Abstract

The recently isolated peptides endomorphin-1 and endomorphin-2 have been suggested to be the endogenous ligands for the mureceptor. In traditional opioid receptor binding assays in mouse brain homogenates, both endomorphin-1 and endomorphin-2 competed bothmu₁ and mu₂receptor sites quite potently. Neither compound had appreciable affinity for either delta orkappa₁ receptors, confirming an earlier report. However, the two endomorphins displayed reasonable affinities for kappa₃ binding sites, withK_i values between 20 and 30 nM. Both endomorphins competed³H-[d-Ala²,MePhe⁴,Gly(ol)⁵] enkephalin binding to MOR-1 receptors expressed in CHO cells with high affinity. In mouse brain homogenates ¹²⁵I-endomorphin-1 and¹²⁵I-endomorphin-2 binding was selectively competed by mu ligands. ¹²⁵I-Endomorphin-1 and¹²⁵I-endomorphin-2 also labeled MOR-1 receptors expressed in CHO cells with high affinity. Autoradiography of the two¹²⁵I-labeled endomorphins demonstrated regional patterns in the brain similar to those previously observed for mudrugs. Pharmacologically, the endomorphins were potent analgesics. Although they were equipotent supraspinally, endomorphin-1 was more potent spinally. Endomorphin analgesia was effectively blocked by naloxone, as well as the mu-selective antagonists β-funaltrexamine and naloxonazine. In CXBK mice, which are insensitive to supraspinal morphine, neither endomorphin was active, consistent with a mu mechanism of action. Finally, the endomorphins inhibited gastrointestinal transit. In conclusion, these results support the mu selectivity of these agents.