Endomorphin 1 is a high affinity and selective agonist of the μ-opioid receptor and displays reasonable affinities for kappa3 binding sites (Ki: 20~30 nM).

κ3 opioid receptor:20-30 nM (Ki), μ opioid receptor:1.11 nM (Ki)

125I-Endomorphin 1 or 125I-Endomorphin 2 binding (0.2 nM) is performed in potassium phosphate buffer (50 mM, pH 7.4; 0.5 mL) with MgCl2 (5 mM) at a tissue concentration of 10 mg wet weight/mL for brains or 0.06 mg protein/mL for MOR-1/CHO cells. Specific binding is determined in the presence and absence of either 1 μM of the corresponding unlabeled peptide. The entire mixture is then incubated at 25°C for 1 hr and filtered over no. 32 glass fiber filters which have been presoaked for 1 hr in 0.5% polyethyleneimine and washed twice with ice-cold Tris buffer using a Brandel cell harvester. The filters are then counted on a Packard Cobra gamma counter. The other opioid receptor binding assays are performed.

Groups of mice are treated i.c.v. with Endomorphin 1 (12 μg) or Endomorphin 2 (3 μg) 15 min before a 0.5-cc charcoal meal (2.5% gum tragacanth,10% activated charcoal in water). The mice are killed 30 min later and the distance the charcoal traveled is measured.