Arachidonic acid-containing phosphatidylcholine species are increased in selected brain regions of a depressive animal model: Implications for pathophysiology

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Abstract

The Flinders Sensitive Line (FSL) rat is a genetic animal model of depression. Following recent findings that the brain fatty acid composition of FSL is characterised by increased arachidonic acid (AA), we used electrospray tandem mass spectrometry and 1H-NMR to examine lipid species in different brain areas. Cholesterol and sphingolipids were increased in the hypothalamus of the FSL rats. Furthermore, arachidonic acid-containing phosphatidylcholine (AA-PC) species were elevated with PC16:0/20:4, PC18:1/20:4 and PC18:0/20:4 (p<0.003) increased in the hypothalamus and striatum. In contrast, there was a decrease in some docosahexaenoic acid (DHA)-containing species, specifically PC18:1/22:6 (p<0.003) in the striatum and PE18:1/22:6 (p<0.004) in the prefrontal cortex. Since no significant differences were observed in the erythrocyte fatty acid concentrations, dietary or environmental causes for these observations are unlikely. The increase in AA-PC species which in this animal model may be associated with altered neuropathy target esterase activity, an enzyme involved in membrane PC homeostasis, may contribute to the depressive phenotype of the FSL rats.

Introduction

The association of depression and essential polyunsaturated fatty acids (PUFA), especially n-3 PUFA, has been variably inferred from epidemiological studies [1], examination of blood tissue in clinical studies [2] and indirectly, by the effect of n-3 PUFA supplementation in clinical trials [3], [4], [5].

Utilizing a unique animal model of depression, the Flinders Sensitive Line (FSL) rat, we showed for the first time that the brain fatty acid composition of the depressive animals differed from that of the Sprague-Dawley controls. Surprisingly, the observed difference was not in the n-3 PUFA fraction, but rather a significantly elevated concentration of arachidonic acid (AA), an n-6 PUFA, was observed in the depressive FSL strain [6]. AA is intimately involved in brain function and altered AA levels may contribute to the pathophysiology of neuropsychiatric disorders through the formation of eicosanoids and endocannabinoids, regulation of neurotransmitter levels, modulation of signal transduction and gene expression [7], [8], [9]. Since the beneficial effect of n-3PUFA treatments may be attributed, in part, to reductions of AA-derived mediators, this finding offers a model for the study of those therapies but, at the same time, raises questions regarding the pathogenesis of depression in the FSL strain and, possibly, depression in general.

The FSL rats were developed by selective breeding of Sprague-Dawley rats for increased sensitivity to the hypothermic effect of the anticholinesterase agent diisopropyl fluorophosphate (DFP) [10], [11]. These rats exhibit behavioral features characteristic of depression, such as reduced locomotion, reduced activity in swim test, increased anhedonia in response to chronic mild stress, increased amount and reduced onset of rapid eye movement sleep, cognitive difficulties and reduced body weight; moreover, these behavioral abnormalities have been normalized by chronic treatment with a number of well-recognized antidepressant drugs [12]. The FSL rats are also more sensitive to directly acting muscarinic and nicotinic agonists compared to their controls, but the mechanisms underlying this hyperresponsiveness and the second messengers involved have not been elucidated [13]. DFP is an organophosphate causing delayed neuropathy, initially shown to act through the phosphorylation of an esterase [14]. Recent reports have shown that this esterase, termed neuropathy target esterase (NTE) (EC 3.1.5), is intimately involved in normal brain development and membrane phosphatidylcholine (PC) homeostasis [15], [16], [17], [18]. Disturbed NTE activity results in altered membrane metabolism, with varying manifestations in different species [17], especially evident in the distal parts of long nerves.

Since establishment almost three decades ago [10] the study and use of FSL rats have offered valuable insights into the neurobiology of depression [19], [20], [21]. Further to our recent findings that this model is characterised by increased AA concentration in several areas of the brain, we have now followed a lipidomic approach and conducted a detailed study of lipids species in the brains in FSL and Sprague-Dawley control rats in order to further our understanding of the involvement of lipids in this animal model of depression.

Section snippets

Materials

l-α-Phosphatidylcholine, dipalmitoyl (PC), l-α-phosphatidylethanolamine, dipalmitoyl (PE), 1,2-diacyl-sn-glycero-3-phospho-d-myo-inositol (PI), 1-palmitoyl, l-α-phosphatidylserine, dipalmitoyl (PS), N-acyl-d-sphingosine-1-phosphocholine (SM), phosphate buffered saline (PBS) tablets, butylated hydroxytoluene (BHT) and anhydrous sodium sulphate were purchased from Sigma (Poole, Dorset, UK). HPLC-grade methanol and chloroform, ammonium hydroxide (10%) and absolute ethanol were purchased from

Lipids in the different areas of the brain

Profiling of lipids in the different areas of the brain was performed by high-field 1H-NMR. The results are summarised in Table 1. Overall, anatomically separate areas of the brain were found to have different proportions of lipids, as noted in both control and FSL animals. In detail, prefrontal cortex had the highest level of PC whilst striatum had the lowest, hypothalamus had proportionally more PE than any of the other three brain areas and prefrontal cortex had relatively low levels of

Discussion

The FSL rat is a unique animal model of depression that has been studied in terms of both understanding the pathogenesis of depression and assessing various therapies. We have recently reported that brain regions from the FSL animals displayed higher concentration of AA compared to control Spraque-Dawley rats [6]. Further detailed studies in anatomically different areas of the brain have now revealed that the observed elevated levels of AA are not random, but can be attributed to increased

Acknowledgements

This study was supported in part by the Institute for Psychobiology, Jerusalem, Israel. The authors are grateful to Prof. Aron Weller of Bar-Ilan University, Israel, for fruitful discussions and helpful suggestions. The authors gratefully acknowledge the excellent technical support provided by Andrew Healey (Analytical Centre, University of Bradford, UK) and Aviva Kluska (FMRC, Tel Aviv University, Israel).

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