Analgesic roles of peripheral intrinsic met-enkephalin and dynorphin A in long-lasting inflammatory pain induced by complete Freund's adjuvant in rats.

Previous studies have focused on strategies for pain relief based on the peripheral opioid system. However, little is known with regard to the profile of the peripheral opioid system in long-lasting inflammatory pain. In the current study, the intrinsic changes of the peripheral opioids were investigated in long-lasting inflammatory pain. A rat model of complete Freund's adjuvant (CFA)-induced inflammatory pain was established. Paw swelling and thermal hyperalgesia (paw withdrawal latency, PWL) were analyzed until day 18 after the CFA injection. The levels of peripheral opioids and their upstream inducers, corticotrophin-releasing factor (CRF) and interleukin (IL)-1ß, were measured, and validation experiments were performed using opioid receptor antagonists. Long-lasting inflammatory pain was successfully induced in the rats, as shown by the significantly increased paw swelling and decreased PWLs. On day 18 after the CFA injection, the IL-1ß levels were significantly elevated, while CRF remained at a normal level in the paw inflammatory tissue. In addition, met-enkephalin (Met-ENK) and dynorphin A (DYN A) levels were significantly increased, while the ß-endorphin level remained normal. Local intraplantar administration of δ- and κ-opioid receptor antagonists resulted in more substantial pain, but did not significantly affect the PWLs of the normal control rats. Therefore, the results indicated that the increased levels of local Met-ENK and DYN A in CFA-induced long-lasting inflammatory pain may be involved in peripheral intrinsic analgesia.

Silibinin activates AMP-activated protein kinase to protect neuronal cells from oxygen and glucose deprivation-re-oxygenation.

In this study, we explored the cytoprotective potential of silibinin against oxygen-glucose deprivation (OGD)-induced neuronal cell damages, and studied underling mechanisms. In vitro model of ischemic stroke was created by keeping neuronal cells (SH-SY5Y cells and primary mouse cortical neurons) in an OGD condition followed by re-oxygenation. Pre-treatment of silibinin significantly inhibited OGD/re-oxygenation-induced necrosis and apoptosis of neuronal cells. OGD/re-oxygenation-induced reactive oxygen species (ROS) production and mitochondrial membrane potential (MMP) reduction were also inhibited by silibinin. At the molecular level, silibinin treatment in SH-SY5Y cells and primary cortical neurons led to significant AMP-activated protein kinase (AMPK) signaling activation, detected by phosphorylations of AMPKα1, its upstream kinase liver kinase B1 (LKB1) and the downstream target acetyl-CoA Carboxylase (ACC). Pharmacological inhibition or genetic depletion of AMPK alleviated the neuroprotective ability of silibinin against OGD/re-oxygenation. Further, ROS scavenging ability by silibinin was abolished with AMPK inhibition or silencing. While A-769662, the AMPK activator, mimicked silibinin actions and suppressed ROS production and neuronal cell death following OGD/re-oxygenation. Together, these results show that silibinin-mediated neuroprotection requires activation of AMPK signaling.

Low Frequency Electroacupuncture Alleviated Spinal Nerve Ligation Induced Mechanical Allodynia by Inhibiting TRPV1 Upregulation in Ipsilateral Undamaged Dorsal Root Ganglia in Rats.

Neuropathic pain is an intractable problem in clinical practice. Accumulating evidence shows that electroacupuncture (EA) with low frequency can effectively relieve neuropathic pain. Transient receptor potential vanilloid type 1 (TRPV1) plays a key role in neuropathic pain. The study aimed to investigate whether neuropathic pain relieved by EA administration correlates with TRPV1 inhibition. Neuropathic pain was induced by right L5 spinal nerve ligation (SNL) in rats. 2 Hz EA stimulation was administered. SNL induced mechanical allodynia in ipsilateral hind paw. SNL caused a significant reduction of TRPV1 expression in ipsilateral L5 dorsal root ganglia (DRG), but a significant up-regulation in ipsilateral L4 and L6 DRGs. Calcitonin gene-related peptide (CGRP) change was consistent with that of TRPV1. EA alleviated mechanical allodynia, and inhibited TRPV1 and CGRP overexpressions in ipsilateral L4 and L6 DRGs. SNL did not decrease pain threshold of contralateral hind paw, and TRPV1 expression was not changed in contralateral L5 DRG. 0.001, 0.01 mg/kg TRPV1 agonist 6'-IRTX fully blocked EA analgesia in ipsilateral hind paw. 0.01 mg/kg 6'-IRTX also significantly decreased pain threshold of contralateral paw. These results indicated that inhibition of TRPV1 up-regulation in ipsilateral adjacent undamaged DRGs contributed to low frequency EA analgesia for mechanical allodynia induced by spinal nerve ligation.