoneurons derived from aged and young adult Sprague-Dawley rats. The main finding was that about half of the examined motoneuron somata from aged rats had a reduced (50%) bouton coverage, which seemed to be caused by a smaller number of axosomatic bouton profiles. Long stretches of the cell body plasma membrane were apposed by pale processes, and immunolabeling for glial fibrillary acidic protein (GFAP) disclosed that a number of the aged motoneurons appeared embedded in GFAP immunopositive processes. Lumbar motoneurons seemed to be more severely affected than cervical motoneurons. At the ultrastructural level, affected motoneurons disclosed plasma membrane irregularities with appendages/sprout-like extensions that in some cases were sites for axosomatic contacts.