Angiotensin-induced vasopressin release and activation of hypothalamic neuron in pre-term fetuses

Our previous studies have shown that central administration of angiotensin II (ANG II) causes vasopressin release in the near-term fetus in utero as evidence that the hypothalamic-neurohypophysial system has relatively matured before birth. However, it is still unknown whether the vasopressin controlling centers have been functionally developed in younger fetuses. This study determined fetal plasma vasopressin levels and hypothalamic vasopressin neuron activity in the chronically instrumented pre-term ovine fetuses. Introcerebroventricular (i.c.v.) administration of ANG II did not affect fetal plasma osmolality and sodium concentrations. However, fetal plasma vasopressin levels were significantly increased ( approximately 3-fold) in response to central injection of ANG II. Central ANG II also induced vasopressin-neuron activity marked with c-fos expression in the fetal hypothalamus at pre-term. In addition, the fetal organum vasculosum of the lamina terminalis and the subfornical organ were activated. The results suggest that hypothalamic-neurohypophysial system has been relatively intact and functional at 70% gestational age, and that central angiotensin is important in inducing fetal vasopressin release in utero.