Presently, lifestyle factors such as chronic high-fat diet (HFD) consumption occurs concomitantly with weight gain and obesity (Gil-Cardoso et al., 2017; Stranahan et al., 2008). In turn, obesity has been associated with impairments to mental functioning, specifically to memory. Human epidemiological studies show that HFD intake containing saturated, and omega-6-fatty acids is associated with worse performance on a cognitive task whereas a lower fat diet containing omega-3-fatty acids is associated with a protective effect against cognitive decline (Zhang et al., 2006; Uranga et al., 2010). One explanation for this is the critical role of the gut bacteria in brain health. In the presence of a suitable host diet, gut bacteria play a fundamental role in influencing the nutritional value of foods by digesting nutrients, synthesizing vitamins, and protecting against disease; however, they can be harmful when the ecosystem undergoes dysbiosis, or abnormal compositional changes, because of malnutrition and lifestyle behaviors, causing memory impairments (Mozaffarian et al., 2011). Recent speculations suggest a profound role of probiotic intervention in influencing brain functioning by altering the gut bacteria in just four weeks (Cryan & Dinan, 2012; Wang et al., 2016). Specifically, one of the brain functions altered by probiotics is hippocampal spatial memory. At present, the question of probiotic influence on gut bacteria, and specifically on obesity, remains actively debated. In different rodent studies, probiotics show some rescue effects of memory impairments, but switching back to control diets also demonstrates reversion (Beillharz et al., 2018, Rahmati et al., 2019). Therefore, it is difficult to know whether probiotics, or avoiding HFD consumption altogether, is important for maintaining intact memory as aging progresses. To test this, the present study observed changes to spatial and non-spatial memory in adult rats using three memory tasks (Novel Object Recognition, Novel Object Location, Radial Arm Maze), as well as sampled blood-glucose levels instantaneously following HFD consumption and instantaneously during recovery. Therefore, this study utilized an interdisciplinary approach to incorporate fields of microbiology, nutrition, and behavioral neuroscience to bridge the gap between HFD-induced memory impairment and dysbiosis, with the possibility of administering probiotics to rescue memory impaired by HFD in adult rats, and further assessed hippocampal volume. The present study hypothesized that chronic HFD consumption will impair spatial memory and probiotic treatment will hasten the memory recovery process.
Sixteen Long-Evans rats were tested on three different memory tasks and had body weight and blood glucose levels measured at three different time points: baseline, post-HFD, and weekly during the recovery phase. The memory tasks were as follows: Novel Object Recognition, Novel Object Location, and Radial Arm Maze. Following six weeks of HFD (60%E fat) consumption, rats were randomly assigned into the control (n=8) or probiotic (n=7) group and given respective treatment daily throughout recovery along with standard diet (13%E fat). One rat from the probiotic group was omitted from memory testing due to high anxiety-like behavior. By week four recovery, all rats were euthanized after one of the two groups showed memory recovery to post-HFD levels, and their brains extracted and stored for neurological analyses of brain tissue Cresyl Violet staining to detect differences in hippocampus structure at the time of memory differences.
The results showed that all rats significantly gained weight, displayed high blood-glucose levels, and impaired spatial and non-spatial memory performance following six weeks of HFD across all three memory tasks. Moreover, all rats displayed significant recovery of non-spatial memory at a similar rate in the Novel Object Recognition (NOR) task by the fourth week of recovery, just by the help of a standard diet. However, only probiotic-treated rats displayed significant spatial memory recovery in the Novel Object Location (NOL) task by the fourth week of recovery. Additionally, probiotic-treated rats displayed significant spatial working memory recovery in the Radial Arm Maze (RAM) by the fourth week of recovery with lower maze completion times. The results of probiotic intervention on hippocampal volume analyses showed that probiotic-treated rats display a larger hippocampus than control rats.
Overall, the results of this study show that following six weeks of HFD, rats have increased blood glucose levels, experienced significant weight gain, and have impaired spatial and non-spatial memory. Following cessation of HFD and return to a standard chow diet during the recovery phase, rats show normalized blood glucose levels, steady weight loss, and recovery of non-spatial memory functions on the NOR task. However, only probiotic-fed rats show recovery of spatial memory functions on the NOL task and RAM. With spatial and non-spatial memory processes impaired by HFD, it follows that failure to explore novel objects or objects in novel locations reflect a failure to recognize object recency. This may be further explained by the negative effects attributed to HFD that alter hippocampal structure (Molteni et al., 2002). Because all rats consumed HFD for six weeks, and then were switched to the standard control diet, any differences were likely due to probiotic intervention. However, because bacteria within the probiotic yogurt samples were not accurately measured nor microbiota fecal samples were collected, the specific changes or effects of food treatment (probiotic or standard diet) on gut health are unknown. As spatial memory is dictated by the hippocampus, it is noteworthy that the hippocampus was larger in probiotic-fed rats, suggesting recovery of hippocampal volume from potential HFD atrophy. Due to the longitudinal nature of the present study and unavailable in vivo MRI imaging to capture brain structures, namely the hippocampus, hippocampal volumes were only evaluated at the week four recovery time point, thus future studies should include this to assess differences in hippocampal volumes at different time points. Together, these data may suggest that brain and behavior differences are attributed to prolonged HFD consumption that can be rescued by probiotic intervention and suggest implications of the gut microbiota in brain and physical health. This study has therefore provided some insight to which the effectiveness of probiotics in rodent models may have altered the gut microbiota, in both attenuating obesity and spatial memory performance and improving hippocampal atrophy.
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El-Hulu, Sanyourah, "Probiotic Intervention Improves Recovery of Hippocampal Memory and Hippocampal Atrophy Following Disruption from High-Fat Diet in Adult Rats" (2021). Honors Theses.