N-palmitoyl-ethanolamine (PEA) induces peripheral antinociceptive effect by ATP-sensitive K+-channel activation

Nitric oxide and potassium channels but not opioid and cannabinoid receptors mediate tramadol-induced peripheral antinociception in rat model of paw pressure withdrawal

Tramadol, an analgesic classified as an "atypical opioid", exhibits both opioid and non-opioid mechanisms of action. This study aimed to explore these mechanisms, specifically the opioid-, cannabinoid-, nitric oxide-, and potassium channel-based mechanisms, which contribute to the peripheral antinociception effect of tramadol, in an experimental rat model. The nociceptive threshold was determined using paw pressure withdrawal. To examine the mechanisms of action, several substances were administered intraplantarly: naloxone, a non-selective opioid antagonist (50 µg/paw); AM251 (80 µg/paw) and AM630 (100 µg/paw) as the selective antagonists for types 1 and 2 cannabinoid receptors, respectively; nitric oxide synthase inhibitors L-NOArg, L-NIO, L-NPA, and L-NIL (24 µg/paw); and the enzyme inhibitors of guanylatocyclase and phosphodiesterase of cGMP, ODQ, and zaprinast. Additionally, potassium channel blockers glibenclamide, tetraethylammonium, dequalinium, and paxillin were used. The results showed that opioid and cannabinoid receptor antagonists did not reverse tramadol's effects. L-NOarg, L-NIO, and L-NPA partially reversed antinociception, while ODQ completely reversed, and zaprinast enhanced tramadol's antinociception effect. Notably, glibenclamide blocked tramadol's antinociception in a dose-dependent manner. These findings suggest that tramadol's peripheral antinociception effect is likely mediated by the nitrergic pathway and sensitive ATP potassium channels, rather than the opioid and cannabinoid pathways.

Neuropathic Pain in Aged People: An Unresolved Issue Open to Novel Drug Approaches, Focusing on Painful Diabetic Neuropathy

A majority of older patients suffer from neuropathic pain (NP) that significantly alters their daily activities and imposes a significant burden on health care. Multiple comorbidities and the risk of polypharmacy in the elderly make it challenging to determine the appropriate drug, dosage, and maintenance of therapy. Age-dependent processes play a contributing role in neuropathy given that diabetic neuropathy (DN) is the most common form of neuropathy. This narrative review is mainly focused on the drug treatment approach for neuropathy-associated pain in aged people including both drugs and dietary supplements, considering the latter as add-on mechanism-based treatments to increase the effectiveness of usual treatments by implementing their activity or activating other analgesic pathways. On one hand, the limited clinical studies assessing the effectiveness and the adverse effects of existing pain management options in this age segment of the population (> 65), on the other hand, the expanding global demographics of the elderly contribute to building up an unresolved pain management problem that needs the attention of healthcare providers, researchers, and health authorities as well as the expansion of the current therapeutic options.

Participation of the cannabinoid system and the NO/cGMP/KATP pathway in serotonin-induced peripheral antinociception

It has already been shown that serotonin can release endocannabinoids at the spinal cord level, culminating in inhibition of the dorsal horn. At the peripheral level, cannabinoid receptors modulate primary afferent neurons by inhibiting calcium conductance and increasing potassium conductance. Studies have shown that after the activation of opioid receptors and cannabinoids, there is also the activation of the NO/cGMP/KATP pathway, inducing cellular hyperpolarization. In this study, we evaluated the participation of the cannabinoid system with subsequent activation of the NO/cGMP/KATP pathway in the peripheral antinociceptive effect of serotonin. The paw pressure test of mice was used in animals that had their sensitivity to pain increased due to an intraplantar injection of PGE2 (2 μg). Serotonin (250 ng/paw), administered locally in the right hind paw, induced antinociceptive effect. CB1 and CB2 cannabinoid receptors antagonists, AM251 (20, 40 and 80 μg) and AM630 (25, 50 and 100 μg), respectively, reversed the serotonin-induced antinociceptive effect. MAFP (0.5 μg), an inhibitor of the FAAH enzyme that degrades anandamide, and JZL184 (3.75 μg), an inhibitor of the enzyme MAGL that degrades 2-AG, as well as the VDM11 (2.5 μg) inhibitor of anandamide reuptake, potentiated the antinociceptive effect induced by a low dose (62. 5 ng) of serotonin. In the evaluation of the participation of the NO/cGMP/KATP pathway, the antinociceptive effect of serotonin was reversed by the administration of the non-selective inhibitor of NOS isoforms L-NOarg (12.5, 25 and 50 μg) and by the selective inhibitor for the neuronal isoform LNPA (24 μg), as well as by the soluble guanylate cyclase inhibitor ODQ (25, 50 and 100 μg). Among potassium channel blockers, only Glibenclamide (20, 40 and 80 μg), an ATP-sensitive potassium channel blocker, reversed the effect of serotonin. In addition, intraplantar administration of serotonin (250 ng) was shown to induce a significant increase in nitrite levels in the homogenate of the plantar surface of the paw of mice. Taken together, these data suggest that the antinociceptive effect of serotonin occurs by activation of the cannabinoid system with subsequent activation of the NO/cGMP/KATP pathway.

