Rats fed the high-sucrose diet (60% sucrose) for 8 weeks did not show obesity or increased food intake, but developed hypertriglyceridemia and insulin resistance, two components of the metabolic syndrome, as reported [50–52]. At 8 weeks, some brain enzymatic markers of AA and DHA metabolism were increased significantly in the high sucrose compared with control diet group (e.g., protein level of phospho-cPLA2, activities of cPLA2 and iPLA2) as was the esterified AA concentration in EtnGpl and unesterified AA concentration. BDNF mRNA and protein and drebrin mRNA were reduced, but synaptophysin mRNA and protein were not altered. Feeding the high-sucrose diet for 8 weeks did not change PGE2, TXB2 or LTB4 concentration significantly. Because the 8-week high-sucrose feeding paradigm represents early-stage metabolic syndrome in the absence of pathological diabetes or obesity [50, 51], these findings demonstrate changes in brain PUFA metabolizing enzymes and composition in association with reduced BDNF and drebrin mRNA at an early disease stage.
The upregulation of brain cPLA2 and iPLA2 enzyme activities (Figure 3 and 4) in the high-sucrose fed rats suggests an increase in brain AA and DHA metabolism. In this regard, disturbed saturated brain fatty acid metabolism has been reported in humans and rats with the metabolic syndrome [60, 61]. A positron emission tomography study demonstrated increased brain uptake of 11C]palmitate and 18F]fluoro-6-thia-heptadecanoic acid in patients with the metabolic syndrome . Hypothalamic concentrations of long-chain saturated acyl-CoAs were increased in a high-fat diet animal model of the metabolic syndrome, also indicating increased metabolism of long-chain saturated fatty acids . Taken together, the results suggest non-specific upregulation in brain fatty acid metabolism, including PUFAs, associated with the metabolic syndrome. Upregulated AA or DHA metabolism could be directly confirmed in this animal model, using quantitative autoradiography to image fatty acid uptake following radiotracer injection, or can be examined in humans using positron emission tomography [31, 38, 62].
The released fatty acids may be alternative energy substrates to glucose for brain metabolism, due to cerebral hypoglycemia caused by insulin-resistance. This is consistent with evidence of increased brain activity of carnitine palmitoyltransferase (which regulates fatty acid entry from the acyl-CoA pool into mitochondria for later β-oxidation) in an animal model of the metabolic syndrome . 14C-palmitate conversion to 14C-CO2 also was increased in mitochondrial brain extracts of diabetic (db/db) mice .
Brain cPLA2 activity and phospho-cPLA2 protein, a marker of activated cPLA2[57, 58], were increased in the high-sucrose fed rats in the absence of changes in cPLA2 mRNA or protein, suggesting post-translational modification and upregulated brain AA metabolism, consistent with the increased unesterified AA concentration (Figure 7). Increased activation of cPLA2 may reflect excitotoxicity associated with increased influx of extracellular calcium into the cell via ionotropic glutamatergic receptors . Since cPLA2 also is functionally coupled via G-proteins to dopaminergic, serotonergic and muscarinic neuroreceptors , an increase in its activity suggests disturbed G-protein neuroreceptor signaling in the metabolic syndrome . Cytokine receptor activation may also initiate cPLA2 activation , although our findings do not suggest an increase in cytokine expression in rats fed the high-sucrose diet.
iPLA2 is insensitive to extracellular calcium influx into the neuron [24, 63], but can be activated by intracellular calcium (at mM concentrations) released from the endoplasmic reticulum by the calcium-releasing ryanodine receptor . Mobilization of intracellular calcium stores can be mediated by increased intracellular unesterified AA levels, which was reported to activate the ryanodine receptor in vitro. This is in agreement with the finding that the unesterified AA concentration was increased in the high-sucrose diet rats (Figure 7). Likely, this increase in AA concentration occurred intracellularly, since sPLA2, which releases AA extracellularly, was not changed significantly (Figure 4).
Concentrations of pro-inflammatory eicosanoids (PGE2, TXB2 and LTB4) did not differ between the groups (Figure 5). It is possible, however, that changes in eicosanoids or cytokines  occurred in specific brain regions such as the hippocampus, as reported in genetically diabetic mice, or that longer administration of the high-sucrose diet sufficient to initiate diabetes would increase whole brain cytokine levels . However, consistent with the lack of significant changes in the three eicosanoids, we did not find significant changes in mRNA levels for COX-1, COX-2, 5- or 15-LOX in the high-sucrose fed rats, nor in TNF-α or GFAP mRNA, suggesting the absence of neuroinflammation, since transcription of these molecular markers occurs within transcriptional circuits related to neuroinflammation [67–69].
Whole brain BDNF mRNA and protein levels were reduced in the high-sucrose group (Figure 6), in agreement with previous studies that showed reduced BDNF levels in animal models of the metabolic syndrome with behavioral impairment [10, 41, 42]. Reduced BDNF expression was not mediated by pro-inflammatory eicosanoids, which were not changed. One possibility is that the increased unesterified AA concentration in the high-sucrose animals decreased BDNF and induced apoptosis, as reported in cultured spinal cord neurons . Reduced BDNF expression in the sucrose-fed rats may have promoted dendritic injury, which was indirectly suggested by the reduction in drebrin mRNA (Figure 6), or have altered the cellular dynamics and structural organization of dendritic spines in the absence of changes in drebrin protein. Changes in dendritic morphology and dynamics could be the topic of future studies. Additionally, more severe changes in synaptic structure are likely to occur with prolonged exposure to the high-sucrose diet, since the 8-week feeding paradigm causes only early-stage metabolic syndrome without obesity, diabetes or liver damage [41, 44, 52].
Contrary to reports using other models involving central insulin resistance [12, 34, 42], we did not find evidence of phospholipid degradation in the brain, since phospholipid mass, derived by the summation of total fatty acids within each phospholipid class, did not differ between the dietary groups (Table 1). Also, lysoPC, a marker of phospholipid breakdown, was not changed (Table 1). The changes in phospholipid fatty acid concentrations were relatively minor, and were significant only for a few n-6 PUFAs in EtnGpl (AA and 22:4n-6) and in lysoPC (20:3n-6).