Decreases in the sustained firing capacity of layer 2/3 pyramidal neurons in the anterior cingulate cortex of aged rats

The anterior cingulate cortex (ACC) is particularly vulnerable to aging, which impairs cognitive functions such as attention and working memory. Although aging is known to alter intrinsic electrophysiological properties in other brain regions, the differences in the properties of aged ACC neurons and young adult neurons remain uninvestigated. In this study, we compared the intrinsic membrane properties and firing characteristics of aged layer 2/3 pyramidal neurons (26-month-old rats) with those of young adult neurons (6-month-old rats) using whole-cell patch-clamp recordings in acute slices from male Long-Evans rats. Although the distribution of the aged neurons based on the firing patterns did not significantly differ from that of the young adult neurons, the aged neurons exhibited depolarized resting membrane potentials, decreased input capacitance, and increased input resistance. In regular-spiking neurons, compared with young adult neurons, aged ACC neurons presented higher action potential thresholds, smaller action potential amplitudes, narrower action potential half-widths, and smaller slow afterhyperpolarization (sAHP) amplitudes. While basic excitability under standard step currents was preserved, aged regular-spiking neurons did not show sustained firing under ramp or triangular current stimulation, with rapid decreases in the firing frequency following the ramp peak and premature spike termination during the triangular descending phase, respectively. These results indicate that the intrinsic membrane properties of aged ACC neurons differ from those of young adult neurons and that aged neurons exhibit a reduced capacity for sustained firing. This cellular dysfunction in aged neurons provides a potential physiological mechanism underlying the age-related decline in ACC-dependent cognitive functions.