Ueda H, Yamaguchi T, Tokuyama S, Inoue M, Nishi M, Takeshima H. Partial loss of tolerance liability to morphine analgesia in mice lacking the nociceptin receptor gene. Neurosci Lett. 1997 Nov 21;237(2-3):136–8. doi: 10.1016/s0304-3940(97)00832-x. PMID: 9453234.