Fluctuations in Monoamine Oxidase Activity in the Hypothalamus of Rat during the Estrous Cycle and after Castration.

It now seems reasonable to postulate that in the hypothalamus there exists a mechanism that is peculiarly concerned with the release of ovulating hormone and through which changes in the external environment and in the ovarian hormones may influence pituitary function. Markee et al. (1948) produced ovulation by instilling epinephrine directly into the pars distalis or into the third ventricle. This pituitary activation was blocked either by atropine or by such an antiadrenergic agent as Dibenamine (Sawyer et al., 1947; Everett et al., 1949; Sawyer et al., 1949a, Van Tienhoven, 1955; Zarrow and Bastian, 1953; Kobayashi, 1956). Since then, there has been extensive support for a hypothesis that adrenergic as well as cholinergic mechanisms in the hypothalamus are involved in the ovulatory release of gonadotropic hormones (Kobayashi, 1956). The use of biochemical analysis in tissue metabolism afford a new approach to the problem of the neural control of ovulation. As previously reported, the characteristic fluctuation in the contents of choline acetylase (Kobayashi et al., 1963a), an enzyme responsible for synthesis of acetylcholine, and of the acetylcholine-like substance (Gitsh, 1952a and b; Kuwajima, 1957) in the hypothalamus of the rat was found to correlate with changes of vaginal smear in the estrous cycle and those after castration. These observations indicate a cholinergic mechanism of the hypothalamus being intimately correlated with gonadotropin secretion. However, a biochemical approach to the adrenergic mechanism has not been made with respect to reproductive function. There is much evidence for the view that norepinephrine of the hypothalamus plays an important role in certain of its function (Dell et al., 1954; Rothballer, 1956; Magoun, 1958; Vogt, 1959; Carlsson, 1959). Monoamine oxidase appears to be the enzyme largely responsible for the inactivation of brain norepinephrine (Blaschko et al., 1937; Blaschko, 1952; Brodie et al., 1959). The present study was designed to investigate the role of monoamine oxidase in the hypothalamus of the rat during the estrous cycle and after castration.