Relationship of Gonadal Hormone Administration, Sex, Reproductive Status and Age to Monoamine Oxidase Activity Within the Hypothalamus

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Abstract
Activity of Type A monoamine oxidase (MAO) was measured in microdissected samples from preoptic‐hypothalamic and hindbrain areas of young male and female rats, and aged female rats. Administration of estradiol and progesterone, in doses sufficient to facilitate lordosis behavior and induce a luteinizing hormone surge in ovariectomized, but not castrated rats, was associated with sexually dimorphic changes of MAO activity within the hypothalamus. Forty‐two h following estradiol benzoate administration, increased MAO activity was measured in the ventromedial nucleus (VML) and midbrain central gray of females, while decreased MAO activity was measured in the VML and arcuate‐median eminence (ArME) of males. Progesterone administration to estradiol benzoate‐primed rats was associated with decreased MAO activity in the VML and medial preoptic nucleus (mPOA) of females and decreased activity in the dorsal raphe nucleus of males. Activity of MAO on diestrus, proestrus and estrus was assessed in ten preoptic‐hypothalamic and hindbrain sites. Differences between days of the cycle were limited to the mPOA, ArME and VML. While activities were generally lowest at estrus, these areas exhibited different patterns of activity across the cycle. Activity was highest at proestrus in the mPOA and highest at diestrus in the VML and ArME. Activity of MAO in some areas of 25‐month old, diestrus rats was altered as compared to young, cycling rats; however, ageing was not associated with widespread changes in MAO activity. In the suprachiasmatic nucleus, aged rats showed approximately 30% less activity than young rats. In the mPOA, VML and ArME, activity in aged females was different from some, but not all, days of the estrous cycle. These results show that MAO activity changes within specific hypothalamic sites when the neuroendocrine axis is altered. Since the changes are present in areas where activity of rnonoaminergic systems is critical for initiating gonadotrophin surges and inducing lordosis behavior, these results provide initial evidence that catabolism of monoamines by MAO may contribute to rnonoaminergic regulation of reproductive function.