Electrical stimulation of the auricular branch of the vagus nerve potentiates analgesia induced by physical exercise in mice with peripheral inflammation.

Objective: This study evaluated the antihyperalgesic and anti-inflammatory effects of percutaneous vagus nerve electrical stimulation (pVNS) associated with physical exercise, i.e., swimming, in mice with peripheral inflammation. Methods: The pain model was induced by intraplantar (i.pl.) injection of Freund's complete adjuvant (CFA). Sixty-four male Swiss mice (35-40 g) received an i.pl. of CFA and underwent behavioral tests, i.e., mechanical hyperalgesia, edema, and paw temperature tests. Additionally, cytokine levels, specifically interleukin-6 (IL-6) and interleukin-10 (IL-10), were determined by enzyme-linked immunosorbent assay. Mice were treated with swimming exercise for 30 min alone or associated with different time protocols (10, 20, or 30 min) of stimulation in the left ear with random frequency during four consecutive days. Results: pVNS for 20 min prolonged the antihyperalgesic effect for up to 2 h, 24 h after CFA injection. pVNS for 30 min prolonged the antihyperalgesic effect for up to 7 h, 96 h after CFA injection. However, it did not alter the edema or temperature at both analyzed times (24 and 96 h). Furthermore, the combination of pVNS plus swimming exercise, but not swimming exercise alone, reduced IL-6 levels in the paw and spinal cord, as well as IL-10 levels in the spinal cord. Conclusion: pVNS potentiates the analgesic effect induced by swimming, which may be, at least in part, mediated by the modulation of inflammatory cytokines in the periphery (paw) and central nervous system (spinal cord). Therefore, the combination of these therapies may serve as an important adjunctive treatment for persistent inflammatory pain.

Antihyperalgesic effect of joint mobilization requires Cav3.2 calcium channels.

The present study was undertaken to explore the relative contributions of Cav3.2 T-type channels to mediating the antihyperalgesic activity of joint manipulation (JM) therapy. We used the chronic constriction injury model (CCI) to induce peripheral neuropathy and chronic pain in male mice, followed by JM. We demonstrate that JM produces long-lasting mechanical anti-hyperalgesia that is abolished in Cav3.2 null mice. Moreover, we found that JM displays a similar analgesic profile as the fatty acid amide hydrolase inhibitor URB597, suggesting a possible converging mechanism of action involving endocannabinoids. Overall, our findings advance our understanding of the mechanisms through which JM produces analgesia.

Preventive Supplementation of Omega-3 Reduces Pain and Pro-inflammatory Cytokines in a Mouse Model of Complex Regional Pain Syndrome Type I.

Complex regional pain syndrome type I (CRPS-I) is a condition that responds poorly to treatments. The role of omega-3 fatty acids in the treatment of inflammatory disorders is well described in the literature; however, few studies have evaluated its therapeutic benefits in different types of pain. We evaluated the potential antihyperalgesic and anti-inflammatory effects of preventive omega-3 supplementation in an animal model of CRPS-I. In experiment 1, Swiss female mice were supplemented for 30 days with omega-3 before the induction of the CRPS-I model and 14 days after. Mechanical hyperalgesia was evaluated at baseline and from the 4th to the 14th day after CPRS-I induction along with open field locomotor activity after 30 days of supplementation. In experiment 2, Swiss female mice were supplemented for 30 days with omega-3 and then subjected to the CRPS-I model. Twenty-four hours later the animals were euthanized, and tissue samples of the spinal cord and right posterior paw muscle were taken to measure pro-inflammatory cytokine TNF and IL-1ß concentrations. Omega-3 supplementation produced antihyperalgesic and anti-inflammatory effects, as well as reducing pro-inflammatory cytokine concentrations, without altering the animals' locomotion. No open field locomotor changes were found. The 30-day supplementation at the tested dose was effective in the CRPS-I model.

Potential Nociceptive Role of the Thoracolumbar Fascia: A Scope Review Involving In Vivo and Ex Vivo Studies.

Nociceptive innervation of the thoracolumbar fascia (TLF) has been investigated over the past few decades; however, these studies have not been compiled or collectively appraised. The purpose of this scoping review was to assess current knowledge regarding nociceptive innervation of the TLF to better inform future mechanistic and clinical TLF research targeting lower back pain (LBP) treatment. PubMed, ScienceDirect, Cochrane, and Embase databases were searched in January 2021 using relevant descriptors encompassing fascia and pain. Eligible studies satisfied the following: (a) published in English; (b) preclinical and clinical (in vivo and ex vivo) studies; (c) original data; (d) included quantification of at least one TLF nociceptive component. Two-phase screening procedures were conducted by a pair of independent reviewers, after which data were extracted and summarized from eligible studies. The search resulted in 257 articles of which 10 met the inclusion criteria. Studies showed histological evidence of nociceptive nerve fibers terminating in lower back fascia, suggesting a TLF contribution to LBP. Noxious chemical injection or electrical stimulation into fascia resulted in longer pain duration and higher pain intensities than injections into subcutaneous tissue or muscle. Pre-clinical and clinical research provides histological and functional evidence of nociceptive innervation of TLF. Additional knowledge of fascial neurological components could impact LBP treatment.

Electroacupuncture decreases inflammatory pain through a pro-resolving mechanism involving the peripheral annexin A1-formyl peptide receptor 2/ALX-opioid receptor pathway.

