Effect of Steroid Administration on N-methyl-D-aspartate (NMDA) Receptor Function in Immature Brain ♦ 342

Abstract

Corticosteroid administration in the fetus and newborn enhances organ maturation and may decrease perinatal brain injury. To test the hypothesis that dexamethasone pretreatment alters development of the NMDA receptor, a mediator of perinatal hypoxic-ischemic brain injury, NMDA receptor ion channel function was investigated in 6 groups: fetal lambs at 84d gestation (n=6), fetal lambs at 120d gestation after maternal pretreatment with either placebo(n=4) or dexamethasone (n=4), and 5d-old term (154d) newborn lambs treated with placebo (PD, n=3), or low-dose (LD, n=5) or high-dose (HD, n=5) dexamethasone. Fetal treatment consisted of dexamethasone, 6 mg IM, or placebo every 12 h × 4 doses to pregnant ewes. Lambs received dexamethasone, 0.02 mg/kg (LD), 0.5 mg/kg (HD), or saline (PD) every 12 h × 4 doses from postnatal day 3. Postnatal doses were chosen to match fetal exposure to maternal steroids (LD) or doses used in premature infants (HD). Tissue was obtained 12 hr after the 4th dose in all animals. Saturation binding assays were performed using 3H-MK801, a noncompetitive NMDA receptor antagonist, and Bmax (apparent number of receptors) and Kd (dissociation constant) were calculated. Bmax (pmoles/mg protein) increased from 0.60± 0.09 at 84d to 0.97 ± 0.12 at 120d and 1.91 ± 0.32 in 5-day-old lambs (p<0.05). Kd did not change significantly with maturation(5.1 ± 0.4 nM at 84d, 4.9 ± 0.8 at 120d, and 5.5 ± 0.4 in lambs). Steroid pretreatment had no effect on Bmax or Kd at 120d. However, Bmax decreased significantly in treated lambs, to 1.65 ± 0.08 and 1.62± 0.06 in LD and HD, respectively (p<0.05 for both vs PD). Kd was also significantly lower in both LD and HD compared to PD (4.4 ± 0.4 and 4.0 ± 0.3, respectively, vs 5.5 ± 0.4 nM), indicating a 20% increase in affinity of the MK-801 binding site within the NMDA receptor ion channel. Previous studies in several species have shown that Bmax peaks in the perinatal period, then declines to adult levels. Thus the decrease in Bmax after postnatal steroid treatment suggests that dexamethasone accelerates the maturational decrease in number of NMDA receptors in the term lamb. The change in Kd associated with steroid treatment could be due to modification of cell membrane lipid composition, leading to altered conformation of the receptor ion channel. These results suggest that steroid administration alters NMDA-type glutamate receptor function in immature brain, and that the effect is dependent on the degree of brain development at the time of administration.