Palmitoylethanolamide reduces pain-related behaviors and restores glutamatergic synapses homeostasis in the medial prefrontal cortex of neuropathic mice

Chronic Pain-Related Cognitive Deficits: Preclinical Insights into Molecular, Cellular, and Circuit Mechanisms

Cognitive impairment is a common comorbidity of chronic pain, significantly disrupting patients' quality of life. Despite this comorbidity being clinically recognized, the underlying neuropathological mechanisms remain unclear. Recent preclinical studies have focused on the fundamental mechanisms underlying the coexistence of chronic pain and cognitive decline. Pain chronification is accompanied by structural and functional changes in the neural substrate of cognition. Based on the developments in electrophysiology and optogenetics/chemogenetics, we summarized the relevant neural circuits involved in pain-induced cognitive impairment, as well as changes in connectivity and function in brain regions. We then present the cellular and molecular alternations related to pain-induced cognitive impairment in preclinical studies, mainly including modifications in neuronal excitability and structure, synaptic plasticity, glial cells and cytokines, neurotransmitters and other neurochemicals, and the gut-brain axis. Finally, we also discussed the potential treatment strategies and future research directions.

Exploring symptomatology and innovative treatment modalities for prefrontal cortex lesions: a systematic review

BACKGROUND

Pre-frontal cortex operates a combination of emotional, cognitive and behavioural functions. Understanding the symptoms of pre-frontal cortex lesions serves as paramount for accurate diagnosis and management.

AIMS

This review aims to determine an association between the causes of prefrontal cortex lesions and the resulting symptoms, as well as the ideal form of treatment.

STUDY DESIGN

A systematic review through utilisation of 3 databases was done using the keywords "Prefrontal cortex lesions," "dysfunction," "symptoms," &" treatment".

METHODOLOGY

RCTs, observational studies, and systematic reviews were searched using Cochrane/EMBASE, PubMed/Medline, and Pedro between 1948 and2024. Studies published in English only were included, and two reviewers were involved in the data extraction process.

RESULTS

Results showed a notable correlation between right-handed individuals and prefrontal cortex lesions with cognition impairment, particularly executive dysfunction, being the most prevalent symptom. Emotional instability followed as the second most common issue, while aphasia remained the primary language deficit. Noninvasive brain stimulation emerged as an effective treatment option for various prefrontal cortex-related disorders.

CONCLUSION

Further investigation is needed to understand the mechanism linking handedness to lesion occurrence. Noninvasive brain stimulation should be prioritised for treating prefrontal injuries.

Role of Brain Derived Neurotrophic Factor and Related Therapeutic Strategies in Central Post-Stroke Pain

Brain-derived neurotrophic factor (BDNF) is vital for synaptic plasticity, cell persistence, and neuronal development in peripheral and central nervous systems (CNS). Numerous intracellular signalling pathways involving BDNF are well recognized to affect neurogenesis, synaptic function, cell viability, and cognitive function, which in turn affects pathological and physiological aspects of neurons. Stroke has a significant psycho-socioeconomic impact globally. Central post-stroke pain (CPSP), also known as a type of chronic neuropathic pain, is caused by injury to the CNS following a stroke, specifically damage to the somatosensory system. BDNF regulates a broad range of functions directly or via its biologically active isoforms, regulating multiple signalling pathways through interactions with different types of receptors. BDNF has been shown to play a major role in facilitating neuroplasticity during post-stroke recovery and a pro-nociceptive role in pain development in the nervous system. BDNF-tyrosine kinase receptors B (TrkB) pathway promotes neurite outgrowth, neurogenesis, and the prevention of apoptosis, which helps in stroke recovery. Meanwhile, BDNF overexpression plays a role in CPSP via the activation of purinergic receptors P2X4R and P2X7R. The neuronal hyperexcitability that causes CPSP is linked with BDNF-TrkB interactions, changes in ion channels and inflammatory reactions. This review provides an overview of BDNF synthesis, interactions with certain receptors, and potential functions in regulating signalling pathways associated with stroke and CPSP. The pathophysiological mechanisms underlying CPSP, the role of BDNF in CPSP, and the challenges and current treatment strategies targeting BDNF are also discussed.

Strategies for measuring non-evoked pain in preclinical models of neuropathic pain: Systematic review

The development of new analgesics for neuropathic pain treatment is crucial. The failure of promising drugs in clinical trials may be related to the over-reliance on reflex-based responses (evoked pain) in preclinical drug testing, which may not fully represent clinical neuropathic pain, characterized by spontaneous non-evoked pain (NEP). Hence, strategies for assessing NEP in preclinical studies emerged. This systematic review identified 443 articles evaluating NEP in neuropathic pain models (mainly traumatic nerve injuries in male rodents). An exponential growth in NEP evaluation was observed, which was assessed using 48 different tests classified in 12 NEP-related outcomes: anxiety, exploration/locomotion, paw lifting, depression, conditioned place preference, gait, autotomy, wellbeing, facial grooming, cognitive impairment, facial pain expressions and vocalizations. Although most of these outcomes showed clear limitations, our analysis suggests that conditioning-associated outcomes, pain-related comorbidities, and gait evaluation may be the most effective strategies. Moreover, a minimal part of the studies evaluated standard analgesics. The greater emphasis on evaluating NEP aligning with clinical pain symptoms may enhance analgesic drug development, improving clinical translation.

Impaired neuronal macroautophagy in the prelimbic cortex contributes to comorbid anxiety-like behaviors in rats with chronic neuropathic pain

A large proportion of patients with chronic pain experience co-morbid anxiety. The medial prefrontal cortex (mPFC) is proposed to underlie this comorbidity, but the molecular and neuronal mechanisms are not fully understood. Here, we reported that impaired neuronal macroautophagy in the prelimbic cortical (PrL) subregion of the mPFC paralleled the occurrence of anxiety-like behaviors in rats with chronic spared nerve injury (SNI). Intriguingly, such macroautophagy impairment was mainly observed in a FOS/c-Fos+ neuronal subpopulation in the PrL. Chemogenetic inactivation of this comorbid anxiety-related neuronal ensemble relieved pain-induced anxiety-like behaviors. Rescuing macroautophagy impairment in this neuronal ensemble relieved chronic pain-associated anxiety and mechanical allodynia and restored synaptic homeostasis at the molecular level. By contrast, artificial disruption of macroautophagy induced early-onset co-morbid anxiety in neuropathic rats, but not general anxiety in normal rats. Taken together, our work identifies causal linkage between PrL neuronal macroautophagy dysfunction and comorbid anxiety in neuropathic pain and provides novel insights into the role of PrL by differentiating its contribution in pain-induced comorbid anxiety from its modulation over general anxiety-like behaviors.Abbreviation: AAV: adeno-associated viruses; ACC: anterior cingulate cortex; ATG5: autophagy related 5; ATG7: autophagy related 7; ATG12: autophagy related 12; CAMK2/CaMKII: calcium/calmodulin-dependent protein kinase II; CNO: clozapine-N-oxide; CQ: chloroquine; DIA: data independent acquisition; DIO: double floxed inverse orf; DLG4/PSD-95: discs large MAGUK scaffold protein 4; Dox: doxycycline; GABA: γ-aminobutyric acid; GFP: green fluorescent protein; GO: gene ontology; Gi: inhibitory guanine nucleotide-binding proteins; HsCHRM4/M4D: human cholinergic receptor muscarinic 4; HsSYN: human synapsin; KEGG: Kyoto encyclopedia of genes and genomes; LAMP1: lysosomal-associated membrane protein 1; LC3-II: PE conjugated microtubule-associated protein 1 light chain3; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; mPFC: medial prefrontal cortex; P2A: 2A self-cleaving peptide; PPI: protein-protein interaction networks; PrL: prelimbic cortex; RBFOX3/NeuN: RNA binding protein, fox-1 homolog (C. elegans) 3; rtTA: reverse tetracycline-transactivator; SDS-PAGE: sodium dodecylsulfate-polyacrylamide gel electrophoresis; SHANK3: SH3 and multiple ankyrin repeat domains 3; SLC1A1/EAAC1: solute carrier family 1 (neuronal/epithelial high affinity glutamate transporter, systemXag), member 1; SNAP23: synaptosomal-associated protein 23; SNI:spared nerve injury; SQSTM1/p62: sequestosome 1; SYT3: synaptotagmin 3; TRE: tetracycline-responsive element; TRE3G: third-generation tetracycline-responsive element.

