ABSTRACT
Neuronal degeneration increases with age in response to stressors, but the sub-cellular mechanism is unknown, partly because of previous difficulty in studying aged neurons in isolation. We studied the mechanism of enhanced neuronal susceptibility to glutamate and beta-amyloid in terms of condensed nuclei and other upstream events in hippocampal neurons cultured from old rats (24 months) compared to middle-age (10 months) and embryonic rats. Treatment of neurons from old animals with beta-amyloid (or glutamate) produced condensed nuclei 1.5x (2x) more frequently than middle-age and 3x (4x) more frequently than embryonic neurons. In addition to age-related baseline levels of caspase activation, neurons from old animals showed a 50% greater increase in caspase activation compared to middle-age and embryonic neurons. In contrast to glutamate treatment, beta-amyloid caused oxyradical damage as protein carbonyls increased 2-fold higher for old neurons than middle-age and 10-fold higher than embryonic neurons. Contrary to expectations, steady-state calcium levels for adult neurons did not increase in response to beta-amyloid. Overall, these results suggest that aged neurons have an inherent increased susceptibility to beta-amyloid toxicity through an early action of oxyradicals followed by caspase activation and nuclear condensation, a common pathway of apoptosis. Age-related glutamate toxicity involves other steps that lead to nuclear condensation, but neuron responses to calcium influx appear more important to cell death than the amount of influx.
