CCR2 upregulation in DRG neurons plays a crucial role in gastric hyperalgesia associated with diabetic gastropathy

Motor Dysfunction of Gastric Antral Smooth Muscle in Diabetic Rats: Contribution of ATP-Dependent Potassium Channels

Background

The goal of the current research was to further elucidate the role of adenosine triphosphate (ATP)-sensitive potassium (KATP) channels in the motility and contractility force of gastric smooth muscle of diabetic rats.

Materials and Methods

Male Wistar rats (190-230 g) were grouped into control and streptozotocin (STZ)-induced diabetes (55 mg/kg) rats. Thirty days later, gastric muscle contractility was measured using a myograph and a force transducer of antral segments immersed in a tissue bath. Gastric emptying response was measured through feeding of standard pellet. Furthermore, the expression of KATP channel subunits in antral smooth muscle was determined by western blot technique.

Results

The amplitude of KCl-evoked twitch contractions of diabetic antral strips was about 25% more than control (P < 0.05). Application of minoxidil, a KATP channel opener, dose dependently decreased the force of twitch contractions in both normal and diabetic antral strips. Application of 10 μM glibenclamide, a KATP channel blocker, did not antagonize the minoxidil-induced relaxation of antral strips. Diabetic gastric emptying was faster than normal, although not significant. Despite the relaxant effect of minoxidil on gastric emptying rate in normal rats (P < 0.05), this effect was not observed in diabetic rats. Also, glibenclamide increased gastric emptying and antagonized minoxidil-induced relaxation in normal rats (P < 0.05). Furthermore, the expression of KATP Kir6.1 and SUR2B subunits was substantially reduced in antral smooth muscle in diabetic condition (P < 0.01).

Conclusion

These results propose that KATP channels may contribute to the development of gastric motility disorders in diabetes.

Targeting the chemokine ligand 2-chemokine receptor 2 axis provides the possibility of immunotherapy in chronic pain

Chronic pain affects patients' physical and psychological health and quality of life, entailing a tremendous public health challenge. Currently, drugs for chronic pain are usually associated with a large number of side effects and poor efficacy. Chemokines in the neuroimmune interface combine with their receptors to regulate inflammation or mediate neuroinflammation in the peripheral and central nervous system. Targeting chemokines and their receptor-mediated neuroinflammation is an effective means to treat chronic pain. In recent years, growing evidence has shown that the expression of chemokine ligand 2 (CCL2) and its main chemokine receptor 2 (CCR2) is involved in its occurrence, development and maintenance of chronic pain. This paper summarises the relationship between the chemokine system, CCL2/CCR2 axis, and chronic pain, and the CCL2/CCR2 axis changes under different chronic pain conditions. Targeting chemokine CCL2 and its chemokine receptor CCR2 through siRNA, blocking antibodies, or small molecule antagonists may provide new therapeutic possibilities for managing chronic pain.

Hedgehog signaling plays a crucial role in hyperalgesia associated with neuropathic pain in mice

Neuropathic pain is a debilitating chronic syndrome of the nervous system caused by nerve injury. In Drosophila, the Hedgehog (Hh) signaling pathway is related to increased pain sensitivity (hyperalgesia) but does not affect the baseline nociceptive threshold. In general, the contribution of the Hh signaling pathway to neuropathic pain in vertebrates is a highly debated issue. Alternatively, we investigated the potential role of Hh signaling in mechanical allodynia using a mouse model of neuropathic pain. Seven days after spinal nerve-transection (SNT) surgery, microglial activation increased in the ipsilateral spinal dorsal horn compared with that in the sham group; however, 21 days after surgery, microglial activation decreased. Contrastingly, astrocyte activation in the spinal cord did not differ between the groups. On day 21 of postsurgery, the SNT group showed marked upregulation of sonic hedgehog expression in peripheral glial cells but not in dorsal root ganglion (DRG) neurons. Intrathecal administration of the Hh signaling inhibitor vismodegib attenuated the mechanical allodynia observed on day 21 postsurgery. Conversely, intrathecal treatment with the Hh signaling activator smoothened agonist in naive mice induced mechanical allodynia, which was abolished by the ATP transporter inhibitor clodronate. Moreover, inhibition of Hh signaling by pretreatment with vismodegib significantly reduced ATP secretion and the frequency/number of spontaneous elevations of intracellular calcium ion levels in cultured DRG cells. Thus, the Hh signaling pathway appears to modulate the neural activity of DRG neurons via ATP release, and it plays an important role in sustaining mechanical allodynia and hypersensitivity in a mouse model of neuropathic pain.

