Magnocellular neuropeptidergic terminals in neurohypophysis: rapid glial release of enclosed axons during parturition.

The ultrastructure of pituicytes as well as their relationship with neurosecretory axons were measured from the following groups of adult rats: virgin females; immediately pre-partum; post-partum; and following 14 days of lactation. Both parturition and lactation brought about a significant decrease in the number of pituicytes with neurosecretory axons completely surrounded by their cytoplasm. Since these two physiological states are each characterized by increased neurohypophysial hormone release, the data suggest that pituicyte enclosure of neurosecretory axons occurs mainly during conditions of low hormone release. Examination of pituicyte size and number of lipid inclusions, and the number of synaptoid contacts from neurosecretory axons onto pituicytes showed no differences among groups.