Monoamine Synthesis and Concentration in Neonatal Rat Brain During Hypercapnia and Recovery

One-day-old rats were exposed to a gas mixture of 15% CO2-21% O2-64% N2 for a 30-min period. Monoamine synthesis in whole brain was measured during, and at various intervals after, hypercapnia by estimating the accumulation of dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) after inhibition of aromatic L-amino-acid decarboxylase with NSD 1015. Endogenous concentrations of tyrosine, dopamine (DA), noradrenaline (NA), tryptophan, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were measured at the same intervals. Exposure to CO2 induced an increased synthesis of catecholamines and 5-HT. Further, an increase in DA concentration was seen during hypercapnia, while NA and 5-HT were unchanged. After the CO2 exposure the increased in vivo synthesis rates of catecholamines and 5-HT were rapidly normalized, as was the endogenous DA concentration. A slight increase in 5-HT and 5-HIAA concentrations was seen immediately after CO2 exposure. These results indicate that in neonatal animals, hypercapnia induces changes in central monoamine neurons, primarily an increased synthesis. These alterations may be relevant to some physiological changes seen during CO2 exposure, such as the alteration in central respiratory performance.