N-palmitoylethanolamide synergizes the antinociception of morphine and gabapentin in the formalin test in mice

OBJECTIVE

The antinociceptive pharmacological interaction between N-palmitoylethanolamide (PEA) and morphine (MOR), as well as gabapentin (GBP), was investigated to obtain synergistic antinociception at doses where side effects were minimal. In addition, the possible antinociceptive mechanism of PEA + MOR or PEA + GBP combinations was explored.

METHODS

Individual dose-response curves (DRCs) of PEA, MOR and GBP were evaluated in female mice in which intraplantar nociception was induced with 2% formalin. Isobolographic method was used to detect the pharmacological interaction in the combination of PEA + MOR or PEA + GBP.

KEY FINDINGS

The ED50 was calculated from the DRC; the order of potency was MOR > PEA > GBP. The isobolographic analysis was obtained at a 1:1 ratio to determine the pharmacological interaction. The experimental values of flinching (PEA + MOR, Zexp = 2.72 ± 0.2 μg/paw and PEA + GBP Zexp = 2.77 ± 0.19 μg/paw) were significantly lower than those calculated theoretically (PEA + MOR Zadd = 7.78 ± 1.07 and PEA + GBP Zadd = 24.05 ± 1.91 μg/paw), resulting in synergistic antinociception. Pretreatment with GW6471 and naloxone demonstrated that peroxisome proliferator-activated receptor alpha (PPARα) and opioid receptors are involved in both interactions.

CONCLUSIONS

These results suggest that MOR and GBP synergistically enhance PEA-induced antinociception through PPARα and opioid receptor mechanisms. Furthermore, the results suggest that combinations containing PEA with MOR or GBP could be of interest in aiding the treatment of inflammatory pain.

Effects of Palmitoylethanolamide (PEA) on Nociceptive, Musculoskeletal and Neuropathic Pain: Systematic Review and Meta-Analysis of Clinical Evidence

Some 30−50% of the global population and almost 20% of the European population actually suffer from chronic pain, which presents a tremendous burden to society when this pain turns into a disability and hospitalization. Palmitoylethanolamide (PEA) has been demonstrated to improve pain in preclinical contexts, but an appraisal of clinical evidence is still lacking. The present study aimed at addressing the working hypothesis for the efficacy of PEA for nociceptive musculoskeletal and neuropathic pain in the clinical setting. The systematic search, selection and analysis were performed in agreement with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 recommendations. The primary outcome was pain reduction, as measured by a pain assessment scale. The secondary outcome was improvement in quality of life and/or of parameters of function. The results obtained for a total of 933 patients demonstrate the efficacy of PEA over the control (p < 0.00001), in particular in six studies apart from the two randomized, double-blind clinical trials included. However, the results are downgraded due to the high heterogeneity of the studies (I2 = 99%), and the funnel plot suggests publication bias. Efficacy in achieving a reduction in the need for rescue medications and improvement in functioning, neuropathic symptoms and quality of life are reported. Therefore, adequately powered randomized, double-blind clinical trials are needed to deepen the domains of efficacy of add-on therapy with PEA for chronic pain. PROSPERO registration: CRD42022314395.

