Differential effect of arachidonic acid and docosahexaenoic acid on age-related decreases in hippocampal neurogenesis
Introduction
Arachidonic acid (ARA) and docosahexaenoic acid (DHA) are important components of brain phospholipids (PLs) that decrease with age (Söderberg et al., 1991, McGahon et al., 1997, McGahon et al., 1999). Age-related deficits in hippocampal functions, including learning and memory, have recently been reported to be ameliorated by ARA or DHA supplementation in aged rats (Okaichi et al., 2005, Kotani et al., 2003, Gamoh et al., 2001); however, the mechanisms involved remain unknown.
Neurogenesis in the dentate gyrus (DG) of the hippocampus occurs throughout the lifetime of humans and rodents (Eriksson et al., 1998, van Praag et al., 2002) and is related to learning and memory in primates and rodents (Aizawa et al., 2009, Imayoshi et al., 2008). Newborn neurons (NNs) are generated from neural stem/progenitor cells (NSPCs) in the subgranular zone (SGZ) of the DG through differentiation and maturation. The numbers of both proliferating NSPCs and NNs in the DG decrease with age (Kuhn et al., 1996, Abrous et al., 2005). These findings suggest that maintaining the number of proliferating NSPCs will ameriorate the age-related decrease in NNs (Rao et al., 2005) and hippocampal function.
Several previous reports have addressed the relationship between neurogenesis and ingestion of ARA and DHA. Maekawa et al. showed that ingestion of ARA during postnatal days 2–31 increases the number of NSPCs in neonatal rats (Maekawa et al., 2009). Administration of DHA for 7 weeks increases the number of NNs in aged n-3 polyunsaturated fatty acid (PUFA)-deficient rats in the third generation of diet-deficient breeding (Kawakita et al., 2006). In addition, DHA promotes neuronal differentiation from embryonic cerebral cortex in a neurosphere assay (Kawakita et al., 2006, Katakura et al., 2009). Recently, Sakayori et al., using a neurosphere assay, showed that ARA and DHA have different effects on maintenance and differentiation of NSPCs (Sakayori et al., 2011). However, the effects of ARA and DHA ingestion on the age-related decline in neurogenesis in normal-aged rats remain unclear, although ARA and DHA may have different roles in neurogenesis.
ARA and DHA are biosynthesized from linoleic acid (LA) and α-linolenic acid (ALA), respectively, in the body (Fig. 1). The conversion reactions from LA and ALA are catalyzed by the same enzymes common to the n-6 and n-3 series, and they compete with each other. The n-6/n-3 ratio is an important factor for brain functions because this ratio in the diet affects cognitive functions in humans and other vertebrates (Loef and Walach, 2013). Therefore, to properly evaluate the effects of ARA and DHA on hippocampal neurogenesis, the effects of variation in the n-6/n-3 ratio should be excluded from the experimental conditions as much as possible.
The purpose of this study was to evaluate the effects of ingestion of ARA and/or DHA on age-related decreases in NSPCs or NNs in rats. We used diets containing ARA and/or DHA with the same n-6/n-3 ratio. We observed age-related decreases in ARA and DHA in the hippocampus and in the number of hippocampal NSPCs, and these phenomena were ameliorated by ARA and/or DHA ingestion.
Section snippets
Animals
Male Fisher 344 rats, aged 7 weeks, were obtained from Oriental Bioservice Inc. (Kyoto, Japan). For 1 week prior to experimentation, the animals were fed a commercial diet (CRF-1, Oriental Yeast Co. Ltd., Tokyo, Japan) and tap water ad libitum and were housed at 25 ± 1 °C and 60 ± 5% humidity under a 12-h light-dark cycle. Four (at 2 and 6 months) and 4–6 rats (at 18 months) in each group were examined. Experiments were approved by the Animal Care and Use Committee of Suntory Holdings Limited
ARA and DHA in hippocampal and red blood cell (RBC) PLs
We found no significant differences in body weight changes during the experimental period in the groups of rats we studied. To assess age-related changes in the hippocampus, we analyzed the fatty acid composition of hippocampal PLs at 3 and 19 months of age (Table S1). In the control group, the ARA content slightly decreased from 13.5% at 3 months to 13.2% at 19 months (Fig. 2A). At 19 months, the ARA content was decreased in the group ingesting DHA, but remained similar to the 3-month control
Discussion
ARA and DHA are important components of brain PLs and an age-related decrease in these fatty acids has been reported in the human or rodent hippocampus (Söderberg et al., 1991, McGahon et al., 1997, McGahon et al., 1999). Results shown here are basically consistent with those of previous studies, in which the concentrations of ARA and DHA in hippocampal PLs of 22-month-old rats are decreased and then improved by the ingestion of ARA and DHA for 8 weeks (McGahon et al., 1997, McGahon et al., 1999
Author contributions
Conceived and designed the experiments: HT MK HK YK NO. Performed the experiments: HT. Analyzed the data: HT MK HK. Contributed reagents/materials/analysis tools: HT MK HK YK HS. Wrote the paper: HT HK NO.
Conflict of interest statement
Authors HT, MK, HK, YK and HS are employees of Suntory Wellness Ltd. This does not alter the authors’ adherence to all the Neuroscience Research policies on sharing data and materials.
Acknowledgements
We thank Drs. Motoko Maekawa and Miho Matsumata (formerly in Tohoku University Graduate School of Medicine, Japan) and Dr. Nobuyuki Sakayori (a postdoc in Tohoku University Graduate School of Medicine) for the valuable advice on the experimental methods of immunostaining and constructive discussion. We are also grateful to Drs. Honoo Satake and Yoshiko Murata (Suntory Foundation for Life Sciences Bioorganic Research Institute, Japan) for their rewarding advice and support on the microscopic
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