Hypothalamic-pituitary corticotroph function after shunting of magnocellular vasopressin and oxytocin to the hypophyseal portal circulation.

Abstract

Surgical pituitary stalk compression (PSC) causes neural lobe denervation and development of ectopic magnocellular terminals in the external zone of the median eminence, resulting in shunting of magnocellular vasopressin (VP) and oxytocin (OT) to the pituitary portal circulation. To determine the effects of PSC on hypothalamic and pituitary function, VP and CRH receptors and POMC messenger RNA (mRNA) in the pituitary, and CRH, VP, and OT mRNA levels in the PVN were examined in 7-day PSC and sham-operated rats. Immunohistochemical staining revealed marked accumulation of immunoreactive VP in the swollen pituitary stalk and the external zone of the distal median eminence of PSC rats. Plasma ACTH and corticosterone levels were significantly increased by PSC, an effect that was attenuated by minipump infusion of desamino-[D-Arg8]-vasopressin (DDAVP) given to correct the diabetes insipidus. [3H]VP binding to anterior pituitary membranes from PSC rats was reduced by 50% compared to that in sham-operated controls, but VP V1b receptor mRNA levels measured by in situ hybridization were unchanged. Pituitary CRH receptors measured by binding autoradiography and CRH receptor mRNA levels did not change after PSC. POMC mRNA was unchanged in the anterior pituitary lobe, but markedly increased in the intermediate lobe of PSC rats, with or without DDAVP infusion. In the hypothalamic supraoptic and paraventricular nuclei, PSC resulted in reduction of VP mRNA (-83%) and OT mRNA (-38%) levels, probably reflecting retrograde degeneration of magnocellular neurons. This decrease was more pronounced for VP mRNA than for OT mRNA CRH mRNA levels in parvicellular cells of the paraventricular nuclei of PSC rats were reduced by 55%. Correction of the diabetes insipidus by DDAVP treatment further decreased hypothalamic VP mRNA levels, but restored CRH mRNA to control levels. The data show that continuous exposure of the pituitary to high VP levels from ectopic magnocellular fibers in the median eminence results in VP, but not CRH, receptor loss. Pituitary VP receptor down-regulation and decreased hypothalamic CRH expression are likely to contribute to the reduced corticotroph responsiveness to VP reported in this experimental model.