Intrathecal Actions of the Cannabis Constituents Δ(9)-Tetrahydrocannabinol and Cannabidiol in a Mouse Neuropathic Pain Model

Abstract

(1) Background: The psychoactive and non-psychoactive constituents of cannabis, Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD), synergistically reduce allodynia in various animal models of neuropathic pain. Unfortunately, THC-containing drugs also produce substantial side-effects when administered systemically. We examined the effectiveness of targeted spinal delivery of these cannabis constituents, alone and in combination. (2) Methods: The effect of acute intrathecal drug delivery on allodynia and common cannabinoid-like side-effects was examined in a mouse chronic constriction injury (CCI) model of neuropathic pain. (3) Results: intrathecal THC and CBD produced dose-dependent reductions in mechanical and cold allodynia. In a 1:1 combination, they synergistically reduced mechanical and cold allodynia, with a two-fold increase in potency compared to their predicted additive effect. Neither THC, CBD nor combination THC:CBD produced any cannabis-like side-effects at equivalent doses. The anti-allodynic effects of THC were abolished and partly reduced by cannabinoid CB1 and CB2 receptor antagonists AM281 and AM630, respectively. The anti-allodynic effects of CBD were partly reduced by AM630. (4) Conclusions: these findings indicate that intrathecal THC and CBD, individually and in combination, could provide a safe and effective treatment for nerve injury induced neuropathic pain.

Keywords: THC; cannabidiol; cannabinoid; intrathecal; mice; neuropathic pain; synergy.

Figure 1

Time course of action of intrathecal THC and CBD. Time plots of the effects of intrathecally injected THC (100 nmol), CBD (100 nmol), or matched vehicle on (A) mechanical paw withdrawal threshold (PWT), (B) acetone responses, (C) rotarod latency and (D) bar latency (n = 6 per treatment group). Animals received a single intrathecal administration at time 0 h, 10-12 days after CCI surgery (post-CCI); pre-CCI data are also displayed. **, *** and **** denote p < 0.01, 0.001 and 0.0001 compared to time 0 h. within each treatment group. Data are shown as the mean ± s.e.mean of raw values.

Figure 2

Dose–response curves for intrathecal THC and CBD. Dose–response curves for the effect of intrathecal THC and CBD on (A) mechanical paw withdrawal threshold (PWT), (B) acetone responses, (C) rotarod latency, (D) bar latency and (E) open field crossings. Where appropriate, the sigmoidal parametric fit is shown. All data are displayed as the mean ± s.e.mean percentage of the maximum possible effect (%MPE), except for open field (mean ± s.e.mean of raw data).

Figure 3

Dose–response curves for the effect of combined intrathecal THC and CBD. Dose–response curves showing the effect of administration of THC and CBD in a 1:1 fixed ratio on (A) mechanical paw withdrawal threshold (PWT), (B) acetone-induced responses, (C) rotarod latency, (D) bar latency and (E) open field crossings. The non-linear curve fits to the experimental combination data (solid lines) and predicted additive effect (dotted lines), where appropriate. Also shown are the sigmoidal fits for THC and CBD alone (solid lines). All data are displayed as the mean ± s.e.mean percentage of the maximum possible effect (%MPE), except for open field (mean ± s.e.mean of raw data are shown). * and ** denote p < 0.05, 0.01 for experimental THC:CBD data points v the predicted additive value at the corresponding dose.

Figure 4

Isoboles for combined intrathecal THC and CBD treatment at a range of effect levels. Isoboles for the effect of intrathecal THC:CBD co-administration on (A) mechanical PWT and (B) acetone-induced responses, at a 1:1 fixed ratio combination. The experimental and predicted (Exp and Pred) ED₅₀s are shown as part of the continuum of fixed-ratio effects. Theoretical isoboles of additivity for effect levels of 20, 30, 40 and 50 of maximum are shown (ED₂₀–ED₅₀, solid lines from Equation (3)); as a comparison the 50% effect level isobole is shown for the simple case where THC and CBD are assumed to have equal 100% maximal effects and Hill slopes of unity (dotted line). The individual ED₅₀s for THC and CBD are shown on the x- and y-axes, respectively. * and *** denote p < 0.05 and 0.001 for the experimental versus predicted additive ED₅₀.

Figure 5

Effect of cannabinoid receptor antagonist on intrathecal THC and CBD induced anti-allodynia. Scatter plots of the effect of co-administration of the cannabinoid CB1 and CB2 receptor antagonists, AM281 and AM630 (30 nmol), on the effect of maximal intrathecal doses of THC (100 nmol), CBD (100 nmol) on (A) mechanical paw withdrawal threshold (PWT) and (B) acetone responses. The bars represent the mean ± s.e.mean of the percentage of the maximum possible effect (%MPE). *, **, **** denote p < 0.05, 0.01, 0.0001 for vehicle/THC/CBD + vehicle versus Vehicle/THC/CBD + AM281/AM630; #, ##, ###, #### denote p < 0.05, 0.01, 0.001, 0.0001 for vehicle + vehicle/AM281/AM630 versus THC/CBD + vehicle/AM281/AM630.