Effect of Pretreatment with Oestradiol, Progesterone and DOPA on Monoamine Oxidase Activity in the Rat

Abstract

THE enzyme monoamine oxidase (MAO) (EC 1.4.3.4), which is widely distributed in the animal kingdom, is probably involved in the in vivo inactivation of a number of biologically active monoamines of physiological importance although its precise function has yet to be defined. Its putative role in the pathogenesis of mental disease has tended to focus interest on the brain in recent years, rather to the exclusion of other anatomical sites. J. S. et al.¹, however, have lately drawn attention to a variant of this enzyme present in human endometrium which undergoes sharp changes in activity during the different phases of the menstrual cycle. Although there is some evidence to suggest that such cyclical variations do not occur outside the female genital tract in the human², small fluctuations of enzyme activity have been noted in several rat and guinea-pig tissues during the oestrus cycle^3–7. The most likely explanation for all these changes must presumably be based on alterations in hormonal levels, and to investigate this possibility we sought to influence the MAO activity of rat tissues by pretreating the animal with oestradiol and progesterone. Although hormones such as these may mediate the observed changes, the available data do not shed any light on how direct is the cause and effect relationship. There may be intermediate stages in the train of events leading to alterations in enzyme activity. A number of reports^8,9 point to variations in tissue concentration of amine substrates of MAO during the phases of the oestrus cycle. It is possible therefore that physiologically occurring cyclical increases in MAO activity might represent an adaptive response to increased availability of substrate. We have attempted to evaluate this possibility indirectly by treating rats before enzyme assay with DOPA which leads to increased tissue levels of the MAO substrate, dopamine.