The pro-resolving mechanism is a recently described endogenous process that controls inflammation. The present study evaluated components of this mechanism, including annexin 1 (ANXA1) and the formyl peptide receptor 2/ALX (FPR2/ALX) receptor, in the antihyperalgesic effect induced by electroacupuncture (EA) in an animal model of persistent peripheral inflammation. Male Swiss mice underwent intraplantar (i.pl.) injection with complete Freund's adjuvant (CFA). Mechanical hyperalgesia was assessed with von Frey monofilaments. Animals were treated with EA (2-10 Hz, ST36-SP6) or subcutaneous BML-111 injection (FPR2/ALX agonist) for 5 consecutive days. In a separate set of experiments, on the first and fifth days after CFA injection, animals received i.pl. WRW4 (FPR2/ALX antagonist) or naloxone (non-selective opioid receptor antagonist) before EA or BML-111 injection. Paw protein levels of FPR2/ALX and ANXA1 were evaluated on the second day after CFA injection by western blotting technique. EA and BML-111 reduced mechanical hyperalgesia. I.pl. naloxone or WRW4 prevented the antihyperalgesic effect induced by either EA or BML-111. EA increased ANXA1 but did not alter FPR2/ALX receptor levels in the paw. Furthermore, i.pl. pretreatment with WRW4 prevented the increase of ANXA1 levels induced by EA. This work demonstrates that the EA antihyperalgesic effect on inflammatory pain involves the ANXA1/FPR2/ALX pro-resolution pathway. This effect appears to be triggered by the activation of FPR2/ALX receptors and crosstalk communication with the opioid system.

Medium- and long-term functional behavior evaluations in an experimental focal ischemic stroke mouse model.

Cerebrovascular accident (CVA) is one of the leading causes of death and disability worldwide, as well as a major financial burden for health care systems. CVA rodent models provide experimental support to determine possible in vivo therapies to reduce brain injury and consequent sequelae. This study analyzed nociceptive, motor, cognitive and mood functions in mice submitted to distal middle cerebral artery (DMCA) occlusion. Male C57BL mice (n = 8) were randomly allocated to control or DMCA groups. Motor function was evaluated with the tests: grip force, rotarod and open field; and nociceptive threshold with von Frey and hot plate assessments. Cognitive function was evaluated with the inhibitory avoidance test, and mood with the tail suspension test. Evaluations were conducted on the seventh- and twenty-eighth-day post DMCA occlusion to assess medium- and long-term effects of the injury, respectively. DMCA occlusion significantly decreases muscle strength and spontaneous locomotion (p < 0.05) both medium- and long term; as well as increases immobility in the tail-suspension test (p < 0.05), suggesting a depressive-type behavior. However, DMCA occlusion did not affect nociceptive threshold nor cognitive functions (p > 0.05). These results suggest that, medium- and long-term effects of DMCA occlusion include motor function impairments, but no sensory dysfunction. Additionally, the injury affected mood but did not hinder cognitive function.

High-intensity swimming exercise reduces inflammatory pain in mice by activation of the endocannabinoid system.

As exercise intervention solely for pain reduction is relatively new, the available research still leaves an incomplete picture of responsible mechanisms and pathways. Nonetheless, evidence indicates that exercise-induced analgesia involves activation of the endocannabinoid (eCB) system. The present study investigated the role of the eCB system on the antihyperalgesic effect of high-intensity swimming exercise (HISE) in an animal model of peripheral persistent inflammation. Male Swiss mice were allocated to non-exercised and exercised groups and subjected to subcutaneous intraplantar injection (i.pl.) of a single dose of complete Freund's adjuvant (CFA) to induce inflammatory pain. Cumulative HISE was performed once a day, and mechanical hyperalgesia and edema were evaluated 0.5 hour after HISE for seven consecutive days. To investigate the role of the eCB system on the antihyperalgesic effect of HISE, non-exercised and exercised mice received intraperitoneal (ip), intrathecal (i.t.) or i.pl. injections of vehicle, AM281 (a CB1 cannabinoid receptor antagonist) or AM630 (a CB2 cannabinoid receptor antagonist) from the 3rd to 5th day after CFA injection. Mechanical hyperalgesia was evaluated 0.5 hour after HISE. In addition, the effect of the fatty acid amide hydrolase [FAAH] inhibitor or monoacylglycerol lipase [MAGL] inhibitor on the antihyperalgesic action of HISE was investigated. HISE reduced mechanical hyperalgesia with effects prevented by AM281 or AM630 pretreatment in all delivery routes tested. The inhibition of FAAH and MAGL prolonged the antihyperalgesic effect of HISE. These data demonstrate evidence for the role of the eCB system upon exercise-induced analgesia in a murine model of inflammatory pain.

Manual Therapy Reduces Pain Behavior and Oxidative Stress in a Murine Model of Complex Regional Pain Syndrome Type I.

Complex regional pain syndrome type I (CRPS-I) is a chronic painful condition. We investigated whether manual therapy (MT), in a chronic post-ischemia pain (CPIP) model, is capable of reducing pain behavior and oxidative stress. Male Swiss mice were subjected to ischemia-reperfusion (IR) to mimic CRPS-I. Animals received ankle joint mobilization 48h after the IR procedure, and response to mechanical stimuli was evaluated. For biochemical analyses, mitochondrial function as well as oxidative stress thiobarbituric acid reactive substances (TBARS), protein carbonyls, antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) levels were determined. IR induced mechanical hyperalgesia which was subsequently reduced by acute MT treatment. The concentrations of oxidative stress parameters were increased following IR with MT treatment preventing these increases in malondialdehyde (MDA) and carbonyls protein. IR diminished the levels of SOD and CAT activity and MT treatment prevented this decrease in CAT but not in SOD activity. IR also diminished mitochondrial complex activity, and MT treatment was ineffective in preventing this decrease. In conclusion, repeated sessions of MT resulted in antihyperalgesic effects mediated, at least partially, through the prevention of an increase of MDA and protein carbonyls levels and an improvement in the antioxidant defense system.