Comment; Palmitoylethanolamide (PEA), a simple, inexpensive supplement available over the counter, without a prescription can have profound neuroprotective effects. I wonder what would happen if we added it to l-Serine to combat neurodegenerative protein misfolding?
- Institute of Physiological Chemistry, University Medical Center of the Johannes Gutenberg University of Mainz, Mainz, Germany
Research on the antiepileptic effects of (endo-)cannabinoids has remarkably progressed in the years following the discovery of fundamental role of the endocannabinoid (eCB) system in controlling neural excitability. Moreover, an increasing number of well-documented cases of epilepsy patients exhibiting multi-drug resistance report beneficial effects of cannabis use. Pre-clinical and clinical research has increasingly focused on the antiepileptic effectiveness of exogenous administration of cannabinoids and/or pharmacologically induced increase of eCBs such as anandamide (also known as arachidonoylethanolamide [AEA]). Concomitant research has uncovered the contribution of neuroinflammatory processes and peripheral immunity to the onset and progression of epilepsy. Accordingly, modulation of inflammatory pathways such as cyclooxygenase-2 (COX-2) was pursued as alternative therapeutic strategy for epilepsy. Palmitoylethanolamide (PEA) is an endogenous fatty acid amide related to the centrally and peripherally present eCB AEA, and is a naturally occurring nutrient that has long been recognized for its analgesic and anti-inflammatory properties. Neuroprotective and anti-hyperalgesic properties of PEA were evidenced in neurodegenerative diseases, and antiepileptic effects in pentylenetetrazol (PTZ), maximal electroshock (MES) and amygdaloid kindling models of epileptic seizures. Moreover, numerous clinical trials in chronic pain revealed that PEA treatment is devoid of addiction potential, dose limiting side effects and psychoactive effects, rendering PEA an appealing candidate as antiepileptic compound or adjuvant. In the present study, we aimed at assessing antiepileptic properties of PEA in a mouse model of acute epileptic seizures induced by systemic administration of kainic acid (KA). KA-induced epilepsy in rodents is assumed to resemble to different extents human temporal lobe epilepsy (TLE) depending on the route of KA administration; intracerebral (i.c.) injection was recently shown to most closely mimic human TLE, while systemic KA administration causes more widespread pathological damage, both in brain and periphery. To explore the potential of PEA to exert therapeutic effects both in brain and periphery, acute and subchronic administration of PEA by intraperitoneal (i.p.) injection was assessed on mice with systemically administered KA. Specifically, we investigated: (i) neuroprotective and anticonvulsant properties of acute and subchronic PEA treatment in KA-induced seizure models, and (ii) temporal dynamics of eCB and eicosanoid (eiC) levels in hippocampus and plasma over 180 min post seizure induction in PEA-treated and non-treated KA-injected mice vs. vehicle injected mice. Finally, we compared the systemic PEA treatment with, and in combination with, pharmacological blockade of fatty acid amide hydrolase (FAAH) in brain and periphery, in terms of anticonvulsant properties and modulation of eCBs and eiCs. Here, we demonstrate that subchronic administration of PEA significantly alleviates seizure intensity, promotes neuroprotection and induces modulation of the plasma and hippocampal eCB and eiC levels in systemic KA-injected mice.
Epilepsy is one of the most common neurological disorders worldwide with severe impact on the life quality of patients and often leading to long-term cognitive impairments (Xu et al., 2013). Although numerous antiepileptic drugs (AEDs) are currently available, they mainly target symptoms rather than underlying molecular mechanisms and often cause massive side effects substantially limiting their therapeutic use (Cully, 2014; Eisenstein, 2014; Savage, 2014). Moreover, a high variability in pharmaco-sensitivity among patients and high incidence of multi-drug resistance in patients pose tremendous challenges in therapy management and development of efficient antiepileptic therapies (Katona, 2015). Multiple causes of epileptogenesis such as head trauma, genetic and metabolic factors, and infections, combined with the diversity of epileptic manifestation and types, and yet unclarified mechanism of epileptogenesis challenge the development of effective antiepileptic therapies (Narain, 2014; Amini et al., 2015). In this context, identification of molecular causes of and/or correlates with epilepsy in various animal models is essential to discover new drug targets and markers for follow-up monitoring.
