Animal Studies

Kappa opioid receptor (KOR) agonists produce robust analgesia with minimal abuse liability and are considered promising pharmacological agents to manage chronic pain and itch. The KOR system is also notable for robust differences between the sexes, with females exhibiting lower analgesic response than males. Sexually dimorphic traits can be due to either the influence of gonadal hormones during development or adulthood, or due to the complement of genes expressed on the X or Y chromosome. Previous studies examining sex differences in KOR antinociception have relied on surgical or pharmacological manipulation of the gonads to determine whether sex hormones influence KOR function. While there are conflicting reports whether gonadal hormones influence KOR function, no study has examined these effects in context with sex chromosomes. Here, we use two genetic mouse models, the four core genotypes and XY*, to isolate the chromosomal and hormonal contributions to sex differences in KOR analgesia. Mice were treated with systemic KOR agonist (U50,488H) and thermal analgesia measured in the tail withdrawal assay. We found that KOR antinociception was influenced predominantly by the number of the X chromosomes. These data suggest that the dose and/or parental imprint on X gene(s) contribute significantly to the sexually dimorphism in KOR analgesia.

The management of persistent postsurgical pain and neuropathic pain remains a challenge in the clinic. Local anesthetics have been widely used as simple and effective treatment for these two disorders, but the duration of their analgesic effect is short. We here reported a new poly lactic-co-glycolic acid (PLGA)-coated ropivacaine that was continuously released in vitro for at least 6 days. Peri-sciatic nerve injection of the PLGA-coated ropivacaine attenuated paw incision-induced mechanical allodynia and heat hyperalgesia during the incisional pain period, and spared nerve injury-induced mechanical and cold allodynia for at least 7 days post-injection. This effect was dose-dependent. Peri-sciatic nerve injection of the PLGA-coated ropivacaine did not produce detectable inflammation, tissue irritation, or damage in the sciatic nerve and surrounding muscles at the injected site, dorsal root ganglion, spinal cord or brain cortex, although the scores for grasping reflex were mildly and transiently reduced in the higher dosage-treated groups. Perspective: Given that PLGA is an FDA-approved medical material, and that ropivacaine is used currently in clinical practice, the injectable PLGA-coated ropivacaine represents a new and highly promising avenue in the management of postsurgical pain and neuropathic pain.

We provided strong proof of concept evidence that somatostatin mediates potent analgesic and anti-inflammatory actions via its receptor subtype 4 (sst) located both at the periphery and the central nervous system. Therefore, sst agonists are promising novel drug candidates for neuropathic pain and neurogenic inflammation, but rational drug design was not possible due to the lack of knowledge about its 3-dimensional structure. We modeled the sst receptor structure, described its agonist binding properties, and characterized the binding of our novel small molecule sst agonists (4-phenetylamino-7H-pyrrolo[2,3-d]pyrimidine derivatives) using an in silico platform. In addition to the in silico binding data, somatostatin displacement by Compound 1 was demonstrated in the competitive binding assay on sst-expressing cells. In vivo effects were investigated in rat models of neurogenic inflammation and chronic traumatic neuropathic pain. We defined high- and low-affinity binding pockets of sst for our ligands, binding of the highest affinity compounds were similar to that of the reference ligand J-2156. We showed potent G-protein activation with the highest potency of 10 nM EC value and highest efficacy of 342%. Oral administration of 100 μg/kg of 5 compounds significantly inhibited acute neurogenic plasma protein extravasation in the paw skin by 40-60%, one candidate abolished and 3 others diminished sciatic nerve-ligation induced neuropathic hyperalgesia by 28-62%. The in silico predictions on sst-ligands were tested in biological systems. Low oral dose of our novel agonists inhibit neurogenic inflammation and neuropathic pain, which opens promising drug developmental perspectives for these unmet medical need conditions.

Latent sensitization is a model of chronic pain in which a persistent state of pain hypersensitivity is suppressed by opioid receptors, as evidenced by the ability of opioid antagonists to induce a period of mechanical allodynia. Our objective was to determine if substance P and its neurokinin 1 receptor (NK1R) mediate the maintenance of latent sensitization. Latent sensitization was induced by injecting rats in the hindpaw with complete Freund's adjuvant (CFA), or by tibial spared nerve injury (SNI). When responses to von Frey filaments returned to baseline (day 28), the rats were injected intrathecally with saline or the NK1R antagonist RP67580, followed 15 min later by intrathecal naltrexone. In both pain models, the saline-injected rats developed allodynia for 2 h after naltrexone, but not the RP67580-injected rats. Saline or RP67580 were injected daily for two more days. Five days later (day 35), naltrexone was injected intrathecally. Again, the saline-injected rats, but not the RP67580-injected rats, developed allodynia in response to naltrexone. To determine if there is sustained activation of NK1Rs during latent sensitization, NK1R internalization was measured in lamina I neurons in rats injected in the paw with saline or CFA, and then injected intrathecally with saline or naltrexone on day 28. The rats injected with CFA had a small amount of NK1R internalization that was significantly higher than in the saline-injected rats. Naltrexone increased NK1R internalization in the CFA-injected rats but nor in the saline-injected rats. Therefore, sustained activation of NK1Rs maintains pain hypersensitivity during latent sensitization.

Transient receptor potential melastatin 8 (TRPM8) ion channel represents a valuable pharmacological option for several therapeutic areas. Here, a series of conformationally restricted derivatives of the previously described TRPM8 antagonist N,N'-dibenzyl tryptophan were prepared and characterized in vitro by Ca-imaging and patch-clamp electrophysiology assays. Molecular modeling studies led to identification of a broad and well-defined interaction network of these derivatives inside the TRPM8-agonist binding site, underlying their antagonist activity. The (5R,11aS)-5-(4-chlorophenyl)-2-(4-fluorobenzyl)-5,6,11,11a-tetrahydro-1H-imidazo[1',5':1,6]pyrido[3,4-b]indole-1,3(2H)-dione () emerged as a potent (IC50 4.10 ± 1.2 nM), selective, and metabolically stable TRPM8 antagonist. In vivo, showed significant target coverage in an icilin-induced WDS (at 11.5 mg/kg i.p.), an oxaliplatin-induced cold allodynia (at 10-30 μg s.c.), and CCI-induced thermal hyperalgesia (at 11.5 mg/kg i.p.) mice models. These results confirm the tryptophan moiety as a solid pharmacophore template for the design of highly potent modulators of TRPM8-mediated activities.