Involvement of central cannabinoid CB2 receptor in reducing mechanical allodynia in a mouse model of neuropathic pain

doi:10.1016/j.ejphar.2008.01.010

European Journal of Pharmacology

Volume 583, Issue 1, 31 March 2008, Pages 56-61

https://doi.org/10.1016/j.ejphar.2008.01.010 Get rights and content

Abstract

We sought to examine the involvement of central cannabinoid CB₂ receptor activation in modulating mechanical allodynia in a mouse model of neuropathic pain. JWH133 was demonstrated to be a selective cannabinoid CB₂ receptor agonist in mice, reducing forskolin-stimulated cAMP production in CHO cells expressing mouse cannabinoid CB₂ and cannabinoid CB₁ receptors with EC₅₀ values of 63 nM and 2500 nM, respectively. Intrathecal administration of JWH133 (50 and 100 nmol/mouse) significantly reversed partial sciatic nerve ligation-induced mechanical allodynia in mice at 0.5 h after administration. In contrast, systemic (intraperitoneal) or local (injected to the dorsal surface of the hindpaw) administration of JWH133 (100 nmol/mouse) was ineffective. Furthermore, the analgesic effects of intrathecal JWH133 (100 nmol/mouse) were absent in cannabinoid CB₂ receptor knockout mice. These results suggest that the activation of central, but not peripheral, cannabinoid CB₂ receptors play an important role in reducing mechanical allodynia in a mouse model of neuropathic pain.

Introduction

Two cannabinoid receptors, cannabinoid CB₁ and cannabinoid CB₂ receptors, have been cloned and characterized (for review, see Walker and Hohmann, 2005). Cannabinoid CB₁ receptors are abundant in the central nervous system (CNS) while cannabinoid CB₂ receptors are predominantly associated with the immune system (Herkenham et al., 1990, Galiegue et al., 1995). Cannabinoid CB₁ receptors are present at sites involved in pain processing, such as primary afferent fibers, spinal cord, thalamus, periaqueductal grey and rostral ventromedial medulla (Tsou et al., 1998). Activation of the cannabinoid CB₁ receptors induces antinociception (Scott et al., 2004, Yoon and Choi, 2003), however, it also produces many neurological effects that limit their therapeutic use (Scott et al., 2004, Sañudo-Peña et al., 2000).

Cannabinoid CB₂ receptor agonists have been also known to produce antinociception without overt behavioral effects in neuropathic, inflammatory, postoperative, and acute pain models (Malan et al., 2001, Malan et al., 2003, Quartilho et al., 2003 Sañudo-Peña et al., 2000, Valenzano et al., 2005). These studies assumed that the primary site of action is peripheral structures, mostly immune cells. However, the role of cannabinoid CB₂ receptor agonists in the CNS has not been well understood in connection to neuropathic pain. Increasing evidence suggests that cannabinoid CB₂ receptor activation in the CNS may also contribute to the analgesic effects of cannabinoids. The cannabinoid CB₂ receptor mRNA and protein are expressed in rat lumbar (L3–L4) spinal cord, though specific cell types were not determined (Beltramo et al., 2006, Walczak et al., 2005). Application of cannabinoid CB₂ receptor agonists inhibits capsaicin-mediated calcitonin gene-related peptide release in rat spinal cord slices (Beltramo et al., 2006) and KCl-stimulated increase in intracellular calcium in rat dorsal root ganglion neurons (Sagar et al., 2005). Recently, Romero-Sandoval and Eisenach (2007) reported that the intrathecally administered cannabinoid CB₂ receptor agonist, JWH015, reduced hypersensitivity in a rat postoperative pain model.

JWH133 has been reported as being a cannabinoid CB₂ receptor-selective agonist with a K_i value against human cannabinoid CB₂ receptors of 3–20 nM (Huffman et al., 1999, Stern et al., 2006) and against whole brain cannabinoid CB₁ receptors of 670 nM (Huffman et al., 1999). Systemic administration of JWH-133 significantly reduced ipsilateral hind paw weight bearing deficit and paw volume in a rat carrageenan-induced inflammatory pain model (Elmes et al., 2005). These effects were abolished by the cannabinoid CB₂ receptor-selective antagonist, SR144528, suggesting that the effect of JWH133 was cannabinoid CB₂ receptor-mediated.

