In addition to its recognized actions on water conservation and blood pressure, the neuropeptide vasopressin is released into the pituitary portal circulation and within distinct brain areas during stressful challenges. The neuroendocrine effects of vasopressin are mediated by specific receptor of subtype V1b in the anterior pituitary and V1a and V1b subtypes in the brain. Vasopressin produced by parvocellular neurons of the PVN potentiates the stimulatory effect of CRH on pituitary ACTH secretion acting through plasma membrane receptors of the V1b subtype (V1bR). During prolonged activation of the HPA axis there is a predominant increase in VP in parvocellular hypothalamic neurons and pituitary V1b receptors compared with CRH and CRH receptors, suggesting that VP becomes the main mediator of pituitary corticotroph responsiveness during chronic stress. However, studies of this laboratory have shown that VP has little impact on HPA axis activity in chronic stress conditions but that the neuropeptide mediates mitogenic effects in the pituitary and has potential neuroprotective effects during stress adaptation. ? ? Studies in the neuronal cell line H32, which expresses endogenous V1 VP receptors, showed that activation of endogenous V1 VP receptors prevents serum deprivation-induced apoptosis, through phosphorylation-inactivation of the pro-apoptotic protein, Bad, and consequent decreases in cytosolic cytochome c and caspase-3 activation. These actions of VP are mediated by mitogen activated protein (MAP) kinase and extracellular signal-regulated kinases (ERK), but also involves Ca2+/calmodulin dependent kinase (CaMK) and protein kinase C (PKC). Additional studies were performed to role of protein kinase C signaling on the antiapoptotic effects of VP in neurons using the cell line H32. Serum deprivation for 6h induced caspase 3 activity and dephosphorylation of the proapoptotic protein, Bad, while co-incubation with VP reversed these changes. The selective PKC subtypes alpha/beta inhibitor, G6976, or PKC alpha and beta dominant-negatives reduced the effect of VP on caspase activation, while PKC delta inhibitors or dominant-negative reduced serum deprivation-induced caspase-3 activity, without affecting the protective action of VP. Consistently, serum deprivation increased PKC delta but not PKC alpha or PKC beta activity, while VP increased PKC alpha and PKC beta activity without affecting PKC delta activity. VP-induced Bad phosphorylation was reduced by PKC alpha and beta or MEK inhibitors, but totally blocked by both inhibitors combined. However, total inhibition of caspase 3 activation required additional inhibition of the PI3K/Akt pathway. The data demonstrate differential roles of PKC subtypes, with PKC delta mediating the apoptotic effect of serum deprivation, and PCK alpha and beta together with the MAP kinase and PI3K/Akt pathways mediating the antiapoptotic actions of VP.
| Aguilera, Greti; Subburaju, Sivan; Young, Sharla et al. (2008) The parvocellular vasopressinergic system and responsiveness of the hypothalamic pituitary adrenal axis during chronic stress. Prog Brain Res 170:29-39 |
| Chen, Jun; Volpi, Simona; Aguilera, Greti (2008) Anti-apoptotic actions of vasopressin in H32 neurons involve MAP kinase transactivation and Bad phosphorylation. Exp Neurol 211:529-38 |
| Chen, Jun; Young, Sharla; Subburaju, Sivan et al. (2008) Vasopressin does not mediate hypersensitivity of the hypothalamic pituitary adrenal axis during chronic stress. Ann N Y Acad Sci 1148:349-59 |
