Effects of postnatal dexamethasone on blood-brain barrier permeability andbrain water content in newborn lambs

We showed that antenatal corticosteroids reduced blood-brain barrier permea bility in fetuses at 60 and 80%, but not 90% of gestation, and decreased br ain water content in fetuses. Our objective was to examine the effects of p ostnatal corticosteroids on regional blood-brain barrier permeability and b rain water content in newborn lambs. Three dexamethasone treatment groups w ere studied in 3- to 5-day-old lambs. A 0.01 mg/kg dose was selected to est imate the amount of dexamethasone that might have reached fetuses via anten atal treatment of ewes in our previous studies. The other doses (0.25 and 0 .5 mg/kg) were chosen to approximate those used clinically to treat infants with bronchopulmonary dysplasia. Lambs were randomly assigned to receive f our intramuscular injections of dexamethasone or placebo given 12 h apart o n days 3 and 4 of age. Blood-brain barrier function was measured with the b lood-to-brain transfer constant (K-i) to alpha -aminoisobutyric acid, brain plasma volume was measured with polyethylene glycol for the calculation of Ki, and brain water was measured by wet-to-dry tissue weights. Postnatal t reatment with corticosteroids did not reduce barrier permeability in newbor n lambs. Brain blood volume was higher in the 0.25 and 0.5 mg/kg dose dexam ethasone groups than in the placebo group. Brain water content did not diff er among the groups. We conclude that postnatal treatment with corticostero ids did not reduce regional blood-brain barrier permeability or brain water content but increased the brain plasma volume in newborn lambs. These find ings are consistent with our previous work indicating that barrier permeabi lity is responsive to corticosteroids at 60 and 80% of gestation and brain water regulation at 60% of gestation, but not in near-term fetuses or newbo rn lambs.