Effect of Oestradiol on Turnover of Type a Monoamine Oxidase in Brain

Abstract— Administration of oestrogen (oestradiol‐17β or oestradiol‐17β‐benzoate) to ovariectomized (OVX) rats for 1–4 weeks results in an approx 30% decrease in the activity of monoamine oxidase (MAO) in the basomedial‐hypothalamus (BM‐Hyp) and corticomedial‐amygdala (CM‐Amy) but not in cerebral cortex. Further investigation shows that (1) decreased MAO activity in the BM‐Hyp and CM‐Amy occurs only in Type A MAO (serotonin as substrate) and does not occur in Type B MAO (phenylethylamine as substrate); (2) decreased MAO activity does not occur when a single large dose of oestrogen is given i. v. or when homogenates from oestrogen treated rats are mixed with homogenates from OVX rats suggesting that direct enzyme inhibition is not responsible for the change in activity; (3) oestrogen administration to OVX rats increases the rate constant of degradation for MAO in BM‐Hyp and CM‐Amy but not in cerebral cortex as determined in turnover studies using pargyline, an irreversible inhibitor of MAO. The increased rate of degradation results in shorter half lives ( t 1/2) for MAO in the BM‐Hyp and CM‐Amy of oestrogen treated rats. In OVX rats the t 1/2 is 9.8 days in BM‐Hyp and 12.7 days in CM‐Amy. Oestrogen administration results in a t 1/2 of 7.6 days in BM‐Hyp and 7.8 days in CM‐Amy. The possible relationship between oestrogen dependent decreased MAO activity and estrogen dependent lordosis behavior is discussed.