Aging of Cholinergic Synapses: Fiction or Reality?

Abstract

Cholinergic system and aging of the human brain. The normal process of aging of the human brain seems to be associated with a loss of both cortical and subcortical neurons (Table 1). A central problem is whether this decrease in number of neurons is related to neurotransmitter-specific systems or whether it involves unspecifically all neuronal systems. Recent autopsy and biopsy studies in humans (Table 1) indicate that aging of the brain is indeed accompanied by a loss of neurons in certain areas, mainly in specific cortical regions. However, no significant changes in cell number are found in other areas, such as brainstem nuclei, cranial nerve nuclei, pons-hypothalamic areas or the medulla oblongata (Table 1). A review of the recent morphological and biochemical literature (Table 1) suggests that the number of brain regions known to escape neuronal loss in the course of the aging process has grown. Therefore, it seems that in the aging human brain two populations of neurons coexist side by side. One undergoing regressive changes leading to cell death and a second surviving and undergoing continuous and dynamic growth of processes. In pathological conditions, such as in Parkinson and Alzheimer diseases, some decreases are found in corresponding areas but not in others, indicating a variance from normal aging (Table 1).