Abstract
Prostaglandins, generated within the fetal brain, are integral components of the mechanism controlling the fetal hypothalamus-pituitary-adrenal (HPA) axis. Previous studies in this laboratory demonstrated that prostaglandin G/H synthase isozyme 2 (PGHS-2) inhibition reduces the fetal HPA axis response to cerebral hypoperfusion, blocks the preparturient rise in fetal plasma ACTH concentration, and delays parturition. We also discovered that blockade of N-methyl-d-aspartate (NMDA) receptors reduces the fetal ACTH response to cerebral hypoperfusion. The present study was designed to test the hypothesis that PGHS-2 action and the downstream effect of HPA axis stimulation are stimulated by NMDA-mediated glutamatergic neurotransmission. Chronically catheterized late-gestation fetal sheep (n = 8) were injected with NMDA (1 mg iv). All responded with increases in fetal plasma ACTH and cortisol concentrations. Pretreatment with resveratrol (100 mg iv, n = 5), a specific inhibitor of PGHS-1, did not alter the magnitude of the HPA axis response to NMDA. Pretreatment with nimesulide (10 mg iv, n = 6), a specific inhibitor of PGHS-2, significantly reduced the HPA axis response to NMDA. To further explore this interaction, we injected NMDA in six chronically catheterized fetal sheep that were chronically infused with nimesulide (n = 6) at a rate of 1 mg/day into the lateral cerebral ventricle for 5-7 days. In this group, there was no significant ACTH response to NMDA. Finally, we tested whether the HPA axis response to prostaglandin E(2) (PGE(2)) is mediated by NMDA receptors. Seven chronically catheterized late-gestation fetal sheep were injected with 100 ng of PGE(2), which significantly increased fetal plasma ACTH and cortisol concentrations. Pretreatment with ketamine (10 mg iv), an NMDA antagonist, did not alter the ACTH or cortisol response to PGE(2). We conclude that generation of prostanoids via the action of PGHS-2 in the fetal brain augments the fetal HPA axis response to NMDA-mediated glutamatergic stimulation.