Non-drug pain relievers active on non-opioid pain mechanisms

This review is aimed to summarize the pain-relieving effect of non-drug substances, mostly prescribed as integrators in treatment of pain, including especially in chronic postoperative pain (CPSP) and in chronic back pain after acute episodes. Their use reflects the fact that the current treatments for these syndromes continue to pose problems of unsatisfactory responses in a significant portion of patients and/or of an excess of side effects like those noted in the present opioid crisis. As integrators are frequently introduced into the market without adequate clinical testing, this review is aimed to collect the present scientific evidence either preclinical or clinical for their effectiveness. In particular, we reviewed the data on the use of: B vitamins; vitamin C; vitamin D; alpha lipoic acid (ALA); N-acetylcysteine; acetyl L-carnitine; curcumin; boswellia serrata; magnesium; coenzyme Q10, and palmitoylethanolamide. The combination of preclinical findings and clinical observations strongly indicate that these compounds deserve more careful attention, some of them having interesting clinical potentials also in preventing chronic pain after an acute episode. In particular, examining their putative mechanisms of action it emerges that combinations of few of them may exert an extraordinary spectrum of activities on a large variety of pain-associated pathways and may be eventually used in combination with more traditional pain killers in order to extend the duration of the effect and to lower the doses. Convincing examples of effective combinations against pain are vitamin B complex plus gabapentin for CPSP, including neuropathic pain; vitamin B complex plus diclofenac against low back pain and also in association with gabapentin, and ALA for burning mouth syndrome. These as well as other examples need, however, careful controlled independent clinical studies confirming their role in therapy.

Ultramicronized Palmitoylethanolamide (um-PEA): A New Possible Adjuvant Treatment in COVID-19 patients

The Coronavirus Disease-19 (COVID-19) pandemic has caused more than 100,000,000 cases of coronavirus infection in the world in just a year, of which there were 2 million deaths. Its clinical picture is characterized by pulmonary involvement that culminates, in the most severe cases, in acute respiratory distress syndrome (ARDS). However, COVID-19 affects other organs and systems, including cardiovascular, urinary, gastrointestinal, and nervous systems. Currently, unique-drug therapy is not supported by international guidelines. In this context, it is important to resort to adjuvant therapies in combination with traditional pharmacological treatments. Among natural bioactive compounds, palmitoylethanolamide (PEA) seems to have potentially beneficial effects. In fact, the Food and Drug Administration (FDA) authorized an ongoing clinical trial with ultramicronized (um)-PEA as an add-on therapy in the treatment of Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) infection. In support of this hypothesis, in vitro and in vivo studies have highlighted the immunomodulatory, anti-inflammatory, neuroprotective and pain-relieving effects of PEA, especially in its um form. The purpose of this review is to highlight the potential use of um-PEA as an adjuvant treatment in SARS-CoV-2 infection.

Broad Lipidomic and Transcriptional Changes of Prophylactic PEA Administration in Adult Mice