Targeting Prefrontal Cortex Dysfunction in Pain

The prefrontal cortex (PFC) has justifiably become a significant focus of chronic pain research. Collectively, decades of rodent and human research have provided strong rationale for studying the dysfunction of the PFC as a contributing factor in the development and persistence of chronic pain and as a key supraspinal mechanism for pain-induced comorbidities such as anxiety, depression, and cognitive decline. Chronic pain alters the structure, chemistry, and connectivity of PFC in both humans and rodents. In this review, we broadly summarize the complexities of reported changes within both rodent and human PFC caused by pain and offer insight into potential pharmacological and nonpharmacological approaches for targeting PFC to treat chronic pain and pain-associated comorbidities. SIGNIFICANCE STATEMENT: Chronic pain is a significant unresolved medical problem causing detrimental changes to physiological, psychological, and behavioral aspects of life. Drawbacks of currently approved pain therapeutics include incomplete efficacy and potential for abuse producing a critical need for novel approaches to treat pain and comorbid disorders. This review provides insight into how manipulation of prefrontal cortex circuits could address this unmet need of more efficacious and safer pain therapeutics.

PolyphyllinVI alleviates the spared nerve injury-induced neuropathic pain based on P2X3 receptor-mediated the release of inflammatory mediators

ETHNOPHARMACOLOGICAL RELEVANCE

PolyphyllinVI (PPⅥ) is the main bioactive component of Chonglou which is a traditional Chinese herbal with various effects, including antitumor, anti-inflammatory, and analgesia.

AIM OF THE STUDY

This study aimed to investigate the properties and mechanisms of the analgesia of PPⅥ by using neuropathic pain (NPP) mice.

MATERIALS AND METHODS

The potential targets and mechanisms of PPⅥ in alleviating NPP were excavated based on the network pharmacology. Subsequently, the construction of a spared nerve injury (SNI) mice model was used to evaluate the effect of PPⅥ on NPP and the expression of the P2X3 receptor. We identified the signaling pathways of PPⅥ analgesia by RNA sequencing.

RESULTS

The results of network pharmacology showed that BCL2, CASP3, JUN, STAT3, and TNF were the key targets of the analgesic effect of PPⅥ. PPⅥ increased the MWT and TWL of SNI mice and decreased the level of P2X3 receptors in the dorsal root ganglion (DRG) and spinal cord (SC). Additionally, PPⅥ reduced the release of pro-inflammatory mediators (TNF-α, IL-1β, and IL-6) in the DRG, SC, and serum. Based on the KEGG enrichment of differentially expressed genes (DEGs) identified by RNA-Seq, PPVI may relieve NPP by regulating the AMPK/NF-κB signaling pathway. Western blotting results showed that the AMPK signaling pathway was activated, followed by inhibition of the NF-κB signaling pathway.

CONCLUSION

PPⅥ increased the MWT and TWL of SNI mice maybe by inhibiting the expression of the P2X3 receptor and the release of inflammatory mediators. The properties of the analgesia of PPⅥ may be based on the AMPK/NF-κB pathway.

Infliximab alleviates memory impairment in rats with chronic pain by suppressing neuroinflammation and restoring hippocampal neurogenesis

Patients with chronic pain commonly report impaired memory. Increasing evidence has demonstrated that inhibition of neurogenesis by neuroinflammation plays a crucial role in chronic pain-associated memory impairments. There is currently a lack of treatment strategies for this condition. An increasing number of clinical trials have reported the therapeutic potential of anti-inflammatory therapies targeting tumour necrosis factor-α (TNF-α) for inflammatory diseases. The present study investigated whether infliximab alleviates chronic pain-associated memory impairments in rats with chronic constriction injury (CCI). We demonstrated that infliximab alleviated spatial memory impairment and hyperalgesia induced by CCI. Furthermore, infliximab inhibited the activation of hippocampal astrocytes and microglia and decreased the release of proinflammatory cytokines in CCI rats. Furthermore, infliximab reversed the decrease in the numbers of newborn neurons and mature neurons in the dentate gyrus (DG) caused by chronic pain. Our data provide evidence that infliximab alleviates chronic pain-associated memory impairments, suppresses neuroinflammation and restores hippocampal neurogenesis in a CCI model. These facts indicate that infliximab may be a potential therapeutic agent for the treatment of chronic pain and associated memory impairments.

Chronic pain accelerates cognitive impairment by reducing hippocampal neurogenesis may via CCL2/CCR2 signaling in APP/PS1 mice

Patients with chronic pain often have cognitive impairment; this is especially true in elderly patients with neurodegenerative diseases such as Alzheimer's disease (AD), but the mechanism underlying this association remains unclear. This was addressed in the present study by investigating the effect of chronic neuropathic pain on hippocampal neurogenesis and cognitive impairment using amyloid precursor protein/presenilin 1 (APP/PS1) double transgenic mice subjected to spared-nerve injury (SNI). The Von Frey test was performed to determine the mechanical threshold of mouse hind limbs after SNI. The Morris water maze test was used to evaluate spatial learning and memory. Doublecortin-positive (DCX+), 5-bromo-2'-deoxyuridine (BrdU)+, BrdU+/neuronal nuclei (NeuN)+, and C-C motif chemokine ligand 2 (CCL2)+ neurons in the dentate gyrus of the hippocampus were detected by immunohistochemistry and immunofluorescence analysis. CCL2 and C-C chemokine receptor type 2 (CCR2) protein levels in the mouse hippocampus were analyzed by western blotting. The results showed that APP/PS1 mice with chronic neuropathic pain induced by SNI had significant learning and memory impairment. This was accompanied by increased CCL2 and CCR2 expression and decreases in the number of DCX+, BrdU+, and BrdU+/NeuN+ neurons. These results suggest that chronic neuropathic pain is associated with cognitive impairment, which may be caused by CCL2/CCR2 signaling-mediated inhibition of hippocampal neurogenesis. Thus, therapeutic strategies that alleviate neuropathic pain can potentially slow cognitive decline in patients with AD and other neurodegenerative diseases.

Potential role of the hydroxyl carboxylic acid receptor type 2 (HCAR2) in microglia pathophysiology: A possible cross-talk with C-X-C chemokine receptor 1 (CXCR1)

Following insults or injury, microglia cells are activated contributing to the cytotoxic response or by promoting an immune-mediated damage resolution. Microglia cells express HCA2R, a hydroxy carboxylic acid (HCA) receptor, which has been shown to mediate neuroprotective and anti-inflammatory effects. In this study we found that HCAR2 expression levels were increased in cultured rat microglia cells after Lipopolysaccharide (LPS) exposure. In a similar fashion, the treatment with MK 1903, a potent full agonist of HCAR2, increased the receptor protein levels. Moreover, HCAR2 stimulation prevented i) cells viability ii) morphological activation iii) pro/anti-inflammatory mediators production in LPS-treated cells. Likewise, HCAR2 stimulation reduced the proinflammatory mediators mRNA expression induced by neuronal chemokine fractalkine (FKN), a neuronal derived chemokine activating its unique receptor, chemokine receptor 1 (CX3CR1) on microglia surface. Interestingly, electrophysiological recordings in vivo revealed that MK1903 was able to prevent the increase of the nociceptive neurons (NS) firing activity mediated by the spinal FKN application in healthy rats. Collectively, our data demonstrate that HCAR2 is functionally expressed in microglia, by showing its capability to shift microglia toward an anti-inflammatory phenotype. Moreover, we indicated the contribute of HCAR2 in the FKN signaling and suggested a possible HCAR2/CX3CR1 functional interaction. This study paves the way for further investigations aimed at understanding the role HCAR2 as potential target in neuroinflammation-based CNS disorders. This article is part of the Special Issue on "The receptor-receptor interaction as a new target for therapy".

Purinergic signaling: a potential therapeutic target for depression and chronic pain

The comorbid mechanism of depression and chronic pain has been a research hotspot in recent years. Until now, the role of purinergic signals in the comorbid mechanism of depression and chronic pain has not been fully understood. This review mainly summarizes the research results published in PubMed during the past 5 years and concludes that purinergic signaling is a potential therapeutic target for comorbid depression and chronic pain, and the purinergic receptors A1, A2A, P2X3, P2X4, and P2X7and P2Y₆, P2Y₁, and P2Y₁₂ may be important factors. The main potential pathways are as follows: A1 receptor-related G protein-dependent activation of introverted K⁺ channels (GIRKs), A2A receptor-related effects on the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) and MAPK/nuclear factor-κB (NF-κB) pathways, P2X3 receptor-related effects on dorsal root ganglia (DRG) excitability, P2X4 receptor-related effects on proinflammatory cytokines and inflammasome activation, P2X7 receptor-related effects on ion channels, the NLRP3 inflammasome and brain-derived neurotrophic factor (BDNF), and P2Y receptor-related effects on the phospholipase C (PLC)/inositol triphosphate (IP3)/Ca²⁺ signaling pathway. We hope that the conclusions of this review will provide key ideas for future research on the role of purinergic signaling in the comorbid mechanism of depression and chronic pain.

