Animal Studies

Chronic administration of opioids produces physical dependence and opioid-induced hyperalgesia. Users claim the Thai traditional tea "kratom" and component alkaloid mitragynine ameliorate opioid withdrawal without increased sensitivity to pain. Testing these claims, we assessed the combined kratom alkaloid extract (KAE) and two individual alkaloids, mitragynine (MG) and the analog mitragynine pseudoindoxyl (MP), evaluating their ability to produce physical dependence and induce hyperalgesia after chronic administration, and as treatments for withdrawal in morphine-dependent subjects. C57BL/6J mice (n = 10/drug) were administered repeated saline, or graded, escalating doses of morphine (intraperitoneal; i.p.), kratom alkaloid extract (orally, p.o.), mitragynine (p.o.), or MP (subcutaneously, s.c.) for 5 days. Mice treated chronically with morphine, KAE, or mitragynine demonstrated significant drug-induced hyperalgesia by day 5 in a 48 °C warm-water tail-withdrawal test. Mice were then administered naloxone (10 mg/kg, s.c.) and tested for opioid withdrawal signs. Kratom alkaloid extract and the two individual alkaloids demonstrated significantly fewer naloxone-precipitated withdrawal signs than morphine-treated mice. Additional C57BL/6J mice made physically dependent on morphine were then used to test the therapeutic potential of combined KAE, mitragynine, or MP given twice daily over the next 3 days at either a fixed dose or in graded, tapering descending doses. When administered naloxone, mice treated with KAE, mitragynine, or MP under either regimen demonstrated significantly fewer signs of precipitated withdrawal than control mice that continued to receive morphine. In conclusion, while retaining some liabilities, kratom, mitragynine, and mitragynine pseudoindoxyl produced significantly less physical dependence and ameliorated precipitated withdrawal in morphine-dependent animals, suggesting some clinical value.

Treatment of chronic pain remains an unmet medical need. The neuronal voltage-gated potassium Kv7/KCNQ/M channel has been implicated as a therapeutic target for chronic pain. However, whether pharmacological activation of Kv7 channel can alleviate pain remains elusive. In this study, we show that selective activation of native M currents by a novel channel opener SCR2682 reduces repetitive firings of dorsal root ganglia (DRG) sensory neurons. Intraperitoneal administration of SCR2682 relieves mechanical allodynia and thermal hyperalgesia in rat models of pain induced by complete Freund's adjuvant (CFA) or spared nerve injury (SNI) in a dose-dependent manner without affecting locomotor activity. The anti-nociceptive efficacy of SCR2682 can be reversed by the channel specific blocker XE991. Furthermore, SCR2682 increases Kv7.2/KCNQ2 mRNA and protein expression in DRG neurons from rats in the SNI model of neuropathic pain. Taken together, pharmacological activation of neuronal Kv7 channels by opener SCR2682 can alleviate pain in rats, thus possessing therapeutic potential for chronic pain or hyperexcitability-related neurological disorders. A novel Kv7 opener SCR2682 inhibits action potential firings in DRG sensory neurons and exhibits an efficacy in antinociception, thus possessing a developmental potential for treatment of chronic pain or epilepsy.

Translational regulation permeates neuronal function. Nociceptors are sensory neurons responsible for the detection of harmful stimuli. Changes in their activity, termed plasticity, are intimately linked to the persistence of pain. While inhibitors of protein synthesis robustly attenuate pain associated behavior, the underlying targets that support plasticity are largely unknown. Here, we examine the contribution of protein synthesis in regions of RNA annotated as non-coding. Based on analyses of previously reported ribosome profiling data, we provide evidence for widespread translation in non-coding transcripts and regulatory regions of mRNAs. We identify an increase in ribosome occupancy in the 5' untranslated regions of the calcitonin gene-related peptide (CGRP/Calca). We validate the existence of an upstream Open Reading Frame (uORF) using a series of reporter assays. Fusion of the uORF to a luciferase reporter revealed active translation in DRG neurons following nucleofection. Injection of the peptide corresponding to the CGRP encoded uORF resulted in pain associated behavioral responses in vivo and nociceptor sensitization in vitro. An inhibitor of heterotrimeric G protein signaling blocks both effects. Collectively, the data suggest pervasive translation in regions of the transcriptome annotated as non-coding in DRG neurons and identify a specific uORF encoded peptide that promotes pain sensitization through GPCR signaling.

Paclitaxel, a widely used anti-cancer drug, is frequently associated with prolonged and severe peripheral neuropathies (PIPN), associated with neuroinflammation. Currently, PIPN effective treatments are lacking. Peroxisome Proliferator-Activated Receptor-α (PPAR-⍺) can modulate inflammatory responses. Thus, the use of PPAR-⍺ agonists, such as fibrates (fenofibrate and choline-fenofibrate), currently used in dyslipidemia treatment, could represent an interesting therapeutic approach in PIPN.

TRPV1, a member of the transient receptor potential (TRP) family, is a nonselective calcium permeable ion channel gated by physical and chemical stimuli. In the skin, TRPV1 plays an important role in neurogenic inflammation, pain and pruritus associated to many dermatological diseases. Consequently, TRPV1 modulators could represent pharmacological tools to respond to important patient needs that still represent an unmet medical demand. Previously, we reported the design of capsaicinoid-based molecules that undergo dermal deactivation (soft drugs), thus preventing their long-term dermal accumulation. Here, we investigated the pharmacological properties of the lead antagonist, 2-((4-hydroxy-2-iodo-5-methoxybenzyl) amino)-2-oxoethyl dodecanoate (AG1529), on heterologously expressed human TRPV1 (hTRPV1), on nociceptor excitability and on an in vivo model of acute pruritus. We report that AG1529 competitively blocked capsaicin-evoked activation of hTRPV1 with micromolar potency, moderately affected pH-induced gating, and did not alter voltage- and heat-mediated responses. AG1529 displays modest receptor selectivity as it mildly blocked recombinant hTRPA1 and hTRPM8 channels. In primary cultures of rat dorsal root ganglion (DRG) neurons, AG1529 potently reduced capsaicin-evoked neuronal firing. AG1529 exhibited lower potency on pH-evoked TRPV1 firing, and TRPA1-elicited nociceptor excitability. Furthermore, AG1529 abolished histaminergic and inflammation mediated TRPV1 sensitization in primary cultures of DRG neurons. Noteworthy, dermal wiping of AG1529, either in an acetone-based formulation or in an anhydrous ointment, dose-dependently attenuated acute histaminergic itch in a rodent model. This cutaneous anti-pruritic effect was devoid of the normal nocifensive action evoked by the burning sensation of capsaicin. Taken together, these preclinical results unveil the mode of action of AG1529 on TRPV1 channels and substantiate the tenet that this capsaicinoid-based soft drug is a promising candidate for drug development as a topical anti-pruritic and anti-inflammatory medication.