A behavioural characterization of neonatal infection-facilitated memory impairment in adult rats

We have reported that exposure to bacteria (Escherichia coli) during the neonatal period in rats is associated with impaired memory for a novel context in adulthood. However, impairment is only observed if a peripheral immune challenge (bacterial lipopolysaccharide (LPS)) is administered immediately following context exposure. The goal of the current study was to more fully characterize this phenomenon. In Experiment 1, memory impairment as a result of neonatal infection and subsequent LPS challenge was observed in juvenile rats, indicating that the changes induced by infection occur early on and are then manifest throughout the lifespan. In Experiment 2, infection in juvenile rats did not lead to LPS-induced memory impairment in adulthood, suggesting there is a critical period for early infection-induced alterations. In Experiments 3 and 4, memory for a novel context was impaired in neonatally infected rats, a task that is dependent on the hippocampus, whereas cued memory for a tone, which does not depend on the hippocampus, was not impaired. Furthermore, long-term, but not short-term contextual memory was impaired in adult rats infected as neonates following an LPS challenge either 24 h before or immediately after conditioning. Finally, in Experiment 5, no neonatal group differences were observed in corticosterone or open field behaviour, suggesting that decreased freezing to a conditioned context reflects impaired memory, and not simply hyperactivity or altered stress reactivity. Taken together, we have demonstrated that neonatal infection results in robust hippocampal-dependent memory impairment following an immune challenge in adulthood using a number of conditioning paradigms.