Regional Distribution of Methionine‐Enkephalin‐Arg6‐Phe7 in the Rat Brain: Comparative Study with the Distribution of Other Opioid Peptides

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Abstract
The distribution of the opioid peptide methionine-enkephalin-arginine6-phenylalanine7 (M-Enk-Arg6-Phe7) has been investigated in various structures of the rat brain by using a highly specific radioimmunoassay (RIA). Immunoreactive M-Enk-Arg6-Phe7 has been further characterized by high performance liquid chromatography. The levels of M-Enk-Arg6--Phe7 in various structures of the rat brain were compared with the levels of several other opioid peptides, including methionineenkephalin (M-Enk), leucine-enkephalin (L-Enk), dynorphin 1–13, and α-neoendorphin, which were also measured by RIA. There was a close relationship between the distribution of M-Enk-Arg6--Phe7 immunoreactive material (ir), M-Enk ir, and L-Enk ir. The distribution of dynorphin 1–13 ir and α-neoendorphin ir appeared to be distinct from that of the enkephalin group. These results are in agreement with recent reports on the cloning and sequencing of the c-DNA coding for the prohormones, in which it has been hypothesized that M-Enk-Arg6-Phe7 and M-Enk are synthesized by the same precursor, called proenkephalin, and that dynorphin-related peptides and α-neoendorphin arise from a separate precursor, prodynorphin.