Vol. 304, Issue 1, 301-309, January 2003

Potency Differences for D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH₂ as an Antagonist of Peptide and Alkaloid µ-Agonists in an Antinociception Assay

Steven N. Sterious and Ellen A. Walker

Department of Biology, Temple University, Philadelphia, Pennsylvania (S.N.S.); Office of Research and Technology Development, Albert Einstein Healthcare Network, Philadelphia, Pennsylvania (S.N.S., E.A.W.); and Department of Psychology, La Salle University, Philadelphia, Pennsylvania (E.A.W.)

D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP) is a peptide antagonist that demonstrates potent and selective affinity for µ-opioid receptors in radioligand binding assays and in vitro bioassays. However, previous studies indicate that CTAP may possess unusual pharmacology under certain conditions. Therefore, CTAP was evaluated as an antagonist of the antinociceptive effects of a range of structurally diverse high- and low-efficacy peptide and alkaloid opioid agonists and compared with the traditional antagonist naltrexone. Male Sprague-Dawley rats (N = 227) were loosely restrained and the latency for tail withdrawal from 55°C water was measured. Morphine s.c. and i.c.v., buprenorphine s.c., etorphine s.c. and i.c.v., [N-Me-Phe³,D-Pro⁴]-morphiceptin and [D-Ala²,N-Me-Phe⁴,Gly⁵-ol]-enkephalin produced antinociceptive effects. CTAP was at least 10-fold more potent than naltrexone as an antagonist of the antinociceptive effects of all five agonists. High doses of CTAP produced a noncompetitive antagonism of etorphine s.c. and morphine s.c. suggesting that CTAP may interact with additional opioid receptors in vivo or produce insurmountable antagonism at these doses. CTAP was approximately 300-fold more potent as an antagonist of DAMGO than the other agonists, indicating that CTAP may distinguish some peptide agonists such as DAMGO from other agonists based on binding interactions within the µ-opioid receptor or pharmacodynamic properties of these peptides. Naltrexone, however, administered by either s.c. or i.c.v. routes of administration was approximately equipotent as an antagonist of the antinociceptive effects of most agonists. Taken together, these data indicate that the peptide antagonist CTAP possesses a unique pharmacology unlike traditional opioid antagonists such as naltrexone.