Activated factor of immature neurons for excessive apoptosis induced by exogenous glucocorticosteroid

OBJECTIVE: To confirm the key inducing factor of excessive apoptosis in immature brain under the exposure of exogenous glucocorticosteroid and observe the characteristics of mitochondrial oxidative stress and intracellular [Ca(2+)]i overload as compared to healthy adult rats. METHODS: A total of 192 healthy Sprague-Dawley (SD) rats selected for the study were divided into three groups including PD7, PD15 and PD60 corresponding human age stages including full-term newborn, one-year infant and adult respectively. To mimic the therapeutic regime for infantile spasms, 8 rats in each group was treated with prednisone (6 mg x kg(-1) x d(-1)), ACTH (150 U x m(-2) x d(-1)), normal saline for 4 days and drug withdrawal for 3 weeks. Rats were deeply anesthetized and sacrificed by decapitation. The mitochondrial concentrations of GSH, SOD, MDA, Na(+)-K(+)-ATPase and intracellular [Ca2+i] were all determined by spectrophotometer. The cellular NMDA-R1 expression was measured by immunohistochemistry. RESULTS: Both prednisone and ACTH caused neuron reduced and TUNEL-positive cell increased in the frontal lobe, specially in PD7 group of prednisone, that were 77.7 +/- 4.7 neurons per VF (control group: 102.3 +/- 5.9) (P = 0.002), 114.8 +/- 8.14 (control group: 56.3 +/- 5.6) (P = 0.029) respectively. The mitochondrial levels of GSH, SOD and Na+-K+-ATPase decreased after prednisone and ACTH administration at different level in different age groups whereas the concentration of MDA increased. These changes were significant in PD7 group after prednisone administration, that were 158.3 +/- 6.1 mg/g prot of GSH (control group: 225.1 +/- 9.5 mg/g prot) (P = 0.006), 155.8 +/- 4.3 U/mg prot of SOD (control group: 228.1 +/- 9.2 U/mg prot) (P = 0.006), 14.6 +/- 3.5 U/mg prot of Na+-K+-ATPase (control group: 20.8 +/- 5.5 U/mg prot) and 10.4 +/- 0.9 nmol/mg prot of MDA (control group: 4.8 +/- 1.9 nmol/mg prot) (all P < 0.01). [Ca(2+)]i level in neuron increased, specially in PD7 group after prednisone administration, that was 164.6 +/- 11.9 nmol/L (control group: 125.8 +/- 6.03 nmol/L) (P = 0.003). Similar to [Ca(2+)]i changes, the density of NMDA-R1 also increased mostly in PD7 group after prednisone administration, that was 0.36 +/- 0.03 (control group: 0.21 +/- 0.05) while that was only 0.18 +/- 0.05 in PD60 group (control group: 0.15 +/- 0.02). At 3 weeks post-dosing, all of them returned to normal level. CONCLUSION: Dysfunctions of oxidation and antioxidation in mitochondria of immature neurocytes are induced by prednisone or ACTH within the therapeutic levels. Enhanced expressions of NMDA-R1, Bax and superoxide contribute to excessive apoptosis of immature neurocytes induced by exogenous glucocorticosteroid. The possible determining factors for immature brain is more vulnerable to exogenous glucocorticosteroid damage. It may be associated with the physiologically high intracellular level of calcium concentration and NMDA-R1 expression.