In the next 10 years nearly a quarter of the US population will be over the age of 65, and advanced age is the strongest risk factor for developing persistent post-operative cognitive declines (POCD). POCD is an array of cognitive impairments experienced following surgery and is thought to stem from increased neuroinflammation. Critically, persistent POCD is predictive of greater susceptibility to developing Alzheimer's disease, yet there is a fundamental gap in understanding the mechanisms underlying this devastating form of POCD. The long-term goal is to develop effective interventions to prevent and reverse this condition. The overall objective for this proposal is to determine the mechanisms that underlie aging-associated persistent POCD. A key oversight in models of POCD that fail to recapitulate the long-lasting POCD symptoms observed in older surgical patients is the pervasive use of morphine for pain management. Preliminary data here demonstrate that post-surgical morphine treatment in aged, but not young rats prolongs POCD to 4 weeks compared to 4 days without mor- phine. The central hypothesis is that aging and morphine synergize in response to surgery through joint activa- tion of the pattern recognition receptor TLR4 resulting in potentiated and protracted neuroinflammatory re- sponses, which inhibit key processes critical for forming long-term memories. This hypothesis has been formu- lated on the basis of two key premises. First, morphine activates TLR4 in the brain, thereby inducing neuroin- flammation. Second, aging is associated with exaggerated neuroinflammatory responses to challenges (e.g. surgery) due to a naturally occurring sensitization of microglia, which express TLR4. Guided by strong prelimi- nary data, this hypothesis will be tested by pursuing 4 specific aims: 1) Characterize the neuroinflammatory re- sponse to surgery and morphine in young and aged rats, and determine the role of TLR4 in persistent POCD; 2) Determine the necessity and/or sufficiency of sensitized microglia in mediating long-lasting POCD; 3) De- termine the specific role of microglia in morphine-induced neuroinflammation; and 4) Characterize POCD in aged female rats. The approach is innovative due to its proposed use of a novel DREADD to selectively inhibit microglia. Furthermore, systematically examining the independent and combined effects of aging, surgery, and morphine, (and in aged female rats, estrogen depletion) on POCD has not been done before. The proposed research is significant because it is expected to advance understanding of the mechanisms underlying persis- tent POCD in the elderly, which has the potential to be clinically informative. For example, it may be that how elderly patients are treated for pain significantly enhances cognitive impairments, suggesting a shift to non- opioid analgesics or a combination treatment in which a TLR4 antagonist is added to mitigate morphine's un- desired effect. Ultimately such knowledge has the potential to slow, halt, or even reverse early POCD process- es that may be precursors to dementia, thus improving quality of life.
The proposed research is relevant to public health because understanding the mechanisms underlying persistent POCD in aging, which is a strong predisposing factor for developing dementia, will ultimately lead to identifying key targets, developing effective therapies, and improving quality of life for the elderly. Thus, the proposed research is relevant to the NIA's mission to further the understanding of neural and behavioral processes associated with the aging nervous system as they relate to Alzheimer's disease and other dementias.
