Extremely low frequency electromagnetic field exposure and restraint stress induce changes on the brain lipid profile of Wistar rats

Brain lipids have different roles such as structural, functional and metabolic. Brain lipids constitute about one-half of brain-tissue dry weight. The brain has fatty acids that are long-chain monocarboxylic acids, either saturated or unsaturated, and cholesterol; the most prevalent families of unsaturated fatty acids are n-3, n-6, and n-9 series and contains some unusual fatty acids, such as very long chain fatty acids [1]. However, fatty acids are associated to other compounds to form glycerophospholipids and sphingolipids. These lipids play important roles in the brain physiology, e.g. in signal transduction across membranes, formation of lipid rafts, and anchoring the cell membranes to the extracellular matrix; furthermore, lipids covalently coupled to proteins play a major role in anchoring marker proteins within membranes [2].

Cholesterol and phospholipids are the main constituents of biomembranes. Changes in membrane lipid content are usually associated to imbalance in lipid homeostasis. In central nervous system (CNS) the lipid imbalance could lead to functional alterations that could result in several pathologies [1] as Alzheimer, Huntington and Parkinson diseases [3].

Stress could be defined as a condition in which homeostasis is modified by several responses, physiological and adaptive, induced by stressing factors. Hypothalamic–pituitary–adrenal (HPA) axis and the autonomic nervous system enhance the stress hormones in plasma, glucocorticoids and catecholamines, respectively [4, 5]. Stress exposure also induces deep changes in brain functions. Chronic stress has been related to increase in oxidative parameters for instance, increase in protein and lipid peroxidation, increased activity of antioxidant enzymes in the cortex, hippocampus and cerebellum [6]. However, the stress effects on brain lipids have not been extensively characterized.

Nowadays, the presence of electromagnetic fields in daily life it has resulted in an increase concerns regarding to the potential adverse effects of exposure to non-ionizing radiation; particularly to extremely low frequency electromagnetic fields (ELF-EMF). The effects induced by ELF-EMF exposure on biological systems are unclear. However, some effects are reported in epidemiological studies due to the incidence of certain types of brain cancer, and mood disorders [7]. Recent studies have reported the possible oxidant effect of ELF-EMF in brain, through the actions of reactive oxygen species [8]. Some authors have considered ELF-EMF as a mild stressor, and the effects related to its exposure have been reviewed [9]. In a previous study, carried out in Wistar rats acutely exposed to ELF-EMF, we have shown that they increase serum non-esterified fatty acids (NEFAs) concentration at 24 h post exposure [10] and impairs the antioxidant status of rat brain [11]. There are reports that show changes on the levels of Thiobarbituric Acid Reactive Substances (TBARS) of rats exposed to ELF-EMF in the cerebral cortex [12] or in various brain areas [13]. However, the effects of ELF-EMF on brain lipids and their metabolism have not been elucidated extensively. The aim of the present study was to evaluate the effects of chronic extremely low frequency electromagnetic field (ELF-EMF) exposure, restraint stress (RS), and both (RS + ELF-EMF) on lipid profile and lipoperoxidation status in cortex, cerebellum, and subcortical structures of Wistar rat brain.

The magnetic flux density used in this work was half the limit recommended for occupational exposures to 50/60 Hz magnetic fields, which is 5 mT for short term exposure (maximum exposure duration is 2 h per workday) [19]. Our results demonstrate that ELF-EMF exposure induces a significant increase in plasma corticosterone levels, indicating a chronic stress status similar to induced by RS. Although major stress was found in RS + ELF + EMF group, there isn’t statistical difference between ELF + EMF and RS + ELF + EMF treatments. Nevertheless, this finding suggests a possible interaction, synergic or additive, that deserves further studies.

The RS model can induce deep changes in rat physiology; Hennebelle found an increment of 30-times basal corticosterone concentration when RS was applied 6 h/day for 21 days [20]. Buynitsky and Mostofsky [21] reported that a daily 3-week period of RS is used in rodents like a chronic stress physiological model, with an increase of corticosterone levels and metabolic perturbations that affects the function of the nervous system. In the present study, the three times increased corticosterone concentration on the exposure protocol supports the statement that RS and ELF-EMF alone, can act as a mild stress condition. These results are in accordance with previous studies, showing that plasma corticosterone levels are one of the most important indicators of stress [22, 23]. Some reports suggest that long-term ELF-EMF exposure may elevate the plasma corticosterone levels in rodents [23]. Taken together, RS + ELF-EMF may count as the addition of two stress situations, whose effects can be added. In the present study, an increase on corticosterone levels was found in RS, ELF-EMF, and RS + ELF-EMF groups, supporting the proposal that ELF-EMF exposure is like a mild stressor.

Lipids play a critical role in structure and function of the nervous system; glucocorticoids may alter lipid metabolism in brain and other tissues; as reported previously, where the administration of glucocorticoids can induce a shift in arachidonic acid metabolism in brain [24]. In the present study, we found changes on TC (increase), and POL (decrease) in the brain cortex of rats exposed to RS + ELF-EMF and ELF-EMF groups, and in subcortical structures in RS + ELF-EMF group, this finding corroborates previous reports in which movement/synthesis of cholesterol is brain are area-dependent. Segatto, found a differential activity pattern of 3-hydroxy-3-methylglutaryl coenzyme A reductase in different brain regions with the highest activity in brain cortex and the lowest activity in brainstem [25]. Cholesterol is essential for membrane structure and stability, it decreases during stress and depressive-like behaviour in rats [26], and in neuronal diseases in human beings [27, 28]. Oliveira found that free cholesterol was the most abundant of the lipids in all brain regions analyzed, showing higher levels in cerebellum [29]. Results of the present work confirm the observation of highest cholesterol levels in cerebellum, and extend the finding of resistance to mild stressor conditions at this brain area.

