Lack of effect of morphine and noxious stimuli on the MET-enkephalin content in the spinal dorsal horn and nucleus reticularis gigantocellularis of the rat.

The Met-enkephalin contents in the dorsal horn of the lumbar enlargement and the nucleus reticularis gigantocellularis of the medulla oblongata of the rat were measured, using a sensitive and specific radioimmunoassay for Met-enkephalin, and the effects of morphine and noxious stimuli on the Met-enkephalin contents in these regions were examined. In this radioimmunoassay, the IC50 and assayable limits for Met-enkephalin were 45 and 5 fmol/tube respectively, and the IC50 for Leuenkephalin was 0.56 nmol/tube (0.008% cross reactivity between Met-and Leu-enkephalins). The contents of Met-enkephalin-like immunoreactivity in the dorsal horn of the lumbar enlargement and the nucleus reticularis gigantocellularis were not altered by either subcutaneous injection of morphine or thermal (hot plate) and chemical (formalin injection) noxious stimuli applied to the hind-paws.