Modulation of endomorphin-2-induced analgesia by dipeptidyl peptidase IV

The tetrapeptide, endomorphin-2 (Tyr-Pro-Phe-PheNH(2)) possesses high affin ity for mu opioid receptors, and produces potent analgesia in mice. Its str ucture appears to satisfy the substrate requirements of the proteinase, dip eptidyl peptidase TV which removes dipeptides from the amino terminus of pe ptides containing proline as the penultimate amino acid. A potent, stable a nd specific inhibitor of this enzyme, Ala-Pyrrolidonyl-2-nitrile, has been described which should potentiate endomorphin-2-induced analgesia. Further, since dipeptidyl peptidase IV has an absolute requirement for L-Pro, a mor e metabolically-stable D-Pro(2)-endomorphin-2 analog should produce longer analgesic actions at lower doses. The present study found that endomorphin- 2 was degraded approximately twice as fast than the chromogenic substrate, Ala-Pro-2naphthylamide, by dipeptidyl peptidase IV, whereas D-Pro(2)-endomo rphin-2 was totally resistant to this enzyme's action. D-Pro(2)-endomorphin -2 (ED50 = 0.05 mu g) was more potent than endomorphin-2 (ED50 = 30 mu g) i n significantly increasing tail-flick latencies with longer durations of ac tion. Both the peptide and analogue were equipotent (ED50 = 0.5 mu g) in si gnificantly increasing jump thresholds. Ala-Pyrrolidonyl-2-nitrile (10-75 n mol) elicited a dose-dependent analgesia, and potentiated the analgesic act ions of endomorphin-2, particularly on the tail-flick test. Whereas systemi c naltrexone (2.5, 10 mg/kg) dose-dependently eliminated each of the three forms of analgesia on the jump test as well as the peak (15 min) effect on the tail-flick test: analgesia elicited by either endomorphin-2, D-Pro2-end omorphin-2 or Ala-Pyrrolidonyl-2-nitrile returned after 30-60 min in naltre xone-treated rats on the tail-flick test. These data strongly suggest that dipeptidyl peptidase IV plays a role in the inactivation of endomorphin-2 i n vivo, and thereby modulates its central analgesic actions. (C) 1999 Elsev ier Science B.V. All rights reserved.