Astrocyte reactivity in the glia limitans superficialis of the rat medial prefrontal cortex following sciatic nerve injury

The glia limitans superficialis (GLS) on the rodent cortical surface consists of astrocyte bodies intermingled with their cytoplasmic processes. Many studies have observed astrocyte reactivity in the medial prefrontal cortex (mPFC) parenchyma induced by a peripheral nerve injury, while the response of GLS astrocytes is still not fully understood. The aim of our study was to identify the reactivity of rat GLS astrocytes in response to sciatic nerve compression (SNC) over different time periods. The alteration of GLS astrocyte reactivity was monitored using immunofluorescence (IF) intensities of glial fibrillary acidic protein (GFAP), glutamine synthetase (GS), and NFκBp65. Our results demonstrated that SNC induced GLS astrocyte reactivity seen as increased intensities of GFAP-IF, and longer extensions of cytoplasmic processes into lamina I. First significant increase of GFAP-IF was observed on post-operation day 7 (POD7) after SNC with further increases on POD14 and POD21. In contrast, dynamic alteration of the extension of cytoplasmic processes into lamina I was detected as early as POD1 and continued throughout the monitored survival periods of both sham and SNC operations. The reactivity of GLS astrocytes was not associated with their proliferation. In addition, GLS astrocytes also displayed a significant decrease in GS immunofluorescence (GS-IF) and NFκB immunofluorescence (NFκB-IF) in response to sham and SNC operation compared with naïve control rats. These results suggest that damaged peripheral tissues (following sham operation as well as peripheral nerve lesions) may induce significant changes in GLS astrocyte reactivity. The signaling mechanism from injured peripheral tissue and nerve remains to be elucidated.

Co-ultramicronized palmitoylethanolamide/luteolin normalizes GABAB-ergic activity and cortical plasticity in long COVID-19 syndrome

OBJECTIVE

Transcranial magnetic stimulation (TMS) studies showed that patients with cognitive dysfunction and fatigue after COVID-19 exhibit impaired cortical GABA_B-ergic activity, as revealed by reduced long-interval intracortical inhibition (LICI). Aim of this study was to test the effects of co-ultramicronized palmitoylethanolamide/luteolin (PEA-LUT), an endocannabinoid-like mediator able to enhance GABA-ergic transmission and to reduce neuroinflammation, on LICI.

METHODS

Thirty-nine patients (26 females, mean age 49.9 ± 11.4 years, mean time from infection 296.7 ± 112.3 days) suffering from persistent cognitive difficulties and fatigue after mild COVID-19 were randomly assigned to receive either PEA-LUT 700 mg + 70 mg or PLACEBO, administered orally bid for eight weeks. The day before (PRE) and at the end of the treatment (POST), they underwent TMS protocols to assess LICI. We further evaluate short-latency afferent inhibition (SAI) and long-term potentiation (LTP)-like cortical plasticity.

RESULTS

Patients treated with PEA-LUT but not with PLACEBO showed a significant increase of LICI and LTP-like cortical plasticity. SAI remained unaffected.

CONCLUSIONS

Eight weeks of treatment with PEA-LUT restore GABA_B activity and cortical plasticity in long Covid patients.

SIGNIFICANCE

This study confirms altered physiology of the motor cortex in long COVID-19 syndrome and indicates PEA-LUT as a candidate for the treatment of this post-viral condition.

A Systematic Review and Meta-Analysis on the Role of Nutraceuticals in the Management of Neuropathic Pain in In Vivo Studies

The control of neuropathic pain is a leading challenge in modern medicine. Traditional medicine has, for a long time, used natural compounds such as nutraceuticals for this purpose, and extensive evidence has supported their role in controlling oxidative stress and persistent pain-related inflammation. Nutraceuticals are natural products belonging to the food sector whose consumption could be related to physiological benefits. Indeed, they are used to improve health, prevent chronic diseases, and delay the aging process. Here, we report a systematic review and meta-analysis to provide a more comprehensive report on the use of nutraceuticals in neuropathic pain, including evaluating confounding factors. A search of the literature has been conducted on principal databases (PubMed, MEDLINE, EMBASE, and Web of Science) following the PRISMA statement, and we retrieved 484 articles, 12 of which were selected for the meta-analysis. The results showed that administration of natural drugs in animals with neuropathic pain led to a significant reduction in thermal hyperalgesia, measured in both the injured paw (SMD: 1.79; 95% CI: 1.41 to 2.17; p < 0.0001) and in the two paws (SMD: −1.74; 95% CI: −3.36 to −0.11; p = 0.036), as well as a reduction in mechanical allodynia and hyperalgesia (SMD: 1.95, 95% CI: 1.08 to 2.82; p < 0.001) when compared to controls. The results of the review indicate that nutraceutical compounds could be clinically relevant for managing persistent neuropathic pain.

A Possible Mechanism for Development of Working Memory Impairment in Male Mice Subjected to Inflammatory Pain

We studied the effects of inflammatory pain on working memory and correlated the pain effects with changes in dendritic spine density and glutamate signaling in the medial prefrontal cortex (mPFC) of male and female mice. Injection of Complete Freund's Adjuvant (CFA) into the hind paw modeled inflammatory pain. The CFA equally decreased the mechanical thresholds in both sexes. The density of dendritic spines, as a marker for neuronal input, increased on the dendrites of both, pyramidal cells and interneurons in males but only on the dendrites of interneurons in CFA injected females. Next, we injected virus with glutamate sensor (pAAV5.hSyn.iGluSnFr) into the mPFC and used fiber photometry to record glutamate signaling during Y-maze spontaneous alternations test, which is a test for working memory in rodents. The detected fluorescent signal was higher during correct alternations when compared to incorrect alternations in both sexes. The CFA injection did not change the pattern of glutamate fluorescence during the test but the female mice made fewer incorrect alternations than their male counterparts. Furthermore, while the CFA injection decreased the expression of the glutamate transporter VGlut1 on the soma of mPFC neurons in both sexes, the decrease was sex dependent. We concluded that inflammatory pain, which increases sensory input into the mPFC neurons, may impair working memory by altering the glutamate signaling. The glutamate deficit that develops as a result of the pain is more pronounced in male mice in comparison to female mice.

Muscarinic receptor regulation of chronic pain-induced atrial fibrillation

Atrial fibrillation (AF), one of the most common arrhythmias, is associated with chronic emotional disorder. Chronic pain represents a psychological instability condition related to cardiovascular diseases, but the mechanistic linkage connecting chronic pain to AF occurrence remains unknown. Wild-type C57BL/6J male mice were randomly divided into sham and chronic pain groups. Autonomic nerve remodeling was reflected by the increased atrial parasympathetic tension and muscarinic acetylcholine receptor M2 expression. AF susceptibility was assessed through transesophageal burst stimulation in combination with electrocardiogram recording and investigating AERP in Langendorff perfused hearts. Our results demonstrated the elevated protein expression of muscarinic acetylcholine receptor M2 in the atria of mice subjected to chronic pain stress. Moreover, chronic pain induced the increase of atrial PR interval, and atrial effective refractory periods as compared to the sham group, underlying the enhanced susceptibility of AF. Thus, autonomic cholinergic nerve may mediate mice AF in the setting of chronic pain.

Palmitoylethanolamide and Related ALIAmides for Small Animal Health: State of the Art

ALIAmides are a family of fatty acid amides whose name comes from their mechanism of action, i.e., the Autacoid Local Injury Antagonism (ALIA). Actually, the ALIAmide parent molecule, palmitoylethanolamide (PEA), is locally produced on demand from a cell membrane precursor in order to control immune-inflammatory cell responses, avert chronic non-resolving inflammation, and limit the resulting clinical signs. ALIAmide sister compounds, such as Adelmidrol and palmitoylglucosamine, share mechanisms of action with PEA and may also increase endogenous levels of PEA. Provided that their respective bioavailability is properly addressed (e.g., through decreasing the particle size through micronization), exogenously administered ALIAmides thus mimic or sustain the prohomeostatic functions of endogenous PEA. The aim of the present paper is to review the main findings on the use of ALIAmides in small animals as a tribute to the man of vision who first believed in this “according-to-nature” approach, namely Francesco della Valle. After briefly presenting some key issues on the molecular targets, metabolism, and pharmacokinetics of PEA and related ALIAmides, here we will focus on the preclinical and clinical studies performed in dogs and cats. Although more data are still needed, ALIAmides may represent a novel and promising approach to small animal health.