The intrinsic role of the endocannabinoid (eCB) system to control neuronal network excitability has shaped the focus of pharmacological approaches in epilepsy on cannabinoid-based therapies (Citraro et al., 2013; Monory et al., 2015). Cannabis has been long used as an effective antiepileptic compound. However, a large variability in patient response, psychoactive effects, possible long-term side effects in young patients, as well as the unpredictable risk of cannabinoid-receptor 1 (CB1) desensitization subsequently reducing antiepileptic effects or even aggravating seizures, remain main concerns for its clinical use (Lutz, 2004; Szaflarski and Bebin, 2014; Blair et al., 2015; Katona, 2015; Mechoulam, 2017).
Alternative approaches to modulate the hyperexcitability with seizure via activation of CB1 and its ligands, include the pharmacological blockade of enzymes involved in the degradation of endogenous neuroprotectants; anandamide (also known as arachidonoylethanolamide [AEA]), palmitoylethanolamide (PEA), and oleoylethanolamide (OEA). Several inhibitors of the fatty acid amide hydrolase (FAAH) which degrades AEA, PEA and OEA were developed and assessed for their anticonvulsant properties (Vilela et al., 2013, 2014; Mikheeva et al., 2017). Although effective anticonvulsant properties were evidenced for many FAAH inhibitors, their biphasic effect has to be carefully considered to prevent pro-convulsant effects (Di Marzo et al., 1994). This is likely the source of inconsistencies among various reports on the use of FAAH inhibitors (Vilela et al., 2013; Rivera et al., 2015). Exogenous administration of AEA has similar biphasic effects, in a dose-dependent manner, in epileptic seizures.
PEA, the fatty acid amide analog of AEA, has become the focus of increasing attention due to its long recognized anti-inflammatory and neuroprotective properties (Lambert et al., 2001; Conti et al., 2002; Mattace Raso et al., 2014). It has been also shown to exert antiepileptic effects in three epilepsy models (Lambert et al., 2001; Sheerin et al., 2004). The mechanism of action of PEA in brain and periphery is still not clarified (Iannotti et al., 2016). Although initially believed that PEA is a cannabinoid CB2 receptor agonist, consensus has emerged that PEA acts through activation of peroxisome proliferator-activated receptor α (PPARα), which is a ubiquitous transcription factor in the periphery (LoVerme et al., 2006; Hansen, 2010). PPARs regulate gene networks by switching off signaling cascades involved in gene transcription leading to the release of pro-inflammatory mediators (Verme et al., 2005; D’Agostino et al., 2007, 2009). PPARα is also present in hippocampus, corpus striatum, spinal cord and frontal cortex, opening new venues for research on the mechanism of PEA-mediated neuroprotection and neuroinflammation in the central nervous system (CNS; Moreno et al., 2004). Through activation of PPARα, PEA activate several different receptors including vanilloid-receptor, and ion channels involved in neuronal firing (Hansen, 2010). The mechanism by which systemically administered PEA mediates neuroprotective and anti-inflammatory effects in CNS has not been yet elucidated, and is of emerging interest. It has been recently demonstrated that at least the anti-inflammatory effects of PEA are mediated by a cross-talk between glia cells and mast cells in periphery and brain, whereby PEA blocks the activation of mast cells in the brain and periphery, thus inhibiting inflammatory signaling pathways involved in the periphery-brain cross-talk (Skaper et al., 2013, 2014).
Neuroinflammatory processes resulting from excitotoxicity in epileptic seizures potentiate the inflammatory responses, mediated by cyclooxygenase-2 (COX-2) derived eicosanoids (eiCs) such as prostaglandin E2 (PGE2) and prostaglandin D2 (PGD2), and accelerate neuronal hyperexcitability, seizure extent and reoccurrence (Serrano et al., 2011; Vezzani et al., 2013; Barker-Haliski et al., 2017; Terrone et al., 2017). Targeting brain inflammation signaling pathways constitutes a complementary approach or adjuvant therapy to AEDs, particularly in patients with refractory epilepsy (Dey et al., 2016).
