Differential effect of age on the brain fatty acid levels and their correlation with animal cognitive status in mice

Abstract

The aim of the present study was to investigate the possible relationship between the levels of various fatty acids (FA) in the brain and learning indices in aged (22–23 months old) and young (2–3 months old) female Swiss Webster (SW) mice. The mice were classified as “good” or “poor” learners based on their performance in a spatial learning task: the Morris Water Maze. The levels of several FA including palmitic, stearic, oleic, linoleic, arachidonic (AA), and docosahexaenoic acid (DHA), were measured by gas chromatography in tissue samples from four different brain areas: hippocampus, frontal cortex, striatum and hypothalamus. The results of behavioral tests confirmed a decline in learning skills with age. However, a great individual variation was revealed in learning scores between aged subjects, indicating that biological aging does not always parallel chronological aging. The relative levels of particular fatty acids across the four examined brain structures were very similar. Interestingly, only in the hypothalamus was the DHA omega-3 acid level significantly higher in young mice compared to the old mice. For the remaining brain structures, no significant correlations were found between the DHA level and the animal's age and/or cognitive status. A significant correlation between learning performance and fatty acid levels in the brain was found only for AA in the young mice hippocampus, a structure known to be critical for spatial learning and memory. The AA level was significantly lower in young “good” learners compared to both young “poor” and old “good” learners with young “good” learners showing significantly better performance than the two other groups. These findings contribute to the current debate on the value of DHA supplementation as an effective protective treatment against senile dementia and the potential role of AA in memory deficits.

