ACUTE-HYPOXIA INDUCES ELEVATION OF ORNITHINE DECARBOXYLASE ACTIVITY IN NEONATAL RAT-BRAIN SLICES

Recent studies in vivo have demonstrated that ornithine decarboxylase (ODC) activity in the fetal rat brain is elevated 4-5-fold by acute ma ternal hypoxia. This hypoxic-associated increase is seen in the rat br ain in both the newborn and the adult. Because of the intimate involve ment of ODC in transcription and translation, as well as in growth and development, it is imperative that the manner in which hypoxia affect s the regulation of this enzyme be better understood. In order to achi eve this, a brain preparation in vitro was required to eliminate the c onfounding effects of the dam on the fetal and newborn brain ODC activ ity in vivo. Therefore, brain slices from 3-4-day-old (P-3) newborn ra ts were utilized to test the hypothesis that ODC activity increases in response to hypoxia in vitro. Cerebral slices from the P-3 rat pups w ere allowed to equilibrate and recover in artificial cerebrospinal flu id (ACSF) continuously bubbled with a mixture of 95% O-2 and 5% CO2 fo r 1h before beginning hypoxic exposures. Higher basal ODC activities w ere obtained by treating the slices with 0.03% fetal bovine serum (FBS ) and 0.003% bovine serum albumin (BSA), rather than with ACSF alone. Hypoxia was induced in the slices by replacing the gas with 40%, 21%, 10%, or 5% O-2, all with 5% CO2 and balance N-2. With FBS and BSA trea tment, ODC activity was maintained at about 0.15-0.11 nM CO2 mg(-1) pr otein h(-1) throughout the experiment, which was 2-3-fold higher than that without FBS and BSA. ODC activity increased significantly and pea ked between 1h and 2h after initiation of hypoxia. For instance, with 21% O-2, ODC activity increased similar to 1.5-fold at 1h and similar to 2-fold at 2 h. These studies demonstrate that: (1) the hypoxic-indu ced increases observed in vivo in the fetal and newborn rat brain ODC activity can be approximated in a newborn rat brain slice preparation in vitro; (2) newborn rat brain slice preparations may provide an alte rnative to methods in vivo or cell culture methods for studying the re gulation of acute hypoxic-induced enzymes; and (3) high, stable baseli ne ODC activities in brain slices suggest that the cells in the slice are capable of active metabolism if FBS and BSA are available to mimic conditions in vivo.