Synaptic Effects of Palmitoylethanolamide in Neurodegenerative Disorders

Increasing evidence strongly supports the key role of neuroinflammation in the pathophysiology of neurodegenerative diseases, such as Alzheimer’s disease, frontotemporal dementia, and amyotrophic lateral sclerosis. Neuroinflammation may alter synaptic transmission contributing to the progression of neurodegeneration, as largely documented in animal models and in patients’ studies. In the last few years, palmitoylethanolamide (PEA), an endogenous lipid mediator, and its new composite, which is a formulation constituted of PEA and the well-recognized antioxidant flavonoid luteolin (Lut) subjected to an ultra-micronization process (co-ultraPEALut), has been identified as a potential therapeutic agent in different disorders by exerting potential beneficial effects on neurodegeneration and neuroinflammation by modulating synaptic transmission. In this review, we will show the potential therapeutic effects of PEA in animal models and in patients affected by neurodegenerative disorders.

The Beneficial Effects of Ultramicronized Palmitoylethanolamide in the Management of Neuropathic Pain and Associated Mood Disorders Induced by Paclitaxel in Mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a common complication of antineoplastic drugs, particularly paclitaxel (PTX). It can affect the quality of patients’ lives and increase the risk of developing mood disorders. Although several drugs are recommended, they yielded inconclusive results in clinical trials. The aim of the present work is to investigate whether the palmitoylethanolamide (PEA) would reduce PTX-induced CIPN and associated mood disorders. Moreover, the role PPAR-α and the endocannabinoid system will also be investigated. CIPN was induced by intraperitoneally injection of PTX (8 mg/kg) every other day for a week. PEA, 30 mg/kg, was orally administrated in a bioavailable form (i.e., ultramicronized PEA, um-PEA) one hour after the last PTX injection, for 7 days. In the antagonism experiments, AM281 (1 mg/kg) and GW6471 (2 mg/kg) were administrated 30 min before um-PEA. Our results demonstrated that um-PEA reduced the development of hypersensitivity with the effect being associated with the reduction in spinal and hippocampal pro-inflammatory cytokines, as well as antidepressive and anxiolytic effects. Moreover, the PPAR-α and CB1 receptor antagonists blocked the behavioral and antinociceptive effects of um-PEA. Our findings suggest that um-PEA is a promising adjunct in CIPN and associated mood disorders through the activation of PPAR-α, which influences the endocannabinoid system.

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.

Influence of Cerebral Glucose Metabolism by Chronic Pain-Mediated Cognitive Impairment in Adolescent Rats

Chronic pain during adolescence can lead to mental health disorders in adulthood, but the underlying mechanism is still unclear. Furthermore, the homeostasis of cerebral glucose metabolism and neurotransmitter metabolic kinetics are closely associated with cognitive development and pain progression. The present study investigated changes in cognitive function and glucose metabolism in adult rats, which had experienced chronic pain during their adolescence. Here, spared nerve injury (SNI) surgery was conducted in 4-week-old male rats. Mechanical nociceptive reflex thresholds were analyzed, and SNI chronic pain (SNI-CP) animals were screened. Based on animal behavioral tests (open field, three-chambered social, novel object recognition and the Y maze), the SNI-CP animals showed learning and memory impairment and anxiety-like behaviors, compared to SNI no chronic pain (SNI-NCP) animals. The cerebral glucose metabolism in the prefrontal cortex and hippocampus of adult SNI-CP animals was decreased with positron emission tomography/computed tomography. GABA₂ and Glu₄ levels in the metabolic kinetics study were significantly decreased in the hippocampus, frontal cortex, and temporal cortex, and the expression of GLUT3 and GLUT4 was also significantly downregulated in the prefrontal cortex and hippocampus of adult rats in the SNI-CP group. These findings suggest that the rats which suffered chronic pain during adolescence have lower cerebral glucose metabolism in the cortex and hippocampus, which could be related to cognitive function during the development of the central nervous system.

MED1/BDNF/TrkB pathway is involved in thalamic hemorrhage-induced pain and depression by regulating microglia

Central post-stroke pain (CPSP) and associated depression remain poorly understood and pharmacological treatments are unsatisfactory. Recently, microglia activation was suggested to be involved in CPSP pathophysiology. The goal of this study was to investigate the effectiveness of a co-ultramicronized combination of N-palmitoylethanolamide and luteolin (PEALut) in a mouse model of thalamic hemorrhage (TH)-induced CPSP. TH was established through the collagenase-IV injection in thalamic ventral-posterolateral-nucleus. PEALut effects in CPSP-associated behaviors were evaluated during a 28-days observation period. We found that repeated administrations of co-ultra PEALut significantly reduced mechanical hypersensitivity after TH, as compared to vehicle, by reducing the early microglial activation in the perilesional site. Moreover, PEALut prevented the development of depressive-like behavior (21 days post-TH). These effects were associated with the restoration of synaptic plasticity in LEC-DG pathway and monoamines levels found impaired in TH mice. Hippocampal MED1 and TrkB expressions were significantly increased in TH compared to sham mice 21 days post-TH, whereas BDNF levels were decreased. PEALut restored MED1/TrkB/BDNF expression in mice. Remarkably, we found significant overexpression of MED1 in the human autoptic brain specimens after stroke, indicating a translational potential of our findings. These results pave the way for better-investigating depression in TH- induced CPSP, together with the involvement of MED1/TrkB/BDNF pathway, proposing PEALut as an adjuvant treatment.

Disruption to normal excitatory and inhibitory function within the medial prefrontal cortex in people with chronic pain

BACKGROUND

Growing evidence indicates a link between changes in the medial prefrontal cortex and the pathophysiology of chronic pain. In particular, chronic pain is associated with altered medial prefrontal anatomy and biochemistry. Due to the comorbid affective disorders seen across all pain conditions, the medial prefrontal cortex is a region of significance as it is involved in emotional processing. We have recently reported that a decrease in medial prefrontal N-acetylaspartate and glutamate is associated with increased emotional dysregulation, indicating there are neurotransmitter imbalances in chronic pain. Therefore, we compared medial prefrontal neurochemistry in 24 people with chronic pain conditions to 24 age and sex-matched healthy controls with no history of chronic pain.

METHOD

GABA-edited MEGA-PRESS was used to measure GABA⁺ levels, and short TE PRESS was used to measure glutamate levels in the medial prefrontal cortex. Psychometric measures regarding pain intensity a week before scanning, during the scan and the total duration of chronic pain, were also recorded and compared to measured GABA⁺ and glutamate levels.

RESULTS

This study reveals that the presence of chronic pain is associated with significant decreases in medial prefrontal GABA⁺ and glutamate. These findings support the hypothesis that chronic pain is associated with altered medial prefrontal biochemistry.

CONCLUSION

The dysregulation of glutamatergic and GABAergic neurotransmitter systems supports a model of disinhibition of chronic pain, which may play a key role in both the experience of persistent pain and its associated affective disturbances.

SIGNIFICANCE

This study reveals a significant reduction in γ-aminobutyric acid (GABA⁺ ) and glutamate within the medial prefrontal cortex in chronic pain sufferers. While the current findings should be considered with reference to a small sample size, the disruption to normal excitatory and inhibitory medial prefrontal cortex function may be key in the development and maintenance of chronic pain and comorbid mental health disorders.

Pharmacological Blockade of PPARα Exacerbates Inflammatory Pain-Related Impairment of Spatial Memory in Rats

Peroxisome proliferator-activated receptors (PPARs) are ligand-dependent transcription factors that exist in three isoforms: PPARα, PPARβ/δ and PPARγ. Studies suggest that the PPAR signalling system may modulate pain, anxiety and cognition. The aim of the present study was to investigate whether endogenous signalling via PPARs differentially modulates innate anxiety responses and mnemonic function in the presence and absence of inflammatory pain. We examined the effects of intraperitoneal administration of GW6471 (PPARα antagonist), GSK0660 (PPARβ/δ antagonist), GW9662 (PPARγ antagonist), and N-palmitoylethanolamide (PEA) on rat behaviour in the elevated plus maze (EPM), open field (OF), light-dark box (LDB), and novel object recognition (NOR) tests in the presence or absence of chronic inflammatory pain. Complete Freund’s Adjuvant (CFA)-injected rats exhibited impaired recognition and spatial mnemonic performance in the NOR test and pharmacological blockade of PPARα further impaired spatial memory in CFA-treated rats. N-oleoylethanolamide (OEA) levels were higher in the dorsal hippocampus in CFA-injected animals compared to their counterparts. The results suggest a modulatory effect of CFA-induced chronic inflammatory pain on cognitive processing, but not on innate anxiety-related responses. Increased OEA-PPARα signalling may act as a compensatory mechanism to preserve spatial memory function following CFA injection.

