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

Abstract

Considerable preclinical research has demonstrated the efficacy of Δ⁹-tetrahydrocannabinol (Δ⁹-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 Δ⁹-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, Δ⁹-THC and CBN bound to the CB₁ receptor and produced antinociceptive effects. The CB₁ receptor antagonist, rimonabant, but not the CB₂ receptor antagonist, SR144528, blocked the antinociceptive effects of both compounds. Although THCV bound to the CB₁ receptor with similar affinity as Δ⁹-THC, it had no effects when administered alone, but antagonized the antinociceptive effects of Δ⁹-THC when both drugs were given in combination. Importantly, the antinociceptive effects of Δ⁹-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 Δ⁹-THC by either eliciting antinociceptive effects (i.e., CBN) or antagonizing (i.e., THCV) the actions of Δ⁹-THC.

Introduction

Cannabis has been used for thousands of years as a therapeutic agent for pain relief, as well as for recreational purposes. Delta-9-Tetrahydrocannabinol (Δ⁹-THC) is the most prevalent and well characterized constituent of the approximately 70 cannabinoids identified in cannabis (Elsohly and Slade, 2005), and largely accounts for the psychoactive properties of this plant. Δ⁹-THC produces antinociceptive effects in a wide range of preclinical assays of pain, including tail-flick, hotplate, inflammatory, cancer, neuropathic, and visceral nociceptive models (Martin et al., 1984, Formukong et al., 1988, Burstein et al., 1988, Compton et al., 1991, Varvel et al., 2005). Visceral pain (e.g., myocardial ischemia, upper gastrointestinal dyspepsia, irritable bowel syndrome, and dysmenorrhea) is one of the most common forms of pain. Importantly, both cannabinoid receptors are expressed in the viscera (Matsuda et al., 1990, Bouaboula et al., 1993, Munro et al., 1993, Galiegue et al., 1995, Wright et al., 2005). Intraperitoneal administration of acetic acid or various other chemicals causes distension of the hollow walled muscular organs and the release of prostaglandins and inflammatory cytokines that induce abdominal stretching. Δ⁹-THC has been well established to produce antinociceptive effects in the acetic acid (Sofia et al., 1975), and phenyl-p-quinone (PPQ) (Welch et al., 1995, Haller et al., 2006) models of visceral nociception.

Other prevalent phytocannabinoids that are structurally similar to Δ⁹-THC include cannabinol (CBN), cannabidiol (CBD), cannabichromene (CBC), and tetrahydrocannabivarin (THCV). CBD has been demonstrated to have anti-edema effects (Lodzki et al., 2003, Costa et al., 2004) and potentiate the antinociceptive effects of Δ⁹-THC (Varvel et al., 2006, Hayakawa et al., 2008). However, orally administered CBD was inactive in the acetic acid stretching model and CBN was only effective at high concentrations (Sofia et al., 1975, Welburn et al., 1976, Sanders et al., 1979). In addition, neither CBC nor THCV has been characterized in visceral pain models. Interestingly, THCV has been shown to act as a competitive cannabinoid receptor antagonist (Thomas et al., 2005). The primary goal of the present study was to compare the antinociceptive effects of Δ⁹-THC to other prevalent phytocannabinoids, including CBC, CBD, CBN, and THCV, in the acetic acid stretching model.

Δ⁹-THC binds to and activates both CB₁ (Matsuda et al., 1990) and CB₂ (Gerard et al., 1991) cannabinoid receptors, both of which are coupled to Gi/o proteins (for review see (Howlett et al., 2002). CB₁ receptors are located extensively throughout the central nervous system (CNS) (Matsuda et al., 1990, Munro et al., 1993, Zimmer et al., 1999), and are believed to mediate marijuana's psychomimetic effects. CB₂ receptors are expressed predominately in cells of the immune and hematopoietic systems (Munro et al., 1993) though CB₂ receptor messenger RNA and protein are expressed in microglia (Carlisle et al., 2002, Nunez et al., 2004) and brainstem neurons (Van Sickle et al., 2005). Consequently, a secondary goal of this study was to determine whether phytocannabinoids produce their antinociceptive effects through a cannabinoid receptor mechanism of action. Accordingly, we examined the involvement of CB₁ and CB₂ receptors using rimonabant and SR144528, selective antagonists for these respective receptors. Because cannabinoids elicit antinociceptive effects as well as motor suppressive effects, in the final set of experiments, we evaluated each active drug for hypomotility.