MARKED CHANGES OF ARGININE-VASOPRESSIN, OXYTOCIN, AND CORTICOTROPIN-RELEASING HORMONE IN HYPOPHYSEAL PORTAL PLASMA AFTER PITUITARY-STALK DAMAGE IN THE RAT

Mechanical compression of the pituitary stalk with the help of a blunt stereotaxic knife results in posterior pituitary denervation (PPD) an d sprouting proximal to the injury, leading to formation of an ectopic neurohypophysis in the stalk. This provides an experimental model for those cases in which traumatic damage severs the nerve fibers to the neural lobe but does not obliterate the hypophysial-portal circulation . The effect of PPD on the hypophysial-portal concentration profile of putative ACTH secretagogues as well as basal and stimulated ACTH secr etion in vitro were investigated at varying times after PPD. The conte nts of arginine vasopressin (AVP) and oxytocin (OT) in extracts of the stalk median eminence 1 week after PPD were markedly elevated, wherea s corticotropin-releasing hormone (CRH) content was unaffected. Levels of these three neuropeptides in hypophysial-portal blood collected un der anesthesia from the proximal stump of the transected stalk (or the ectopic neural lobe) were measured at weekly intervals in groups of r ats after sham or PPD surgery. Hypophysial-portal AVP levels showed a monotonic increase with time after PPD from a 1.8-fold elevation at 1 week post-PPD to a maximum concentration B-fold greater than that in s ham groups at 4 weeks post-PPD. Portal plasma OT levels also exhibited extreme elevation. In contrast, portal plasma CRH levels showed an in itial 72% decline 1 week post-PPD. We suggest that mechanical damage t o the pituitary stalk. and the subsequent sprouting redirected secreti on of AVP and OT from the neural lobe to the pituitary stalk. This cau sed sustained elevations of portal plasma concentrations of AVP and OT . The resulting tonic exposure to AVP and/or OT may down-regulate ante rior pituitary receptors to these neurohypophyseal peptides and indire ctly decrease CRH release into the portal circulation.