Beside diverse therapeutic properties of palmitoylethanolamide (PEA) including: neuroprotection, inflammation and pain alleviation, prophylactic effects have also been reported in animal models of infections, inflammation, and neurological diseases. The availability of PEA as (ultra)micronized nutraceutical formulations with reportedly no side effects, renders it accordingly an appealing candidate in human preventive care, such as in population at high risk of disease development or for healthy aging. PEA’s mode of action is multi-facetted. Consensus exists that PEA’s effects are primarily modulated by the peroxisome proliferator-activated receptor alpha (PPARα) and that PEA-activated PPARα has a pleiotropic effect on lipid metabolism, inflammation gene networks, and host defense mechanisms. Yet, an exhaustive view of how the prophylactic PEA administration changes the lipid signaling in brain and periphery, thereby eliciting a beneficial response to various negative stimuli remains still elusive. We therefore, undertook a broad lipidomic and transcriptomic study in brain and spleen of adult mice to unravel the positive molecular phenotype rendered by prophylactic PEA. We applied a tissue lipidomic and transcriptomic approach based on simultaneous extraction and subsequent targeted liquid chromatography-multiple reaction monitoring (LC-MRM) and mRNA analysis by qPCR, respectively. We targeted lipids of COX-, LOX- and CYP450 pathways, respectively, membrane phospholipids, lipid products of cPLA₂, and free fatty acids, along with various genes involved in their biosynthesis and function. Additionally, plasma lipidomics was applied to reveal circulatory consequences and/or reflection of PEA’s action. We found broad, distinct, and several previously unknown tissue transcriptional regulations of inflammatory pathways. In hippocampus also a PEA-induced transcriptional regulation of neuronal activity and excitability was evidenced. A massive downregulation of membrane lipid levels in the splenic tissue of the immune system with a consequent shift towards pro-resolving lipid environment was also detected. Plasma lipid pattern reflected to a large extent the hippocampal and splenic lipidome changes, highlighting the value of plasma lipidomics to monitor effects of nutraceutical PEA administration. Altogether, these findings contribute new insights into PEA’s molecular mechanism and helps answering the questions, how PEA prepares the body for insults and what are the “good lipids” that underlie this action.

Novel CB1-ligands maintain homeostasis of the endocannabinoid system in ω3- and ω6-long-chain-PUFA deficiency

Mammalian ω3- and ω6-PUFAs are synthesized from essential fatty acids (EFAs) or supplied by the diet. PUFAs are constitutive elements of membrane architecture and precursors of lipid signaling molecules. EFAs and long-chain (LC)-PUFAs are precursors in the synthesis of endocannabinoid ligands of G_i/o protein-coupled cannabinoid receptor (CB)1 and CB2 in the endocannabinoid system, which critically regulate energy homeostasis as the metabolic signaling system in hypothalamic neuronal circuits and behavioral parameters. We utilized the auxotrophic fatty acid desaturase 2-deficient (fads2^−/−) mouse, deficient in LC-PUFA synthesis, to follow the age-dependent dynamics of the PUFA pattern in the CNS-phospholipidome in unbiased dietary studies of three cohorts on sustained LC-PUFA-free ω6-arachidonic acid- and DHA-supplemented diets and their impact on the precursor pool of CB1 ligands. We discovered the transformation of eicosa-all cis-5,11,14-trienoic acid, uncommon in mammalian lipidomes, into two novel endocannabinoids, 20:3^5,11,14-ethanolamide and 2-20:3^5,11,14-glycerol. Their function as ligands of CB1 has been characterized in HEK293 cells. Labeling experiments excluded Δ8-desaturase activity and proved the position specificity of FADS2. The fads2^−/− mutant might serve as an unbiased model in vivo in the development of novel CB1 agonists and antagonists.

The role of transient receptor potential vanilloid receptor 1 and peroxisome proliferator-activated receptors-α in mediating the antinociceptive effects of palmitoylethanolamine in rats

Palmitoylethanolamine (PEA) is a ligand at peroxisome proliferator-activated receptors-α (PPARα), a nuclear receptor that has anti-inflammatory effects. Herein, complete Freund's adjuvant (CFA)-induced inflammatory pain model in rats and in-vitro calcium imaging studies were used to evaluate the mechanisms that underlie the antinociceptive effects of PEA, including modulating the activity of the transient receptor potential vanilloid receptor 1, which is a key receptor involved in the development of inflammatory pain. Adult male Sprague-Dawley rats (180-250 g) received subcutaneous injections of CFA (0.1 ml) into the plantar surface of the left hind paw. Von Frey filaments were used to determine the paw withdrawal threshold. PEA (50 µg), WY14643 (50 µg, a selective PPARα agonist) were injected into the plantar surface of the left hind paw at day 7 after CFA injection, and behavioral tests were repeated 6 h after drug administration. Rats were killed and dorsal root ganglia neurons were dissected and prepared for calcium imaging. Neurons were loaded with the calcium-sensitive ratiometric dye Fura-2AM. Changes in [Ca]i were measured as ratios of peak florescence at excitation wavelengths of 340 and 380 nm and expressed as a percentage of the KCl (60 mM) response. Both PEA and WY14643 significantly restored the paw withdrawal threshold in a PPARα-dependent fashion (P<0.01). Capsaicin of 15 nM produced 63.9±13.4% of KCl response. Preincubation of dorsal root ganglia neurons with PEA 6 h before stimulation with capsaicin, significantly reduce capsaicin-evoked calcium responses (42.9±6.4% of KCl response, n=54, P<0.001). In conclusion, modulating transient receptor potential vanilloid receptor 1 activity could provide the mechanism that underlies PEA antinociceptive effects observed in vivo.

