The development of monoamine oxidase, glutamate decarboxylase and choline acetyltransferase in the guinea pig brain.

The activities of three enzymes involved in the metabolism of neurotransmitter substances in the brain of the guinea pig were studied during gestation and the first month after birth. Monoamine oxidase (MAO) activity towards dopamine, tryptamine, 5-hydroxytryptamine and phenylethylamine in whole brain homogenates was, at day 30 of gestation, already similar to that found in the adult animal. When specific brain regions were analysed, a significant increase in enzyme activity towards dopamine, tyramine and tryptamine between day 40 and 60 of gestation became apparent. In the guinea pig, as in the rat, there appears to be a parallelism in the development of neuronal systems containing monoamines and MAO activity. Glutamate decarboxylase and choline acetyltransferase activities were trebled during the last two weeks of gestation. A further small increase to adult values occurred during the first three weeks after birth.