Cannabinoids suppress inflammatory and neuropathic pain by targeting α3 glycine receptors

Abstract

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.

Figure 1.

Suppression of inflammatory pain by CBD and DH-CBD in mice. (A) Time course of heat pain hypersensitivity in mice after 20 µl CFA paw injection (1:4 in saline). Each data point represents the mean from 10–15 mice. (B) The analgesic effect of i.p. (50 mg/kg, n = 10) and i.t. (50 µg, n = 10) injection of DH-CBD or vehicle in mice 2 h after CFA injection. (C) The analgesic effect of the repeated application of i.p. injection of 50 mg/kg DH-CBD (n = 10) in mice after CFA injection. Note that the analgesic effect was nearly identical after repeated applications of DH-CBD during a period of two consecutive days after CFA injection. (D) Traces of I_Gly in the absence and presence of 1 µM DH-CBD and 1 µM CBD in HEK 293 cells expressing the α3 GlyRs. (E) Time course of CBD- (n = 7) and DH-CBD (n = 9)–induced potentiation on I_Gly in HEK 293 cells expressing the α3 GlyRs_. (F) Dose-dependent analgesic effects of CBD (n = 10) and DH-CBD (n = 10) applied i.p. and i.t. in mice with CFA paw injection. Data are representative of two to three independent experiments (*, P < 0.05; ***, P < 0.001) and expressed as mean ± SEM.

Figure 2.

DH-CBD potentiation of I_Gly in spinal neurons and suppression of inflammatory pain in rats. (A) Original recordings showing currents activated by puff application of 30 µM glycine to lamina II neurons with and without sustained DH-CBD application in rat spinal cord slices. 1 µM strychnine, a specific GlyR antagonist, was used to determine whether GlyRs are the target that mediates the DH-CBD–induced potentiation. (B) Bar graphs of DH-CBD–induced potentiation on I_Gly of spinal lamina II neurons (n = 7). (C and D) Effect of i.t. application of DH-CBD or vehicle on the ipsilateral (C) and contralateral (D) PWL to noxious heat stimulation in rats before and after intraplantar CFA injection (n = 6–7). (E and F) As in C and D, but rats were subjected to punctuate mechanical stimuli (n = 8). Data are representative of three (A and B) and two (C–F) independent experiments (*, P < 0.05; **, P < 0.01; ***, P < 0.001, post-drug vs. pre-drug) and expressed as mean ± SEM.

Figure 3.

DH-CBD suppression of neuropathic pain in rats. (A) Time courses of the ipsilateral and contralateral PWT to mechanical stimulation after the fifth lumbar SNL in rats (n = 12). ***, P < 0.001, as compared with pre-SNL. (B and C) The analgesic effect of i.t. application of DH-CBD at 100 µg on PWT to mechanical stimulation (B) or heat stimulation (C) on day 14 after SNL (n = 12). *, P < 0.05; **, P < 0.01; ***, P < 0.001, as compared with pre DH-CBD. Data are representative of three independent experiments and expressed as mean ± SEM.

Figure 4.

DH-CBD rescue of PGE₂-induced inhibition of the α3 GlyRs and chronic pain. (A) Trace records of I_Gly in HEK 293 cells coexpressing EP2 receptors with either the α3 or α1 GlyRs before and after sustained PGE₂ application. (B) Time courses of I_Gly in the cells coexpressing EP2 receptors with either the α3 (n = 6) or α1 (n = 5) GlyRs after sustained PGE₂ application. ***, P < 0.001 (α1 vs. α3). (C) Trace records of I_Gly in cells coexpressing EP2 receptors with the α3 GlyRs without and with DH-CBD after sustained PGE₂ application. (D) Time courses of DH-CBD potentiation of I_Gly in cells coexpressing the α3 GlyRs and EP2Rs after sustained PGE₂ application (n = 6). ***, P < 0.001 as compared with vehicle application. (E) Time courses of PWL to thermal stimulation before, during, and after i.t. PGE₂ injection in WT and α3^−/− mice (n = 10). ***, P < 0.001, as compared with the WT mice. (F) The analgesic effect of 50 mg/kg DH-CBD i.p. (n = 10) on PWL to thermal stimulation after i.t. PGE₂ injection. *, P < 0.05; ***, P < 0.001, as compared with i.p. vehicle (n = 10). Data are representative of two independent experiments and expressed as mean ± SEM.

Figure 5.

