Neuropathological Responses to Chronic NMDA in Rats Are Worsened by Dietary n 3 PUFA Deprivation but Are Not Ameliorated by Fish Oil Supplementation

Background  Dietary long chain n3 polyunsaturated fatty acid (PUFA) supplementation may be beneficial for chronic brain illnesses, but the issue is not agreed on. We examined effects of dietary n3 PUFA deprivation or supplementation, compared with an n3 PUFA adequate diet (containing alpha linolenic acid but not docosahexaenoic acid), on brain markers of lipid metabolism and excitotoxicity, in rats treated chronically with NMDA or saline. Methods  Male rats after weaning were maintained on one of three diets for 15 weeks. After 12 weeks, each diet group was injected i.p. daily with saline (1 ml per kg) or a subconvulsive dose of NMDA (25 mg per kg) for 3 additional weeks. Then, brain fatty acid concentrations and various markers of excitotoxicity and fatty acid metabolism were measured. Results  Compared to the dietadequate group, brain DHA concentration was reduced, while n  6 docosapentaenoic acid concentration was increased in the n  3 deficient group; arachidonic acid concentration was unchanged. These concentrations were unaffected by fish oil supplementation. Chronic NMDA increased brain cPLA2 activity in each of the three groups, but n  3 PUFA deprivation or fish oil did not change cPLA2 activity or protein compared with the adequate group. sPLA2 expression was unchanged in the three conditions, whereas iPLA2 expression was reduced by deprivation but not changed by supplementation. BDNF protein was reduced by NMDA in N  3 PUFA deficient rats, but protein levels of IL  1b, NGF, and GFAP did not differ between groups. Conclusions  N  3 PUFA deprivation significantly worsened several pathological NMDA induced changes produced in diet adequate rats, whereas n  3 PUFA supplementation did not affect NMDA induced changes. Supplementation may not be critical for this measured neuropathology once the diet has an adequate n  3 PUFA content. (Authors abstract)

The central nervous system is highly enriched in the polyunsaturated fatty acids (PUFAs) arachidonic acid and docosahexaenoic acid . AA and DHA cannot be synthesized de novo by vertebrates, but must be obtained from the diet or by hepatic elongation and desaturation of their dietary essential precursors, linoleic acid and ALA, respectively. North American diets tend to contain low levels of DHA and high levels of n  6 PUFAs derived primarily from LA. Some epidemiological studies suggest that reduced dietary n  3 PUFA content is associated with an increased risk of neuropsychiatric and neurodegenerative disorders , in which excitotoxicity is present .Effects of n  3 PUFA deprivation and supplementation on brain markers during physiological or pharmacological insults are uncertain. Several studies reported that dietary n  3 PUFA deficiency affected brain glutamatergic, dopaminergic and serotonergic systems. Hyperglutamatergic activity is reported in neurodegenerative and neuropsychiatric illnesses such as Alzheimer disease (AD), Parkinson disease and bipolar disorder. Glutamate is the main excitatory neurotransmitter in brain and is involved in learning and memory, brain development and aging, and excitotoxicity. This study was designed to understand whether dietary n  3 PUFA deficiency or supplementation could modify chronic NMDA  induced changes in rat brain AA signaling, neuroinflammatory markers and trophic factors. To do this, we maintained rats on an n3 PUFA deprived, adequate or supplemented (by fish oil) diet for 15 weeks after weaning. After the initial 12 weeks, each of the three groups of animals was administered saline or a subconvulsive dose of NMDA (25 mg per kg, i.p.) daily for 3 weeks. The rats were sacrificed at 15 weeks and their brains were used to measure lipid concentrations, and molecular markers of lipid metabolism, neuroinflammation, BDNF and nerve growth factor (NGF).This study confirms that chronic NMDA administration compared with saline control produces a number of changes in brain lipid metabolism, markers of neuro inflammation and growth factors in n  3 PUFA diet adequate rats. Dietary n  3 PUFA deprivation below the adequate level exacerbated these changes, whereas supplementation did not dampen them. In n  3 PUFA adequate rats fed 4.6 per cent ALA but no DHA, NMDA compared with saline increased body weight, cPLA2 activity and cPLA2 IVA protein, but decreased iPLA2 protein and BDNF. Deprivation to an ALA content of 0.2 per cent of total fatty acid, significantly changed NMDA responses compared with saline treated group and increase cPLA2 activity and protein was observed. In rats supplemented with n  3 PUFA via fish oil, NMDA compared with saline also increased cPLA2 activity and cPLA2 IVA protein. Significant diet effects as noted by ANOVAs and post hoc comparisons with the adequate group occurred in DHA and DPAn  6 concentrations (DHA decreased and DPAn  6 increased with n  3 PUFA deprivation), body weight (decreased by supplementation), cPLA2 activity (increased by deprivation) and iPLA2 activity (decreased by deprivation). Thus, deprivation worsened or did not change pathological markers induced by chronic NMDA in rats fed the adequate diet, whereas supplementation did not affect these markers significantly but did reduce body weight. In agreement with our findings, dietary n  3 PUFA supplementation reduced weight gain and improved postprandial lipidemia and the associated inflammatory response in the obese JCR LA  cp rat, whereas dietary n  3 PUFA deprivation did not change body weight.

The findings may clarify whether dietary n  3 PUFA supplementation is necessary in human subjects. The brain DHA concentration did not significantly differ between adequate vs. fish oil supplemented diets and it seems that slight reductions in circulating DHA in humans would not necessarily have pathophysiological consequences. Supporting this suggestion, a 33 per cent  lesser blood DHA concentration in vegetarians than in omnivores  was not associated with a significant difference in mood, or in general mortality or mortality from any cause. Moreover, 4 month dietary DHA supplementation in preschool children, which increased blood DHA from 1.0 per cent  to 3.2 per cent  of total fatty acids, did not enhance scores on any of four cognitive tests. 
In conclusion, dietary deprivation of DHA precursor ALA in the absence of dietary DHA reduced brain DHA and exacerbated NMDA induced AA and inflammatory markers in rat brain. (Editors comments)