Evaluation of prevalent phytocannabinoids in the acetic acid model of visceral nociception

Abstract

Considerable preclinical research has demonstrated the efficacy of Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the primary psychoactive constituent of Cannabis sativa, in a wide variety of animal models of pain, but few studies have examined other phytocannabinoids. Indeed, other plant-derived cannabinoids, including cannabidiol (CBD), cannabinol (CBN), and cannabichromene (CBC) elicit antinociceptive effects in some assays. In contrast, tetrahydrocannabivarin (THCV), another component of cannabis, antagonizes the pharmacological effects of Delta(9)-THC. These results suggest that various constituents of this plant may interact in a complex manner to modulate pain. The primary purpose of the present study was to assess the antinociceptive effects of these other prevalent phytocannabinoids in the acetic acid stretching test, a rodent visceral pain model. Of the cannabinoid compounds tested, Delta(9)-THC and CBN bound to the CB(1) receptor and produced antinociceptive effects. The CB(1) receptor antagonist, rimonabant, but not the CB(2) receptor antagonist, SR144528, blocked the antinociceptive effects of both compounds. Although THCV bound to the CB(1) receptor with similar affinity as Delta(9)-THC, it had no effects when administered alone, but antagonized the antinociceptive effects of Delta(9)-THC when both drugs were given in combination. Importantly, the antinociceptive effects of Delta(9)-THC and CBN occurred at lower doses than those necessary to produce locomotor suppression, suggesting motor dysfunction did not account for the decreases in acetic acid-induced abdominal stretching. These data raise the intriguing possibility that other constituents of cannabis can be used to modify the pharmacological effects of Delta(9)-THC by either eliciting antinociceptive effects (i.e., CBN) or antagonizing (i.e., THCV) the actions of Delta(9)-THC.

Figure 1

Subcutaneous administration of Δ⁹-THC reduced abdominal stretching in a dose-dependent manner; ED₅₀ (95% confidence interval) value = 1.1 mg/kg (0.8–1.6). Each data point represents 6–8 mice. ^**p < 0.01 compared with vehicle. Data reflect the mean ± SEM number of abdominal stretches during the 20 min observation period.

Figure 2

The antinociceptive effects of Δ⁹- THC in the acetic acid model of visceral nociception are mediated through a CB₁ receptor mechanism of action. A) The CB₁ receptor antagonist, rimonabant (Rim; 3.0 mg/kg, i.p.), but the CB₂ receptor antagonist, SR144528 (SR2; 3.0 mg/kg, i.p.), blocked the antinociceptive effects of Δ⁹- THC (3 mg/kg, s.c.). # indicates significant difference from Vehicle (Veh)/Δ⁹-THC control p < 0.01; ^** indicates significant difference from Vehicle/Vehicle control p < 0.01. B) Neither rimonabant nor SR144528 given alone affected acetic acid-induced abdominal stretching. N = 6–8 mice/group. Data reflect the mean ± SEM number of abdominal stretches.

Figure 3

Evaluation of prevalent marijuana constituents in the acetic acid abdominal stretching model. A) The marijuana constituents, CBC and CBD, did not produce antinociceptive effects. However, CBN (50 mg/kg) significantly suppressed the stretching response compared to vehicle )Veh), ^**p < 0.01 vs. Vehicle. B) The CB₁ receptor antagonist, rimonabant (Rim), but not by the CB₂ receptor antagonist, SR144528 (SR2), significantly blocked the antinociceptive effects of CBN (50 mg/kg). ^**p < 0.01 vs. Veh/Veh, and ^#p < 0.01 vs. Veh/CBN. N = 6–10 mice/group. Data reflect the mean ± SEM abdominal stretches.

Figure 4

Delta 8-tetrahydrocannabivarin (THCV, 50 mg/kg, s.c.) had no effects on its own, but blocked the antinociceptive effects of Δ⁹-THC (3.0 mg/kg, s.c.). ^**p < 0.01 vs. Veh/Veh group. ^#p < 0.05 vs. Veh/Δ⁹-THC. N = 6–8 mice/group. Data reflect the mean ± SEM abdominal stretches.

Figure 5

Activity of prevalent phytocannabinoids at rat cannabinoid receptor type 1. The affinity of Δ⁹-THC was determined for rat CB₁ receptor (filled circles/solid line), THCV (open circle/solid line), CBN (filled triangle/dash-dotted line), CBD (open triangle/dashed line), and CBC (filled square/dashed line). Details for competition binding experiments are described in the methods section. The points on the graph represent the mean ± SEM of three independent experiments with duplicate wells on each plate. The data were normalized to the signal in the absence of unlabeled competitor (defined as 100%) and in the presence of excess unlabeled CP55,940 (defined as 0%).