Certain types of nonpsychoactive cannabinoids can potentiate glycine receptors (GlyRs), an important target for nociceptive regulation at the spinal level. However, little is known about the potential and mechanism of glycinergic cannabinoids for chronic pain treatment. We report that systemic and intrathecal administration of cannabidiol (CBD), a major nonpsychoactive component of marijuana, and its modified derivatives significantly suppress chronic inflammatory and neuropathic pain without causing apparent analgesic tolerance in rodents. The cannabinoids significantly potentiate glycine currents in dorsal horn neurons in rat spinal cord slices. The analgesic potency of 11 structurally similar cannabinoids is positively correlated with cannabinoid potentiation of the α3 GlyRs. In contrast, the cannabinoid analgesia is neither correlated with their binding affinity for CB1 and CB2 receptors nor with their psychoactive side effects. NMR analysis reveals a direct interaction between CBD and S296 in the third transmembrane domain of purified α3 GlyR. The cannabinoid-induced analgesic effect is absent in mice lacking the α3 GlyRs. Our findings suggest that the α3 GlyRs mediate glycinergic cannabinoid-induced suppression of chronic pain. These cannabinoids may represent a novel class of therapeutic agents for the treatment of chronic pain and other diseases involving GlyR dysfunction.

α2δ-1, commonly known as a voltage-activated Ca2+ channel subunit, is a binding site of gabapentinoids used to treat neuropathic pain and epilepsy. However, it is unclear how α2δ-1 contributes to neuropathic pain and gabapentinoid actions. Here, we show that Cacna2d1 overexpression potentiates presynaptic and postsynaptic NMDAR activity of spinal dorsal horn neurons to cause pain hypersensitivity. Conversely, Cacna2d1 knockdown or ablation normalizes synaptic NMDAR activity increased by nerve injury. α2δ-1 forms a heteromeric complex with NMDARs in rodent and human spinal cords. The α2δ-1-NMDAR interaction predominantly occurs through the C terminus of α2δ-1 and promotes surface trafficking and synaptic targeting of NMDARs. Gabapentin or an α2δ-1 C terminus-interfering peptide normalizes NMDAR synaptic targeting and activity increased by nerve injury. Thus, α2δ-1 is an NMDAR-interacting protein that increases NMDAR synaptic delivery in neuropathic pain. Gabapentinoids reduce neuropathic pain by inhibiting forward trafficking of α2δ-1-NMDAR complexes.

Chronic pain is a major clinical problem with limited treatment options. Previous studies have demonstrated that activation of adenosine monophosphate-activated protein kinase (AMPK) can attenuate neuropathic pain. Inflammation/immune response at the site of complete Freund’s adjuvant (CFA) injection is known to be a critical trigger of the pathological changes that produce inflammatory pain. However, whether activation of AMPK produces an analgesic effect through inhibiting the proinflammatory cytokines, including interleukin-1β (IL-1β), in inflammatory pain remains unknown. Inflammatory pain was induced in mice injected with CFA. The effects of AICAR (5-aminoimidazole-4-carboxyamide ribonucleoside, an AMPK activator), Compound C (an AMPK inhibitor), and IL-1ra (an IL-1 receptor antagonist) were tested at day 4 after CFA injection. Inflammatory pain was assessed with von Frey filaments and hot plate. Immunoblotting, hematoxylin and eosin (H&E) staining, and immunofluorescence were used to assess inflammation-induced biochemical changes. The AMPK activator AICAR produced an analgesic effect and inhibited the level of proinflammatory cytokine IL-1β in the inflamed skin in mice. Moreover, activation of AMPK suppressed CFA-induced NF-κB p65 translocation from the cytosol to the nucleus in activated macrophages (CD68+ and CX3CR1+) of inflamed skin tissues. Subcutaneous injection of IL-1ra attenuated CFA-induced inflammatory pain. The AMPK inhibitor Compound C and AMPKα shRNA reversed the analgesic effect of AICAR and the effects of AICAR on IL-1β and NF-κB activation in inflamed skin tissues. Our study provides new information that AMPK activation produces the analgesic effect by inhibiting NF-κB activation and reducing the expression of IL-1β in inflammatory pain.

Chronic neuropathic pain is associated with K+ channel downregulation in primary sensory neurons. The authors show that G9a, a histone-modifying enzyme, is required for transcriptional repression of K+ channel–associated gene families caused by nerve injury. G9a in primary sensory neurons is a key epigenetic regulator involved in acute-to-chronic pain transition. Neuropathic pain is a debilitating clinical problem and difficult to treat. Nerve injury causes a long-lasting reduction in K+ channel expression in the dorsal root ganglion (DRG), but little is known about the epigenetic mechanisms involved. We found that nerve injury increased dimethylation of Lys9 on histone H3 (H3K9me2) at Kcna4, Kcnd2, Kcnq2 and Kcnma1 promoters but did not affect levels of DNA methylation on these genes in DRGs. Nerve injury increased activity of euchromatic histone-lysine N-methyltransferase-2 (G9a), histone deacetylases and enhancer of zeste homolog-2 (EZH2), but only G9a inhibition consistently restored K+ channel expression. Selective knockout of the gene encoding G9a in DRG neurons completely blocked K+ channel silencing and chronic pain development after nerve injury. Remarkably, RNA sequencing analysis revealed that G9a inhibition not only reactivated 40 of 42 silenced genes associated with K+ channels but also normalized 638 genes down- or upregulated by nerve injury. Thus G9a has a dominant function in transcriptional repression of K+ channels and in acute-to-chronic pain transition after nerve injury.

Myocardial ischaemia causes the release of metabolites such as bradykinin, which stimulates cardiac sensory receptors to evoke a sympathoexcitatory reflex. However, the molecular identity of the afferent neurons and fibres mediating this reflex response is not clear. In this study, we tested the hypothesis that the cardiogenic sympathoexcitatory reflex is mediated by capsaicin‐sensitive afferent fibres. Enhanced immunofluorescence labelling revealed that vanilloid receptor 1 (VR1)‐containing afferent nerve fibres were present on the epicardial surface of the rat heart. Resiniferatoxin (RTX), a potent analogue of capsaicin, was used to deplete capsaicin‐sensitive afferent fibres in rats. Depletion of these fibres was confirmed by a substantial reduction of VR1 immunoreactivity in the epicardium and dorsal root ganglia. The thermal sensitivity was also diminished in RTX‐treated rats. Renal sympathetic nerve activity (RSNA) and blood pressure were recorded in anaesthetized rats during epicardial application of bradykinin or capsaicin. In vehicle‐treated rats, epicardial bradykinin (10 μg ml⁻¹) or capsaicin (10 μg ml⁻¹) application produced a significant increase in RSNA and arterial blood pressure. The RSNA and blood pressure responses caused by bradykinin and capsaicin were completely abolished in RTX‐treated rats. Furthermore, epicardial application of iodo‐RTX, a highly specific antagonist of VR1 receptors, blocked capsaicin‐ but not bradykinin‐induced sympathoexcitatory responses. Thus, these data provide important histological and functional evidence that the heart is innervated by VR1‐expressing afferent nerves and these afferent nerves are essential for the cardiogenic sympathoexcitatory reflex during myocardial ischaemia.