Vitamin D Deficiency Induces Chronic Pain and Microglial Phenotypic Changes in Mice

The bioactive form of vitamin D, 1,25-dihydroxyvitamin D (1,25D3), exerts immunomodulatory actions resulting in neuroprotective effects potentially useful against neurodegenerative and autoimmune diseases. In fact, vitamin D deficiency status has been correlated with painful manifestations associated with different pathological conditions. In this study, we have investigated the effects of vitamin D deficiency on microglia cells, as they represent the main immune cells responsible for early defense at central nervous system (CNS), including chronic pain states. For this purpose, we have employed a model of low vitamin D intake during gestation to evaluate possible changes in primary microglia cells obtained from postnatal day(P)2-3 pups. Afterwards, pain measurement and microglia morphological analysis in the spinal cord level and in brain regions involved in the integration of pain perception were performed in the parents subjected to vitamin D restriction. In cultured microglia, we detected a reactive-activated and proliferative-phenotype associated with intracellular reactive oxygen species (ROS) generation. Oxidative stress was closely correlated with the extent of DNA damage and increased β-galactosidase (B-gal) activity. Interestingly, the incubation with 25D3 or 1,25D3 or palmitoylethanolamide, an endogenous ligand of peroxisome proliferator-activated-receptor-alpha (PPAR-α), reduced most of these effects. Morphological analysis of ex-vivo microglia obtained from vitamin-D-deficient adult mice revealed an increased number of activated microglia in the spinal cord, while in the brain microglia appeared in a dystrophic phenotype. Remarkably, activated (spinal) or dystrophic (brain) microglia were detected in a prominent manner in females. Our data indicate that vitamin D deficiency produces profound modifications in microglia, suggesting a possible role of these cells in the sensorial dysfunctions associated with hypovitaminosis D.

Chronic Pain in Dogs and Cats: Is There Place for Dietary Intervention with Micro-Palmitoylethanolamide?

The management of chronic pain is an integral challenge of small animal veterinary practitioners. Multiple pharmacological agents are usually employed to treat maladaptive pain including opiates, non-steroidal anti-inflammatory drugs, anticonvulsants, antidepressants, and others. In order to limit adverse effects and tolerance development, they are often combined with non-pharmacologic measures such as acupuncture and dietary interventions. Accumulating evidence suggests that non-neuronal cells such as mast cells and microglia play active roles in the pathogenesis of maladaptive pain. Accordingly, these cells are currently viewed as potential new targets for managing chronic pain. Palmitoylethanolamide is an endocannabinoid-like compound found in several food sources and considered a body's own analgesic. The receptor-dependent control of non-neuronal cells mediates the pain-relieving effect of palmitoylethanolamide. Accumulating evidence shows the anti-hyperalgesic effect of supplemented palmitoylethanolamide, especially in the micronized and co-micronized formulations (i.e., micro-palmitoylethanolamide), which allow for higher bioavailability. In the present paper, the role of non-neuronal cells in pain signaling is discussed and a large number of studies on the effect of palmitoylethanolamide in inflammatory and neuropathic chronic pain are reviewed. Overall, available evidence suggests that there is place for micro-palmitoylethanolamide in the dietary management of chronic pain in dogs and cats.

Neuropathic pain-induced cognitive dysfunction and down-regulation of neuronal pentraxin 2 in the cortex and hippocampus

Evidence from both basic and clinical science suggests that neuropathic pain can induce cognitive dysfunction. However, these results are mainly based on a series of behavioral tests, there is a lack of quantitative variables to indicate cognitive impairment. Neuronal activity-regulated pentraxin (NPTX2) is a ubiquitously expressed, secreted protein in the nervous system. NPTX2 has been implicated to be involved in a variety of neuropathic diseases including Parkinson's disease, ischemia, and Alzheimer's disease. In a mouse model of chronic pain, NPTX2 is involved in the regulation of inflammatory responses. Here, we employ a variety of behavioral approaches to demonstrate that mice with chronic neuropathic pain have cognitive impairment and exhibit an increased anxiety response. The expression of NPTX2, but not NPTX1, was down-regulated in the hippocampus and cortex after chronic neuropathic pain exposure. The modulation effect of NPTX2 on cognitive function was also verified by behavioral tests using Nptx2 knock-out mice. Above all, we conclude that downregulation of NPTX2 induced by neuropathic pain may serve as an indicator of a progressive cognitive dysfunction during the induction and maintenance of spared nerve injury.

N-palmitoyl-D-glucosamine, a Natural Monosaccharide-Based Glycolipid, Inhibits TLR4 and Prevents LPS-Induced Inflammation and Neuropathic Pain in Mice

Toll-like receptors (TLRs) are key receptors through which infectious and non-infectious challenges act with consequent activation of the inflammatory cascade that plays a critical function in various acute and chronic diseases, behaving as amplification and chronicization factors of the inflammatory response. Previous studies have shown that synthetic analogues of lipid A based on glucosamine with few chains of unsaturated and saturated fatty acids, bind MD-2 and inhibit TLR4 receptors. These synthetic compounds showed antagonistic activity against TLR4 activation in vitro by LPS, but little or no activity in vivo. This study aimed to show the potential use of N-palmitoyl-D-glucosamine (PGA), a bacterial molecule with structural similarity to the lipid A component of LPS, which could be useful for preventing LPS-induced tissue damage or even peripheral neuropathies. Molecular docking and molecular dynamics simulations showed that PGA stably binds MD-2 with a MD-2/(PGA)3 stoichiometry. Treatment with PGA resulted in the following effects: (i) it prevented the NF-kB activation in LPS stimulated RAW264.7 cells; (ii) it decreased LPS-induced keratitis and corneal pro-inflammatory cytokines, whilst increasing anti-inflammatory cytokines; (iii) it normalized LPS-induced miR-20a-5p and miR-106a-5p upregulation and increased miR-27a-3p levels in the inflamed corneas; (iv) it decreased allodynia in peripheral neuropathy induced by oxaliplatin or formalin, but not following spared nerve injury of the sciatic nerve (SNI); (v) it prevented the formalin- or oxaliplatin-induced myelino-axonal degeneration of sciatic nerve. SIGNIFICANCE STATEMENT We report that PGA acts as a TLR4 antagonist and this may be the basis of its potent anti-inflammatory activity. Being unique because of its potency and stability, as compared to other similar congeners, PGA can represent a tool for the optimization of new TLR4 modulating drugs directed against the cytokine storm and the chronization of inflammation.

Development of depression-like behavior and altered hippocampal neurogenesis in a mouse model of chronic neuropathic pain

Chronic-pain patients often suffer from depression. In rodent models of neuropathic pain, animals develop depression-like and anxiety behaviors, indicating a relationship between chronic pain and affective disorders. However, the underlying neurobiological mechanisms linking chronic pain and depression are not yet fully understood. Neurogenesis in the hippocampus is a fundamental process related to brain plasticity. Reduced neurogenesis has been associated with the development of mood disorders and cognitive impairments. The current study aims to elucidate the underlying long-term changes in brain plasticity induced by neuropathic pain in mice at a time point when depression-like behavior has already developed. Furthermore, our focus is set on alterations in neurogenesis in the hippocampus. We found that manifestation of anxiety- and depressive-like behavior as well as cognitive impairment co-occur with decreased survival of newly generated cells but not with impaired proliferative activity or reduced number of immature neurons in the dentate gyrus area of the hippocampus. Moreover, we detected an impairment of differentiation of newly generated cells into mature calbindin-positive neurons, accompanied with a shift towards increased differentiation into astroglial cells. These findings indicate that a reduction in mature functional neurons, rather than reduced proliferation or neuronal progenitor cells, are the long-term changes in hippocampal plasticity that manifest in neuropathic pain conditions after depression-like behavior has developed.

