Influence of δ‐Opioid Receptors on Production of Labeled Methionine⁵‐Enkephalin in Murine Neuroblastoma Cells

Abstract: Synthesis of methionine⁵‐enkephalin by intact cells of murine neuroblastoma clone N1E‐115 has been demonstrated both immunocytochemically and biochemically. In addition, N1E‐115 cells possess homogeneous enkephalin (δ) receptors which inhibit prostaglandin E₁‐ induced intracellular cyclic AMP formation. An assay was developed for measuring de novo synthesis of methionine⁵‐enkephalin by pulsing cells in culture with radioactive methionine and isolating this pentapeptide to radiochemical purity by a procedure that included immunoaffinity chromatography specific for oxidized methionine⁵‐enkephalin. This assay indicated that production of radiolabeled‐methionine⁵‐enkephalin was increased upon lengthy exposure of intact N1E‐115 cells in the late logarithmic phase of growth to a nonproteolyz‐able analog of methionine⁵‐enkephalin. This increase in synthesis of intracellular methionine⁵‐enkephalin relative to control cells was prevented by prior incubation of the clone with naloxone, indicating that the response was mediated by the δ receptor.