Low-dose cannabinoid receptor 2 agonist induces microglial activation in a cancer pain-morphine tolerance rat model

Research progress of p38 as a new therapeutic target against morphine tolerance and the current status of therapy of morphine tolerance

With the development of the medical industry, new painkillers continue to appear in people's field of vision, but so far no painkiller can replace morphine. While morphine has a strong analgesic effect, it is also easy to produce pain sensitivity and tolerance. Due to the great inter-individual differences in patient responses, there are few clear instructions on how to optimise morphine administration regimens, which complicates clinicians' treatment strategies and limits the effectiveness of morphine in long-term pain therapy. P38MAPK is a key member of the MAPK family. Across recent years, it has been discovered that p38MAPK rises dramatically in a wide range of morphine tolerance animal models. Morphine tolerance can be reduced or reversed by inhibiting p38MAPK. However, the role and specific mechanism of p38MAPK are not clear. In this review, we synthesise the relevant findings, highlight the function and potential mechanism of p38MAPK in morphine tolerance, as well as the present status and efficacy of morphine tolerance therapy, and underline the future promise of p38MAPK targeted morphine tolerance treatment.

Microglial Cannabinoid CB2 Receptors in Pain Modulation

Pain, especially chronic pain, can strongly affect patients' quality of life. Cannabinoids ponhave been reported to produce potent analgesic effects in different preclinical pain models, where they primarily function as agonists of G_i/o protein-coupled cannabinoid CB₁ and CB₂ receptors. The CB₁ receptors are abundantly expressed in both the peripheral and central nervous systems. The central activation of CB₁ receptors is strongly associated with psychotropic adverse effects, thus largely limiting its therapeutic potential. However, the CB₂ receptors are promising targets for pain treatment without psychotropic adverse effects, as they are primarily expressed in immune cells. Additionally, as the resident immune cells in the central nervous system, microglia are increasingly recognized as critical players in chronic pain. Accumulating evidence has demonstrated that the expression of CB₂ receptors is significantly increased in activated microglia in the spinal cord, which exerts protective consequences within the surrounding neural circuitry by regulating the activity and function of microglia. In this review, we focused on recent advances in understanding the role of microglial CB₂ receptors in spinal nociceptive circuitry, highlighting the mechanism of CB₂ receptors in modulating microglia function and its implications for CB₂ receptor- selective agonist-mediated analgesia.

Transcriptomic Profiling in Mice With CB1 receptor Deletion in Primary Sensory Neurons Suggests New Analgesic Targets for Neuropathic Pain

Type 1 and type 2 cannabinoid receptors (CB1 and CB2, respectively) mediate cannabinoid-induced analgesia. Loss of endogenous CB1 is associated with hyperalgesia. However, the downstream targets affected by ablation of CB1 in primary sensory neurons remain unknown. In the present study, we hypothesized that conditional knockout of CB1 in primary sensory neurons (CB1cKO) alters downstream gene expression in the dorsal root ganglion (DRG) and that targeting these pathways alleviates neuropathic pain. We found that CB1cKO in primary sensory neurons induced by tamoxifen in adult Advillin-Cre:CB1-floxed mice showed persistent hyperalgesia. Transcriptome/RNA sequencing analysis of the DRG indicated that differentially expressed genes were enriched in energy regulation and complement and coagulation cascades at the early phase of CB1cKO, whereas pain regulation and nerve conduction pathways were affected at the late phase of CB1cKO. Chronic constriction injury in mice induced neuropathic pain and changed transcriptome expression in the DRG of CB1cKO mice, and differentially expressed genes were mainly associated with inflammatory and immune-related pathways. Nerve injury caused a much larger increase in CB2 expression in the DRG in CB1cKO than in wildtype mice. Interfering with downstream target genes of CB1, such as antagonizing CB2, inhibited activation of astrocytes, reduced neuroinflammation, and alleviated neuropathic pain. Our results demonstrate that CB1 in primary sensory neurons functions as an endogenous analgesic mediator. CB2 expression is regulated by CB1 and may be targeted for the treatment of neuropathic pain.

Cannabinoids in Chronic Pain: Therapeutic Potential Through Microglia Modulation

Chronic pain is a complex sensory, cognitive, and emotional experience that imposes a great personal, psychological, and socioeconomic burden on patients. An estimated 1.5 billion people worldwide are afflicted with chronic pain, which is often difficult to treat and may be resistant to the potent pain-relieving effects of opioid analgesics. Attention has therefore focused on advancing new pain therapies directed at the cannabinoid system because of its key role in pain modulation. Endocannabinoids and exogenous cannabinoids exert their actions primarily through Gi/o-protein coupled cannabinoid CB1 and CB2 receptors expressed throughout the nervous system. CB1 receptors are found at key nodes along the pain pathway and their activity gates both the sensory and affective components of pain. CB2 receptors are typically expressed at low levels on microglia, astrocytes, and peripheral immune cells. In chronic pain states, there is a marked increase in CB2 expression which modulates the activity of these central and peripheral immune cells with important consequences for the surrounding pain circuitry. Growing evidence indicate that interventions targeting CB1 or CB2 receptors improve pain outcomes in a variety of preclinical pain models. In this mini-review, we will highlight recent advances in understanding how cannabinoids modulate microglia function and its implications for cannabinoid-mediated analgesia, focusing on microglia-neuron interactions within the spinal nociceptive circuitry.

Characterization of Subtype Selective Cannabinoid CB2 Receptor Agonists as Potential Anti-Inflammatory Agents

Activation of the CB₂ receptor has been shown to have anti-inflammatory and antinociceptive effects without causing psychoactive effects. Previously, we reported that the compound ethyl 2(2-(N-(2,3-dimethylphenyl) phenylsulfonamido)acetamido)benzoate (ABK5) is a CB₂ subtype selective agonist with anti-inflammatory and antinociceptive effects. In the present study, we tested four ABK5 derivatives, ABK5-1, ABK5-2, ABK5-5, and ABK5-6, to analyze the structure of ABK5 to obtain CB₂-selective agonists with higher affinity and efficacy. Affinity, subtype selectivity, and G-protein coupling were determined by radioligand binding assays. Selected compounds were then subjected to evaluation of anti-inflammatory effects using two different cell lines, Jurkat (ABK5-1 and 5-2) and BV-2 cells (ABK5-1), which are models of T cells and microglia, respectively. ABK5-1, ABK5-2, and ABK5-6 had comparable CB₂ binding affinity with ABK5 (and stimulated G-protein coupling), while only ABK5-1 and ABK5-2 maintained CB₂-subtype selectivity. ABK5-5 did not bind CB₂ in the detectable range. RT-PCR and ELISA analysis showed that the two compounds also inhibit IL-2 and TNF-α production, and they were more efficacious than ABK5 in inhibiting TNF-α production. CXCL-12 mediated chemotaxis was also evaluated by the transwell migration assay, and both ABK5-1 and ABK5-2 inhibited chemotaxis with a stronger effect observed in ABK5-1. In the microglia cell line BV-2, ABK5-1 inhibited IL-1β and IL-6 production, which suggests this compound has anti-inflammatory effects through targeting multiple immune cells, and may be a candidate for treatment of inflammation.