Activation of the Notch signaling pathway in the anterior cingulate cortex is involved in the pathological process of neuropathic pain

Plastic changes in the anterior cingulate cortex (ACC) are critical in pain hypersensitivity caused by peripheral nerves injury. The Notch signaling pathway has been shown to regulate synaptic differentiation and transmission. Therefore, this study was to investigate the function of the Notch signaling pathway in the ACC during nociceptive transmission induced by neuropathic pain. We adopted Western blotting, N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT) microinjections, RNA interference targeting Notch1, Hairy and enhancer of split (Hes) 1 or Hes5, electrophysiological recordings, and behavioral tests to verify the link between Notch signaling in ACC and neuropathic pain with adult male Sprague-Dawley rats. Levels of the Notch intracellular domain were increased in ACC on day 7 after chronic constriction injury surgery or spared nerve injury. Meanwhile, the mRNA level of the downstream effector of Notch signaling Hes1 was increased, whereas the level of Hes5 mRNA did not change. Microinjection of DAPT, a γ-secretase (a key enzyme involved in Notch pathway) inhibitor, into ACC significantly reversed neuropathic pain behaviors. Intra-ACC injection of short hairpin RNA-Notch reduced Notch intracellular domain expression and decreased the potentiation of synaptic transmission in the ACC. Moreover, pain perceptions were also alleviated in rats subjected to chronic constriction injury or spared nerve injury. This process was mainly mediated by the downstream effector Hes1, but not Hes5. Based on these results, the activation of the Notch/Hes1 signaling pathway in the ACC participates in the development of neuropathic pain, indicating that the Notch pathway may be a new therapeutic target for treating chronic pain.

Palmitoylethanolamide: A Nutritional Approach to Keep Neuroinflammation within Physiological Boundaries-A Systematic Review

Neuroinflammation is a physiological response aimed at maintaining the homodynamic balance and providing the body with the fundamental resource of adaptation to endogenous and exogenous stimuli. Although the response is initiated with protective purposes, the effect may be detrimental when not regulated. The physiological control of neuroinflammation is mainly achieved via regulatory mechanisms performed by particular cells of the immune system intimately associated with or within the nervous system and named "non-neuronal cells." In particular, mast cells (within the central nervous system and in the periphery) and microglia (at spinal and supraspinal level) are involved in this control, through a close functional relationship between them and neurons (either centrally, spinal, or peripherally located). Accordingly, neuroinflammation becomes a worsening factor in many disorders whenever the non-neuronal cell supervision is inadequate. It has been shown that the regulation of non-neuronal cells-and therefore the control of neuroinflammation-depends on the local "on demand" synthesis of the endogenous lipid amide Palmitoylethanolamide and related endocannabinoids. When the balance between synthesis and degradation of this bioactive lipid mediator is disrupted in favor of reduced synthesis and/or increased degradation, the behavior of non-neuronal cells may not be appropriately regulated and neuroinflammation exceeds the physiological boundaries. In these conditions, it has been demonstrated that the increase of endogenous Palmitoylethanolamide-either by decreasing its degradation or exogenous administration-is able to keep neuroinflammation within its physiological limits. In this review the large number of studies on the benefits derived from oral administration of micronized and highly bioavailable forms of Palmitoylethanolamide is discussed, with special reference to neuroinflammatory disorders.

New Evidence on the Role of D-Aspartate Metabolism in Regulating Brain and Endocrine System Physiology: From Preclinical Observations to Clinical Applications

The endogenous amino acids serine and aspartate occur at high concentrations in free D-form in mammalian organs, including the central nervous system and endocrine glands. D-serine (D-Ser) is largely localized in the forebrain structures throughout pre and postnatal life. Pharmacologically, D-Ser plays a functional role by acting as an endogenous coagonist at N-methyl-D-aspartate receptors (NMDARs). Less is known about the role of free D-aspartate (D-Asp) in mammals. Notably, D-Asp has a specific temporal pattern of occurrence. In fact, free D-Asp is abundant during prenatal life and decreases greatly after birth in concomitance with the postnatal onset of D-Asp oxidase expression, which is the only enzyme known to control endogenous levels of this molecule. Conversely, in the endocrine system, D-Asp concentrations enhance after birth during its functional development, thereby suggesting an involvement of the amino acid in the regulation of hormone biosynthesis. The substantial binding affinity for the NMDAR glutamate site has led us to investigate the in vivo implications of D-Asp on NMDAR-mediated responses. Herein we review the physiological function of free D-Asp and of its metabolizing enzyme in regulating the functions of the brain and of the neuroendocrine system based on recent genetic and pharmacological human and animal studies.

Altered thalamic neurotransmitters metabolism and functional connectivity during the development of chronic constriction injury induced neuropathic pain

Background

To investigate the thalamic neurotransmitters and functional connections in the development of chronic constriction injury (CCI)-induced neuropathic pain.

Methods

The paw withdrawal threshold was measured by mechanical stimulation the right hind paw with the von frey hair in the rats of CCI-induced neuropathic pain. The N-acetylaspartate (NAA) and Glutamate (Glu) in thalamus were detected by magnetic resonance spectrum (MRS) process. The thalamic functional connectivity with other brain regions was scanned by functional magnetic resonance image (fMRI).

Results

The paw withdrawal threshold of the ipsilateral side showed a noticeable decline during the pathological process. Increased concentrations of Glu and decreased levels of NAA in the thalamus were significantly correlated with mechanical allodynia in the neuropathic pain states. The thalamic regional homogeneity (ReHo) decreased during the process of neuropathic pain. The functional connectivity among the thalamus with the insula and somatosensory cortex were significantly increased at different time points (7, 14, 21 days) after CCI surgery.

Conclusion

Our study suggests that dynamic changes in thalamic NAA and Glu levels contribute to the thalamic functional connection hyper-excitation during CCI-induced neuropathic pain. Enhanced thalamus-insula functional connection might have a significant effect on the occurrence of neuropathic pain.

ALIAmides Update: Palmitoylethanolamide and Its Formulations on Management of Peripheral Neuropathic Pain

Neuropathic pain results from lesions or diseases of the somatosensory nervous system and it remains largely difficult to treat. Peripheral neuropathic pain originates from injury to the peripheral nervous system (PNS) and manifests as a series of symptoms and complications, including allodynia and hyperalgesia. The aim of this review is to discuss a novel approach on neuropathic pain management, which is based on the knowledge of processes that underlie the development of peripheral neuropathic pain; in particular highlights the role of glia and mast cells in pain and neuroinflammation. ALIAmides (autacoid local injury antagonist amides) represent a group of endogenous bioactive lipids, including palmitoylethanolamide (PEA), which play a central role in numerous biological processes, including pain, inflammation, and lipid metabolism. These compounds are emerging thanks to their anti-inflammatory and anti-hyperalgesic effects, due to the down-regulation of activation of mast cells. Collectively, preclinical and clinical studies support the idea that ALIAmides merit further consideration as therapeutic approach for controlling inflammatory responses, pain, and related peripheral neuropathic pain.

A New Palmitoylethanolamide Form Combined with Antioxidant Molecules to Improve Its Effectivess on Neuronal Aging

Palmitoylethanolamide is a nutraceutical compound naturally produced in many plants and animal source foods, but the natural form is poorly water-soluble. It has demonstrated an anti-inflammatory role as a neuroprotective mediator, acting on several molecular targets of the central nervous system involved on brain aging process. In healthy adults, palmitoylethanolamide is an endogenous PPAR-α (peroxisome proliferator-activated receptor α) agonist through which it performs anti-inflammatory activity and provides its effects by activating the cannabinoid receptor. The different formulations of palmitoylethanolamide (micronized palmitoylethanolamide, FM-LipoMatrix^® palmitoylethanolamide and FM-LipoMatrix^® palmitoylethanolamide plus lipoic acid and vitamin D3) were analyzed starting from intestinal barrier, to verify their bioavailability, to in primary astrocytes in which cell viability, reactive oxygen species (ROS) and nitric oxide (NO) production, NFKB activity, MAPK, p53 and PPARα activities were investigated. Additionally, cannabinoid and estrogen receptors were analyzed using the western blot technique. The combination of palmitoylethanolamide in FM-LipoMatrix^®, lipoic acid and vitamin D3 shows better absorption predicting an improvement on plasma concentration; this formulation also shows a reduction in ROS and NO production and the data show the interaction of palmitoylethanolamide with cannabinoids and estrogen receptors inhibiting neuroinflammatory markers. All these data support the hypothesis of a new potential strategy to restore brain function and slow down brain aging in humans.

Palmitoylethanolamide and Related ALIAmides: Prohomeostatic Lipid Compounds for Animal Health and Wellbeing

Virtually every cellular process is affected by diet and this represents the foundation of dietary management to a variety of small animal disorders. Special attention is currently being paid to a family of naturally occurring lipid amides acting through the so-called autacoid local injury antagonism, i.e., the ALIA mechanism. The parent molecule of ALIAmides, palmitoyl ethanolamide (PEA), has being known since the 1950s as a nutritional factor with protective properties. Since then, PEA has been isolated from a variety of plant and animal food sources and its proresolving function in the mammalian body has been increasingly investigated. The discovery of the close interconnection between ALIAmides and the endocannabinoid system has greatly stimulated research efforts in this field. The multitarget and highly redundant mechanisms through which PEA exerts prohomeostatic functions fully breaks with the classical pharmacology view of “one drug, one target, one disease”, opening a new era in the management of animals’ health, i.e., an according-to-nature biomodulation of body responses to different stimuli and injury. The present review focuses on the direct and indirect endocannabinoid receptor agonism by PEA and its analogues and also targets the main findings from experimental and clinical studies on ALIAmides in animal health and wellbeing.

