Influence of δ-opioid receptors on production of labeled methionine⁵-enkephalin in murine neuroblastoma cells

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 nonproteolyzable 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.