Conditional deletion of CB2 cannabinoid receptors from peripheral sensory neurons eliminates CB2-mediated antinociceptive efficacy in a mouse model of carrageenan-induced inflammatory pain

The cannabinoid CB2 agonist LY2828360 suppresses neuropathic pain behavior and attenuates morphine tolerance and conditioned place preference in rats

Cannabinoid CB2 agonists show promise as analgesics because they lack unwanted side effects associated with direct activation of CB1 receptors. CB2 receptor activation suppresses pathological pain in animal models, but the types of pain that best respond to CB2 agonists are incompletely understood. This gap in knowledge may contribute to failures in clinical translation. We previously showed that the G protein-biased CB2 receptor agonist LY2828360 attenuated the maintenance of neuropathic pain behavior in mouse models of inflammatory and neuropathic pain. Whether this finding generalizes to neuropathic pain induced by traumatic nerve injury or occurs in multiple rodent species remains unknown. Here we show that LY2828360 (3 and 10 mg/kg i.p.), administered acutely, reversed paclitaxel-induced mechanical hypersensitivity in male rats. By contrast, LY2828360 (10 mg/kg i.p.), administered acutely, attenuated mechanical hypersensitivity in a spared nerve injury (SNI) rat model, whereas the low dose (3 mg/kg i.p.) was ineffective. In both models, efficacy of LY2828360 was sustained following 10 days of repeated dosing. LY2828360 (3 mg/kg i.p.) also prevented development of tolerance to the opioid analgesic morphine (6 mg/kg i.p.) in rats with SNI when co-administered. LY2828360 (3 mg/kg i.p.) did not produce preference or aversion in the conditioned place preference (CPP) test in rats when administered alone but blocked CPP to morphine (6 mg/kg i.p.). Lastly, LY2828360 (3 mg/kg i.p.) did not alter the acquisition of i.v. morphine self-administration under fixed ratio 1 (FR1) and 3 (FR3) or motivation to work for morphine under a progressive ratio (PR) schedule of reinforcement.

Discovery of Novel pH-Sensitive μ-Opioid Receptor Agonists as Potent Analgesics with Reduced Side Effects

Although μ-opioid receptor (MOR) agonists are the most effective drugs for relieving acute pain, nonselective activation of MOR can also lead to serious side effects. There is an urgent need for novel analgesics that can selectively activate MOR under pathological conditions while avoiding side effects under normal physiological conditions. In this study, a series of pH-sensitive 4-propionamide piperidine derivatives were synthesized and evaluated for their MOR activities and antinociceptive effects. Among them, compound 22 showed high pH sensitivity for MOR with a K i pH 7.4/K i pH 6.4 ratio of 6.6. Compound 22 acted as an MOR agonist in the functional test. Compound 22 exhibited good antinociceptive effects in the acetic acid-induced writhing test (ED50 = 1.5 mg/kg) and carrageenan-induced inflammatory pain model (ED50 = 3.3 mg/kg) in mice. Moreover, compound 22 showed reduced side effects when compared to the equianalgesic dose of fentanyl, including physical dependence, hyperlocomotion, and constipation. Compound 22 holds promise as a safe candidate for further development as an analgesic with diminished side effects.

Peripherally restricted cannabinoid and mu-opioid receptor agonists synergistically attenuate neuropathic mechanical hypersensitivity in mice

ABSTRACT

Many medications commonly used to treat neuropathic pain are associated with significant, dose-limiting adverse effects, including sedation, dizziness, and fatigue. These adverse effects are due to the activity of these medications within the central nervous system. The objective of this work was to investigate the interactions between peripherally restricted cannabinoid receptor and mu-opioid receptor (MOR) agonists on ongoing and evoked neuropathic pain behaviors in mouse models. RNAscope analysis of cannabinoid receptor type 1 (CB1R) and MOR mRNA demonstrated that the mRNA of both receptors is colocalized in both mouse and human dorsal root ganglion. Single-cell RNAseq of dorsal root ganglion from chronic constriction injury mice showed that the mRNA of both receptors ( Cnr1 and Oprm1 ) is coexpressed across different neuron clusters. Myc-CB1R and FLAG-MOR were cotransfected into immortalized HEK-293T cells and were found to interact at a subcellular level. We also find that CB-13 (a peripherally restricted dual CB1R and cannabinoid receptor type 2 agonist) and DALDA (a peripherally restricted MOR agonist) both attenuate mechanical hypersensitivity in a murine model of neuropathic pain. Using isobolographic analysis, we demonstrate that when coadministered, these agents synergistically attenuate mechanical hypersensitivity. Importantly, combination dosing of these agents does not cause any detectable preferential behaviors or motor impairment. However, repeated dosing of these agents is associated with the development of tolerance to these drugs. Collectively, these findings suggest that leveraging synergistic pain inhibition between cannabinoid receptor and MOR agonists in peripheral sensory neurons may be worth examining in patients with neuropathic pain.

