Evidence That Increased Hippocampal Expression of the Cytokine Interleukin-1β Is a Common Trigger for Age- and Stress-Induced Impairments in Long-Term Potentiation

Several cytokines and their receptors are identified in brain; one of these is the proinflammatory cytokine interleukin-1β that is synthesized and released from neurons and glia in response to stress or insult. Among the actions of interleukin-1β is its ability to inhibit long-term potentiation in the hippocampus in vitro , an action that mimics one of the consequences of stress and age. It has been shown that the concentration of interleukin-1β in brain tissue is increased in neurodegenerative conditions, and recent evidence from our laboratory has indicated an increase in the concentration of interleukin-1β in the hippocampus of aged rats. These observations led us to consider that the underlying common cause of impaired long-term potentiation in aged and stressed rats might be increased endogenous interleukin-1β concentration in hippocampus. The data presented here indicate that there was an inverse relationship between concentration of interleukin-1β in the dentate gyrus and long-term potentiation in perforant path→granule cell synapses in aged rats, stressed rats, and rats pretreated with interleukin-1β. The evidence suggested that the cytokine induces formation of reactive oxygen species that triggers lipid peroxidation in vivo , as well as in vitro , and that these changes lead to depletion of membrane arachidonic acid that correlates with impaired long-term potentiation. We propose that three theories of aging, the glucocorticoid theory, the membrane theory, and the free radical theory, constitute three facets of age with one underlying trigger: an increase in the endogenous concentration of interleukin-1β in hippocampus.