Involvement of TRPM7 Channel on the Induction of Diabetic Neuropathic Pain in Mice: Protective Role of Selenium and Curcumin

TRPM7 Mediates Neuropathic Pain by Activating mTOR Signaling in Astrocytes after Spinal Cord Injury in Rats

In this study, we investigated whether transient receptor melastatin 7 (TRPM7), known as a non-selective cation channel, inhibits neuropathic pain after spinal cord injury (SCI) and how TRPM7 regulates neuropathic pain. Neuropathic pain was developed 4 weeks after moderate contusive SCI and TRPM7 was markedly upregulated in astrocytes in the lamina I and II of L4-L5 dorsal horn. In addition, both mechanical allodynia and thermal hyperalgesia were significantly alleviated by a TRPM7 inhibitor, carvacrol. In particular, carvacrol treatment inhibited mechanistic target of rapamycin (mTOR) signaling, which was activated in astrocytes. When rats were treated with rapamycin, an inhibitor of mTOR signaling, neuropathic pain was significantly inhibited. Furthermore, blocking TRPM7 and mTOR signaling by carvacrol and rapamycin inhibited astrocyte activation in lamina I and II of dorsal spinal cord and reduced the level of p-JNK and p-c-Jun, which are known to be activated in astrocytes. Finally, inhibiting TRPM7/mTOR signaling also downregulated the production of pain-related factors such as tumor necrosis factor-α, interleukin-6, interleukin-1β, chemokine (C-C motif) ligand (CCL) 2, CCL-3, CCL-4, CCL-20, chemokine C-X-C motif ligand 1, and matrix metalloproteinase 9 which are known to be involved in the induction and/or maintenance of neuropathic pain after SCI. These results suggest an important role of TRPM7-mediated mTOR signaling in astrocyte activation and thereby induction and/or maintenance of neuropathic pain after SCI.

Alpha-lipoic acid modulates the diabetes mellitus-mediated neuropathic pain via inhibition of the TRPV1 channel, apoptosis, and oxidative stress in rats

Diabetes mellitus (DM) is a chronic syndrome involving neuropathic pain. Increased oxidative stress in DM is assumed to increase free reactive oxygen radicals (ROS) and causes diabetic damage. The sciatic nerve (ScN) and dorsal root ganglion (DRG) both contain high levels of the TRPV1 channel, which is triggered by capsaicin and ROSs and results in increased Ca²⁺ entry into the neurons. Alpha-lipoic acid (ALA) is considered an important part of the antioxidant system. To better characterize the protective effects of ALA on the DM-induced neuronal through TRPV1 modulation, we investigated the role of ALA on DM-induced neuropathic pain, oxidative ScN, and DRG damage in diabetic rats. Forty adult Wistar albino female rats were divided into four groups as control, ALA (50 mg/kg for 14 days), streptozotocin (STZ and 45 mg/kg and single dose), and STZ + ALA. Rats were used for the pain tests. After obtaining the DRGs and ScN, they were used for plate reader, patch-clamp, and laser confocal microscope analyses. We observed the modulator role of ALA on the thresholds of mechanical withdrawal pain (von Frey test) and hot sensitivity pain (hot plate test) in the STZ + ALA group. The treatment of ALA decreased STZ-induced increase of TRPV1 current densities, intracellular free Ca²⁺ concentrations (Fura-2 and Fluo - 3/AM), ROS, caspase 3, caspase 9, mitochondrial membrane potential, and apoptosis values in the ScN and DRG neurons, although its treatment induced the increase of cell viability and body weight gain. The treatment of ALA acted a neuroprotective role on the TRPV1 channel stimulation-mediated Ca²⁺ influx, neuropathic pain, and neuronal damage in diabetic rats. The neuroprotective role of ALA treatment can be explained by its modulating the TRPV1 channel activity, intracellular Ca²⁺ increase-induced oxidative stress, and apoptosis.

Intraganglionic reactive oxygen species mediate inflammatory pain and hyperalgesia through TRPA1 in the rat

Reactive oxygen species (ROS) are generated in nociceptive pathways in response to inflammation and injury. ROS are accumulated within the sensory ganglia following peripheral inflammation, but the functional role of intraganlionic ROS in inflammatory pain is not clearly understood. The aims of this study were to investigate whether peripheral inflammation leads to prolonged ROS accumulation within the trigeminal ganglia (TG), whether intraganglionic ROS mediate pain hypersensitivity via activation of TRPA1, and whether TRPA1 expression is upregulated in TG during inflammatory conditions by ROS. We demonstrated that peripheral inflammation causes excess ROS production within TG during the period when inflammatory mechanical hyperalgesia is most prominent. Additionally, scavenging intraganglionic ROS attenuated inflammatory mechanical hyperalgesia and a pharmacological blockade of TRPA1 localized within TG also mitigated inflammatory mechanical hyperalgesia. Interestingly, exogenous administration of ROS into TG elicited mechanical hyperalgesia and spontaneous pain-like responses via TRPA1, and intraganglionic ROS induced TRPA1 upregulation in TG. These results collectively suggest that ROS accumulation in TG during peripheral inflammation contributes to pain and hyperalgesia in a TRPA1 dependent manner, and that ROS further exacerbate pathological pain responses by upregulating TRPA1 expression. Therefore, any conditions that exacerbate ROS accumulation within somatic sensory ganglia can aggravate pain responses and treatments reducing ganglionic ROS may help alleviate inflammatory pain.