Palmitoylethanolamide Modulates Central Sensitisation and Neuroinflammation in Chronic Low Back Pain

Overview

Low back pain (LBP) is a public and occupational health problem that is a major professional, economic and social burden. Up to 84% of the general population will experience an episode of LBP during its life time, and recurrence rates are high. Acute LBP is the second reason for consultations in general medicine, and chronic LBP is the eighth. One in five LBP episodes result in sick leave. LBP represents 30% of sick leaves that are longer than 6 months, and 20% of work accidents. LBP has become the leading cause of exclusion from work before the age of 45, and the third cause of work disabilities in France. In 2020 globally, LBP affected approximately 619 million people, making it the leading cause of disability worldwide. This number is projected to rise to 843 million by 2050, driven by population growth and aging.

Chronic LBP is a heterogenous condition with mixed nociceptive and neuropathic components in many patients. Mechanisms that sustain chronicity include ongoing peripheral nociceptor sensitization, neuroimmune activation (mast cells, microglia), and maladaptive central nervous system plasticity (altered cortical excitability, changes in descending modulatory systems). Pro-inflammatory cytokines such as TNF-α and IL-1β have been implicated in maintenance of peripheral and central sensitization and the transition from acute to chronic pain. Measuring an inflammatory biomarker (e.g., serum TNF-α) alongside clinical pain and neurophysiological markers can therefore help test an anti-neuroinflammatory treatment hypothesis. Today's management of chronic LBP includes analgesics (paracetamol), non-steroidal anti-inflammatory drugs, antidepressants, anticonvulsants, opioids, and topical treatments , with oral agents recommended as first-line therapy. Analgesics and non-steroidal anti-inflammatory drugs target the nociceptive component of LBP without affecting neuropathic pain components, while opioids target both nociceptive and (to a lesser degree) neuropathic pain, and antidepressants target only the neuropathic component, although data concerning their efficacy is conflicting. Analgesics and non-steroidal anti-inflammatory drugs target the nociceptive component of LBP without affecting neuropathic pain components, while opioids target both nociceptive and (to a lesser degree) neuropathic pain, and antidepressants target only the neuropathic component, although data concerning their efficacy is conflicting. Although relieving neuropathic pain to some extent, classical opioid analgesics suffer from frequent side effects, particularly at the gastrointestinal level that limit their long-term use. An innovative approach in the management of chronic pain diseases is represented by palmitoylethanolamide (PEA), a member of the N-acylethanolamine family, produced by most mammalian cells and which is particularly abundant in brain tissues. PEA is involved in endogenous protective mechanisms activated by stimulation of inflammatory responses. PEA exerts its effects on cellular targets involved in the generation and maintenance of pain by down-modulating mast cell activation and controlling microglial cell behaviors. PEA acts through several complementary mechanisms that are relevant to chronic pain: PEA is a ligand of peroxisome proliferator-activated receptor alpha (PPAR-α); activation has anti-inflammatory and analgesic consequences., Mast cell stabilization / Autacoid Local Inflammation Antagonism (ALIA) mechanism: PEA reduces mast cell degranulation and local release of pro-nociceptive mediators, historically described as ALIA. Indirect endocannabinoid-modulating effects: PEA can indirectly modulate the endocannabinoid system, contributing to analgesia without psychotropic effects. Biomarkers can be used to aid diagnosis, clarify disease pathophysiology, classify the extent of a disease, indicate disease prognosis, and predict or monitor the disease over time and in response to interventions. Central sensitization is defined by the International Association for the Study of Pain (IASP) as an increased responsiveness of nociceptive neurons in the central nervous system to their normal or subthreshold afferent input . Historically, the discovery that injury at peripheral tissues induces hyperexcitability of spinal cord nociceptive neurons has prompted a massive research effort that has consistently confirmed enhanced responsiveness of central nociceptive pathways with different animal models. One proposed biomarker for assessing CNS pathophysiology in chronic pain is transcranial magnetic stimulation (TMS). TMS uses electromagnetic induction to noninvasively generate an electrical current in the brain.TMS involves a high-current pulse generator that discharges an electrical current of several thousand amperes through an insulated metal stimulating coil for a period of \< 1ms, generating a brief and focal magnetic field of approximately 1 to 2 Tesla-\]. When the coil is placed on an individual's scalp overlying the primary motor cortex (M1), the magnetic field can induce an intracortical electric current sufficient to depolarize superficial corticospinal neurons and activate a target muscle leading to a measurable electromyographic responseThe neurotransmitters involved in various TMS-evoked measurements have been well characterized thus making TMS a putative biomarker for the study of chronic pain. Mechanistic link between TNF-α and TMS measures:Neuroinflammation, characterized by elevated levels of proinflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), can profoundly influence cortical excitability. TNF-α modulates synaptic transmission by enhancing AMPA receptor trafficking to the postsynaptic membrane and reducing GABA\_A receptor surface expression, leading to an imbalance between excitatory and inhibitory neurotransmission. This cytokine-driven shift toward hyperexcitability alters the functional state of intracortical circuits. Transcranial magnetic stimulation (TMS) provides a noninvasive means to quantify these changes: short-interval intracortical inhibition (SICI) reflects GABA\_A-mediated inhibitory tone, while intracortical facilitation (ICF) reflects glutamatergic excitatory activity.