Time Course of Vasopressin and Oxytocin Secretion after Stress in Adrenalectomized Rats

Abstract

To characterize the participation of vasopressin (AVP) and oxytocin (OT) in hypothalamus-pituitary-adrenal regulation after adrenalectomy (ADX), we evaluated corticosterone, ACTH, AVP and OT plasma concentrations and AVP and OT content of the paraventricular nucleus (PVN) at different periods (3 h, 1, 3, 7 and 14 days) in sham or ADX rats under basal conditions and after immobilization stress. ADX animals showed undetectable corticosterone levels, while sham animals showed a marked increase in corticosterone and ACTH 3 h after surgery, then lowering to basal control levels. ADX rats showed high basal ACTH levels with a triphasic response without changes after immobilization. After three hours, the ADX group showed higher OT levels than the sham group. OT was increased after immobilization stress in sham and ADX groups. AVP plasma levels did not change throughout the basal or stress studies in either group. There was a decrease in hypothalamic AVP content 1 and 3 days after ADX under basal and stress conditions. Plasma osmolality showed a significant decrease in the ADX group at 3, 7, and 14 days. In conclusion, there are different pituitary-adrenal axis set points after removal of the glucocorticoid negative feedback. The role of vasopressinergic and oxytocinergic neurons in the ACTH secretion after ADX or immobilization stress appears to differ. Magnocellular AVP is unlikely to contribute to ACTH secretion in response to ADX or immobilization stress. On the other hand, OT is elicited by immobilization stress and might contribute to the ACTH secretion during short-term ADX.

Changes in prolactin secretion in the short- and long-term after adrenalectomy

“…Moreover, chronic hypoos molality inhibits PRL secretion induced by the stress of restraint, indicating the involvement of magnocellular AVP neurons in these responses (36). Although AVP induces prolactin release in vivo (37), we did not observe changes in plasma AVP throughout the time course after ADX surgery (24), suggesting that this peptide might not have a role in prolactin se cretion in this experimental model. We and others have shown an increase in plasma PRL induced by increased water and salt ingestion or after hypertonic EVE infu sion (21,38).…”

“…Our data de monstrated a triphasic response pattern of ACTH levels induced by ADX: at first, there was a marked increase in ACTH release, leading to depletion of pituitary ACTH storage, which explains the decrease in circulating ACTH levels during the second phase. Finally, com pensatory changes occur, the most important being an increase in ACTH synthesis, as confirmed by increased pituitary ACTH content and proopiomelanocortin mRNA expression (19,24,25). These findings indicate a profound alteration in the HPA axis induced by ADX in the short and longterm.…”

“…However, we cannot rule out the effect of plasma hypoosmolality observed in the longterm after ADX. Indeed, we previously reported a significant decrease in plasma osmolality associated with severe dehydration in rats that underwent ADX after 3, 7, and 14 days (24). Moreover, chronic hypoos molality inhibits PRL secretion induced by the stress of restraint, indicating the involvement of magnocellular AVP neurons in these responses (36).…”

“…In addition, we can not rule out that, in our model, PRL levels had already returned to basal levels. Previously, our group descri bed an increase in OT concentration 3 hours after ADX (19,24,25). It is important to point out that subcuta neous administration of oxytocin induces a reduction in basal, as well as in stressinduced prolactin secretion, in male rats (3335).…”

Corticotropin-releasing factor and its receptors in the brain of rats with insulin and corticosterone deficits

“…In addition, magnocellular CRF via the neurohypophysis may reach the circulation and directly stimulate the adrenal glands using the extrapituitary mechanisms (Fehm et al 1988). It is not clear as to whether CRF can activate the release of OT, which could either activate or block the HPA-axis activity (Petersson et al 1999, Windle et al 2004, Laguna-Abreu et al 2005) depending on various factors. At the level of anterior pituitary, OT has been reported to act as an ACTH secretagog, with significant effects seen at low concentrations (Schlosser et al 1994).…”