In recent years, evidences accumulated also on the role of systemic inflammation and/or peripheral immunity activation in rendering propensity to seizure occurrence and progression. Overexpression of pro-inflammatory mediators, e.g., cytokines, prostaglandins, nitric oxide signaling, and/or endogenous opioids in hippocampal tissue was evidenced to follow peripheral inflammation with subsequently increased neuronal excitability in animal models underscoring the occurrence of brain immune system communication (Riazi et al., 2010; Murta et al., 2015). Peripheral inflammation induced by lipopolisacharides led to increased neuroinflammatory processes, oxidative stress, and seizure susceptibility in a rat brain of kainic acid (KA)-induced excitotoxicity. This effect was mainly reversible by COX-2 inhibitor mediated neuroprotection (Ho et al., 2015). Peripheral anti-inflammatory treatment with anti-interleukin (IL)-1β has been shown to reduce seizure severity in a pilocarpine model (Marchi et al., 2009). In human studies, a relation between inflammatory processes, immunity and seizure susceptibility, occurrence and intensity were evidenced for patients suffering from different epilepsy types including temporal lobe epilepsy (TLE; Gupta and Appleton, 2005; Buzatu et al., 2009; Hancock et al., 2013; van den Munckhof et al., 2015). Interleukin 6 (IL-6) levels in cerebrospinal fluid (CSF) and plasma from epilepsy patients were significantly increased 24 h post tonic-clonic seizures, and in patients with TLE serum levels of IL-6 and IL-β1 remain upregulated, indicating a chronic immune mechanism (Silveira et al., 2012; Uludag et al., 2015; de Vries et al., 2016).
We previously evidenced that systemic administration of KA in mice leads to widespread damage in brain and peripheral organs and increased peripheral inflammation at acute seizure state (Lerner et al., 2016), the latter resembling thus a pathological feature of TLE. We specifically aimed at investigating the PEA treatment effectiveness in acute injury phase of the KA-induced excitotoxicity, and therefore we chose a time course of 180 min to reflect this epileptogenesis phase, whereby at 1 h post injection a maximum of seizure activity typically occurs. Therefore, a mouse model of KA-systemic induced epileptic seizure was chosen as suitable to assess the neuroprotective and anti-inflammatory role of PEA, its potential to affect both periphery and brain by inhibiting inflammatory signals, and the anticonvulsive properties. We determined anticonvulsant effects of acute and subchronic administration of PEA by behavioral examination for 180 min post KA injection. Neuroprotective effects of subchronic PEA administration (double injection) were demonstrated in brain sections of PEA-treated vs. non-treated KA-injected animals. Additionally, we evaluated by targeted mass spectrometry-based quantitative profiling the temporal dynamics of eCB- and eiC-levels with epileptic seizures, at 20, 60, 120, and 180 min post KA-seizure induction, and its response upon PEA administration. Anticonvulsive effects of PEA were associated with a modulation of both peripheral and hippocampal levels of eCBs and eiCs. Finally, antiepileptic properties of PEA were compared to those exhibited by systemic pharmacological blockade of FAAH (URB597) and peripheral FAAH blockade by URB937, as well as in combinatorial therapeutic administration.
Conclusions and Perspectives
Subchronic PEA treatment is an effective anticonvulsant therapy in KA-induced acute mouse model of epilepsy by significantly decreasing the seizure intensity at and after-acute seizure state, exerting significant neuroprotective effects in brain and downregulating the peripheral and hippocampal inflammatory responses to the excitotoxicity. These results highlight PEA as a promising candidate for chronic antiepileptic treatment. Prospective application of PEA treatment on other models resembling TLE in humans will help elucidating its applicability and potential beyond the current epilepsy model. To our knowledge, this is the first study that reveals time-point specific PEA-mediated antiepileptic effects, as well as eCB and eiC dynamics in KA-mouse model of epilepsy. Moreover, plasma lipids are shown to serve as viable candidates for follow-up drug monitoring, hence facilitating the quest of AED-therapies. The positive effects of PEA pre-treatment revealed here highlight its potential applicability in translational human studies that aim at establishing novel antiepileptic pharmaco-therapeutic strategies.