Regulation of the cerebral circulation in the fetus during gestation and during delivery is very important for protecting the developing brain from hypoxic, hypotensive and hypertensive insults that could lead to cerebral palsy, mental retardation and neonatal seizures. Sheep have proven a useful model because cerebrovascular regulation can be studied in the fetus in utero as early as 90 days gestation (term= 145 days) by measuring pial arteriolar diameter and cerebral blood flow (CBF). Hypoxic vasodilation is underdeveloped at 90 days gestation and is depressed in offspring of ewes chronically administered ethanol during the first trimester.
In aim 1, we will test the hypothesis that activation of ATP- sensitive potassium (KATP) channels, which contribute to hypoxic vasodilation in mature brain, is underdeveloped at mid-gestation, and with maternal ethanol treatment. Underdeveloped hypoxic vascular reactivity could render the brain severely hypoxic during mid-gestation or premature delivery and might contribute to poor brain growth and mental retardation alcohol syndrome.
In aim 2, we will use an inhibitor of cytochrome P450 RA, whose 02 dependent hydroxylation of arachidonic aid produce a potent vasoconstrictor, 20 HETE. The role of this pathway in the cerebral vasoconstrictor response to increased oxygenation and hypertension, which normally occurs after birth, will be investigated. This vasoconstrictor response limits over-perfusion and may help prevent intraventricular hemorrhage.
In aim 3, the coupling of CBF to neural activity, which partly depends on stimulation of nitric oxide synthase (NOS), will be investigated. Cell localization and in vitro activity of neuronal NOS do not mature until late gestation. We will determine when during fetal development neuronal N-methyl-D-aspartate (NMDA) receptor activation becomes capable of stimulating NOS in vivo, and when such stimulation becomes functionally coupled to cause vasodilation.
In aim 4, ischemic neuropathology was found to be less at 90 days than at 130 days gestation, and the increase in extracellular K+ was smaller and more delayed at 90 days. Astrocyte-based glutamate reuptake transporters largely develop after 90 days. The profile of excitatory amino acids in microdialysates and extracellular K+ activity in neocortex will be compared between 90 and 130 days gestation. The role of NMDA receptors, calcium-activated K channels and KATP channels in the maturation of the anoxic depolarization response will be evaluated. Experiments in these four aims provide a unique opportunity to investigate in utero the development of critical regulatory mechanisms by which the fetal brain defends itself against insults that can cause severe damage and debilitation.
Showing the most recent 10 out of 301 publications
