Localization of FSH-Releasing Factor in the Hypothalamus and Neurohypophysis as Determined byin VitroAssay

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Abstract
FSH [follicle-stimulating hormone]-re-leasing action of rat hypothalamic extracts was assessed by an in vitro assay with male anterior pituitaries. The endpoint was the increase in FSH in the medium after a 6 hr. period of incubation. FSH was assayed by the rat ovarian weight augmentation method. Doses of crude extract prepared from 1/16-1/8 of a rat stalk-median eminence/pituitary produced a significant increase of SH release into the media and a dose-response relationship was obtained. Similarly prepared extracts from cerebral cortex or muscle were inactive, as was vasopressin. When FSH concentration of pituitaries was added to that of the medium in order to determine the effect of stalk median eminence extract on total FSH, there was an indication of an increase in total FSH with low doses of extract which was not maintained at higher doses. Frozen sections were cut in 3 planes through the hypothalamus to determine the localization of the FSH-releasing factor. Activity was localized to the medial, basal, tuberal region, an area which included the median eminence and arcuate nucleus. Lesser activity was obtained with extracts of the pituitary stalk, whereas posterior lobe extracts were inactive. It is concluded that the FSH-releasing factor is localized to the stalk-median eminence region in the rat.