RELATIONSHIP OF EXTRACELLULAR DOPAMINE IN STRIATUM OF NEWBORN PIGLETSTO CORTICAL OXYGEN-PRESSURE

The present studies describes the relationship between extracellular d opamine in striatum of newborn piglets and cortical oxygen pressure. T he extracellular level of dopamine was measured by in vivo microdialys is and the oxygen pressure in the cortex was measured by phosphorescen ce lifetime of oxygen probe in the blood. Controlled, graded levels of hypoxic insult to the brain of animals were generated by decreasing o f the oxygen fraction in the inspired gas (FiO(2)) from 21% to 14%, 11 %, and 9%. This resulted in decrease in the cortical oxygen pressure f rom 31-35 Torr to about 24 Torr, 15 Torr and 4 Torr, respectively. The changes in extracellular level of dopamine, DOPAC and HVA were depend ent on changes in cortical oxygen pressure. Stepwise decrease in the c ortical oxygen pressure (see above) caused increases in extracellular dopamine of about 80%, 200% and 550%, respectively The levels of DOPAC and HVA progressively decreased and when cortical oxygen decreased to 4-6 Torr were about 50% and 70% of control, respectively. After retur n of FiO(2) to control (21%), the cortical oxygen pressure rapidly inc reased to above normal, then returned to control values. The extracell ular levels of dopamine, DOPAC, and HVA recovered more slowly, attaini ng control values in about 30 minutes. The data show that extracellula r levels of dopamine increase with even very small decreases in oxygen pressure. Thus, there is no ''oxygen reserve'' which protects dopamin e release and metabolism from decrease in oxygen pressure.