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Chronic pain often occurs in the elderly, particularly in the patients with neurodegenerative disorders such as Alzheimer's disease (AD). Although studies indicate that chronic pain correlates with cognitive decline, it is unclear whether chronic pain accelerates AD pathogenesis. In this review, we provide evidence that supports a link between chronic pain and AD and discuss potential mechanisms underlying this connection based on currently available literature from human and animal studies. Specifically, we describe two intertwined processes, locus coeruleus noradrenergic system dysfunction and neuroinflammation resulting from microglial pro-inflammatory activation in brain areas mediating the affective component of pain and cognition that have been found to influence both chronic pain and AD. These represent a pathological overlap that likely leads chronic pain to accelerate AD pathogenesis. Further, we discuss potential therapeutic interventions targeting noradrenergic dysfunction and microglial activation that may improve patient outcomes for those with chronic pain and AD.
Learn More >This study assessed the participation of spinal TWIK-related acid-sensitive K channels 1 and 3 (TASK-1 and TASK-3) in inflammatory (formalin test) and neuropathic (spinal nerve ligation, SNL) pain in rats. Intrathecal pre-treatment (-10 min) with the TASK-1 blocker ML365 or TASK-3 blocker PK-THPP, but not vehicle, enhanced in a dose-dependent manner 1% formalin-induced acute and long-lasting secondary mechanical allodynia and mechanical hyperalgesia in rats. In contrast, intrathecal pre-treatment with terbinafine, an activator of TASK-3, reduced formalin-induced flinching and allodynia/hyperalgesia. Both blockers and terbinafine had similar effects on female and male rats. In addition, intrathecal injection of ML365 or PK-THPP blocked the terbinafine-induced antiallodynic effect in neuropathic rats, but they did not modify baseline withdrawal threshold in naïve or sham-operated rats. TASK-1 and TASK-3 mRNA and protein were expressed in L4 and L5 dorsal root ganglia (DRG) and dorsal and ventral spinal cord of naïve animals. Interestingly, formalin injection increased TASK-1 expression in ipsilateral L5 DRG, but not in the spinal cord. Moreover, formalin injection transiently enhanced TASK-3 expression in ipsilateral L5 DRG and dorsal spinal cord. In contrast, SNL down-regulated TASK-3 expression in the ipsilateral L4 and L5 DRG but not in dorsal or ventral spinal cord, while SNL did not modify TASK-1 expression at any tissue. The pharmacological and molecular results suggest that TASK-1 and TASK-3 have a relevant antinociceptive role in inflammatory and neuropathic pain.
Learn More >Pain is a major primary health care problem. Emerging studies show that inhibition of spinal microglial activation reduces pain. However, the precise mechanisms by which microglial activation contributes to nociceptive synaptic transmission remain unclear. In this study, we measured spontaneous synaptic activity of miniature excitatory postsynaptic currents (mEPSCs) in rat spinal cord superficial dorsal horn (SDH, laminae I and II) neurons. Lipopolysaccharide (LPS) and adenosine triphosphate (ATP) increased the frequency, but not amplitude, of mEPSCs in SDH neurons. Microglial inhibitors minocycline and paeonol, as well as an astrocyte inhibitor, a P2Y1 receptor (P2Y1R) antagonist, and a metabotropic glutamate receptor 5 (mGluR5) antagonist, all prevented LPS-induced enhancement of mEPSC frequency. In mouse behavioral testing, minocycline and paeonol effectively reduced acetic acid-induced writhing and LPS-induced hyperalgesia. These results indicate that LPS-activated microglia release ATP, which stimulates astrocyte P2Y1Rs to release glutamate, triggering presynaptic mGluR5 receptors and increasing presynaptic glutamate release, leading to an increase in mEPSC frequency and enhancement of nociceptive transmission in SDH neurons. We propose that these effects can serve as a new electrophysiological model for evaluating pain. Moreover, we predict that pharmacologic agents capable of inhibiting the LPS-induced enhancement of mEPSC frequency in SDH neurons will have analgesic effects.
Learn More >Pain involves an intrinsically dynamic connectome characterized by fluctuating spontaneous brain activity and continuous neuroplastic changes of relevant circuits. Activity in the hippocampus-medial prefrontal cortex (mPFC) pathway has been suggested to correlate with spontaneous pain and pain chronicity, but causal evidence is lacking. Here we combine longitudinal in vivo electrophysiological recording with behavioral testing and show that persistent spontaneous pain disrupts ventral hippocampal CA1-infralimbic cortex (vCA1-IL) connectivity and hippocampal modulation of IL neuronal activity in rats with peripheral inflammation. Chemo- and optogenetic rescue of vCA1-IL dysfunction relieves spontaneous pain. Circuit-specific overexpression of brain-derived neurotrophic factor (BDNF) in vCA1-IL reverses electrophysiological changes, relieves spontaneous pain, and accelerates overall recovery from inflammatory pain. Our work identifies a neural pathway that specifically correlates with spontaneous pain and supports the significance of using a circuit dynamics-based strategy for more comprehensive understanding of circuitry mechanisms underlying chronic pain.
Learn More >Chronic musculoskeletal pain affects more than 20% of the population, and the prevalence is increasing, causing suffering, loss of quality of life, disability, and an enormous expenditure on healthcare resources. The most common location for chronic pain is the spine. Many of the treatments used are mainly passive (pharmacological and invasive) and poor outcomes. The treatments currently applied in the public health system do not comply with the recommendations of the main clinical practice guidelines, which suggest the use of educational measures and physical exercise as the first-line treatment. A protocol based on active coping strategies is described, which will be evaluated through a clinical trial and which could facilitate the transfer of the recommendations of the clinical practice guidelines to a primary care setting.
Learn More >Given the readily increasing membership of the pain physician community, efforts toward correcting notable gender disparities are instrumental. The under-representation of women is particularly prevalent within leadership roles in academic medicine, thought to be driven largely by diminished research efforts. Consequently, we aimed to characterize gender differences among the highest impact pain literature.
Learn More >To support or refute the hypothesis that opioid tapering in chronic pain patients (CPPs) improves pain or maintains the same pain level by taper completion but does not increase pain.
Learn More >Preclinical studies suggest that type 2 hyperpolarization-activated cyclic nucleotide gated ion channels (HCN2) are necessary for neuropathic pain. This trial assessed the influence of ivabradine, a non-selective HCN channel blocker, on capsaicin-induced hyperalgesia and pain in healthy human subjects. An enriched population comprising subjects who developed >20cm2 of punctate hyperalgesia from topical capsaicin (0.5% cream applied onto 9cm2 area) was identified. These subjects then received ivabradine (15mg) or placebo one hour prior to capsaicin application in randomly allocated order in a crossover study. The forearm site for capsaicin alternated with each application of the cream. The interval of time from screening to the 1st and to the 2nd treatment visits were at least 3 and 5 weeks respectively to minimize carry-over effects. 55 participants were screened, of which 25 completed at least one treatment visit. Intention-to-treat hierarchical analysis revealed no significant effects of the drug on primary trial outcome, defined as a difference in effects of placebo and ivabradine on the area of punctate hyperalgesia (ivabradine – placebo: mean=3.22 cm2, 95% CI: = -4.04, 10.48, p=0.37). However, ivabradine caused a slowing of heart rate (difference of 10.10 beats per min (95% CI – 6.48, – 13.73; p-value <0.0001)). We conclude that ivabradine lacks analgesic effects in the capsaicin pain model at a dose that caused appreciable slowing of heart rate, and hence is unlikely to prove a useful analgesic in humans. More selective drugs are required to establish a role of HCN2 for pain in humans.
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