Central angiotensin I increases fetal AVP neuron activity and pressor responses

Abstract

Angiotensin (Ang) II plays a critical role in cardiovascular homeostasis and neuroendocrine regulation. Little is known about whether central angiotensin-converting enzyme (ACE) is functional in the fetal brain. We investigated cardiovascular and neuroendocrinological responses to intracerebroventricular (icv) application of Ang I in the chronically prepared near-term ovine fetus in utero and examined the action sites marked by c-fos expression in the fetal hypothalamus. ACE mRNA was detected in the specific central areas. Intracerebroventricular Ang I significantly increased fetal blood pressure and c-fos expression in the supraoptic nuclei (SON) and the paraventricular nuclei (PVN) in the hypothalamus, accompanied by an increase of fetal plasma arginine vasopressin (AVP). Double labeling demonstrated that AVP neurons in the fetal SON and PVN were expressing c-fos. Captopril, an inhibitor of ACE, significantly suppressed fetal pressor responses and plasma AVP. Double labeling experiments showed colocalization of AT(1) receptor (AT(1)R) and c-fos expression in both SON and PVN following icv Ang I. The results indicate that central endogenous ACE has been functional at least at the last third of gestation and the endogenous brain renin-angiotensin system-mediated pressor responses and AVP release via AT(1)Rs by acting at the sites consistent with the cardiovascular network in the hypothalamus.

Fig. 1.

Angiotensin-converting enzyme (ACE) mRNA expression was detected by RT-PCR analysis in the ovine fetal brain areas at near-term gestation.

Fig. 2.

The effect of fetal intracerebroventricular (icv) injection of angiotensin I (Ang I) or captopril + Ang I on fetal arterial blood pressure and heart rate (HR). 0 min: time for icv injection of vehicle, Ang I (5 μg/kg), or captopril (2.5 mg/kg) with Ang I (5 μg/kg). **P < 0.01; *P < 0.05 compared with the baseline level (1-way ANOVA followed by Tukey post hoc test). SP, systolic pressure; DP, diastolic pressure; MAP, mean arterial pressure.

Fig. 3.

The effect of fetal icv injection of vehicle, Ang I, or captopril with Ang I on fetal and maternal plasma arginine vasopressin (AVP levels). 0 min: time for icv injection of vehicle, Ang I (5 μg/kg), or captopril (2.5 mg/kg) with Ang I (5 μg/kg). *P < 0.01 compared with the baseline level (1-way ANOVA followed by Tukey post hoc test).

Fig. 4.

Number of positive c-fos-immunoreactive (FOS-ir) cells in the fetal supraoptic (SON) and paraventricular nuclei (PVN). *P < 0.01 compared with the control and captopril + Ang I icv injected fetuses.

Fig. 5.

FOS-ir induced by icv Ang I or captopril + Ang I in the fetal PVN (A–C) and SON (D–F). A and D: icv vehicle. B and E: icv Ang I. C and F: icv captopril + Ang I. 3V, 3rd ventricle. Scale bar, 50 μm.

Fig. 6.

FOS-ir and AVP-ir in the PVN and SON following icv Ang I or captopril + Ang I injection. A–C: PVN. D–F: SON. A and D: icv vehicle. B and E: icv Ang I. C and F: icv captopril + Ang I. Solid arrows indicate colocalization of FOS-ir and AVP-ir. Dashed arrows indicate AVP-ir positive cells without FOS-ir. Scale bar, 20 μm. Green represents FOS-ir positive, and red represents AVP-ir positive.

Fig. 7.

FOS-ir and AT₁ receptor (AT₁R)-ir in the PVN and SON following icv Ang I or captopril + Ang I injection. A–C: double labeling of FOS-ir and AT₁R-ir in the PVN. D–F: double labeling of FOS-ir and AT₁R-ir in the SON. A and D: icv vehicle. B and E: icv Ang I. C and F: icv captopril + Ang I. Scale bar, 20 μm. Arrows indicate colocalization of FOS-ir and AT₁R-ir. Green represents FOS-ir positive, and red represents AT₁R-ir positive.