Carbidopa effect on rat brain monoamine oxidase and pineal melatonin

Abstract

A number of investigators have observed a diurnal rhythm in brain monoamine oxidase (MAO) activities (Chevillard et al. 198 l; Bhaskaran and Radha 1984), and it has been suggested that the pineal gland may influence pituitary MAO-A activity (Oxenkrug et al. 1984a). The substrates of MAO-A (serotonin and noradrenaline) are involved in the regulation of melatonin synthesis, whereas MAO-B apparently does not participate in melatonin metabolism (Lewy 1983). In accordance with this suggestion, selective inhibition of MAO-A but not MAO-B has been found to stimulate melatonin synthesis (Oxenkrug et al. 1984a). However, the negative correlation between age-associated changes in rat pineal melatonin and brain MAOB suggests the possibility of melatonin influence on MAO-B activity (Filipowicz et al. submitted). This report represents the attempt to explore such a possibility. In these experiments, rats were treated with a dose of the aromatic amino acid decarboxylase inhibitor, carbidopa, that was sufficient to inhibit the synthesis of pineal serotonin and melatonin but not brain serotonin (Oxenkrug et al. 1984b). This differential effect can be obtained because carbidopa does not readily penetrate the blood-brain barrier. MAO activities were then measured in the