Short-loop feedback mechanism of luteinizing hormone: LH stimulated hypothalamic L-cystine arylamidase to inactivate LH-RH in the rat hypothalamus.

The effect of the intravenous administration of several hormones on L-cystine arylamidase in the hypothalamus of the rat was examined. The injection of 10 mug LH into adult male and female rats was followed within 90 min by an increase of the enzyme activity by 50%, while infantile animals were not affected. When prepuberal rats were pretreated sc with testosterone and progesterone. The iv injection of 0.1 mug oestradiol-17beta into female, and of 0.5 mug testosterone into male intact mature rats also resulted in an increase of hypothalamic enzyme activity. The maximal increase in enzyme activity was seen 16 h after steroid treatment. As it had previously been shown that L-cystine arylamidase inactivates LH-RH, it may be assumed that this enzyme is involved in the short-loop feedback of LH. This assumption is based on the observation that an elevation of plasma LH brings about an activation of the enzyme system, which subsequently leads to an increased inactivation of LH-RH in the hypothalamus. This mechanism seems to depend on the presence of certain plasma levels of oestrogen in female, and of testosterone in male animals.