Enkephalin: radioimmunoassay and radioreceptor assay in morphine dependent rats.

A sensitive and specific radioimmunoassay for enkephalins is described which discriminates between leucineenkephalin and methionine-enkephalin. Total opioid peptide activity was assayed by the ability of brain extracts to compete for opiate receptor binding. In rats treated with morphine and/or naloxone, opiates were separated from opioid peptides by Dowex AG-50W column chromatography prior to radioreceptor assay. Chronic administration of morphine failed to alter immunoreactive enkephalin levels or total opioid activity in radioreceptor assays.