Vasopressin-Containing and Vasopressin-Deficient Subpopulations of Corticotropin-Releasing Factor Axons Are Differentially Affected by Adrenalectomy

CRF-containing parvocellular axons in the external zone of the rat median eminence were classified as vasopressin-containing (CRF+/AVP+) and vasopressin-deficient (CRF+/AVP-) subpopulations based on post-embedding electron microscopic immunocytochemical staining of serial ultrathin sections for CRF, AVP and the other peptides derived from the AVP precursor: AVP-associated neurophysin (NP-AVP) and the C-terminal glycopeptide (GP). In normal animals, the CRF+/AVP+ and CRF+/AVP- subpopulations were approximately equal in terms of detectable axonal swellings. Three to 14 days after adrenalectomy (ADX), the CRF+/AVP+ and CRF+/AVP- subpopulations represented about 95% and 5%, respectively, of total CRF+ swellings. This change was due to a 90% decrease in the absolute number of detectable CRF+/AVP- swellings after ADX, whereas the absolute number of detectable CRF+/AVP+ swellings rose by less than 20%. These changes were completely blocked by administering the glucocorticoid agonist dexamethasone throughout the period after ADX. The results suggest that the CRF+/AVP+ and CRF+/AVP- subpopulations of neurosecretory axons in the external zone of the median eminence respond differently to ADX, indicating that they are independently regulated by glucocorticoids.