The opioid peptide dynorphin A antagonizes morphine-induced analgesia in vivo and inhibits opiate binding in vitro, although most of it is rapidly degraded under both conditions. The inhibitory effect was present even in tissue treated in vivo with dynorphin A and assayed in vitro without it. Shorter fragments of this peptide lacked these effects, indicating that the apparent potency did not result from a metabolite. Na+ ion specifically reversed both agonist and antagonist binding from in vitro inhibition by dynorphin A. These results are discussed in terms of current opioid receptor theories.
Bibliographical noteFunding Information:
Acknowledgements---Supported by NIDA 02643 and 2K02-DA-70554 (HHL) and 2-K02-DA-00020 (NML). PSB is a recipient of a Fulbright-MEC Fellowship. JG is a recipient of a NIH Fellowship 705-TW-03080. We thank Professor J. Del Rio for the facilities given in the preparation of this manuscript.