Differential role of adenosine signaling cascade in acute and chronic pain

Hyperbaric oxygen attenuates chronic postsurgical pain by regulating the CD73/adenosine/A1R axis of the spinal cord in rats

Chronic postsurgical pain (CPSP) affects postoperative rehabilitation and quality of life in patients, but its mechanisms are still poorly understood. Hyperbaric oxygen (HBO) attenuates neuropathic pain in animal and human studies, but its efficacy for CPSP treatment and its underlying mechanism have not been elucidated. This study aimed to investigate the analgesic effect of HBO in a CPSP rat model and the role of spinal cord adenosine circulation in HBO-induced analgesia. A skin/muscle incision and retraction (SMIR) rat model was used to mimic CPSP, and HBO treatment (2.5 ATA, 60 min) was administered once daily for five consecutive days beginning three days after surgery. The role of spinal cord adenosine circulation in HBO-induced analgesia was investigated using APCP (a CD73 inhibitor), DPCPX (an A1R antagonist) or an intrathecal injection of adenosine. The mechanical paw withdrawal threshold (PWT) was determined at different time points before and after surgery. The spinal cord adenosine and ATP contents were analyzed using high-performance liquid chromatography (HPLC), and the spinal cord expression of A1R, CD73, and ADK was examined by Western blotting and immunofluorescence staining. The results showed that the mechanical PWT of the ipsilateral hind paw and the adenosine content decreased, and the spinal cord expression of A1R, CD73, and ADK and ATP content increased within 14 days after surgery. HBO treatment alleviated mechanical allodynia, reduced ATP content, and increased adenosine content by activating CD73 but downregulated the spinal cord expression of A1R, CD73, and ADK. Intrathecal adenosine alleviated mechanical allodynia after SMIR and downregulated the spinal cord expression of A1R and CD73, and intrathecal APCP or DPCPX attenuated the analgesic effect of HBO treatment on SMIR-induced CPSP. PERSPECTIVE: Spinal cord adenosine is involved in the occurrence and development of CPSP, and HBO treatment alleviates CPSP by regulating adenosine production/metabolism in the spinal cord. Thus, HBO may be employed for the treatment of CPSP with favorable efficacy.

Adenosine signaling mediate pain transmission in the central nervous system

Pain is a common clinical symptom that seriously affects the quality of life in a variety of patient populations. In recent years, research on the role of adenosine signaling in pain modulation has made great progress. Adenosine is a purine nucleoside and a neuromodulator, and regulates multiple physiological and pathophysiological functions through the activation of four G protein-coupled receptors, which are classified as A₁, A_2A, A_2B, and A₃ adenosine receptors (ARs). Adenosine and its receptors that are widespread in the central nervous system (CNS) play an important role in the processing of nociceptive sensory signals in different pain models. A₁Rs have the highest affinity to adenosine, and the role in analgesia has been well investigated. The roles of A_2ARs and A_2BRs in the modulation of pain are controversial because they have both analgesic and pronociceptive effects. The analgesic effects of A₃Rs are primarily manifested in neuropathic pain. In this article, we have reviewed the recent studies on ARs in the modulation of neuropathic pain, inflammatory pain, postoperative pain, and visceral pain in the CNS. Furthermore, we have outlined the pathways through which ARs contribute to pain regulation, thereby shedding light on how this mechanism can be targeted to provide effective pain relief.

The real-time detection of acupuncture-induced extracellular ATP mobilization in acupoints and exploration of its role in acupuncture analgesia

Our and in vitro studies had confirmed that mechanosensitive ATP release and accumulation in acupoints was elicited by acupuncture (AP), which might be a pivotal step for triggering AP analgesia. But to date, the dynamics of extracellular ATP (eATP) in the interstitial space during AP process was poorly known, mainly due to the low temporal resolution of the current detection approach. This study attempted to capture rapid eATP signals in vivo in the process of needling, and further explored the role of this eATP mobilization in initiating AP analgesic effect. Ipsilateral 20-min needling was applied on Zusanli acupoint (ST36) of complete Freund's adjuvant (CFA)-induced ankle arthritis rats. Pain thresholds were assessed in injured-side hindpaws. eATP in the interstitial space was microdialyzed and real-time quantified by luciferin-luciferase assay at 1-min interval with the aid of the microfluid chip. We revealed in behavioral tests that modulation of eATP levels in ST36 influenced AP analgesic effect on ankle arthritis. A transient eATP accumulation was induced by needling that started to mobilize at 4 min, climbed to the peak of 11.21 nM within 3.25 min and gradually recovered. Such AP-induced eATP mobilization was significantly impacted by ankle inflammation, needling depth, needle manipulation, and the presence of local ecto-nucleotidases. This work reveals that needling elicits a transient eATP mobilization in acupoints, which contributes to initiating AP analgesia. This study will help us better understand the peripheral mechanism of AP analgesia and guide clinicians to optimize the needle manipulations to improve AP efficacy.

