[D-1-Nal4]endomorphin-2 is a potent micro-opioid receptor antagonist in the aequorin luminescence-based calcium assay4398, Fichna, J., Piestrzeniewicz M., Gach K., Poels J, Burgeon E., J. Broeck Vanden, and Janecka A.E. , Life Sci., 8/8/2006, Volume 79, Issue 11, Number 1099, p.1094 - 1099, (2006)