Evidence for an FSH-releasing factor in the posterior portion of the rat median eminence.

To test the hypothesis that an FSH-releasing factor might be contained within the posterior portion of the median eminence (ME), the anterior half of the ME (aME) and the posterior half of the ME (pME) were removed separately from the brains of adult male rats and extracted in 0.2 N acetic acid. LH and FSH-releasing activities of the extracts were measured in vitro by incubating 8 hemipituitaries from adult male rats for 6 h at a dose of 5 tissue equivalents and determining the radioimmunoassayable LH and FSH released into the medium. LH release induced by the aME extracts was significantly greater than that induced by the pME in both experiments, whereas there were no differences in FSH release between aME and pME extracts. A significant dose-related increase in FSH release was noted in this system when 1 and 2 ng of synthetic LHRH were tested which indicates that the assay was sensitive to different amounts of LHRH with regard to FSH-releasing action. The content of immunoreactive LHRH in the extracts was almost twice as high in the aME as in the pME. Therefore, the results indicate that the pME has greater FSH-releasing activity than can be accounted for by its content of LHRH. The additional FSH-releasing activity is presumably due to an FSH-releasing factor distinct from LHRH.