In silico identification of A1 agonists and A2a inhibitors in pain based on molecular docking strategies and dynamics simulations

Most recently, the adenosine is considered as one of the most promising targets for treating pain, with few side effects. It exists in the central nervous system, and plays a key role in nociceptive afferent pathway. It is reported that the A1 receptor (A1R) could inhibit Ca²⁺ channels to reduce the pain like analgesic mechanism of morphine. And, A2a receptor (A2aR) was reported to enhance the accumulation of AMP (cAMP) and released peptides from sensory neurons, resulting in constitutive activation of pain. Much evidence showed that A1R and A2aR could be served as the interesting targets for the treatment of pain. Herein, virtual screening was utilized to identify the small molecule compounds towards A1R and A2aR, and top six molecules were considered as candidates via amber scores. The molecular dynamic (MD) simulations and molecular mechanics/generalized born surface area (MM/GBSA) were employed to further analyze the affinity and binding stability of the six molecules towards A1R and A2aR. Moreover, energy decomposition analysis showed significant residues in A1R and A2aR, including His1383, Phe1276, and Glu1277. It provided basics for discovery of novel agonists and antagonists. Finally, the agonists of A1R (ZINC19943625, ZINC13555217, and ZINC04698406) and inhibitors of A2aR (ZINC19370372, ZINC20176051, and ZINC57263068) were successfully recognized. Taken together, our discovered small molecules may serve as the promising candidate agents for future pain research.

Inosine as a Tool to Understand and Treat Central Nervous System Disorders: A Neglected Actor?

Since the 1970s, when ATP was identified as a co-transmitter in sympathetic and parasympathetic nerves, it and its active metabolite adenosine have been considered relevant signaling molecules in biological and pathological processes in the central nervous system (CNS). Meanwhile, inosine, a naturally occurring purine nucleoside formed by adenosine breakdown, was considered an inert adenosine metabolite and remained a neglected actor on the purinergic signaling scene in the CNS. However, this scenario began to change in the 1980s. In the last four decades, an extensive group of shreds of evidence has supported the importance of mediated effects by inosine in the CNS. Also, inosine was identified as a natural trigger of adenosine receptors. This evidence has shed light on the therapeutic potential of inosine on disease processes involved in neurological and psychiatric disorders. Here, we highlight the clinical and preclinical studies investigating the involvement of inosine in chronic pain, schizophrenia, epilepsy, depression, anxiety, and in neural regeneration and neurodegenerative diseases, such as Parkinson and Alzheimer. Thus, we hope that this review will strengthen the knowledge and stimulate more studies about the effects promoted by inosine in neurological and psychiatric disorders.

Lack of efficacy of a partial adenosine A1 receptor agonist in neuropathic pain models in mice

Previous studies suggest that adenosine A₁ receptors (A₁R) modulate the processing of pain. The aim of this study was to characterize the distribution of A₁R in nociceptive tissues and to evaluate whether targeting A₁R with the partial agonist capadenoson may reduce neuropathic pain in mice. The cellular distribution of A₁R in dorsal root ganglia (DRG) and the spinal cord was analyzed using fluorescent in situ hybridization. In behavioral experiments, neuropathic pain was induced by spared nerve injury or intraperitoneal injection of paclitaxel, and tactile hypersensitivities were determined using a dynamic plantar aesthesiometer. Whole-cell patch-clamp recordings were performed to assess electrophysiological properties of dissociated DRG neurons. We found A₁R to be expressed in populations of DRG neurons and dorsal horn neurons involved in the processing of pain. However, administration of capadenoson at established in vivo doses (0.03–1.0 mg/kg) did not alter mechanical hypersensitivity in the spared nerve injury and paclitaxel models of neuropathic pain, whereas the standard analgesic pregabalin significantly inhibited the pain behavior. Moreover, capadenoson failed to affect potassium currents in DRG neurons, in contrast to a full A₁R agonist. Despite expression of A₁R in nociceptive neurons, our data do not support the hypothesis that pharmacological intervention with partial A₁R agonists might be a valuable approach for the treatment of neuropathic pain.