Increased cholesterol levels found in cortex and subcortical structures in response to ELF-EMF exposure or RS + ELF-EMF could be an adaptive response for cellular protection, as observed in cerebellum. On the contrary, reduced cholesterol levels could affect the animal behaviour [26], suggesting a decrease in brain function. The increasing CT observed in the present study suggest that brain cortex CT turnover increases in response to ELF-EMF exposure, as seen in neuroinflammation and in other pathologic conditions, according to different reports [27, 28, 30]. On the other hand, we found a decrease in POL content of the cortex in the groups exposed to ELF-EMF, and RS + ELF-EMF groups, this finding is in accordance with previous reports in which some noxious conditions as maternal deprivation, RS, and ELF-EMF stimulation have effects on phospholipids and phospholipid-dependent pathways [20, 31].

According to our results, Oliveira [29] found POL changes in cortex and cerebellum after deep chronic stress, sphingolipid and phospholipid metabolism were deeply affected, showing a decrease in: phosphatidylethanolamine, ether phosphatidylcholine, and an increase in lysophosphatidylethanolamine levels similar to Lee et al. [32].

It has been observed that stress modifies the profile of mainly POL and fatty acid in different brain regions; in our results, we found a decrease of POL in cortex and in subcortical structures of ELF-EMF and RS + ELF-EMF groups, suggesting an association of this effect to ELF-EMF stimulation.

Fatty acids are usually bound to complex lipids in the cell membrane, and many stimulus can induce the breakdown of these complex lipids, which can be converted to signalling molecules, second messengers, and other molecules involved in neuronal metabolism and survival [33]. In the present study, we found an increase of NEFAs (as breakdown index for fatty acid released from complex lipids) in subcortical structures of rats exposed to RS + ELF-EMF, but not in other brain regions studied. We suggest that this increase in NEFAs could be through phospholipase activation by ELF-EMF, as described by Piacentini et al. [34]; nevertheless, the regional effects in NEFAs composition remains unclear [31, 35].

In further analysis, we found changes in total FAMEs composition in cortex, cerebellum and subcortical structures. According to our chromatographic method, eicosatetraenoic, docosahexaenoic, docosatetranoic, octadecanoic, and eicosenoic acids were found with major abundance in the tissues analyzed. Under physiologic conditions, the balance of membrane lipid metabolism, particularly of arachidonic and docosahexaenoic acids, lead a very small and tightly controlled cellular pool of free arachidonic acid, but their levels increase very quickly upon cell activation, cerebral ischemia, seizures or other types of brain stress [30, 35, 36]. However, in the present study a decrease in the relative abundance of FAMEs was found, especially in brain cortex in RS group. This effect could be due to membrane-bound fatty acids transformation into other metabolites as suggested by Malcher-Lopes et al. [24]; similar findings were observed in the cerebellum, with a decrease in eicosatetraenoic and docosahexaenoic acids in rats exposed to ELF-EMF, but not in the RS group, this may suggest that an ELF-EMF-mediated mechanism is involved in metabolism of these lipids, this mechanism could be through phospholipase activation as suggested by some authors [30, 37, 38]. We also observed a differential effect on the subcortical structures; RS induces a decrease in eicosatetraenoic and docosahexaenoic acids, while ELF-EMF stimulation induces an increase in these lipids. These findings agree with those reported by Clejan et al. [39], who found a differential effect of ELF-EMF on phospholipases and their patterns in second messengers in hematopoietic cell lines.

The cellular effects of extremely low frequency electromagnetic fields remain unknown, but several hypothesis about the mechanism of action have been proposed; one is the lifetime extension of free radicals, and radical-mediated damages on macromolecules [8, 40]. Some authors have hypothesized that ELF-EMF can act on living organisms in a similar way to other stressors, like heat and RS, by inducing the neuroendocrine stress response [41, 42]. According to that proposal, the findings that cortex and cerebellum of experimental groups showed higher lipoperoxide levels than control group, but without differences in subcortical structures, could be explained because these areas were the closest and most exposed to ELF-EMF, so producing free radicals that induce lipid damage and increase of saturated-/unsaturated-fatty acids ratio. In a previous report we observed that acute exposure to EMF induces reduction in catalase and superoxide dismutase activities, without changes in lipoperoxidation [11]. In the present study, lipid damage could be by induction of oxidative imbalance due to chronic exposure, suggesting that chronic ELF-EMF exposure could be like a mild-stressor; this finding is supported by the increase levels in plasma corticosterone concentration and brain lipid peroxidation. Changes in lipid composition, could have deep effects on membrane function by affecting membrane-associated enzymes, receptors and ion channels [43]. In the present study, different effects of RS and ELF-EMF were found in different brain regions, we speculate that these effects may be mediated by specific mechanisms, like phospholipase [30] activation by ELF-EMF and genomic and non-genomic effects of glucocorticoids, but these observations deserve further research as has been suggested previously in clinical trials [44].

Changes found in the present study are in accordance with previous reports indicating the effects of chronic stress on brain lipid metabolism and suggest that the actions of extremely low frequency electromagnetic fields are similar to physiological stress. The effects found in different brain regions indicate that the extremely low frequency electromagnetic fields could be distance-dependent from the source of exposure due to findings in the analyzed brain regions. The effects in EMF versus RS were found differences on total cholesterol and polar lipids in cortex. In addition, in subcortical zone was found differences on total lipids. Although cerebellum’s lipid peroxidation induced by any stressor was similar to the found in cortex, minor changes in lipid profile were found in cerebellum and subcortical structures were more susceptible to increase NEFAs content in response to RS + ELF-EMF exposure.