The similarity of FSH-releasing factor to lamprey gonadotropin-releasing hormone III (l-GnRH-III)

To validate further the existence of a specific hypothalamic follicle stimulating hormone releasing factor (FSHRF), stalk-median eminence (SME) fragments from sheep and whole hypothalami from male rats were purified by gel filtration on Sephadex G-25, and the gonadotropin-releasing activity on hemipituitaries of rats incubated in vitro was determined by bioassay and compared with the radioimmunoassayable luteinizing hormone releasing hormone (LHRH) and lamprey gonadotropin releasing hormone (l-GnRH) activities in the fractions. The FSH-releasing fractions eluted in the same sequence of tubes from the Sephadex column found earlier by in vivo bioassay and were clearly separated from the immunoassayable and bioassayable LHRH. The radioimmunoassay (RIA) for l-GnRH recognized equally l-GnRH-I and -III but had negligible cross-reactivity with LHRH. Fractionation of rat hypothalamic extract by gel filtration on Sephadex G-25 revealed three peaks of l-GnRH determined by RIA, all of which eluted prior to the peak of LHRH. Only the second peak had FSH-releasing but not LH-releasing activity. To determine if this FSH-releasing activity was caused by the presence of l-GnRH in the fraction, the pituitaries were incubated with normal rabbit serum or the l-GnRH antiserum (1:1000), and the effect on the FSH- and LH-releasing activity of the FSH-releasing fraction and the LH-releasing activity of LHRH was determined. The antiserum had no effect on basal release of either FSH or LH but eliminated the FSH-releasing activity of the active fraction without altering the LH-releasing activity of LHRH. Since l-GnRH-I has little activity to release FSH or LH, and its activity is nonselective, whereas previous experiments have shown that l-GnRH-III highly selectively releases FSH with a potency equal to that of LHRH to release LH, the results support the hypothesis that the FSH-releasing activity observed in these experiments was caused by l-GnRH-III or a closely related peptide.