The activities of the A and B forms of monoamine oxidase in liver, hypothalamus and cerebral cortex of the female rat: effects of administration of ethinyloestradiol and the progestogens norethisterone acetate and d-norgestrel.

Most of the actions of sex hormones are mediated by specific cytosolic receptors in target tissues, translocating to the cell nucleus where they exert their effects (Buller & O'Malley, 1976; McEwen ef al., 1978). An effective means of monitoring changes in gene products resulting from hormonal stimulation is to measure enzyme activities in target tissues (McEwen et al., 1978). Sex steroids affect monoamine oxidase (E.C. 1.4.3.4) activities in gonadal and other tissues (reviewed by Youdim & Holzbauer, 1976). Phasic changes in monoamine oxidase activity in different rat organs during the oestrous cycle have been attributed to cyclic variations in the secretions of progesterone and oestrogens (Youdim & Holzbauer, 1976). It has been reported that progesterone stimulates, whereas oestrogens inhibit, monoamine oxidase (Youdim & Holzbauer, 1976; McEwen et al., 1978). However, different laboratories have reported apparently conflicting results (Youdim & Holzbauer, 1976). Since monoamine oxidase may exist in multiple forms (Houslay et al., 1976), it is possible that sex steroids may influence these multiple forms differently. A useful classification into two major forms, monoamine oxidases A and B, was first introduced by Johnston (1968). Enzyme A preferentially oxidizes serotonin and noradrenaline and is particularly sensitive to inhibition by clorgyline. Enzyme B is specifically inhibited by deprenyl and selectively deaminates benzylarnine and phenethylamine (Johnston, 1978; Owen et al., 1977). As the effects of synthetic sex steroids have not been fully elucidated, we have studied the effects of ethinyloestradiol, norethisterone acetate and d-norgestrel on the activities of monoamine oxidases A and B in liver, hypothalamus and cerebral cortex of the intact adult female rat. The experimental procedures for steroid administration, brain dissection and homogenization of tissues were as described in the accompanying paper (Lai et al., 1980). Monoamine oxidase activities were determined in a medium containing 40mlrl-potassium phosphate buffer, pH 7.4, and 1 m ~ ~ ~ C l a b e l l e d amine substrate as detailed elsewhere (Owen ef al., 1977; T. K. C. Leung, J. C. K. Lai & L. Lim, unpublished work). Enzyme A was assayed with 5-hydroxy[ *4Cltryptamine and the reaction product was extracted with 2ml of ethyl acetate/benzene ( 1 : 1; v/v). Enzyme B was determined by using I 14Clbenzylamine and the product was extracted with 2ml of toluene. Chronic administration of the progestogens, but not of ethinyloestradiol, resulted in significant decreases in monoamine oxidase A and B activities in the liver (Table 1). These results should be compared with the accompanying paper (Lai el al., 1980) showing that the converse occurs when glucose 6phosphate dehydrogenase activity is monitored, i.e. progestogens were without effect whereas ethinyloestradiol inhibited enzyme activity. These results confirm that, although injection of either norethisterone acetate or ethinyloestradiol resulted in increases in nuclear oestrogen receptor content (Marr et al., 1980b), the mechanism of genomic action of the oestrogen and the progestogen is clearly dissimilar. Although progestogen receptors have not been measured, it is likely that they do not alter since the concentration of hepatic nuclear oestrogen receptor does not vary during the oestrous cycle (Marr et al., 1980a). Because there are no marked differences in hepatic monoamine oxidase A and B activities in dioestrus compared with oestrus (T. K. C. Leung, J. C. K. Lai & L. Lim, unpublished work), the change we observed can be attributed to administration of exogenous steroids. Although cyclic changes in hypothalamic nuclear oestrogen receptor concentration occur (White et al., 1978), we have not observed pronounced differences in either monoamine oxidase A or B in dioestrus compared with oestrus, despite reports to the contrary in the literature (Youdim & Holzbauer, 1976; McEwen et al., 1978). Chronic administration of ethinyloestradiol gave rise to a small decrease (1 3%) of monoamine oxidase A activity and a consequent decrease (from 2.79 to 2.30) of the enzyme A/enzyme B ratio in the hypothalamus (Table 1). This result is consistent with that of McEwen et al. (1978). Administration of the progestogens norethisterone acetate and d-norgestrel did not appear to have any marked effect on monoamine oxidase