Title: Narita M, et al. Evidence for the existence of the beta-endorphin-sensitive "epsilon-opioid receptor" in the brain: the mechanisms of epsilon-mediated antinociception. Jpn J Pharmacol. 1998 Mar;76(3):233-53.
Title: Luan YH, et al. Action of β-endorphin and nonsteroidal anti-inflammatory drugs, and the possible effects of nonsteroidal anti-inflammatory drugs on β-endorphin. J Clin Anesth. 2017 Feb;37:123-128.