Tetrodotoxin (TTX) is a potent neurotoxin found mainly in puffer fish and other marine and terrestrial animals. TTX blocks voltage-gated sodium channels (VGSCs) which are typically classified as TTX-sensitive or TTX-resistant channels. VGSCs play a key role in pain signaling and some TTX-sensitive VGSCs are highly expressed by adult primary sensory neurons. During pathological pain conditions, such as neuropathic pain, upregulation of some TTX-sensitive VGSCs, including the massive re-expression of the embryonic VGSC subtype Na_V1.3 in adult primary sensory neurons, contribute to painful hypersensitization. In addition, people with loss-of-function mutations in the VGSC subtype Na_V1.7 present congenital insensitive to pain. TTX displays a prominent analgesic effect in several models of neuropathic pain in rodents. According to this promising preclinical evidence, TTX is currently under clinical development for chemo-therapy-induced neuropathic pain and cancer-related pain. This review focuses primarily on the preclinical and clinical evidence that support a potential analgesic role for TTX in these pain states. In addition, we also analyze the main toxic effects that this neurotoxin produces when it is administered at therapeutic doses, and the therapeutic potential to alleviate neuropathic pain of other natural toxins that selectively block TTX-sensitive VGSCs.

Postoperative pain management is challenging. We used mice with a transverse laparotomy to study tactile allodynia measured by the von Frey test, pain at rest measured by facial pain expressions detected by an artificial intelligence algorithm, and movement-induced pain measured by reductions in exploratory activity. The standard analgesics morphine and ibuprofen induced distinct patterns of outcome-dependent effects. Whereas morphine was more effective in reversing pain at rest compared to tactile allodynia, it was unable to alter movement-induced pain. Ibuprofen showed comparable effects across the three outcomes. Administered together, the compounds induced synergistic effects in the three aspects of postoperative pain, mirroring the known advantages of multimodal analgesia used in clinical practice. We explored the impact of neuroimmune interactions using a neutrophil depletion strategy. This reversed pain at rest and movement-induced pain, but did not alter cutaneous sensitivity. Non-peptidergic (IB4+) and peptidergic (CGRP+) nociceptors are segregated neuronal populations in the mouse. We tested the effects of gefapixant, an antitussive drug targeting non-peptidergic nociceptors through P2X3 antagonism, and olcegepant, an antimigraine drug acting as a CGRP antagonist. Both compounds reversed tactile allodynia, while only gefapixant reversed pain at rest, and none of them reversed movement-induced pain. In conclusion, tactile allodynia, pain at rest, and movement-induced pain after surgery have different pharmacological profiles, and none of the three aspects of postoperative pain can predict the effects of a given intervention on the other two. Combining these measures provides a more realistic view of postoperative pain and has the potential to benefit analgesic development.

Postoperative pain remains a major clinical challenge, as it often persists despite analgesic treatment even with opioids. We studied the effects of sigma-1 receptor antagonists (BD-1063 and S1RA), administered alone or in combination with the µ-opioid morphine, on three key aspects of postoperative pain in mice with a transverse laparotomy: tactile allodynia, pain at rest, and movement-induced pain. Sigma-1 antagonism and morphine induced antiallodynic effects sensitive to peripheral opioid antagonism by naloxone methiodide, although only sigma-1 antagonism was sensitive to the sigma-1 agonist PRE-084. The antiallodynic effect of sigma-1 antagonism was also reversed by the μ-opioid antagonist cyprodime and by depletion of neutrophils, which express high levels of proopiomelanocortin, the precursor of the µ-opioid agonist ß-endorphin. Morphine, but not sigma-1 antagonism, reversed pain at rest, and none of the drugs tested improved movement-induced pain. Notably, the combination of S1RA and morphine at doses ineffective when administered alone, fully reversed tactile allodynia, pain at rest, and movement-induced pain, and in a manner sensitive to PRE-084 and naloxone methiodide, indicating the simultaneous participation of both sigma-1 and peripheral opioid receptors. Therefore, sigma-1 antagonism boosts the actions of endogenous opioid peptides from neutrophils only to reverse tactile allodynia, but when combined with morphine, it enhances peripheral opioid analgesia to reverse all aspects of postoperative pain. Finally, S1RA did not enhance morphine-induced inhibition of gastrointestinal transit or rewarding effects. Modulation of opioid analgesia by sigma-1 receptors might have potential clinical application to increase the therapeutic range of opioids in the treatment of postoperative pain.

The soluble epoxide hydrolase (sEH) has recently emerged as a promising target for the treatment of several pain-related conditions. Herein, we report the design and synthesis of a peripherally restricted sEH inhibitor with high potency and good Drug Metabolism and Pharmacokinetics (DMPK) properties. Molecular dynamics and X-ray crystallography helped reveal the binding of these inhibitors to sEH. The selected compound showed a robust analgesic effect in a dose-dependent manner in a murine model of chemotherapy-induced neuropathic pain (CINP). Moreover, the compound also prevented the development of paclitaxel-induced neuropathic pain. Overall, these results suggest that peripheral inhibition of sEH might constitute a novel therapy to prevent and treat CINP.

Background/Objectives: Postoperative pain pharmacology is complex. We investigated the sensitivity of analgesic-like effects induced by morphine, ibuprofen, and their combination to peripheral opioid antagonism in a mouse laparotomy model. Methods: Mechanical hypersensitivity was assessed using von Frey filaments, and ongoing pain (abdominal licking and facial expressions) was evaluated using artificial intelligence algorithms. We tested the sensitivity of the analgesic treatments to the opioid antagonist naloxone or its peripherally restricted analog, naloxone methiodide. We also tested the effects of neutrophil depletion using an anti-Ly6G antibody. Gastrointestinal transit and pupillary diameter were measured to assess non-analgesic opioid effects. Results: Morphine reversed all pain-related behaviors; its effect on mechanical hypersensitivity was reversed by peripheral opioid antagonism, whereas its effects on ongoing pain were not. Ibuprofen reduced mechanical hypersensitivity and facial expressions but failed to alter licking. Interestingly, the ibuprofen effect on mechanical hypersensitivity depended on peripheral opioid receptors and neutrophils at the injury site. The morphine–ibuprofen combination produced synergistic analgesia across all endpoints without enhancing opioid-induced gastrointestinal inhibition or mydriasis. Peripheral opioid antagonism reversed the effect of the combination on mechanical hypersensitivity and facial expressions but not on licking. Conclusions: Our results replicate the key clinical phenomena relevant to the postoperative pain context, including the potentiation of morphine analgesia by ibuprofen without the exacerbation of adverse effects. Our results suggest that drug effects on different postoperative pain measures rely on distinct neurobiological mechanisms and are not interchangeable. Therefore, the use of a battery of complementary pain endpoints in preclinical pharmacology studies is advisable.