Abstract
The prevention of age-related cognitive decline and dementia is becoming a high priority because of the rapid growth of aging populations. We have previously shown that hop bitter acids such as iso-α-acids (IAAs) and matured hop bitter acids (MHBAs) activate the vagus nerve and improve memory impairment. Moreover, supplements with MHBAs were shown to improve memory retrieval in older adults. However, the underlying mechanisms have not been entirely elucidated. We aimed to investigate the effects of MHBAs and the common β-tricarbonyl moiety on memory impairment induced by the activation of microglia and the loss of the noradrenergic system. MHBAs and a model compound with β-tricarbonyl moiety were administered to LPS-inoculated mice and 5 × FAD Alzheimer's disease (AD) model mice, following the evaluation in behavioral tests and microglial activation. To evaluate the association of noradrenaline with MHBAs effects, mice treated with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), a noradrenergic neurotoxin that selectively damages noradrenergic projections from the locus coeruleus, were subjected to the behavioral evaluation. MHBAs reduced brain inflammation and improved LPS-induced memory impairment. A model compound possessing the β-tricarbonyl moiety improved the LPS-induced memory impairment and neuronal loss via the vagus nerve. Additionally, the protective effects of MHBAs on memory impairment were attenuated by noradrenaline depletion using DSP-4. MHBAs suppressed the activation of microglia and improved the memory impairment in 5 × FAD mice, which was also attenuated by noradrenaline depletion. Treatment with MHBAs increased cholecystokinin production from the intestinal cells. Generally, cholecystokinin activates the vagal nerve, which stimulate the noradrenergic neuron in the locus ceruleus. Taken together, our results reveal that food ingredients such as hop bitter acids with a β-tricarbonyl moiety suppress microglial activation and improve memory impairment induced by inflammation or AD pathology via the activation of the gut-brain axis and noradrenergic system. Supplements with hop bitter acids, including MHBAs, might be a novel approach for the prevention of cognitive decline and dementia.