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Distribution pattern of metorphamide compared with other opioid peptides from proenkephalin and prodynorphin in the
Abstract:
Metorphamide 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 extracts. 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 metorphamide's mu opioid receptor binding and bioactivities suggest that the amounts of metorphamide in the bovine brain are sufficient to make this peptide a candidate for a physiologically significant endogenous mu opioid receptor ligand.
Insights
Metorphamide, an endogenous opioid peptide, shows high affinity for mu opioid receptors. Its concentrations in bovine brain tissues suggest it may play a significant physiological role as a mu opioid receptor ligand.
Area of Science:
- Neuroscience
- Endocrinology
- Pharmacology
Background:
- Metorphamide is a unique endogenous opioid peptide derived from proenkephalin.
- It exhibits a high affinity for mu opioid receptors, with lesser affinity for kappa and low affinity for delta opioid receptors.
- Previous research indicates metorphamide's potential physiological significance based on its receptor binding and bioactivities.
Purpose of the Study:
- To quantify metorphamide concentrations in various bovine neural tissues.
- To compare metorphamide levels with other endogenous opioid peptides derived from proenkephalin and prodynorphin.
- To assess if metorphamide concentrations support its role as a physiologically significant endogenous mu opioid receptor ligand.
Main Methods:
- Radioimmunoassay and chromatography separation were employed to determine peptide concentrations.
- Concentrations of metorphamide were measured in bovine caudate nucleus, hypothalamus, spinal cord, and neurointermediate pituitary.
- Comparative analysis included eight other opioid peptides from proenkephalin and prodynorphin precursors.
Main Results:
- Metorphamide was detected in all four investigated bovine neural tissues.
- Concentrations of metorphamide were lower than other proenkephalin-derived peptides but comparable to prodynorphin-derived peptides.
- Significant regional variations in prodynorphin-derived opioid ratios suggest differential processing, while proenkephalin-derived opioid ratios showed less variation.
Conclusions:
- The quantified levels of metorphamide in bovine brain regions are substantial.
- These concentrations suggest metorphamide is a viable candidate for a physiologically significant endogenous ligand at mu opioid receptors.
- Observed regional differences in opioid peptide ratios highlight complex post-translational processing within the brain.