Regulation of the number of pituitary vasopressin (VP) receptors plays an important role in controlling pituitary responsiveness during alterations of the hypothalamic pituitary adrenal axis. The mechanisms regulating these VP receptors were studied by analysis of the effects of adrenalectomy and glucocorticoid administration on V1b receptor (V1b-R) messenger RNA (mRNA) by Northern blot and by in situ hybridization in the rat. Adrenalectomy transiently decreased V1b-R mRNA levels by 18 h (77% and 62% for the 3.7-kb and 3.2-kb bands in the Northern blots, and 50% by in situ hybridization), returning to basal levels after 6 days. The decrease in V1b-R mRNA after 18 h adrenalectomy was fully prevented by dexamethasone (100 μg sc) but not by elimination of hypothalamic CRH and VP by paraventricular nucleus lesions or median eminence deafferentation. In sham-operated rats, dexamethasone increased receptor mRNA by 50% after 6 days. In contrast to Sprague-Dawley rats, in Brattleboro rats (di/di), which lack hypothalamic VP, adrenalectomy caused a sustained decrease in V1b-R mRNA levels (<50% of controls by 6 days). The data show that pituitary V1b-R mRNA is positively regulated by glucocorticoids and that the recovery of V1b-R mRNA levels after prolonged adrenalectomy is probably mediated by VP. In addition, the data suggest that the down-regulation of VP binding after long-term adrenalectomy is due to posttranscriptional events rather than to changes in V1b-R mrNA.
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