Co-existence of unrelated peptides in oxytocin and vasopressin terminals of rat neurohypophyses: Immunoreactive methionine5-enkephalin-, leucine5- enkephalin- and cholecystokinin-like substances

In neurohypophyses of normal rats and of vasopressin-deficient Brattleboro rats, applying immunocytochemical methods, we have found (1) in oxytocin terminals co-existing methionine-enkephalin-, cholecystokinin- and possibly leucine-enkephalin-like substances and (2) in vasopressin terminals, co-existing leucine-enkephalin-like material. The correspondence between the enkephalin immunoreactivity patterns and the oxytocin or vasopressin immunoreactivity pattern in serial 0.5 μ thick sections was so close that occurrence of enkephalin in separate nerves appears unlikely. At a subcellular level the enkephalin- and the cholecystokinin-like immunoreactivities were localized mainly in the neurosecretory granules that also contain oxytocin or vasopressin; there was no evidence for separate enkephalingranule populations. The intensity and frequency of leucine-enkephalin immunostaining in vasopressin terminals was much enhanced by treatment of the deplasticized sections with trypsin prior to incubation with antibodies. This suggests incorporation of the leucine-enkephalin sequence into longer peptide chains, presumably dynorphin and/or α-neo-endorphin. In oxytocin endings tryptic cleavage enhanced cholecystokinin-like immunoreactivity, while methionine-enkephalin immunostaining was as intense without enzyme treatment.The opioid peptide forms of vasopressin neurons appear to be elongated chains containing exclusively the leucine-enkephalin sequence, whereas in oxytocin neurons methionine-enkephalin as free pentapeptide seems to prevail. Association of enkephalins with vasopressin and oxytocin was also indicated when methionine- and leucine-enkephalin contents of neurointermediate lobes of homozygous Brattleboro rats were compared with peptide stores in heterozygous controls in a high pressure liquid chromatography system and highly specific leucine- and methionine-enkephalin radioimmunoassays. We assume that the opioid peptides in the oxytocin system and in the vasopressin system are derived from different opioid precursor molecules.