Microglia dependent BDNF and proBDNF can impair spatial memory performance during persistent inflammatory pain

Inflammatory pain is commonly associated with cognitive impairment. However, its molecular mechanisms are poorly understood. Thus, this study was conducted to investigate the molecular mechanisms of behavioral changes associated with inflammatory pain. Briefly, 36 Wistar rats were randomly divided into two main groups: CFA group treated with 100 μL of Complete Freunds' Adjuvant (CFA) and CFA + Minocycline group treated with 100 μL of CFA+40 mg/kg/day of minocycline). After that, each group was divided into three subgroups based on different time points of the study. The pain was induced using CFA and subsequent behavioral changes (i.e., hyperalgesia and learning and spatial memory) were analyzed by the Morris Water Maze (MWM) task and Radiant Heat. Then, the cellular and molecular changes were assessed using Western Blotting, Immunohistochemistry, and Terminal deoxynucleotidyl transferase dUTP Nick End Labeling (TUNEL) techniques. Results of the study indicated that CFA-induced pain impaired spatial learning and memory functions. Studying the cellular changes showed that persistent inflammatory pain increased the microglial activity in CA1 and Dentate Gyrus (DG) regions. Furthermore, an increase was observed in the percentage of TUNEL-positive cells. Also, pro-Brain-Derived Neurotrophic Factor (BDNF)/BDNF ratio, Caspase3, and Receptor-Interacting Protein kinase 3 (RIP3) levels increased in the rats' hippocampus following induction of persistent inflammatory pain. These changes were reversed following the cessation of pain as well as the injection of minocycline. Taking together, the results of the current study for the first time revealed that an increase in the microglia dependent proBDNF/BDNF ratio following persistent inflammatory pain leads to cell death of the CA1 and DG neurons that subsequently causes a cognitive deficit in the learning and spatial memory functions.

The Medial Prefrontal Cortex as a Central Hub for Mental Comorbidities Associated with Chronic Pain

Chronic pain patients frequently develop and suffer from mental comorbidities such as depressive mood, impaired cognition, and other significant constraints of daily life, which can only insufficiently be overcome by medication. The emotional and cognitive components of pain are processed by the medial prefrontal cortex, which comprises the anterior cingulate cortex, the prelimbic, and the infralimbic cortex. All three subregions are significantly affected by chronic pain: magnetic resonance imaging has revealed gray matter loss in all these areas in chronic pain conditions. While the anterior cingulate cortex appears hyperactive, prelimbic, and infralimbic regions show reduced activity. The medial prefrontal cortex receives ascending, nociceptive input, but also exerts important top-down control of pain sensation: its projections are the main cortical input of the periaqueductal gray, which is part of the descending inhibitory pain control system at the spinal level. A multitude of neurotransmitter systems contributes to the fine-tuning of the local circuitry, of which cholinergic and GABAergic signaling are particularly emerging as relevant components of affective pain processing within the prefrontal cortex. Accordingly, factors such as distraction, positive mood, and anticipation of pain relief such as placebo can ameliorate pain by affecting mPFC function, making this cortical area a promising target region for medical as well as psychosocial interventions for pain therapy.

Behavioral, Biochemical and Electrophysiological Changes in Spared Nerve Injury Model of Neuropathic Pain

Neuropathic pain is a pathological condition induced by a lesion or disease affecting the somatosensory system, with symptoms like allodynia and hyperalgesia. It has a multifaceted pathogenesis as it implicates several molecular signaling pathways involving peripheral and central nervous systems. Affective and cognitive dysfunctions have been reported as comorbidities of neuropathic pain states, supporting the notion that pain and mood disorders share some common pathogenetic mechanisms. The understanding of these pathophysiological mechanisms requires the development of animal models mimicking, as far as possible, clinical neuropathic pain symptoms. Among them, the Spared Nerve Injury (SNI) model has been largely characterized in terms of behavioral and functional alterations. This model is associated with changes in neuronal firing activity at spinal and supraspinal levels, and induces late neuropsychiatric disorders (such as anxious-like and depressive-like behaviors, and cognitive impairments) comparable to an advanced phase of neuropathy. The goal of this review is to summarize current findings in preclinical research, employing the SNI model as a tool for identifying pathophysiological mechanisms of neuropathic pain and testing pharmacological agent.

Altered gut microbiota and endocannabinoid system tone in vitamin D deficiency-mediated chronic pain

Recent evidence points to the gut microbiota as a regulator of brain and behavior, although it remains to be determined if gut bacteria play a role in chronic pain. The endocannabinoid system is implicated in inflammation and chronic pain processing at both the gut and central nervous system (CNS) levels. In the present study, we used low Vitamin D dietary intake in mice and evaluated possible changes in gut microbiota, pain processing and endocannabinoid system signaling. Vitamin D deficiency induced a lower microbial diversity characterized by an increase in Firmicutes and a decrease in Verrucomicrobia and Bacteroidetes. Concurrently, vitamin D deficient mice showed tactile allodynia associated with neuronal hyperexcitability and alterations of endocannabinoid system members (endogenous mediators and their receptors) at the spinal cord level. Changes in endocannabinoid (anandamide and 2-arachidonoylglycerol) levels were also observed in the duodenum and colon. Remarkably, the anti-inflammatory anandamide congener, palmitoylethanolamide, counteracted both the pain behaviour and spinal biochemical changes in vitamin D deficient mice, whilst increasing the levels of Akkermansia, Eubacterium and Enterobacteriaceae, as compared with vehicle-treated mice. Finally, induction of spared nerve injury in normal or vitamin D deficient mice was not accompanied by changes in gut microbiota composition. Our data suggest the existence of a link between Vitamin D deficiency - with related changes in gut bacterial composition - and altered nociception, possibly via molecular mechanisms involving the endocannabinoid and related mediator signaling systems.

Pharmacological Blockade of PPAR Isoforms Increases Conditioned Fear Responding in the Presence of Nociceptive Tone

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors with three isoforms (PPARα, PPARβ/δ, PPARγ) and can regulate pain, anxiety, and cognition. However, their role in conditioned fear and pain-fear interactions has not yet been investigated. Here, we investigated the effects of systemically administered PPAR antagonists on formalin-evoked nociceptive behaviour, fear-conditioned analgesia (FCA), and conditioned fear in the presence of nociceptive tone in rats. Twenty-three and a half hours following fear conditioning to context, male Sprague-Dawley rats received an intraplantar injection of formalin and intraperitoneal administration of vehicle, PPARα (GW6471), PPARβ/δ (GSK0660) or PPARγ (GW9662) antagonists, and 30 min later were re-exposed to the conditioning arena for 15 min. The PPAR antagonists did not alter nociceptive behaviour or fear-conditioned analgesia. The PPARα and PPARβ/δ antagonists prolonged context-induced freezing in the presence of nociceptive tone without affecting its initial expression. The PPARγ antagonist potentiated freezing over the entire trial. In conclusion, pharmacological blockade of PPARα and PPARβ/δ in the presence of formalin-evoked nociceptive tone, impaired short-term, within-trial fear-extinction in rats without affecting pain response, while blockade of PPARγ potentiated conditioned fear responding. These results suggest that endogenous signalling through these three PPAR isoforms may reduce the expression of conditioned fear in the presence of nociceptive tone.

Reduced Glutamate in the Medial Prefrontal Cortex Is Associated With Emotional and Cognitive Dysregulation in People With Chronic Pain

A decrease in glutamate in the medial prefrontal cortex (mPFC) has been extensively found in animal models of chronic pain. Given that the mPFC is implicated in emotional appraisal, cognition and extinction of fear, could a potential decrease in glutamate be associated with increased pessimistic thinking, fear and worry symptoms commonly found in people with chronic pain? To clarify this question, 19 chronic pain subjects and 19 age- and gender-matched control subjects without pain underwent magnetic resonance spectroscopy. Both groups also completed the Temperament and Character, the Beck Depression and the State Anxiety Inventories to measure levels of harm avoidance, depression, and anxiety, respectively. People with chronic pain had significantly higher scores in harm avoidance, depression and anxiety compared to control subjects without pain. High levels of harm avoidance are characterized by excessive worry, pessimism, fear, doubt and fatigue. Individuals with chronic pain showed a significant decrease in mPFC glutamate levels compared to control subjects without pain. In people with chronic pain mPFC glutamate levels were significantly negatively correlated with harm avoidance scores. This means that the lower the concentration of glutamate in the mPFC, the greater the total scores of harm avoidance. High scores are associated with fearfulness, pessimism, and fatigue-proneness. We suggest that chronic pain, particularly the stress-induced release of glucocorticoids, induces changes in glutamate transmission in the mPFC, thereby influencing cognitive, and emotional processing. Thus, in people with chronic pain, regulation of fear, worry, negative thinking and fatigue is impaired.

