1. Adams, J.E.: Pain 2, 161–166 (1976)
2. Akil, H., Mayer, J.D., Liebeskind, J.C.: C.R. Acad. Sci. (Paris) 274, 3603–3605 (1972)
3. Akil, H., Madden, J., Patrick, R.L., Barchas, J.D.: In: Opiates and Endogenous Opioid Peptides. H.W. Kosterlitz (ed.) pp. 63–70. Elsevier: Amsterdam 1976
4. Akil, H., Richardson, D.E., Hughes, J., Barchas, J.D.: Science 201, 463–465 (1978)
5. Berkowitz, B.A., Ngai, S.H., Finck, A.D.: Science 194, 967–968 (1976)