Abstract
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.
Highlights
Neuropathic pain, caused by a lesion or disease affecting the somatosensory system, is a major clinical problem and has a considerable impact on the life quality of patients (Burke et al, 2017)
More than 90% of CB1 were knocked out on dorsal root ganglion (DRG) neurons in CB1cKOmice (Figure 1A, thick white arrow). qPCR assay showed that compared with wild-type littermates male mice (WT mice), CB1 mRNA was significantly reduced in the DRG of CB1 conditional knockout from peripheral sensory neurons (CB1cKO) mice (Figure 1B, p < 0.001)
Our data showed that CB1cKO mice presented persistent hyperalgesia and changed transcriptome expression in DRG in early and late phases of CB1cKO
Summary
Neuropathic pain, caused by a lesion or disease affecting the somatosensory system, is a major clinical problem and has a considerable impact on the life quality of patients (Burke et al, 2017). Single-cell sequencing technology revealed the gene expression patterns in subtypes of DRG neurons after peripheral nerve injury (Wang et al, 2021). CB1 knockout may exaggerate pain, it remains unknown how transcriptomic profiling changes in mice with CB1 deletion in peripheral sensory neurons and whether the downstream genes of CB1 may provide new analgesic targets in neuropathic pain. In this study, we investigated whether CB1 conditional knockout from peripheral sensory neurons (CB1cKO) in mice may present mechanical allodynia, thermal hyperalgesia, and a change in transcriptome expression in DRG. We determined the effect of chronic constriction injury (CCI) model of CB1cKO mice on pain behavior and transcriptome expression of DRG. Our findings provided new evidence of the role of CB1 in the development of neuropathic pain and the downstream genes of CB1 may serve as therapeutic targets for neuropathic pain
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