Contributions of dorsal root reflex and axonal reflex to formalin-induced inflammation

Inhibition of Spinal Interleukin-33 Attenuates Peripheral Inflammation and Hyperalgesia in Experimental Arthritis

Accumulating evidence indicates that the continuous and intense nociceptive from inflamed tissue may increase the excitability of spinal dorsal horn neurons, which can signal back and modulate peripheral inflammation. Previous studies have demonstrated that spinal interleukin (IL)-33 contributes to the hyperexcitability of spinal dorsal horn neurons. This study was undertaken to investigate whether spinal IL-33 can also influence a peripheral inflammatory response in a rat model of arthritis. Lentivirus-delivered short hairpin RNA targeting IL-33 (LV-shIL-33) was constructed for gene silencing. Rats with adjuvant-induced arthritis (AIA) were injected intrathecally with LV-shIL-33 3 days before the complete Freund's adjuvant (CFA) injection. During an observation period of 21 days, pain-related behavior and inflammation were assessed. In addition, the expression of spinal proinflammatory cytokines and the activation of spinal extracellular signal-regulated kinase (ERK) and nuclear factor-κB (NF-κB) pathways were evaluated on 9 days after CFA treatment. The existence of tissue injury or inflammation in rats with AIA resulted in the upregulation of spinal IL-33, which is predominantly expressed in neurons, astrocytes, and oligodendrocytes. Intrathecal administration of LV-shIL-33 significantly alleviated hyperalgesia, paw swelling, and joint destruction, and attenuated the expression of proinflammatory cytokines [IL-6, IL-1β, and tumor necrosis factor-α (TNF-α)], as well as the activation of ERK and NF-κB/p65 in the spinal cord. Our data suggest that spinal IL-33 contributes to the development of both peripheral inflammation and hyperalgesia. Thus, interference with IL-33 at the spinal level might represent a novel therapeutic target for painful inflammatory disorders.

Antiepileptic drugs as analgesics/adjuvants in inflammatory pain: current preclinical evidence

Inflammatory pain is the most common type of pain that is treated clinically. The use of currently available treatments (classic analgesics - NSAIDs, paracetamol and opioids) is limited by insufficient efficacy and/or side effects/tolerance development. Antiepileptic drugs (AEDs) are widely used in neuropathic pain treatment, but there is substantial preclinical evidence on their efficacy against inflammatory pain, too. In this review we focus on gabapentinoids (gabapentin and pregabalin) and dibenzazepine AEDs (carbamazepine, oxcarbazepine, and recently introduced eslicarbazepine acetate) and their potential for relieving inflammatory pain. In models of somatic, visceral and trigeminal inflammatory pain, that have a translational value for inflammatory conditions in locomotor system, viscera and head/face, AEDs have demonstrated analgesic activity. This activity was mostly consistent, dependent on the dose and largely independent on the site of inflammation and method of its induction, nociceptive stimuli, species, specific drug used, its route of administration and dosing schedule. AEDs exerted comparable efficacy with classic analgesics. Effective doses of AEDs are lower than toxic doses in animals and, when expressed as equivalent human doses, they are largely overlapping with AEDs doses already used in humans for treating epilepsy/neuropathic pain. The main mechanism of antinociceptive/antihyperalgesic action of gabapentinoids in inflammatory pain models seems to be α₂δ-dependent suppression of voltage-gated calcium channels in primary sensory neurons that leads to reduced release of neurotransmitters in the spinal/medullar dorsal horn. The suppression of NMDA receptors via co-agonist binding site primarily at spinal sites, activation of various types of K⁺ channels at spinal and peripheral sites, and activation of noradrenergic and serotonergic descending pain modulatory pathways may also contribute. Inhibition of voltage-gated sodium channels along the pain pathway is probably the main mechanism of antinociceptive/antihyperalgesic effects of dibenzazepines. The recruitment of peripheral adrenergic and purinergic mechanisms and central GABAergic mechanisms may also contribute. When co-administered with classic/other alternative analgesics, AEDs exerted synergistic/additive interactions. Reviewed data could serve as a basis for clinical studies on the efficacy/safety of AEDs as analgesics/adjuvants in patients with inflammatory pain, and contribute to the improvement of the treatment of various inflammatory pain states.

Activities of 2-phthalimidethyl nitrate and 2-phthalimidethanol in the models of nociceptive response and edema induced by formaldehyde in mice and preliminary investigation of the underlying mechanisms

The activities of 2-phthalimidethyl nitrate (PTD-NO) and 2-phthalimidethanol (PTD-OH) were recently demonstrated in models of pain and inflammation. We expanded our investigation by evaluating their activities in models of nociceptive and inflammatory pain and inflammatory edema, the preliminary pharmacokinetic parameter for PTD-NO and the role of opioid and cannabinoid pathways in the activity of analogs. Per os (p.o.) administration of PTD-NO or PTD-OH, 1h before intraplantar injection of formaldehyde, inhibited both phases of the nociceptive response (500 and 750 mg/kg) and paw edema (125, 250, 500 and 750 mg/kg). After p.o. administration of PTD-NO, peak plasma concentrations of PTD-NO and PTD-OH were found 0.92 and 1.13 h, respectively. The plasma concentrations of PTD-NO were higher than those of PTD-OH. Intraperitoneal (i.p.) administration of CB1 (AM251) or CB2 (AM630) cannabinoid receptor antagonists (4 or 8 mg/kg, -30 min) or opioid antagonist naltrexone (5 or 10mg/kg, -30 min) did not affect the antinociceptive activities of the analogs. AM251 (8 mg/kg, i.p., -30 min) attenuated the antiedematogenic activity of both analogs, while naltrexone (10mg/kg, i.p., -30 min) only attenuated the antiedematogenic activity of PTD-NO. The antiedematogenic activities of both analogs were not affected by the CB2 cannabinoid antagonist AM630 (4 or 8 mg/kg, i.p., -30 min). Concluding, we expanded the knowledge on the activities of PTD-NO and PTD-OH by showing that these phthalimide analogs also exhibit marked activity in models of nociceptive and inflammatory pain and inflammatory edema. Opioid and cannabinoid mechanisms partially mediate the anti-inflammatory, but not the antinociceptive activity.

