Distribution Pattern of Metorphamide Compared with Other Opioid Peptides from Proenkephalin and Prodynorphin in the Bovine Brain

Publisher Website
Google Scholar
Abstract
Metrophamide is a [Met]-enkephalin-containing opioid octapeptide with a C-terminal .alpha.-amide group. It is derived from proenkephalin and is, so far, the only endogenous opioid peptide with a particularly high affinity for .mu. opioid (morphine) receptors, a somewhat lesser affinity for .kappa. opioid receptors, and a relatively low affinity for .delta. opioid receptors. The concentrations of metorphamide in the bovine caudate nucleus, the hypothalamus, the spinal cord, and the neurointermediate pituitary were determined by radioimmunoassay and chromatography separation procedures. Metorphamide concentrations were compared with the concentrations of eight other opioid peptides from proenkephalin and prodynorphin in identical exstracts. The other opioid peptides were [Met]-enkephalyl-Arg6-Phe7 and [Met]-enkephalyl-Arg6-Gly7-Leu8 from proenkephalin; .alpha.-neoendorphin, .beta.-neoendorphin, dynorphin A(1-8), dynorphin A(1-17), and dynorphin B from prodynorphin; and [Leu]-enkephalin, which can be derived from either precursor. All opioid peptides were present in all four bovine neural tissues investigated. Metorphamide concentrations were lower than the concentrations of the other proenkephalin-derived opioid peptides. They were, however, similar to the concentrations of the prodynorphin-derived opioid peptides in the same tissues. Marked differences in the relative ratios of the opioids derived from prodynorphin across brain regions were observed, a finding suggesting differential posttranslational processing. Differences in the ratios of the proenkephalin-derived opioids across brain regions were less pronounced. The results from this study together with previous findings on metophamide''s .mu. opioid receptor binding and bioactivities suggest that the amounts of metorphamidein the bovine brain are sufficient to make this peptide a candidate for a physiologically significant endogenous .mu. opioid receptor ligand.