Co-localization Of Corticotropin-releasing Factor And Vasopressin In Median Eminence Neurosecretory Vesicles

Vasopressin (VP) potentiates the effect of corticotropin-releasing factor (CRF) on the secretion of adrenocorticotropic hormone (ACTH) from anterior pituitary cells in vitro1–3, and both CRF4–7 and VP6,8 have been found in portal blood. These data support the hypothesis that VP acts synergistically with CRF to cause the secretion of ACTH in vivo1–4,8,9 but the origin of the CRF and VP, and the physiology of their release, have not been precisely defined. Parvocellular cell bodies in the paraventricular nucleus (PVN) which project to the external zone of the median eminence can be stained for both CRF and VP after adrenalectomy10–12, and there is light microscopic imunocytochemical evidence that neurophysin (NP) may be located within some of the CRF-containing axons13. Electron microscopic immunocytochemical studies have demonstrated the presence of CRF14–16, VP17,18 and its ‘carrier’ protein, VP-associated neurophysin (NP-VP)17,18 in 100-nm neurosecretory vesicles (NSVs) in axons terminating near the portal capillary plexus in the external zone of the median eminence. If these peptides are extensively co-localized in the same NSVs in the median eminence, then coordinate secretion of CRF and VP in vivo is obligatory, at least in some physiological circumstances. We demonstrate in this report, using post-embedding electron microscopic immunocytochemistry on serial ultrathin sections, that CRF, VP and NP-VP are contained not only in the same axons and terminals, but in the same 100-nm NSVs in the median eminence of both normal and adrenalectomized rats. In addition, in the normal rat median eminence 44% of the CRF-positive axons and terminals stained strongly for VP and NP-VP, whereas in the adrenalectomized rat virtually all the CRF-positive structures in the median eminence showed strong staining for VP and NP-VP, indicating a transformation of one subpopulation of CRF-positive axons and terminals by adrenalectomy.