Functional activity of the cannabinoid 1 receptor is not affected by opioid antagonists in the rat brain

Background: WIN55212-2 is a synthetic cannabinoid agonist and selective to cannabinoid 1 (CB1) receptors, which are distributed mainly in the central nervous system. Opioid receptors and CB1 receptors have several similarities in terms of their intracellular signal transduction mechanisms, distributions, and pharmacological action. Several studies have therefore sought to describe the functional interactions between opioids and cannabinoids at the cellular and behavioral levels. The present study investigated agonist-stimulated [³⁵S]GTPγ{baby}S binding by WIN55212-2 in rat brain membranes and determined the antagonism by selective opioid antagonists at the level of receptorligand interaction and intracellular signal transduction. Methods: Sprague-Dawley rats (male, n = 20) were euthanized for the preparation of brain membranes. In agoniststimulated [³⁵S]GTPγ{baby}S binding by WIN55212-2, the values of EC₅₀ and maximum stimulation (% over basal) were determined in the absence or presence of the μ, κ and δ opioid receptor antagonists naloxone (20 nM), norbinaltorphimine (3 nM), and naltrindole (3 nM), respectively. Ke values for opioid antagonist inhibition in the absence or presence of each opioid receptor antagonist were calculated using the following equation: [nanomolar antagonist] / (dose ratio of EC₅₀ - 1). Results: In WIN55212-2-stimulated [³⁵S]GTPγ{baby}S binding in the rat brain membranes, the values of EC₅₀ and maximum stimulation (% over basal) were 154 ± 39.5 nM and 27.6 ± 5.3% over basal, respectively. Addition of selective opioid antagonists did not produce a significant rightward shift in the WIN55212-2 concentration-response curve, and Ke values were not applicable. Conclusions: Our results suggest that the functional activity of WIN55212-2-stimulated [ ³⁵S]GTPγ{baby}S binding was not affected by opioid antagonists in the rat brain membranes. Although the exact mechanism remains unclear, our results may partially elucidate their actions.