Incorporation of ¹⁸O into homovanillic acid of rat brain during exposure to oxygen 18 containing atomospheres

Rats were exposed to air containing ¹⁸O₂ at atmospheric pressure. In vivo incorporation of ¹⁸O in brain homovanillic acid (HVA) was determined by gas chromatography-mass spectrometry. One ¹⁸O atom was incorporated into each molecule of HVA indicating that tyrosine is the predominant precursor of brain dopamine and that the oxygen in the 3-position is of atmospheric origin. Intraperitoneal administration of ¹⁸O-enriched water did not alter the ¹⁸O content of brain HVA Mass fragmentography (2) was used to measure the increase in ¹⁸O and the decrease in ¹⁶O in HVA from rat brain over several hours of exposure to an ¹⁸O enriched atmosphere. These experiments demonstrate the possibility to pulse label brain dopamine and its metabolites by in vivo inhalation of stable oxygen isotopes. The procedure should be useful for quantitative determinations of the turnover of brain dopamine in animals and man.