1. Hughes I, Smith TW, Kosterlitz, HW, Fothergill LA, Morgan BA, Morris, HR (1975). Identification of two related pentapeptides from the brain with potent opioid agonist activity. Nature (London)258: 277–279.
2. Höllt V (1986). Opioid peptide processing and receptor selectivity. Ann Reu Pharmacol Toxicol26: 59–77.
3. Watson SJ, Akil H, Khachaturian H, Young E, Lewis, ME (1984). Opioid systems: Anatomical, physiological and clinical perspectives. In J Hughes, HOJ Collier, MJ Rance, MB Tyers, eds. Opioids: Past, Present and Future. Taylor and Francis, London, pp 145–178.
4. Zukin RS, Zukin SR (1984). The case for multiple opiate receptors. Trends Neurosci, May: 160–164.
5. Martin WR, Eades CG, Thompson JA, Huppler RE, Gilbert PE (1976). The effects of morphine-and nalorphine-like drugs in the nondependent and morphine-dependent chronic spinal dog. J Pharmacol Exp Ther197: 517–532.