PALMITOYLETHANOLAMIDE, AN ENDOGENOUS PPAR-ALPHA AGONIST, MODULATES NEUROSTEROIDS DE NOVO SYNTHESIS

Abstract

Palmitoylethanolamide (PEA), a member of fatty-acid ethanolamide family, is an endogenous neuromodulator with a broad spectrum of pharmacological properties, including analgesic (Calignano et al., 1998, 2001), anti-inflammatory (Costa et al., 2002; D’Agostino et al., 2007), anticonvulsant (Lambert et al., 2001) and antiproliferative (Di Marzo et al., 2001) effects. Less investigated are the functions of PEA in the central nervous system (CNS), where PEA is present in detectable levels (Cadas et al., 1997), showing diurnal variation (Rodriguez et al., 2006). Despite its molecular target remain debate, PEA failed to exert its analgesic and anti-inflammatory properties in mice lacking peroxisome proliferator-activated receptor alpha (PPAR-α) (LoVerme et al., 2005; D’Agostino et al., 2007). Although PPAR-α is a well characterized transcription factor (Lemberger et al., 1996), it seems to be pivotal for other effects not strictly related to its transcriptional activity, such as the effect on calcium-activated K+ channels, mediating rapid analgesia (Lo Verme et al., 2006). Likewise PEA, PPAR-α has been localized in discrete areas of the CNS with a peculiar distribution (Kainu et al., 1994; Moreno et al., 2004; Benani et al., 2004), although its role in these areas is unclear and poorly defined. In the last ten years it has emerged that neurosteroids are strongly involved in several physiological cognitive and emotive functions of the CNS (Jung-Testas and Baulieu, 1998). Neurosteroids are known to exert several rapid effects, including modulation of hypnosis, through activation of GABAA receptors, these studies have been mainly conducted with the action of pregnenolone-like neuroactive steroid (Mendelson et al., 1987), and metyrapone, a blocker of the enzyme 11β-hydroxylase, which is essential for the biosynthesis of corticosteroids (Burade et al., 1996). Keller et al. (2004) have shown that 5α-reduced neurosteroids act on GABAA, as well as peripheral benzodiazepine receptor (PBR), modulating the GABA-induced Cl- currents that result in neuron hyperpolarization. Among 5α-reduced neuroactive steroids, the 3α-hydroxy-5α-pregnan-20-one (allopregnanolone, ALLO) displays anxiolytic, sedative, analgesic and anaesthetic properties (Rupprecht et al., 2001), causing a great pharmacological interest. The main pharmacological properties above reported are shared with those of PEA. Particularly, it was previously reported that N-acylethanolamines of unsaturated fatty acid (N-linoleoyl-, N-oleoyl-) and of saturated fatty acid (PEA), significantly prolonged pentobarbital-induced hypnosis in mice (Watanabe et al., 1999). Here, we hypothesize that exogenous administration of PEA, through a PPAR-α-dependent mechanism, participates in neurosteroid formation increasing their levels and leading to a positive modulation of GABAA receptor. For this aim, we study the effect of PEA in vitro using C6 glioma cell line and primary murine astrocytes, which, as other glial cells and neurons, have the enzymatic machinery for de novo synthesis of neurosteroids and in vivo using pentobarbital induced LORR duration and EEG recordings in mouse. In particular, Fast Fourier Transformer (FFT) power spectral analysis was applied to reveal the time change of slow wave activity in the EEG, which would provide some quantitative measures of LORR EEG. The demonstration of the capacity of a neural centre to synthesize neurosteroids requires the localization in that centre of active forms of key steroidogenic enzymes, and local increase of neurosteroids. Therefore, we evaluated the ALLO content and the expression of StAR and P450scc, two proteins implicated in the early step of neurosteroidogenesis in mitochondrial fraction, supernatant and brainstem. As well known, this area serves integrative functions, including, pain sensitivity control, alertness, and consciousness, and shows appreciable and functional levels of PEA (Petrosino et al., 2007; Melis et al., 2008), as well as of PPAR-α (Moreno et al., 2004; Melis et al., 2008).