Antagonism of DD-CBD to DH-CBD–induced potentiation of I_Gly and analgesia in inflammatory pain. (A) Structures of DH-CBD and DD-CBD. Trace records of I_Gly in HEK 293 cells expressing the α3 GlyRs without and with co-application of DD-CBD and DH-CBD. (B) DD-CBD inhibition of DH-CBD–induced potentiation of I_Gly in HEK 293 cells expressing the α3 GlyRs (n = 6). (C) The concentration-response curves of DH-CBD–induced potentiation of I_Gly without and with 1 µM DD-CBD (n = 5–7). (D) The effect of i.t. and i.p. application of DD-CBD on PWL to thermal stimulation after CFA paw injection in mice (n = 7–10). (E) Dose-dependent inhibition of i.p. DH-CBD–induced analgesic effect by i.p. application of DD-CBD in post-CFA mice (n = 7–10). (F) Dose-dependent inhibition of i.p. DH-CBD–induced analgesic effect by i.t. application of DD-CBD in post-CFA mice (n = 8–10). *, P < 0.05; ***, P < 0.001, as compared with vehicle injection. Data are representative of two independent experiments and expressed as mean ± SEM.

Figure 6.

A decrease in CBD and DH-CBD–induced analgesia in the α3^−/− mice but not in the CB1^−/− and CB2 ^−/− mice. (A and B) The analgesic effect of 50 mg/kg CBD i.p. (A) or 50 mg/kg DH-CBD i.p. (B) on PWL to thermal stimulation after CFA injection in the α3^−/− mice (n = 9–10). ***, P < 0.001, as compared with WT mice. (C and D) The analgesic effect of 50 mg/kg CBD i.p. (C) or 50 mg/kg DH-CBD i.p. (D) on PWL to thermal stimulation after CFA injection in the CB1^−/− mice (n = 6). (E and F) The analgesic effect of 50 mg/kg CBD i.p. (E) or 50 mg/kg DH-CBD i.p. (F) on PWL to thermal stimulation after CFA injection in the CB2^−/− mice (n = 6). Data are representative of two independent experiments and expressed as mean ± SEM.

Figure 7.

Correlation analysis: relationships between cannabinoid potentiation and cannabinoid-induced analgesic and psychoactive effects. Correlation analysis: (A) Cannabinoid (1 µM)-induced potentiation of I_Gly versus the analgesic potency (percent changes of PWL) of 50 mg/kg cannabinoids i.p. in CFA-induced inflammatory pain (linear regression, n = 11). (B and C) The K_i values of cannabinoid binding affinity for CB1 receptors (B) or CB2 receptors (C) versus the analgesic potency of cannabinoids. (D–F) Cannabinoid (1 µM)-induced potentiation of I_Gly versus the effects of 50 mg/kg cannabinoids i.p. on body temperature (D), locomotor activity (E), or rotarod performance (F). (G–I) The analgesic potency of 50 mg/kg cannabinoids i.p. versus the effect of cannabinoids on body temperature (G), locomotor activity (H), or rotarod performance (I). Data are representative of two to three independent experiments.

Figure 8.

NMR analysis: a direct interaction between CBD and S296-containing domain of α3 GlyR. (A) A homology model of α3 GlyR-TM domain structure derived from the high-resolution NMR structure of α1 GlyR TM domains. S296 residue is highlighted in red. (B) Contour plots of the S296 resonance in the HSQC spectrum of the α3 GlyR TM domains in LPPG micelles as a function of CBD concentrations: red, 0 µM; orange, 5 µM; blue, 105 µM; and green, 546 µM. (C) Strip plots of the 2D ¹H-¹H NOESY spectra in the absence and presence of α3 GlyR TM domains, showing an unambiguous cross peak between the aromatic protons of CBD and the Hβ of S296. (D) Side views of a proposed binding interaction between CBD and the α3 GlyR-TM involving the S296, as revealed by a molecular dynamic simulation of 20 ns. The S296 residue is indicated in red and CBD is indicated in yellow. Lipids surrounding the S296-containing domain are indicated in blue. (E) Traces of CBD-induced potentiation on I_Gly in HEK 293 cells expressing the WT (n = 5) and mutant S296A (n = 6) α3 GlyRs. (F) The concentration-response curves of CBD-induced potentiation of the α3 GlyRs. ***, P < 0.001 (WT vs. S296A). Data are representative of two independent experiments and expressed as mean ± SEM.

Figure 9.

The effect of DH-CBD on α3 GlyR gating kinetics and agonist affinity. (A) Traces of currents activated by 1 mM glycine in HEK 293 cells expressing WT or S296A mutant α3 GlyRs in the absence and presence of 3 µM DH-CBD. Bar graph shows the mean activation time constant (n = 6–9). Asterisks indicate a significant difference as compared with vehicle (*, P < 0.05). (B) Traces of currents during a washout after a 10-ms application of 1 mM glycine in the absence and presence of 3 µM DH-CBD. The bar graph shows mean deactivation time constants (n = 6–7). The asterisk indicates a significant difference as compared with vehicle (***, P < 0.001). (C) The glycine concentration-response curves in the absence (n = 6) and presence of 1 µM (n = 8) and 10 µM (n = 5) DH-CBD. Data are representative of two independent experiments and expressed as mean ± SEM.