Cardiovascular regulation and expressions of NO synthase-tyrosine hydroxylase in nucleus tractus solitarius of ovine fetus

[Changes in spatial relationships between catecholamine- and nitroxidergic neurons in the nuclei of the caudal brain stem in the development of hypertension]

AIM

To obtain the data on the spatial relationships between catecholamine (TH-positive) and nitroxidergic (nNOS-positive) neurons in vasomotor nuclei of the medulla in different periods of hypertension development.

MATERIAL AND METHODS

The experiment was performed on male Wistar rats (n=45) with induced renovascular hypertension (RVH). TH and nNOS in neurons of solitary tract nuclei, reticular small-and giant cell nuclei were detected using immunohistochemical methods.

RESULTS AND CONCLUSION

The most early and severe changes in the intensity of reaction and amount of nNOS-positive neurons were noted in the solitary tract nucleus. Significant changes in the quantitative parameters of TH-positive neurons in RVH were identified only in the reticular giant cell nucleus but they appeared later and were less expressed compared to nNOS-positive cells. This resulted in the changes of spatial relationships between two types of neurons and remodeling of the bulbar region of the cardiovascular center.

Nitric oxide reduces vagal baroreflex sensitivity in the late gestation fetus

Goals to understand the etiology of essential hypertension have proposed that this problem arises, in part, because of changes within brainstem circuits involved in arterial blood pressure (ABP) control. It has been suggested that nitric oxide (NO) exerts inhibitory influences on the integration of afferent discharge from the arterial baroreceptors. This study tested the hypothesis that the inhibitory influence of NO on the arterial baroreflex is present in fetal life. Fetal baroreflex sensitivity was calculated in fetal sheep, before and during the NO-clamp; a technique that permits NO synthase (NOS) blockade with l-NAME while maintaining basal cardiovascular function with sodium nitroprusside. Under halothane anesthesia, five fetal sheep at 0.8 gestation were instrumented with vascular catheters. Five days later, fetuses received a range of bolus doses of phenylephrine (5-75 microg I.A.) in randomized order either during saline or treatment with the NO clamp. Basal fetal ABP and heart rate before (50 +/- 4 mm Hg, 170 +/- 3 bpm) or during (51 +/- 4 mm Hg, 173 +/- 3 bpm) the NO-clamp were similar. The gradient of the pulse interval-ABP relationship was nearly doubled during NOS blockade (14.2 =/- 2.5 versus 7.8 +/- 1.6 ms/mm Hg). The data provide in vivo evidence that NO attenuates the sensitivity of the cardiac baroreflex during fetal life.

Expression of nitric oxide synthase isoforms is reduced in late-gestation ovine fetal brainstem

Fetal baroreflex responsiveness increases in late gestation. An important modulator of baroreflex activity is the generation of nitric oxide in the brainstem nuclei that integrate afferent and efferent reflex activity. The present study was designed to test the hypothesis that nitric oxide synthase (NOS) isoforms are expressed in the fetal brainstem and that the expression of one or more of these enzymes is reduced in late gestation. Brainstem tissue was rapidly collected from fetal sheep of known gestational ages (80, 100, 120, 130, 145 days gestation and 1 day and 1 wk postnatal). Neuronal (nNOS), inducible (iNOS), and endothelial (eNOS) mRNA was measured using real-time PCR methodology specific for ovine NOS isoforms. The three enzymes were measured at the protein level using Western blot methodology. In tissue prepared for histology separately, the cellular pattern of immunostaining was identified in medullae from late-gestation fetal sheep. Fetal brainstem contained mRNA and protein of all three NOS isoforms, with nNOS the most abundant, followed by iNOS and eNOS, respectively. nNOS and iNOS mRNA abundances were highest at 80 days' gestation, with statistically significant decreases in abundance in more mature fetuses and postnatal animals. nNOS and eNOS protein abundance also decreased as a function of developmental age. nNOS and eNOS were expressed in neurons, iNOS was expressed in glia, and eNOS was expressed in vascular endothelial cells. We conclude that all three isoforms of NOS are constitutively expressed within the fetal brainstem, and the expression of all three forms is reduced with advancing gestation. We speculate that the reduced expression of NOS in this brain region plays a role in the increased fetal baroreflex activity in late gestation.