A Pharmacological Rationale to Reduce the Incidence of Opioid Induced Tolerance and Hyperalgesia: A Review

Chronic pain is an important health and social problem. Misuse and abuse of opioids in chronic non-cancer pain management seem to be a huge problem, in some countries. This could probably affect the normal use of such analgesics in patients in need of them. Basic and clinical researches should find the solution to mitigate the potential damage. Dysregulation of mast cell and microglia activation plays an important role in the pathogenesis and management of chronic pain. Persistent mast cell activation sensitizes nociceptors and initiates central nervous system inflammatory processes, involving microglial cell activation and sensitization of spinal somatosensory neurons. Exposure of mast cells and microglia to opioids is well known to provoke activation of these non-neuronal immune cell populations, thereby contributing to an exacerbation of pro-inflammatory and pro-nociceptive processes and promoting, over the long-term, opioid-induced hyperalgesia and tolerance. This review is intended to provide the reader with an overview of the role for these non-neuronal cells in opioid-induced chronic pain and tolerance as a consequence of prolonged exposure to these drugs. In addition, we will examine a potential strategy with the aim to modulate opioid-induced over-activation of glia and mast cells, based on endogenous defense mechanisms and fatty acid amide signaling molecules.

Palmitoylethanolamide for the treatment of pain: pharmacokinetics, safety and efficacy

Palmitoylethanolamide (PEA) has been suggested to have useful analgesic properties and to be devoid of unwanted effects. Here, we have examined critically this contention, and discussed available data concerning the pharmacokinetics of PEA and its formulation. Sixteen clinical trials, six case reports/pilot studies and a meta‐analysis of PEA as an analgesic have been published in the literature. For treatment times up to 49 days, the current clinical data argue against serious adverse drug reactions (ADRs) at an incidence of 1/200 or greater. For treatment lasting more than 60 days, the number of patients is insufficient to rule out a frequency of ADRs of less than 1/100. The six published randomized clinical trials are of variable quality. Presentation of data without information on data spread and nonreporting of data at times other than the final measurement were among issues that were identified. Further, there are no head‐to‐head clinical comparisons of unmicronized vs. micronized formulations of PEA, and so evidence for superiority of one formulation over the other is currently lacking. Nevertheless, the available clinical data support the contention that PEA has analgesic actions and motivate further study of this compound, particularly with respect to head‐to‐head comparisons of unmicronized vs. micronized formulations of PEA and comparisons with currently recommended treatments.

Synergistic antinociceptive interaction between palmitoylethanolamide and tramadol in the mouse formalin test

Pharmacological synergism has been used to obtain a higher efficacy using drug concentrations at which side effects are minimal. In this study, the pharmacological antinociceptive interaction between N-palmitoylethanolamide (PEA) and tramadol was investigated. The individual concentration-response curves for PEA (0.1-56.2 μg/paw) and tramadol (1-56.2 μg/paw) were evaluated in mice in which nociception was induced by an intraplantar injection of 2% formalin. Isobolographic analysis was used to evaluate the pharmacological interaction between PEA (EC50=23.7±1.6 μg/paw) and tramadol (EC50=26.02±2.96 μg/paw) using the EC50 and a fixed 1:1 ratio combination. The isobologram demonstrated that the combinations investigated in this study produced a synergistic interaction; the experimental values (Zexp=9.5±0.2 μg/paw) were significantly smaller than those calculated theoretically (Zadd=24.8±0.2 μg/paw). The antinociceptive mechanisms of the PEA and tramadol combination involved the opioid receptor, transient receptor potential cation channel subfamily V member 1 (TRPV1), and peroxisome proliferator-activated receptor alpha (PPAR-α). The sedative effect of the combination of PEA and tramadol was less than that generated by individual treatments. These findings suggest that the PEA and tramadol combination produced enhanced antinociceptive efficacy at concentrations at which side effects are minimal.