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 unwanted CB1-mediated side effects. The G protein-biased CB2 receptor agonist LY2828360 attenuates the maintenance of chemotherapy-induced neuropathic nociception in male mice and blocks development of morphine tolerance in this model. However, the cell types involved in this phenomenon are unknown and whether this therapeutic profile is observed in female mice has never been investigated. We used conditional deletion of CB2 receptors to determine the cell 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 and in mice lacking CB2 receptors in CX3CR1 expressing microglia/macrophages (CX3CR1CRE/+; CB2f/f), but were absent in mice lacking CB2 receptors in peripheral sensory neurons (AdvillinCRE/+; CB2f/f). The morphine-sparing effect of LY28282360 occurred in a sexually-dimorphic manner, being present in male, but not female, mice. LY2828360 treatment (3 mg/kg per day i.p. x 12 days) blocked the development of morphine tolerance in male CB2f/f and CX3CR1CRE/+; CB2f/f mice with established paclitaxel-induced neuropathy but was absent in male (or female) AdvillinCRE/+; CB2f/f mice. Co-administration of morphine with a low dose of LY2828360 (0.1 mg/kg per day i.p. x 6 days) reversed morphine tolerance in paclitaxel-treated male CB2f/f mice, but not AdvillinCRE/+; CB2f/f mice of either sex. LY2828360 (3 mg/kg per day i.p. x 8 days) delayed, but did not prevent, the development of paclitaxel-induced mechanical or cold allodynia in either CB2f/f or CX3CR1CRE/+; CB2f/f mice of either sex. Our findings have potential clinical implications.

Cannabinoid CB 2 receptors in primary sensory neurons are implicated in CB 2 agonist-mediated suppression of paclitaxel-induced neuropathic nociception and sexually-dimorphic sparing of morphine tolerance

Cannabinoid CB 2 agonists show therapeutic efficacy without the unwanted side effects commonly associated with direct activation of CB 1 receptors. The G protein-biased CB 2 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 CB 2 receptors from specific cell populations to determine the population(s) mediating the anti-allodynic and morphine-sparing effects of CB 2 agonists. Anti-allodynic effects of structurally distinct CB 2 agonists (LY2828360 and AM1710) were present in paclitaxel-treated CB 2 f/f mice of either sex. The anti-allodynic effect of the CB 2 agonists were absent in conditional knockout (KO) mice lacking CB 2 receptors in peripheral sensory neurons (Advillin CRE/+ ; CB 2 f/f ) but preserved in mice lacking CB 2 receptors in CX3CR1 expressing microglia/macrophages (CX3CR1 CRE/+ ; CB 2 f/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 CB 2 f/f mice but was absent in male (or female) Advillin CRE/+ ; CB 2 f/f mice. LY2828360-induced sparing of morphine tolerance was preserved in male CX3CR1 CRE/+ ; CB 2 f/f mice, but this effect was not observed in female CX3CR1 CRE/+ ; CB 2 f/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 CB 2 f/f mice, but this effect was absent in female CB 2 f/f mice and Advillin CRE/+ ; CB 2 f/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 CB 2 f/f or CX3CR1 CRE/+ ; CB 2 f/f mice of either sex. Our studies reveal that CB 2 receptors in primary sensory neurons are required for the anti-allodynic effects of CB 2 agonists in a mouse model of paclitaxel-induced neuropathic nociception. We also find that CB 2 agonists acting on primary sensory neurons produce a sexually-dimorphic sparing of morphine tolerance in males, but not female, paclitaxel-treated mice.

Targeting sensory neuron GPCRs for peripheral neuropathic pain

Despite the high prevalence of peripheral neuropathic pain (NP) conditions and significant progress in understanding its underlying mechanisms, the management of peripheral NP remains inadequate. Existing pharmacotherapies for NP act primarily on the central nervous system (CNS) and are often associated with CNS-related adverse effects, limiting their clinical effectiveness. Mounting preclinical evidence indicates that reducing the heightened activity in primary sensory neurons by targeting G-protein-coupled receptors (GPCRs), without activating these receptors in the CNS, relieves pain without central adverse effects. In this review, we focus on recent advancements in GPCR-mediated peripheral pain relief and discuss strategies to advance the development of more effective and safer therapies for peripheral NP by shifting from traditional CNS modulatory approaches toward selective targeting of GPCRs on primary sensory neurons.

Imaging and Genetic Tools for the Investigation of the Endocannabinoid System in the CNS

As central nervous system (CNS)-related disorders present an increasing cause of global morbidity, mortality, and high pressure on our healthcare system, there is an urgent need for new insights and treatment options. The endocannabinoid system (ECS) is a critical network of endogenous compounds, receptors, and enzymes that contribute to CNS development and regulation. Given its multifaceted involvement in neurobiology and its significance in various CNS disorders, the ECS as a whole is considered a promising therapeutic target. Despite significant advances in our understanding of the ECS's role in the CNS, its complex architecture and extensive crosstalk with other biological systems present challenges for research and clinical advancements. To bridge these knowledge gaps and unlock the full therapeutic potential of ECS interventions in CNS-related disorders, a plethora of molecular-genetic tools have been developed in recent years. Here, we review some of the most impactful tools for investigating the neurological aspects of the ECS. We first provide a brief introduction to the ECS components, including cannabinoid receptors, endocannabinoids, and metabolic enzymes, emphasizing their complexity. This is followed by an exploration of cutting-edge imaging tools and genetic models aimed at elucidating the roles of these principal ECS components. Special emphasis is placed on their relevance in the context of CNS and its associated disorders.