Biomarkers of response to alpha-lipoic acid ± palmitoiletanolamide treatment in patients with diabetes and symptoms of peripheral neuropathy

PURPOSE

To evaluate the effect of oral alpha-lipoic acid (ALA) ± palmitoyl-ethanolamide (PEA) on neuropathic symptoms in patients with diabetic peripheral neuropathy (DPN) and to identify factors related to the efficacy of the treatment.

METHODS

This is a retrospective observational pilot study evaluating 49 patients with diabetes and positive Neuropathy Symptoms Score (NSS). Clinical and biochemical variables, including NSS, were compared between untreated patients and patients treated with oral 600 mg/day ALA ± 600 mg/day PEA at baseline (first occurrence of NSS ≥ 3) and at least 2 months after baseline. Number of days between treatment initiation and symptoms' relief and related factors were also investigated.

RESULTS

Thirty subjects were treated with ALA ± PEA and 19 subjects did not receive any specific treatment for neuropathy symptoms. Follow-up visits occurred after 98 ± 46 days. NSS significantly decreased in patients treated with ALA ± PEA (5.4 ± 1.3 at baseline vs. 1.7 ± 2.4 at follow-up, p < 0.001), but not in untreated patients (p = 0.164). Subjects treated with ALA ± PEA reported a mean time from treatment initiation to symptoms' relief of 18.4 ± 9.0 days. The number of days of treatment needed for symptoms' relief was inversely related to HDL-cholesterol levels (r = -0.503, p = 0.010) and to eGFR (r = -0.428, p = 0.033), whereas there was no significant relationship between time to symptoms' relief and age, HbA1c, lipid profile and the severity of symptoms at baseline.

CONCLUSIONS

This study documents that oral administration of ALA ± PEA helps in controlling neuropathy symptoms in diabetes. Moreover, our data show that higher HDL-c levels and better renal function are associated to a faster therapeutic effect, suggesting them as biomarkers of response to therapy with ALA ± PEA.

Acupuncture Improves Comorbid Cognitive Impairments Induced by Neuropathic Pain in Mice

Growing evidence indicates that neuropathic pain is frequently accompanied by cognitive impairments, which aggravate the quality of life of chronic pain patients. Here, we investigated whether acupuncture treatments can improve cognitive dysfunction as well as allodynia induced by neuropathic pain in mice. One week after the left partial sciatic nerve ligation (PSNL), acupuncture treatments on the acupoints GB30-GB34 (AP1), HT7-GV20 (AP2), or control points (CP) were performed for 4 weeks. Notably, the significant attenuations of mechanical allodynia and cognitive impairment were observed in the AP1 group, but not in PSNL, AP2, or CP groups. A random decision forest classifier based on the pain and cognitive functions displayed that the acupuncture group was clearly segregated from the other groups. We also demonstrated that acupuncture restored the reduced field excitatory post-synaptic potentials and was able to elevate the expression levels of glutamate receptors (NR2B and GluR1) in the hippocampus. Moreover, the expressions of Ca²⁺/calmodulin-dependent protein kinase II and synaptic proteins (pPSD-95 and pSyn-1) were enhanced by acupuncture treatment. These results suggest that acupuncture can enhance hippocampal long-term action through the regulation of the synaptic efficacy and that acupuncture may provide a viable option for managing both pain and cognitive functions associated with chronic neuropathic pain.

Characterization of the sensory, affective, cognitive, biochemical, and neuronal alterations in a modified chronic constriction injury model of neuropathic pain in mice

The chronic constriction injury (CCI) of the sciatic nerve is a nerve injury-based model of neuropathic pain (NP). Comorbidities of NP such as depression, anxiety, and cognitive deficits are associated with a functional reorganization of the medial prefrontal cortex (mPFC). Here, we have employed an adapted model of CCI by placing one single loose ligature around the sciatic nerve in mice for investigating the alterations in sensory, motor, affective, and cognitive behavior and in electrophysiological and biochemical properties in the prelimbic division (PrL) of the mPFC. Our adapted model of CCI induced mechanical allodynia, motor, and cognitive impairments and anxiety- and depression-like behavior. In the PrL division of mPFC was observed an increase in GABA and a decrease in d-aspartate levels. Moreover an increase in the activity of neurons responding to mechanical stimulation with an excitation, mPFC (+), and a decrease in those responding with an inhibition, mPFC (-), was found. Altogether these findings demonstrate that a single ligature around the sciatic nerve was able to induce sensory, affective, cognitive, biochemical, and functional alterations already observed in other neuropathic pain models and it may be an appropriate and easily reproducible model for studying neuropathic pain mechanisms and treatments.

Effects of the glial modulator palmitoylethanolamide on chronic pain intensity and brain function

Background: Chronic neuropathic pain (NP) is a complex disease that results from damage or presumed damage to the somatosensory nervous system. Current treatment regimens are often ineffective. The major impediment in developing effective treatments is our limited understanding of the underlying mechanisms. Preclinical evidence suggests that glial changes are crucial for the development of NP and a recent study reported oscillatory activity differences within the ascending pain pathway at frequencies similar to that of cyclic gliotransmission in NP. Furthermore, there is evidence that glial modifying medications may be effective in treating NP. The aim of this Phase I open-label clinical trial is to determine whether glial modifying medication palmitoylethanolamide (PEA) will reduce NP and whether this is associated with reductions in oscillatory activity within the pain pathway.

Methods: We investigated whether 6 weeks of PEA treatment would reduce pain and infra-slow oscillatory activity within the ascending trigeminal pathway in 22 individuals (17 females) with chronic orofacial NP.

Results: PEA reduced pain in 16 (73%) of the 22 subjects, 11 subjects showed pain reduction of over 20%. Whilst both the responders and non-responders showed reductions in infra-slow oscillatory activity where orofacial nociceptor afferents terminate in the brainstem, only responders displayed reductions in the thalamus. Furthermore, functional connections between the brainstem and thalamus were altered only in responders.

Conclusion: PEA is effective at relieving NP. This reduction is coupled to a reduction in resting oscillations along the ascending pain pathway that are likely driven by rhythmic astrocytic gliotransmission.

Role of palmitoylethanolamide (PEA) in depression: Translational evidence: Special Section on "Translational and Neuroscience Studies in Affective Disorders". Section Editor, Maria Nobile MD, PhD. This Section of JAD focuses on the relevance of translational and neuroscience studies in providing a better understanding of the neural basis of affective disorders. The main aim is to briefly summaries relevant research findings in clinical neuroscience with particular regards to specific innovative topics in mood and anxiety disorders

BACKGROUND

Antidepressants have a low rate of response paired with a delayed onset of action. Translational studies are thus seeking for novel targets for antidepressant drug development. Preclinical evidence has demonstrated that the endocannabinoid system plays an important role in mood and stress response, even if drugs targeting this system have not yet become available for clinical use. The dietary supplement N-Palmitoylethanolamide (PEA) is a fatty acid amide belonging to the endocannabinoid system with potential antidepressant properties.

METHODS

We performed a bibliographic search to review current knowledge on the potential antidepressant effects of PEA and its underlying mechanism of action.

RESULTS

PEA targets not only the peroxisome proliferator-activated receptor-alpha (PPAR-α), but also the endocannabinoid system, binding the G-protein-coupled receptor 55, a non-CB₁/CB₂ cannabinoid receptor, and also the CB₁/CB₂ receptors, although with a weak affinity. Preclinical studies have shown antidepressant activity of PEA in animal paradigms of depression and of depression associated with neuropathic pain and traumatic brain injury. In a translational perspective, PEA is increased in stress conditions, and a randomized, double-blind study in depressed patients indicated a fast-antidepressant action of PEA when associated with citalopram.

LIMITATIONS

There are still limited preclinical and clinical studies investigating the effect of PEA upon the endocannabinoid system and its potential as antidepressant.

CONCLUSIONS

PEA has potential antidepressant effects alone or in combinations with other classes of antidepressants. Future studies in depressed patients are needed to confirm the mood-modulating properties of PEA and its role as a biomarker of depression.