The parvocellular vasopressinergic system and responsiveness of the hypothalamic pituitary adrenal axis during chronic stress

Abstract

Vasopressin (VP) secreted from parvocellular neurons of the hypothalamic paraventricular nucleus (PVN) stimulates pituitary adrenocorticotropic hormone (ACTH) secretion, through interaction with receptors of the V1b subtype (V1bR) in the pituitary corticotroph, mainly by potentiating the stimulatory effects of corticotrophin-releasing hormone (CRH). Chronic stress paradigms associated with corticotroph hyperresponsiveness lead to preferential expression of hypothalamic VP over CRH and upregulation of pituitary V1bR, suggesting that VP has a primary role during adaptation of the hypothalamic pituitary adrenal (HPA) axis to long-term stimulation. However, studies using pharmacological or genetic ablation of V1bR have shown that VP is required for full ACTH responses to some stressors, but not for the sensitization of ACTH responses to a novel stress observed during chronic stress. Studies using minipump infusion of a peptide V1 antagonist in long-term adrenalectomized rats have revealed that VP mediates proliferative responses in the pituitary. Nevertheless, only a minor proportion of cells undergoing mitogenesis co-express markers for differentiated corticotrophs or precursors, suggesting that new corticotrophs are recruited from yet undifferentiated cells. The overall evidence supports a limited role of VP regulating acute ACTH responses to some acute stressors and points to cell proliferation and pituitary remodelling as alternative roles for the marked increases in parvocellular vasopressinergic activity during prolonged activation of the HPA axis.

Figure 1

Effect of acute restraint stress for 1 h on CRH hnRNA levels in the PVN of naive rats (B) or 24 h after the last restraint in rats subjected to 1h repeated restraint for 14 days (D). Basal CRH hnRNA were almost undetectable in both naïve rats (A) and repeatedly restrained rats 24 h after the last episode (C). Darkfield photographs were taken through the medial parvocellular subdivision of the PVN (MP). 3^rd V, Third ventricle; RR × 14, repeated restraint, 1h daily for 14 days.

Figure 2

Effect of acute restraint stress for 1 h on VP hnRNA levels in the PVN of naive rats (B) or 24 h after the last restraint in rats subjected to 1h repeated restraint for 14 days (D). Basal VP hnRNA levels were undetectable in the parvocellular region of naïve rats (A), while they were markedly elevated 24 hr after the last stress in repeatedly restrained rats (C). Darkfield photographs of emulsion dipped slides were taken through the medial parvocellular subdivision of the PVN (MP). 3^rd V, Third ventricle; PM, posterior magnocellular; MP, medial parvocellular; RR × 14, repeated restraint, 1h daily for 14 days.

Figure 3

The effect of chronic osmotic minipump administration of the non-selective V1 receptor antagonist, dGly[Phaa1,D-tyr(et), Lys, Arg]VP (V1R Ant), or vehicle for 14 days on plasma ACTH responses to the novel stress of i.p. hypertonic saline injection (ipHS) in conscious naïve control rats (A), or rats subjected to restraint stress for 1h daily for 14 days (B). Data are the mean and SE of values obtained in 8 to 10 rats per group. Both groups showed a significant effect of ipHS on plasma ACTH levels (*, p<0.001). ACTH responses to ipHS in naïve rats were significantly reduced by the V1R Ant compared with the vehicle infused controls ($, p<0.04). ACTH responses to ipHS in repeatedly restrained rats were significantly higher than those in naïve rats (#, p<0.02), and these enhanced responses were unaffected by the V1R Ant.

Figure 4

The effect of chronic administration of the non-selective V1 receptor antagonist, dGly[Phaa1,D-tyr(et), Lys, Arg]VP (V1R Ant), or vehicle on adrenalectomy (ADX)-induced changes in the number of cells immunostained for ACTH or deoxybromouridine (BrdU). Rats were subjected to adrenalectomy and implantation of osmotic minipumps (Alza, model 2004) containing the V1 antagonist (230 ng/h/28 days) or vehicle, and BrdU injections for 7 days before sacrifice at day 28. Rats were killed by decapitation and pituitaries fixed for BrdU and ACTH immunohistrochemistry. Bars represent the mean and SE of immunopositive cells counted in pituitary sections of 4 rats per experimental group. *p<0.05 vs. sham; ** p<0.002 vs. sham; *** p<0.001 vs. sham, $p<0.05 vs ADX