Guenther, K. G. | bioRxiv Science

bioRxiv2025

Assessments of Evoked and Spontaneous Pain Following Administration of Gabapentin and the Cannabinoid CB2 agonist LY2828360 in a Rat Model of Spared Nerve Injury

Cannabinoid CB2 agonists reduce stimulus-evoked behavioral hypersensitivities in preclinical pain models, but their ability to modulate spontaneous pain remains unexplored. Spontaneous pain has been assessed in rodents using the conditioned place preference (CPP) approach, given that the relief of pain is described as rewarding and results in negative reinforcement (i.e. removal of an aversive pain state). LY2828360 is a CB2 agonist that failed in a clinical trial for osteoarthritis pain. We compared impact of LY2828360 on evoked and spontaneous pain using a spared nerve injury (SNI) model in rats. First, we verified that an analgesic dose of gabapentin (100 mg/kg i.p.) produces CPP in rats with SNI, but not in sham-operated rats, consistent with a previous report (Griggs et al. 2015). We then used a within-subjects design to ascertain whether the CB2 agonist LY2828360 (10 mg/kg i.p., chronic) would suppress both evoked and spontaneous pain in rats with SNI. To assess evoked pain behavior, mechanical paw withdrawal thresholds were measured and revealed that LY2828360 reliably suppressed mechanical hypersensitivity in the paw ipsilateral, but not contralateral to SNI. Furthermore, efficacy was sustained across repeated injections without development of tolerance. To assess spontaneous pain behavior, we tested the ability of LY2828360 to prevent gabapentin-induced CPP in the SNI model, as failure to develop CPP to gabapentin following treatment with an analgesic has been considered evidence of suppression of spontaneous pain. The same rats that showed suppression of mechanically-evoked responses following chronic LY282860 treatment did not develop CPP to gabapentin. However, rats that were tested in parallel and treated chronically with vehicle showed robust mechanical hypersensitivity, but also did not develop CPP to gabapentin. These studies document that CB2 agonist-induced suppression of mechanically evoked pain is highly robust and reproducible, whereas CPP, used to assess spontaneous pain, is vulnerable to disruption and requires rigorous controls to rule out alternative explanations (e.g. failure to learn).

pharmacology and toxicology↗

bioRxiv2024

Cannabinoid CB2 receptors in primary sensory neurons are implicated in CB2 agonist-mediated suppression of paclitaxel-induced neuropathic nociception and sexually-dimorphic sparing of morphine tolerance

Cannabinoid CB2 agonists show therapeutic efficacy without the unwanted side effects commonly associated with direct activation of CB1 receptors. The G protein-biased CB2 receptor agonist LY2828360 attenuates the maintenance of chemotherapy-induced neuropathic nociception in male mice and blocks the development of morphine tolerance in this model. However, the specific cell types involved in this phenomenon have never been investigated and whether this therapeutic profile is observed in female mice remains poorly understood. We used conditional deletion of CB2 receptors from specific cell populations to determine the population(s) mediating the anti-allodynic and morphine-sparing effects of CB2 agonists. Anti-allodynic effects of structurally distinct CB2 agonists (LY2828360 and AM1710) were present in paclitaxel-treated CB2f/f mice of either sex. The anti-allodynic effect of the CB2 agonists were absent in conditional knockout (KO) mice lacking CB2 receptors in peripheral sensory neurons (AdvillinCRE/+; CB2f/f) but preserved in mice lacking CB2 receptors in CX3CR1 expressing microglia/macrophages (CX3CR1CRE/+; CB2f/f). The morphine-sparing effect of LY28282360 occurred in a sexually-dimorphic manner, being present in male mice but absent in female mice of any genotype. In mice with established paclitaxel-induced neuropathy, prior LY2828360 treatment (3 mg/kg per day i.p. x 12 days) blocked the subsequent development of morphine tolerance in male CB2f/f mice but was absent in male (or female) AdvillinCRE/+; CB2f/f mice. LY2828360-induced sparing of morphine tolerance was preserved in male CX3CR1CRE/+; CB2f/f mice, but this effect was not observed in female CX3CR1CRE/+; CB2f/f mice. Similarly, co-administration of morphine with a low dose of LY2828360 (0.1 mg/kg per day i.p. x 6 days) reversed tolerance to the anti-allodynic efficacy of morphine in paclitaxel-treated male CB2f/f mice, but this effect was absent in female CB2f/f mice and AdvillinCRE/+; CB2f/f mice of either sex. Additionally, LY2828360 (3 mg/kg per day i.p. x 8 days) delayed, but did not prevent, the development of paclitaxel-induced mechanical and cold allodynia in either CB2f/f or CX3CR1CRE/+; CB2f/f mice of either sex. Our studies reveal that CB2 receptors in primary sensory neurons are required for the anti-allodynic effects of CB2 agonists in a mouse model of paclitaxel-induced neuropathic nociception. We also find that CB2 agonists acting on primary sensory neurons produce a sexually-dimorphic sparing of morphine tolerance in males, but not female, paclitaxel-treated mice.

neuroscience↗