Relation between brain tissue pO(2) and dopamine synthesis of basal ganglia - A (18)FDOPA-PET study in newborn piglets

Perinatal hypoxic-ischemic cerebral injury is a major determinant of neurol ogic morbidity and mortality in the neonatal period and later in childhood. There is evidence that the dopaminergic system is sensitive to oxygen depr ivation. However, the respective enzyme activities have yet not been measur ed in the living neonatal brain. In this study, we have used F-18-labelled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron emissi on tomography (PET) to estimate the activity of the aromatic amino acid dec arboxylase (AADC), the ultimate enzyme in the synthesis of dopamine, in the brain of newborn piglets under normoxic and moderate asphyxial conditions. The Study was performed on 8 newborn piglets (2-5 days old). In each pigle t PET studies were performed under control conditions and during 2-hour asp hyxia. Simultaneously, brain tissue pO(2) was recorded, cerebral blood flow (CBF was measured with colored microspheres and cerebral metabolic rate of oxygen (CMRO2) was determined. Asphyxia was induced by lowering the inspir ed fraction of oxygen from 0.35 to 0.10 and adding about 6 % CO2 to the ins pired gas. Asphyxia elicited a more than 3-fold increase of the CBF (p < 0.01) so that CMRO2 remained unchanged throughout the asphyxial period. Despite this, br ain tissue pO(2) was reduced from 19 +/- 4 mm Hg to 6 +/- 3 mm Mg ( p < 0.0 1). Blood-brain transfer of FDOPA as well as permeability-surface area prod uct (PS) from striatum were unchanged. Striatal synthesis rate of fluoro-do pamine from FDOPA (k(3)) was, however, significantly increased (p < 0.01). This increase of the AADC activity is associated with reduced brain tissue pO(2). Asphyxia-induced CBF increase impedes an alteration of brain oxidati ve metabolism.