Activation of spinal NF-κB/p65 contributes to peripheral inflammation and hyperalgesia in rat adjuvant-induced arthritis

OBJECTIVE

It is known that noxious stimuli from inflamed tissue may increase the excitability of spinal dorsal horn neurons (a process known as central sensitization), which can signal back and contribute to peripheral inflammation. However, the underlying mechanisms have yet to be fully defined. A number of recent studies have indicated that spinal NF-κB/p65 is involved in central sensitization, as well as pain-related behavior. Thus, the aim of this study was to determine whether NF-κB/p65 can facilitate a peripheral inflammatory response in rat adjuvant-induced arthritis (AIA).

METHODS

Lentiviral vectors encoding short hairpin RNAs that target NF-κB/p65 (LV-shNF-κB/p65) were constructed for gene silencing. The spines of rats with AIA were injected with LV-shNF-κB/p65 on day 3 or day 10 after treatment with Freund's complete adjuvant (CFA). During an observation period of 20 days, pain-related behavior, paw swelling, and joint histopathologic changes were evaluated. Moreover, the expression levels of spinal tumor necrosis factor α (TNFα), interleukin-1β (IL-1β), and cyclooxygenase 2 (COX-2) were assessed on day 14 after CFA treatment.

RESULTS

The presence of peripheral inflammation in rats with AIA induced an increase in NF-κB/p65 expression in the spinal cord, mainly in the dorsal horn neurons and astrocytes. Delivery of LV-shNF-κB/p65 to the spinal cord knocked down the expression of NF-κB/p65 and significantly attenuated hyperalgesia, paw edema, and joint destruction. In addition, spinal delivery of LV-shNF-κB/p65 reduced the overexpression of spinal TNFα, IL-1β, and COX-2.

CONCLUSION

These findings indicate that spinal NF-κB/p65 plays an important role in the initiation and development of both peripheral inflammation and hyperalgesia. Thus, inhibition of spinal NF-κB/p65 expression may provide a potential treatment to manage painful inflammatory disorders.

Neuroinflammation, neuroautoimmunity, and the co-morbidities of complex regional pain syndrome

Complex Regional Pain Syndrome (CRPS) is associated with non-dermatomal patterns of pain, unusual movement disorders, and somatovisceral dysfunctions. These symptoms are viewed by some neurologists and psychiatrists as being psychogenic in origin. Recent evidence, however, suggests that an autoimmune attack on self-antigens found in the peripheral and central nervous system may underlie a number of CRPS symptoms. From both animal and human studies, evidence is accumulating that neuroinflammation can spread, either anterograde or retrograde, via axonal projections in the CNS, thereby establishing neuroinflammatory tracks and secondary neuroinflammatory foci within the neuraxis. These findings suggest that neuroinflammatory lesions, as well as their associated functional consequences, should be evaluated during the differential diagnosis of non-dermatomal pain presentations, atypical movement disorders, as well as other "medically unexplained symptoms", which are often attributed to psychogenic illness.

When does acute pain become chronic?

The transition from acute to chronic pain appears to occur in discrete pathophysiological and histopathological steps. Stimuli initiating a nociceptive response vary, but receptors and endogenous defence mechanisms in the periphery interact in a similar manner regardless of the insult. Chemical, mechanical, and thermal receptors, along with leucocytes and macrophages, determine the intensity, location, and duration of noxious events. Noxious stimuli are transduced to the dorsal horn of the spinal cord, where amino acid and peptide transmitters activate second-order neurones. Spinal neurones then transmit signals to the brain. The resultant actions by the individual involve sensory-discriminative, motivational-affective, and modulatory processes in an attempt to limit or stop the painful process. Under normal conditions, noxious stimuli diminish as healing progresses and pain sensation lessens until minimal or no pain is detected. Persistent, intense pain, however, activates secondary mechanisms both at the periphery and within the central nervous system that cause allodynia, hyperalgesia, and hyperpathia that can diminish normal functioning. These changes begin in the periphery with upregulation of cyclo-oxygenase-2 and interleukin-1β-sensitizing first-order neurones, which eventually sensitize second-order spinal neurones by activating N-methyl-d-aspartic acid channels and signalling microglia to alter neuronal cytoarchitecture. Throughout these processes, prostaglandins, endocannabinoids, ion-specific channels, and scavenger cells all play a key role in the transformation of acute to chronic pain. A better understanding of the interplay among these substances will assist in the development of agents designed to ameliorate or reverse chronic pain.