Expression of Nitric Oxide Synthase Isoforms in the Ovine Fetal Brain: Alteration by Hormonal and Hemodynamic Stimuli

Objective: Nitric oxide (NO) is synthesized in the brain through the action of three isoforms of nitric oxide synthase (NOS). The local generation of NO in neurons, glia, and vasculature modulates neuronal activity, as well as regional cerebral bloo flow. We propose that, in the fetal brain, cerebral hypoprefusion alters the expression of NOS isoforms, and that estrogen administration modulates the NOS response to hypoperfusion.

Methods: Sixteen chronically catheterized fetal sheep of known gestational age (124 to 128 days' gestation) were subjected to a 10-minute period of brachiocephalic occlusion (BCO) or to sham BCO; half of these fetuses wee subjected to subcutaaneous implant, which released 17β-estradiol (E₂; 0.25 mg/d) or placebo. Brain tissue was collected for mRNA and protein extraction 1 hour after the start of the BCO or sham BCO.

Results: All three isoforms of NOS were identified in fetal brain at both the mRNA and protein levels. BCO increased NOS1 (hippocampus, brainstem), NOS2 (hypothalamus), and NOS3 (hippocampus, cortex) at the protein level. Estradiol alone increased NOS1 (brainstem, cortex), NOS2 (hippocampus, hypothalamus), and NOS3 (brainstem, cerebellum) at the protein level, changes tht were not mirrored at the mRNA level. The combination of BCo and estradiol produced smaller changes in NOS1 (braainstem, cortex), NOS2 (hippocampus, hypothalamus), and NOS3 (braainstem) protein levels than those produced by either stimulus alone.

Conclusions: We conclude that the fetal brain expresses all isoforms of NOS, and that NOS expression is altered by both BCo and estradiol, but that the most prevalent effect of estradiol is to reduce specific NOS responses to cerebral hypoperfusion. The present results suggest the possibility that the neuroendocrine responses to estradiol and BCo are modulated by central nervous system (CNS) NO biosynthesis.