Tingenone, a pentacyclic triterpene, induces peripheral antinociception due to NO/cGMP and ATP-sensitive K(+) channels pathway activation in mice

Substances derived from plants play an important role in the development of new analgesic drugs, among them, triterpenoids. The connection between the participation of L-arginine/NO/cGMP pathway and the activation of ATP-sensitive K(+) channels (KATP) has been established on the peripheral antinociception induced by various drugs. The study assessed the involvement of L-arginine/NO/cGMP/KATP pathway in the antinociceptive effect induced by tingenone, from Maytenus imbricata, against the hyperalgesia evoked by prostaglandin E2 (PGE2) in peripheral pathway. The paw pressure test was used, with hyperalgesia induced by intraplantar injection of PGE2 (2 μg). Tingenone (200 µg/paw) administered into the right hind paw induced a local antinociceptive effect, that was antagonized by l-NOArg, nonselective nitric oxide synthase (NOS) inhibitor and by L-NPA, selective neuronal NOS (nNOS) inhibitor. The L-NIO, selective inhibitor of endothelial (eNOS), and the L-NIL, selective inhibitor of inducible (iNOS), did not alter the peripheral antinociceptive effect of the tingenone. The ODQ, selective soluble guanylyl cyclase inhibitor, prevented the antinociceptive effect of tingenone, and zaprinast, inhibitor of the phosphodiesterase that is cyclic guanosine monophosphate (cGMP) specific, intensified the peripheral antinociceptive effect of the smaller dose of tingenone. Glibenclamide, ATP-sensitive K(+) channels (KATP) blocker, but not tetraethylammonium chloride, voltage-dependent K(+) channel blocker; dequalinium dichloride, blocker of the small conductance Ca(2+)-activated K(+) channel, and paxilline, a potent blocker of high-conductance Ca(2+)-activated K(+) channels, respectively, prevented the peripheral antinociceptive effect of tingenone. The results demonstrate that tingenone induced a peripheral antinociceptive effect by L-arginine/NO/cGMP/KATP pathway activation, with potential for a new analgesic drug.

Ang-(1-7) activates the NO/cGMP and ATP-sensitive K+ channels pathway to induce peripheral antinociception in rats

Angiotensin-(1-7) is a bioactive component of the renin-angiotensin system that is formed endogenously and induces nitric oxide release in several tissues. The L-arginine/NO/cyclic GMP pathway and ATP-sensitive K+ channels have been proposed as the mechanism of action for the peripheral antinociception of several groups of drug and endogenous substances, including opioids, non-steroidal analgesics, acetylcholine and others. The aim of the present study was to investigate the involvement of the L-arginine/NO/cGMP and KATP+ pathway on antinociception induced by angiotensin-(1-7). Paw pressure in rats was used to induce hyperalgesia via an intraplantar injection of prostaglandin E2 (2 μg/paw). Ang-(1-7) (2, 3 and 4 μg/paw) elicited a local peripheral antinociceptive effect that was antagonized by the nonselective NO synthase (NOS) inhibitor L-NOarg and the selective neuronal NOS (nNOS) inhibitor L-NPA. The selective inhibition of endothelial (eNOS) and inducible (iNOS) NOS by L-NIO and L-NIL, respectively, was ineffective at blocking the effects of a local Ang-(1-7) injection. In addition, the level of nitrite in the homogenized paw tissue, as determined by a colorimetric assay, indicated that exogenous Ang-(1-7) is able to induce NO release. The soluble guanylyl cyclase inhibitor ODQ and the specific blocker of ATP-sensitive K+ channels glibenclamide (40, 80 and 160 μg/paw) antagonized the Ang-(1-7) response. The results provide evidence that Ang-(1-7) most likely induces peripheral antinociceptive effects via the L-arginine/NO/cGMP pathway and KATP+ pathway activation.