Animal Studies

Long-term neuropathic pain can lead to anxiety, depression, and other issues, which seriously affect patients' quality of life. For this reason, it is important to find effective treatments. Studies have shown that glial cell-derived neurotrophic factor (GDNF) can relieve neuropathic pain. However, its mechanism of action is unknown. Our previous study of GDNF suggested that the N-cadherin-β-catenin transmembrane signaling system might play a role in GDNF transmembrane signaling. Based on this, the current study aimed to produce a neuropathic pain model to confirm the activation of the N-cadherin-β-catenin signaling system in the spinal dorsal horn under pain conditions and to study the impact of GDNF intrathecal injection on central sensitization of dorsal horn neurons. The results showed that N-cadherin expression, as well as the expression of membrane-associated β-catenin, was reduced in the dorsal horn of the spinal cord in the chronic pain model. Intrathecal injection of GDNF could reactivate the N-cadherin-β-catenin system, improve central sensitization, and relieve pain. Knockdown of N-cadherin or β-catenin could significantly reduce the analgesic effect of GDNF. These results provide clear experimental evidence that the N-cadherin-β-catenin signaling system participates in the analgesic effect of GDNF in neuropathic pain and help identify transmembrane and intracellular signal transduction mechanisms associated with GDNF's analgesic effects.

We recently demonstrated in rat spinal cord that a regimen of escalating doses of systemic morphine, analogous to regimens used clinically for chronic pain management, selectively upregulates the mu-opioid receptor (MOR) splice variants MOR-1B2 and MOR-1C1 mRNA and functional protein. The current study investigated the potential relevance of upregulating MOR-1B2 and MOR-1C1 to the ability of chronic morphine to shift MOR signaling from predominantly G /G inhibitory to G stimulatory. Specifically, we tested the hypotheses that chronic morphine induces phosphorylation of carboxyl terminal sites unique to MOR-1B2 and MOR-1C1, and that this phosphorylation is causally related to augmented association of these variants with G α. Hypotheses were validated by (1) abolition of the chronic morphine-induced increment in MOR-1C1 and MOR-1B2 association with G α by inhibitors of protein kinase A and Casein kinase 2, respectively; (2) failure of chronic morphine to augment MOR variant G α interactions in Chinese hamster ovary cells transiently transfected with either rat MOR-1C1 or MOR-1B2 in which targeted protein kinase A and Casein kinase 2 serine phosphorylation sites, respectively, were mutated to alanine; (3) abrogation of chronic morphine-induced augmented MOR G α association in spinal cord of male rats following intrathecal administration of dicer substrate small interfering RNAs targeting MOR-1B2/MOR-1C1 mRNA. The ability of chronic morphine to not only upregulate specific MOR variants but also their carboxyl terminal phosphorylation and consequent augmented association with G α may represent a novel component of opioid tolerance mechanisms, suggesting novel potential targets for tolerance abatement. This article is protected by copyright. All rights reserved.

Metformin, an adenosine monophosphate (AMP)-activated protein kinase activator, as well as a common drug for type 2 diabetes, has previously been shown to decrease mechanical allodynia in mice with neuropathic pain. The objective of this study is to determine if treatment with metformin during the first 3 weeks after fracture would produce a long-term decrease in mechanical allodynia and improve a complex behavioral task (burrowing) in a mouse tibia fracture model with signs of complex regional pain syndrome.

Oxaliplatin is a cornerstone chemotherapeutic used in the treatment of colorectal cancer, the third leading cause of death in Western countries. Most side effects of this platinum-containing drug are adequately managed in the clinic, although acute and long-term neurotoxicity still severely compromises the quality of life of patients treated with oxaliplatin.We have previously demonstrated that therapeutically relevant concentrations/doses of oxaliplatin lead to a reduction in intracellular pH in mouse dorsal root ganglia neurons in vitro and in vivo and that this alteration sensitizes TRPA1 and TRPV1 channels, which most likely mediate the allodynia associated with treatment. In the present manuscript, we show that oxaliplatin leads to a reduction of intracellular pH by forming adducts with neuronal haemoglobin, which acts in this setting as a proton buffer. Furthermore, we show that FDA-approved drugs that inhibit carbonic anhydrase (an enzyme that is linked to haemoglobin in intracellular pH homeostasis), i.e. topiramate and acetazolamide, revert (i) oxaliplatin-induced cytosolic acidification and TRPA1 and TRPV1 modulation in dorsal root ganglia neurons in culture, (ii) oxaliplatin-induced cytosolic acidification of dorsal root ganglia of treated animals, and (iii) oxaliplatin-induced acute cold allodynia in mice while not affecting OHP-induced cytotoxicity on cancer cells.Our data would therefore suggest that reversal of oxaliplatin-induced cytosolic acidification is a viable strategy to minimize acute oxaliplatin-induced symptoms.

Agonists at the δ opioid receptor are known to be potent anti-hyperalgesics in chronic pain models and to be effective in models of anxiety and depression. However, some δ opioid agonists have pro-convulsant properties whilst tolerance to the therapeutic effects can develop. Previous evidence indicates that different agonists acting at the δ opioid receptor differentially engage signalling and regulatory pathways with significant effects on behavioural outcomes. As such, interest is now growing in the development of biased agonists as a potential means to target specific signalling pathways and potentially improve the therapeutic profile of δ opioid agonists. Here we report on PN6047 (3-[[4-(dimethylcarbamoyl)phenyl]-[1-thiazol-5-ylmethyl)-4-piperidylidene]methyl]benzamide) a novel G protein-biased and selective δ opioid agonist. In cell based assays PN6047 fully engages G protein signalling but is a partial agonist in both the arrestin recruitment and internalisation assays. PN6047 is effective in rodent models of chronic pain but shows no detectable analgesic tolerance following prolonged treatment. In addition, PN6047 exhibited antidepressant-like activity in the forced swim test and importantly, the drug had no effect on chemically induced seizures. PN6047 did not exhibit reward-like properties in the conditioned place preference test or induce respiratory depression. Thus, δ opioid ligands with limited arrestin signalling such as PN6047 may be therapeutically beneficial in the treatment of chronic pain states. SIGNIFICANCE STATEMENT: PN6047 is a selective, G protein-biased δ opioid agonist with efficacy in preclinical models of chronic pain. No analgesic tolerance was observed after prolonged treatment and PN6047 does not display pro-convulsant activity or other opioid-mediated adverse effects. Our data suggest that δ opioid ligands with limited arrestin signalling will be beneficial in the treatment of chronic pain.