Indication for a granule-free form of vasopressin in immobilization-stressed rats.

Following immobilization stress the supraoptic nucleus exhibits an increased number of coarse, heavily immunostained fibers in the basal glia labyrinth. Ultrastructural immunocytochemistry demonstrates a labeling of the rough endoplasmic reticulum in the neurosecretory perikarya and granule-free, immunoreactive material in the axons adjacent to the basal glia labyrinth. Furthermore, a labeling of the intercellular clefts of the neuropil is demonstrable in the supraoptic nucleus. These results lead to the hypothesis that 1) vasopressin is synthesized and released in two forms, in a granule-bound form and in a granule-free, probably more soluble form, and that 2) the latter might be released already in the nuclear area into the intercellular clefts from where it may reach its target cells via the cerebrospinal fluid of the subarachnoid space.