We sought to examine the involvement of central cannabinoid CB₂ receptor activation in modulating mechanical allodynia in a mouse model of neuropathic pain induced by partial sciatic nerve ligation using JWH133. We first evaluated the in vitro agonist activity and selectivity of JWH133 for mouse cannabinoid CB₂ and cannabinoid CB₁ receptors. We next examined and compared the effects of JWH133 following intrathecal (i.t.), systemic (intraperitoneal; i.p.) and local (injected to the dorsal surface of the hindpaw; i.pw.) administration on partial sciatic nerve ligation-induced mechanical allodynia to determine a target site of action of cannabinoid CB₂ receptor agonist. Finally, we used cannabinoid CB₂ receptor knockout (KO) mice to confirm that the effect of JWH133 was mediated by the cannabinoid CB₂ receptor.

Section snippets

Reagents

Selective cannabinoid CB₂ ligands JWH133 (3-[1.1-Diethylbutyl]-1deoxy-Δ8-tetrahydrocannabinol) and AM1241 ((R,S)-3-(2-Iodo-5-nitrobenzoyl)-1-(1-methyl-2-piperidinylmethyl)-1H-indole) were purchased from Sigma (St. Louis, Mo., USA) and synthesized by Pfizer Global Research and Development Nagoya Laboratories, respectively. Non-selective cannabinoid agonists WIN55212-2 ((R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate)

cAMP assay

The ability of JWH133 to activate mouse cannabinoid CB₂ and cannabinoid CB₁ receptors was assessed in a functional cAMP assay using CHO cells expressing recombinant mouse cannabinoid receptors. In mouse cannabinoid CB₂ receptors, JWH133 showed a concentration-dependent inhibition of forskolin-induced cAMP accumulation with an EC₅₀ value of 63 nM and a maximum inhibition of 106% (Fig. 1A). In mouse cannabinoid CB₁ receptors, JWH133 demonstrated a weak potency with an EC₅₀ value of 2500 nM and a

Discussion

Cannabinoid CB₂ receptor agonists reduce hypersensitivity in neuropathic pain models (for review, see Whiteside et al., 2007) such as spinal nerve ligation and partial sciatic nerve ligation models. In this report we used a mouse model of neuropathic pain induced by partial sciatic nerve ligation. The reasons why we used this model and detected the mechanical allodynia instead of mechanical hyperalgesia were as follows: (1) preliminary experiments conducted in our laboratory demonstrated that

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For instance, administration of CB2R agonists hindered capsaicin-induced calcitonin gene-related peptide release in rat spinal cord slices (Beltramo et al., 2006) and KCl-mediated increases in intracellular calcium in rat dorsal root ganglion neurons (Sagar et al., 2005). Yamamoto et al. (2008) reported that intrathecal administration of JWH133 markedly inhibited mechanical allodynia in mice induced by partial sciatic nerve ligation, whereas systemic or local administration did not show any effect. Moreover, the analgesic activity of intrathecal JWH133 was abolished in CB2R-deficient mice, suggesting that stimulation of central, but not peripheral, CB2Rs attenuates mechanical allodynia in this neuropathic pain model.
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Involvement of central cannabinoid CB2 receptor in reducing mechanical allodynia in a mouse model of neuropathic pain

Abstract

Introduction

Section snippets

Reagents

cAMP assay

Discussion

Eur. J. Pharmacol.

Pain

Bioorg. Med. Chem.

Eur. J. Pharmacol.

Curr. Opin. Pharmacol.

Pain

Neuroscience

Eur. J. Pharmacol.

Pain

Pain

Neuroscience

Neuropharmacology

Neuroscience

Neurochem. Int.

CB2 receptor-mediated antihyperalgesia: possible direct involvement of neural mechanisms

Eur. J. Neurosci.

Species-specific in vitro pharmacological effects of the cannabinoid receptor 2 (CB2) selective ligand AM1241 and its resolved enantiomers

Br. J. Pharmacol.,

Involvement of central cannabinoid CB₂ receptor in reducing mechanical allodynia in a mouse model of neuropathic pain