ORNITHINE DECARBOXYLASE ACTIVITY IN-VITRO IN RESPONSE TO ACUTE-HYPOXIA - A NOVEL USE OF NEWBORN RAT-BRAIN SLICES

In fetal as well as newborn rats, acute hypoxic exposure results in si gnificantly elevated brain ornithine decarboxylase (ODC) activity, pol yamine concentrations, and ODC mRNA. The interpretations of these in v ivo hypoxic-induced changes, however, are complicated by maternal conf ounding effects. To test the hypothesis that acute hypoxia will also i ncrease ODC activity in vitro, we developed a brain slice preparation which eliminates such maternal effects. Sections of whole cerebrum, ap proximately 300-500 mu m thick, were made from 3-to 4-day old Sprague- Dawley rat pups. The slices were equilibrated for 1 h in artificial ce rebrospinal fluid (ACSF) continuously bubbled with 95% O-2/5% CO2 prio r to induction of hypoxia. We induced hypoxia by changing the oxygen c oncentration to 40%, 30%, 21%, 15%, 10%, or O% O-2 all with 5% CO2 and balance N-2. In the normoxic control brain slices, low but stable bas al ODC activity persisted for up to 5 h post-sacrifice. Slices in ACSF treated with bovine serum albumin (BSA), or both BSA and fetal bovine serum (FBS), however, showed stable ODC activity values 2-to 3-fold h igher than slices in ACSF alone, for up to 5 h. In response to acute h ypoxia (i.e., 15, 21, and 30% O-2), ODC activity was elevated 1.5-to 2 -fold above control values between 1 and 2 h after initiation of hypox ia. Qualitative light and electron microscopic examination of the neon atal brain slices following 2 h hypoxic exposure suggested that the gr eat majority of cells did not show severe hypoxic damage or necrosis. It was concluded that: (1) in neonatal rat brain slices in vitro, stab le ODC activity values approximating the whole brain ODC activity seen at sacrifice, can be maintained for several hours; (2) the in vivo hy poxic-induced increase in ODC activity can be approximated in vitro; ( 3) the neonatal rat brain slice preparation may be an alternative to o ther methods for studying hypoxic-induced ODC enzyme kinetics, or othe r brain enzymes, without maternal confounding effects; and (4) ODC act ivity may be an indicator of active metabolism within the newborn brai n slice both in normoxia and hypoxia.