Activation of the immune system by an infection may temporarily disrupt formation of long-term memories in healthy, aging rats by reducing levels of a protein required for brain cells to make new connections, suggests new research published in eNeuro.

Cognitive decline in old age is thought to be gradual, as in Alzheimer's disease. However, an abrupt or more rapid deterioration of brain function called delirium is also common in aging, but much less studied. Although most individuals will recover from periods of delirium, these episodes, which can be triggered by an exaggerated immune response, may increase risk of dementia.

Susan Patterson and colleagues previously found that 24-month-old rats infected with the bacteria Escherichia coli have a prolonged immune response and memory impairments compared to 3-month-old rats. The researchers have now extended these findings to show that the immune response in older rats persists for eight to 14 days and is associated with reduced levels of brain-derived neurotrophic factor (BDNF), a protein that enables the hippocampus to create long-term memory. The rodent model of delirium described in this study could inform future research to understand how surgery, infection, and injury -- factors that activate the immune system -- affect cognitive function in elderly humans.