Increased excitability of aged rabbit CA1 neurons after trace eyeblink conditioning

Cellular properties of CA1 neurons were studied in hippocampal slices 24 hr after acquisition of trace eyeblink conditioning in young adult and aging rabbits. Aging rabbits required significantly more trials than young rabbit s to reach a behavioral criterion of 60% conditioned responses in an 80 tri al session. Intracellular recordings revealed that CA1 neurons from aging c ontrol rabbits had significantly larger, longer lasting postburst afterhype rpolarizations (AHPs) and greater spike frequency adaptation (accommodation ) relative to those from young adult control rabbits. After learning, both young and aging CA1 neurons exhibited increased postsynaptic excitability c ompared with their respective age-matched control rabbits (naive and rabbit s that failed to learn). Thus, after learning, CA1 neurons from both age gr oups had reduced postburst AHPs and reduced accommodation. No learning-rela ted differences were seen in resting membrane potential, membrane time cons tant, neuron input resistance, or action potential characteristics. Further more, comparisons between CA1 neurons from trace-conditioned aging and trac e-conditioned young adult rabbits revealed no statistically significant dif ferences in postburst AHPs or accommodation, indicating that similar levels of postsynaptic excitability were attained during successful acquisition o f trace eyeblink conditioning, regardless of rabbit age. These data represe nt the first in vitro demonstration of learning-related excitability change s in aging rabbit CA1 neurons and provide additional evidence for involveme nt of changes in postsynaptic excitability of CA1 neurons in both aging and learning.