Blockade of CCR4 Diminishes Hypersensitivity and Enhances Opioid Analgesia - Evidence from a Mouse Model of Diabetic Neuropathy

Chemokine signaling has been implicated in the pathogenesis of diabetic neuropathy; however, the role of chemokine CC motif receptor 4 (CCR4) remains unknown. The goal was to examine the function of CCR4 in hypersensitivity development and opioid effectiveness in diabetic neuropathy. Streptozotocin (STZ; 200 mg/kg, intraperitoneally administered)-induced mouse model of diabetic neuropathy were used. An analysis of the mRNA/protein expression of CCR4 and its ligands was performed by qRT-PCR, microarray and/or Western blot methods. C021 (CCR4 antagonist), morphine and buprenorphine were injected intrathecally or intraperitoneally, and pain-related behavior was evaluated by the von Frey, cold plate and rotarod tests. We observed that on day 7 after STZ administration, the blood glucose level was increased, and as a consequence, hypersensitivity to tactile and thermal stimuli developed. In addition, we observed an increase in the mRNA level of CCL2 but not CCL17/CCL22. The microarray technique showed that the CCL2 protein level was also upregulated. In naive mice, the pronociceptive effect of intrathecally injected CCL2 was blocked by C021, suggesting that this chemokine acts through CCR4. Importantly, our results provide the first evidence that in a mouse model of diabetic neuropathy, single intrathecal and intraperitoneal injections of C021 diminished neuropathic pain-related behavior in a dose-dependent manner and improved motor functions. Moreover, both single intrathecal and intraperitoneal injections of C021 enhanced morphine and buprenorphine effectiveness. These results reveal that pharmacological modulation of CCR4 may be a good potential therapeutic target for the treatment of diabetic neuropathy and may enhance the effectiveness of opioids.

Overexpression of Purinergic P2X4 Receptors in Hippocampus Rescues Memory Impairment in Rats with Type 2 Diabetes

Purinergic receptors have been reported to be involved in brain disorders. In this study, we explored their roles and mechanisms underlying the memory impairment in rats with type 2 diabetes mellitus (T2DM). T2DM rats exhibited a worse performance in the T-maze and Morris water maze (MWM) than controls. Microglia positive for P2X purinoceptor 4 (P2X4R) in the hippocampus were reduced and activated microglia were increased in T2DM rats. Long Amplicon PCR (LA-PCR) showed that DNA amplification of the p2x4r gene in the hippocampus was lower in T2DM rats. Minocycline significantly reduced the number of activated microglia and the mean distance traveled by T2DM rats in the MWM. Most importantly, P2X4R overexpression suppressed the activated microglia and rescued the memory impairment of T2DM rats. Overall, T2DM led to excessive activation of microglia in the hippocampus, partly through the DNA damage-mediated downregulation of P2X4Rs, thus contributing to memory impairment.

CCL2-CCR2 Axis Potentiates NMDA Receptor Signaling to Aggravate Neuropathic Pain Induced by Brachial Plexus Avulsion

Brachial plexus avulsion (BPA) represents the most devastating nerve injury in the upper extremity and is always considered as a sophisticated problem due to its resistance to most standard pain relief medications or neurosurgical interventions. There is also a lack of understanding on the underlying mechanisms. Our study aimed to investigate whether spinal CCL2-CCR2 signaling contributed to the development of neuropathic pain following BPA via modulating glutamate N-methyl-d-aspartate receptor (NMDAR). A rat model of BPA on lower trunk (C8-T1) was established, and the sham- and BPA-operated animals were intrathecally injected with saline, C-C chemokine receptor type 2 (CCR2) inhibitor INCB3344 and NMDAR antagonist DL-AP5 one week postoperatively, the behavioral performance of the treated animals and expressions of C-C motif ligand 2 (CCL2), CCR2, and N-methyl-D-aspartic acid receptor 2B (NR2B) in spinal cord sections of each group were examined. It was shown that BPA injury significantly reduced mechanic withdrawal thresholds the next day after surgery until the end of the observation. Both CCL2 and CCR2 expressions increased in BPA rats compared to those in sham rats. CCL2 was mainly localized in astrocytes, and CCR2 was preferably expressed on astrocytes and neurons. Besides, NMDAR subunit NR2B increased in BPA-operated rats, which was reversed in response to CCR2 and NR2B inhibition. However, these inhibitors didn't change the spinal NMDAR level in sham rats. CCR2 and NMDAR inhibition efficiently alleviated mechanical allodynia caused by BPA either at early or late phase of neuropathic pain. Collectively, CCL2-CCR2 axis is associated with mechanical pain after BPA by elevating NMDAR signaling.

