An attempt to characterize by immunocytochemical methods the enkephalin-like material in oxytocin endings of the rat neurohypophysis.

An antibody raised against dynorphin 1-8 was immunoreactive in the rat neurohypophysis in vasopressin but not in oxytocin endings. Oxytocin but not vasopressin terminals were immunostained when a Met5-enkephalin, a N-terminally directed monoclonal beta-endorphin antibody and an enkephalin-Met5-Arg6-Phe7 antibody were applied. The immunocytochemical reactions indicate that oxytocin terminals and their granules store pro-enkephalin-A derived opioid peptide fragments.