Differential regulation of corticotropin-releasing hormone and vasopressin transcription by glucocorticoids.

CRH and vasopressin (VP), the main regulators of pituitary ACTH secretion, co-exist in parvocellular cells of the PVN, but their levels of expression are regulated differentially during manipulations of the hypothalamic pituitary adrenal (HPA) axis. The effects of glucocorticoids on this system was studied using in situ hybridization with intronic and exonic probes to measure changes in CRH and VP messenger RNA (mRNA) and heteronuclear (hn) RNA in 48-h adrenalectomized (ADX) rats receiving injections of corticosterone (2.8 mg/100 g, ip) or vehicle. We also determined the time course of changes in VP expression following the first 72 h of ADX. Levels of VP heteronuclear (hn) RNA and the number of parvocellular cells containing VP hnRNA remained very low in sham operated rats, whereas biphasic changes were observed after ADX. Grain density levels increased 11.5-fold over sham-operated controls by 6 h, declined to 2-fold by 18 h, to increase again to 10- and 20-fold by 48 and 72 h, respectively. In 48-h ADX rats, vehicle injection increased CRH hnRNA levels transiently (11-fold the basal by 15 and 30 min), returning to basal at 60 min, whereas VP hnRNA levels increased progressively up to 28-fold the basal by 2 h. Corticosterone injection had no significant effect on vehicle-induced increases in CRH hnRNA, in spite of marked elevations in circulating corticosterone. In contrast to CRH, VP hnRNA levels increased only transiently by 15 min, and then decreased below basal (near sham-ADX levels) by 2 h. The data show that in normal conditions the responsiveness of parvocellular neurons to stress is under marked inhibition by the low resting levels of glucocorticoids, and that the sensitivity of CRH and VP transcription to glucocorticoid feedback is markedly different.