The role of peripheral corticotropin-releasing factor signaling in a rat model of stress-induced gastric hyperalgesia

BACKGROUND

Functional dyspepsia (FD) is a common gastrointestinal disorder associated with persistent or recurrent upper gastrointestinal tract symptoms such as pain without any obvious pathological changes. Psychological and psychiatric factors might have a pathogenic role in FD. Changes in the sensation of stomach pain were determined after application of stress to adult rats. The involvement of corticotropin-releasing factor (CRF), Type 2 CRF receptor (CRF₂) and inflammatory cytokine interleukin-6 (IL-6) was also investigated in the gastric hyperalgesia observed in this model.

RESULTS

Repeated water avoidance stress (WA-S) produced gastric hyperalgesia, with no obvious lesions in the gastric mucosa. Gastric hyperalgesia was inhibited by CRF and CRF₂ antagonists, suggesting their involvement in gastric hyperalgesia observed after application of stress. Gastric hyperalgesia was inhibited by IL-6 neutralizing antibody. Immunofluorescence staining demonstrated CRF, CRF₂, urocortin (Ucn)1, and Ucn2-positive cells in the gastric mucosa. CRF₂-positive cells increased after WA-S, compared to sham stress. CRF₂ and Ucn2 were expressed in the mast cells in the gastric mucosa.

CONCLUSIONS

CRF₂ plays an important role in gastric hyperalgesia produced by stress. CRF₂ signaling may be a useful therapeutic target for functional dyspepsia.

Downregulation of glucose-6-phosphate dehydrogenase contributes to diabetic neuropathic pain through upregulation of toll-like receptor 4 in rats

Background and aim

Diabetic neuropathic pain is a refractory and disabling complication of diabetes mellitus. The pathogenesis of the diabetic neuropathic pain is still unclear, and treatment is insufficient. The aim of this study is to investigate the roles of glucose-6-phosphate dehydrogenase (G6PD) and toll-like receptor 4 (TLR4) in neuropathic pain in rats with diabetes.

Methods

Type 1 diabetes model was induced by intraperitoneal injection of streptozotocin (STZ, 75 mg/kg) in adult female Sprague-Dawley rats. Paw withdrawal threshold and paw withdrawal latency of rats were measured by von Frey filaments and thermal radiation, respectively. The expressions of G6PD and TLR4 in L4-L6 dorsal root ganglions (DRGs) were measured by western blotting and quantitative real-time polymerase chain reaction analysis. Fluorescent immunohistochemistry was employed to detect expressions of G6PD and TLR4 and co-location of G6PD with TLR4.

Results

The mRNA and protein expression levels of G6PD in DRGs were significantly decreased in diabetic rats when compared with age-matched control rats. Upregulation of G6PD by intrathecal injection of G6PD overexpression adenovirus markedly attenuated hindpaw pain hypersensitivity of diabetic rats. The mRNA and protein expression levels of TLR4 in DRGs of diabetic rats were significantly increased when compared with control rats. Intrathecal injection of TLR4-selective inhibitor CLI-095 attenuated diabetic pain in dose- and time-dependent manners. Furthermore, G6PD and TLR4 were co-localized in DRG neurons. Intrathecal injection of G6PD overexpression adenovirus greatly reduced TLR4 expression, while intrathecal injection of CLI-095 had no significant effect on G6PD expression in diabetic rats.

Conclusions

Our results suggest that decrease in G6PD expression was involved in diabetic peripheral neuropathic pain, which was most likely through upregulation of TLR4 expression in the DRGs of rats.