ApoE4 attenuates cortical neuronal activity in young behaving apoE4 rats

The E4 allele of apolipoprotein E (apoE4) is the strongest genetic risk factor for late-onset Alzheimer's disease (AD). However, apoE4 may cause innate brain abnormalities before the appearance of AD-related neuropathology. Understanding these primary dysfunctions is vital for the early detection of AD and the development of therapeutic strategies. Recently we reported impaired extra-hippocampal memory in young apoE4 mice, a deficit that was correlated with attenuated structural pre-synaptic plasticity in cortical and subcortical regions. Here we tested the hypothesis that these early structural deficits impact learning via changes in basal and stimuli evoked neuronal activity. We recorded extracellular neuronal activity from the gustatory cortex (GC) of three-month-old humanized apoE4 (hApoE4) and wildtype rats expressing rat apoE (rAE), before and after conditioned taste aversion (CTA) training. Despite normal sucrose drinking behavior before CTA, young hApoE4 rats showed impaired CTA learning, consistent with our previous results in target-replacement apoE4 mice. This behavioral deficit was correlated with decreased basal and taste-evoked firing rates in both putative excitatory and inhibitory GC neurons. Further taste coding analyses at the single neuron and ensemble levels revealed that GC neurons of the hApoE4 group correctly classified tastes, but were unable to undergo plasticity to support learning. These results suggest that apoE4 impacts brain excitability and plasticity early in life that may act as an initiator for later AD pathologies.