Highly selective fluorescent analogue of the potent delta-opioid receptor antagonist Dmt-Tic.

A fluorescent tripeptide probe derived by coupling fluorescein to H-Dmt-Tic-Glu-NH2 was developed to interact with delta-opioid receptors with high affinity (Ki = 0.035 nM) and selectivity (Ki(mu)/Ki(delta) = 4371). It acts as an irreversible delta-opioid receptor antagonist, and binding to NG108-15 cells is blocked by the standard nonpeptidic delta-opioid receptor antagonist naltrindole. This probe should prove useful in the study of the distribution of delta-opioid receptors in tissues and the internalization of opioid peptides during signal transduction.