Neuron-microglia dialogue and hippocampal neurogenesis in the aged brain

Age-related changes in innate immune function and glial-neuronal communication are early and critical events in brain aging and neurodegenerative disease, and lead to a chronic increase in oxidative stress and inflammation, which initiates neuronal dysfunction and reduced synaptic plasticity, and ultimately disruption in learning and memory in the aged brain. Several lines of evidence suggest a correlation between adult neurogenesis and learning. It has been proposed that a decline in hippocampal neurogenesis contributes to a physiologic decline in brain function. Recently, new and important insights relating to the production of new neurons affecting hippocampal-dependent memory ability have been provided. A multitude of factors have been shown to regulate the production of new neurons in the adult hippocampus, many of which change as a result of aging. Yet, the potential importance of neurogenesis in some affective and cognitive behaviors, as well as endogenous tissue repair mechanisms, makes further investigation of neurogenic regulators warranted. We have recent evidence that key regulators of communication between neurons and microglia are disrupted in the aged brain and may be one of the factors that precedes and initiates the observed increase in chronic inflammatory state. In this review the role of dysfunction in these neuronal-glial communication regulators underlying age-related impairments in cognition and hippocampal neurogenesis will be discussed. An understanding of these mechanisms will lead to the development of preventive or protective therapies.