Possible recognition of the GnRH receptor by an antiserum against a peptide encoded by nucleotide sequence complementary to mRNA of a GnRH precursor peptide

Two peptides, rHRnG and hproHRnG, which were encoded by the nucleotide sequences complementary to mRNA of rat hypothalamic gonadotropin-releasing hormone (GnRH) and human placental proGnRH(−3–13), respectively, were synthesized. A remarkable hydropathic anti-complementarity was observed in the N-terminal region between hproHRnG and human proGnRH(−3–13). Neither hproHRnG nor rHRnG bound GnRH in ELISA unless exremely high concentrations of peptides were used. ¹²⁵I-GnRH failed to bind with either rHRnG or hproHRnG previously coated polypropylene tubes. Antisera against these peptides were generated in rabbits. All the rabbits produced antibodies with high titer as tested by ELISA. One rabbit immunized with hproHRnG showed markedly reduced serum testosterone levels as compared with those of other rabbits. Intravenous administration of 1 ml serum from this rabbit, antiserum R281, into ovariectomized rats significantly decreased plasma LH. Using antiserum R281, about 10% of female rat pituitary cells were stained by immunohistochemistry. The staining was specific to hproHRnG since it was abolished by preabsorption of the antiserum with hproHRnG, but not with rHRnG, GnRH, LH nor any other peptide tested. This particular antiserum may have recognized the GnRH receptor, and thereby interfered with the action of endogenous GnRH. These results appear to be in agreement with the view that there is a structural similarity between the receptor for a peptide and the so-called complementary peptide.