Introduction

Biochemical, biophysical, and pharmacological studies indicate an important role of fatty acids in the brain in neural functions. As a major constituent of cell membranes, fatty acids regulate membrane fluidity (Aricha et al., 2004, Yang et al., 2011) and through interactions with membrane proteins, have an impact on the membrane's functional properties such as conductance of ion channels (Lundbaek, 2008), clustering or dispersal of membrane proteins (Chapkin et al., 2008), receptors sensitivity to signaling molecules and activity of G-proteins and other enzymes involved in the intracellular signal transduction pathways (Bruno et al., 2007, Simons and Toomre, 2000, Stillwell et al., 2005). They are also metabolized into intracellular signaling molecules such as prostaglandins, leukotrienes, thromboxanes, and platelet activating factor (Shimizu, 2009). It was demonstrated that some of the free fatty acids, both saturated and polyunsaturated, such as stearic and arachidonic acid, show neuroprotective effects against oxidative stress. Their antioxidant effects were postulated to be mediated by the activation of perioxisomal proliferator-activated receptor-ɣ (PPAR-ɣ) and the synthesis of new proteins (Wang et al., 2006, Wang et al., 2007). Oleic acid was shown to be a major constituent of myelin (Garbay et al., 2000). Long chain polyunsaturated fatty acids (LC-PUFAs), especially the omega-6 linoleic acid (LA) and its derivative arachidonic acid (AA) as well as omega-3 acids (.-linolenic (ALA), eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids) for long have been recommended as dietary supplements preventing or ameliorating symptoms of various human diseases including neurodegenerative diseases. The omega-6 AA and omega-3 DHA were reported to carry many diverse functions in the brain (Crawford and Sinclair, 1971). AA itself is known as a retrograde neurotransmitter regulating signaling enzymes, such as phospholipase C (PLC) and protein kinase C (PKC) (McGahon and Lynch, 1998, Shearman et al., 1991, Shinomura et al., 1991). AA and LA were shown to activate syntaxin-3 (STX-3), a protein involved in the growth and repair of neurons, and thus facilitate growth cone membrane expansion and neurite outgrowth (Darios and Davletov, 2006). It has also been reported that in neural stem cells culture, DHA facilitated neurogenesis by promoting cell cycle exit, suppressing cell death, and mediating differentiation of neural stem cells into neurons (Kawakita et al., 2006). Moreover, the same authors demonstrated an increase in the number of new born dentate granular cells in adult rats receiving dietary DHA supplementation in in vivo experiments. This is important because neurogenesis in the dentate gyrus, a critical area considered to be the “gateway” of information to the hippocampus, has been positively associated with an animals' learning ability by some authors (Bruel-Jungerman et al., 2007, Deng et al., 2010, Drapeaut et al., 2003, Koehl and Abrous, 2011). Oral DHA supplementation was also shown to elevate phospholipids and synaptic proteins such as synapsin-1, syntaxin-3, the postsynaptic protein PSD-95, F-actin, and the mGluR1 receptors (Wurtman et al., 2006, Wurtman et al., 2009), and to increase the number of dendritic spines in the adult gerbil hippocampus (Sakamoto et al., 2007). In line with these findings are reports showing that low levels of circulating or brain DHA correlate with cognitive decline in neurocognitive disorders such as Alzheimer's disease (AD), while the DHA supplementation contributes to amelioration of cognitive deficits in AD patients (Prasad et al., 1998, Soderberg et al., 1991). Data from experimental animal studies suggest that dietary deficiency in PUFA such as DHA, increases the risk of developing cognitive impairments that appears to be the result of changes in the composition of fatty acid in different brain regions (Lim and Suzuki, 2000, Lim and Suzuki, 2002, Skinner et al., 1993). Consistent with these reports, a reduction of amyloid plagues and tau tangles was observed after dietary DHA supplementation in the mouse model of AD (Issa et al., 2006, Lukiw et al., 2005, Yurko-Mauro et al., 2010). Along with these findings it has been postulated that dietary deficiency in LC-PUFAs during gestation and/or the early postnatal period can have adverse effects on brain development and results in learning and memory deficits (De Souza et al., 2011). Long-term maintenance on an omega-3 deficient diet was demonstrated to cause DHA depletion in the brain and impairments in olfactory stimulus discrimination and place learning as assessed by the Morris water maze task. This was true, even in the second generation of adult male rats (Geiner et al., 1999, Xiao et al., 2006). On the other hand, a compensatory DHA-rich diet applied over a 10-week period to either young adult or aged rats, showed a beneficial effect on the reference (but not working) spatial memory as assessed by animals' performance in a partially baited eight-arm radial maze (Gamoh et al., 1999, Gamoh et al., 2001). Interestingly, in young rats, improved performance in the spatial memory task correlated with a higher DHA/AA ratio in both the hippocampus and the cerebral cortex, while in aged animals it correlated with a lower level of lipid peroxide in the hippocampus (Green et al., 2007, Tanabe et al., 2004). Tanabe et al. (2004) linked the improved performance of rats in the partially baited eight-arm radial maze observed after chronic DHA supplementation to increased Fos expression in the CA1 hippocampus. Enhanced maze-learning was also reported for young and old mice exposed to DHA and/or an ethyl ester and egg-phosphatidylcholine-rich diet (Lim and Suzuki, 2000). Some dietary and population studies in humans also suggest a beneficial effect of LC-PUFA supplementation on cognitive functions during normal aging and in dementia conditions (Issa et al., 2006). The beneficial effect of elevated plasma and brain DHA levels on cognitive processes in AD was associated by some authors with the formation of DHA-derived 10,17S-docosatriene (neuroprotectin D1) which was shown to suppress ß amyloid-induced neurotoxicity in some brain regions including the hippocampal CA1 area (Lukiw et al., 2005), and with a decrease in steady-state levels of presenilin 1 (Green et al., 2007). AA supplementation was also reported to ameliorate cognitive dysfunction caused by either organic brain damage or age but not AD (Kotani et al., 2006).

Most experimental and clinical studies published to date have investigated the effects of either dietary restrictions or dietary supplementations on the cognitive status of the subjects. Therefore, to further elucidate the potential relationship between fatty acid levels in the brain and individual cognitive skills, we examined the content of brain fatty acids in young and aged animals classified as “good” or “poor” learners based on their performance in a spatial learning task, the Morris water maze.