High-affinity interaction of (des-tyrosyl)dynorphin A(2-17) with NMDA receptors

The opioid peptide dynorphin A elicits non-opioid receptor-mediated, neurotoxic response in vivo, which is blocked by pretreatment with MK-801, a noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist. In the present study, we examined the possible direct interaction of dynorphin A on the NMDAR. A nonopioid dynorphin A analog, ¹²⁵I-(des-tyrosyl) dynorphin A(2-17), was used in radioligand binding analysis on rat cortical brain membranes. This radioligand exhibited a saturable, specific binding at high affinity with a K(d) value of 9.4 ± 1.6 nM and maximal binding of 2.4 ± 0.6 pmol/mg protein. This binding site was associated with the NMDAR complex because it was modulated by a number of NMDAR ligands. Transient expression of the rat NR1a/NR2A complex in human embryonic kidney 293 cells confirmed a coexpression of ¹²⁵I-(des-tyrosyl) dynorphin A(2-17), [³H]CGP39,653, and [³H]MK-801 binding. These data provide direct evidence of the presence of a high-affinity binding site for dynorphin A on the NMDAR. The modulatory effect of the various NMDAR-selective ligands on dynorphin A binding suggests that dynorphin A may bind preferentially to the closed/desensitized state of the NMDAR. The physiological role of dynorphin A binding to the NMDAR remains to be established.