An immunoelectronmicroscopic method for the specific localization of
neurohypophyseal hormones was developed in neurohypophyses of Wistar and Brattleboro rats, the latter strain being homozygous for
diabetes insipidus. If the proper precautions were omitted, a marked cross reactivity between anti-
vasopressin and anti-
oxytocin preparations was found. Cross reaction of an anti-
vasopressin plasma with
oxytocin, at a dilution of less than 1:1600, resulted in electron density of all granules within neurosecretory fibres of the Brattleboro and Wistar neurohypophyses. However, this cross reactivity could be eliminated either by sufficient dilution of the anti-plasma, or by its purification. Purification of the
antibodies was performed by absorption to
agarose beads coated with the cross reacting component. Upon incubation with anti-
vasopressin (diluted unpurified 1:1600 or purified 1:80) and unpurified anti-
oxytocin (1:400) plasma, sections of a Wistar neurohypophysis revealed two types of neurosecretory fibres, containing either electron dense or lucent granules.
Oxytocin and
vasopressin containing neurosecretory fibres were found as clusters in the neurohypophysis. The specificity of both unpurified anti-
vasopressin (1:1600) and anti-
oxytocin (1:400) plasma was confirmed on serial sections of a Wistar neurohypophysis, alternately incubated with the solutions of the two
antibodies. These data prove that the one-cell-one-
hormone hypothesis holds true for the hypothalamic-neurohypophyseal system.