Effects of Chronic Pain Treatment on Altered Functional and Metabolic Activities in the Brain: A Systematic Review and Meta-Analysis of Functional Neuroimaging Studies

Previous studies have identified altered brain changes in chronic pain patients, however, it remains unclear whether these changes are reversible. We summarized the neural and molecular changes in patients with chronic pain and employed a meta-analysis approach to quantify the changes. We included 75 studies and 11 of these 75 studies were included in the activation likelihood estimation (ALE) analysis. In the 62 functional magnetic resonance imaging (fMRI) studies, the primary somatosensory and motor cortex (SI and MI), thalamus, insula, and anterior cingulate cortex (ACC) showed significantly decreased activity after the treatments compared to baseline. In the 13 positron emission tomography (PET) studies, the SI, MI, thalamus, and insula showed significantly increased glucose uptake, blood flow, and opioid-receptor binding potentials after the treatments compared to baseline. A meta-analysis of fMRI studies in patients with chronic pain, during pain-related tasks, showed a significant deactivation likelihood cluster in the left medial posterior thalamus. Further studies are warranted to understand brain reorganization in patients with chronic pain compared to the normal state, in terms of its relationship with symptom reduction and baseline conditions.

Allopurinol for fibromyalgia pain in adults: A randomized controlled trial

BACKGROUND

Allopurinol is a potent inhibitor of the enzyme xanthine oxidase used in the treatment of hyperuricemia and gout. Because it is well known that purines exert multiple affects on pain transmission, we hypothesized that the inhibition of xanthine oxidase by allopurinol could be a valid strategy to treat pain in humans. This study aimed to compare the analgesic efficacy of oral allopurinol versus placebo as an adjuvant therapy in patients displaying fibromyalgia.

METHODS

This randomized, double-blinded, placebo-controlled study included 60 women with the diagnosis of fibromyalgia. Patients were randomly assigned to receive either oral allopurinol 300 mg (n = 31) or placebo (n = 29) twice daily during 30 days. The patients were submitted to evaluation for pain sensitivity, anxiety, depression, and functional status before treatment, and 15 and 30 days thereafter.

RESULTS

Oral administration of allopurinol 300 mg twice daily was ineffective in improving pain scores measured by several tools up to 30 days of treatment (P > 0.05). Additionally, no significant effects of allopurinol over anxiety, depressive symptoms, and functional status of fibromyalgia patients were observed in the present study.

CONCLUSIONS

Although previous findings indicated that allopurinol could present intrinsic analgesic effects in both animals and humans, this study showed no benefit of the use of oral allopurinol as an adjuvant strategy during 30 days in women displaying fibromyalgia. However, considering previous promising results, new prospective studies are still valid to further investigate allopurinol and more selective purine derivatives in the management of pain syndromes.

Sustained adenosine exposure causes endothelial mitochondrial dysfunction via equilibrative nucleoside transporters

Adenosine is a potent signaling molecule that has paradoxical effects on lung diseases. We have previously demonstrated that sustained adenosine exposure by inhibition of adenosine degradation impairs lung endothelial barrier integrity and causes intrinsic apoptosis through equilibrative nucleoside transporter_1/2-mediated intracellular adenosine signaling. In this study, we further demonstrated that sustained adenosine exposure increased mitochondrial reactive oxygen species and reduced mitochondrial respiration via equilibrative nucleoside transporter_1/2, but not via adenosine receptor-mediated signaling. We have previously shown that sustained adenosine exposure activates p38 and c-Jun N-terminal kinases in mitochondria. Here, we show that activation of p38 and JNK partially contributed to sustained adenosine-induced mitochondrial reactive oxygen species production. We also found that sustained adenosine exposure promoted mitochondrial fission and increased mitophagy. Finally, mitochondria-targeted antioxidants prevented sustained adenosine exposure-induced mitochondrial fission and improved cell survival. Our results suggest that inhibition of equilibrative nucleoside transporter_1/2 and mitochondria-targeted antioxidants may be potential therapeutic approaches for lung diseases associated with sustained elevated adenosine.